General Relativity: Gravitational-Red Shift Confused with Doppler Effect?

prime axiom

Okay, if I think about the signal, and not just the photon, seems like the same result.

The signal frequency will change. How could it not? If it passes through spacetime that is warped, it will be stretched or compressed, since it passed through warped spacetime and it took a longer or shorter time to do so.

Does anyone disagree that photon(s) are not effect and changed dynamically as they travel through different areas of warped spacetime?

Jonathan Scott

The effect of gravity on space is indeed like stretching or compressing when mapped to a coordinate system, so waves travel faster and slower, in the same way for example as light is slowed when travelling through a transparent medium such as glass with a refractive index greater than one.

However, if you consider the amount of time it takes a particular event in the signal to travel along the whole path, and you are considering a STATIC situation (where the gravitational sources are not moving and the distance is fixed) then that amount of time is a constant, so the received signal is an identical copy of the transmitted one.

This is no different for example to watching someone wave a flag a long way away; if you are a fixed distance away, the frequency you see is exactly the same as the original frequency at which it happened. However, if your local clocks are running at a different rate because of a difference in gravitational potential, the received frequency according to your local clocks may appear to be different to the transmitted frequency according to a clock associated with the transmitting location.

jartsa

Fast moving clock has a low ticking frequency, and a high Compton frequency. Time dilation does not apply to Compton frequency.

Free falling clock has a decreasing ticking frequency, and a constant Compton frequency.

A clock falling through atmosphere at terminal velocity has ticking frequency and Compton frequency decreasing the same way.

jartsa

Frequency is conserved, in a closed system.

Let us consider a planet-photon system. The system has the same frequency whether the photon is close to the planet, or whether the photon has moved far away from the planet.

The simplest way that the frequency can be conserved is that the frequency of the planet stays the same, and the frequency of the photon stays the same.

Can we prove that the simplest way is the way that it happens?

Austin0

This is a good example. In an accelerating system there is both red shift and motion Doppler. The light radiating from the system in all directions is motion Doppler and is essentially relativistic Doppler, the same as an inertial system.

The equivalence to gravitational red shift is operative internally. With the dilation differential between back and front of the system being equivalent to the varying potential in a G field.

SO light emitted at the back [lower potential] is red shifted when received at the front [higher altitude/potential] and vice verse. And also similarly ,the coordinate speed of light can change with location and direction depending on the coordinate systems. But there is no suggestion that the change in light speed is an actual effect resulting from space time geometry, because in this case that is flat .

In this situation there is also a relativistic Doppler between the front and the back due to the relative velocity between the two but this is purely due to motion so doesn't seem to be equivalent to gravity at all.

yuiop

It does not have to be an accelerating source. A receding source with constant velocity will have a velocity doppler redshift indistinguishable from gravitational redshift. For single galaxies it is difficult to put the redshift down to the Doppler redshift due to the recession speed or gravitational time dilation or "the stretching of space itself" as the universe expands, without looking at the larger global picture.

You are right that the frequency of a photon (in coordinate terms) does not change but that does not mean that nothing happens to the photon in transit. When we consder the equation c = w*f where w is wavelength and f is the frequency, we normally think of the speed of light as a constant, but in a gravitational field the coordinate speed of light is NOT constant and it is the frequency that is the constant. This means the wavelength is also NOT constant. As light falls, the coordinate wavelength contracts by a factor of gamma^2 as does the coordinate speed of light. Gravitational time dilation causes local observers to measure the frequency as speeding up by a factor of gamma and gravitational length contraction causes local observers to measure the wavelength as shorter by only a factor of gamma and the combined effects cause local observers to measure the speed of light as constant.

pervect

Thanks for the reference. I'd put in in the category of a "popularization", because it has little mathematical content.

I don't really think the idea that "The curvature of spacetime by matter ... stretches or shrinks distances" is a very good idea :-(. For instance, the Earth's surface is an example of a curved two-dimensional surface. But - would one say that the curvature of the Earth's surface "stretches or shrinks distances?" I wouldn't. And offhand I don't know of anyone who does.

In some general sense, it's partially true - 1 minute of arc of lattitude does represent a different distance at the equator than it does near the poles. But this has more to do with the relation between coordinates and distances, i.e. the metric, than it does with distances themselves "shrinking or expanding".

