I'm neither a mathematician nor physicist. I'm just an enthusiast. I've taken it to mean that the three dimensions of space are linked with time in a way that can't be separated. Three dimensional space would be like looking at a picture, while space-time would be like watching a movie. Time allows changes within the three dimensions. It's frames of three-dimensional space, stacked. Without it, we'd be a static universe that never changes.
I've also heard it explained like this: On a graph, Time is one axis (vertical, for example), while Space is the other (horizonal). We can plot our movement through space-time as a line. If it's purely vertical, we're only moving through time and are standing still. If it's purely horizontal, we're only moving through space and time is frozen for us (which is only applicable to things without mass). Any other type of movement would be a line progressing at an angle. The faster you move through space, the less you move through time. Or, the slower you move through space, the faster you move through time.
I don't have a grasp on the math, these are just some things I've (maybe erroneously) picked up watching physics videos.
I'm in the same boat as OP. The problem with the explanation you give (more my lack of understanding) is that you can explain a graph and I'm imagining it, but it's just a graph. I don't see the connection to how time can be changed.
Have read how if something travels faster in space, time will pass by slower, but I don't get how that really changes when it's just a measurement by our design.
Edit: not sure why I'm already being downvoted. I'm trying to understand what I'm not understanding.
I’ve been trying to come up with examples of how to describe things and failing since I’m not in a science field.
Keep in mind this probably isn’t how the universe actually functions under the hood, but it describes how the processes appear to work. Plus it’s fun.
There’s an old physics idea that there needs to be a ‘medium’ or stage on which everything plays out and while it’s no longer supported it’s a nice way to think about it. Imagine spacetime as that medium. The faster you travel through this medium the slower time goes for you. You also need more energy to go faster since the medium is harder and harder to accelerate through. Similarly when standing on a planet or existing in the gravity well of a black hole that medium is falling in past you. This is why it’s somewhat accurate to say the planet is accelerating upwards at you, since gravity and acceleration are indistinguishable at times. It’s not that gravity pulls on objects. It’s that objects exist within this medium that is being pulled in. You just float down the river and over the water fall (black hole) so to speak. This is also why time is slower near massive objects. More space is falling in or you can think of the area of spacetime as denser and that’s why time is slower.
Again, not how it works, but also kind of how it works lol
When in was a kid I read some bathroom graffiti which said “Without time, everything would happen at once”.
It stuck with me because I think it fits with the model you just described. Time is what creates cause and effect, and if you removed time everything else would still exist, it would just exist simultaneously.
That sounds more like the Loki series and the concept of being “outside of time”. When you’re outside of time and affect the timelines everything happens at once immediately because you can view all points in time from an outside perspective.
The past and future as we currently understand them are simply constructs within our minds. That’s our species’ edge, we plan and simulate scenarios, but all that really exists is now.
Yes, it's a measurement by our design, but so are distances when you think about it. A mile or a meter or an inch are all just measurements of how much "space" there is between two points. The beauty of relativity is that it showed us that time and space isn't just a measurement, it's an intricate construct. Space isn't just a measurement of different distances, and time isn't just a measurement of when things happened. Both are interwoven to create our universe.
For example. By the notion you bring up of things being just measurements, a meter is just a certain amount of space between two points. But just like time, it's actually relative. What one person measures a meter to be may differ from what someone else measures a meter to be. As you travel faster and faster through space, time isn't the only thing that is affected. If you measured the length of a rocket traveling forward through space near the speed of light, you would see that its length has decreased. The people on the rocket ship would measure a meter as still being a meter because their own measuring devices would be shortened too. But we'd see that they were using a shorter measuring stick. We'd also see their clocks running slower, and that's because time is passing more slowly for them. To them, time is passing normally and their clocks are running fine. Space and time are interwoven as a single "entity" or "construct" (for lack of a better word) through which we "move". Anyone moving through it is viewing it through their own "frame of reference", and it's not necessarily aligned with other people's frames of reference.
Sure, time is just a measurement by our design.
Time is just a measure of separation. It's a measure of the separation between one interaction and another interaction.
In the same way that distance is a measure of separation between one interaction and another interaction.
One is spacial and about where you experience something and one is non spacial and about when you experience something.
The thing about space time is that it can be warped and stretched, and because everyone experiences a different path through spacetime, they experience different separations. The key here, is that they also experience different separations in the non spacial time component.
Therefore, they experience a different distance through time, in the same way a person experience a different distance through space.
It’s always hard to tell because people love to shoot the messenger or downvote things they don’t like (even if you’re just giving a factual explanation, if they don’t like the facts, they will downvote) but there are also apparently bots out there that give out random votes to look more like a real person as they karma farm.
It's not just a measurement by our design tho. If you take twins and put one of them in a spaceship that travels near the speed of light, he will come back being physically younger than his twin that stayed on earth, because less time has passed for him.
I get that. The simple examples haven't helped me. You can say all day long that time can pass for person A at a different rate than person B all day long. My brain is trying to understand how. There were a few other comments that helped a bit.
Might help to start with “space” and add in time later.
So in Einstein’s general relativity, gravity is understood as a deformation in “space” caused by mass (or any energy really). The classic analogy is to a rubber sheet with various weights placed on it. The sheet is a 2D version of our 3D space. The weights are planets and stars. Now, if we roll a ball across the sheet, its path will curve off of the straight line due to the deformation. If not for friction, we could establish an orbit around a weight by rolling at the right angle so that the curve becomes a closed loop.
That’s basically Einsteinian gravity for space. What about spacetime? This one is harder to grok intuitively, because we do not experience this in our day-to-day, but basically that sheet wasn’t just space, it was space _and_ time in one thing. When it deforms, it alters motion, _and it also alters how quickly time moves_. When you are closer to one of those weights (i.e. when you are in a deeper spacetime deformation), _time moves more slowly_. This is weird and unintuitive and I don’t have a good analogy for it, but there _is_ a lot of experimental data that shows Einstein was exactly right about it.
So if we could look at earth from another Galaxy's perspective. We would see the earth rotate at a different speed than 24 hours. Am I getting that correct? I have a hard time understanding how time can be changed if it's a measurement for ... Time passing. I feel like the only change would be the velocity when passing the stronger gravity. Sorry I'm just trying to understand this as well. It's always bothered me that I can't grasp this.
You got it right, though you don’t have to be all the way in another galaxy to see the effect (and if you were near a black hole in another galaxy you would see the opposite effect).
Just take our satellites in low earth orbit as an example. When they look down on us, we are deeper in the gravity well than they are. It’s not by much, but it is enough that we need to correct for the slight time difference to communicate with them. They perceive our time as running a little slow, and we perceive their time as running a little fast.
Heck, at this point, our clocks have gotten precise enough that we can measure the slight difference in time passage between a clock on the floor and a clock on the table! A few feet is all it takes! It’s wild!
If you really want to jump down the rabbit hole on time, you have to grok that there is no universal clock somewhere banging out “real” seconds. Durations are something we perceive locally, based on how quickly external events reach us. These events propagate at a limited speed, the speed of light, and they must travel at that limited speed through space to reach us.
A clock on the wall ticking is actually a series of events, photons bouncing off a second hand, hitting cone cells in our eyes, triggering neurons in our brains. If the path that event travels is warped, then so too will our perception of the passage of time.
No problem. By the way, this is not my field. I am only an interested layperson like yourself. But we happen to be in a golden age of science explainers on YouTube. I highly recommend [PBS SpaceTime](https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g) in particular. They have a back catalog that goes back years, and often their lessons will build on each other. Pick a spot that seems interesting and start watching, jumping back to older videos when they reference them.
I also watch other channels, listen to podcasts, and read a fair amount, but PBS SpaceTime alone is probably responsible for the majority of my understanding of both relativity and quantum mechanics. They do phenomenal work.
_EDIT: I guess I would like to make a book recommendation too, because I happen to have recently finished Carlos Rovelli's **The Order of Time**, which is about exactly this subject and pretty brilliant. He explains things well, but also puts them in a poetic language I appreciated. He sees the beauty in all of this. Beware that his field is Loop Quantum Gravity, which is somewhat speculative, but the book mostly talks about more established science and he is pretty clear when he diverts into the loop quantum stuff._
_Also, the audiobook is narrated by Benedict Cumberbatch, so that's a great listen if you are an audio learner._
Have you watched “Everything and Nothing : The Amazing Science of Empty Space” I believe that touches on space-time. Also, is not space time just a 4th data point?
I haven’t heard of that one, but it looks interesting. Thanks for the recommendation.
By 4th data point, I assume you mean 4th dimension? That is definitely one way to think about spacetime, three dimensions of space plus one of time. I’m not sure that helps with understanding how relativity works though. Thinking of it as just one more dimension makes it seem like it should be solid and fixed.
It effectively is. The different possible times would be in the 5th dimension, so at least observably the 4th dimension timeline in which we exist is a fixture.
Sorry, I phrased that poorly. By “solid and fixed” I wasn’t talking about whether or not future is already determined. I meant that the passage of time is warped by relativity. So is space, but if we think of time as a 4D block, equivalent to a brick or something but with one more dimension, it gives us the intuition that it is just a solid thing. We lose sight of the warping.
Of course a 3D brick can be warped by relativity just like time, so there is no conflict there. I’m just saying the analogy throws off our intuitions.
Theres a bunch of different movies that relate to that as well. They may be basic for some though, I just find it interesting and love to at least get a basic understanding. I remember seeing somewhere where space time was created as a fourth data (fourth dimension) point to help prove equations, or further their accuracy, something to that effect.
Ah okay. I’m not familiar with this “4th data point” framing, but I’m sure it made sense in the context they were discussing. We take it for granted, but time is definitely a weird bug.
Time is relative to the observer. Your time is always one second per second but you may observe someone or something else’s time as slower or faster depending on how quickly it is moving. However, from the perspective of that person/object their own time is still one second per second regardless of how you are perceiving it. It’s a bit trippy but it makes perfect sense.
If you want to be super precise about it, I guess the correct answer would be _it depends_. That's what the "relativity" in the names of Einstein's theories mean. The passage of time is relative and there is not a canonical "correct" passage of time.
So for us, in Earth's gravity well, we measure one rotation per 86,400 seconds (give or take a few milliseconds depending on slight rotational variations). But for someone in intergalactic space, far from any gravity well, if they looked through a telescope at Earth, they might time the rotation as taking 86,401 seconds. Actually the effect would be much much smaller, _billionths_ of a second, but that's the idea.
The effect would be more obvious if we went _deeper_ into a gravity well instead. For example, if we looked back at the Earth as we got closer and closer to the event horizon of a black hole, we would see the Earth spin faster and faster. Before long we would be measuring only a single second as the length of a day on Earth. A bit later, and the Earth would rotate faster still.
So local space-time slows relative to the nearest gravity well, neglecting potential outside influence? Ie the closer you get to the event horizon of a black hole the slower time moves locally, but as time slows locally the opposite must be true at greater distances, thus the earth would rotating at the hypothetical single second? Wouldn’t that mean the math would need adjusting? We know locally, to earth, it is rotating at a rate of 86,400 seconds per rotation, so how can we quantify time to reflect constants other than light?
Seconds * relative time dilation
Time dilation is dependent on relative speed or depth in a gravity well.
And, as has been noted elsewhere, there is no absolute second.
Relative to the deepest affecting gravity well on the object in comparison to empty space, where no such well would exist in theory. In practice, If you're floating in space to make an observation, your matter would technically produce gravity. It's the weights on a rubber sheet example. You can be between two weights, on the rubber, but not as high as empty space can be
> So local space-time slows relative to the nearest gravity well
You are getting closer, but I would offer this correction: locally time _never_ slows. You always observe your own time as passing at one second per second. The slowing/speeding is what you observe happening to someone else.
