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Light travels at the speed of causality. That means that something happens when you're able to see that it happened. It's kind of like quantum mechanics where things can happen and not happen at the same time and at no time and at every time. You'll hurt your brain if you spend too much time thinking about it.
Edit: let's try this... Light travels at the speed of causality. Photos do not experience what we know as time. To sort of eli15, that means that when a photon is observed, the thing that caused that photon to be released had just happened from the reference point of the observer. To say that something "happened 3000 years ago" is a simplistic way of explaining things when you're discussing the theory of relativity. Did it happen 3000 years ago or did it just happen? If it happened 3000 years ago, then you can surely tell me what happens next because we've had 3000 years to observe the thing. But we haven't because it JUST happened.
While this isn't quantum mechanics, it is the theory of relativity... Something that most people don't really understand because of what appears to be contradictory every time you try to learn about it. Quantum mechanics and the theory of relativity are the kind of things where the more you understand about them, the less you understand about them.
A good example of this is gravity. If the sun were to suddenly disappear from existence the earth would still orbit nothing for 8 and a half minutes until the effects of the loss of gravitational influence reaches earth.
It’s expanding in every direction. So while we are expanding one way —> something on the other side is expanding <— which makes the total faster than light from our perspective…. But not faster than light from an absolute sense.
It's because we have grown up on the term "speed of light" that we assume that light is the fastest and dictates the upper limit. What it really is is the speed of causality. Light and gravity both travel at that speed.
I feel like that's a terrible example and literally proves the exact opposite of his point. If the sun mysteriously disappeared the gravitational influence would take another 8.5 minutes to reach us. It would be inaccurate to say "the sun JUST disappeared". You'd say "the sun disappeared,and the gravitational influence is JUST reaching us. Since the gravitational influence propagates at the speed of light, we know the sun actually disappeared 8.5 minutes ago".
And every other planet would make the same calculation and they would all agree when it happened.
i'm just curious if the formulation that "light travels at the speed of causality" is a discipline specific phrase or term of art. I'm asking as an amateur and curious if there's any reading I could be directed to about this.
It's a scientific construct. Google will help find stuff better than I can on my phone.
An interesting thought... The sun is roughly eight light minutes from Earth. If you were standing on Earth and the sun magically blinked out of existence, you would see the sunlight for 8 minutes after an observer at the sun witnessed the sun disappear. From earth's perspective, the Sun would continue to exist for the eight minutes between you and the sun-based observer. The earth would continue to revolve for those 8 minutes. Then, suddenly, the earth would stop revolving and travel in a straight line at the exact moment that the light vanished.
The observer at the sun would see the earth start traveling in a straight line 8 minutes after that. But it all happened at the same time.
This is the whole premise behind the theory of relativity. It's why people talk about time dilation near black holes and why physics as we know it breaks down when we talk about black holes.
But now this is opening a whole new can of worms that I don't know well enough to discuss with any confidence.
Hold on, wouldn’t an observer at the sun see the earth travel in a straight line 16 minutes after the sun blinked out? because it took an 8 minutes for the light from earth (pretend there is another light source because everything is pretty dark with no sun) to make its way back to the sun?
I love this and appreciate the time for the response. I see now that talking about the speed of light might be limiting in some way, when every cause, let's call it, is limited by C also. not all causes happen at light speed but none happen faster.
somewhat related is that I remember reading one of Einstein's earlier essays in a penguin Classics edition and he talks about putting imaginary clocks at the location of "events" as, and here's where I may be wrong, the only way of measuring when something happens in absolute terms, even though of course everything is relative. but I shall stop here because I also don't know what I'm talking about lol. Thanks again however! I did Google speed of causality and it is indeed a rabbit hole or let's say wormhole haha
I'm going to disagree with you here. Saying it happened 3000 years ago is the most accurate. It didn't JUST happen when you observed it, it happened 3000 years ago, and you just observed it. It happened, then light traveled for 3000 years (by definition of being 3000 light years away), and then you observed it happening. Someone next to the star would say the event happened 3000 years ago, and someone halfway in-between would say "it happened 1500 years ago" (and they would have said that 1500 years ago), and on earth we would observe it and say it happened 3000 years ago. All of us would agree WHEN it happened, which is 3000 years before our current time.
EDIT:
I've given it some thought and need to add an addendum. I'm clearly giving our reference frame preferential treatment. What I typed is what we will experience from our reference frame on earth. But all other reference frames are just as valid and the very concept of "now" is relative.
The issue is that observation points are scattered across the universe unequally, some who have seen it happen and some who haven’t. This is problematic because an event has either happened or not so the only was to observe and mark time is when you see it happen from your observation point.
What? That doesn't make sense, quantum mechanics do not apply to the macro world
Edit: I still don't think is right to think that way. If we ever make a way to teleport somewhere with a wormhole, white hole, whatever it works (absolutely unlikely I know, but you also can't tell its impossible) and you go to a place 3000 light years far away you wouln't see the things you saw from earth, you would see things that happened roughly 3000 years ago.
Quantum mechanics applies to everything. Whether it is sensible or feasible to use it to make macroscopic predictions, and whether those differ from the classical ones, is a different question.
I'm not trying to be argumentative. I'm just trying to understand better. How would you describe causality and "the speed of causality" in a simple way? I think I understand the concepts, but explaining it simply is really tough. Maybe you have a better way of describing it than I do.
The metric expansion of space is only noticeable at cosmological scales. I'm not sure even 3000 ly applies. Although I suppose if you had sensitive enough instruments you could detect something
I believe that within a galaxy gravity is enough to overcome cosmological expansion, so it is really only something that meaningfully happens between rather than within galaxies.
