A question about relativity

[Alan Grayson]

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

[Alan Grayson]

On Wednesday, January 29, 2020 at 1:57:25 AM UTC-7, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

Obviously, the temperature of the CMBR declines exceedingly slowly, making it an inconvenient clock, but it's still a clock, making the question above sensible. AG 

[smitra]

It's equivalent to a purely SR problem with 3 observers, two of them
moving away with opposite velocities from the third.

Saibal

[Alan Grayson]

TY, but I can't relate your comment to the issue I've raised. AG 

[Brent Meeker]

The idea that some clocks run slower than others is a confusion.  Talk of clocks in general relativity always refers to ideal clocks that, by definition, run at identical rates when compared at the same place.  "Running slow" really refers to taking a shorter path (less elapsed proper time) thru spacetime, as reflected in the metric.  As AG noted the "running slow" relation is symmetric; so it can't be invariant.

Using the CMB is an operational way to define a global time.  It is the same as co-moving coordinates in which matter is, on average, stationary.  But it is a good/useful coordinate system because it makes the representation of an FLRW model simple.  There's an implicit assumption that the universe is homogenous and isotropic, which implies that it satisfies an FLRW model.  With that assumption a measurement of curvature locally can be extended to infer the whole spacetime.  Space can be flat while spacetime is curved, so as to be open or closed.

Brent

On 1/29/2020 12:57 AM, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

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[Alan Grayson]

On Wednesday, January 29, 2020 at 12:18:40 PM UTC-7, Brent wrote:

The idea that some clocks run slower than others is a confusion.  Talk of clocks in general relativity always refers to ideal clocks that, by definition, run at identical rates when compared at the same place.  "Running slow" really refers to taking a shorter path (less elapsed proper time) thru spacetime, as reflected in the metric.  As AG noted the "running slow" relation is symmetric; so it can't be invariant.

Using the CMB is an operational way to define a global time.  It is the same as co-moving coordinates in which matter is, on average, stationary.  But it is a good/useful coordinate system because it makes the representation of an FLRW model simple.  There's an implicit assumption that the universe is homogenous and isotropic, which implies that it satisfies an FLRW model.  With that assumption a measurement of curvature locally can be extended to infer the whole spacetime.  Space can be flat while spacetime is curved, so as to be open or closed.

Brent

Can you answer the question? If we have two clocks at the distant galaxy; some observer's clock which is running slower compared to a local clock in this galaxy, and the CMBR clocks at every location in the universe which are synchronized, what is the status of time dilation? Do it exist or not? TIA, AG 

On 1/29/2020 12:57 AM, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

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[Brent Meeker]

On 1/29/2020 7:21 PM, Alan Grayson wrote:

On Wednesday, January 29, 2020 at 12:18:40 PM UTC-7, Brent wrote:

The idea that some clocks run slower than others is a confusion.  Talk of clocks in general relativity always refers to ideal clocks that, by definition, run at identical rates when compared at the same place.  "Running slow" really refers to taking a shorter path (less elapsed proper time) thru spacetime, as reflected in the metric.  As AG noted the "running slow" relation is symmetric; so it can't be invariant.

Using the CMB is an operational way to define a global time.  It is the same as co-moving coordinates in which matter is, on average, stationary.  But it is a good/useful coordinate system because it makes the representation of an FLRW model simple.  There's an implicit assumption that the universe is homogenous and isotropic, which implies that it satisfies an FLRW model.  With that assumption a measurement of curvature locally can be extended to infer the whole spacetime.  Space can be flat while spacetime is curved, so as to be open or closed.

Brent

Can you answer the question? If we have two clocks at the distant galaxy; some observer's clock which is running slower compared to a local clock in this galaxy, and the CMBR clocks at every location in the universe which are synchronized, what is the status of time dilation? Do it exist or not? TIA, AG

If you're syncing clocks everywhere to the CMB then you've defined a global time.  You've also implicitly defined a space reference frame that's stationary with respect to the CMB.  There will still be relativistic doppler shift between clocks because of the expansion rate of the universe, i.e. if you look at a distant galaxy you will see shifts in its spectral lines and you will be seeing the galaxy as it was in the past.  It's not though that clocks are running slower on that galaxy; rather it's definition of the future direction is different (see my popular lecture explanation of the twin's paradox).