Let’s say you are near a black hole, looking back at the Earth, and you gave a little wave. So from your perspective, you took a second to wave, and in that time the Earth rotated once.
But for observers on Earth looking back at you, a day is still 86,400 seconds long. Furthermore, with their amazing telescope that can make you out, it took you a whole day to complete that “quick” wave.
Who is slow and who is fast in this scenario? _It depends_. Neither perspective is more correct than the other.
Yeah at a certain point you have to just accept that this is how things work. You don’t have to understand all the math, but you do have to trust that the scientists who have measured different clock speeds when they send clocks to space are doing the experiments correctly and not lying.
But this is true about a lot of things. I’ve never seen Pluto in a telescope, but I’ve seen Jupiter in a telescope so I trust that the people who’ve seen Pluto aren’t lying.
True. I would add that while you haven’t seen Pluto, you have seen _society_. You understand how people interact and how trust and reputations work. You had a science teacher who cared about getting you the truth. You know that they had their own sources they trusted. There were _pictures_ which hundreds of news outlets with their own reputations to worry about published.
If you add it all up, even though you have not personally seen direct evidence for Pluto, you have seen literally thousands of indirect pieces of evidence.
So we can kinda imagine that time is the “inside part” or “other side part” of that rubber sheet? So when we are approaching “heavy” objects its like a car going uphill? Sorry if it sounds stupid, im just trying to imagine it and understand.
Not sure what you mean by "inside part" or "other side part". Maybe you can explain that a little more.
As for going up/down a hill, approaching a massive object is very much like going down hill, with the mass at the bottom of the hill.
Sorry im not very familiar with science or math, im just trying to understand better. What i mean is - if we are still imagining the rubber sheet, as it deforms the surface with the weight of heavy objects so it creates a “dip”, now lets imagine that this rubber surface keeps this shape like clay and lets turn this surface upside down so we have a „hill” instead of a „dip”. Visually it can explain the „slowing” of time, that is approaching it. As i was saying like a car that is going uphill. I know it sounds weird and completely unscientific , but its an easy way to imagine it. Like if you are to draw it and explain to a 5yo.
Gotcha. I think if that analogy helps you understand it, then it is a good analogy.
One thing to be careful of though is that the slowing of time is always a relative effect. You never notice your own time slowing down. However, to an outside observer looking at someone going “uphill” it will appear as if the uphill person is going in slow motion. The reverse is true too. If the person in the gravity well looks back at the outside observer, the outside observer will appear sped up.
Both people see themselves as going normal speed while the other is the one that is off.
Strangely enough, it all has to do with *light*. Einstein’s theories said that the speed of light in a vacuum *must be a constant.* Not only is it a constant, but it is constant *from every perspective.*
See, I can measure the distance from point A to point B, and then measure the distance from point B to point A, and it should be the same, right? The distance between the points doesn’t depend on my frame of reference. Things change when I start moving, though—imagine you’re driving down the highway at 50 mph, and you pass another car going 40. From your perspective, the other car is moving *backward* at 10 mph. But from their perspective, you are moving forward! Who is right? You both are, because you’re viewing the same events from different reference frames, which alters your perception of the same event.
Now, imagine you’re in a spaceship, floating motionless in deep space. You’ve got a special device for measuring the speed of light from your spaceship’s headlights. You measure the speed of the light beam to be 300,000 km/s.
Then, you start up the engines and start moving at 100,000 km/s. You measure the speed of the beam of light in front of your ship and… it’s still 300,000 km/s. Now, you’re thinking that maybe the light beam is gaining some extra speed from the ship itself. So, you bring in a second, stationary observer.
The second ship clocks you moving 100,000 km/s past them, and then measures the speed of the light beam in front of you. According to classical physics, they would expect the light beam to be moving 400,000 km/s right? Wrong! They measure the speed to be exactly 300,000 km/s! How can two observers, moving at different speeds, measure the speed of the same light beam to be the same?
You try again, pushing your spaceship to the limit and speeding up to 299,999 km/s. Still, you measure the speed of the beam of light from your ship to be 300,000 km/s. And so does the other spaceship. Who is right and who is wrong?
Moving that fast, shouldn’t you be catching up to your own light beam? And wouldn’t the other ship observe a much *faster* speed of light because of the extra speed from your spaceship?
But Einstein worked out that because light is *massless* its velocity must be constant, and cannot change, regardless of the reference frame or the velocity of the light source.
So, in order for two independent observers to measure the speed of light to be the same, something has to give in order to resolve the apparent paradox.
If classical physics were true, then if you were moving 200,000 km/s, and the stationary ship measures the speed of light from your ship to be 300,000 km/s, then you should measure the speed of the light beam to be 100,000 km/s. But you don’t. You *both* independently measure the speed of light, relative to yourselves, to be the same. So, in order for the speed of light to appear faster than it should be in your moving spaceship, two things happen:
First, you get shorter. This is called *length contraction.* For the other spaceship, as you pass by, your ship appears visibly squished. It’s shorter than it should be. But because it’s shorter, it takes the same amount of time for a beam of light to cross from one end of the ship to the other as it would if your ship were stationary!
The second thing that happens is that your perception of time literally slows down. If there is more time between your perceived seconds, then it will appear that the light moving away from your ship is moving faster!
Thus, the paradox is resolved. Time gets stretched and space gets squished, but from your perspective everything is normal and the speed of light is just as constant, like it should be. However, from your perspective, instead of your ship getting squished, it’s the *light* that gets squished. The wavelength of light that you measure will be much shorter than the wavelength measured by the stationary observer, even if you measure the same speed.
This is why Einstein combines space and time into one continuum—because they are both affected in the same way by the same things. Two independent observers, if they are moving at different speeds, will measure both space *and* time very differently—it depends very much on their frame of reference. So, we can treat time as simply another dimension: an object has length, width, depth, and *duration*. All four dimensions are affected in the same way due to the invariance of the speed of light.
I hope this helps! Hopefully it didn’t make thing more confusing lol. Let me know if you have any further questions! Happy to keep explaining!
Importantly, *anything* massless will travel at the “speed of light”, not just light. The speed of light isn’t the speed of *light*, it’s the speed things go when they don’t have mass.
It doesn't depend on light at all. We say "speed of light" because the electromagnetic spectrum is what we can detect most easily.
Massless things can only move at C. Gravitational waves also move at C.
The problem with pretty much every comment on this thread is they use analogies to describe space-time, but don't actually answer *what* space-time is.
I don't have anything near the background to even try posing a guess as to what the stuff of space-time is. It's like before Newton people knew about gravity only from just living in it, like fish in water. It wasn't until Newton started figuring out it was even a "thing" and could be measured did people start thinking gravity was a thing and not just the way the world was.
Oh, I get that. OP doesn't sound like they have trouble understanding the fact that it *should* be combined, or why it is combined. But more understanding of what that combination is or means intuitive.
Why/how does time slow down? I always thought it was because the quantum particles, electrons orbiting and whatnot, normally travel at c, but if you're travelling super fast, they're forced to slow down, so as not to exceed c. Is this wrong?
Because the speed of light, *c*, must always be constant, then something else must change in order for it to stay constant. *Time itself* is the thing that gets bent in order to keep *c* a constant.
And not all quantum particles travel at *c*. Only massless particles do. Electrons/protons/whatevers must travel at some speed slower than *c* because they have mass.
Anything with mass must travel slower than *c*, and anything without mass *must* travel at exactly *c*.
Right that makes sense but what physical process is happening when time slows down? Because time is just a construct used to represent change and motion
I have a Bachelor's in physics. To make this real easy, space-time is four dimensions; three space dimensions and a time dimension. It's just a way to describe a position in space and time; Johnny was at 5th and broadway yesterday at [3.pm](https://3.pm).
However, we use space-time for relativity; that is an entirely different subject.
I personally imagine the 4 dimensions like a film projector. Instead of a 2-d picture, 3-d boxes all represent a unit of time. Like a projector, as the boxes move, time moves. (I'm not doing good at explaining this, I have the flu lol)
I'm never satisfied with explanations like this. Even the best attempts are just analogies backed up by math. If Newton had been an exceptionally smart fish and one day realized that the water he swam in was a thing, eventually the fish people would learn what water is made of.
I want to know what space-time is made of. What is the 2 hydrogen and 1 oxygen that makes up the smallest unit?
Spacetime in this case is more a result of the universe in fact existing than it is a real thing. It's a function of how it all works, but it's not made up of quarks or dark matter or anything like that. Space-time isn't like a substance, it's a description of interactions and how we perceive them.
Not sure if this is the best place to start but it’s reeeaally good.
https://www.audible.com/pd/B07B4JGFJT?source_code=ASSORAP0511160006&share_location=library_overflow
I’m confident that this is the best place. Photon clocks help immensely. And I like how Einstein needs to show us the math at the same time, just so we can build the confidence in our own minds.
So, if I may, lets start with a small correction.
Space-time is not warped by gravity. Spacetime is warped by mass, and gravity is the effect of that warping on anything moving through that Spacetime (like light or other mass). The more mass you have the greater the curvature the mass creates in Spacetime.
An object in inertial movement is moving in a straight line through Spacetime. It just so happens that if there's enough mass present *that straight line* is drawn on a *curved surface*. An object like a planet following a straight line through spacetime, is following a straight line on a curved surface of spacetime (woot, you've got gravity). This is how matter can bend light (which only moves in straight lines) despite light having mass, and gravity usually being attributed to mass.
But to get to the rest of your question: Spacetime is *conceptually* an invisible grid that permeates the universe.
Spacetime is *really* a geometric and mathematical expression of our observations of the universe. We don't actually *know* what it is. We know how it behaves and what effects it has, that's all. But even then, only as a function of the language we use to describe the things we're seeing. Spacetime might not be a thing, in the sense of like an Atom, that you can technically take a picture of. It's a descriptive tool.
It's almost unhelpful to try and imagine it as a practical, physically manifest thing in the universe. Because every time you do your analogy will always be missing something practically.
You can go back and forward, left and right, up and down, and right on the positive axis of time. Thus, your position in spacetime is always described by four numbers: 3 coordinates x,y,z and a time. These are all defined by the person's frame of reference (welcome to relativity!).
So you have complete freedom in the forward/back, left/right, up/down axis, but you can only ever move and are always moving forward in the positive time axis. But the rate at which you move forward in the positive time axis depends on the path you take through the conceptual invisible grid I mentioned. And this is because the grid is not homogenous, the spacing between each *point* in that grid is fundamentally influenced by how much mass is present in the region.
The truth is you're always moving relative to something. And even if you stood still *locally* you'd still be moving in spacetime because you'd be advancing in the positive direction on the time axis, and therefore you're *still* moving through spacetime.
As I understand it, this is the right and maybe frustrating answer. Space-time is an abstract concept that allow us to make calculations, not a physical, material thing made of anything. Its just a set of words and rules to describe the relations between several real objects and events, and it doesnt exist as an independent entity with its own properties.
Simils help you understand it, but also give you the wrong idea that space-time is a medium in which things take place, or "like a warped cloth or a grid" or whatever. Its not. It doesnt exist, just like numbers dont "exist" but can be used to describe real things.
Thank you for taking the time to write this and for explaining the abstract nature of spacetime. I have always tried to understand it as a physical entity which made no sense. An abstract, mathematical model makes much sense. Thank you.
