Basic understanding of time

[Strange]

Common understanding says that we are able to discern the Big bang while future astronomers (in billion years from now) will not.

In that way, we are privileged.

I don't understand. What observations will future astronomers not be able to make that we can now? They will still observe the cosmic microwave background (a little cooler than now), the redshift of distant objects increasing with distance, the same relative abundance of hydrogen and helium, etc., the large scale homogeneity and structure of the universe, etc.

 

I suppose they may have replaced the big bang model based on future theories of quantum gravity, or something. But I can't see that there is anything special about what we can observe now.

 

Of course, there will come a point where the universe has expanded so much that nothing outside our local galaxy (or cluster) is visible. But even that fact would be evidence for the big bang!

[michel123456]

I don't understand. What observations will future astronomers not be able to make that we can now? They will still observe the cosmic microwave background (a little cooler than now), the redshift of distant objects increasing with distance, the same relative abundance of hydrogen and helium, etc., the large scale homogeneity and structure of the universe, etc.

 

I suppose they may have replaced the big bang model based on future theories of quantum gravity, or something. But I can't see that there is anything special about what we can observe now.

 

Of course, there will come a point where the universe has expanded so much that nothing outside our local galaxy (or cluster) is visible. But even that fact would be evidence for the big bang!

I don't think so.

 

Eminent cosmologist Lawrence Krauss explaing standard cosmology here answers some questions and says the following:

 

Quote

 

FLATOW: 1-800-989-8255. You mentioned in your book that we are lucky to be living in this time in the universe.

 

KRAUSS: Yeah, I mean for a variety of reasons. One is in the far future, and by the far future I mean hundreds of billions of years, astronomers and radio hosts on planets around other stars will look out at the universe, and what they'll see is the universe we thought we lived in 100 years ago, all of the other galaxies will have disappeared expect for our own, and people will assume, or beings will assume, they live in a universe that's basically infinite, dark and empty except for one galaxy, with no evidence of the Big Bang.

 

[Strange]

You seem to be demanding that the universe should be forever unchanging. But that is not the way it is. And it is not what the Copernican principle says.

 

And you say, "I don't think so" but, as far as I can tell, everything I said is in agreement with the passage from Krauss you quote.

[swansont]

The CMB will still be there, redshifted even further, with no context for its presence. But what Krauss is talking about is not a privileged reference frame as privileged is commonly used. All observers will be in the same situation, with nobody else in their visible horizon, and the laws of physics will work for all of them.

 

The fortunate happenstance Krauss mentions is akin to the fortune we have that solar eclipses happen the way they, often showing us the sun's corona. In the remote past (a billion years or more) the moon was closer and would block out a larger diameter. Some years from now, the moon will be more distant and we won't have total eclipses at all. So we are lucky to be observing the moon now, though our perspective is not privileged.

[michel123456]

You seem to be demanding that the universe should be forever unchanging. But that is not the way it is. And it is not what the Copernican principle says.

What does say the Copernican principle? It says that we cannot consider ourselves as privileged observers.

 

And you say, "I don't think so" but, as far as I can tell, everything I said is in agreement with the passage from Krauss you quote.

You are in agreement with him except that Krauss said that there will be no evidence for the Big Bang.

The CMB will still be there, redshifted even further, with no context for its presence. But what Krauss is talking about is not a privileged reference frame as privileged is commonly used. All observers will be in the same situation, with nobody else in their visible horizon, and the laws of physics will work for all of them.

 

The fortunate happenstance Krauss mentions is akin to the fortune we have that solar eclipses happen the way they, often showing us the sun's corona. In the remote past (a billion years or more) the moon was closer and would block out a larger diameter. Some years from now, the moon will be more distant and we won't have total eclipses at all. So we are lucky to be observing the moon now, though our perspective is not privileged.

You mean "all observers at the same time stamp", not "all observers in the Universe".

What you say is that an extraterrestrial astronomer on a galaxy 10 billion light-years away from us (that is about 10 billion years in the past) was not observing the same universe as we do today. And that some future astronomer will not see the same universe as we do today, that he will observe a single galaxy in an empty universe.

I don't understand how it is possible to agree with such concepts.

[Strange]

I don't understand how it is possible to agree with such concepts.

 

All it takes is to accept evidence over personal preference.

[swansont]

 

You mean "all observers at the same time stamp", not "all observers in the Universe".

 

 

Unless you can come up with a situation where you can see my galaxy but I can't see yours, Krauss means all observers in the universe. To the extent that you can equate the times, the scenario already implies it's for observers with the same time stamp, i.e. existing in an extremely old universe.

[MigL]

My two cents...

 

Saying 'Time is Change' seems to be circular reasoning, as we define the passage of time by change.

It would be akin to saying distance is metres.

 

At a point in time so far in the future that all other galaxies have disappeared from our observable universe, recall that the galaxies aren't moving away, rather spatial separation between them is increasing.

This will happen for all other objects in the observable universe ( vast size ), not just galaxies, and I'm sure any surviving intelligent civilization will be able to measure the rate of expansion, and 're-wind' the expansion back to the big bang.