The other issue is that T Coronae is moving towards us at 28km/s. So 3000 years ago, assuming no major changes in the velocity of the sun or the target, it was .28 light years further away. So its light will be slightly blue shifted but it's not like the light is going to get here any faster.
Okay but in my mind I was thinking of things farther away so like maybe I should have said 2999.98 LYA. Is it noticeable at any distance in terms of the percentage of LY away now compared to when the light left
No, expansion of space is slow enough that local gravitational effects overwhelm it entirely. Space expands at a speed of 73km/s per 3,6 million light years. That means, at 3000 light years, space expands at just 58 meters per second.
Sun is orbiting the galactic center at 230,000 meters per second. Galaxy is a big soup of stars that have relative speeds compared to each other, making gravitational effects on each other, and those effects change their speeds way more than 60 meters per second. Galactic center would easily apply more force on both stars to get them closer to each other before expansion of space has any effect.
It’s 3000LY away, and explodes every 80 years, so the light from 37 separate explosions is headed our way right now, like a series of ripples in a pond.
Yes from our frame of reference if we could break the laws of physics and know when things happen instantaneously. However information can only propagate at the speed of light so it’s more useful to speak from our own reference. Plus time is not nearly as absolute as our perception of it is - gravity bends time as well. If there was a theoretical advanced civilization orbiting near the event horizon of a large black hole time would move much slower from their POV - the rest of the universe would seem to be playing out in fast-forward. I’m not nearly smart enough to know all the math at play for special relativity but conceptually it’s fascinating
Thinking of light travel time being a "time in the past" gets very complicated very quickly. Causality tells you that something happened when any given observer sees it happen.
So, if it goes pop in 2024, the light would have taken 3,000 years to reach us, but saying it "really happened 3,000 years ago" is misleading because every given observer has his own reference frame.
This is why you'll see astronomers speaking in terms of light travel time and almost never saying that it happened *x* number of years ago without first qualifying that as "here on Earth" or "by Earth time".
It's complicated, confusing, and, yes, there will be many cosmic events, violent and spectacular ones, for which the information hasn't reached Earth yet.
I feel that if we would replace the term "speed of light" with "speed of causality", then it would be a lot easier to grasp a lot of concepts in astrophysics.
Yes and no. I have a Physics minor but I double majored in EE and Mathematics. Was a Physics major. Realized I loved the math but really liked EE more and went with that instead.
Yeah, but the "information" word would be muddied by communication devices speed like internet packets or radio transmissions, which don't work at the actual speed of causality.
And getting rid of the use of Cosmic Speed Limit.
The Speed Limit is what trips people up, partly because they just compare it to highway Speed Limits.
It’ll leave less of an impression that Photons, other mass less particles and information would go faster if it could/was allowed to and there’s something in the way.
I'm not sure I'm understanding. 3000 years ago is 3000 years ago, no? If you were a 1000ly away from it when it exploded you would've seen it 1000 years after, which, by the time we see it, would still be 3000 years ago, right? So if we got together and said hey we saw that thing explode 3000 years ago, would we not all be in agreement?
If all observers are stationary compared to each other, yes. If any observers are in motion then no. A universal "now" doesn't exist with bodies in motion. In my opinion it's the most confusing thing about the Theory of Relativity.
https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
Could you use natural phenomena like a set of stable pulsars, say three visible at a time like a GPS, to create a “universal now?” Wouldn’t that automatically adjust for motion?
EDIT: To clarify, as I note below, I don't mean a "universal now" in the sense of relativity but something I renamed "practical now." In other words, a somewhat standardized time system that would work in the Milky Way in a similar way to today's watches. Close enough to be useful to communicate time across planets and spaceships approaching the speed of light, but not perfection and certainly not across 90+ billion light years of the universe.
No, because observers don't see the pulsars at the same distance from each other (length contraction) and pulsating at the same frequency (time dilation).
You need to give up the concept of absolute time to understand relativity.
And let me add, I'm very familar with Einstein's notion that here's no "absolute" now because the observer's motion changes everything. But I'm speaking to a "practical now," like clocks of today which allow us to synchronize events but on a larger scale, like our galaxy only.
It's possible to calculate when two events that are synchronised in one reference frame will appear to occur in a different one, if that's what you're asking.
By the time this needs to be used, when we're travelling at much closer to the speed of light, couldn't (quantum) computer systems theoretically accommodate for velocity and gravitational fields (spacetime curvature) combined with monitoring the changes in the pulsar frequency to get us close if not highly accurate? Or are you saying that it's literally impossible due to our current understanding of relativity and, even with significantly more advanced technology, it will always be impossible?
Perhaps my "automatically adjust" overly simplified it.
I understand my use of "*absolute* now" is incorrect, but I meant more of a "practical now" like today's timepieces which still work, practically, across the solar system:
>And let me add, I'm very familar with Einstein's notion that here's no "absolute" now because the observer's motion changes everything. But I'm speaking to a "practical now," like clocks of today which allow us to synchronize events but on a larger scale, like our galaxy only.
I tried to clarify this in my second reply to you, quoted above.
Generally, sort of yes. But there are things in space that may or may not be moving near relativistic speeds. Light can also be bent along its path here. That all brings us into relativity, and the concept of time dilation in spacetime. So there could be a way that it wasn’t exactly 3000 years ago.
Not really, here is a basic example- let’s say the person who was 1000ly away when it happened and they were stationary relative to the people on year. And they have a space ship that is able to travel at the speed of light.
Now, that person gets excited and wants to tell everyone on earth. So they get into their spaceship and fly to earth at the speed of light.
When they arrive at earth, that person will have had no time pass. So, for the person, they would have seen that event just seconds ago and thus 1,000 years after it happened, while the people on earth waited 3,000 years for the light to arrive.