A rather better online reference, IMO, is Sean Carrol's lecture notes on General Relativity. http://arxiv.org/pdf/gr-qc/9712019v1.pdf. This is the draft version of his GR textbook. It's got the math to back up the generalizations, though it has those, too.

One of the quotes about redshift from Carroll's paper:

Baez also makes the same point about the tangent spaces, and the fact that the comparison of velocities at distant events is an ambiguous operation defined by convention rather than mathematics.

Note also that, while the idea that "space is expanding" is used in Carroll's description, said expansion of space is not directly due to gravity.

There's another section in Carrol's paper about gravitational redshift that might be of interest, but I'm not going to quote it here. I hope it suffices to say that light does experience some gravitational redshift when it leaves a large mass (the galaxy, say), and some blueshift when the reverse happens (it enters a target galaxy, for instance).

Cosmology makes the approximation that the universe is homogeneous and isotropic as a whole - lumps of matter like stars and galaxies are treated, at all, as a pertubation.

I also don't see the need to drag quantum mechanics into the picture at all. GR is a purely classical theory - dragging quantum mechanics into it doesn't serve any purpose unless your'e trying to do quantum gravity.

Austin0

Yes . The change referred to in my above quote was specifically change in frequency I.e. red/blue shift,,the subject of inquiry. Not a suggestion that other changes didn't occur but that they weren't relevant.
You have provided a nice description of why they aren't relevant to red shift ,
I understood that the change in coordinate speed c was 2x(gamma) not gamma^2.
Is this incorrect?
Thanks
PS Do you have some good idea how to actually measure the wavelength of photons with our contracted rulers :-)

yuiop

I am pretty sure it is gamma^2, not 2x(gamma). It is very easily obtained from the Schwarzschild metric by setting ds=0 for a photon and solving for dr/dt.

I have always wondered how I would do that and hoped know one would ever put me on the spot and ask me that directly. Damn you! :P

Seriously, does anyone know if it is possible to measure wavelength directly, rather than infer it indirectly from frequency?

prime axiom

Yes, the questions is exactly whether photons change dynamically with local conditions, by which I would mean local spacetime.

This is what I'm asking here, and looking for support helping prove that photons do in fact change dynamically with local conditions. From new research into the quantum mechanics of quantum spacetime, this seems to be the case.

I want these questions to be looked at in terms of General Relativity, and hopefully, down to the tiniest of scales, the realm of quantum mechanics. Photons change dynamically traveling along warped spacetime, and we have to look at quantum mechanics to tell us how this process happens.


I never said that. Those are 2 different phenomenon.

However I did say:

The 2 can be confused with one another, because they have the same effect. Source: http://hyperphysics.phy-astr.gsu.edu...gratim.html#c1

If we receive a photon from a galaxy, and note a red-shift in it, there could also be gravitational red & blue shifting in its overall wavelength change.

prime axiom

Helpful. Thanks.

Yes, if the clock rate changes, the frequency would also change. That is a fact. Frequency change of photons is possible.

In terms of General Relativity, and imagining photons moving through spacetime, down at the quantum level, and imagining the quantum spacetime grid. Now imagine that quantum spacetime grid as it would be disturbed by a large mass. Photons traveling through this warped spacetime, would be traveling through spacetime that is either stretched or compressed, depending on relativity and gravitational field direction. These photons would be undergoing gravitational red or blue shifting, due to gravitational time dilation.

Since these photons are traveling through spacetime that is stretched or compressed, the clock rate for these photons would be changing. When the photons are traveling stretched space, for example, they are taking longer in doing so, and c stays c, clock rate slows, frequency stretches.

So we have it that photons change dynamically as they travel through warped spacetime. At least in terms of the Quantum mechanics of General Relativity.

Wouldn't you say so?

This is the key question. Do photons change dynamically as they travel through warped spacetime?

prime axiom

Gravitational red & blue shifting means there is a process effecting the wavelength. To look into the process would mean getting down to the quantum mechanics of General Relativity.

Photons change dynamically due to gravitational wavelength shifting, as they pass through curved spacetime. Only quantum mechanics can tell us how. This seems to be a hard to find, and recent area of study. Would love some insight into it.

prime axiom

Yes, thanks.

Yes, great stuff.

