Now

[Mart]

Is every part of the universe equally old? The universe is estimated to be several billion years old. If I had a really accurate clock that measured the time from the big bang would it show the same time as anyone elses similar clock?

[timo]

A simplified answer: No. The only time you could measure is eigentime. Eigentime does not need to be equal for different objects even if they were created in the same event. The "twins paradoxon" would be an analogy.

[Mart]

What is eigentime?

[timo]

The time passed measured by a particle itself. This stands in contrast to the "coordinate time" which is ... well, the time value of your coordinate system. Both "times" do not have to coincide. For a particle travelling at a very high velocity from point A to point B you´ll measure larger time (coordinate time) than the particle does (eigentime).

 

Seems like it´s called "proper time" on wikipedia: http://en.wikipedia.org/wiki/Proper_time

[Martin]

 

Seems like it´s called "proper time" on wikipedia: http://en.wikipedia.org/wiki/Proper_time

 

eigenwert ----- eigenvalue ---- proper value

eigenzeit ------- eigentime ----- proper time

 

some english-speaking mathematicians say "eigenvalue" and some say "proper value"

 

we have the habit of sometimes, when we adopt a German word, only translating PART of the word at first, and then later we translate the rest.

 

in english "proper" can have the meaning of "one's own" or "it's own" what belongs to itself alone.

what is proper to me is my own possessions, my personal features, my identity, my own distinctive character. these are my "properties"

 

Unfortunately in English the word "proper" also has a second meaning of "correct" or "customary"

 

maybe we should change and for a thing's eigentime we should

say its OWN-time.

[Martin]

Is every part of the universe equally old? The universe is estimated to be several billion years old. If I had a really accurate clock that measured the time from the big bang would it show the same time as anyone elses similar clock?

 

 

there is a ROUGH APPROXIMATE notion of a universal timekeeping that corresponds to the time measured by all observers who are at REST relative to the cosmic microwave background.

 

even observers who are all at rest relative to the CMB can disagree somewhat, because some are deeper down in various gravity wells than others

 

someone who lives near the center of his galaxy, his clock runs at a different rate from someone who lives on the outskirts of the galaxy

 

even if you discount for relative motion and set it up so both guys are not moving relative to CMB.

 

so you can never get it quite right, but still in an approximate sense, if you consider observers who are all out in the big emptyspaces between clusters of galaxies so none are very deep in anybody's gravity well and who all have gotten so they are at rest relative CMB, then you have a prettygood universe-wide system of observers whose clocks prettymuch agree.

 

And in fact cosmologists ordinarily USE this approximate universal time as if it was well-defined. they assume a homogeneous universe with uniformly distributed mass anyway. the popular FRW metric has such a time built into it----likewise the Friedmann model universe that is the basic model they mostly work with.

 

Atheist is right----there isnt any one absolute universal time. but cosmologists fudge a little and use a time that is practically that

 

===============

the way you tell if you are moving relative CMB is if the microwave background looks a little bit "bluer" (higher frequency) in one direction and correspondingly "redder" (lower frequency) in the other direction.

then you know you are moving and causing this kind of dopplershift effect.

 

if you are at rest relative CMB then you are also at rest relative to the expansion of the universe-----that means that galaxies at some given distance will seem to be receding at the same speed whichever direction you look

 

if you were moving relative to the expansion, then ahead of you galaxies at that distance wouldnt be receding from you as fast as galaxies behind you at the same distance.

 

the recession speed is supposed to be proportional to the distance away it is, but that can only be true if you are at rest relative to the expansion (which is the same as at rest relative to the background microwaves)

 

this is not to contradict Atheist. He is correct. but cosmologists get around the difficulty by singling out this approximate universal rest frame

[Mart]

Originally Posted by Martin

 

that means that galaxies at some given distance will seem to be receding at the same speed

 

Thanx for the info. In the above quote you use the word "seem". Is that the same as "are"? Seem is often taken to mean some phenomenon that is not actually as it appears.

