Some stars burnt out?

Cecil's Columns/Staff Reports
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SDSTAFF David:

regarding whether some stars may be burnt out and all we see is the light they emitted long ago, I am not an expert on this but I think the answer is so oversimplified that it comes very close to being erroneous. I do not say it is wrong as I know what you mean and it is the correct oversimplified answer to a very simple question, but…

in the theory of relativity there is no such thing as an absolute concept of now or simultaneously. There is only “absolute before”, “absolute after” and “depends how you look at it”

To say that we do not know how the star looks like “now” maybe a way to express it that we can all understand but strictly speaking it is not true. What you see now is what the star looks like now in space-time. Time is a very elastic concept and there is no such thing as a universal now. Time depends on the conditions of the observer.

An example: suppose I am here on Earth and my cousin is in a spaceship where my light or radio signals take 1 hour to arrive. Further assume he can answer my message instantly. I send him my message and he receives it instantly and sends his reply which I receive instantly and yet from my viewpoint his reply has taken two hours since i sent my message (one hour for my mesage to go there and one more for his reply to get back here). Everything that happened here before I sent my message happened absolutely before. Everything that happened after i received the reply happened absolutely after but, everything that happened between those two events is relative. if my cousin flipped a switch when he received my message, and I flipped a switch after i sent mine but before i received his, there is no absolute answer as to who flipped it first as this depends on the observer.

The concept of simultaneity in relativity is that things that happen along the same ray of light are simultaneous so, from that point of view, you are seeing the star as it is now even if the light took a billion years to get here.

Well, I am not able to express this better. Can someone knowledgeable please do a better job of explaining what I am trying to say?

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I remember I saw a pretty good explanation of this on the Net but I cannot find it now. Still, I found this one: http://www.physics.purdue.edu/~hinson/ftl/html/FTL_part1.html#sec:lightcone which explains it quite directly: in space-time, and in my frame of reference, when i see the star burn out, that’s when it burns out. It is not that that’s when it appears to burn out but when it actually does (again, for my frame of reference)

It also explains later on that there is no such thing as absolute simultaneity but it depends on your frame of reference.

I realize this may be counterintuitive but how do you think people felt some hundreds of years ago when they were told “vertical” is not an absolute concept and people in the souther hemisphere lived upsidedown?

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But I wasn’t talking about what they “look like” now – I was talking about what they actually are now (which was the question. Obviously, if we can see them, that is what they look like now. But they may actually be something completely different (as in burned out or blown up).

Yes, I know what you’re talking about in terms of frame of reference and all that, but it’s not the point of the question. Somewhere, 12 billion light years away, there was a star that emitted some light 12 billion years ago. That star may still be there. More likely it’s not. That is completely independent of the light that is just reaching us now.

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Hey Sailor -

You have seem to have misunderstood a bit there. In fact, when we observe a supernova, that is not when it actually happened in our frame of reference. Rather, we must correct for the time it took the light to get here (as measured in out frame of reference). For example, humans observed supernova 1987A in February of 1997 on earth, but since the star that died there was 169 kilo-lightyears away, we say that the supernova actually happened 169 millenia ago, not 13 years ago.

Also, simultaneity does exists. Thw events that happen at the same time AND at the same location in one frame happen in the same time and location in all inertail frames. However, if two events happen at the same time at different places, you can find a frame such that they happened at different times. A similar argument exists for events that happen at the same location at different times.

Joe.

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The mailbag item being referenced is

Are the stars really all burned out, and all we see is the glow? (05-Apr-2000)

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I understand what you’re saying, but have you worked out what the inaccuracy in the definition of "now’ would be billions light years away due to a person’s varying motion? For people on Earth, their velocity varies due to the Earth’s rotation and orbit around the Sun. Although the author of the question didn’t explicitly say so, he or she probably isn’t travelling at some substantial fraction of the speed of light.