Brent

On 1/29/2020 12:57 AM, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

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[John Clark]

On Wed, Jan 29, 2020 at 10:21 PM Alan Grayson <agrays...@gmail.com> wrote:

> If we have two clocks at the distant galaxy; some observer's clock which is running slower compared to a local clock in this galaxy, and the CMBR clocks at every location in the universe which are synchronized, what is the status of time dilation? Do it exist or not?

It does. Regardless of what the measured temperature of the CMBR is if you're moving relative to your twin brother then you will observe that his local clock is running slower than your local clock and he will observe that your local clock is running slower than his local clock. And it's not just clocks that are affected, minds are too. You will see that it takes your brother longer to solve a long division problem than it takes you and he will observe that it takes you longer to solve it than it takes him. And none of this has anything to do with the temperature of the CMBR, the only important thing that affects local clocks is the relative motion between you and your twin brother.

The reason this is odd but not a paradox is because of the relativity of simultaneity which Dr. Don Lincoln made clear in the 3 videos I recommended yesterday which you obviously didn't watch. If you're still interested Wikipedia has a rather good article on the subject:

Relativity of simultaneity

And I'll recommend yet another video although I doubt your attention span is long enough to watch all of it, it is after all nearly 4 minutes long:

Relativity of Simultaneity | Special Relativity Ch. 4

Or just ask Google to search for "Simultaneity" and have your pick of lots of good articles and videos.

John K Clark

[Alan Grayson]

Now you seem retarded. Every galaxy observer can use the CMB clock, and they ALL read the SAME temperature! NO TIME DILATION! AG  

[Alan Grayson]

On Thursday, January 30, 2020 at 6:22:13 AM UTC-7, John Clark wrote:

On Wed, Jan 29, 2020 at 10:21 PM Alan Grayson <agrays...@gmail.com> wrote:

> If we have two clocks at the distant galaxy; some observer's clock which is running slower compared to a local clock in this galaxy, and the CMBR clocks at every location in the universe which are synchronized, what is the status of time dilation? Do it exist or not?

It does. Regardless of what the measured temperature of the CMBR is if you're moving relative to your twin brother then you will observe that his local clock is running slower than your local clock and he will observe that your local clock is running slower than his local clock. And it's not just clocks that are affected, minds are too. You will see that it takes your brother longer to solve a long division problem than it takes you and he will observe that it takes you longer to solve it than it takes him. And none of this has anything to do with the temperature of the CMBR, the only important thing that affects local clocks is the relative motion between you and your twin brother.

 

Relative motion? I would never have guessed! Why not consider the CMBR as the local clock for each galaxy? What then? Does AE precisely define a "clock"? AG 

The reason this is odd but not a paradox is because of the relativity of simultaneity which Dr. Don Lincoln made clear in the 3 videos I recommended yesterday which you obviously didn't watch. If you're still interested Wikipedia has a rather good article on the subject:

Relativity of simultaneity

And I'll recommend yet another video although I doubt your attention span is long enough to watch all of it, it is after all nearly 4 minutes long:

Why the fuck should I view it? It has nothing to do with the problem I posed. Obviously, you have no clue. Oh, one other thing; why is it that everything that CAN happen, MUST happen? Why do you refuse to answer this question? Oh, I know; you have no clue. End of story. AG

[Lawrence Crowell]

On Wednesday, January 29, 2020 at 2:57:25 AM UTC-6, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

The physics with distant galaxies is general relativistic, not special relativity. The redshift factor v = Hd, in the near linear form, has the redshift factor v/c = z = Hd/c. In the FLRW metric this is a bit more general with z = e^{Ht} - 1, where for small HT << 1 then t = d/c and z =~ Ht. The reshift factor for the CMB is z = 1100, which means that anIR photon with wavelength 1000nm is expanded to 1100 microns, or a millimeter. The peak of the CMB blackbody radiation is 160 GHz and this was produced by radiation peaked at 17.6x10^{4}GHz. This is in the IR region with a wavelength of 5,87x10^{-5}cm, in the IR, The z multiplicative factor is the same as a time dilation, where we can think of these red shifted photons are representing the slowdown of clocks (clocks being the quantum oscillations of atoms etc) in this surface of last opaque scatter.

