No redshift in a freely falling frame

[stevendaryl]

Repeating the same error ad nauseaum doesn't make it right. Your so-called "counter-example" has the source and the emitter at rest wrt each other.

Whether two objects are at rest wrt each other is a COORDINATE-DEPENDENT fact. In Rindler coordinates, two clocks at different values of the X coordinate are at rest relative to one another. In inertial coordinates, they are not at rest relative to one another.

You are desperately trying to prove that the method does not apply when the emitter and the detector are moving wrt each other

It clearly doesn't. You know that's the case. If the receiver and the sender are at the SAME height, and are moving relative to one another, then there will be a nonzero redshift. The redshift formula in that case is not the same as the position-dependent gravitational time dilation formula. I can't believe you're disputing that.

(you changed the goal posts when I showed you that the method works when there is no relative motion). The GPS calculations , as posted by Ashby, disprove your statement.

No, they DON'T. They are in complete agreement. What is true is that the Schwarzschild relative clock rate calculation gives the same answer as the redshift calculation in the case where the sender and receiver are stationary in the Schwarzschild coordinates. If they are NOT stationary in the Schwarzschild coordinates, then there is an additional Doppler effect that must be taken into account. Are you seriously disputing this?

 

[GAsahi]

. What is true is that the Schwarzschild relative clock rate calculation gives the same answer as the redshift calculation in the case where the sender and receiver are stationary in the Schwarzschild coordinates.

Good, you finally got this right despite multiple previous protestations.

If they are NOT stationary in the Schwarzschild coordinates, then there is an additional Doppler effect that must be taken into account. Are you seriously disputing this?

You are either missing the point or you are trying desperately to move the goalposts. If the source and the receiver are moving wrt each other, the effect is WHOLLY described by using the Schwarzschild solution, Doppler AND gravitational effect all rolled in ONE formula, the one formula derived SOLELY using the Schwarzschild solution. You can find that solution posted in this forum. You seem to be disputing that the solution is valid though it is the standard approach to solving such problems (see the references to Neil Ashby).

 

[stevendaryl]

I showed you how to do the calculations using the Schwarzschild solution for the case of relative motion between source and detector. You do not need any "additional Doppler term", the answer is fully contained in the Schwarzschild solution.

You didn't do the case in which the sender and receiver are at the SAME radius r, and have a relative velocity v in the direction perpendicular to the radius. Your method gives the WRONG answer for this case, if you don't include the Doppler effect.

 

[PeterDonis]

(see the references to Neil Ashby).

GAsahi, I can't find a link in this thread to the Ashby paper you are referencing. Do you mean this one?

http://relativity.livingreviews.org/Articles/lrr-2003-1/

 

[stevendaryl]

You are either missing the point or you are trying desperately to move the goalposts. If the source and the receiver are moving wrt each other, the effect is WHOLLY described by using the Schwarzschild solution, Doppler AND gravitational effect all rolled in ONE formula, the one formula derived SOLELY using the Schwarzschild solution.

No, they're NOT. You are deeply confused about this point. Consider the case in which the sender and the receiver are at the SAME radius r. For example, they are both on the surface of the Earth, on the equator. But they are in relative motion. They are traveling in opposite directions, one traveling east and the other traveling west. One observer sends a signal to the other. Let f₁ be the frequency of the signal as measured by the sender, and let f₂ be the frequency as measured by the receiver.

In this case, the two frequencies will NOT be the same. They will differ by a Doppler shift. How are you proposing to compute that Doppler shift solely using the Schwarzschild metric?

The answer is: you can't. f₁/f₂ is NOT equal to d\tau₁/d\tau₂ in that case.

 

[stevendaryl]

Hm, ok, I need to go back and read your original posts more carefully. However, I'm not sure GAsahi is talking about flat spacetime (but maybe I need to go back and read his original posts more carefully too).

He's not, but flat spacetime is a special case of curved spacetime. If the technique works in general, then it should work in flat spacetime, as well.

This is not correct as you state it; the circumstances are not coordinate-dependent.