Defining “now” a billion light years away to an accuracy of +/- 1000 years is not unreasonable. In fact, I doubt the inaccuracy of "now’ a billion light years away is any more or less accurate than the inaccuracy of saying “a billion light years away” in the first place.

It is too clear, and so it is hard to see.

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David,

I understood perfectly what you said the first time around and if you read my first post carefully you will see it is exactly what I am trying to disagree with. No need to explain it further. We disagree.

You are assuming there is such a thing as an abslute now and my contention is that there is no such concept as absolute now and that for us in Earth, what you see is what is. If you can see the star, it is there. If you cannot see it, it is not there.

There are many paradoxes and this is one of them. Another one is that you cannot just add speeds. If i am travelling away from a stationary observer at half the speed of light and I shine a light forward it will travel away from me at the speed of light but it will travel away from the stationary observer also at the speed of light. This is an a pparent contradiction as we would have to add the speeds.

I realize it is too much to ask that you study this carefully and I do not want to start a thread where I am clearly going to be outnumbered by people who know a lot about everyday physics and nothing about relativity.

Did you consult any person who is knowledgeable about relativity before you wrote that?

I am quite sure I am right on this one but, obviously, I do not have the credentials that you should take my word for it. I propose we try to find some authority on this matter who can come in and settle it for us.

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I have emailed this guy http://www.physics.purdue.edu/~hinson/homepage.html and asked him to give us his view.

Maybe you can find some other people who would be qualified. If they all agree I think we can all be sure they are right on whatever they agree on. While my reasoning may not be totally rigorous I feel confident it is closer to the truth.

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Hi all…

Responding to the request for my input on this topic, I would say that sailor is only half right.

It is true that what is happening to a star “right now” is a purely frame-dependent question. However, it does have an answer for any given frame, and no matter what frame you ask, the answer says that what’s going on with the star “right now” is not what you are seeing right now.

For the practicality of the question being addressed, I’d assume that what’s going on right now means “in the frame of reference of the Earth” (which doesn’t change all that much with respect to a given distant star over a person’s lifetime). If it takes X years for the light to reach us in our frame of reference, then what’s happening “right now” in our frame of reference will be seen by us X years from now.

As an example, let’s say that in our frame of reference, the star is burning out right…oh…say… NOW! Okay, X years from now we will see it burning out, we will calculate how long it took the light to reach us, and we will THEN know that it actually burned out at this time.

Of course, another observer who is seeing the same light we are seeing right NOW but who (at this very moment) is passing by us at a high speed towards the distant star will also at some point in the future see the light from the star as it burned out. He too will make a completely accurate calculation of how long the light took to reach him in his frame of reference, and he will NOT agree with us that the star burned out around the same time he was passing by us. In his frame, the star will have trully burned out some time before he passed by us.

Finally, another observer traveling AWAY from the star could be passing by us just as we both are seeing the stars light at this moment, and he will eventually calculate that the star was really, trully burrning out some time AFTER he passed by us according to his frame of reference.

All of this is due to the relative nature of simultaneity in relativity, but for each of these observers, what the star was doing “right now” was still not what he was seeing “right now” in his frame of reference.
Hope that helps.
-Jason

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My mistake. The orbital motion of the Earth causes a variation in the definition of “now” of +/- 100,000 years at a distance of 1 billion light years. I still don’t see what would be the point of David B bringing this up in the mailbag answer.

It is too clear, and so it is hard to see.

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ftlguy, thanks for your input and I stand corrected. I once read something that I interpreted differently but obviously you are the expert and so I have to accept your answer.

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I think the answer given by David as well as this discussion missed the boat. The question was “Are the stars really all burned out, and all we see is the glow?”.

This is really a question about how long stars “live” and what stages they go through as they age and how we know about these things. Relativity doesn’t enter into it, nor even the distance to the stars. After all, he’s asking about what we are seeing, not what’s happening at the star “right now”.

One other thing. The question doesn’t evince much knowledge of this subject, so a fairly basic explanation is in order, avoiding advanced subjects.

Dan Tilque