Age of Universe relative to what?

[goodabouthood]

People always say the Universe is 14 billions year old.

But what does that mean?

What is the time of the Universe relative to?

 

[alexg]

It means that the Big Bang took place about 13.8 billion years ago.

 

[goodabouthood]

It means that the Big Bang took place about 13.8 billion years ago.

But isn't time relative?

Did the Big Bang happen at different times in different reference frames?

When we say 13.8 billion years ago, what exactly are we referring to?

 

[phinds]

It is possible to imagine a frame of reference that is stationary relative to the CMB. If you were in such a frame of reference, and had enough snacks to last you for the duration, and a reliable watch that used our current system of duration, you would have observed that amount of time since one Plank Time following the singularity (aka the "Big Bang Event").

There are considerations that would have made this physically difficult so this is just a thought experiment.

 

[pervect]

The age of the universe is always giving in cosmological time, which is time as measured in the frame comoving with the Hubble flow, the unique frame where the universe (operationally, the cosmic microwave background radiation) is isotropic.

 

[lalbatros]

Assuming humans are bound to a co-moving frame, we could as well say that these 14 billion year are as-if measured by our common watches. This would be a time as measured in our reference frame.

However, I still wonder how time can be "continued" to periods where references clocks -like our atomic clocks- could even not be envisaged. Atomic clocks could not possibly even exist before atoms were there.

Considering that time is a measured quantity, we need a series of reference clocks bringing us back to the BB. Would it be enough to calibrate these clocks with respect ot each other?
Has such a time accounting been actually performed by astonomers?

 

[my_wan]

Relative to a comoving inertial clock like what's on your wall.

 

[phinds]

Considering that time is a measured quantity, we need a series of reference clocks bringing us back to the BB. Would it be enough to calibrate these clocks with respect ot each other?
Has such a time accounting been actually performed by astonomers?

There is no evidence that the flow of time has had any different characteristics since the Plank Time, so it seems perfectly reasonable to project backwards as we do.

 

[petm1]

The atom is the edge of my universe, all the atoms together form one edge, and this one edge was created at the same time, relative to my present. It is the atom that appears eternal to me but thanks to Einstein's calendar I can see that even the atom is temporal with a beginning and an end in time. Everything is relative to the present and each of our local clocks.

 

[Passionflower]

It is the age of an ideal co-moving clock.

 

[my_wan]

There is no evidence that the flow of time has had any different characteristics since the Plank Time, so it seems perfectly reasonable to project backwards as we do.

Not strictly true:
Time Dilation in the Light Curve of the Distant Type Ia Supernovae SN 1995K
Observation of Cosmological Time Dilation using Type Ia Supernovae as Clocks
Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae
Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications
Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves

 

[phinds]

Not strictly true:
Time Dilation in the Light Curve of the Distant Type Ia Supernovae SN 1995K
Observation of Cosmological Time Dilation using Type Ia Supernovae as Clocks
Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae
Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications
Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves

I checked the first couple of these and I completely fail to see what their points have to do with this thread. They talk about the time dilation of accelerating object ... no surprise there but what has that to do with a frame of refernce that is comoving with the CMB?

 

[my_wan]

I checked the first couple of these and I completely fail to see what their points have to do with this thread. They talk about the time dilation of accelerating object ... no surprise there but what has that to do with a frame of refernce that is comoving with the CMB?

The comoving frame seems to be what is responsible for the 'apparent' global acceleration of these objects in the universe relative to any distant comoving frame. Just because a pair of distant observers are both comoving with the CMB does not mean they escape the time dilation with respect to each other.

 

[phinds]

The comoving frame seems to be what is responsible for the 'apparent' global acceleration of these objects in the universe relative to any distant comoving frame. Just because a pair of distant observers are both comoving with the CMB does not mean they escape the time dilation with respect to each other.

I didn't realize that anyone had suggested that they would.

