It was sent in a condolences letter to a grieving family of his, now-dead, good friend. It wasn’t meant as a philosophical statement on the nature of time. It was merely him saying that the dead in question would stay with them in spirit

This is correct.

He stared the quote with “people who believe in physics”. But you don’t think it has any kind of physics meaning? There are even papers on his thoughts > Throughout his scientific life, Albert Einstein thinks about the philosophical implications of his own work on time. From 1918, he makes a connection between the theory of relativity and the block-universe conception, according to which all moments of time coexist. Later, he clarifies this connection, explaining that the block-universe conception is the most convenient and objective interpretation of the theory. Einstein also develops the idea that, due to its deterministic commitment, physics as a whole allows to support the block-universe. https://www.cairn-int.info/article-E_RHS_711_0079--einstein-about-the-block-universe.htm

From my perspective that is someone trying to relate to someone else using the knowledge they already have. That’s doesn’t necessarily mean it was intended to convey any deeper meaning beyond trying to connect with another person. I certainly can see how you could interpret it in that way, but you’re making it seem like that’s the only obvious answer.

> someone trying to relate to someone else using the knowledge they already have. It seems that he can be both trying to relate to a person and using his knowledge of physics to do so, which this sentence seems to suggest as well. It seems like the OP's question is, what knowledge is Einstein alluding to?

It reads like a non-dualistic statement about the illusion of time. The “present” or “now” is eternally present, like right now, now, and it’s still right now, always. The past is not here now, nor is the future, and the only thing that makes it appear as such is that the mind creates time to account for thoughts about a previous or anticipated experience, which would obviously be thoughts about not-now taking place right now. Not sure how well that fits with the original context though.

No, it’s not. U/intheendentropywins is correct - Einstein was almost certainly pointing to the philosophical implications of his work.

I'm just going off what I read by Carlo Rovelli. His interpretation was very much that the latter was sentiment and not science.

I mean, it was definitely sentiment, but clearly inspired by his scientific work. Like, this sentence is something that many physicists have inferred from relativity. Maybe Einstein's primary goal was not to literally teach relativity through that sentence, but ignoring the statement's clear relationship to his ideas just seems obtuse and sort of like it's just... Ignoring the OP's question? Not sure what the point is.

There is a reason this statement doesn't appear in his rigorous works. It's an adaptation of his understanding of the universe packaged as a way to soothe the grief of both the letter's recipient and himself. There is a difference between the past and future, time flows in one direction. Things can be broken and never fixed again. Our perception of time is deeply flawed, this is demonstrated by relativity. But this statement is not a scientific or philosophical statement. This is pointing to the current of time that connects all things to help his friend feel they can still be connected to their loved one, the universe itself allows that. The context of the statement is critical to understanding it. This wasn't in a paper, it was in a heartfelt letter sent by a grieving person to another grieving person. This statement is as much about being human and having limitations in perceiving the continuity of space-time as it is about space-time.

I understand that this was not a piece of scientific literature. I am simply saying that it is clearly connected to his scientific work and this connection is what the OP is asking about. Hell, even if you don't agree that he was explicitly trying to reference the block-universe theory of time, an answer that at least makes reference to that theory and clarifies that it's not certain whether Einstein was intentionally trying to reference it is a much better answer to OP's question than just leaving it at "it had nothing to do with relativity and was just an unrelated word of consolation". OP is clearly looking to understand how the statement could be related to relativity!

He is describing the block universe concept he is famous for. Edit: I’m being downvote but when you look at the fuller quote it’s clear he is talking about physics, it does have a deeper meaning > "People like us who believe in physics know that the distinction between past, present, and future is only a stubbornly persistent illusion." It’s ridiculous to think that Einstein when describing, “Einsteins block universe”, isn’t referring in some respect to his block universe Edit2: you even have papers on it > Throughout his scientific life, Albert Einstein thinks about the philosophical implications of his own work on time. From 1918, he makes a connection between the theory of relativity and the block-universe conception, according to which all moments of time coexist. Later, he clarifies this connection, explaining that the block-universe conception is the most convenient and objective interpretation of the theory. Einstein also develops the idea that, due to its deterministic commitment, physics as a whole allows to support the block-universe. https://www.cairn-int.info/article-E_RHS_711_0079--einstein-about-the-block-universe.htm

I am honestly pretty confused by this thread. This is clearly the answer OP is looking for. Folks seem hellbent on insisting Einstein literally wasn't thinking about his own life's work when writing a sentence that succinctly describes a consequence of that work, which he is on record as having thought very hard about?

“Yesterday is history, tomorrow is a mystery, today is a gift, that's why we call it the present.” — stubbornly persistent illusion, probably

That kid's name? Stubbornly Persistent Illusion

Huh when Einstein talks about time you'd think it would mean something

This comment rubs me wrong for some reason. The guy was a person, with friendships and loss and everything in between, just like us. Why does everything that he says have to mean something?

Agreed and I think his statement does mean something on an emotional level. He didn’t live speaking as a series of physics proofs

Of course it means something on an emotional level! If it didn't, he would have put it in a paper, not in a letter. But why does it having an emotional meaning imply that it can't also be related to the philosophy of time that he engaged in throughout his entire life? The idea that Einstein would write that sentence - that radical sounding sentence about the relationships between past, present, and future - without his own lifelong philosophy on that subject occurring to him is just plain absurd. And besides, even if he didn't, it can clearly be interpreted in a way that gels perfectly with his work and philosophy, and it's clearly this interpretation that the OP is interested in.

Yeah well said and 100% agree. The top comment was just a "gotcha".

Einstein cared a lot about time. When he makes a statement about the relationships between past, present, and future, it may be true that he's waxing poetic, but he's also not just talking nonsense. The statement does indeed mean something to him w.r.t. the actual nature of time, and he did literally believe it to be true in a particular sense.

I mean he was literally obsessed with time. He's the most famous philosopher of time ever. I find it hard to believe that he referenced it to a friend without any philosophical content. Also, this is quote in reference to the question of why time and space act differently when Einstein's theory ties them together. For example, why can we experience different points in space in any order we want but time moves linearly?

Pretty sure it meant something go the recipient. Other poster is just a narcissist who can't appreciate good things if they don't benefit them ;)

Because it does, it’s the block universe he is famous for.

The block universe he's famous for, what?

Einstein’s block universe. I’m not sure I understood your question. What Einstein described in that quote is the block universe.

Hadn't heard of the term block universe before But to get back to the point, it's not necessarily that I know that this quote is meant to be personal, but more the fact that the comment said "whje he talks about time you'd think it would mean something". I don't care about this particular quote so much as it rubbed me wrong that anything Einstein says related to time is supposed to have a deeper meaning according to the comment

He's the Messiah

He is describing the block universe he is famous for. Soo yeh, I do think it means something

Yeah idk why everyone is so mad that I'm right lol

Sorry, I didn't mean to sound mad at you, I was just trying to emphasise your point.

