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This question might sound quite odd and is a mix of philosophy and physics. Suppose we observe a star that is 50 million lightyears away, and suppose it is a type of star that has a lifespan of only 30 million years. The question is, does this star "exist" in our reference frame?

The reasoning for why you would say "no" is simple: the star's lifespan is shorter than the time it took for light to travel to us. People who believe this would say that the star doesn't exist anymore and what we are seeing is only the photons from the star.

But the reasoning for why you might say "yes" is a bit more abstract. According to general relativity, all information must travel below the speed of light. This means that we don't just see the star, we also feel it's faint gravitational force, we also receive it's heat, and any other ways the star could affect us. By all measurable accounts, the star is still there. There is no measurement we can do that shows it's absence, so how can you argue it isn't real?

Think about it this way: if I were to try and prove the screen infront of me is real, I would do so by making measurements. I could say I see it, and I can feel it(electromagnetic effects from the atomic bonds). This is the exact same argument as with the distant star, I can see it and "feel" the gravitational effects. The star is real in the same way everything nearby to me is real.

One last extension of this idea for those who lie in the "yes" camp: suppose we live in a universe with positive curvature. This would mean that if you departed from earth in a straight line and mainted this without acceleration you would eventually reach earth again(okay not exactly but you know what I mean). Now imagine that both you and the earth are immortal. Eventually the observable universe would grow large enough that you would be able to "see" the earth in the distance, and would see a much younger version of yourself. Now if that distant version of you is "real" does that mean there are now two yous?

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  • We receive the light emitted long time ago by distant stars. If in the meantime they disappeared... Feb 1 at 16:01
  • We still read the De rerum natura written centuries ago by Lucretius. Feb 1 at 16:03
  • 1
    The distant star is real: maybe is now dead, but when it emitted the light it was alive. Feb 1 at 16:14
  • 4
    "does X exist" is a blunt tool of a question. Santa Claus may or may not exist, depending on exactly how you choose to parse the question. Better, in my opinion, to ask more detailed questions about the manner in which the X can be interacted with, including interacted with in conception, or what features/characteristics it has (including the characteristic of just being an idea) and so on.
    – Dave
    Feb 1 at 18:05
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    The star doesn't exist anymore, but the perturbation it made in the electromagnetic field (light) and in space-time (gravity) do still exist and this is what you perceive. When you hear the thunder from far away you know the lightning is already over, but you perceive the perturbation it did in the air (sound wave). This is just the same except the perturbation travels at light speed so it's harder to figure out.
    – armand
    Feb 13 at 6:57

8 Answers 8

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The conventional answer in physics is that "right now far away" is a phrase that doesn't quite mean anything. As soon as you define what it means in some way that seems intuitively natural - for instance, "fifty million years after the event fifty million light-years away measured here now, according to a clock comoving with us now but located next to the event fifty million light-years away" - you've defined it in a way that lets all times in all places be "right now far away" for an appropriate choice of reference frame. This is a consequence of relativity of simultaneity. Nearly any conceivable question you have about it, you will find asked and answered already on Physics SE, so I will not expound here. You can always pick an observer "right now far away" which has the same "right here and right now" as another observer in the same place but at a very different velocity for whom "right now far away" is what for us would be a long time ago or a long time from now.

Your statement about a positive curvature universe leading to duplication is false. We live in a positive curvature region of the universe, which makes it easy to experimentally test this. You can depart Earth in a straight line and maintain that course without acceleration (neglecting air resistance) by bouncing up into the air from a trampoline. You will indeed eventually return to Earth and the trampoline attached to it. You will note that you do not see or crash into any younger versions of yourself.