The claim that I'm making, which is really an indisputable claim, it's pure mathematics, is that the ratio of two clock rates for distant clocks is a coordinate-dependent quantity. This is easily seen to be true in SR: In the twin paradox, during the outward journey, each twin's clock is running slow, as measured in the coordinate system in which the other twin is at rest. The ratio of two clock rates is a coordinate-dependent quantity. It's true in SR, and it doesn't become less true in GR.

It would be better if you stated these conditions in coordinate-free terms, which can be done:

(1) The spacetime has a timelike Killing vector field;

(2) The sender and receiver's worldlines are both orbits of the timelike Killing vector field.

That should make it clear that the conditions you are talking about depend on particular properties of the spacetime and the worldlines, but *not* on coordinates; the mathematical description of the conditions looks simpler in Schwarzschild coordinates (or Rindler in flat spacetime), but that doesn't mean it's only "true in" those coordinates.

My point is that there are two different ratios to compute:

(1) The ratio f₁/f₂ of a light signal sent from one observer to another, where f₁ is the frequency as measured by the sender, and f₂ is the frequency as measured by the receiver.

This quantity is completely independent of coordinates, and you can calculate it using whatever coordinates you like.

(2) The ratio R₁/R₂ of clock rates for the clocks of the two observers.

This quantity is coordinate-dependent. If you use different coordinates, you get a different ratio.

Specifically, R₁ = d\tau/dt = √(g_αβ dx^α/dt dx^β/dt. This rate has different values in different coordinate systems.

What's special about Schwarzschild coordinates (or Rindler coordinates) is that ratio (2) is equal to ratio (1) for those coordinates, but not for other coordinates.

You are right, that if there is a Killing vector field, then we can come up with a corresponding ratio by defining R₁ = d\tau/dt, where dt is the timelike Killing vector, instead of a coordinate. In that case, R₁ is no longer coordinate-dependent.

 

[GAsahi]

GAsahi, I can't find a link in this thread to the Ashby paper you are referencing. Do you mean this one?

http://relativity.livingreviews.org/Articles/lrr-2003-1/

yes,of course.

 

[PeterDonis]

The claim that I'm making, which is really an indisputable claim, it's pure mathematics, is that the ratio of two clock rates for distant clocks is a coordinate-dependent quantity.

For one definition of "ratio of two clock rates", yes this is true. But there are other possible definitions.

This is easily seen to be true in SR: In the twin paradox, during the outward journey, each twin's clock is running slow, as measured in the coordinate system in which the other twin is at rest.

Yes, but when the twins come together, the traveling twin has experienced less elapsed proper time, which of course is *not* a coordinate-dependent statement. So if I define "ratio of two clock rates" in terms of elapsed proper time between some pair of events common to both worldlines, then the ratio is not coordinate-dependent.

Of course, if the two worldlines don't cross, there won't be a pair of events common to both worldlines. But there may still be a coordinate-independent way to pick out "common" events on both worldlines. For example, if the spacetime has a timelike Killing vector field which is hypersurface orthogonal (as Schwarzschild spacetime does), I can pick two spacelike hypersurfaces orthogonal to the Killing vector field and say that the "common" events on each worldline are the events where the worldlines intersect the two surfaces. This, of course, is a roundabout way of saying "pick the events on each worldline with Schwarzschild coordinate times t1 and t2", but you'll note that I've stated it in a coordinate-independent way. I could do the same thing for a pair of Rindler observers with non-intersecting worldlines.

In a sense all these choices of "common events" are arbitrary; but they do match up with particular symmetries of the spacetime, so they're not completely arbitrary. They do have some coordinate-independent physical meaning.

(1) The ratio f₁/f₂ of a light signal sent from one observer to another, where f₁ is the frequency as measured by the sender, and f₂ is the frequency as measured by the receiver.

This quantity is completely independent of coordinates, and you can calculate it using whatever coordinates you like.

Yes, agreed.

(2) The ratio R₁/R₂ of clock rates for the clocks of the two observers.

This quantity is coordinate-dependent. If you use different coordinates, you get a different ratio.

This depends, as above, on how you define "relative clock rates". You note this as well, since you agree that we could use a timelike Killing vector field as the "dt" in R1.