The objects at the edge of our observable universe are receding from us at about 3c, so relative to each other we most certainly are seeing time dilation. BUT ... a comoving frame out there would see the universe at 13.7B years old, as do we (well, almost ... we are a hair off of being comoving). I fail to see what point you are making relative to this thread, which is about the age of the universe.

 

[Naty1]

Age of the Universe:
(Here are my notes from a very long discussion in these forums)

Do all observers agree on the age of our Universe?

Crowell:
No, they don't all agree.

But in an FRW cosmological model, there are preferred observers, who are essentially observers who detect no dipole asymmetry in the CMB. Such observers agree with one another on the amount of clock time since the Big Bang, and this is what we mean when we speak of the age of the universe in such a model.

In the real universe, a clock on the Earth's surface is not a bad approximation to such a clock. The solar system isn't moving at any large fraction of c relative to the CMB, and there is not a huge amount of gravitational time dilation between the Earth's surface and a point that is, say, outside the local group of galaxies.

There is not just one such frame for the whole cosmos. There is one such frame for every point in the cosmos. Global frames of reference don't exist in GR.

The existence of these preferred frames is also not a general characteristic of GR. It's just a characteristic of this particular solution of the GR field equations.


The age of the universe as usually discussed is for an observer who is at rest relative to the average motion of the matter and radiation in the universe (the "Hubble flow"), and is in the context of homogeneous models, which wouldn't include any structure such as black holes, etc. Yes, you're right, different observers can measure different ages of the universe on their clocks. You can't be "on" a black hole, but an observer hovering just outside a black hole's event horizon would say that according to her clock, the universe is very young. There is no limit on how young the universe could be according to such an observer. The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.

 

[goodabouthood]

What does CMB stand for?

So we are taking this 13.7 billion year time from the Earth's reference frame correct?

How do we even take this measurement?

 

[phinds]

What does CMB stand for?

Google is a nice tool. You should learn to use it.

 

[TrickyDicky]

... The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.

Right, such an observer's clock would measure the age of the universe arbitrarily close to t=0 as it approaches arbitrarily close to c, therefore the age of the universe is totally relative and depends on the state of motion of the observer; but there are limits to this age for any observer, brought by the absolute velocity c at one side and by the "conventional" CMB velocity that actually also puts an absolute maximum limit to the age of the universe (observer's proper time) that any observer could measure (meaning there seems to be no way to go slower than the comoving frame).
The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

 

[Dale]

The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

That is very basic SR. C is absolute not because there is an absolute rest frame but because something which is moving at c in one frame is moving at c in all frames.

 

[TrickyDicky]

That is very basic SR. C is absolute not because there is an absolute rest frame but because something which is moving at c in one frame is moving at c in all frames.

did I say anything contradicting that? Your statement is just a tautological explanation of what absolute means. Very basic indeed.

 

[Dale]

did I say anything contradicting that?

Yes:

in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest

 

[TrickyDicky]

Yes:

That is not contradicting it at all. Would you deny that a way to define an absolute velocity is referencing it to an absolute rest. If you think about it a little you'll realize that in abstract terms one thing implies the other. That also implies that all frames agree about c, that is another way to say c is absolute.

 

[phinds]

That is not contradicting it at all. Would you deny that a way to define an absolute velocity is referencing it to an absolute rest. If you think about it a little you'll realize that in abstract terms one thing implies the other. That also implies that all frames agree about c, that is another way to say c is absolute.

You may be right, but I too felt that you had made an incorrect statement. Your wording does seem to lead to that conclusion.

 

[Dale]

If you think about it a little you'll realize that in abstract terms one thing implies the other.

No, it does not. Since relativity has one but not the other they clearly do not imply each other.

 

[OCR]

Your statement is just a tautological explanation of what absolute means.

What is the meaning of the word, tautological?

Wait, wait, on 5 seconds thought, I'll bet it means...

Google is a nice tool. You should learn to use it.

What does CMB stand for?

It means "cosmic microwave background radiation", well, that is if you tack an "r" on the end.

I'll post a link goodabouthood, so you can read the definition, without using tautological...