You will be in the company of credentialed physicists if you believe any of the following non-exhaustive list about right now far away:

  • It really is intrinsically meaningless.
  • I have faith that it will mean something someday when we have a proper theory of cosmology, but right now we don't know how to mean precisely whatever it means.
  • It is meaningful; however, concepts of size and shape are not meaningful at a distance. That is: we can say "There is a box right now far away, and there is a cat here and now", but it means nothing to say "that box now and far away is the right size and shape for my cat here and now to sit in."
  • I can't actually imagine distances like 50 million light years anyway, so I already can't really mean anything about anything at all on that scale except as a description of my own approximate model. The best way I can express my best model is with a mathematical calculation or an empirical prediction. I can calculate. I can use those calculations to tell you what you'll measure. I mean what I predict and how I predict it, and for that relativity works great.
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  • Suppose I see evidence of a supernova right now and I know the distance in light-years to the event. Can I assume that the event took place at some time in the past relative to my clock (local oscillator?) and that the star is perhaps long dead? Can I know its time of death or only the time I measure with my clock? Can we estimate how old the universe is if time is relative then how do we decide the big bang is 13.8 billion years (or whatever the estimate) is in the past? I am just trying to comprehend your intended meaning of "right now far away". Model is just events + propagation delay. Feb 2 at 0:09
  • @SystemTheory we can decide that the Big Bang is ~14 Gy old because the Big Bang happened right here (and right everywhere else) and was pretty close to comoving with us. We are roughly stationary with respect to more or less all the matter in the early universe, which we can tell because the CMB is isotropic. If we were in a very improbable galaxy translating at 0.87c with respect to the CMB, we'd measure the universe to be 28Gy old.
    – g s
    Feb 2 at 1:24
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    In practice, almost everything in the universe that we can detect, except for light-speed propagating stuff which doesn't have a reference frame to put a clock in anyway, is traveling at approximately zero (compared to the speed of light) with respect to the CMB, so picking that as a privileged frame, while arbitrary, is useful and natural.
    – g s
    Feb 2 at 1:29
  • Note that we'd also measure the universe to have begun behind us if we were in such an improbable galaxy, not everywhere all at once.
    – g s
    Feb 2 at 1:31
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Lorentz Transformations, I believe, still maintain a certain sense of realism, and I don't believe the "truth of the matter" of a question like "is this star still alive?" in relativity relies on the information that star is dead reaching the people who you're asking this question relative to.

So let me just lay out what I'm saying more explicitly: let's say some civilization is 80million years in the future and they know, quite clearly - because they have observational data - that the star died. They know when it died. The know it died in the year 2000, but they also know that because it's 50m lightyears away, people in 2024 wouldn't have known that it died 24 years ago relative to them. So you can frame the question like this: is it reasonable for these people, 80m years in the future, to say "In the year 2024, that star was objecctively dead, but the people on Earth didn't know it?"

I think the answer is, yes. If you calculate the truth-of-the-universe as it was in 2024, in Earth's reference frame, knowing what they know in 80m years in the future, and calculate it from the reference frame of a person on earth in 2024 - doing whatever lorenz transform is necessary - that truth-of-the-universe (in that reference frame) will include that star being dead in that reference frame, but the information of its death not having reached earth yet.

I know naive realism can be a bit boring for philosophers, but I think it really is as simple as that. Even in relativity.

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  • +1 for "naive realism can be a bit boring for philoshopers". Feb 1 at 18:20
  • If you take this view, does that mean you believe that obects in an accelerating reference frame can go backwards and forwards in time? Taking the lorrentz transform at face value, as you accelerate, distant objects are shifting in time. Consider the twin paradox. Imagine me and my twin are both at rest relative to each other, I am on the moon and they are on earth. If I start accelerating away from them quickly enough, my "plane of the present" will change such that even though I am travelling forwards in time, their point will go backwards as I accelerate(think about the plane tilting).
    – Augs
    Feb 1 at 19:11
  • People who believe that what they see is what's real(people who say "the star exists") don't have to deal with that because even though the "plane of the present" shifts, they will not see their twin travel backwards in time. Everything they see(and measure) will travel only forwards in time.
    – Augs
    Feb 1 at 19:15
  • [my "plane of the present" will change such that even though I am travelling forwards in time, their point will go backwards as I accelerate] This is false. The comments section of PhilSE is not the right place to address the error. Similar questions have been asked and answered on Physics SE such as: physics.stackexchange.com/questions/316425/… and physics.stackexchange.com/questions/494124/…
    – g s
    Feb 1 at 23:59
  • @Augs I'm not sure you've understood relativity properly with that last comment. No sub-light-speed velocity will result in anything going back in time. And going faster than light speed is impossible.
    – TKoL
    Feb 2 at 9:30
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Let's consider a slightly different version of your question, which should highlight the principles concerned. Suppose there is a robot in New York that prints letters which it posts to you in London, the letters taking two days to arrive. If I smash the robot to smithereens in New York on Sunday, do you consider the robot to continue existing until its final letter is delivered to you in London on Tuesday?