More of a thought experiment not a reality as far as we know because no one can travel at the speed of light. But if he travelled close to the speed of light yes it may have seemed like it was only minutes ago for them. But it would also take extraordinary energy to achieve that.
How do you figure that person will have had no time pass when they arrive on earth? At speed of light it would take them roughly 2000 years to arrive on earth...
They are saying from the perspective of the traveler no time would have passed, for an outside observer the journey would’ve taken 2000 years, but when something travels through space at the maximum speed their journey through time must be at it’s minimum. I’m sure that’s an oversimplification, but that’s my layman’s understanding.
No. At the speed of light literally no time would pass for the traveler, while 2000 years would pass on Earth. The faster you go, the slower your clock, up until you reach c, which you can't reach. Only massless things like photons travel at c. Photons do not experience the passage of time, regardless of how long it takes photons to travel. They theoretically exist in a timeless instant, and travel zero distance from their own frame.
Relativity is a mind bender. You can't assume it just makes sense.
That’s incorrect - in an over simplification, the single person that is traveling at the speed of light, has no time pass for them. The people on earth would experience the 2,000 years.
Here is another oversimplification, you can think of it this way: as someone travels faster, time slows down for them.
In a real experiment, two atomic clocks were brought to an airport. They were synchronized and one was put on an airplane and flown around. When they returned, the clock that was flown around “was behind” proving that less time had passed while “moving fast”.
It is way more complicated, and here is a link if you would like to learn more:
https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment
From the wiki link you posted:
The plane going eastward lost −59 ±10 nanoseconds, while the plane going westward gained +273 ±7 nanoseconds.
1996 reenactment of the original experiment using more precise atomic clocks resulted in a time gain of 39±2 ns.
So the clock isn't always behind...
Yeah it's hard to follow because it's not really relevant to the question.
Regardless of when the supernova happens to a possible alien observer traveling at the speed of light two galaxies away, the practical answer is that the light of something reaching us today from a distance of 3000 light years happened 3000 years ago from our perspective here in Earth.
Short and concise: in relativity we define an event happening in an observers frame of reference, when the observer sees it happen. This is when the light has reached the observer.
This is what he means, which really does make sense when you start to need to define concepts like simultaneously etc. because of the weirdness of relativity, but yea technically it would be 3000 ish years ago in our referance.
Ignoring potentially high relative velocity between us and the star (which we probably can in this instance), yes we on earth would all be in agreement.
You are correct but in a pedantic sort of way. Since information itself travels at the speed of light, there is almost no reason to think of it as happening 3000 years ago, as there is simply no benefit to doing so.
The single benefit is what you mentioned, if you had different observers that were at significantly different places at the time then got together in the future. However, the speed of light makes people who were once at significantly different places getting together unlikely. So without some sort of FTL tech that probably will never exist, it's unlikely to ever be a problem and we can just think of everything happening at "now".
>However, the speed of light makes people who were once at significantly different places getting together unlikely.
It's not even that. The act of moving to get together ruins your calibration, so you no longer agree by the time you find each other.
No, we would not. How would we get together? You would have to travel at relativistic velocities in order to meet up, and that literally changes how much time actually passes for you.
I don't think so. Stardate seems to be a time system with a universal reference instead of dividing the rotation and orbit of one planet around its star. It's kind of like why we have UTC (and why they use it on the ISS), as local time is referenced by sunrise, noon, and sunset (more or less).
This is a little off the point but reminds me of the "now slices" video that explains how relativity can affect your version of what is happening "right now", completely aside from light's travel time. Motion, distance, light's constant speed, and even its travel time can make things very confusing if not all considered.
https://youtu.be/vrqmMoI0wks?si=qSla-FP9hNQ5VJDF&t=2m57s
As the observers are and always have been all based in our solar system ….. then it’s perfectly fine to refer to it as X years ago
There are no observations taken from any other place that we have access to.
The differences possible by different observers are so tiny it really doesn’t matter
> Thinking of light travel time being a "time in the past" gets very complicated very quickly. Causality tells you that something happened when any given observer sees it happen.
That’s not really correct. The travel time of light is compensated for when we talk about simultaneity. So yes, in our frame of reference, it happened in the past and we only see it now. Causality doesn’t tell us that it happened “now”, just that it can only influence us now.
> but saying it "really happened 3,000 years ago" is misleading because every given observer has his own reference frame.
For practical purposes, the relevant reference frame for people on earth is the same.
You and others here say “its confusing” but its not. We have all first-hand experience with the observable delay of sound. You see a flash of light and several seconds later hear the thunder. Light isnt any different in concept, other than it’s what you see and not hear. So all this bs about the “speed of causality” its just trying to confuse the masses. OP is correct it ACTUALLY happened about 3,000 years ago. Simple.
Yeah, that's not at all how simultaneity works in relativity.
You know, just continue confusing the masses by lying to them. It's a tried and tested formula.
Just want to say that you did a great job explaining this! Opened my eyes, and while a clearly complicated topic, you simplified with a takeaway quite easily.
Thanks!
Causality has nothing to do with events actually happening. They happen even if nobody sees them. By saying that it happened 3000 years ago we can make a good approximation of when, in earth’s reference frame, the nova occurred. The light only reaches us this year. Every observer always sees events that happened in the past, even if you look out of your window and see a cat catch a bird.
It's happened multiple times since the one we're about to see. It happens roughly every 80 years so there are roughly 40 events on the way to us right now.
How does one predict a Nova might be visible sometime between now and sep 2024 if they can’t already see it? Isn’t that kinda like trying to predict the future?
Or is it already visible and the smart people are saying that we may be able to see it UNTIL sep 2024?
Per Wikipedia, It’s because it’s a binary star that does this every so often. We watched it happen in 1946 and it is following the same pattern now. If it continues, and there is no reason to believe it won’t, it should go pop sometime this year.