LC

[Alan Grayson]

On Thursday, January 30, 2020 at 10:16:48 AM UTC-7, Lawrence Crowell wrote:

On Wednesday, January 29, 2020 at 2:57:25 AM UTC-6, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

The physics with distant galaxies is general relativistic, not special relativity.

I know. Now, if you can, please answer my question. AG

[Lawrence Crowell]

On Thursday, January 30, 2020 at 11:21:25 AM UTC-6, Alan Grayson wrote:

On Thursday, January 30, 2020 at 10:16:48 AM UTC-7, Lawrence Crowell wrote:

On Wednesday, January 29, 2020 at 2:57:25 AM UTC-6, Alan Grayson wrote:

Considering the distant galaxies, they're receding at near light speed. So according to SR, their clocks should be ticking at a much slower rates than, say, a local clock in our galaxy. OTOH, there's a physical clock for the entire universe; namely, the temperature of the CMBR. If we tell time by this clock, all clock readings of all galaxies are identical. So which is it? Are clocks in distant galaxies running slower than a local clock in our galaxy, or are both clocks running at the same rate? TIA, AG

The physics with distant galaxies is general relativistic, not special relativity.

I know. Now, if you can, please answer my question. AG

I did below

LC

[Alan Grayson]

Maybe I was making the wrong assumption; namely, that the CMBR "clock" reads the same "time" for the far galaxy as compared to its reading in our galaxy. But this is probably wrong since CMBR as viewed from the far galaxy is from a much earlier epoch, so the reading cannot be identical. Do you agree? AG

[Alan Grayson]

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

[Brent Meeker]

We've defined "the same time" as when they see the same temperature.  But if they look at one another's clocks it appears that other clocks are running slow.  You could create a grid of clocks that stayed at the same distance from you (by radar measure) and they would appear to run the same speed as your clock...but they would be moving rapidly relative to the matter near them and the CMB.

Brent

[Brent Meeker]

"Reads the same time" when?, is the relevant question...and there's no absolute answer.  If you take the CMB as your clock, you are defining what "simultaneous at distant points" means.  Simultaneity is a relative attribute.  It's relative to state of motion in special relativity and it's relative to motion through curved spacetime in general relativity...so you can define it in different ways.

Brent

But this is probably wrong since CMBR as viewed from the far galaxy is from a much earlier epoch, so the reading cannot be identical. Do you agree? AG

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[Brent Meeker]

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

[Alan Grayson]

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

[Alan Grayson]

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG 

[Alan Grayson]

IOW, since the CMBR clock reading is the same everywhere, this strongly suggests that by THIS clock, there is no time dilation. AG 

[Brent Meeker]

On 1/30/2020 5:29 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t,

You're missing the point.  "At any time t" is defined as when the CMB has a certain temperature.  It's the same everywhere at time t, because that's how we defined t.  Our model (FLRW) says the expansion is uniform, so that's the same proper time from the Big Bang.  But it's not the time that photons we're seeing now left the distant galaxy; the photons we would use to read a distant clock and see that it runs slow.

we can in principle determine if the two measurements are simultaneous or not. AG 

In relativity, simultaneity is not an objective property.  It's motion and coordinate dependent.  So think of t as just one of four numbers, (t x y z) used to label a spacetime event.  Then we choose our labeling system to make our equations look simple.  But the labels don't have physical significance.

Brent

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[Brent Meeker]

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

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[Alan Grayson]

On Thursday, January 30, 2020 at 10:37:13 PM UTC-7, Brent wrote:

On 1/30/2020 5:37 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 6:29:18 PM UTC-7, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

Given that the temperature of the CMBR is the same for every location in space-time, it follows that time dilation is not a property of THIS clock. For this clock, which is NOT moving through space-time, paths through space-time are irrelevant. AG 

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[Lawrence Crowell]

On Friday, January 31, 2020 at 2:04:00 AM UTC-6, Alan Grayson wrote:

On Thursday, January 30, 2020 at 10:37:13 PM UTC-7, Brent wrote:

On 1/30/2020 5:37 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 6:29:18 PM UTC-7, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

Given that the temperature of the CMBR is the same for every location in space-time, it follows that time dilation is not a property of THIS clock. For this clock, which is NOT moving through space-time, paths through space-time are irrelevant. AG 

You are making this a whole lot more difficult. The time dilation associated with red shift of radiation is uniform out to a certain distance of around 46 billion light years. It is fairly uniform to within 10^{-5} in isotropy.