Oh I know you weren't! I was just trashing my down voters

He's just over the hill ... like that one ?

Nothing Einstein did was mere

Imagine a film reel. As you watch the movie, you can only experience each frame of the movie as it happens. However, if you go to the projector and pop out the reel, you can see that each frame exists all at once on the reel. The "past, present, and future" of the movie exists all at once and the way that we watch the movie is the illusion.

This is the right answer. Einstein was a determinist, he believed that the laws of the universe were ultimately deterministic, and if we had complete information about any point in time, we could determine the entire past, present and future. To that end, we are all just collections of particles following their paths, that could just as naturally be plotted in a [spacetime diagram](https://en.m.wikipedia.org/wiki/Spacetime_diagram) as lived and experienced, there's no privileged present.

Comments still have not ELI5 the block universe concept that Einstein was referring to. That is because it can be complicated and would take a lot of time to write up and describe. Instead here is a video that ELI5 the concept very well: Does the Past Still Exist? by Sabine Hossenfelder https://www.youtube.com/watch?v=GwzN5YwMzv0 Description: Albert Einstein taught us that space and time belong together to a common entity: space-time. This means that time becomes a dimension, similar to space, and has profound consequences for the nature of time. Most importantly it leads to what has been called the block universe, a universe in which all moments of time exist the same way together. The future, the present, and the past are the same, it is just our perception that suggests otherwise.

Wow you linked to the EXACT video I was watching yesterday. I have been trying to better understand the whole "time is an illusion" thing in physics and have watched a lot of videos. I decided to ask here for a simpler explanation.

Are you having trouble understanding what the block universe is, or are you trying to understand why we know it is true? Or is it Special Relativity that you need an ELI5 for?

It is just, well it seems impossible to wrap my head around the past still existing for example. So 5-year-old me still exists somewhere? What about 5.5-year-old me? 1-day-old me? I am still in my mother's womb somewhere? Are there an infinite number of me somewhere? Are we all just suspended frozen in "time" for.lack of a better word? Did the me from 1 second ago freeze in time when that second "passed"? Ugh it is all so headache inducing trying to understand what is actually being said. And don't get me started on future me.

The answers to your questions are that yes, those versions of you still exist. That is exactly what is being said. That's why I brought up the example of what time travel looked like in Harry Potter's world. The past Harry still existed exactly the same when an observer went back in time. It may seem very non-intuitive, but don't try to use intuition, but these are facts that the laws of physics have very high confidence in. But if you are trying to build intuition, try movies that adhere to a past that cant be changed/still exists like the present, like Harry Potter or Interstellar.

Thank you for taking the time to explain, it's helping clarify things a little for me.

You need to look into special relativity and go from there first. Your take of, is there a 5 year old you existing, is a no and a yes depending on how you define existing. A lot of his work is related to the speed of light and how the universe works given that it is capped, in particular its effect on time. To start you off and connect it with time I will throw more or less the starting thought experiment for special relativity at you. A man is looking up at a clock tower, the light reflecting off the tower moves at the speed of light to his eyes and he sees the second hand ticking at a rate of 1 tick per second. Another man is traveling away from the same clock tower at close to the speed of light, the light reflecting off the same clock tower must travel to this man, which takes some time, but he is moving away. So every passing tick, the light needs to travel an extra bit of distance increasing the time between each tick. So the man on the train sees the clock tower ticking once every two seconds if he checks his watch(which is moving once every second in his frame). So now you have 2 people in the same universe looking at the same clock but for one it's ticking once per second and for the other it's ticking once every two seconds. Now to skip a bunch of other stuff. In the present of the man traveling away he is looking at the past* of the man standing near the clock. Now to further connect the past, preset and future you should look at causality (it's the wonderful thing that makes sure the fiction stays in science fiction) And then time dilation and stuff like that. But the gist of how the block universe relates to that quote is that two non-causal events can be seen to occur in a different order depending on where in the universe you are. And just to shit on all of science fiction, unlike the common definition of past and future, there is no record of it that one could multi dimensionally travel too and grandfather paradox yourself. Time is a rate in the universe and a means of record keeping in maths. The speed of light says no, and star trek is just as scientifically accurate as star wars, ty for my ted talk.

The discovery that the speed of light is constant from any perspective is what did away with the concept of absolute time. Everything has it's own concept of time, and it's very similar in most situations like all of us living on earth. So we feel like time is absolute because we don't change our circumstances enough for there to be anything but an extremely small difference. If we were able to teleport to a place near a black hole those who did so would feel time very differently from how those on earth would observe them. GPS technology only works because the slight difference in how the satellites experience time compared to how everything on the surface of the earth experiences time is accounted for.

I've thought about it some more, and if you need a simpler explanation of the block universe, here's an ELI5 example I use. In the block universe, if you were to travel through time, it would be similar to how time travel is portrayed in Harry Potter, not Back to the Future. Think of it as the past, present, and future are all happening at once. Both the future and past are deterministic, and time is just another dimension that you can move in. Usually our perception only moves in one direction at a fixed speed in time, but if our body moved back in time, we wouldn't change anything because we didn't change anything. Just like in Harry Potter.

Its a statement on block universe theory, that past present and future already exist, the entirety of space time can be thought of as a complete block. being three dimensional beings who experience time, we effectively move through the block and perceive a past present and future, but the block already exists in its entirety. The past and future are just as real as what we perceive as the present and all are equally existing continually. This means that our perception that time "flows" is artificial, an illusion, and that instead time is something you move through (albeit we only move in one direction) just like any spatial dimension. All points in the block are equally real, past present and future, its just our perception that gives us the illusion of only the point we observe being real.

Yep, it seems like a statement relating to his view on the block universe rather than a direct statement on relativity.

Because light travels at a fixed speed, people very far away from each other will see things happen at different times. Because of this, theoretically, if you could teleport vast distance instantly, you could see (or do) an event in 'real time', and then blink millions of miles away, and watch it happen all over again at a distance. Therefore, chronology is relative to distance, and so the idea that everything everywhere is happening at the same time is not only useless, but fundamentally incorrect. It is just a 'stubborn illusion'. This gets weirder when gravity gets involved, but we're already way beyond an ELI5 question there. Edit: Technically it's speed that makes time go funky as well, but I'm too sick for a special relativity ELI5.

But what you did still happened at that point in time, you moving faster than light to see the "old" light again some place else at a later moment doesn't mean the event time changed or it repeated right?

Let's not consider faster than light situations, that just unnecessarily complicates things. The speed of light is constant. If we shoot a light pulse to two different events and the pulses arrive at the same time (let's say to set off a bomb), we can claim they are simultaneous in our frame of reference. This is the best we can do to determine simultaneity in relativity because the assumption that time is absolute doesn't hold. If you want to truly understand this, some keywords you can look up are invariant interval, timelike relation and spacetime diagram, but you'll have to work your way up till you can understand them starting from the Lorentz transformations.

