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goodabouthood
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People always say the Universe is 14 billions year old.
But what does that mean?
What is the time of the Universe relative to?
But what does that mean?
What is the time of the Universe relative to?
alexg said:It means that the Big Bang took place about 13.8 billion years ago.
lalbatros said: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?
Not strictly true:phinds said: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.
my_wan said: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 said: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 said: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.
goodabouthood said:What does CMB stand for?
Naty1 said:... The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.
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 said: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?
DaleSpam said: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.
Yes:TrickyDicky said:did I say anything contradicting that?
TrickyDicky said:in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest
DaleSpam said:Yes:
TrickyDicky said: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.
No, it does not. Since relativity has one but not the other they clearly do not imply each other.TrickyDicky said:If you think about it a little you'll realize that in abstract terms one thing implies the other.
What is the meaning of the word, tautological?TrickyDicky said:Your statement is just a tautological explanation of what absolute means.
phinds said:Google is a nice tool. You should learn to use it.
goodabouthood said:What does CMB stand for?
DaleSpam said:No, it does not. Since relativity has one but not the other they clearly do not imply each other.
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.TrickyDicky said: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.
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 said:If it has one it has the other, it is in the postulates.
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.ghwellsjr said: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.
DaleSpam said: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.
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.TrickyDicky said: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 said: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?
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.TrickyDicky said: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.DaleSpam said: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.
ghwellsjr said: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.
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."TrickyDicky said:That's not what the special principle of relativity says.
TrickyDicky said:there has to be some absolute rest
:rofl:TrickyDicky said:there is obviously no absolute rest