LC 

[John Clark]

On Fri, Jan 31, 2020 at 5:59 AM Lawrence Crowell <goldenfield...@gmail.com> wrote:

> The time dilation associated with red shift of radiation is uniform out to a certain distance of around 46 billion light years. It is fairly uniform to within 10^{-5} in isotropy.

And if you have time dilation you've got to also have length contraction if the measured speed of light is to be the same for all observers.

 John K Clark

[Alan Grayson]

Why difficult? I'm just pointing out an inconvenient fact; namely, if you use the CMBR as a clock (inconvenient to be sure since the temperature decline of the CMBR is exceedingly slow), simultaneity for all observers in all galaxies exists to one part in 100,000.  What are the implications? AG

[Lawrence Crowell]

This simultaneity is the Hubble frame.

LC

[Alan Grayson]

On Friday, January 31, 2020 at 11:23:49 AM UTC-7, Lawrence Crowell wrote:

This simultaneity is the Hubble frame.

LC

No Wiki article on the "Hubble frame" and no definition that I can find on Internet. What exactly is it? TIA, AG 

[John Clark]

On Fri, Jan 31, 2020 at 1:18 PM Alan Grayson <agrays...@gmail.com> wrote:

> Why difficult? I'm just pointing out an inconvenient fact; namely, if you use the CMBR as a clock (inconvenient to be sure since the temperature decline of the CMBR is exceedingly slow), simultaneity for all observers in all galaxies exists to one part in 100,000.  What are the implications? AG

If you are heading for the Andromeda Galaxy at 99.999% the speed of light then to you the CMBR would not look even remotely symmetrical, one hemisphere would look much brighter than the other, which would indicate that you and everything in your spaceship, including your clock and your brain, were moving at a very different speed than most of the matter in the universe. But so what? Time dilation would still be in effect, when you used your telescope to look at the Earth (which happens to be moving at a speed closer to the average speed of matter) you'd see things back on Earth were moving at only 0.4472%  the speed they are on your spaceship (assuming 99.999% of light speed). And when observers on Earth look at you they'd see that you and everything on your spaceship were were moving at only 0.4472%  the speed they are on Earth. Both would see the other moving slowly. And none of this has anything whatsoever to do with the CMBR, both see that it takes the other 237 seconds to do things that only takes them one second to do. As I said before this is odd but not a logical paradox because of the disagreement among observers over the meaning of "now". This is explained in more detail in the videos on the Twin Paradox that I recommended yesterday, the ones you refused to look at because you thought they were irrelevant.

John K Clark

[Alan Grayson]

How about telling me something I don't already know, like why MUST everything happen, that CAN happen?  I won't waste time reading your articles. AG 

[Lawrence Crowell]

It is a frame where this gravitational redshift of CMB or equivalently a time dilation of quantum oscillations that emitted this radiation is homogeneous.

LC

[Alan Grayson]

I'll view your article if they give a clear explanation of the breakdown in simultaneity, when each observer sees the (other) traveling clock having a different "now". AG 

[Alan Grayson]

Wouldn't that be everywhere at rest wrt the CMB? AG 

[Alan Grayson]

I'll view your articleS if they give a clear explanation of the breakdown in simultaneity, when each observer sees the (other) traveling clock having a different "now". AG 

The issue of the CMB has nothing to do with the Twin Paradox. I was just postulating the IF the CMB can be used as clock, there seems to be a time defined for the entire universe, as well as absolute rest, contrary to the claims of relativity. AG 

[Brent Meeker]

On 1/31/2020 12:04 AM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 10:37:13 PM UTC-7, Brent wrote:

On 1/30/2020 5:37 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 6:29:18 PM UTC-7, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

Given that the temperature of the CMBR is the same for every location in space-time, it follows that time dilation is not a property of THIS clock.