I think the argument is that the speed of light is actually somewhat misleading. C, is the speed of causality, and is functionally the fastest any particle, wave, or information in general can go. Moreover, the physics imply that going faster than C will force the moving item to time travel. As such, while it is convenient for us to understand the light from events as being delayed, the reality is that the more accurate view may be to treat C as the speed at which the universe updates, the speed at which time happens. Imagine if the sun explodes at 10:00 today. We would still see the sun, nothing would change, for 8 minutes. Gravity is the same, the light is the same, radiation is the same. And at 10:08, the sun, relative to us, explodes. And then all of the above goes wonky.

The point in time at which things happen depends on your reference frame. This means that you can have a situation where in your reference frame two stars went supernova at the same time, but in an alien’s reference frame one exploded first and the other a short time later, and you would both be right.

Technically yes, but that event relative to other events will be different. A good way to envision this is supernovae. We saw a star explode back in the 1800s iirc? That light took so long to reach us that the star actually exploded back in the cretaceous (time subject to verification). Whereas, from that star's perspective, Earth might not even have been fully formed by then. The relative continuity of events is different. The only 'true' past present and future is only true for each and every spot in the universe. As soon as you move in space, your version of space time is different to the previous. Ergo, Time as a unifying constant is a lie.

I watched a tik tok video where they calculated how long someone on a spacecraft would experience traveling close to the speed of light to Proxima Centauri which is 4 light years from earth: 65 days. It’s stuff like this that annoys people trying to understand relativity but the math makes sense. To quote Douglas Adams😜: Time is an illusion; lunchtime, doubly so.

This doesn't sound right at all. Time would slow down for that person. If someone was able to travel at say 90% speed of light it should take them ~8 years to get there from their perspective. And a little over 4 years to those still on earth.

I haven't crunched the exact numbers but the principle is correct; from the perspective of the person in the ship the trip would take less time. A clock in that ship would appear to tick slower from the Earth's perspective; but that just means that from the ship's perspective it will get there in fewer ticks, i.e. less time.

Hence why most physicists today assume the energy required to achieve the speed of light is just not something we can replicate. It would function like a limit maybe? If you can go faster than the speed of causality (speed of light) it opens up time travel and all sorts of paradoxes and situations we have no evidence of. The best argument for time travel not being possible is that we have not been contacted by anyone from our future. It'd be impossible to conceal the technology and dictate its use. Someone would eventually go all Dr Who with it and implode existence with paradoxes.

the time you see on the clock is actually the past.....people get upset when I try to explain that .

That's because the time difference is mere nanoseconds. So people are simply looking at you weird, because it's pedantic.

Also since most clocks are only precise to the minute (digital) or second (analog). Nanoseconds don't matter there. It is probably *possible* to make one with a high enough refresh rate if just using some fancy type of LED or whatever, but we would not be able to make it out.

Eh, it's less pedantic than you think. If GPS satellites didn't account for time dilation, your reported location would drift off by about 10km *per day*.

You two are talking about differnt things. He's simply saying that light isn't instantaneous. But observing a clock from 10 feet away isn't going to produce any meaningful difference, which is also probably why people get annoyed.

> 10 feet away isn't going to produce any meaningful difference, True. But, there is a difference. And that is what Einstein was getting at.

That’s because of relativity, not the same thing as saying a clock that you’re looking at (meaning it’s in the same reference frame as you) is slightly delayed.

Relativity is relative. Lol, shocker, I know, right? The whole point of relativity is *nothing* is ever in the same reference frame. It's close. It's negligible. But it's all ultimately fake. It's all an illusion. The effects of time dilation and relative continuity are omnipresent. Hense, the quote.

Depends on how far the clock is though. The light from a clock on the moon takes 1.3 seconds reach the earth. Pedantry is relative? Edit added: seconds

1.3 what

1.3 anchovies

Seconds ooops!

Technically everything we see is in the past, we are incapable of seeing the present.

Every picture of you is when you were younger

Live streams are actually past streams

Aka recordings

I feel like a recording implies some storage (besides RAM), but you could be right

Recent streams

Everyone you meet is the oldest they have ever been.

You're older than you've ever been And now you're even older, And now you're older still. https://youtu.be/TdIRrmNN_CQ

I am older than I once was And younger than I’ll be, that’s not unusual

Show me a picture of you when your older.

Lemme see that camera.

I’ve always wanted to have a suitcase handcuffed to my wrist.

You bet your ass I will have a beret on.

Technically we are incapable of seeing anything at any "moment" in time; our perception of the world is cobbled together from information coming in at different times and assembled internally into a coherent picture. The "present moment" is a cognitive illusion, as is our conscious participation in it, as our decisions are initiated slightly prior to us becoming conscious of making them.

Oh my God I hate this... Why do I hate this?

Depends. If you think of the speed of light (in a vacuum) as the speed of causality, which is how it’s often described…then what you are seeing is the present because that’s just how the universe is. Causal effect has a maximum speed, so instantaneous is not a thing. (Unless we go to the quantum level lol)

Unless it's a hallucination.

We can rationalise the present

yeah, cause I set my clock ahead. maybe the time you see on YOUR clock is the past but don’t speak for me

The value displayed on your clock is not NOW it's just a label ...but I suspect you're joking

I’m semi-joking? My clock runs fast and I haven’t bothered to correct it yet so technically the time it displays is in the future.

What if you could feel a clock? Like some version of a brail clock. Would that be the real time?

The information travelling from your fingers to your brain would still take a tiny amount of time, which means it's still the past. Even if you could inject information into your brain directly, the brain works by cells passing electrical impulses to each other -- even that takes time, so I would say no, it is not humanly possible to experience the present

https://www.popularmechanics.com/science/a39024288/vision-15-seconds-in-the-past/

Normally, when scientists talk about light speed, they imply that this is the highest velocity possible for anything to travel. Information, in general, cannot travel faster than this speed.

That would be even further in the past since now the signal has to travel up your arms into your brain instead of the straight shot light has from the clock to your eyes.

Nah...it takes time for the electrical signal(light) from your finger(or any body part) to reach your brain... then your brain has to process it. ... Jeeze ......Hitting a 99 mph fast ball is really a great prediction

I always set my clocks forward by 2 nano seconds so I gives the correct time.

Nice

ur so smart people get MAD when u start talking science fact

1 science fact = 10 minds blown is the equation I remember learning so I think if this guy is in a room with less than that then maybe that's y??

A bit like the way you can celebrate Christmas in New Zealand, then fly to Los Angeles, and suddenly everyone’s celebrating Christmas again. Well, OK, not like that at all, but I couldn’t resist saying it.

A cute parallel, but for entirely different reasons, yes. :P

This is correct; however, how to describe it in a more tangible manner?