You won't discover that the robot has been destroyed until Tuesday, but that does not mean that the robot existed until Tuesday. If the robot's last letter is the only thing you have to go on, you will not be able to pinpoint the time of the robot's destruction. However, if I send you a letter explaining that I destroyed the robot at 10:30 am, you would. Do you think the time of the robot's destruction depends on whether or not I send such a letter?

It seems to me that you are conflating two events, namely the destruction of the star and your learning of the destruction of the star.

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  • 1
    This is different because I could make a measurement from my reference frame and know immediately about the robot. E.g. I might have a powerful xray that could look through the earth and at the robot. With the case of the star, the counter-example doesn't work because there is no possible measurement that could be made earlier than when the light arrives in my eyes. No information about the star can travel faster than the speed of light. There is no analogous thing to you sending that latter at 10:30 am. In fact, I would maintain that the star is destroyed even if I never learn about it.
    – Augs
    Feb 1 at 21:43
  • You couldn't know immediately about the robot, because your X-ray would take a finite amount of time to reach you. Feb 1 at 21:46
  • Well yes I meant effectively immediately. But if we are considering the time it takes for the x-ray to travel then it is basically the same as my star question but at a closer range(the letters become irrelevant again if we are considering speed-of-light information travel). My point about "the present" is not about consciousness as I am not talking about anyone knowing if the star in question exists, just if it exists or not at all.
    – Augs
    Feb 1 at 22:33
  • Go back to my example. Do you think the robot continues to exist during the interval between my smashing it to pieces and you learning about it? Feb 1 at 22:35
  • No because I don't think knowledge of the robot is important here. However there is still some question as to if the robot exists during the brief time period where you smash it but the light cone hasn't had time to propagate to London. If we are talking about physics, the robot still effects objects in London such as through gravitational forces.
    – Augs
    Feb 1 at 22:47
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If a star ends its life, then in general its mass – at least some part of his original mass – still exists, but the star is no longer active, i.e. supporting nuclear reaction in its interior. Hence the answer to the OP’s question depends on how to define the end of the star.

  • In general, the ending of the nuclear reactions and the cooling down of the star is considered the end of the lifespan of the star. Accordingly, a star like in your question does not exist any longer.
  • But if you define a star as a collection of massive particles then those stars, which do not explode and spread all their mass into the galactic space, continue to exist - in a different state.
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    Let's suppose that at the end of it's lifespan the star just pops out of existence and all the atoms are deleted(say it collides with a star made entirely of anti-matter). This is more a question about relativity and "what is the present?" than "what counts as a star?".
    – Augs
    Feb 1 at 19:18
  • @Augs The annihilation of the star with an equal amount of antimatter creates a gigantic blow of gamma-rays. They burst into all directions - assuming that secondary processes are neglectible. Hence there is no resulting collection of mass. The star ended his existence. - I do not see why relativistic effects should play a role. Of course we do know about this end of existence not until when the effect has reached the observer after a finite non-zero time.
    – Jo Wehler
    Feb 1 at 20:39
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    Relativistic effects play a role because I am asking about what constitutes the present, and not really that interested in what constitutes a star, that was just an example. The mechanics of time are going to be closely related to relativistic physics so it makes sense to me.
    – Augs
    Feb 1 at 21:47
  • @Augs That an event happens or not, does not depend on the reference system of an observer, it is not a relativistic quantity, it is relativistically invariant. - I do not see where in your question you speak about presence. Anyhow, presence is a 4-dimensional domain already in Minkowski space. – Of course, time is a relativistic coordinate. But one may assume that the effect propagates along a light-like geodesic, which is a well-determined concept, invariant with respect to Lorentz-transformation. – How travelling along closed geodesics relates to causality seems still an open problem.
    – Jo Wehler
    Feb 1 at 22:09
  • There is no doubt that the star will eventually burn out, I am not debating if it happens or not. My question was though "does it exist right now from the view point of earth if we can still see it?" That is how the question was asking about what constitutes the present. Or do you believe we can see stars that don't exist right now?
    – Augs
    Feb 1 at 22:28
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Star as Source