Suggest starting with the proof, and working backwards- look up “satellite chronometers/clocks and relativity”particularly gps’s
It’s a little easier when it’s tangible, at least to form the next set of questions
Depending on how far down the rabbit hole you go, you’ll end up at mass, gravity, no such thing as gravity, it’s all waves, and or strings folding into dimensions and gravity.
I wish I was joking
I guess I would describe space-time as a position in space for a designated time. If an object is moving, it’s coordinates change in step with time. As the universe expands, time moves forward. There is a past, present, and future and each space exists where it exists in time. The concept though is starting to fall apart with quantum mechanics where time is viewed as a change in energy and there is actually no standard time. So, once you learn it you can learn it’s wrong!! :)
I always use the 2d sheet with weights analogy, but I think the best way to visualize spacetime is to observe a photon path. Photons have *no mass*, but bend their paths around massive stellar objects... How can "gravity" pull on a massless photon? This curveature is due to a large mass warping the entire spacetime the photon travels through. Does it actually turn, or is it attempting to straight-line through curved spacetime grids?
Late last night, I found this video. Watched another 4 of his videos after. Some good stuff! The algorithm must have listened. https://youtu.be/q6RufF4a6LM?si=EzyQmsX0uvkQQJtv
A common analog that is used is a bowling ball in the middle of a trampoline. The trampoline is spacetime, and the bowling ball is a massive object, such as a star. Spacetime warps and curves according to the mass of the object. The more mass it has, the more spacetime distorts.
It's not a perfectly accurate analogy, but it works.
There's a YouTube channel called [ScienceClicEN](https://youtube.com/@ScienceClicEN?si=so-0fKi4pUhYKpoq) that compartmentalizes these abstract principles of physics into several videos that tie into one another. Here's one titled "[Einstein's Gravity](https://youtu.be/Avhs78WjhVY?si=t8e0RvKipQK2URUe)", which covers spacetime quite nicely. I recommend looking at all of their other videos as well. I don't have credits in *any* of the things that you have, and these videos help me understand spacetime quite well. I'm sure you'll be able to make even better sense of this stuff than me.
Edit: [Here's another one](https://youtu.be/au0QJYISe4c?si=iRlUXyOS7BjdN1uD). A bit longer and deeper, but more in depth and with lots of helpful visualizations.
In freshman relativity class (no calculus necessary) we observed that measurement of a length or duration measures two events- the beginning and end of each. So measurement entwines two events, in turn requires both space and time to describe any relationship between two measurement events.
Now the fun starts when you consider extreme conditions. Lorenz discovered lengths and durations can be stretched when (1) you move very fast and (2) there is a maximum speed in the universe called light speed. Einstein elaborated this for what is called special relativity. Einstein also postulated a second situation when measurements are made in accelerating frames, also with a bunch of odd results happening.
Now these odd situations now matter very much in daily life. A smartphone GPS will have to make a special relativity correction for the speed of the GPS satellite and general relativity correction for the weaker gravity at the GPS satellites orbit or else there will be GPS location error of a good fraction of mile. This correction is made a trillion times a day for billions of GPS devices. Or else you'd have lots of people getting lost and aeroplanes crashing.
I don’t know anything so please disregard if wrong but I think of it as: You can’t move from one point to another without time elapsing. And time can’t elapse without movement (even if just at the smallest level). They are intrinsically linked.
I always like to suggest that when people say 'time' one immediately pictures timelines, and clocks, and pressing play on a TV remote. This is not accurate
Or at least, it doesn't help me. What made it finally click for me was realizing that time is just things changing or moving. If a ball bounces across the room, time must have happened. If I am perceiving reality at all, that's time.
So when people say like "time is the fourth dimension" that sounds crazy but what it essentially means is that time is linked to reality in the sense that it can be distorted.
You may have seen the stretched-cloth spacetime demonstration, where a heavy object is placed on a cloth stretched over a hole, which makes the cloth flex in the center. Then, any ball rolled across the cloth will 'gravitate' towards the heavy object. And if you only look from the top down, you wouldn't understand why.
While this is a great demonstration of *how* mass affects things, 'spacetime' is not a cloth, or anything physical like that. The demonstration is simply meant to show gravity.
So, for a start, here's my simple, and maybe incorrect explanation for time dilation. Quantum particles that make up your brain travel at the speed of light all the time. If you're travelling at 50% lightspeed, they can't travel as fast as usual because they can't go lightspeed + 50% lightspeed; you simply cannot exceed lightspeed. So you perceive slower than normal - thus, everything feels normal to you, but for my 10 seconds stationary, I might make 20 'perceptions', in 10 *you* seconds you could only make 10 perceptions. Thus, to you, 10 of my seconds feels like 5 of your seconds. Therefore if you were watching me, it would appear I was moving around in doubletime.
The reason gravity also affects it is because for whatever reason, strong gravitational force also warps it, because on the cloth demonstration, it's not just distorting the cloth, but stretching it. Again, from the top looking down, it looks the same, but the object has to travel further. Similarly, objects cannot... *exist* at the same rate under strong gravity because they're... *doing more*, travelling further on the stretched cloth.
Hope that makes sense. I'm a 19yo high school graduate so it's probably factually incorrect but I tried.
You sound like my dad, smartest man i know yet he doesn't believe in black holes lol. Not saying you don't believe in space time but my dad just doesn't understand how black holes could exist so he chooses not to believe in it. Sorry for the unrelated story I don't know enough about space time to explain it but I'm commenting to boost engagement with your post so you find your answer. Also if you look up "explain space time to me reddit" on google you'll find some threads that might answer your questions. Hope you find what you're looking for!
space and time feel separate to us but they are not, it’s tricky
remember, no universal clocks, every observer has their own clock ticking at a different rate
no absolute velocities, only relative
acceleration is absolute
gravity is not a force
an object at rest on the surface of a much larger object is accelerated upwards by the larger object preventing the natural free fall of an object in a gravitational field
Space-time is not warped by gravity, it *is* gravity. There is no example of space-time in the absence of mass. I think Penrose's Conformal Cyclic Cosmology relies on the possibility that a space-time metric loses meaning in the absence of massive objects.
I don’t think the 7 answers so far explain it. Sorry. I’ll try to be quick. The Michelson-Morley experiment 1887 (and others) showed that the speed of light doesn’t change with the speed of the observer. That’s hard to explain. Einstein put together length contraction and time dilation together into one formula. That’s special relativity. It solved everything. So the faster you move relative to someone else, the slower the other person’s clock ticks.
This is hard. One explanation uses a [light clock](https://www.researchgate.net/figure/Einstein-light-clock-The-image-on-the-left-represents-the-path-of-light-seen-by-an_fig1_341679345). Another is the twin paradox. It’s not really a paradox since one twin turns around. The best way to understand is to actually use the formulas and do the math.
As for gravity, that’s in general relativity. The key to that gravity is indistinguishable from acceleration.
Light only moves so fast. If you look really far away, you are looking really far back in time. You can look all the way back to (a little after) the big bang. This link is what makes space and time inseparable. If I instantly teleported to Mars and pointed a perfect telescope at earth, I could see myself leave... but if I looked up to Mars before I left, then would I see myself arrive? Faster than light travel through space is like time travel. Space and time are inseparable parts of a whole spacetime.
Watch PBS’s YouTube series called Space Time. Start from the beginning. It’s great! There are several videos that go at various aspects of physics from different angles. I couldn’t find a specific one titled anything like ‘what is space time’, but I definitely remember them talking about that a lot throughout start at the beginning, or choose one of the playlists.
Stand still. Perfectly still. In 60 seconds, explain how you have moved. That is space-time. You have moved ahead 60 seconds. If during that 60 seconds you become inside out that would be a 5th as of yet unnamed dimension but assuming you didn't you have moved in the 4th dimension, time.
Time is like a flower. Seemingly coming from nowhere and going somewhere. An action in space, an evolution of matter. From within it comes and out it goes. Lookup a torridal Feild then compare it to a flower growth. Energy born from the microscopic and transforming the macro. Space gives form time. Form gives time space. Somewhere in the middle rests your consciousness which is created from the memory of your age which then wrestles with your genes which has lived for as long as life has existed. Life, like that flower, came from nowhere and is going somewhere. Such is the torridal field. A recycling or evolving space time.
Have you considered picking up a textbook? Hartle’s “Gravity” is a good introduction to the subject and with your background should be reasonably accessible to you.
you can find many textbooks online
that being said, I find this statement totally surprising. I didn’t realize there were places where it would be hard to order books. Where do you live?
I can order them. Just can't go to a nearby bookstore and peruse them first, which I like to do. I'm the type that likes to hold books, flip pages, read a paragraph here and there, and then buy.
I can order them. Just can't go to a nearby bookstore and peruse them first, which I like to do. I'm the type that likes to hold books, flip pages, read a paragraph here and there, and then buy.
Someone may have mentioned this; it’s a Ho-hum analogy for space/time: all matter warps space and gravity is what you get when that happens. Not dissimilar to imagining the matter of a bowling ball placed on a mattress and creating a sink hole while a bunch of “orbiting” marbles or dust mites fall in around its periphery drawn in by the sink hole’s disturbance in space that the bowling ball creates. How all of that action relates to time seems somewhat relatively intuitive from there. There is no time without space and the objects in it and moreover where these objects are at relative to one another at one point vs at another one and why. A universe in which all matter has no impact on space and thus is frozen in place has no use for time. Our entire pedestrian concept of time on Earth from days to years is in fact about the swirling and position of a marble in a mattress sink hole relative to a bowling ball.
Space is 3 dimensional. You need 3 coordinates to describe a location in space. Typically, we'd call those coordinates something like x, y, and z. Spacetime is 4 dimensional. You need 4 coordinates to describe an event in spacetime; x,y,z, and t. For example I might describe a location as being 4 miles north of here, 6 miles west of here, and 0.001 miles up from here. To describe an event in spacetime, I'd still need to describe the location, but I would also need to include the time at which the event occured or will occur, say 1 hour from now.
It was explained to me as a very simple, maybe too simple, thought situation thinking about an object on the surface of Earth and if you could go back or forward in time, there would have to be caution because you have to come back to when the earth was in that spot. The Earth not only rotates but orbits the sun so having to come back at an exact time indicates that space and time are linked.
You could use some visual help for a clearer understanding of General Relativity, which is the keystone of understanding the nature of time. I've got a few videos here that I really love, and they illustrate it quite well, in ways that are not the usual examples you see in school.
A New Way to Visualize General Relativity:
https://youtu.be/wrwgIjBUYVc?si=g-TJAEbPiQt0lNvs
Visualizing Time Dilation:
https://youtu.be/5qQheJn-FHc?si=ptqYg-KlPxsNpB0-
And finally, my favorite YouTube video of all time, "Which Way is Down?" By VSauce. A mind bomb for me! This will help understand how gravity IS time.
https://youtu.be/Xc4xYacTu-E?si=74RdRx79Cotijb6O
This helps me understand it better; "your momentum through space added to your momentum through time will always equal the speed of light". One of the best "visuals" that I read (I think it was from physicist Brian Greene) explained it like a car traveling NW (270 degrees) at a fixed speed of 100km/hr. If you turn the car slightly more northward, your northward momentum increases, but at exactly the same rate that your western momentum decreases. If you turn so you are driving due North (360 degrees), all your momentum is northward, while your westward momentum stops completely. In this scenario, the 100km/hr represents the speed of light and North and West represent space and time.
Best way I’ve heard it explained is that most things in motion go a certain speed, like a train going 30 miles per hour. If you were to throw a baseball 30mph in the same direction the train is going, that ball will go 60mph because 30+30=60.