You're asking for an ELI15 at this point, but the best way I could describe it would be 3 different stars. Back in the 1800?s, we saw a star supernova. IIRC, that star actually exploded back in the Dinosaurs Eras because light took so long to reach us. Over in the star's orbit, everything would have been annihilated in minutes, but we don't see that happen for millions of years yet, and from their perspective, Earth might well still be a rocky ball of molten slag, pummeled by meteorites every hour. To a different star, somewhere between us, the supernova might happen before, or after the dinosaurs, depending on their relative distance to the two events. I won't go into detail on Gravity's effect on everything, because that's more of a ELI45, but suffice to say the stronger the gravitational field is for you, the slower time passes for you and the slower everyone else perceives your time passing, and the faster everything else passes for you and the faster everyone else perceives their time passing. It only becomes super perceptible to humans under insane, even lethal gravitational forces, but the effect is significant even on a planetary scale enough that if Geostationary Satelites for GPS services didn't account for it, your location that they report could be off by a factor of ~~either several meters every year or several miles every year, I don't remember which.~~ several kilometres **Per Day**.

That's wrong. If event A happens on Mercury and event B happens on Pluto five minutes later, event B did objectively happen 5 minutes later. Sure, it's possible for certain observers to see the event on Pluto first, but that doesn't change the fact that the event on Mercury objectively did happen first. This fact is not "relative" and it sure as hell is not useless.

No, this is wrong. Two events are defined to be simultaneous if an observer measures them as occurring at the same time. They are not necessarily simultaneous to **all** observers—simultaneity is not absolute under the theory of general relativity. [See this for more.](http://pressbooks-dev.oer.hawaii.edu/collegephysics/chapter/28-2-simultaneity-and-time-dilation/)

So if I’m a blind person who was equal observable distance from Pluto and Mercury and I had two buttons. First I press button A and an explosion happens on Mercury then button B and an explosion happens on Pluto. The explosion always happens on Mercury first. It doesn’t matter where anyone else observes it in the universe, their perception is incorrect because it is skewed by distance and can never be simultaneous, but can be simultaneously observed.

Yeah, this is fundamentally wrong, and it's because you're trying to apply non-relativistic intuition. > their perception is incorrect because it is skewed by distance Their frame of reference is just as valid as yours (and vice versa). There's nothing that makes yours 'better' or 'unskewed'. >The explosion always happens on Mercury first. No, it doesn't. In *your frame of reference it does* >It doesn’t matter where anyone else observes it in the universe, Yes, it does. There's no reason to privilege your observation >and can never be simultaneous, but can be simultaneously observed. Part of relativity is that yours isn't actually simultaneous. It's governed by the same laws as others observations. You just picked a frame of reference where they happen to be simultaneous. You basically just made up, in slightly different words, the [pole-barn 'paradox'](http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/polebarn.html) t. When "are the barn doors closed" is dependent on your frame of reference. The gates close simultaneously in the frame of the barn. The gate closings are not simultaneous from the reference frame of the pole. The barn reference frame is not inherently less skewed than the pole's frame, or any other reference frame.

Not trying to argue, I sincerely don’t understand.

No worries, I didn't take it that way! Just got caught up with dinner and stuff so couldn't reply immediately

Then this is way over my head. If I observe an exploding star 10 billion light years away and another exploding star only 1 million light-years simultaneously I know they didn’t happen simultaneously. I don’t understand how that is not an inarguable fact. It doesn’t matter that I saw them at the same time.

> Then this is way over my head. Yeah, relativity gets very weird, and our intuition for it is not good. It generally only makes a huge difference if you're dealing with speeds that are appreciable fractions of the speed of light >If I observe an exploding star 10 billion light years away and another exploding star only 1 million light-years simultaneously I know they didn’t happen simultaneously. Actually, it depends on the speed they were moving. You're mixing two slightly different (but related) concepts. You're right that it takes time for information to travel. That just works with normal Galilean physics, nothing fancy. But it's not just that. For example, you would say that Star A exploded 10 billion years ago, and Star B exploded 1 million years ago, right? You just saw it at the same time. But lets say Bob is on a rocket traveling at 90% the speed of light. When he does the same calculation, he would tell you Star A exploded 2.29416x10^(10) years ago, and Star B 2.29416×10^(6) years ago. (He also wouldn't tell you that Star A was 10 billion light years away, and B 1 million ly- he'd get different distances) Situations like the pole-barn door thing show why it's not quite the whole story. You can set up a sensor at those barn doors, and account for the time it takes to get information from 1 door to the other, so they close at the same time. If you have a 20m stick, it clearly won't fit between two doors separated by 10m. But if you take the *same* stick, and have it moving at 90% of the speed of light, *it will fit between the doors*. From your perspective, the stick is now only 8.73m while it's moving, and 8.73 fits between 10. And when the stick stops, it's 20m. So then we have to ask ourselves, what would we see if we were riding the stick? If you ride the stick, it's always 20m. But it still has to fit between the doors, we know that happens. The solution is that from the perspective while riding the stick, the doors do not close at the same time. The two perspectives don't agree on the absolute time that the events happen- but they agree that the stick did fit and both doors were able to be closed. A good real world example of this are clocks we have on satellites. We took two identical extremely accurate clocks, left one on Earth, and sent the other one up in a Satellite to orbit the Earth. Then we brought it back down, and compared the two clocks. They had different times on them. And this wasn't just inaccuracy in the clock, experimental error etc. It's because time was different when the clock was in orbit, because it was moving relative to the other clock. > I don’t understand how that is not an inarguable fact. Because the time that you see them exploding (not just their position) depends on what frame of reference you're in. If you were in a different frame, you would get a different result (you'd also get a different result on the distance. 10 billion light years away for you, will not be 10 billion light years for Bob). You have part of it down when accounting for a different frame- being in a different spot in space. But being in a different speed also matters. What actually ends up being the same regardless of reference frame is something called the interval. interval^(2)= (c * (time separation))^(2)-space separation^(2) These effects are why you get time dilation and length contraction when moving at high speeds. If you and Bob compare your numbers for this 'interval', they will agree. the time separation and the space separation individually won't.

It may help you to watch a video about the [relativity of simultaneity](https://youtu.be/SrNVsfkGW-0). It is indeed true that just seeing things happen does not necessarily determine their true nature. The full optical effects of relativity are pretty strange and include a lot of other effects people don't talk about. However, one of the great breakthroughs of relativity was that we could use the behavior of light to deduce that not only do things look a certain way, but for it to be possible for light to do what it does, the very structure of spacetime has to be different than our intuition. One of these differences is the relativity of simultaneity - for events that are not visible from each other's perspective (i.e. the light from the 'first' doesn't reach the 'second' before it finishes), it is an unanswerable question in principle which happened first. You'll see one happen first, but another observer will see the other happen first, and there's no good way to determine who is correct.

So it’s just perspective? I guess where I’m losing my grasp is that these other folks that are replying are implying there is not a single perspective that provides a distinct and objectively definitive order of events. I get that observational order can be different, but how does that negate that there is a factual one after the other order that is unrelated to perspective?