[imatfaal]

My two cents...

 

Saying 'Time is Change' seems to be circular reasoning, as we define the passage of time by change.

It would be akin to saying distance is metres.

 

At a point in time so far in the future that all other galaxies have disappeared from our observable universe, recall that the galaxies aren't moving away, rather spatial separation between them is increasing.

This will happen for all other objects in the observable universe ( vast size ), not just galaxies, and I'm sure any surviving intelligent civilization will be able to measure the rate of expansion, and 're-wind' the expansion back to the big bang.

 

Not sure about your last paragraph; unless they have historical records they would only have astronomical observations - and in the very distant future civilization may arise in a gravitationally bound supercluster (ie everything they see is actually getting closer) and no object that is not gravitational bound is still visible due to accelerated expansion. The idea is that they will have nothing that can give evidence for an expanding universe; this could be speculated just as we have speculated on solid state, on big rip, on crunch and bounce, and big bang - we have evidence as well though!

[Schneibster]

I know it is the standard point of vue.

But I really wonder why scientists are comfortable with that idea. To me it is monstrous, it should ring a bell because, as far as I can understand, it violates the copernican principle since it makes us today privileged observers of the universe.

 

How "privileged?" You'll see the same thing from everywhere; just not the same location for the horizon.

In relation to time and the foregoing discussion about physical and real, consider the following:

 

In space, positions x, (x+10) and (x-10) are all physical and real since I can move my boat from one to the other and back again as many times as I like and place it in any one of them if I am a good enough sailor.

 

But in time I can only observe my boat in the present. I cannot move it to the future or the past or observe it there.

 

So does this make the present physical and real, but the future and past non-physical and non-real?

 

Actually you are hurtling into the future at the speed of light.

But could it not be argued that it is doing something but we can't measure it ...something must be happening to cause the decay?

 

Yes: time is passing.

 

But inside a muon? No. Muons have no internal structure. They are Generation II fundamental leptons. They have no more internal structure than quarks.

Well I guess my point is that time is only meaningful relative to change. So I sort of agree with the OP (Substituting "change" for "movement")

 

No. Time is as real as space; in fact, the Lorentz transform lets us convert one into the other. If you go fast enough you see time as space; of course it just looks like space to you, but everyone else can see you think space is time.

[MigL]

I don't think so Imatfaal. Even in the very distant future there will be objects transiting out of the observable universe due to expansion. Unless that expansion stops. This means that there will always be objects which aren't gravitationally bound and measurements can be made to determine the rate of expansion.

Maybe once we get a grip on vacuum energy, we may even be able to deduce the rate of expansion of flat ( non gravitational ) space without measuring recession rates.

[Strange]

No. Time is as real as space

 

Quite. And every argument made by the "time doesn't exist" crowd, can be equally applied to space: "space is just a change in position over time; it doesn't really exist, it's an illusion created by motion."

[Lizzie L]

 

No. Time is as real as space; in fact, the Lorentz transform lets us convert one into the other. If you go fast enough you see time as space; of course it just looks like space to you, but everyone else can see you think space is time.

 

I didn't mean to imply it wasn't real. I just mean that it only makes sense conceptually in regard to change, including changing place (i.e. moving) but also changing state. It makes sense if you can talk about things happening over time. But I guess it's a philosophical point one rather than a physics one.

 

Quite. And every argument made by the "time doesn't exist" crowd, can be equally applied to space: "space is just a change in position over time; it doesn't really exist, it's an illusion created by motion."

 

The first clause seems OK, but the second and third don't seem to me to follow from the first.

[Schneibster]

 

I didn't mean to imply it wasn't real. I just mean that it only makes sense conceptually in regard to change, including changing place (i.e. moving) but also changing state. It makes sense if you can talk about things happening over time. But I guess it's a philosophical point one rather than a physics one.

 

Actually it's a physiological point, bound up with how humans perceive time. Which is due both to that, and to the fact that time is hyperbolic, whereas space is right circular.

 

ETA: Well, flat space is anyway.

Edited February 21, 2014 by Schneibster

[Strange]

The first clause seems OK, but the second and third don't seem to me to follow from the first.

 

It isn't supposed to follow or to make sense: it is a parody of the "logic" used by those who have some sort of quasi-religious belief that time doesn't exist. (The way the argument gets endlessly repeated, makes time seem cyclical.)

[Lizzie L]

 

Actually it's a physiological point, bound up with how humans perceive time. Which is due both to that, and to the fact that time is hyperbolic, whereas space is right circular.

 

ETA: Well, flat space is anyway.

 

Yes indeed. Well, I don't know about the hyperbolic vs circular part, but it does seem to me that we can't leave ourselves out of the picture.

[studiot]

 

Which is due both to that, and to the fact that time is hyperbolic, whereas space is right circular.

 

You have made that mathematical statement several times, but there is the view that time is a simply useful parametrisation for many equations.