What's special about Schwarzschild coordinates (or Rindler coordinates) is that ratio (2) is equal to ratio (1) for those coordinates, but not for other coordinates.

Didn't you point out that there are some pairs of observers (such as two observers on Earth's equator but at opposite points) for whom ratio (1) different from ratio (2) even in Schwarzschild coordinates? Perhaps what you meant to say is that ratio (1) is equal to ratio (2) for observers who are *static* in these coordinates?

It may also be worth noting that Schwarzschild coordinates and Rindler coordinates both have a Killing vector field as "dt", so the two definitions of R1 amount to the same thing in those coordinates.

 

[GAsahi]

In this case, the two frequencies will NOT be the same. They will differ by a Doppler shift. How are you proposing to compute that Doppler shift solely using the Schwarzschild metric?

Easy, as already explained for the case of radial motion:

\frac{d \tau_1^2}{d \tau_2^2}=\frac{1-r_s/r_1}{1-r_s/r}\frac{1}{1-\frac{v^2}{(1-r_s/r)^2}}

The first factor represents the "gravitational redshift" component, the second factor (speed dependent) represents the "Doppler" component.

I can easily do that for the case of circular motion but I will leave that as an exercise for you. Hint: you use the fact that dr=0 and you use the full Schwarzschild metric (you do not drop the rotational term in d \theta.

The answer is: you can't. f₁/f₂ is NOT equal to d\tau₁/d\tau₂ in that case.

I frankly do not understand how you got this bee under your bonnet.

 

[stevendaryl]

The answer is: you can't. f₁/f₂ is NOT equal to d\tau₁/d\tau₂ in that case.

I should elaborate on what I mean by that:
What I thought was being proposed was that a way to compute
f₁/f₂
is the following:

The Schwarzschild metric:

d\tau² = (1-r/r_s) dt² - 1/(1 -r/r_s) dr² - r² dθ²

So let one observer be at "rest" at r=R. Then we have for that observer:
d\tau₁ = √(1-R/r_s) dt

Let the other observer be also at r=R, moving at speed v along the θ direction; that is R dθ/dt = v. Then we have:
d\tau₂ = √(1-R/r_s - v²) dt

Now, my claim is that d\tau₁/d\tau₂ will NOT give the correct redshift for signals sent from the first observer to the second observer.

 

[GAsahi]

I should elaborate on what I mean by that:
What I thought was being proposed was that a way to compute
f₁/f₂
is the following:

The Schwarzschild metric:

d\tau² = (1-r/r_s) dt² - 1/(1 -r/r_s) dr² - r² dθ²

So let one observer be at "rest" at r=R. Then we have for that observer:
d\tau₁ = √(1-R/r_s) dt

Correct.

Let the other observer be also at r=R, moving at speed v along the θ direction; that is R dθ/dt = v. Then we have:
d\tau₂ = √(1-R/r_s - v²) dt

Correct.

Now, my claim is that d\tau₁/d\tau₂ will NOT give the correct redshift for signals sent from the first observer to the second observer.

Why not? This is the bee under your bonnet that you keep repeating with no formal justification.

 

[PeterDonis]

yes,of course.

Well, I suppose it's true that the paper wasn't hard to find, but despite the "of course", it's helpful to post an explicit link, to be sure there's no question about what you are referring to.

\frac{d \tau_1^2}{d \tau_2^2}=\frac{1-r_s/r_1}{1-r_s/r}\frac{1}{1-\frac{v^2}{(1-r_s/r)^2}}

Can you point out where in Ashby's paper you are deriving this from?

I can easily do that for the case of circular motion but I will leave that as an exercise for you. Hint: you use the fact that dr=0 and you use the full Schwarzschild metric (you do not drop the rotational term in d \theta.

There is no "rotational term in d \theta" in the Schwarzschild metric. If you are referring to the metric in Ashby's paper, that metric is *not* "the Schwarzschild metric". It uses features of the Schwarzschild metric, but it's not the same thing. Also, there is more than one metric referred to in Ashby's paper; if there are particular equations in Ashby's paper that you are using, it would be helpful to give explicit references.