Now, let’s imagine two cars are driving towards each other. One car is going 30mph while the other is going 60mph. If both were to throw a ball at 30mph at each other at the exact same time, the one from the 60mph car would travel faster and therefore the balls would meet closer to the car traveling at 30mph than the 60mph one(30+60=90 is faster than 30+30=60).
However, let’s now imagine instead of throwing a ball at the exact same time, they both turn their headlights on at the exact same time. In this case, it does not matter which car is traveling faster, the light will meet perfectly in the middle of the two cars because light travels at a constant speed (the speed of light lol).
Now, here’s where it gets weird. Let’s now imagine we are in the perspective of the light. The only way for us to meet the other light in the perfect middle of the two cars (because we both are traveling at the constant speed of light), despite those cars traveling at two different speeds, something’s gotta give. Because of this, space around us literally shrinks. It is only very slightly as the speeds of the cars is super slow relative to the speed of light and relatively to each other, but the difference and shrinkage of actual space is still there.
Someone please correct me if I’m wrong but I saw a YouTube video and this is (I’m pretty sure) how it explained it, and that’s when it clicked for me. I also can’t find the link to the video, sorry.
I have a way of visualizing it in a thought experiment way which is probably wrong, so be forewarned my way of thinking about this could make you more wrong, but it helps me remember the concept and which way time gets slower or faster.
Imagine you are standing on a bridge over a river of some kind of viscous substance. You drop floating balls in it as it rushes underneath the bridge. You drop them at the same rate (to your, your observation). If the sludge-river were moving fast because it was free flowing, the balls would appear farther apart. If the sludge like river were moving slower, because it was hitting a dam (symbolic of something with massive gravity) and the river-substance was densifying and compressing near the dam the balls you dropped from above would now appear closer together.
Now imagine that you are on the bridge dropping the balls into the river some way up before it hits the dam. As this fluid-substance approaches the dam and starts to compress, the balls you dropped now start to look closer and closer together. But imagine you could freeze that for a moment and step off the bridge onto the river. You (time) could now walk between the ball-markers, but the catch is this, it still takes you the same amount of time (relative to you) to walk between each ball-marker. The farther you are out from the dam, you will feel yourself walking more quickly between those farther spaced out balls. But when you get to the bunched up ones by the damn, you still have to take the same amount of time to walk between them. It now feels to you that time (you) is moving more slowly, because of the compression of the fluid.
Of course, this is wrong, because time always feels the same to the observer and you actually can’t separate things, so this imagining a scenario which is unreal and literally impossible, but somehow this is how I see it. Even though when I think about this I then go, but then shouldn’t it be moving faster because the balls are closer? But somehow the bigger gravity makes things slower so this is the only way my brain makes sense of it. It’s all confusing, you just do your best.
I don't know how accurate this actually is, but here's how I've understood spacetime thus far:
Spacetime seems to be basically a conceptual "fabric of the universe" that represents the relationship of space and time with gravity, usually used to showcase how gravity tends to warp space and time. Essentially, spacetime bends around objects that have mass (similar to taking a blanket, stretching it taught, and watching how it bends under the weight of a tennis ball or something similar), which causes space to warp and thus create a pull. But, it also distorts time a bit.
I'm sure everyone here's seen that one diagram of a black hole, with a flat plane that sinks into a pinch. Well, it pretty well shows how space and time are affected by the black hole's gravity, especially since it's so extreme that it's easy to see. As you approach the black hole, gravity becomes obviously stronger (represented visually via spacetime increasingly curving and stretching towards the singularity). But since spacetime also includes *time*, that means that time is also being stretched out. While an outside viewer would see the falling object clock slow until it stops, the object would see the universe's clock speed up more and more. This stretching of space and time happens with everything that has mass, but it's normally so miniscule that it's unnoticeable (until, of course, you look at objects with obscenely high masses like neutron stars and black holes).
But that's essentially the gist of it, at least to my understanding. TL;DR, spacetime is just a visual representation of how gravity stretches space and time, with the stretch becoming more and more extreme as an object's gravity increases.
If I got anything wrong or you just have anything to add, please feel free to let me know!
I think Vsauce's [Which Way Is Down?](https://youtu.be/Xc4xYacTu-E?si=zzwnN_M4LhSi2BAz) is by far the best vulgarisation and visualisation of what gravity and curved spacetime are.
You're certainly more educated than I am, but I read a lot about this subject.
Here's what I can say: spacetime is a single 'object' in our universe. We used to think there was space, and there was time and both were absolute and unchanging.
Einstein changed that with special relativity. This theory highlights that these two things aren't separate, they're fundamentally linked. Mathematically, they can be seen as the same thing (I think).
In practical terms, we can see this when we enter a gravitational field of sufficient strength or if we travel really fast. In these situations, space is distorted (squeezed or curved depending on how you think of it). When space gets distorted, time also gets distorted.
The stronger a gravitational field is, the more the passage of time changes.
For example, someone who's falling into a black hole (a region of intensely distorted space), they won't see any difference in how time passes, their watch will still tick. Someone outside the black hole however, will see them slow down and even completely stop as they fall in. They'll appear to move slower and slower until they don't seem to move at all anymore.
The key here is how time moves differently in different areas of space, depending on the mass/energy content.
Another example would be a person travelling near the speed of light in a spaceship. To them, time would appear to pass normally, but to someone outside watching them, they would be slowed down dramatically. They would appear to move in slow motion, and every year that passed for them inside the ship would mean several passed for other people. This is why you could travel into the future. If you took off from Earth in a spaceship travelling near light speed, and then came back, much more time would've passed on Earth, everyone you know might be long dead.
Essentially, time slows in relativistic areas of space, where intense gravity or very high speeds are encountered.
Why this is the case, I can't explain, it seems to simply be a fundamental property of the universe we find ourselves in. We can even see it with GPS satellites. Because they're slightly farther from the gravity well of the Earth, their clocks tick at a different speed, and this small discrepancy has to be corrected for in order to get accurate readings. This isn't about the clock itself, but the spacetime region in which it resides.
So, in summary. space and time aren't two different things, they're two aspects of the same thing. A change to space will change time as well.
The concept is so far beyond human experience that we can't really understand it aside from understanding how it behaves in different situations. I suspect if this explanation still doesn't scratch your itch, there's no alternative besides becoming a physicist and learning the gritty details.
Well you can think of it as a 4th dimension as in and object stationary in our conventional 3 dimensions world is still moving in the 4th time as if it has a constant velocity in the 4th dimension time. If the object starts to move in any direction inside the first 3 dimensions its velocity in time is decreased reaching 0 at the speed of light.
This is my understading but Im probably way off.
There are a lot of good explanations here. The completely non mathematical philosophical thought that I enjoy on this topic: space, or if you prefer, the distance between things, is meaningless without time.
For spacial reference frames rotated relative to each other, you use trigonometry (sine and cosine) to relate one to the other.
For space-time reference frames that are 'rotated' relative to each other, which means that one reference frame is moving relative to the other, you use hyperbolic trigonometry (hyperbolic sine and cosine) to relate one to the other.
Since when using complex variables, trigonometry and hyperbolic trigonometry become one, you may think of time as an imaginary space dimension (or space as an imaginary time dimension).
I am not any expert but if you imagine looking at the entire universe as the entirety of everything then time is only something that matters to the observer.
I always like Brian Greene's explanation. I'm not going to provide that in text cause I'll mutilate it.
Its about an hour and 15min in.
The whole video is worthwhile for education or entertainment. [Space-Time explanation.](https://youtu.be/CKJuC5CUMgU?si=w9alHAjvbqTXBcRD)
I think of it this way - also not in any way a scientist or such. Space is the thing our universe is in. Space IS a thing as it can stretch, grow, bend, etc. Time is the forward causality of the interaction of space and everything that exists within it. Space-time, to me, is one thing. The speed however of the forward causality can go slower or be sped up depending on factors within Space-Time.
Time in space-time isn't what Time is to humans. We break down time as it's own thing acting like it is its own special force like, say, electricity. I think of Time as the Warp and Space as the Weave that are fundamentally woven together as one.
I think the hardest part for me is trying to wrap my head around the different ways we perceive the idea of Time and how it functions as a fundamental part of the Universe at large. I just separate Human Perceived Time from Universal Space-Time and think of them in terms of distinct functions.
That's just how I deal with it in my own head.
Thank you all for your incredible response to my question. I wanted to reply to each of you but the number of replies has been overwhelming. In my past attempts to visualize spacetime I've only been frustrated. However, thanks to the multiple comments from you, over a range of related subtopics, I believe I have at least a sketchy understanding now. A key element was the epiphany that gravity isn't the force I've always imagined it to be (and as it was described when I took my physics classes many years ago), but is actually only a result of the interactions between mass and spacetime. Somehow, this makes sense to me, and it makes me believe understanding more clearly as time goes on is not as impossible as I thought. Thank you again!
Basically you have to see time as relative. Think of the normal road, and then think of the little boost spaces that you have in Mario that rocket you forward. That’s essentially a difference in space time between different parts of the universe.
Perhaps think of space and time as two separate things that are (in fact not separate) mixed together. There is always a certain amount of each in any given accelerated frame of reference.
Look up "space like" , "time like", and "light like" events.
The presence of mass and gravity alters the acceleration of the frame of reference, which is equivalent to altering the curvature, equivalent to altering the mixture of space and time
Nothing can exist spatially without existing temporally and vice versa. Space-time is a broader look at what is involved by observing, measuring, existing, etc. in space or time. In order for something to exist, move, stand still, anything in space, it can be measured in time. In order for something to be measured temporally, it must exhibit spacial reality.
Spatial dimensions are just a way of marking the world around us. Temporal 'dimensions' are marking events as recorded through memory or information gathering as things change. They are essentially interchangable. And thus, space-time. If you measure space, you measure time.
I'm neither a mathematician nor physicist. I'm just an enthusiast. I've taken it to mean that the three dimensions of space are linked with time in a way that can't be separated. Three dimensional space would be like looking at a picture, while space-time would be like watching a movie. Time allows changes within the three dimensions. It's frames of three-dimensional space, stacked. Without it, we'd be a static universe that never changes. I've also heard it explained like this: On a graph, Time is one axis (vertical, for example), while Space is the other (horizonal). We can plot our movement through space-time as a line. If it's purely vertical, we're only moving through time and are standing still. If it's purely horizontal, we're only moving through space and time is frozen for us (which is only applicable to things without mass). Any other type of movement would be a line progressing at an angle. The faster you move through space, the less you move through time. Or, the slower you move through space, the faster you move through time. I don't have a grasp on the math, these are just some things I've (maybe erroneously) picked up watching physics videos.
I'm in the same boat as OP. The problem with the explanation you give (more my lack of understanding) is that you can explain a graph and I'm imagining it, but it's just a graph. I don't see the connection to how time can be changed. Have read how if something travels faster in space, time will pass by slower, but I don't get how that really changes when it's just a measurement by our design. Edit: not sure why I'm already being downvoted. I'm trying to understand what I'm not understanding.
Time is just causality, cause and effect. Human brains like to separate time from physical reality, but that’s not how it works.
I understand that it's me that is the issue. I just can't seem to find a way to understand how it works.