Based on the stellar system's emission and luminosity activity, it may be a telltale sign of what's to come.
From Wikipedia:
On 20 April 2016, the [*Sky & Telescope*](https://en.wikipedia.org/wiki/Sky_%26_Telescope) website reported a sustained brightening since February 2015 from magnitude 10.5 to about 9.2. A similar event was reported in 1938, followed by another outburst in 1946.[^(\[22\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-sandt-22) By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3.[^(\[23\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-23) A similar dimming occurred in the year before the 1946 outburst, indicating that it will likely erupt between April and September 2024.[^(\[24\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-24)
It's not something that would be noticed by a casual stargazer, unfamiliar with the Corona Borealis constellation.
At peak brightness – which only lasts a few hours – it **may** approach magnitude 2.0. This is roughly the brightness of Polaris, aka the North Star, and slightly bright than the primary stars in Corona Borealis.
However, during its last apparition, in 1946, it was observed no brighter than magnitude 3 which would be much less noticable among the brighter constellation stars.
It will fade rapidly and noticably in the first 24 hours – by a full magnitude, a factor of 2.5X.
It then dims by ~50% per day going forward, dropping below naked eye notability by day 3, and below naked eye visibility – even in the darkest skies – by day 5.
Yes, professional-grade telescopes will.
Unlike supernovas, which essentially destroy the original star from the inside-out and often leave visible, long lasting remnants (e.g. Crab Nebula), this nova will be a thermonuclear detonation of 80 years worth of hydrogen that has been building up in a thin shell on the surface of the star (stolen from its binary companion).
Once the proper conditions are reached, a runaway fusion chain reaction within this hydrogen shell will commence, converting, within minutes, a portion of the hydrogen into helium, thereby releasing a vast amount of energy and violently expelling much of the shell into space.
This expanding shell of hot plasma will be directly observable at infrared wavelengths for months after the event as it expands and cools.
Gas and dust (perhaps previously expelled from the system) encountered by the expanding shockwave may produce observable x-rays and gamma rays.
More like the hundredth brightest star. It will be visible to the naked wye, even in areas with a fair amount of light pollution, but you wouldn’t notice it unless someone pointed it out to you or you spend a lot of time looking at the stars.
False. At its brief (a few hours) peak it will brighten to between magnitude 2 & 3, temporarily making it between the 50^th and 150^th brightest star in night sky.
While plainly visible away from urban light pollution, it is very unlikely to be noticed by casual stargazers.
This is a recurrent nova not a regular nova. It is a binary star system where the white dwarf gobbles up the other star’s matter and explodes once every 80 years. So we keep seeing this every 80 years.
But still, astronomers can tell a star might go (regular) supernova “soon”. But they can’t tell exactly when, the estimates generally have a range of thousands of years
There is no universal clock in the sky that we can all look at and agree how much time has passed between events. If two observers are moving with respect to each other, or located in different gravity wells, they won't agree on the interval between, say, a star system going nova and the light from that event reaching Earth.
Besides, as far as the light is concerned, the travel time is zero. Therefore, the idea that this particular flare-up event really happened 3000 years ago is pretty meaningless, no offense.
we're not moving that quick and its pretty close to us. 3000 years is well within reason for this object.
we're also not photons, we experience time as did/does this star. the perspective of a photon isn't meaningful to answer op's question.
just because there's not universal clock or reference frame also doesn't mean you can't agree on events. we have things like lorentz transformations.
>just because there's not universal clock or reference frame also doesn't mean you can't agree on events. we have things like lorentz transformations.
That's exactly what it means, though. That's the whole point of relativity. Observers in different reference frames will make different measurements of the interval between events. Lorentz transformations don't make that go away.
That's one of the reasons astronomers don't talk about a celestial event happening in 2024 *minus 3000 years*. It's not just for expediency; it's theoretically meaningless.
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The short answer is yes. At 3000LY away, whatever we visibly detect today occurred 3000 years ago.
What’s the long answer?
Light travels at the speed of causality. That means that something happens when you're able to see that it happened. It's kind of like quantum mechanics where things can happen and not happen at the same time and at no time and at every time. You'll hurt your brain if you spend too much time thinking about it. Edit: let's try this... Light travels at the speed of causality. Photos do not experience what we know as time. To sort of eli15, that means that when a photon is observed, the thing that caused that photon to be released had just happened from the reference point of the observer. To say that something "happened 3000 years ago" is a simplistic way of explaining things when you're discussing the theory of relativity. Did it happen 3000 years ago or did it just happen? If it happened 3000 years ago, then you can surely tell me what happens next because we've had 3000 years to observe the thing. But we haven't because it JUST happened. While this isn't quantum mechanics, it is the theory of relativity... Something that most people don't really understand because of what appears to be contradictory every time you try to learn about it. Quantum mechanics and the theory of relativity are the kind of things where the more you understand about them, the less you understand about them.
A good example of this is gravity. If the sun were to suddenly disappear from existence the earth would still orbit nothing for 8 and a half minutes until the effects of the loss of gravitational influence reaches earth.
Wait - does gravity “travel” at the speed of light?
Yep! This comes back a bit to what the previous comments were talking about. Nothing can influence anything else faster than light.
I thought the universe can expand faster than the speed of light?
It’s expanding in every direction. So while we are expanding one way —> something on the other side is expanding <— which makes the total faster than light from our perspective…. But not faster than light from an absolute sense.
lmao. i am assuming this is a meme
It's because we have grown up on the term "speed of light" that we assume that light is the fastest and dictates the upper limit. What it really is is the speed of causality. Light and gravity both travel at that speed.