Time dilation is a property of one clock (or one path) relative to another.  It's called relatvity theory of a reason.

For this clock, which is NOT moving through space-time, paths through space-time are irrelevant. AG

Clocks that aren't moving thru spacetime are stopped.  You're thinking of clocks that aren't moving thru space.

Brent

[Alan Grayson]

On Friday, January 31, 2020 at 7:34:18 PM UTC-7, Brent wrote:

On 1/31/2020 12:04 AM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 10:37:13 PM UTC-7, Brent wrote:

On 1/30/2020 5:37 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 6:29:18 PM UTC-7, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

Given that the temperature of the CMBR is the same for every location in space-time, it follows that time dilation is not a property of THIS clock.

Time dilation is a property of one clock (or one path) relative to another.  It's called relatvity theory of a reason.

My point is that the CMB "clock" exists everywhere, and that it has no relative motion wrt anything, so how can time dilation be applied to it? AG

For this clock, which is NOT moving through space-time, paths through space-time are irrelevant. AG 

Clocks that aren't moving thru spacetime are stopped.  You're thinking of clocks that aren't moving thru space.

I'm thinking about motion through space. That's what the "v" in the Lorentz transformation means; motion through space. AG 

Brent

[Alan Grayson]

I did refer to motion through space-time to demonstrate that your model of space-time motion to explain that time dilation doesn't work in the case of CMB clocks. But time does change for these clocks, however slowly, which are synchronized and located everywhere.  AG

Brent

[Brent Meeker]

Only relative motion is meaningful.  Clocks synced to the CMB are moving relative to one another if the universe is expanding.

Brent

[Lawrence Crowell]

But of course that is the case, where here rest just means isotropic distribution of the CMB radiation. There are small deviations from local motion. This however, does mean there is radial motion of galaxies and ultimately the last ionized surface of scatter away, and this motion away is the same for all observers.

LC

[John Clark]

On Fri, Jan 31, 2020 at 10:47 PM Alan Grayson <agrays...@gmail.com> wrote:

> My point is that the CMB "clock" exists everywhere, and that it has no relative motion wrt anything, so how can time dilation be applied to it? AG

It can't. Nobody said the CMB looks the same for everybody regardless of their motion. It doesn't. But if you and I are in relative motion then I will see my local clock running faster than your local clock, and you will see your local clock running faster than my local clock. And the CMB has absolutely nothing to do with it because Time Dilation is about what local clocks do.

 John K Clark 

[Alan Grayson]

But what if the CMB is the local clock? How could it manifest time dilation, compared to a clock in some moving frame, if its "clock" reading doesn't change? AG 

[Brent Meeker]

There no moving vs stationary frames.  All motion is relative.  If a clock's reading doesn't change, it's stopped.  It's not marking time.  It is easy to observe time dilation of a clock synced to the CMB...just be in motion relative to it.

Brent

[John Clark]

On Sat, Feb 1, 2020 at 7:41 AM Alan Grayson <agrays...@gmail.com> wrote:

>But what if the CMB is the local clock? 

I'm not sure what you mean by that, but if all the hemispheres of the CMB look about the same to you then you'd know you're motion was about the same as the average motion of matter in the universe, if the hemispheres looked radically different then you'd know you were moving at a different speed than most matter in the universe. But so what? If you and I want to compare our local clocks the only relevant factors are our relative speed (Special Relativity) and the relative gravitational fields (General Relativity) we're in, how the CMB looks to either of us is irrelevant.  As Brent said "it's called relativity theory for a reason".

Einstein and even Galileo said if you're in a sealed room moving at a constant velocity you can't tell if you're moving or not, but you don't need to invoke the CMB to know that if you look out a window on a moving train you can see that there is a lot more stuff outside that window than inside the train, and so you could determine you're moving relative to most of the stuff around you. And if I was in a smaller train than you on a parallel track that was moving even faster than you compared to most of the stuff around us then the only thing you would need to know to figure out the time dilation is our relative motion. And both of our local clocks will be different not just from each other but also different from the clock on the station platform.