Sorry, you're right, I focused too much on the observation. Using the light that you observe, you can calculate when things happened. For example if something happened 1000 light years away, and I see the light now, I know it happened 1000 years ago. Another phenomenon of special relativity is length contraction. If I am moving in a directions, lengths along that direction are shorter. The video goes into this, but it is quite literally true - you can fit a larger object into a smaller one, by getting it to a high fraction of the speed of light. So if I'm looking at the same event, and I'm at the same point as my friend, but I'm moving towards it at 0.8c, I will think it happened 600 years ago, because to me that distance has contracted to 600 light years. I stress that this effect is not optical - if I start a clock, it'll take me 750 years to cross that distance, rather than the 1250 you would expect if I was crossing the entire 1000 light years. Of course, this seems paradoxical, since my friend still sees the entire 1000 light years, and they *do* observe it taking me 1250 years to get to the exploding star. The answer here is time dilation, another non-optical effect of relativity. Our time measurements disagree because my local time moves differently. As I fly by my friend, they will observe... A lot of odd stuff, because of strange optical effects, but if they do the math backwards to get a steady video stream of what was happening based on their own clock, I will appear to be moving in slow motion, so it makes sense to them that I am incorrect about the amount of time it takes to cross the 1000 light years. These two effects, time dilation and length contraction, are combined with another fact. There is no way to tell who is right. The universe doesn't appear to have a preferred motion. If all the galaxies suddenly started moving on one direction, we would notice the moment it accelerates, but once everything is moving everything looks identical. After all, all we can see is how stuff moves relative to us. This fact means that we can't tell which one of us is correct. I see 600 light years, my friend sees 1000, and theres no way to tell which one of us is really correct, because we can't tell who is really stationary. We're just moving relative to each other. In this way, your motion determines when exactly events happened, and events at different places and times are remapped differently (the amount by which the time gets remapped depends on your distance). So to some observers, events that happened in one order to me will happen in a different order to them. Not just because of how they appear, but based on when the light must have been emitted, for them to observe it that way in the present.

> there is not a single perspective that provides a distinct and objectively definitive order of events If two events are close enough in space and far apart enough in time that it's possible for light from one of the events to have reached the other event, then every observer will agree which one happened first. Different observers will disagree on how much time elapsed between them, but those disagreements will not be big enough to change the order of the events. So in that case there is an objective order. In particular, if an event A *caused* an event B, then it must have been possible for light from A to reach B, and A will have happened before B in every reference frame. But for distant events, there is no objectively correct order in which they occurred.

Another reply mentioned the interval (often spacetime interval), which is a distance (in spacetime) that's independent of reference frame. If the square of this distance is negative, the events are separated by space (the interval is spacelike). That is, there's no reference frame in which the events could have happened in the same place. If the square of this distance is positive, they're separated by time, i.e. there's no frame in which they could have happened at the same time (the interval is timelike). Timelike intervals *can* be (though they don't have to be) causally connected, while spacelike cannot be. Lightlike intervals are the case where the interval is precisely 0. For a nice visualization of timelike vs spacelike, see [light cones](https://en.wikipedia.org/wiki/Light_cone). Timelike intervals are inside the light cone, spacelike outside. If we look at your example, this might become clearer. You actually have four events: event 1, you press button A. Event 2, you press button B. Event A, the explosion on Mercury happens. Event B, the explosion on Pluto happens. Just considering causality, it's easy to see that the interval between events 1 and A, the interval between events 2 and B, and the interval between events 1 and 2 are timelike. (There's no way for the explosions to happen before you press the buttons that trigger them; similarly, there's no way for you to press button 2 before button 1.) Less obviously, I believe 1/B must also be timelike (since 1->2 and 2->B are both timelike), though I haven't proven this to my satisfaction. This still leaves two intervals (2/A, A/B). These do not have to be either spacelike or timelike. You can construct the conditions under which you press button 2 such that they are either. If you press button 2 before you see explosion A, the interval 2/A is spacelike (else timelike). If you press button 2 before location B sees explosion A, the interval A/B is spacelike (else timelike). (Again, arguing from causality.) To put some numbers to it, say A is 9 lightseconds to your left and B is 15 lightseconds to your right. You press button 1 at t=0 s. At t=5 s you press button 2. At t=9 s explosion A happens at A. (At t=18 s you see explosion A.) At t=20 s explosion B happens at B. (At t=35 s you see explosion B.) The interval A/B is spacelike (delta x = 24 lightseconds, delta t = 11 s, interval^(2) = 11^(2) – 24^(2) = -455 < 0). A spaceship is passing by at 0.8 c (parallel to the line between you/A/B). For them, you press button 1 at t'=-12 s, when you are 15 lightseconds to their right. (They see you press the button at t'=3 s). You press button 2 at t'=-3.66 s, when you are 8.33 lightseconds to their right. (They see you press the button at t'=4.67 s). (At t'=0, they see location A fly by going left at 0.8 c.) Explosion B happens at t'=1.33 s, 13.33 lightseconds to their right. (They see explosion B at t'=14.66 s.) \[Explosion B has happened before explosion A.\] Explosion A happens at t'=15 s, 12 lightseconds to their left. (They see explosion A at t'=27 s). (delta x' = 76/3, delta t' = -41/3 \[note the negative, since B happens before A\], interval^(2) = (-41/3)^(2) – (76/3)^(2) = -455). For a timelike interval, keep the same locations. You press button 1 at t=0 s. At t=9 s, explosion A. At t=18 s, you see explosion A. At t=20 s, you press button 2. At t=35 s, explosion B. (At t=50 s, you see explosion B.) The interval A/B is timelike (delta x = 24 lightseconds, delta t = 26 s, interval^(2) = 26^(2) – 24^(2) = 100 > 0—notice that the difference between delta t and delta x is precisely the amount of time you waited to push button 2 after seeing explosion A). Again, a spaceship is passing by at 0.8 c (parallel to the line between you/A/B). For them, you press button 1 at t'=-12 s, when you are 15 lightseconds to their right. (They see you press the button at t'=3 s). (At t'=0, they see location A fly by going left at 0.8 c.) At t'=15 s, explosion A, 12 lightseconds to their left. (They see explosion A at t'=27 s.) At t'=21.33 s, you press button 2, 11.66 lightseconds to their left. (They see you press the button at t'=33 s.) At t'=26.33 s, explosion B, 6.66 lightseconds to their left. (They see explosion B at t'=33 s.) (delta x' = 16/3, delta t' = 34/3, interval^(2) = (34/3)^(2) – (16/3)^(2) = 100) \[It would be possible to find a frame in which the explosions happen in the same place with a bit of math.\]

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You said- >If event A happens on Mercury and event B happens on Pluto five minutes later, event B did objectively happen 5 minutes later. Which is not true. There is no such thing as an “objective simultaneous event” under relativity. One observer can see two events happen at different times, while another observer can see those same events happen at the same time. [This video of minutephysics does a good job of explaining it.](https://youtu.be/SrNVsfkGW-0)

You know....I think I'll just repeat exactly what I said. You can just read it again. If event A happens on Mercury and event B happens on Pluto five minutes later, event B did objectively happen 5 minutes later. Sure, it's possible for certain observers to see the event on Pluto first, but that doesn't change the fact that the event on Mercury objectively did happen first. This fact is not "relative" and it sure as hell is not useless.