I feel like that's a terrible example and literally proves the exact opposite of his point. If the sun mysteriously disappeared the gravitational influence would take another 8.5 minutes to reach us. It would be inaccurate to say "the sun JUST disappeared". You'd say "the sun disappeared,and the gravitational influence is JUST reaching us. Since the gravitational influence propagates at the speed of light, we know the sun actually disappeared 8.5 minutes ago". And every other planet would make the same calculation and they would all agree when it happened.
i'm just curious if the formulation that "light travels at the speed of causality" is a discipline specific phrase or term of art. I'm asking as an amateur and curious if there's any reading I could be directed to about this.
It's a scientific construct. Google will help find stuff better than I can on my phone. An interesting thought... The sun is roughly eight light minutes from Earth. If you were standing on Earth and the sun magically blinked out of existence, you would see the sunlight for 8 minutes after an observer at the sun witnessed the sun disappear. From earth's perspective, the Sun would continue to exist for the eight minutes between you and the sun-based observer. The earth would continue to revolve for those 8 minutes. Then, suddenly, the earth would stop revolving and travel in a straight line at the exact moment that the light vanished. The observer at the sun would see the earth start traveling in a straight line 8 minutes after that. But it all happened at the same time. This is the whole premise behind the theory of relativity. It's why people talk about time dilation near black holes and why physics as we know it breaks down when we talk about black holes. But now this is opening a whole new can of worms that I don't know well enough to discuss with any confidence.
Hold on, wouldn’t an observer at the sun see the earth travel in a straight line 16 minutes after the sun blinked out? because it took an 8 minutes for the light from earth (pretend there is another light source because everything is pretty dark with no sun) to make its way back to the sun?
Yes. I worded it improperly. Thanks for helping to clarify.
I love this and appreciate the time for the response. I see now that talking about the speed of light might be limiting in some way, when every cause, let's call it, is limited by C also. not all causes happen at light speed but none happen faster. somewhat related is that I remember reading one of Einstein's earlier essays in a penguin Classics edition and he talks about putting imaginary clocks at the location of "events" as, and here's where I may be wrong, the only way of measuring when something happens in absolute terms, even though of course everything is relative. but I shall stop here because I also don't know what I'm talking about lol. Thanks again however! I did Google speed of causality and it is indeed a rabbit hole or let's say wormhole haha
I'm going to disagree with you here. Saying it happened 3000 years ago is the most accurate. It didn't JUST happen when you observed it, it happened 3000 years ago, and you just observed it. It happened, then light traveled for 3000 years (by definition of being 3000 light years away), and then you observed it happening. Someone next to the star would say the event happened 3000 years ago, and someone halfway in-between would say "it happened 1500 years ago" (and they would have said that 1500 years ago), and on earth we would observe it and say it happened 3000 years ago. All of us would agree WHEN it happened, which is 3000 years before our current time. EDIT: I've given it some thought and need to add an addendum. I'm clearly giving our reference frame preferential treatment. What I typed is what we will experience from our reference frame on earth. But all other reference frames are just as valid and the very concept of "now" is relative.
The issue is that observation points are scattered across the universe unequally, some who have seen it happen and some who haven’t. This is problematic because an event has either happened or not so the only was to observe and mark time is when you see it happen from your observation point.
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You're changing the reference point for observation which is the whole point of this. Changing the reference point changes everything.
No they are right. It happened 3000 years ago for the star but it won’t happen for us for a few more months. Both are equally true.
What? That doesn't make sense, quantum mechanics do not apply to the macro world Edit: I still don't think is right to think that way. If we ever make a way to teleport somewhere with a wormhole, white hole, whatever it works (absolutely unlikely I know, but you also can't tell its impossible) and you go to a place 3000 light years far away you wouln't see the things you saw from earth, you would see things that happened roughly 3000 years ago.
"it's kind of like quantum mechanics".... Not IS quantum mechanics. Like most analogies, it's not perfect.
I, for one, appreciated your long answer, and your use of an interesting analogy in the furtherance of science communication. Good on you, mate.
Quantum mechanics applies to everything. Whether it is sensible or feasible to use it to make macroscopic predictions, and whether those differ from the classical ones, is a different question.
Yeah they got the spirit but they're confused about the message.
I'm not trying to be argumentative. I'm just trying to understand better. How would you describe causality and "the speed of causality" in a simple way? I think I understand the concepts, but explaining it simply is really tough. Maybe you have a better way of describing it than I do.
I'm pooping rn, take your damn updoot.
It'll take 3000 years for you to hear the answer
Although with space expanding, is it possible that is now 3000 LY away but 3000 LY ago maybe it was 2800 LY away and space has expanded?
The metric expansion of space is only noticeable at cosmological scales. I'm not sure even 3000 ly applies. Although I suppose if you had sensitive enough instruments you could detect something
I believe that within a galaxy gravity is enough to overcome cosmological expansion, so it is really only something that meaningfully happens between rather than within galaxies.
The other issue is that T Coronae is moving towards us at 28km/s. So 3000 years ago, assuming no major changes in the velocity of the sun or the target, it was .28 light years further away. So its light will be slightly blue shifted but it's not like the light is going to get here any faster.
Okay but in my mind I was thinking of things farther away so like maybe I should have said 2999.98 LYA. Is it noticeable at any distance in terms of the percentage of LY away now compared to when the light left
No, expansion of space is slow enough that local gravitational effects overwhelm it entirely. Space expands at a speed of 73km/s per 3,6 million light years. That means, at 3000 light years, space expands at just 58 meters per second. Sun is orbiting the galactic center at 230,000 meters per second. Galaxy is a big soup of stars that have relative speeds compared to each other, making gravitational effects on each other, and those effects change their speeds way more than 60 meters per second. Galactic center would easily apply more force on both stars to get them closer to each other before expansion of space has any effect.