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



OCR

 

[TrickyDicky]

No, it does not. Since relativity has one but not the other they clearly do not imply each other.

If it has one it has the other, it is in the postulates. Maybe you are conflating light with massive objects that can never reach c.

 

[TrickyDicky]

Short SR lesson: Usually motion or velocity of massive objects is defined in reference to some other object that is considered to be momentarily at rest wrt the first, but if we choose a different object as reference we may obtain a different velocity, the velocity is relative in this sense. And the rest state by which we measure it is a conventional frame, there is nothing special or absolute about it.
The case of the velocity of light is different , regardless of the state of motion of the object we choose as reference the velocity doesn't vary. The nature of the rest frame that allows us to measure a specific invariant speed must be different than what was mentioned above.
Unless one prefers to say that this particular velocity is not a motion by any of the usual meanings of motion in physics and therefore it is not defined wrt some kind of rest frame, but that's kind of like saying that when someone sells something to someone the other one is not buying. Motion and rest are logically linked.

 

[ghwellsjr]

Short SR lesson: Usually motion or velocity of massive objects is defined in reference to some other object that is considered to be momentarily at rest wrt the first, but if we choose a different object as reference we may obtain a different velocity, the velocity is relative in this sense. And the rest state by which we measure it is a conventional frame, there is nothing special or absolute about it.
The case of the velocity of light is different , regardless of the state of motion of the object we choose as reference the velocity doesn't vary. The nature of the rest frame that allows us to measure a specific invariant speed must be different than what was mentioned above.
Unless one prefers to say that this particular velocity is not a motion by any of the usual meanings of motion in physics and therefore it is not defined wrt some kind of rest frame, but that's kind of like saying that when someone sells something to someone the other one is not buying. Motion and rest are logically linked.

You can only measure the round-trip speed of light and there is nothing special about the state of motion in which it is measured as long as it is inertial. This is covered in Einstein's first postulate, just like all other measurements and observations of physics and its laws.

The second postulate defines the one-way speed to be equal to the two-way speed. This cannot be measured but rather defines what a Frame of Reference is. Any state of motion that can be used to measure the round-trip speed of light as c can also be used as a Frame of Reference in which the propagation of light is defined to be c.

So although the one-way speed of light is absolutely defined to be c in any reference frame, that in no way implies that there is an absolute rest.

 

[Dale]

If it has one it has the other, it is in the postulates.

No, your suggestion is directly contrary to the postulates. The principle of relativity postulate says that there is no absolute rest. The light speed postulate says that c is absolute.

 

[TrickyDicky]

You can only measure the round-trip speed of light and there is nothing special about the state of motion in which it is measured as long as it is inertial. This is covered in Einstein's first postulate, just like all other measurements and observations of physics and its laws.

The second postulate defines the one-way speed to be equal to the two-way speed. This cannot be measured but rather defines what a Frame of Reference is. Any state of motion that can be used to measure the round-trip speed of light as c can also be used as a Frame of Reference in which the propagation of light is defined to be c.

So although the one-way speed of light is absolutely defined to be c in any reference frame, that in no way implies that there is an absolute rest.

What I discussed in my last posts is independent of the one way vs. round trip speed considerations or the state of motion in which is measured.
do you think that motion is logically defined wrt a rest frame or not?

 

[TrickyDicky]

No, your suggestion is directly contrary to the postulates. The principle of relativity postulate says that there is no absolute rest. The light speed postulate says that c is absolute.

That's not what the special principle of relativity says. But nevermind that, there is obviously no absolute rest frame, I'm simply inquiring what is the difference between the rest frame of massive objects and light.

 

[ghwellsjr]

What I discussed in my last posts is independent of the one way vs. round trip speed considerations or the state of motion in which is measured.
do you think that motion is logically defined wrt a rest frame or not?

In Special Relativity, motion is defined with respect to an arbitrarily chosen inertial Frame of Reference. The motion we measure between two objects does not depend on any Frame of Reference. But we cannot measure the one-way speed of light under any circumstance.