> How could it manifest time dilation, compared to a clock in some moving frame, if its "clock" reading doesn't change? AG 

I don't understand the question. You never see your local clock rate change, you observe other people's local clock rate change. Everything always seems normal to you, it's other people's clocks that behave oddly.

 John K Clark

[Alan Grayson]

When you use the Lorentz transformation to calculate the slower clock rate in another frame, what you get is the real clock rate in that frame. It's what the other observer measures, even though that observer notices nothing different. IOW, the calculation of the other observer's clock rate is not just an appearance, but what is experienced by the other observer. Now suppose we have an observer moving wrt the CMB, and the other observer at rest wrt the CMB, what I was calling the local clock. The local clock rate never changes, but it should according to relativity, from the pov of the observer in motion wrt the CMB.  AG

[Brent Meeker]

I think it is unfortunate that the idea of time dilation and length contraction was ever introduced.  Just compare time dilation to ordinary Doppler shift.  We don't make a big deal of the oscillator appearing slower when it's going away from us.  We didn't invent a "frequency contraction" and puzzle over it.  We just see it is just a temporal-geometric effect and the oscillator didn't do anything, it didn't slow down or speed up.  When someone measures the frequency of an oscillator they would never attribute the measured value to the oscillator without correcting for Doppler due any relative motion that was present.  Relativistic effects should be looked at the same way.  Time dilation is not a clock slowing down compared to your stationary clock.  It is the relativistic Doppler effect due to the two clocks measuring time in different directions.  It should not be attributed to the clocks, any more than Doppler shift is changing an oscillator.  It's just the paths they take thru spacetime and each one correctly measures duration along their path.  How one looks from a different frame is interesting from the standpoint of instruments and measurements, but that's so you can correct for the spatio-temporal effects of motion and curvature, or you can invert the relation and infer the motion and curvature from the effects.  But it should be kept clear that the motion and curvature are not effecting anything locally, they are only a relative effect of the intervening space and motion.

Brent

[Alan Grayson]

But the doppler effect is apparent only; it's what the observer receiving the signal measures or perceives; not what is reality for, say, the engineer of the passing train. In contrast, IIUC, the LT tells us what the observer in the transformed coordinate system actually measures, and experiences. AG 

[Alan Grayson]

Another problem; using space-time paths, one gets the differential elapsed time for different paths, say for the TP. But the LT, gives the actual clock rate in the transformed frame. I don't think using space-time paths gives this information. AG 

 

[Brent Meeker]

The observer in which transformed system?  The system of the observer who is moving relative to the thing observed?  The Doppler shift tells you what that observer actually measures and experiences.  Do you think the engineer on the train (assuming it's closed) hears  the Doppler shift?  The Lorentz transform tells you what is measured in system by someone in a relatively moving system.  It's exactly analogous.  Having made an observation of a moving system you can use the inverse transformation to predict what observed moving with the system will see.

Brent

[Alan Grayson]

I don't think so. Not an exact analogy. CMIIAW, but the LT gives the observer using the transformation, what the other frame observer will actually measure; its real value. What the doppler effect gives is NOT what the train engineer measures or hears, which doesn't change while he moves with constant velocity, but what an observer external to the train measures, which depends on whether the train is coming toward him or receding. Motion in space-time gives the actual total elapsed time of, say, the traveling twin, at any point along his path, namely, the proper time. AG 

[Brent Meeker]

Sure it does.  The Lorentz transformation transforms a moving path to a stationary path and vice versa.

Brent

[Brent Meeker]

Same as when he measures the frequency of a radio wave.  It's a real value he measures. But it's not a property of the transmitter.  The transmitter isn't affected by this measurement.

What the doppler effect gives is NOT what the train engineer measures or hears, which doesn't change while he moves with constant velocity, but what an observer external to the train measures, which depends on whether the train is coming toward him or receding. Motion in space-time gives the actual total elapsed time of, say, the traveling twin, at any point along his path, namely, the proper time. AG

Right.  And the train engineer hears the actual pitch of his whistle at every point along his track.  And people on other trains really do hear it differently.

But the simpler analogy for the twin paradox is the example of the car odometers I gave earlier.