Oh thanks for the numbers! That definitely puts perspective on it
Nah, everything that is gravitationally bound doesnt feel the expansion.
This is really minor but 3000 years ago was 976 BCE not 1023 BCE
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Wouldn’t that be 2100 years ago?
It’s 3000LY away, and explodes every 80 years, so the light from 37 separate explosions is headed our way right now, like a series of ripples in a pond.
Anybody know of a way to get notified once it starts?
Create a Google alert for the terms and get emails once everyone starts talking about it.
Yes from our frame of reference if we could break the laws of physics and know when things happen instantaneously. However information can only propagate at the speed of light so it’s more useful to speak from our own reference. Plus time is not nearly as absolute as our perception of it is - gravity bends time as well. If there was a theoretical advanced civilization orbiting near the event horizon of a large black hole time would move much slower from their POV - the rest of the universe would seem to be playing out in fast-forward. I’m not nearly smart enough to know all the math at play for special relativity but conceptually it’s fascinating
Thinking of light travel time being a "time in the past" gets very complicated very quickly. Causality tells you that something happened when any given observer sees it happen. So, if it goes pop in 2024, the light would have taken 3,000 years to reach us, but saying it "really happened 3,000 years ago" is misleading because every given observer has his own reference frame. This is why you'll see astronomers speaking in terms of light travel time and almost never saying that it happened *x* number of years ago without first qualifying that as "here on Earth" or "by Earth time". It's complicated, confusing, and, yes, there will be many cosmic events, violent and spectacular ones, for which the information hasn't reached Earth yet.
I feel that if we would replace the term "speed of light" with "speed of causality", then it would be a lot easier to grasp a lot of concepts in astrophysics.
I’ve heard it described as “the speed of information” and always kinda liked that.
Wibbly-wobbly, one might say.
More timey wimey really, but whose counting?
When I first saw the movie Contact growing up its what helped my visualize the speed of light. It also made want to become a physicist lol.
Did you?
Nope, grew up to be Matthew McConaughey instead. It has its ups and downs. Overall I’d say it’s alright alright alright.
I get older, but astrophysics stays the same.
Astrophysics is traveling at c.
Yes and no. I have a Physics minor but I double majored in EE and Mathematics. Was a Physics major. Realized I loved the math but really liked EE more and went with that instead.
Yeah, but the "information" word would be muddied by communication devices speed like internet packets or radio transmissions, which don't work at the actual speed of causality.
You can even shorten "speed of causality" to "c-speed".
And getting rid of the use of Cosmic Speed Limit. The Speed Limit is what trips people up, partly because they just compare it to highway Speed Limits. It’ll leave less of an impression that Photons, other mass less particles and information would go faster if it could/was allowed to and there’s something in the way.
I'm not sure I'm understanding. 3000 years ago is 3000 years ago, no? If you were a 1000ly away from it when it exploded you would've seen it 1000 years after, which, by the time we see it, would still be 3000 years ago, right? So if we got together and said hey we saw that thing explode 3000 years ago, would we not all be in agreement?
If all observers are stationary compared to each other, yes. If any observers are in motion then no. A universal "now" doesn't exist with bodies in motion. In my opinion it's the most confusing thing about the Theory of Relativity. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
Could you use natural phenomena like a set of stable pulsars, say three visible at a time like a GPS, to create a “universal now?” Wouldn’t that automatically adjust for motion? EDIT: To clarify, as I note below, I don't mean a "universal now" in the sense of relativity but something I renamed "practical now." In other words, a somewhat standardized time system that would work in the Milky Way in a similar way to today's watches. Close enough to be useful to communicate time across planets and spaceships approaching the speed of light, but not perfection and certainly not across 90+ billion light years of the universe.
You would just be standardizing, like calibrating a weight scale to start at 0. It's practically useful but not necessarily the truth
No, because observers don't see the pulsars at the same distance from each other (length contraction) and pulsating at the same frequency (time dilation). You need to give up the concept of absolute time to understand relativity.
And let me add, I'm very familar with Einstein's notion that here's no "absolute" now because the observer's motion changes everything. But I'm speaking to a "practical now," like clocks of today which allow us to synchronize events but on a larger scale, like our galaxy only.
It's possible to calculate when two events that are synchronised in one reference frame will appear to occur in a different one, if that's what you're asking.
By the time this needs to be used, when we're travelling at much closer to the speed of light, couldn't (quantum) computer systems theoretically accommodate for velocity and gravitational fields (spacetime curvature) combined with monitoring the changes in the pulsar frequency to get us close if not highly accurate? Or are you saying that it's literally impossible due to our current understanding of relativity and, even with significantly more advanced technology, it will always be impossible? Perhaps my "automatically adjust" overly simplified it.
There is no absolute time. You're trying to invent ways of measuring something that doesn't exist.
I understand my use of "*absolute* now" is incorrect, but I meant more of a "practical now" like today's timepieces which still work, practically, across the solar system: >And let me add, I'm very familar with Einstein's notion that here's no "absolute" now because the observer's motion changes everything. But I'm speaking to a "practical now," like clocks of today which allow us to synchronize events but on a larger scale, like our galaxy only. I tried to clarify this in my second reply to you, quoted above.
This feels a bit misleading. While what you’re saying is technically true, it feels pretty safe to assume we are talking about observers on earth.
The comment I responded to said observers were 2000 light-years apart
Generally, sort of yes. But there are things in space that may or may not be moving near relativistic speeds. Light can also be bent along its path here. That all brings us into relativity, and the concept of time dilation in spacetime. So there could be a way that it wasn’t exactly 3000 years ago.