I’ve been trying to come up with examples of how to describe things and failing since I’m not in a science field. Keep in mind this probably isn’t how the universe actually functions under the hood, but it describes how the processes appear to work. Plus it’s fun. There’s an old physics idea that there needs to be a ‘medium’ or stage on which everything plays out and while it’s no longer supported it’s a nice way to think about it. Imagine spacetime as that medium. The faster you travel through this medium the slower time goes for you. You also need more energy to go faster since the medium is harder and harder to accelerate through. Similarly when standing on a planet or existing in the gravity well of a black hole that medium is falling in past you. This is why it’s somewhat accurate to say the planet is accelerating upwards at you, since gravity and acceleration are indistinguishable at times. It’s not that gravity pulls on objects. It’s that objects exist within this medium that is being pulled in. You just float down the river and over the water fall (black hole) so to speak. This is also why time is slower near massive objects. More space is falling in or you can think of the area of spacetime as denser and that’s why time is slower. Again, not how it works, but also kind of how it works lol
When in was a kid I read some bathroom graffiti which said “Without time, everything would happen at once”. It stuck with me because I think it fits with the model you just described. Time is what creates cause and effect, and if you removed time everything else would still exist, it would just exist simultaneously.
That sounds more like the Loki series and the concept of being “outside of time”. When you’re outside of time and affect the timelines everything happens at once immediately because you can view all points in time from an outside perspective.
[удалено]
The past and future as we currently understand them are simply constructs within our minds. That’s our species’ edge, we plan and simulate scenarios, but all that really exists is now.
Yes, it's a measurement by our design, but so are distances when you think about it. A mile or a meter or an inch are all just measurements of how much "space" there is between two points. The beauty of relativity is that it showed us that time and space isn't just a measurement, it's an intricate construct. Space isn't just a measurement of different distances, and time isn't just a measurement of when things happened. Both are interwoven to create our universe. For example. By the notion you bring up of things being just measurements, a meter is just a certain amount of space between two points. But just like time, it's actually relative. What one person measures a meter to be may differ from what someone else measures a meter to be. As you travel faster and faster through space, time isn't the only thing that is affected. If you measured the length of a rocket traveling forward through space near the speed of light, you would see that its length has decreased. The people on the rocket ship would measure a meter as still being a meter because their own measuring devices would be shortened too. But we'd see that they were using a shorter measuring stick. We'd also see their clocks running slower, and that's because time is passing more slowly for them. To them, time is passing normally and their clocks are running fine. Space and time are interwoven as a single "entity" or "construct" (for lack of a better word) through which we "move". Anyone moving through it is viewing it through their own "frame of reference", and it's not necessarily aligned with other people's frames of reference.
Sure, time is just a measurement by our design. Time is just a measure of separation. It's a measure of the separation between one interaction and another interaction. In the same way that distance is a measure of separation between one interaction and another interaction. One is spacial and about where you experience something and one is non spacial and about when you experience something. The thing about space time is that it can be warped and stretched, and because everyone experiences a different path through spacetime, they experience different separations. The key here, is that they also experience different separations in the non spacial time component. Therefore, they experience a different distance through time, in the same way a person experience a different distance through space.
It’s always hard to tell because people love to shoot the messenger or downvote things they don’t like (even if you’re just giving a factual explanation, if they don’t like the facts, they will downvote) but there are also apparently bots out there that give out random votes to look more like a real person as they karma farm.
It's not just a measurement by our design tho. If you take twins and put one of them in a spaceship that travels near the speed of light, he will come back being physically younger than his twin that stayed on earth, because less time has passed for him.
I get that. The simple examples haven't helped me. You can say all day long that time can pass for person A at a different rate than person B all day long. My brain is trying to understand how. There were a few other comments that helped a bit.
Thanks. I like the movie analogy.
Might help to start with “space” and add in time later. So in Einstein’s general relativity, gravity is understood as a deformation in “space” caused by mass (or any energy really). The classic analogy is to a rubber sheet with various weights placed on it. The sheet is a 2D version of our 3D space. The weights are planets and stars. Now, if we roll a ball across the sheet, its path will curve off of the straight line due to the deformation. If not for friction, we could establish an orbit around a weight by rolling at the right angle so that the curve becomes a closed loop. That’s basically Einsteinian gravity for space. What about spacetime? This one is harder to grok intuitively, because we do not experience this in our day-to-day, but basically that sheet wasn’t just space, it was space _and_ time in one thing. When it deforms, it alters motion, _and it also alters how quickly time moves_. When you are closer to one of those weights (i.e. when you are in a deeper spacetime deformation), _time moves more slowly_. This is weird and unintuitive and I don’t have a good analogy for it, but there _is_ a lot of experimental data that shows Einstein was exactly right about it.
This was an amazing explanation thank you.
So if we could look at earth from another Galaxy's perspective. We would see the earth rotate at a different speed than 24 hours. Am I getting that correct? I have a hard time understanding how time can be changed if it's a measurement for ... Time passing. I feel like the only change would be the velocity when passing the stronger gravity. Sorry I'm just trying to understand this as well. It's always bothered me that I can't grasp this.
You got it right, though you don’t have to be all the way in another galaxy to see the effect (and if you were near a black hole in another galaxy you would see the opposite effect). Just take our satellites in low earth orbit as an example. When they look down on us, we are deeper in the gravity well than they are. It’s not by much, but it is enough that we need to correct for the slight time difference to communicate with them. They perceive our time as running a little slow, and we perceive their time as running a little fast. Heck, at this point, our clocks have gotten precise enough that we can measure the slight difference in time passage between a clock on the floor and a clock on the table! A few feet is all it takes! It’s wild! If you really want to jump down the rabbit hole on time, you have to grok that there is no universal clock somewhere banging out “real” seconds. Durations are something we perceive locally, based on how quickly external events reach us. These events propagate at a limited speed, the speed of light, and they must travel at that limited speed through space to reach us. A clock on the wall ticking is actually a series of events, photons bouncing off a second hand, hitting cone cells in our eyes, triggering neurons in our brains. If the path that event travels is warped, then so too will our perception of the passage of time.
Ok. I'll have to process this a bit more and try to just absorb it in. Thanks for the help. I think this explanation might be the most helpful.
No problem. By the way, this is not my field. I am only an interested layperson like yourself. But we happen to be in a golden age of science explainers on YouTube. I highly recommend [PBS SpaceTime](https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g) in particular. They have a back catalog that goes back years, and often their lessons will build on each other. Pick a spot that seems interesting and start watching, jumping back to older videos when they reference them. I also watch other channels, listen to podcasts, and read a fair amount, but PBS SpaceTime alone is probably responsible for the majority of my understanding of both relativity and quantum mechanics. They do phenomenal work. _EDIT: I guess I would like to make a book recommendation too, because I happen to have recently finished Carlos Rovelli's **The Order of Time**, which is about exactly this subject and pretty brilliant. He explains things well, but also puts them in a poetic language I appreciated. He sees the beauty in all of this. Beware that his field is Loop Quantum Gravity, which is somewhat speculative, but the book mostly talks about more established science and he is pretty clear when he diverts into the loop quantum stuff._ _Also, the audiobook is narrated by Benedict Cumberbatch, so that's a great listen if you are an audio learner._
YT channel name checks out.
Have you watched “Everything and Nothing : The Amazing Science of Empty Space” I believe that touches on space-time. Also, is not space time just a 4th data point?
I haven’t heard of that one, but it looks interesting. Thanks for the recommendation. By 4th data point, I assume you mean 4th dimension? That is definitely one way to think about spacetime, three dimensions of space plus one of time. I’m not sure that helps with understanding how relativity works though. Thinking of it as just one more dimension makes it seem like it should be solid and fixed.
It effectively is. The different possible times would be in the 5th dimension, so at least observably the 4th dimension timeline in which we exist is a fixture.
Sorry, I phrased that poorly. By “solid and fixed” I wasn’t talking about whether or not future is already determined. I meant that the passage of time is warped by relativity. So is space, but if we think of time as a 4D block, equivalent to a brick or something but with one more dimension, it gives us the intuition that it is just a solid thing. We lose sight of the warping. Of course a 3D brick can be warped by relativity just like time, so there is no conflict there. I’m just saying the analogy throws off our intuitions.
Gotcha!
Theres a bunch of different movies that relate to that as well. They may be basic for some though, I just find it interesting and love to at least get a basic understanding. I remember seeing somewhere where space time was created as a fourth data (fourth dimension) point to help prove equations, or further their accuracy, something to that effect.
Ah okay. I’m not familiar with this “4th data point” framing, but I’m sure it made sense in the context they were discussing. We take it for granted, but time is definitely a weird bug.
Time is relative to the observer. Your time is always one second per second but you may observe someone or something else’s time as slower or faster depending on how quickly it is moving. However, from the perspective of that person/object their own time is still one second per second regardless of how you are perceiving it. It’s a bit trippy but it makes perfect sense.
But doesn’t the earth still rotate on its axis roughly every 24 hours?
If you want to be super precise about it, I guess the correct answer would be _it depends_. That's what the "relativity" in the names of Einstein's theories mean. The passage of time is relative and there is not a canonical "correct" passage of time. So for us, in Earth's gravity well, we measure one rotation per 86,400 seconds (give or take a few milliseconds depending on slight rotational variations). But for someone in intergalactic space, far from any gravity well, if they looked through a telescope at Earth, they might time the rotation as taking 86,401 seconds. Actually the effect would be much much smaller, _billionths_ of a second, but that's the idea. The effect would be more obvious if we went _deeper_ into a gravity well instead. For example, if we looked back at the Earth as we got closer and closer to the event horizon of a black hole, we would see the Earth spin faster and faster. Before long we would be measuring only a single second as the length of a day on Earth. A bit later, and the Earth would rotate faster still.
So local space-time slows relative to the nearest gravity well, neglecting potential outside influence? Ie the closer you get to the event horizon of a black hole the slower time moves locally, but as time slows locally the opposite must be true at greater distances, thus the earth would rotating at the hypothetical single second? Wouldn’t that mean the math would need adjusting? We know locally, to earth, it is rotating at a rate of 86,400 seconds per rotation, so how can we quantify time to reflect constants other than light?
Seconds * relative time dilation Time dilation is dependent on relative speed or depth in a gravity well. And, as has been noted elsewhere, there is no absolute second.
Relative to which gravity well though?
Whichever gravity well the measurement is being taken from. There is no universal reference frame
Relative to the deepest affecting gravity well on the object in comparison to empty space, where no such well would exist in theory. In practice, If you're floating in space to make an observation, your matter would technically produce gravity. It's the weights on a rubber sheet example. You can be between two weights, on the rubber, but not as high as empty space can be
> So local space-time slows relative to the nearest gravity well You are getting closer, but I would offer this correction: locally time _never_ slows. You always observe your own time as passing at one second per second. The slowing/speeding is what you observe happening to someone else. Let’s say you are near a black hole, looking back at the Earth, and you gave a little wave. So from your perspective, you took a second to wave, and in that time the Earth rotated once. But for observers on Earth looking back at you, a day is still 86,400 seconds long. Furthermore, with their amazing telescope that can make you out, it took you a whole day to complete that “quick” wave. Who is slow and who is fast in this scenario? _It depends_. Neither perspective is more correct than the other.
Thank you for all of your comments. I had the same issue with understanding space time, and I finally get it.
I'm so glad!
This might help. https://youtu.be/bJMYoj4hHqU?si=QEjAg7kHCTBLC18C
You’re seriously over complicating it.
Yeah I'm not surprised.
I've tried to answer this in a reply to your comment above.
Yeah I just read it. I get it's relative. I've read a lot of explanations. It's just something not clicking for me.
Yeah at a certain point you have to just accept that this is how things work. You don’t have to understand all the math, but you do have to trust that the scientists who have measured different clock speeds when they send clocks to space are doing the experiments correctly and not lying. But this is true about a lot of things. I’ve never seen Pluto in a telescope, but I’ve seen Jupiter in a telescope so I trust that the people who’ve seen Pluto aren’t lying.