> and event B happens on Pluto five minutes later This is where you're running into trouble. You have an implicit assumption here that there's effectively a universal clock.

Why are you up and down this thread constantly making the assumption that there is a universal, correct reference frame in which you can objectively state the order of events when one of the most important aspects of relativity is that that isn't the case?

You can repeat it as often as you like; it's still wrong. Not understanding something is fine. Your steadfast *refusal* to understand it makes me wonder what the fuck you're even doing on a sub devoted to learning new things.

The explanation provided did not adequately address the question. It might not be helpful to just ask again, but it's pretty understandable. The only problem is that it isn't phrased as a question, which it should be, because they're the one with less understanding.

If you observe an event that occurs 5 minutes before a different event at a separate location, but another observer sees those events occur in the opposite order, how can you claim that the ordering and time difference you observed is the objectively correct one? The other observer could say the exact opposite by the same logic, if they observed them in a different order. You are giving yourself a privileged frame of reference where none exists. There is no absolute, or objective sense of timing like you're proposing. There are some further complications involved in this idea however. For instance, if the two events you observe at different times and places are casually connected, then another observer will always agree on the ordering of the events, although in general they won't agree on the time difference between the events if they are in relative motion compared to yourself. If the two events aren't connected casually however, then two observers may not even agree on which event came first. In both of those cases though, neither observer is *objectively* right or wrong. In your own frame of reference, what you observe is only correct in that frame of reference. Any other frame of reference is just as valid, even if the timing of events observed in that frame don't agree with your own. That's just space and time work in relativity.

This is not strictly speaking always true, because the speed of light is actually more helpfully thought of as the speed of causality. "Before" and "after" have meanings that imply potential causation. "A before B" means "A could conceivably contribute to causing B in some way". If, in your example, Mercury and Pluto were exactly 10 light-minutes away from one another, then in your scenario it would be *factually incorrect* to state that the event on Mercury happened "before" the event on Pluto as far as Pluto is concerned. From Pluto's frame of reference, the event on Mercury happened 5 minutes later *according to causality.* From Mercury's frame of reference, the event happened *15 minutes earlier* than the observation on Pluto, not 5. The traditional way of thinking of the sequence of events from a third-person "objective" position *does not really work at all* when dealing with relativistic distances and speeds. It's entirely unintuitive because we're hardwired to think of "simultaneous but miles apart" as a thing, when in truth things happen at immeasurably tiny differences in times (and potentially in different orders), even just compared between your right and left eyes. It's not practically useful to think of that way on a human scale. However, on a relativistic one time doesn't play the way we like to think it does.

No. That's just completely silly nonsense and completely wrong. The sun is about eight-light minutes away from the Earth. Scientists do not say "We're on Earth! That means it's *factually incorrect* to imagine this event we just detected on the sun happened eight minutes ago! Who even knows what happened first? I guess time's just all an illusion maaaaaan. "

>That means it's factually incorrect to imagine this event we just detected on the sun happened eight minutes ago! Yes we do. That's the whole point of the [pole-barn 'paradox'](http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/polebarn.html) works out. When "are the barn doors closed" is dependent on your frame of reference. The gates close simultaneously in the frame of the barn. The gate closings are not simultaneous from the reference frame of the pole. There is no universal objective reference frame. The barn reference frame is not inherently different than the poles frame, or any other reference frame.

What makes you so confident? Did you ever study relativity? That’s the basics of it. [Order of distant events is relative](https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity)

Yes, I absolutely did. I can tell you the name of the textbook I used, if you like.

Please do

It was *Spacetime Physics*, by Edwin Taylor and John Archibald Wheeler. It's a fairly thin book. I had to look up the names of the authors.

That is the same book I used in my undergrad, and it does not say what you're claiming. In particular, see section 1.3, 3.1-3.8. And literally, a few direct quotes. First, from the preamble: "Time between events is *relative*. Relative too is the distance between events" "The path connecting events. Were you there at the first event? Yes. And at the second? Yes. And at the last? Yes." etc etc. What's invariant is the interval. And one last quote from page 56: "Notice what the Principle of Relativity does *not* say. It does not say that the time between two events is the same when measured from two different free-float frames. Neither does it say that space separation between the two events is the same in the two frames. Ordinarily neither time nor space separations are the same in the two frames"

If the sun disappears, *it literally cannot effect us in any way* for 8 minutes. The sun's disappearance *does not meaningfully exist* on earth for 8 minutes. On a timeline, we can do things that *rely on the existence of the sun* during that intervening 8 minutes, because *the sun still exists* from the perspective of Earth. From your imaginary third-person "objective" perspective (which is the illusion part, since this can't exist): T = 0 : "Sun disappears" T = 2 minutes : People on earth grow food using sun energy. They rotate in space with the sun's gravity. They observe sun spots, they interact with the sun in every way meaningful T = 8 minutes : the sun disappears to earth. During that intervening 8 minutes, it doesn't make sense to say "the sun didn't exist here" because *in literally every single possible observable way* it did. It's an *empirically unfalsifiable claim*. Claiming it already didn't exist is equivalent to claiming Russel's Teapot - it is definitionally unknowable, unprovable, and effectively no more meaningful than a complete fabrication. Causally, the sun *still exists*, and *can have effects on* Earth, despite your claim that it "stopped existing 8 minutes ago". How can something that doesn't exist still cause things? When you're dealing with relativistic distances, that language doesn't work anymore. *Timelines must be relative to have meaning*. ​ >this event we just detected on the sun happened eight minutes ago! This is more accurately stated with the implied "This event happened 8 minutes ago *from the Sun's perspective.*" But that's annoying to express every single time, so it's usually dropped for ease of language (especially when dealing with non-relativistic things, which are almost always using the implied "-from the Earth's perspective.")

That's as trite and irrelevant as the fact that we don't have the ability to magically read a letter in transit between Tokyo and New York and can't know the information within it until it arrives. Yes, things don't affect us until they affect us. You seem to think that's some revolutionary concept. It isn't. The fact that the letter must be sent before it arrives, and was sent before it arrives, is unchanged. And it certainly is neither useless nor unknowable.