  1. source of light (generates electromagnetic field)

  2. source of gravity (generates gravitational field)

  3. propagation delay in EM or Gravitational field

  4. observer detects EM radiation or gravity and infers time delay

Light Curves - Supernova Detection

https://imagine.gsfc.nasa.gov/science/toolbox/timing1.html

If, instead, the light curve we measured looked like the one below, we would know that this object was the death of a star by a massive explosion called a supernova!

enter image description here

Gravitational lensing

https://universe.nasa.gov/news/290/how-gravity-warps-light/

Let’s recap – mass warps space-time. The more mass, the stronger the warp, and the bigger its gravitational lensing effects. In fact, by studying “lensed” objects, we can map out the quantity and location of the unseen matter causing the distortion!

Gravitational Propagation Delay

https://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result, although so far with less precision.)

What Exists in Physics??

In physics what exists are perceptions coupled to the conceptual models of physical phenomena and the inferences one draws from the combination of perceptions and models!

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The question is trying to frame something in a way that doesn't make any sense to relativity. The idea that there is an objective "now" both here and over there doesn't work on those scales.

https://en.wikipedia.org/wiki/Relativity_of_simultaneity

means that depending on the observer, event A and event B could occur in a different order apparent to their local time. Pretty weird but causality is maintained by the speed of light limit.

Trying to describe a "now" to everywhere leads to even weirder paradoxes where two people walking past each other can't even agree on what has and hasn't happened:

https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Andromeda_paradox

(swap the alien invasion fleet for your star going supernova) One observer would say it appears to still exists, and the other would say it appears to have died.

It's like trying to answer the question does the "past" exist? In some sense yes because because to us the past for distance objects hasn't yet happened.

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  • [two people walking past each other can't even agree on what has and hasn't happened... One observer would say it appears this way, and the other would say it appears that way] This is false, too. The information is unavailable to the observers. It does not appear like anything to either of them. They will disagree in the future about what had been happening when they passed each other, if they magically keep going in straight lines at their present velocity with respect to the aliens for the whole time it takes the information to get there.
    – g s
    Feb 2 at 17:37
  • Conversely, if the light from the event has already reached the place where the driver and the pedestrian are passing one another, they both agree about what events they have seen, but they disagree slightly about exactly how far away the events were in space and how long ago they were in time.
    – g s
    Feb 2 at 17:40
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The question is, does this star "exist" in our reference frame?

Everything exists in every reference frame. A reference frame is just a way of parameterize the relationship between events. Events being space-time locations. The B theory of time is the natural language for discussing conditions where the impacts of Relativity cannot be ignored.

The relevant point is that: not every event is causally connected to every event in any given reference frame.

So the event that corresponds to "photon emitted from surface of the surface of the star (that will hit our instruments now)" is causally connected to the event of "me, here, now".

The event that corresponds to "the location of the remnants of that start propagated 50 million years (by it's clock's reckoning)" is not causally connected to the event of "me, here, now".

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Every object in the universe carries with it its very own "right now."

Anytime anyone anywhere in the universe moves in any direction at any speed, time slows down for them. If their clock read the same time as someone else's clock, it doesn't anymore.

If the sun disappeared suddenly, the Earth would still orbit it for 8 1/2 minutes. If you understand special relativity, you say, "Of course it works like that." It really is the case that you have to suddenly see it all at once.

I found it very confusing until I read a grad school textbook with a blue cover, I think It was called 'An Introduction to Special Relativity." That was magnificent. I remember the moment that I suddenly "got it." I was riding on the DC Metro, and I began giggling out loud.

Every object in the universe carries with it its very own "right now."

As far as flying out your front door really fast and eventually returning to your back door, yes the universe Is finite and yes it is not simply connected (in 4 dimensions).

But that can't happen because of the Hubble expansion. The distance that the universe is expanding away at the speed of light is coincidentally(?) almost exactly the same as its age, 14 billion (light) years. I suspect it's not a coincidence.

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