Edited February 21, 2014 by studiot

[Lizzie L]

 

It isn't supposed to follow or to make sense: it is a parody of the "logic" used by those who have some sort of quasi-religious belief that time doesn't exist. (The way the argument gets endlessly repeated, makes time seem cyclical.)

 

Sure. I just meant that you can hold the view that time only makes sense relative to change without indulging in quasi-religious woo about it. In any case "doesn't exist" is a bit of a non-starter IMO. We model the world with our perceptual apparatus, and with more sophisticated mathematical models, and if the models make good predictions, we keep them. Are the models "real"? Dunno. But if they are part of a good predictive system then they are as good as anything else we have. The fact that we can make predictive models suggests there is something consistently "there" that we can call "real", but all we have access to is models.

 

*gets off hobby horse*

[Schneibster]

 

Yes indeed. Well, I don't know about the hyperbolic vs circular part, but it does seem to me that we can't leave ourselves out of the picture.

 

 

Well, certainly we have to allow for our biases and our sensory equipment's sensitivity and range.

 

Instrumentation is intended to "make something happen" where we can see it when our experiment pops.

Edited February 21, 2014 by Schneibster

[Lizzie L]

 

Well, certainly we have to allow for our biases and our sensory equipment's sensitivity and range.

 

Instrumentation is intended to "make something happen" where we can see it when our experiment pops.

 

And the fact that we are the authors of the very models we are debating. Without people there are no models, right?

[Schneibster]

 

And the fact that we are the authors of the very models we are debating. Without people there are no models, right?

 

Sure, but remember that the universe is in a curious and mysterious accord with math.

 

You have made that mathematical statement several times, but there is the view that time is a simply useful parametrisation for many equations.

 

Well, that is not the relativistic view. Relativity proves time is hyperbolic. Spacetime's basic group is the Poincare group. Time is not right circular.

[studiot]

 

studiot, on 21 Feb 2014 - 11:27 PM, said:

 

You have made that mathematical statement several times, but there is the view that time is a simply useful parametrisation for many equations.

 

 

Well, that is not the relativistic view. Relativity proves time is hyperbolic. Spacetime's basic group is the Poincare group. Time is not right circular.

 

 

What proof do you have that they are incompatible for you to be able to say this?

[michel123456]

 

All it takes is to accept evidence over personal preference.

Show me evidence of a far away galaxy getting out of sight because of the expansion.

 

Sure, but remember that the universe is in a curious and mysterious accord with math.

 

Well, that is not the relativistic view. Relativity proves time is hyperbolic. Spacetime's basic group is the Poincare group. Time is not right circular.

Please explain why time is hyperbolic.

[Strange]

 

Sure. I just meant that you can hold the view that time only makes sense relative to change without indulging in quasi-religious woo about it.

 

Not really. There are never any scientific arguments presented that time doesn't exist. Just personal beliefs.

 

 

In any case "doesn't exist" is a bit of a non-starter IMO.

 

I suppose the whole thing hinges on the definition of "exist"; some of people who say "time is only motion [or change]" will concede that such motion/change requires time but will still insist that time itself doesn't exist. That is a staggering level of cognitive dissonance.

Show me evidence of a far away galaxy getting out of sight because of the expansion.

 

Good grief. Really?

 

http://arxiv.org/abs/astro-ph/0310808

http://en.wikipedia.org/wiki/Observable_universe (there is a "simple English" version of that you might want to tackle first).

http://en.wikipedia.org/wiki/Cosmological_horizon

http://en.wikipedia.org/wiki/Particle_horizon

[Lizzie L]

 

Not really. There are never any scientific arguments presented that time doesn't exist. Just personal beliefs.

 

 

I suppose the whole thing hinges on the definition of "exist"; some of people who say "time is only motion [or change]" will concede that such motion/change requires time but will still insist that time itself doesn't exist. That is a staggering level of cognitive dissonance.

 

I didn't say that time didn't exist, nor that it was "only motion" or "only change". What I'm saying is that time only makes sense in terms of change. Or, if you prefer, that time is intrinsic to the concept of change and that change is intrinsic to the concept of time. And possibly that that is what the people you are referring to mean.

 

More interestingly (because that seems fairly uncontroversial to me), the irreversible direction of time as we model it is intrinsic to our properties as observers with memories of the past, but only contingent imaginings of the future. As a thought experiment it's interesting (well, I find it so!) to imagine a kind world in which all we know is the present, but in which our computational capacity is such that, given the present, we can compute the future with a high degree of confidence - a kind of cellular automaton world, in which many things could have resulted in its current configuration, but given its current configuration, only one future is possible. In such a world "causality" would seem to be reversed - the future would be knowable with certainty, but the past would be highly speculative.

 

In that sense I like Schneibster's hyperbolic view of time (presumably he's referring to something like Minkowski space time) - with the observer at the apex of a double cone. In my (heh) model, though, the flow of time is simply a function of what the observer can know, given what she can observe at the apex: which is more certain - the changes in the upper cone, or the changes in the lower? I'm saying that her perceived flow of time will be in the direction of the less knowable.