[Martin]

Thanx for the info. In the above quote you use the word "seem". Is that the same as "are"? Seem is often taken to mean some phenomenon that is not actually as it appears.

 

yes they are receding

recession velocity is the rate the distance to something is increasing because of the expansion of the space in between you and it.

the distances to the galaxies are increasing, so they are receding

so likewise is the space around them, also receding from us.

 

I should not have said "seem"

 

they appear to be receding and they are

[Mart]

Originally Posted by Martin

they appear to be receding and they are

They appear to be receding in our now. No problem. But how sure can we be that they are receding in their now? After all, the data we receive from very distant sources is very old.

[Martin]

we CANT be sure

but we are modeling the universe with a model that assumes the laws of physics----stuff like newton law and the einstein equation of Gen Rel.

 

there is a limit to how weird things can be, without violating basic laws

 

Like an airplane flies over and you dont see it how it is now but only how it was 10,000 nanoseconds ago (light goes one foot in one nanosecond if the thing is 10,000 feet away then you see it as it was that long ago)

 

we dont KNOW that a big bird didnt materialize and eat the airplane or that it didnt suddenly freeze in its tracks

 

but if it froze in its tracks that would look mighty like a violation of conservation of momentum, or if a giant bird materialized and it bumped into it that seems awfully fishy like a violation of conservation of energy.

 

so all we know for sure is the past, but we have expectations based on basic lawful behavior. we have a model of spacetime which has checked out remarkably well (Einst. 1915 Gen Rel, einstein equation etc.)

 

we cant say for SURE that it is, at this moment, still receding as the model says it should be, but if it isnt then something really weird must have happened----because the model has checked out in so many other situations.

 

I think you can take it from here. i have to go run some errands.

[Mart]

Originally Posted by Martin

we CANT be sure

but we are modeling the universe with a model that assumes the laws of physics----stuff like newton law and the einstein equation of Gen Rel.

 

there is a limit to how weird things can be, without violating basic laws

 

I thought the laws of physics (Newton's, Einstein's theories) were descriptions of how things behave. They can't tell things how to behave unless they describe laws (natural ones) that do tell things how to behave. If there are natural laws (prescriptive ones obviously) where are they? How do they interact with physical entities?

[Martin]

I thought the laws of physics (Newton's, Einstein's theories) were descriptions of how things behave. They can't tell things how to behave unless they describe laws (natural ones) that do tell things how to behave. If there are natural laws (prescriptive ones obviously) where[/i'] are they? How do they interact with physical entities?

 

too much logic-chopping, too philosophicall a type of discussion for me.

descriptive, prescriptive.

(also don't forget predictive, empirically testable)

so many things to define.

physical laws are mathematical.

you should probably get someone else to discuss this with. thanks.

[Martin]

upon reflection, I'd say this thread would be happier in the subforum called Philosophy and Religion

(because of the philosophical flavor)

 

what is the true character of physical law?

what do we know (Epistomology)?

what do we know about a galaxy NOW that we cant see now,

since we only see it as it was?

 

Good philosophy questions.

 

Astronomers, on the other hand, mostly concerned with describing and modeling what they can see HAS ALREADY HAPPENED. that is what is in our backwards lightcone basically. and like any scientists they would like to be able to test the correctness of their models by making predictions.

if the prediction turns out wrong that is always good because then you learn that something is wrong with the model and you can improve it.

 

someone who wants to talk about the abstract notion of NOW or the meaning of physical law, or what it is that we know, that person would be happiest in the Philosophy section of SFN, in my humble

[starbug1]

What HAS HAPPENED THEN is our NOW. For example, somewhere in the center of the universe there was a massive collison or the presence of a massive black hole, and while the universe is ultimately eating itself, we don't receive anything from our earth view point. This is an extraordinary gap in time dilation and proper time. We are living billions of years when maybe the universe is disentegrating. And this same process can start in our galaxy; but the thing is, we live in such a great space that time and the speed at which it travels varies greatly everywhere in the universe. That is as simple as I can put it.