Brent

[Alan Grayson]

I'm confused concerning your last point. ISTM, that the LT gives the time in transformed frame, or differences between two times in that frame, allowing for the comparison of rates in two frames (the frame applying the LT, and the transformed frame). OTOH, if you have a space-time path, there aren't two frames, only one, and all you can calculate is the proper elapsed time from one point to another along that path in space-time. AG 

[Alan Grayson]

Another issue for me is the breakdown of simultaneity. Although events in one frame aren't necessarily simultaneous in another frame, is it possible, given two frames in relative motion, for each to use the LT to determine the time in the other frame simultaneously, or is this impossible? AG 

[Jesse Mazer]

On Fri, Jan 31, 2020 at 3:04 AM Alan Grayson <agrays...@gmail.com> wrote:

On Thursday, January 30, 2020 at 10:37:13 PM UTC-7, Brent wrote:

On 1/30/2020 5:37 PM, Alan Grayson wrote:

On Thursday, January 30, 2020 at 6:29:18 PM UTC-7, Alan Grayson wrote:

On Thursday, January 30, 2020 at 5:09:56 PM UTC-7, Brent wrote:

On 1/30/2020 12:45 PM, Alan Grayson wrote:

That's not it. I think the two observers, one in a galaxy far removed and one here, would read the same CMBR "time", regardless of the distant galaxy's speed of recession.  But relativity says otherwise. This is what puzzles me. AG

Ask yourself when do they read the same time.

Brent

I don't know if this helps. Since the temperature of the CMBR is the same everywhere, at any time t, we can in principle determine if the two measurements are simultaneous or not. AG  

But regardless of simultaneity or not, there's no dilation of this clock! (And AE doesn't say what a clock is.) What the hell is going on? AG

The clocks used in relativity examples are the whatever the most perfect and stable clock in existence are (in this case cesium atom clocks).  They always measure proper time thru spacetime.  The only reason that when compared they seem to register different durations is because they traveled different paths thru spacetime and these paths had different proper length.  "Time dilation" is not some function of the clock...it's a function of the path the clock is measuring.  Remember my odometer analogy?

Brent

Given that the temperature of the CMBR is the same for every location in space-time

The same for every location in spaceTIME? That would imply that if you had one clock created at the big bang, and an observer next to that clock measured the local CMBR temperature when its own proper time showed an elapsed time of say 2 million years, and then if you had a second clock created at the big bang, and an observer next to that clock measured the local CMBR temperature when its proper time showed an elapsed time of 10 *billion* years, they would show the same temperature--which clearly isn't true. And as long as neither of those two clock readings occurs within the other's past or future light cone, you are perfectly free to construct a cosmological coordinate system where they are both simultaneous (both assigned the same value of the time-coordinate in that coordinate system), in relativity there is no coordinate system whose judgment about simultaneity is considered more objectively correct than any other (though some are certainly more *useful* than others in a pragmatic sense, the coordinate system where surfaces of constant CMBR temperature are also surfaces of simultaneity is widely used in cosmology for that reason).

[Brent Meeker]

It's impossible in the sense that one observers "simultaneous" will not agree with the other.  Here's a spacetime diagram of A who stays on Earth and B who travels away, and C who travels to Earth from far away.   I did this to show the twin paradox without acceleration: If B hands off his elapsed time to C as they pass, then when C passes A he will have clock elapsed time measure less than A.  The tic marks are for equal proper (clock) time along the paths.



Notice that A, using radar (dashed red line), sees that B and C pass at A's time 10, while by counting tics we see that it happened at time 8.5 for B.

Now apply a Lorentz transform to that B is stationary, i.e. the same events in B's reference frame (remember every tic is an event).  B sees the event of passing C at his clock time 8.5; not the same as A's time of 10.  So A and B don't agree on what it simultaneous.  The two events A's tic=10 and B's tic=8.5 are simultaneous in A's frame, but not in B's frame.



Notice that the tics, the clock proper time rates, are the same for all three clocks; meaning that when I transform them so they look stationary their tics are the same.  Compare B's tics in the second graph to A's in the first.  Clocks don't slow down...they just measure time in a different direction.  It's only unintuitive because in Minkowski space closer you get to 45 deg (light speed) the shorter (in proper time) the line is (the fewer tics it has).

Brent

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