And none of this has anything to do with a state of absolute rest. That's the point of contention. You say that because there is an absolute speed of light, there must also be an absolute speed of zero, which is wrong. Here's where you said it:

The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

 

[ghwellsjr]

No, your suggestion is directly contrary to the postulates. The principle of relativity postulate says that there is no absolute rest. The light speed postulate says that c is absolute.

That's not what the special principle of relativity says. But nevermind that, there is obviously no absolute rest frame, I'm simply inquiring what is the difference between the rest frame of massive objects and light.

You state that the principle of relativity does not claim that there is no absolute rest. Are you changing your mind? First you argue for an absolute rest and now you say the opposite.

And you ask about the rest frame of massive objects (which exist) and the rest frame of light (which doesn't exist). It's getting very difficult to tell what you are asking or what you are promoting.

 

[TrickyDicky]

You state that the principle of relativity does not claim that there is no absolute rest. Are you changing your mind? First you argue for an absolute rest and now you say the opposite.

And you ask about the rest frame of massive objects (which exist) and the rest frame of light (which doesn't exist). It's getting very difficult to tell what you are asking or what you are promoting.

I'm not actually promoting anything, the OP was about the age of the universe so I'd say it involves GR rather than SR. SR is a theory limited to idealized spacetimes where there's only uniform motion, the special relativity principle was confined to a "local only" principle by Einstein in 1915, it's about time the die hard fans of SR take note; the phrase that originated this discussion was in the context of GR and it simply was exploring what "appears natural" logically and it's really no big deal, just ignore it.

 

[Dale]

That's not what the special principle of relativity says.

Yes, it is: "In the real world, there exists no such state of absolute rest. That's the content of the so-called principle of relativity, which is one of the basic postulates of the special theory of relativity."
http://www.einstein-online.info/elementary/specialRT/RelativityPrinciple

there has to be some absolute rest

there is obviously no absolute rest

 

[TrickyDicky]

Yes, it is: "In the real world, there exists no such state of absolute rest. That's the content of the so-called principle of relativity, which is one of the basic postulates of the special theory of relativity."
http://www.einstein-online.info/elementary/specialRT/RelativityPrinciple

Here your troll soul shines at its brightest, you ignore my previous post that settled the argument by giving it context, then you quote some words as if they were from the first postulate when in fact are taken from some website for kids, which suggests you haven't even read the original postulates. And finally you take a couple of sentences out of context and put them together for effect:as I said pure trolling. I must admit I had a good laugh too.

 

[Dale]

And finally you take a couple of sentences out of context and put them together for effect.

The quotes were deliberately brief, for effect as you mention, but hardly taken out of context. Or are you honestly going to try to claim that your posts 18, 22, 26, and 27 didn't all try to promote the idea of absolute rest?

Look, I'm glad you changed your mind, but don't try to pretend that I took something out of context when it was clearly in line with what you had been repeatedly saying in multiple posts.

 

[phinds]

Here your troll soul shines at its brightest, you ignore my previous post that settled the argument by giving it context, then you quote some words as if they were from the first postulate when in fact are taken from some website for kids, which suggests you haven't even read the original postulates. And finally you take a couple of sentences out of context and put them together for effect:as I said pure trolling. I must admit I had a good laugh too.

TrickyDicky, you are indeed living up to your name. "Out of context" ? Please.

 

[DaveC426913]

TrickyDicky, members of 6 years with 9000+ posts are not trolls, pretty much by definition, but regardless, we don't use that word here; it is an infractible offense and a personal attack.

If you have a beef with the argument, attack the argument, not the arguer. You'll notice right up until your trolling comments, everyone else was doing you the courtesy targeting your arguments, not you personally. Let's keep it that way.

 

[Dale]

It's OK, I understand his irritation with me, and I don't take offense at it. He changed his mind to agree with me, but instead of accepting it graciously and allowing him to save face I emphasized it and laughed at him. I provoked him, and I hope no infractions result.