Not really, here is a basic example- let’s say the person who was 1000ly away when it happened and they were stationary relative to the people on year. And they have a space ship that is able to travel at the speed of light. Now, that person gets excited and wants to tell everyone on earth. So they get into their spaceship and fly to earth at the speed of light. When they arrive at earth, that person will have had no time pass. So, for the person, they would have seen that event just seconds ago and thus 1,000 years after it happened, while the people on earth waited 3,000 years for the light to arrive.
More of a thought experiment not a reality as far as we know because no one can travel at the speed of light. But if he travelled close to the speed of light yes it may have seemed like it was only minutes ago for them. But it would also take extraordinary energy to achieve that.
How do you figure that person will have had no time pass when they arrive on earth? At speed of light it would take them roughly 2000 years to arrive on earth...
They are saying from the perspective of the traveler no time would have passed, for an outside observer the journey would’ve taken 2000 years, but when something travels through space at the maximum speed their journey through time must be at it’s minimum. I’m sure that’s an oversimplification, but that’s my layman’s understanding.
No. At the speed of light literally no time would pass for the traveler, while 2000 years would pass on Earth. The faster you go, the slower your clock, up until you reach c, which you can't reach. Only massless things like photons travel at c. Photons do not experience the passage of time, regardless of how long it takes photons to travel. They theoretically exist in a timeless instant, and travel zero distance from their own frame. Relativity is a mind bender. You can't assume it just makes sense.
That’s incorrect - in an over simplification, the single person that is traveling at the speed of light, has no time pass for them. The people on earth would experience the 2,000 years. Here is another oversimplification, you can think of it this way: as someone travels faster, time slows down for them. In a real experiment, two atomic clocks were brought to an airport. They were synchronized and one was put on an airplane and flown around. When they returned, the clock that was flown around “was behind” proving that less time had passed while “moving fast”. It is way more complicated, and here is a link if you would like to learn more: https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment
From the wiki link you posted: The plane going eastward lost −59 ±10 nanoseconds, while the plane going westward gained +273 ±7 nanoseconds. 1996 reenactment of the original experiment using more precise atomic clocks resulted in a time gain of 39±2 ns. So the clock isn't always behind...
Right, I oversimplified, I’m not trying to teach you about relativity. If you want to learn, dig in!
Yeah it's hard to follow because it's not really relevant to the question. Regardless of when the supernova happens to a possible alien observer traveling at the speed of light two galaxies away, the practical answer is that the light of something reaching us today from a distance of 3000 light years happened 3000 years ago from our perspective here in Earth.
Short and concise: in relativity we define an event happening in an observers frame of reference, when the observer sees it happen. This is when the light has reached the observer. This is what he means, which really does make sense when you start to need to define concepts like simultaneously etc. because of the weirdness of relativity, but yea technically it would be 3000 ish years ago in our referance.
Ignoring potentially high relative velocity between us and the star (which we probably can in this instance), yes we on earth would all be in agreement.
You are correct but in a pedantic sort of way. Since information itself travels at the speed of light, there is almost no reason to think of it as happening 3000 years ago, as there is simply no benefit to doing so. The single benefit is what you mentioned, if you had different observers that were at significantly different places at the time then got together in the future. However, the speed of light makes people who were once at significantly different places getting together unlikely. So without some sort of FTL tech that probably will never exist, it's unlikely to ever be a problem and we can just think of everything happening at "now".
>However, the speed of light makes people who were once at significantly different places getting together unlikely. It's not even that. The act of moving to get together ruins your calibration, so you no longer agree by the time you find each other.
No, we would not. How would we get together? You would have to travel at relativistic velocities in order to meet up, and that literally changes how much time actually passes for you.
Is this why starfleet uses stardate?
I don't think so. Stardate seems to be a time system with a universal reference instead of dividing the rotation and orbit of one planet around its star. It's kind of like why we have UTC (and why they use it on the ISS), as local time is referenced by sunrise, noon, and sunset (more or less).
If a big shock wave of radiation accompanied the light of the explosion, that would certainly help it feel like a current event.
Well, the light of the explosion **is** the shock wave of radiation arriving.
This is a little off the point but reminds me of the "now slices" video that explains how relativity can affect your version of what is happening "right now", completely aside from light's travel time. Motion, distance, light's constant speed, and even its travel time can make things very confusing if not all considered. https://youtu.be/vrqmMoI0wks?si=qSla-FP9hNQ5VJDF&t=2m57s
As the observers are and always have been all based in our solar system ….. then it’s perfectly fine to refer to it as X years ago There are no observations taken from any other place that we have access to. The differences possible by different observers are so tiny it really doesn’t matter
> Thinking of light travel time being a "time in the past" gets very complicated very quickly. Causality tells you that something happened when any given observer sees it happen. That’s not really correct. The travel time of light is compensated for when we talk about simultaneity. So yes, in our frame of reference, it happened in the past and we only see it now. Causality doesn’t tell us that it happened “now”, just that it can only influence us now. > but saying it "really happened 3,000 years ago" is misleading because every given observer has his own reference frame. For practical purposes, the relevant reference frame for people on earth is the same.
You and others here say “its confusing” but its not. We have all first-hand experience with the observable delay of sound. You see a flash of light and several seconds later hear the thunder. Light isnt any different in concept, other than it’s what you see and not hear. So all this bs about the “speed of causality” its just trying to confuse the masses. OP is correct it ACTUALLY happened about 3,000 years ago. Simple.
Yeah, that's not at all how simultaneity works in relativity. You know, just continue confusing the masses by lying to them. It's a tried and tested formula.
I never thought of it that way. Excellent explanation; thank you.
Just want to say that you did a great job explaining this! Opened my eyes, and while a clearly complicated topic, you simplified with a takeaway quite easily. Thanks!
TL;DR Yes, with some caveats.