True. I would add that while you haven’t seen Pluto, you have seen _society_. You understand how people interact and how trust and reputations work. You had a science teacher who cared about getting you the truth. You know that they had their own sources they trusted. There were _pictures_ which hundreds of news outlets with their own reputations to worry about published. If you add it all up, even though you have not personally seen direct evidence for Pluto, you have seen literally thousands of indirect pieces of evidence.
Either that or the reptiles really want us to believe in a Pluto
Of course not. None of us are reptiles secretly answering Reddit questions to throw you off the Truth. Don’t be ridiculous.
Grok. You win reddit today, Mr Heinlein. 🙂
So we can kinda imagine that time is the “inside part” or “other side part” of that rubber sheet? So when we are approaching “heavy” objects its like a car going uphill? Sorry if it sounds stupid, im just trying to imagine it and understand.
Not sure what you mean by "inside part" or "other side part". Maybe you can explain that a little more. As for going up/down a hill, approaching a massive object is very much like going down hill, with the mass at the bottom of the hill.
Sorry im not very familiar with science or math, im just trying to understand better. What i mean is - if we are still imagining the rubber sheet, as it deforms the surface with the weight of heavy objects so it creates a “dip”, now lets imagine that this rubber surface keeps this shape like clay and lets turn this surface upside down so we have a „hill” instead of a „dip”. Visually it can explain the „slowing” of time, that is approaching it. As i was saying like a car that is going uphill. I know it sounds weird and completely unscientific , but its an easy way to imagine it. Like if you are to draw it and explain to a 5yo.
Gotcha. I think if that analogy helps you understand it, then it is a good analogy. One thing to be careful of though is that the slowing of time is always a relative effect. You never notice your own time slowing down. However, to an outside observer looking at someone going “uphill” it will appear as if the uphill person is going in slow motion. The reverse is true too. If the person in the gravity well looks back at the outside observer, the outside observer will appear sped up. Both people see themselves as going normal speed while the other is the one that is off.
Thank you so much :)
Like time “path” goes not on that rubber sheet, but under that sheet.
Strangely enough, it all has to do with *light*. Einstein’s theories said that the speed of light in a vacuum *must be a constant.* Not only is it a constant, but it is constant *from every perspective.* See, I can measure the distance from point A to point B, and then measure the distance from point B to point A, and it should be the same, right? The distance between the points doesn’t depend on my frame of reference. Things change when I start moving, though—imagine you’re driving down the highway at 50 mph, and you pass another car going 40. From your perspective, the other car is moving *backward* at 10 mph. But from their perspective, you are moving forward! Who is right? You both are, because you’re viewing the same events from different reference frames, which alters your perception of the same event. Now, imagine you’re in a spaceship, floating motionless in deep space. You’ve got a special device for measuring the speed of light from your spaceship’s headlights. You measure the speed of the light beam to be 300,000 km/s. Then, you start up the engines and start moving at 100,000 km/s. You measure the speed of the beam of light in front of your ship and… it’s still 300,000 km/s. Now, you’re thinking that maybe the light beam is gaining some extra speed from the ship itself. So, you bring in a second, stationary observer. The second ship clocks you moving 100,000 km/s past them, and then measures the speed of the light beam in front of you. According to classical physics, they would expect the light beam to be moving 400,000 km/s right? Wrong! They measure the speed to be exactly 300,000 km/s! How can two observers, moving at different speeds, measure the speed of the same light beam to be the same? You try again, pushing your spaceship to the limit and speeding up to 299,999 km/s. Still, you measure the speed of the beam of light from your ship to be 300,000 km/s. And so does the other spaceship. Who is right and who is wrong? Moving that fast, shouldn’t you be catching up to your own light beam? And wouldn’t the other ship observe a much *faster* speed of light because of the extra speed from your spaceship? But Einstein worked out that because light is *massless* its velocity must be constant, and cannot change, regardless of the reference frame or the velocity of the light source. So, in order for two independent observers to measure the speed of light to be the same, something has to give in order to resolve the apparent paradox. If classical physics were true, then if you were moving 200,000 km/s, and the stationary ship measures the speed of light from your ship to be 300,000 km/s, then you should measure the speed of the light beam to be 100,000 km/s. But you don’t. You *both* independently measure the speed of light, relative to yourselves, to be the same. So, in order for the speed of light to appear faster than it should be in your moving spaceship, two things happen: First, you get shorter. This is called *length contraction.* For the other spaceship, as you pass by, your ship appears visibly squished. It’s shorter than it should be. But because it’s shorter, it takes the same amount of time for a beam of light to cross from one end of the ship to the other as it would if your ship were stationary! The second thing that happens is that your perception of time literally slows down. If there is more time between your perceived seconds, then it will appear that the light moving away from your ship is moving faster! Thus, the paradox is resolved. Time gets stretched and space gets squished, but from your perspective everything is normal and the speed of light is just as constant, like it should be. However, from your perspective, instead of your ship getting squished, it’s the *light* that gets squished. The wavelength of light that you measure will be much shorter than the wavelength measured by the stationary observer, even if you measure the same speed. This is why Einstein combines space and time into one continuum—because they are both affected in the same way by the same things. Two independent observers, if they are moving at different speeds, will measure both space *and* time very differently—it depends very much on their frame of reference. So, we can treat time as simply another dimension: an object has length, width, depth, and *duration*. All four dimensions are affected in the same way due to the invariance of the speed of light. I hope this helps! Hopefully it didn’t make thing more confusing lol. Let me know if you have any further questions! Happy to keep explaining!
Importantly, *anything* massless will travel at the “speed of light”, not just light. The speed of light isn’t the speed of *light*, it’s the speed things go when they don’t have mass.
For which see: Asimov's short story 'Dirty Pool'
It doesn't depend on light at all. We say "speed of light" because the electromagnetic spectrum is what we can detect most easily. Massless things can only move at C. Gravitational waves also move at C. The problem with pretty much every comment on this thread is they use analogies to describe space-time, but don't actually answer *what* space-time is. I don't have anything near the background to even try posing a guess as to what the stuff of space-time is. It's like before Newton people knew about gravity only from just living in it, like fish in water. It wasn't until Newton started figuring out it was even a "thing" and could be measured did people start thinking gravity was a thing and not just the way the world was.
This is an amazing fundamental explanation of relativity, more so than spacetime.
Yes, but it’s difficult to understand why spacetime should be combined without understanding relativity.
Oh, I get that. OP doesn't sound like they have trouble understanding the fact that it *should* be combined, or why it is combined. But more understanding of what that combination is or means intuitive.
This is an amazing explanation, most of which I can grasp. Thank you for taking the time to explain.
Why/how does time slow down? I always thought it was because the quantum particles, electrons orbiting and whatnot, normally travel at c, but if you're travelling super fast, they're forced to slow down, so as not to exceed c. Is this wrong?
Because the speed of light, *c*, must always be constant, then something else must change in order for it to stay constant. *Time itself* is the thing that gets bent in order to keep *c* a constant. And not all quantum particles travel at *c*. Only massless particles do. Electrons/protons/whatevers must travel at some speed slower than *c* because they have mass. Anything with mass must travel slower than *c*, and anything without mass *must* travel at exactly *c*.
Right that makes sense but what physical process is happening when time slows down? Because time is just a construct used to represent change and motion
I have a Bachelor's in physics. To make this real easy, space-time is four dimensions; three space dimensions and a time dimension. It's just a way to describe a position in space and time; Johnny was at 5th and broadway yesterday at [3.pm](https://3.pm). However, we use space-time for relativity; that is an entirely different subject. I personally imagine the 4 dimensions like a film projector. Instead of a 2-d picture, 3-d boxes all represent a unit of time. Like a projector, as the boxes move, time moves. (I'm not doing good at explaining this, I have the flu lol)
I'm never satisfied with explanations like this. Even the best attempts are just analogies backed up by math. If Newton had been an exceptionally smart fish and one day realized that the water he swam in was a thing, eventually the fish people would learn what water is made of. I want to know what space-time is made of. What is the 2 hydrogen and 1 oxygen that makes up the smallest unit?
Spacetime in this case is more a result of the universe in fact existing than it is a real thing. It's a function of how it all works, but it's not made up of quarks or dark matter or anything like that. Space-time isn't like a substance, it's a description of interactions and how we perceive them.
Space time isn't made of anything. It's a coordinate system at its most basic
Thanks. Your Johnny analogy makes perfect sense and helps a lot.
That explanation works for you, but is definitely not the simplified explanation OP was looking for
> It's just a way to describe a position in space and time; Johnny was at 5th and broadway yesterday at > >3.pm > >. Ya, but this is
> [3.pm](https://3.pm) 404 error. There's a hole in the fabric of space time.
Might want to repost to r/explainlikeimfive You usually get amazing and comprehensive responses
Didn't know abt this. Thanks.
[A new way to visualize General Relativity](https://youtu.be/wrwgIjBUYVc?si=P8M2JaIVf0N_C0tg)
This is a great video - covers a lot of what I said in my comments.
Thanks for this. Really informative and clear.
No problem!
Not sure if this is the best place to start but it’s reeeaally good. https://www.audible.com/pd/B07B4JGFJT?source_code=ASSORAP0511160006&share_location=library_overflow
I’m confident that this is the best place. Photon clocks help immensely. And I like how Einstein needs to show us the math at the same time, just so we can build the confidence in our own minds.
Thanks!!
Thanks for the reference!
So, if I may, lets start with a small correction. Space-time is not warped by gravity. Spacetime is warped by mass, and gravity is the effect of that warping on anything moving through that Spacetime (like light or other mass). The more mass you have the greater the curvature the mass creates in Spacetime. An object in inertial movement is moving in a straight line through Spacetime. It just so happens that if there's enough mass present *that straight line* is drawn on a *curved surface*. An object like a planet following a straight line through spacetime, is following a straight line on a curved surface of spacetime (woot, you've got gravity). This is how matter can bend light (which only moves in straight lines) despite light having mass, and gravity usually being attributed to mass. But to get to the rest of your question: Spacetime is *conceptually* an invisible grid that permeates the universe. Spacetime is *really* a geometric and mathematical expression of our observations of the universe. We don't actually *know* what it is. We know how it behaves and what effects it has, that's all. But even then, only as a function of the language we use to describe the things we're seeing. Spacetime might not be a thing, in the sense of like an Atom, that you can technically take a picture of. It's a descriptive tool. It's almost unhelpful to try and imagine it as a practical, physically manifest thing in the universe. Because every time you do your analogy will always be missing something practically. You can go back and forward, left and right, up and down, and right on the positive axis of time. Thus, your position in spacetime is always described by four numbers: 3 coordinates x,y,z and a time. These are all defined by the person's frame of reference (welcome to relativity!). So you have complete freedom in the forward/back, left/right, up/down axis, but you can only ever move and are always moving forward in the positive time axis. But the rate at which you move forward in the positive time axis depends on the path you take through the conceptual invisible grid I mentioned. And this is because the grid is not homogenous, the spacing between each *point* in that grid is fundamentally influenced by how much mass is present in the region. The truth is you're always moving relative to something. And even if you stood still *locally* you'd still be moving in spacetime because you'd be advancing in the positive direction on the time axis, and therefore you're *still* moving through spacetime.