 

[stevendaryl]

Easy, as already explained for the case of radial motion:

\frac{d \tau_1^2}{d \tau_2^2}=\frac{1-r_s/r_1}{1-r_s/r}\frac{1}{1-\frac{v^2}{(1-r_s/r)^2}}

The first factor represents the "gravitational redshift" component, the second factor (speed dependent) represents the "Doppler" component.

I thought you were saying that there was no Doppler component! So maybe we are on the same page now. The ratio of clock rates is given by the "gravitational redshift" component. This is NOT equal to the observed redshift, except in the special case in which the sender and receiver are at rest (no Doppler component).

So you cannot compute redshifts by simply taking a ratio of "clock rates", which is what I thought you were claiming.

 

[stevendaryl]

There is no "rotational term in d \theta" in the Schwarzschild metric.

Sure there is. In all its gory detail, the Schwarzschild metric is:

d\tau² =
(1-2GM/(c²r)) dt²
- 1/(1-2GM/(c²r) dr²/c²
- r²/c² d\theta²
- r² sin²(\theta)/c² d\phi²

 

[PeterDonis]

There is no "rotational term in d \theta" in the Schwarzschild metric.

I hadn't read the Word document GAsahi posted earlier when I wrote this; having looked at it, I see that by "rotation term" he meant what would usually be called the "angular term". Sorry for the confusion.

 

[GAsahi]

I thought you were saying that there was no Doppler component! So maybe we are on the same page now.

The whole thing started with your claim that my derivation is not correct. The formulas were posted since the beginning of this thread. Are we done now?

 

[GAsahi]

Can you point out where in Ashby's paper you are deriving this from?

I derived it myself, at the beginning of this thread, here.

 

[stevendaryl]

Why not? This is the bee under your bonnet that you keep repeating with no formal justification.

At first, I was not clear whether there was just some miscommunication going on, or there was a serious error of understanding on your part. This shows me that, whatever miscommunication there may have been, you are mistaken about some fundamental facts about GR and SR and Doppler shift.

Try taking the limit as r_s → 0 and v << c. Then the redshift formula should reduce to the nonrelativistic Doppler shift formula.

Your way of doing things would give:
d\tau₁ = dt
d\tau₂ = √(1-(v/c)²) dt
So the ratio d\tau₁/d\tau₂ gives
1/√(1-(v/c)²)

That's NOT correct. It should be, instead
√((1-v/c)/(1+v/c))

 

[stevendaryl]

The whole thing started with your claim that my derivation is not correct.

Your derivation is certainly not correct, even though it happens to give the right answer in a specific case. In the case of two observers at the same radius R, moving away from each other at speed v, your derivation gives the wrong answer.

 

[GAsahi]

At first, I was not clear whether there was just some miscommunication going on, or there was a serious error of understanding on your part. This shows me that, whatever miscommunication there may have been, you are mistaken about some fundamental facts about GR and SR and Doppler shift.

Try taking the limit as r_s → 0 and v << c. Then the redshift formula should reduce to the nonrelativistic Doppler shift formula UNLESS one replaces v with its function of the relative speed between source and receiver.

Your way of doing things would give:
d\tau₁ = dt
d\tau₂ = √(1-(v/c)²) dt
So the ratio d\tau₁/d\tau₂ gives
1/√(1-(v/c)²)

That's NOT correct. It should be, instead
√((1-v/c)/(1+v/c))

In my formula \frac{d \tau_1^2}{d \tau_2^2}=\frac{1-r_s/r_1}{1-r_s/r}\frac{1}{1-\frac{v^2}{(1-r_s/r)^2}}
v=\frac{dr}{dt}, so v is NOT the relative speed between source and receiver. This is why the above formula will not reduce to the relativistic Doppler formula.

On the other hand, in the formula \sqrt{\frac{1-v/c}{1+v/c}} v IS the speed of the receiver wrt the source (see my .doc attachment on the subject. Though you are using the symbol "v" in both cases, the meaning is different. We have been over this before.
If you are fixated on finding an "error" in my derivation , you would do best to ask the meaning of the variables first.