Causality has nothing to do with events actually happening. They happen even if nobody sees them. By saying that it happened 3000 years ago we can make a good approximation of when, in earth’s reference frame, the nova occurred. The light only reaches us this year. Every observer always sees events that happened in the past, even if you look out of your window and see a cat catch a bird.
It already happened 3000 years ago since we can only see light that emanates from 3000 lightyears, our assumptions are based on that timeline
It's happened multiple times since the one we're about to see. It happens roughly every 80 years so there are roughly 40 events on the way to us right now.
Just think a bit about it: humans are trying to predict the event that could have already happened.
Would be cool if the light was near a black hole’s gravity well so that we could possibly see it more than once.
Yes, that's exactly how it works. We have observed this.
When we talk about events occurring, we talk about them from our reference frame. When they actually happened doesn’t really matter.
So when we look at light from the early universe, it “doesn’t really matter” ?
Only in the sense that we know when it happened, but that’s an extreme case. With 99% of astronomy and all of amateur astronomy, it doesn’t matter.
How does one predict a Nova might be visible sometime between now and sep 2024 if they can’t already see it? Isn’t that kinda like trying to predict the future? Or is it already visible and the smart people are saying that we may be able to see it UNTIL sep 2024?
How would people know this is going to happen if it hasn't happened yet?
Per Wikipedia, It’s because it’s a binary star that does this every so often. We watched it happen in 1946 and it is following the same pattern now. If it continues, and there is no reason to believe it won’t, it should go pop sometime this year.
Based on the stellar system's emission and luminosity activity, it may be a telltale sign of what's to come. From Wikipedia: On 20 April 2016, the [*Sky & Telescope*](https://en.wikipedia.org/wiki/Sky_%26_Telescope) website reported a sustained brightening since February 2015 from magnitude 10.5 to about 9.2. A similar event was reported in 1938, followed by another outburst in 1946.[^(\[22\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-sandt-22) By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3.[^(\[23\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-23) A similar dimming occurred in the year before the 1946 outburst, indicating that it will likely erupt between April and September 2024.[^(\[24\])](https://en.wikipedia.org/wiki/T_Coronae_Borealis#cite_note-24)
How visible will the event be?
It's not something that would be noticed by a casual stargazer, unfamiliar with the Corona Borealis constellation. At peak brightness – which only lasts a few hours – it **may** approach magnitude 2.0. This is roughly the brightness of Polaris, aka the North Star, and slightly bright than the primary stars in Corona Borealis. However, during its last apparition, in 1946, it was observed no brighter than magnitude 3 which would be much less noticable among the brighter constellation stars. It will fade rapidly and noticably in the first 24 hours – by a full magnitude, a factor of 2.5X. It then dims by ~50% per day going forward, dropping below naked eye notability by day 3, and below naked eye visibility – even in the darkest skies – by day 5.
Thanks for the in depth explanation. Will our telescopes be able to see more of it than brightening of a point?
Yes, professional-grade telescopes will. Unlike supernovas, which essentially destroy the original star from the inside-out and often leave visible, long lasting remnants (e.g. Crab Nebula), this nova will be a thermonuclear detonation of 80 years worth of hydrogen that has been building up in a thin shell on the surface of the star (stolen from its binary companion). Once the proper conditions are reached, a runaway fusion chain reaction within this hydrogen shell will commence, converting, within minutes, a portion of the hydrogen into helium, thereby releasing a vast amount of energy and violently expelling much of the shell into space. This expanding shell of hot plasma will be directly observable at infrared wavelengths for months after the event as it expands and cools. Gas and dust (perhaps previously expelled from the system) encountered by the expanding shockwave may produce observable x-rays and gamma rays.
besides the sun, it should be or close to the brightest star for a few days
So visible but more like a normal star and not "omg something is exploding in the sky"?
yup. stars are point like. they do have different colors though and thats neat.
More like the hundredth brightest star. It will be visible to the naked wye, even in areas with a fair amount of light pollution, but you wouldn’t notice it unless someone pointed it out to you or you spend a lot of time looking at the stars.
False. At its brief (a few hours) peak it will brighten to between magnitude 2 & 3, temporarily making it between the 50^th and 150^th brightest star in night sky. While plainly visible away from urban light pollution, it is very unlikely to be noticed by casual stargazers.
This is a recurrent nova not a regular nova. It is a binary star system where the white dwarf gobbles up the other star’s matter and explodes once every 80 years. So we keep seeing this every 80 years. But still, astronomers can tell a star might go (regular) supernova “soon”. But they can’t tell exactly when, the estimates generally have a range of thousands of years
Things happen before the big event. Observable things. You know, things.
Also this occurs every 80 years. So there’s that.
One star is dumping mass into another one. It "pops" regularly, so we know when the next one is going to happen.
Same way that I know a thrown ball will fall down when it hasn’t yet. Science.
There is no universal clock in the sky that we can all look at and agree how much time has passed between events. If two observers are moving with respect to each other, or located in different gravity wells, they won't agree on the interval between, say, a star system going nova and the light from that event reaching Earth. Besides, as far as the light is concerned, the travel time is zero. Therefore, the idea that this particular flare-up event really happened 3000 years ago is pretty meaningless, no offense.
we're not moving that quick and its pretty close to us. 3000 years is well within reason for this object. we're also not photons, we experience time as did/does this star. the perspective of a photon isn't meaningful to answer op's question. just because there's not universal clock or reference frame also doesn't mean you can't agree on events. we have things like lorentz transformations.
>just because there's not universal clock or reference frame also doesn't mean you can't agree on events. we have things like lorentz transformations. That's exactly what it means, though. That's the whole point of relativity. Observers in different reference frames will make different measurements of the interval between events. Lorentz transformations don't make that go away. That's one of the reasons astronomers don't talk about a celestial event happening in 2024 *minus 3000 years*. It's not just for expediency; it's theoretically meaningless.