In first post:

point O: origin point of emitted photo
point R: receiving point of photon

GlxO: Galaxy of origin
GlxR: Galaxy of reception

Lets say GlxO is relatively more massive than GlxR: gravity well of GlxO relatively deeper than the gravity well of GlxR

As the photon travels from Point O to R,

GlxO, O ---------->----------------R, GlxR

it would undergo gravitational wavelength shifting due to gravitational time dilation. Since GlxO is heavier, from any frame of reference, the photon would undergo a net red-shifting on its trip to R.

Would you folks not agree?
Would you yuiop?

Austin0

Sorry for the lack of clarity here. I meant; that was not the question because everybody already agrees there are dynamic changes. The area of disagreement and the actual question was: did those changes effect the end result [frequency shift] or not?


???? ;-)
Another analogy would be:
Two sound sources in atmosphere. A low pressure front has created a pressure gradient between the two locations. The low pressure area being comparable to a low G potential.
Signals sent out from the low pressure locale starting off slower and then speeding up as they progressed towards the receiver. Of course the opposite in the other direction.

DO you think in this circumstance there would be any frequency shift between emission and reception??

prime axiom

Great analogy. I haven't run into that one, but it seems to work quite well.

In the simpler model you set up, you basically have an emission point and reception point, with a gravity well in the middle.

In your case, there would be no frequency shift, because the frequency would have been compressed and stretched by the same amount, as it passed the low pressure area. Great analogy, once again.

In my case, however, there would be a NET frequency shift (observed by any reference frame), as modelled in my first post.

Jonathan Scott

This is still wrong.

Relative to some coordinate system that can be used to describe the whole path, c does not "stay c", as it is not possible to map a path containing gravitational fields to Minkowski space as used in Special Relativity. The apparent speed of light relative to the coordinate system varies, and may locally even be different in different directions. In most cases it varies approximately as the square of the time dilation factor.

Relative to that same coordinate system, if the situation is static, the photon frequency is constant. Local observers at each point may have clocks that run at different rates because of the time dilation effect of the gravitational potential, so they may record different frequencies relative to their local clocks, but that does not mean that the photon frequency speeds up or slows down.

This also applies to massive objects as well as photons. If they are freely falling through a static gravitational field, the total energy of the object as seen within the relevant coordinate system is constant. This matches the Newtonian concept that potential plus kinetic energy is constant.

yuiop

Here is an even better analogy, because you can actually do this at home and demonstrate to yourself what really happens. Get a piece of gutter or flexible curtain track or even a track for toy matchbox cars. Set up the track so that start is higher than the finish. Get some marbles and roll them down the track starting them off at one minute intervals. Notice that whatever the incline of the track they always arrive at the far end in one minute intervals. The conclusion you should reach is that the receiving frequency is always the same as the transmitting frequency. Now vary the track so that it slopes down at the beginning and then goes upwards on the last half. The marbles initially speed up on the first half of the track and then slow down on the last half of the track and yet they still arrive at the far end of the track at the same frequency. What does change is the speed and the distance between the marbles. You can think of individual marbles as peaks in a wave and the length of the gap between the peaks is the wavelength. Therefore a wave passing through various media at different speeds always maintains its frequency, but the speed and wavelength can change. If you have any doubts about the truth of that then actually do the experiment for real. It will only cost you some loose change. If that is still too much do a computer simulation with two sections of track with points travelling at one speed one first section of track and different speed on the second section of track and demonstrate that at any point along the track, the points always pass with the same frequency. If after all that you still not convinced yourself that frequency stays constant then I can only assume you lost your marbles.

The same is true for gravitational redshift. Using Schwarzschild coordinates, the wavelength and speed of a photon climbing out of a gravity well increases, but the frequency remains constant. It is only because the clocks of different observers at different heights run at different rates, that the frequency appears to slow down from the point of view of local observers.

In cosmology the redshift of light from distant galaxies is put down to stretching of space between galaxies as the universe expands which in turn stretches the wavelength of the light in transit and slows it down to a certain extent. (There are corrections for gravitational redshift due to the mass of the galaxies but this is a minor effect.) This is a difficult concept because it is difficult to imagine how a vacuum can stretch. How do we know the redshift is not simply due to the galaxies receding away from us in static space? The main clue is the cosmic microwave background (CMB) radiation. The frequency of the CMB is consistent with a extreme high frequencies during the big bang followed by billions of years of expanding space stretching the wavelength to the values we observe today.