 

[stevendaryl]

If you are fixated on finding an "error" in my derivation , you would do best to ask the meaning of the variables first.

I gave you a formula for d\tau₁/d\tau₂ for the special case in which the first observer is at the constant value of r=R, and constant \theta and \phi, the second observer is at the constant value of r=R, but is traveling so that R d\theta/dt = v. I thought you AGREED with that formula. You certainly said that you did.

I said: "Now, my claim is that dτ₁/dτ₂ will NOT give the correct redshift for signals sent from the first observer to the second observer."

You replied: "Why not?"

Which to me seemed to indicate that you thought it WOULD give the correct redshift formula. Do you now agree that it doesn't?

 

[GAsahi]

I gave you a formula for d\tau₁/d\tau₂ for the special case in which the first observer is at the constant value of r=R, and constant \theta and \phi, the second observer is at the constant value of r=R, but is traveling so that R d\theta/dt = v. I thought you AGREED with that formula. You certainly said that you did.

I said: "Now, my claim is that dτ₁/dτ₂ will NOT give the correct redshift for signals sent from the first observer to the second observer."

You replied: "Why not?"

Which to me seemed to indicate that you thought it WOULD give the correct redshift formula. Do you now agree that it doesn't?

I am done wasting my time.

 

[stevendaryl]

Yes, but when the twins come together, the traveling twin has experienced less elapsed proper time, which of course is *not* a coordinate-dependent statement. So if I define "ratio of two clock rates" in terms of elapsed proper time between some pair of events common to both worldlines, then the ratio is not coordinate-dependent.

Well, that's kind of a funny way to define "rate". By that definition, the rate is undefined for clocks that DON'T get back together eventually.

But anyway, if we don't get hung up on word choice, I think that you agree that
the instantaneous ratio (d\tau₁/dt)/(d\tau₂/dt) is always defined for any coordinate system, but is coordinate-dependent.

Of course, if the two worldlines don't cross, there won't be a pair of events common to both worldlines. But there may still be a coordinate-independent way to pick out "common" events on both worldlines. For example, if the spacetime has a timelike Killing vector field which is hypersurface orthogonal (as Schwarzschild spacetime does), I can pick two spacelike hypersurfaces orthogonal to the Killing vector field and say that the "common" events on each worldline are the events where the worldlines intersect the two surfaces. This, of course, is a roundabout way of saying "pick the events on each worldline with Schwarzschild coordinate times t1 and t2", but you'll note that I've stated it in a coordinate-independent way. I could do the same thing for a pair of Rindler observers with non-intersecting worldlines.

I would put it a different way, which is that in cases in which there is a special symmetry, there is a "natural" choice for a time coordinate.

Didn't you point out that there are some pairs of observers (such as two observers on Earth's equator but at opposite points) for whom ratio (1) different from ratio (2) even in Schwarzschild coordinates?

Yes, that's why I had my full qualification: If there are coordinates such that (A) the metric components are independent of time, and (B) the sender and receiver are both at rest in that coordinate system, then the ratio of "clock rates" is the same as the redshift for a light signal sent from one to the other. If (A) or (B) fails, then they won't be the same.

It may also be worth noting that Schwarzschild coordinates and Rindler coordinates both have a Killing vector field as "dt", so the two definitions of R1 amount to the same thing in those coordinates.

That was part of my point (Isn't "there is a timelike Killing vector field" and "there is a coordinate system in which the components of the metric are time-independent" the same thing? Is there some circumstance in which one holds and not the other). For specific coordinates and for "stationary" observers in that coordinate system, you can compute a ratio of "clock rates" and get the same answer as the redshift formula, but not in other coordinates or for other observers. For example, in the case of an accelerating rocket with clocks in the front and rear, if you use the inertial coordinates of the "launch" frame, then the ratio of clock rates will not be the same as the redshift.

 

[stevendaryl]

I am done wasting my time.

I can't tell whether that means that you now realize that d\tau₁/ d\tau₂ is not equal to f₁/f₂, or still think that they're always the same.

 

[GAsahi]

I can't tell whether that means that you now realize that d\tau₁/ d\tau₂ is not equal to f₁/f₂, or still think that they're always the same.