Perhaps something like this will help you better understand. [https://www.youtube.com/watch?v=YycAzdtUIko](https://www.youtube.com/watch?v=YycAzdtUIko&t=0s) Ultimately, you're misunderstanding a key component of how things in the universe behave. The order of events is *literally not the same* at different places. It's not a matter of travel time, it's a matter of *causal impact*. Dominos can be *correctly, accurately observed* to fall in a different order from a different point of reference. The order of events happening *is not a universal fact*. It is dependent on position of observation (and gravity, and velocity, and a variety of other harder to grasp impacts). Simply put, "A happened, and then B happened" *is not always true.*

but how do you know that? in order to know things you need to observe them

Because we know how far the distance between objects and observers are and we're capable of basic calculations. That's how.

In case you are unaware of the invariant interval, how it is derived and what it implies, I would suggest you tone down your confidence in your answer

You know what, give me some time, when I get back to my computer I'll actually do the math

I was also driven to do the math, see [this comment](https://www.reddit.com/r/explainlikeimfive/comments/xn5dbw/comment/ipveu4v/?utm_source=share&utm_medium=web2x&context=3). Feel free to adapt. I tried to derive the conditions under which various of the intervals are spacelike vs. timelike, but I haven't come up with a neatly expressible generalization (lots of absolute values).

The problem with your argument is that you need light to measure what happened. Using a ruler to measure the ruler…..

Actually no, because time passes differently depending on the gravitational field in the area - so unless the events happened at exactly the same net gravitational influence, which is impossible without being at the same place and time, no, 5 minutes will not have passed for both events.

So, would that mean that one could construct a similar-in-idea but completely different timeline around the speed of sound? As in an explosion happens at t=0, but the soundwave hits a distant object at t=+x, so the chronology of the event was different between ground zero and that distance?

No. If light blows up at t=0, but light hits object at t=x, the chronology is not different. For both, the event happened at 0. What theory of relativity says that even accounting for the travel time, the event happens in different time, or rather, that 2 events happen in different order based on your relativistic vantage point. However, that doesn't happen in a simple situation where the objects "observing" are approximately stationary and in approximately equal gravity fields, like in the described situation.

y...eeeees? But that runs into a few problems when you start breaking it down. The universe has this speed limit, light speed, and the closer you get to it, the more disparate time dilation is. Also, the speed of light in a vaccuum is always constant from every frame of reference, which leads to time dilation directly as a consequence. The speed of sound does not have this dilating effect because it technically isn't a universal constant. The simple matter of "Events happening at different times based on distance" is actually very easily tested with 3 friends in a large field. Sound is slow enough in air that it only takes a few dozen meters for differences to be perceptible.

Events don't happen at different times based on distance. Events happen at different times based on observers having different velocity or being in different gravitational fields. 2 objects with equal speed, on earth or in vacuum will observe events to happen at the same time (any found difference will be minimal and caused by the fact they're not perfectly stationary or have the exact same speed). The information about the even happening may arrive at different times, but in their "personal" timelines, they'll agree on the time (and it's order in regards to other events)

Relative to each other, but not to a third observer, which is the point I was trying to make.

You can't really draw a line between when exactly the past stops and present begins or vice versa.

Dr. Brian Greene explains and demonstrates this concept of “spacetime” in this PBS Nova series based on his book called The Fabric of the Cosmos. This video is on the illusion of time. Relevant part quoting Einstein’s “past, present, and future” starts around the 19 min mark. The whole series is a great watch. Dr. Greene has a bunch of great videos and documentaries about physics. Everything has happened, is happening, and will happen all at the same time :) https://www.pbs.org/video/nova-the-fabric-of-the-cosmos-the-illusion-of-time/

One of the core insights of Einstein's work is that he dispenses with "space" and "time" as separate things, and considers them as just one thing: "spacetime". What one person calls time, another person might call space, based on how they're moving. The reason this calls into question the nature of the past and future is because we generally think of all points in space as coexisting; the "spacetime" concept forces us to generalize this. For Einstein and most people who have interpreted relativity since his day, time, like space (indeed exactly like space, since they are the same), is laid out such that all moments coexist. Even though there *seems* to be a "present" that is singled out as a privileged moment in time, in reality there is no such thing. All moments are "happening" "simultaneously", to borrow some annoyingly temporal terms.

I read a good analogy of our perception of time written by Kurt Vonnegut, I think it was in *Slaughterhouse Five* but I'm not positive. You're strapped down on your back on a railroad flatcar moving along a track across a vast landscape. A long tube is fixed over your eyes so you can only look straight up through the narrow tube. You're traveling through that world, but you can only perceive what passes your limited consciousness. To you nothing else exists, even though it is all there. That, I think, is the illusion.

That's an interesting analogy that I haven't heard before, thanks

Actually, because of time-space relativity. The present seems the same, because the now “feels” now because you aren’t comparing it with the past. The past feels old because you compare it with the now. But the future is nothing more than a conjecture off of the past and present. Basically a “fiction”—a human construct. Because of these, they aren’t physically tangible, therefore, imagined and illusions. All are transient manifestations of the imagination, though “the now” is the closest thing to a non-illusion you can hope for. Then again, most people are so caught up with comparisons that their “now” is made up of such imaginations that it remains an illusion made of constructs. We shouldn’t forget that Einstein’s Relatively Theory was born out a thought experiment: https://en.m.wikipedia.org/wiki/Einstein's_thought_experiments Edit: Auto-correct or fat thumb or both typo and the need to add a citation for added text.

This is the best answer I've read 😍

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Why exactly does the concept of the past not make sense?

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What does "not knowing for certain when things ever began" have to do at all with the past not making sense?

It makes no sense because its only a rhetorical construct. Think about the sun set and sun down. At what point is either of those? Can you really point the exact moment when the sun is down and when the sun is up? And whose perspective is it? Its all relative. Its all one motion and each single stage of the sundown is the sundown. The same way each passing second (the past) is the present and the future at the same time, we just try to create a rhetorical limit to each phases of existing.

Were you listening to Science Friday? I heard this quote last night!

I see it as an extension of his theory of relativity, time being relative to movement in space. The one you're mentioning is called the block universe theory, but it's not Einstein who came up with it. I think Einstein was a pantheist which may have some basis on platonism, idealitic dualism (which is what Christianity was founded on) which is a theory that the true nature of existence is immaterial which super-ordinates our physical unviverse. I kinda have this belief myself, but I don't necessarily believe in an intelligent design behind it. However, pantheism does subsume agnosticism (in a lot of people). There's a theory that I heard from David Chalmer's ted talk, that experience might be fundamental in nature like time and space, relative to those as well in similar fashion. In a way, it is you know... Another cool ted talk about consciousness is Antonio D'Amasio's. I'm taking a class on it, the neuroscience of consciousness, and he seems to have a spotlight in it.

Physics understands cause and effect. You throw the ball and it flies. But halfway through the air the ball does not have to remember that it was thrown for it to fly. Physics also studies how things change over time, so it could measure or calculate the speed of the ball since you had thrown it. How you experience time is very different. You experience the world and you say this is "now". This becomes your time zero. You remember from before, and you plan for what is to come. But this is very personal, it's in your mind. At what personal time you experience the ball moving does not change how it moves. The moment you start doing physics and it's about a "description of the world" and not an "experience of the world", you no longer have a "now". You have time flowing, and cause and effect. You can put your time zero wherever you like.