 

[Snip3r]

You can only measure the round-trip speed of light

a quick question why can't 1-way speed of light be measured?

 

[Dale]

a quick question why can't 1-way speed of light be measured?

In order to measure the one-way speed of light requires two synchronized clocks. In order to synchronize the clocks you have to adopt some synchronization convention. Your measurement of the one-way speed of light then depends on the synchronization convention you have chosen, so you measure whatever number you chose to measure.

 

[TrickyDicky]

It's OK, I understand his irritation with me, and I don't take offense at it. He changed his mind to agree with me, but instead of accepting it graciously and allowing him to save face I emphasized it and laughed at him. I provoked him, and I hope no infractions result.

If it makes you happy to think that...lol oh, you enjoy provoking people eh noughty boy?, yeah, you got such power to make me change my mind to agree with you whenever you want.
Anyway why should you take offence if I just described your behaviour wrt a particular post, no personal attack involved at all. I'm sure you and your defenders are honourable men. (well maybe honourable dog in phinds case).

 

[Dale]

If it makes you happy to think that...lol oh, you enjoy provoking people eh noughty boy?, yeah, you got such power to make me change my mind to agree with you whenever you want.

Are you saying you didn't change your mind from

there has to be some absolute rest

in post 18 and emphasized in posts 22, 26, and 27 to

there is obviously no absolute rest

in post 31?

 

[TrickyDicky]

Do you mean you didn't read #34 yet?

And why don't you quote the complete phrase?

You are getting boring, I won't respond to any more of your instigations, don't find it fun anymore.

 

[Dale]

Do you mean you didn't read #34 yet?

If you had just posted 18 and 34 then I would have chalked it up to miscommunication, that from the beginning you had meant "it appears natural but it is wrong".

But in your followup posts (especially 22 and 26) you went far beyond that and explicitly stated that it didn't just "appear natural" but that logically "one implies the other" and "it is in the postulates". If anything your emphasis of the "seems natural" comment is out of context wrt the rest of your comments.

And why don't you quote the complete phrase?

For effect, as you already realized and I already agreed. It isn't misrepresenting your comments in any way, so I picked the most effective quotes.

I won't respond to any more of your instigations, don't find it fun anymore.

I am not surprised you don't find it fun anymore. I wouldn't either if I were in your position. But that is why, when I make a mistake or change my mind during a discussion, I usually admit it unambiguously. I do that to preempt anyone who would point out the change at my expense.

 

[phinds]

DaleSpam, your patience and equanimity amaze me. Thanks for setting a good example.

 

[Dale]

Thanks, although if I were really to be a good example I do feel like I shouldn't have rubbed it in his face in post 35. I am sad that he was banned or quit; I hope it is temporary.

 

[phinds]

Thanks, although if I were really to be a good example I do feel like I shouldn't have rubbed it in his face in post 35. I am sad that he was banned or quit; I hope it is temporary.

See that's why you're a good example. You're sad he was banned. I found him so offensive I'm glad he was banned. I just have no patience with his kind of behavior even though I sometimes have to struggle to keep myself from doing similar things, although I don't think even in my worst moments I'd call another forum member a dog, and I have no difficulty in admitting when I'm wrong.

 

[my_wan]

I didn't take TrickyDicky's quoted text to be contradictory in the context given. It seemed to me that if you take the quotes in the context given as contradictory implies relativity is contradictory. In fact I'll search up a quote from Einstein's book making a major point of saying that in the context of GR the constancy of light cannot be considered an "absolute" constant. Simply choosing to apply "absolute" in the sense certain fringe interpretations use it as a beating stick on relativity isn't necessarily valid. The speed of light is itself not an absolute constant, it is an observationally bound constant.

It is also possible to measure the one way speed of light, though I know of no explicit examples of it actually being done. Simply take a rotating hollow disk with holes on opposite side and note the RPM ranges in which a very short flash of light makes it through the disk to be detected, or some variation thereof.