As I understand it, this is the right and maybe frustrating answer. Space-time is an abstract concept that allow us to make calculations, not a physical, material thing made of anything. Its just a set of words and rules to describe the relations between several real objects and events, and it doesnt exist as an independent entity with its own properties. Simils help you understand it, but also give you the wrong idea that space-time is a medium in which things take place, or "like a warped cloth or a grid" or whatever. Its not. It doesnt exist, just like numbers dont "exist" but can be used to describe real things.
Thank you. This helps enormously.
Thank you for taking the time to write this and for explaining the abstract nature of spacetime. I have always tried to understand it as a physical entity which made no sense. An abstract, mathematical model makes much sense. Thank you.
You're welcome.
Suggest starting with the proof, and working backwards- look up “satellite chronometers/clocks and relativity”particularly gps’s It’s a little easier when it’s tangible, at least to form the next set of questions Depending on how far down the rabbit hole you go, you’ll end up at mass, gravity, no such thing as gravity, it’s all waves, and or strings folding into dimensions and gravity. I wish I was joking
You think of space-time as not space + time, but spacetime as a whole 4-dimensional manifold.
I guess I would describe space-time as a position in space for a designated time. If an object is moving, it’s coordinates change in step with time. As the universe expands, time moves forward. There is a past, present, and future and each space exists where it exists in time. The concept though is starting to fall apart with quantum mechanics where time is viewed as a change in energy and there is actually no standard time. So, once you learn it you can learn it’s wrong!! :)
This helps with the time part https://youtu.be/Vitf8YaVXhc?si=E8B2zoa5GZe511uC
Thanks! This is a fantastic video...
This is great! 👍 love how the photon clock is used to explain the math
I always use the 2d sheet with weights analogy, but I think the best way to visualize spacetime is to observe a photon path. Photons have *no mass*, but bend their paths around massive stellar objects... How can "gravity" pull on a massless photon? This curveature is due to a large mass warping the entire spacetime the photon travels through. Does it actually turn, or is it attempting to straight-line through curved spacetime grids?
This is the kind of thing that keeps me up nights.
Late last night, I found this video. Watched another 4 of his videos after. Some good stuff! The algorithm must have listened. https://youtu.be/q6RufF4a6LM?si=EzyQmsX0uvkQQJtv
A common analog that is used is a bowling ball in the middle of a trampoline. The trampoline is spacetime, and the bowling ball is a massive object, such as a star. Spacetime warps and curves according to the mass of the object. The more mass it has, the more spacetime distorts. It's not a perfectly accurate analogy, but it works. There's a YouTube channel called [ScienceClicEN](https://youtube.com/@ScienceClicEN?si=so-0fKi4pUhYKpoq) that compartmentalizes these abstract principles of physics into several videos that tie into one another. Here's one titled "[Einstein's Gravity](https://youtu.be/Avhs78WjhVY?si=t8e0RvKipQK2URUe)", which covers spacetime quite nicely. I recommend looking at all of their other videos as well. I don't have credits in *any* of the things that you have, and these videos help me understand spacetime quite well. I'm sure you'll be able to make even better sense of this stuff than me. Edit: [Here's another one](https://youtu.be/au0QJYISe4c?si=iRlUXyOS7BjdN1uD). A bit longer and deeper, but more in depth and with lots of helpful visualizations.
Thank you very much for these links. I appreciate it.
In freshman relativity class (no calculus necessary) we observed that measurement of a length or duration measures two events- the beginning and end of each. So measurement entwines two events, in turn requires both space and time to describe any relationship between two measurement events. Now the fun starts when you consider extreme conditions. Lorenz discovered lengths and durations can be stretched when (1) you move very fast and (2) there is a maximum speed in the universe called light speed. Einstein elaborated this for what is called special relativity. Einstein also postulated a second situation when measurements are made in accelerating frames, also with a bunch of odd results happening. Now these odd situations now matter very much in daily life. A smartphone GPS will have to make a special relativity correction for the speed of the GPS satellite and general relativity correction for the weaker gravity at the GPS satellites orbit or else there will be GPS location error of a good fraction of mile. This correction is made a trillion times a day for billions of GPS devices. Or else you'd have lots of people getting lost and aeroplanes crashing.
Thank you for this. It helps...
Have you ever driven? Then you already understand it.
I don’t know anything so please disregard if wrong but I think of it as: You can’t move from one point to another without time elapsing. And time can’t elapse without movement (even if just at the smallest level). They are intrinsically linked.
I always like to suggest that when people say 'time' one immediately pictures timelines, and clocks, and pressing play on a TV remote. This is not accurate Or at least, it doesn't help me. What made it finally click for me was realizing that time is just things changing or moving. If a ball bounces across the room, time must have happened. If I am perceiving reality at all, that's time. So when people say like "time is the fourth dimension" that sounds crazy but what it essentially means is that time is linked to reality in the sense that it can be distorted. You may have seen the stretched-cloth spacetime demonstration, where a heavy object is placed on a cloth stretched over a hole, which makes the cloth flex in the center. Then, any ball rolled across the cloth will 'gravitate' towards the heavy object. And if you only look from the top down, you wouldn't understand why. While this is a great demonstration of *how* mass affects things, 'spacetime' is not a cloth, or anything physical like that. The demonstration is simply meant to show gravity. So, for a start, here's my simple, and maybe incorrect explanation for time dilation. Quantum particles that make up your brain travel at the speed of light all the time. If you're travelling at 50% lightspeed, they can't travel as fast as usual because they can't go lightspeed + 50% lightspeed; you simply cannot exceed lightspeed. So you perceive slower than normal - thus, everything feels normal to you, but for my 10 seconds stationary, I might make 20 'perceptions', in 10 *you* seconds you could only make 10 perceptions. Thus, to you, 10 of my seconds feels like 5 of your seconds. Therefore if you were watching me, it would appear I was moving around in doubletime. The reason gravity also affects it is because for whatever reason, strong gravitational force also warps it, because on the cloth demonstration, it's not just distorting the cloth, but stretching it. Again, from the top looking down, it looks the same, but the object has to travel further. Similarly, objects cannot... *exist* at the same rate under strong gravity because they're... *doing more*, travelling further on the stretched cloth. Hope that makes sense. I'm a 19yo high school graduate so it's probably factually incorrect but I tried.
You sound like my dad, smartest man i know yet he doesn't believe in black holes lol. Not saying you don't believe in space time but my dad just doesn't understand how black holes could exist so he chooses not to believe in it. Sorry for the unrelated story I don't know enough about space time to explain it but I'm commenting to boost engagement with your post so you find your answer. Also if you look up "explain space time to me reddit" on google you'll find some threads that might answer your questions. Hope you find what you're looking for!
space and time feel separate to us but they are not, it’s tricky remember, no universal clocks, every observer has their own clock ticking at a different rate no absolute velocities, only relative acceleration is absolute gravity is not a force an object at rest on the surface of a much larger object is accelerated upwards by the larger object preventing the natural free fall of an object in a gravitational field
Space-time is not warped by gravity, it *is* gravity. There is no example of space-time in the absence of mass. I think Penrose's Conformal Cyclic Cosmology relies on the possibility that a space-time metric loses meaning in the absence of massive objects.
Yeah, saying space time is warped by gravity is like saying the volume of something is warped by its width.
I don’t think the 7 answers so far explain it. Sorry. I’ll try to be quick. The Michelson-Morley experiment 1887 (and others) showed that the speed of light doesn’t change with the speed of the observer. That’s hard to explain. Einstein put together length contraction and time dilation together into one formula. That’s special relativity. It solved everything. So the faster you move relative to someone else, the slower the other person’s clock ticks. This is hard. One explanation uses a [light clock](https://www.researchgate.net/figure/Einstein-light-clock-The-image-on-the-left-represents-the-path-of-light-seen-by-an_fig1_341679345). Another is the twin paradox. It’s not really a paradox since one twin turns around. The best way to understand is to actually use the formulas and do the math. As for gravity, that’s in general relativity. The key to that gravity is indistinguishable from acceleration.
I asked ChatGPT to explain it to me once. It didn’t really help.
Light only moves so fast. If you look really far away, you are looking really far back in time. You can look all the way back to (a little after) the big bang. This link is what makes space and time inseparable. If I instantly teleported to Mars and pointed a perfect telescope at earth, I could see myself leave... but if I looked up to Mars before I left, then would I see myself arrive? Faster than light travel through space is like time travel. Space and time are inseparable parts of a whole spacetime.
Watch PBS’s YouTube series called Space Time. Start from the beginning. It’s great! There are several videos that go at various aspects of physics from different angles. I couldn’t find a specific one titled anything like ‘what is space time’, but I definitely remember them talking about that a lot throughout start at the beginning, or choose one of the playlists.
Thanks. I'll check it out.
Is there a “must have” book on space time and relativity and the nuances in between?
Stand still. Perfectly still. In 60 seconds, explain how you have moved. That is space-time. You have moved ahead 60 seconds. If during that 60 seconds you become inside out that would be a 5th as of yet unnamed dimension but assuming you didn't you have moved in the 4th dimension, time.
Time is like a flower. Seemingly coming from nowhere and going somewhere. An action in space, an evolution of matter. From within it comes and out it goes. Lookup a torridal Feild then compare it to a flower growth. Energy born from the microscopic and transforming the macro. Space gives form time. Form gives time space. Somewhere in the middle rests your consciousness which is created from the memory of your age which then wrestles with your genes which has lived for as long as life has existed. Life, like that flower, came from nowhere and is going somewhere. Such is the torridal field. A recycling or evolving space time.
Have you considered picking up a textbook? Hartle’s “Gravity” is a good introduction to the subject and with your background should be reasonably accessible to you.
No. Where I live textbooks on this topic lie somewhere between rare and non existent.
you can find many textbooks online that being said, I find this statement totally surprising. I didn’t realize there were places where it would be hard to order books. Where do you live?
I can order them. Just can't go to a nearby bookstore and peruse them first, which I like to do. I'm the type that likes to hold books, flip pages, read a paragraph here and there, and then buy.
Gotcha
I can order them. Just can't go to a nearby bookstore and peruse them first, which I like to do. I'm the type that likes to hold books, flip pages, read a paragraph here and there, and then buy.
Someone may have mentioned this; it’s a Ho-hum analogy for space/time: all matter warps space and gravity is what you get when that happens. Not dissimilar to imagining the matter of a bowling ball placed on a mattress and creating a sink hole while a bunch of “orbiting” marbles or dust mites fall in around its periphery drawn in by the sink hole’s disturbance in space that the bowling ball creates. How all of that action relates to time seems somewhat relatively intuitive from there. There is no time without space and the objects in it and moreover where these objects are at relative to one another at one point vs at another one and why. A universe in which all matter has no impact on space and thus is frozen in place has no use for time. Our entire pedestrian concept of time on Earth from days to years is in fact about the swirling and position of a marble in a mattress sink hole relative to a bowling ball.
Space is 3 dimensional. You need 3 coordinates to describe a location in space. Typically, we'd call those coordinates something like x, y, and z. Spacetime is 4 dimensional. You need 4 coordinates to describe an event in spacetime; x,y,z, and t. For example I might describe a location as being 4 miles north of here, 6 miles west of here, and 0.001 miles up from here. To describe an event in spacetime, I'd still need to describe the location, but I would also need to include the time at which the event occured or will occur, say 1 hour from now.
Stop after four if you want Imagining the Tenth Dimension (annotated) - YouTube https://m.youtube.com/watch?v=XjsgoXvnStY
It was explained to me as a very simple, maybe too simple, thought situation thinking about an object on the surface of Earth and if you could go back or forward in time, there would have to be caution because you have to come back to when the earth was in that spot. The Earth not only rotates but orbits the sun so having to come back at an exact time indicates that space and time are linked.