You need to get your ratios right:

\frac{d \tau_1}{d \tau_2}=\frac{f_2}{f_1}

 

[PeterDonis]

I derived it myself, at the beginning of this thread, here.

Ah, I see. But this derivation only holds for purely radial motion; in fact it only holds when one object is static at a constant height and the other is moving purely radially. The case stevendaryl talks about, as here...

In the case of two observers at the same radius R, moving away from each other at speed v, your derivation gives the wrong answer.

...is more general. You gave a "hint" as to how a more general formula could be derived, but you haven't actually done the derivation. (A fully general formula would need to include the effects of motion, both radial and non-radial, for *both* objects, not just one.)

Also, as you noted in a later post, what you call "v" in your formula is a coordinate velocity, not an actual observed relative velocity.

Finally, since you mentioned Ashby's paper, it's worth noting that your derivation uses Schwarzschild coordinates, but his paper does not. He uses Earth Centered Inertial (ECI) coordinates, which differ from Schwarzschild coordinates in several important respects:

(1) They are isotropic;

(2) The coordinate time dt is scaled to the rate of time flow of observers on the "geoid" (the equipotential surface at "sea level") who are at rest relative to the actual Earth (i.e., rotating with it); however, the simultaneity convention is that of hypothetical inertial observers moving with the Earth's center of mass but *not* rotating with it (which would be the simultaneity convention of Schwarzschild coordinates centered on the Earth);

(3) The "potential" \Phi_{0} in the metric, which is the potential on the "geoid", includes not only the effect of the Earth's rotation, but also includes corrections for the Earth's quadrupole moment, so it differs in two ways from the Schwarzschild potential.

These differences don't affect the general points under discussion, but since you referenced Ashby's paper, I think it's worth pointing out the ways in which his equations and notation differ from those being used in this thread.

 

[GAsahi]

Ah, I see. But this derivation only holds for purely radial motion; in fact it only holds when one object is static at a constant height and the other is moving purely radially.

1. stevendaryl denied for the longest time that it is correct, resulting into a monumental waste of time

2. the derivation is easy to extend to more complex cases, like the one of a stationary receiver and a rotating transmitter, the methodology is the same.

3. the derivation extends easily to other metrics (as in the Ashby paper). The point is that the methodology is STANDARD and that it produces predictions that are confirmed by experiment (Pound Rebka for radial motion, GPS for more complex motion, etc)

...is more general. You gave a "hint" as to how a more general formula could be derived, but you haven't actually done the derivation. (A fully general formula would need to include the effects of motion, both radial and non-radial, for *both* objects, not just one.)

...because the derivation is trivial

These differences don't affect the general points under discussion, but since you referenced Ashby's paper, I think it's worth pointing out the ways in which his equations and notation differ from those being used in this thread.

Agreed. The point was to show stevendaryl that the applied methodology is STANDARD.

 

[stevendaryl]

You need to get your ratios right:
\frac{d \tau_1}{d \tau_2}=\frac{f_2}{f_1}

My mistake, but either way, it's incorrect in the case that I was talking about, namely two observers, at the equator, moving at relative speed v in opposite directions (east for one, west for the other). In that case
\frac{d \tau_1}{d \tau_2} = 1.
\frac{f_2}{f_1} = √((1-v/c)/(1+v/c))

 

[stevendaryl]

1. stevendaryl denied for the longest time that it is correct, resulting into a monumental waste of time

No, I said that your DERIVATION was wrong. From the very beginning, I said that it was your method that was incorrect, not the result. It happens to give the right answer in one situation, but not in other situations.

 

[PeterDonis]

Agreed. The point was to show stevendaryl that the applied methodology is STANDARD.

I agree, but it's worth noting that part of what is standard is the adoption of particular coordinates--you used Schwarzschild coordinates, the Ashby paper used ECI coordinates (which seem to be the "standard" for these kinds of computations). That includes adopting a particular simultaneity convention, which is crucial for defining the time differentials and intervals that appear in the equations. I agree it's a "natural" choice of simultaneity convention for the purpose, but it's still a specific choice.