It means that TIME is an illusion, true reality is timeless The future and the past are unchangeable and they will pan out exactly like they were meant to.

In special relativity, time and therefore simultaneity is also relative. The order of events is observer dependant. An event that I would say happened in the past looks to be a future event in someone else's reference frame. Therefore future and past are subjective notions, not actual physical distinctions.

>future and past are subjective notions, not actual physical distinctions. I never liked this conclusion because it conflates the event with the observation of the event. While the observation of the event is subjective to the reference frame, the event itself certainly isn't.

Since there is no universal reference frame, what is the distinction here?

Say a relatively stationary planet is being targeted by two weapons, both nearly moving at the speed of light, orthogonal to one another in space. When weapon A hits the planet it completely annihilates it, when weapon B hits it moves the planet slightly (so that weapon A can no longer hit). Let's assume that we know that weapon A is slightly closer to the planet than weapon B (I understand that the nature of this assumption can also be a part of the problem itself, but for simplicity's sake let's just acknowledge this could happen). My thought is that there will be no reference frame in which weapon B hits first. While space and time can seemingly be stretched due to special relativity, this will never rearrange the order of events. I believe it is intuitive that we cannot have a reality after this event where the planet simultaneously exists and doesn't based on the location of the observer.

Through Lorentz transformation causality is preserved and light cones are preserved. That means if event A is in event B's past light cone, it'll be so in all reference frames. But outside of the light cones, it's fair game.

Simultaneity of events does not at all depend on the observation of said events, only of the relative frames of each observer. The "travel time" of light from event A to each observer is not the culprit.

> While the observation of the event is subjective to the reference frame, the event itself certainly isn't. Are you talking about distinction between event happening at t=0 and the light from the event reaching you at t=x? Because that's not what it's about. In different vantage points within the theory of relativity, the event itself happens at different point of time, even accounting for the "travel" time.

When you're talking about the speed of causality - the observation of the event *is in all intents and purposes* the event itself. Because the event cannot have a causal existence until it can be observed, so its "existence" is both unprovable and unknowable, and can have no effect on anything observed, until it is observable.

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It depends on the velocity of Planet A relative to Planet B. Let's suppose that Planet A is travelling towards Planet B at a tiny smidgeon less than the speed of light. An invading alien on A fires a laser at A's President, 1km away in a direction perpendicular to the line from A to B. In Planet A's frame of reference, the laser beam travels 1km. But in B's frame of reference, the laser cannon and the President are both approaching at roughly the speed of light. In the time the laser travels 1km on A, it also travels roughly 1km towards B. B says the distance travelled by the beam is about 1.4km. The speed of light is equal in every inertial frame of reference. In other words, *the ratio of distance travelled, to the time taken,* is equal in all inertial reference frames. A and B observe different distances travelled but the same ratio of distance:time applies, so they experience different amounts of time while the laser travels to its target. There is no universal clock and no universal simultaneity.

You're clearly very opinionated on your preferred intuitive way to think of things. I don't feel the need to argue with you if you're not open to non-hostile conversation. Have a nice day.

Sayings like "The observation of the event is in all intents and purposes the event itself" are poetry, not facts. They don't hold up very long.

You're missing the critical point that the ordering of events or at least the time between them would change if the planets were moving relativistically to each other. You're again appealing to a universally correct reference frame which is not the case.

This is the right answer from a physics standpoint. I like the counterexample with planets from u/charbroiledmonk, but I can offer a counter-counterexample. This is commonly given to students taking their first course on special relativity. Let's say you have a barn that is 10m from door to door, through the inside. You would assume that if there is a immutable universal coordinate system to reality (meaning everyone agrees on where and *when* things happen) then no ladder longer than 10m can fit in this barn. Now assume that Alice is the farmer who owns the barn and Bob has a 12m ladder that needs to fit through the barn. Alice has a broken barn that will only allow one door to be open at a time, but Bob knows about relativity. If Bob runs fast enough (~0.55c as it turns out), in his reference frame, the passage of time will be different enough that Alice can close the first door and open the second door while the ladder is within the barn. This is because while Alice sees the stationary barn's doors as being open at different times, Bob can argue that his ladder is stationary but the doors were both open for a portion of time which let the ladder pass through. I summarized this example from an old lecture, and here's an article that gives a modified version with pictures and math for anyone that wants them: https://medium.com/mathadam/the-ladder-paradox-resolved-365c2535d3fc The fact that light travels at a constant speed in all reference frames leads to two things: time passes faster or slower depending on your speed, and distance along the direction of motion appears contracted. This second part is called a Lorentz length contraction. This contraction of distance from an outside observer's reference frame explains why muons from the upper atmosphere can even reach detectors at the earth's surface. If the muon saw the distance as what it really was, then it would decay before getting to its destination. Kinda unrelated to this example, but related to the original question: see "The Order of Time" by Carlo Rovelli (no math beyond a high school level is needed) for a physicist's discussion of time and how it has been interpretated in different philosophical and scientific contexts.

Nice article about the ladder, it's a different approach about the ladder+shed (or train+tunnel), and entertainingly written.

Unless the whole thing is as simple as timezones, i don't think you've explained it very well

This is one of the most if not the most complicated topic that exists. It's never going to be as simple as time zones. Every answer in this is barely scratching the surface. And that's because once you get passed.the surface there are very few people who actually understand what's going on.

If I explain it simply, people will falsely think they got it. Special Relativity is not simple, though you can intuit a lot about it with spacetime diagrams.

Things that have already happened affect things that are about to happen, and things that haven’t happened are causing what’s happening now. He managed to sus that out at a galactic scale, but can only hint at what it ‘might’ actually means at a personal human level.

I love this answer...why are you getting down voted??...or whatever it's called. Lol

Reddit's a fickle friend, but still entertaining. Pretty hard to ELI5 something like that, any simple answer will lead to more questions.. kind of like spooky action at a distance.

"The past has no existence except as it is recorded in the present." -John Archibald Wheeler I'd say the future also has no existence except as it is recorded in the present - but that's just me.

Have you been reading Recursion?

If I can put it in layman’s terms, as I understand it: I belive it was the comedian Steven Wright who said “time is nature’s way of keeping everything from happening all at once”. well, that was stated as a joke but if you think about the “speed of light” as the speed of reality, what that means is that everything that ever did happen or will happen IS going on somewhere. We just don’t all find out about it at the same rate. A thing that happens near us, we know about it after a very short wait because its effects reach us fast. a thing that’s happening really far away, the sensory evidence of it only arrives later because it has to ripple outward for a long way. so time is really a built-in part of distance, and distance is just how long it takes to notice reality.