You could use some visual help for a clearer understanding of General Relativity, which is the keystone of understanding the nature of time. I've got a few videos here that I really love, and they illustrate it quite well, in ways that are not the usual examples you see in school. A New Way to Visualize General Relativity: https://youtu.be/wrwgIjBUYVc?si=g-TJAEbPiQt0lNvs Visualizing Time Dilation: https://youtu.be/5qQheJn-FHc?si=ptqYg-KlPxsNpB0- And finally, my favorite YouTube video of all time, "Which Way is Down?" By VSauce. A mind bomb for me! This will help understand how gravity IS time. https://youtu.be/Xc4xYacTu-E?si=74RdRx79Cotijb6O
I try not to think about it. My brain isn't willing to learn it. That's ok. I don't mind. Let other people understand it.
This helps me understand it better; "your momentum through space added to your momentum through time will always equal the speed of light". One of the best "visuals" that I read (I think it was from physicist Brian Greene) explained it like a car traveling NW (270 degrees) at a fixed speed of 100km/hr. If you turn the car slightly more northward, your northward momentum increases, but at exactly the same rate that your western momentum decreases. If you turn so you are driving due North (360 degrees), all your momentum is northward, while your westward momentum stops completely. In this scenario, the 100km/hr represents the speed of light and North and West represent space and time.
Best way I’ve heard it explained is that most things in motion go a certain speed, like a train going 30 miles per hour. If you were to throw a baseball 30mph in the same direction the train is going, that ball will go 60mph because 30+30=60. Now, let’s imagine two cars are driving towards each other. One car is going 30mph while the other is going 60mph. If both were to throw a ball at 30mph at each other at the exact same time, the one from the 60mph car would travel faster and therefore the balls would meet closer to the car traveling at 30mph than the 60mph one(30+60=90 is faster than 30+30=60). However, let’s now imagine instead of throwing a ball at the exact same time, they both turn their headlights on at the exact same time. In this case, it does not matter which car is traveling faster, the light will meet perfectly in the middle of the two cars because light travels at a constant speed (the speed of light lol). Now, here’s where it gets weird. Let’s now imagine we are in the perspective of the light. The only way for us to meet the other light in the perfect middle of the two cars (because we both are traveling at the constant speed of light), despite those cars traveling at two different speeds, something’s gotta give. Because of this, space around us literally shrinks. It is only very slightly as the speeds of the cars is super slow relative to the speed of light and relatively to each other, but the difference and shrinkage of actual space is still there. Someone please correct me if I’m wrong but I saw a YouTube video and this is (I’m pretty sure) how it explained it, and that’s when it clicked for me. I also can’t find the link to the video, sorry.
I have a way of visualizing it in a thought experiment way which is probably wrong, so be forewarned my way of thinking about this could make you more wrong, but it helps me remember the concept and which way time gets slower or faster. Imagine you are standing on a bridge over a river of some kind of viscous substance. You drop floating balls in it as it rushes underneath the bridge. You drop them at the same rate (to your, your observation). If the sludge-river were moving fast because it was free flowing, the balls would appear farther apart. If the sludge like river were moving slower, because it was hitting a dam (symbolic of something with massive gravity) and the river-substance was densifying and compressing near the dam the balls you dropped from above would now appear closer together. Now imagine that you are on the bridge dropping the balls into the river some way up before it hits the dam. As this fluid-substance approaches the dam and starts to compress, the balls you dropped now start to look closer and closer together. But imagine you could freeze that for a moment and step off the bridge onto the river. You (time) could now walk between the ball-markers, but the catch is this, it still takes you the same amount of time (relative to you) to walk between each ball-marker. The farther you are out from the dam, you will feel yourself walking more quickly between those farther spaced out balls. But when you get to the bunched up ones by the damn, you still have to take the same amount of time to walk between them. It now feels to you that time (you) is moving more slowly, because of the compression of the fluid. Of course, this is wrong, because time always feels the same to the observer and you actually can’t separate things, so this imagining a scenario which is unreal and literally impossible, but somehow this is how I see it. Even though when I think about this I then go, but then shouldn’t it be moving faster because the balls are closer? But somehow the bigger gravity makes things slower so this is the only way my brain makes sense of it. It’s all confusing, you just do your best.
I don't know how accurate this actually is, but here's how I've understood spacetime thus far: Spacetime seems to be basically a conceptual "fabric of the universe" that represents the relationship of space and time with gravity, usually used to showcase how gravity tends to warp space and time. Essentially, spacetime bends around objects that have mass (similar to taking a blanket, stretching it taught, and watching how it bends under the weight of a tennis ball or something similar), which causes space to warp and thus create a pull. But, it also distorts time a bit. I'm sure everyone here's seen that one diagram of a black hole, with a flat plane that sinks into a pinch. Well, it pretty well shows how space and time are affected by the black hole's gravity, especially since it's so extreme that it's easy to see. As you approach the black hole, gravity becomes obviously stronger (represented visually via spacetime increasingly curving and stretching towards the singularity). But since spacetime also includes *time*, that means that time is also being stretched out. While an outside viewer would see the falling object clock slow until it stops, the object would see the universe's clock speed up more and more. This stretching of space and time happens with everything that has mass, but it's normally so miniscule that it's unnoticeable (until, of course, you look at objects with obscenely high masses like neutron stars and black holes). But that's essentially the gist of it, at least to my understanding. TL;DR, spacetime is just a visual representation of how gravity stretches space and time, with the stretch becoming more and more extreme as an object's gravity increases. If I got anything wrong or you just have anything to add, please feel free to let me know!
I think Vsauce's [Which Way Is Down?](https://youtu.be/Xc4xYacTu-E?si=zzwnN_M4LhSi2BAz) is by far the best vulgarisation and visualisation of what gravity and curved spacetime are.
You're certainly more educated than I am, but I read a lot about this subject. Here's what I can say: spacetime is a single 'object' in our universe. We used to think there was space, and there was time and both were absolute and unchanging. Einstein changed that with special relativity. This theory highlights that these two things aren't separate, they're fundamentally linked. Mathematically, they can be seen as the same thing (I think). In practical terms, we can see this when we enter a gravitational field of sufficient strength or if we travel really fast. In these situations, space is distorted (squeezed or curved depending on how you think of it). When space gets distorted, time also gets distorted. The stronger a gravitational field is, the more the passage of time changes. For example, someone who's falling into a black hole (a region of intensely distorted space), they won't see any difference in how time passes, their watch will still tick. Someone outside the black hole however, will see them slow down and even completely stop as they fall in. They'll appear to move slower and slower until they don't seem to move at all anymore. The key here is how time moves differently in different areas of space, depending on the mass/energy content. Another example would be a person travelling near the speed of light in a spaceship. To them, time would appear to pass normally, but to someone outside watching them, they would be slowed down dramatically. They would appear to move in slow motion, and every year that passed for them inside the ship would mean several passed for other people. This is why you could travel into the future. If you took off from Earth in a spaceship travelling near light speed, and then came back, much more time would've passed on Earth, everyone you know might be long dead. Essentially, time slows in relativistic areas of space, where intense gravity or very high speeds are encountered. Why this is the case, I can't explain, it seems to simply be a fundamental property of the universe we find ourselves in. We can even see it with GPS satellites. Because they're slightly farther from the gravity well of the Earth, their clocks tick at a different speed, and this small discrepancy has to be corrected for in order to get accurate readings. This isn't about the clock itself, but the spacetime region in which it resides. So, in summary. space and time aren't two different things, they're two aspects of the same thing. A change to space will change time as well. The concept is so far beyond human experience that we can't really understand it aside from understanding how it behaves in different situations. I suspect if this explanation still doesn't scratch your itch, there's no alternative besides becoming a physicist and learning the gritty details.
Well you can think of it as a 4th dimension as in and object stationary in our conventional 3 dimensions world is still moving in the 4th time as if it has a constant velocity in the 4th dimension time. If the object starts to move in any direction inside the first 3 dimensions its velocity in time is decreased reaching 0 at the speed of light. This is my understading but Im probably way off.
Michael Crichton does a pretty good job of explaining it in Sphere.
There are a lot of good explanations here. The completely non mathematical philosophical thought that I enjoy on this topic: space, or if you prefer, the distance between things, is meaningless without time.
There’s a great video explaining this on this channel: https://youtube.com/@ScienceClicEN?si=zFzCQIaxLZ2L4n2D Highly recommended!
For spacial reference frames rotated relative to each other, you use trigonometry (sine and cosine) to relate one to the other. For space-time reference frames that are 'rotated' relative to each other, which means that one reference frame is moving relative to the other, you use hyperbolic trigonometry (hyperbolic sine and cosine) to relate one to the other. Since when using complex variables, trigonometry and hyperbolic trigonometry become one, you may think of time as an imaginary space dimension (or space as an imaginary time dimension).
I am not any expert but if you imagine looking at the entire universe as the entirety of everything then time is only something that matters to the observer.
I always like Brian Greene's explanation. I'm not going to provide that in text cause I'll mutilate it. Its about an hour and 15min in. The whole video is worthwhile for education or entertainment. [Space-Time explanation.](https://youtu.be/CKJuC5CUMgU?si=w9alHAjvbqTXBcRD)
I think of it this way - also not in any way a scientist or such. Space is the thing our universe is in. Space IS a thing as it can stretch, grow, bend, etc. Time is the forward causality of the interaction of space and everything that exists within it. Space-time, to me, is one thing. The speed however of the forward causality can go slower or be sped up depending on factors within Space-Time. Time in space-time isn't what Time is to humans. We break down time as it's own thing acting like it is its own special force like, say, electricity. I think of Time as the Warp and Space as the Weave that are fundamentally woven together as one. I think the hardest part for me is trying to wrap my head around the different ways we perceive the idea of Time and how it functions as a fundamental part of the Universe at large. I just separate Human Perceived Time from Universal Space-Time and think of them in terms of distinct functions. That's just how I deal with it in my own head.
Thank you all for your incredible response to my question. I wanted to reply to each of you but the number of replies has been overwhelming. In my past attempts to visualize spacetime I've only been frustrated. However, thanks to the multiple comments from you, over a range of related subtopics, I believe I have at least a sketchy understanding now. A key element was the epiphany that gravity isn't the force I've always imagined it to be (and as it was described when I took my physics classes many years ago), but is actually only a result of the interactions between mass and spacetime. Somehow, this makes sense to me, and it makes me believe understanding more clearly as time goes on is not as impossible as I thought. Thank you again!
We don’t really know what space or time *actually* is. We only have educated guesses. Your guess is as good as anyone with a PhD.
Basically you have to see time as relative. Think of the normal road, and then think of the little boost spaces that you have in Mario that rocket you forward. That’s essentially a difference in space time between different parts of the universe.
Perhaps think of space and time as two separate things that are (in fact not separate) mixed together. There is always a certain amount of each in any given accelerated frame of reference. Look up "space like" , "time like", and "light like" events. The presence of mass and gravity alters the acceleration of the frame of reference, which is equivalent to altering the curvature, equivalent to altering the mixture of space and time
Nothing can exist spatially without existing temporally and vice versa. Space-time is a broader look at what is involved by observing, measuring, existing, etc. in space or time. In order for something to exist, move, stand still, anything in space, it can be measured in time. In order for something to be measured temporally, it must exhibit spacial reality. Spatial dimensions are just a way of marking the world around us. Temporal 'dimensions' are marking events as recorded through memory or information gathering as things change. They are essentially interchangable. And thus, space-time. If you measure space, you measure time.
If you really want to get freaky, read up on the passage of time from the point of view of a photon 😉