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General Relativity: GravitationalRed Shift Confused with Doppler Effect? 
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#19
Mar3112, 01:56 PM

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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. 


#20
Mar3112, 05:31 PM

P: 476

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. 


#21
Mar3112, 09:25 PM

P: 476

Frequency is conserved, in a closed system.
Let us consider a planetphoton 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? 


#22
Apr112, 12:41 AM

P: 1,162

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. 


#23
Apr112, 01:54 AM

P: 3,967

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. 


#24
Apr112, 02:17 AM

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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 twodimensional 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/grqc/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: 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. 


#25
Apr112, 08:19 PM

P: 1,162

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 :) 


#26
Apr112, 09:31 PM

P: 3,967

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


#27
Apr112, 09:47 PM

P: 26

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.phyastr.gsu.edu...gratim.html#c1 If we receive a photon from a galaxy, and note a redshift in it, there could also be gravitational red & blue shifting in its overall wavelength change. 


#28
Apr112, 10:16 PM

P: 26

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? 


#29
Apr112, 10:33 PM

P: 26

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. 


#30
Apr112, 10:51 PM

P: 26

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 redshifting on its trip to R. Would you folks not agree? Would you yuiop? 


#31
Apr212, 12:16 AM

P: 1,162

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?? 


#32
Apr212, 01:53 AM

P: 26

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. 


#33
Apr212, 06:21 AM

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P: 1,159

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. 


#34
Apr212, 07:47 AM

P: 3,967

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. 


#35
Apr212, 12:47 PM

P: 1,162

the two sources. One of the sources is in the middle of it and the other source is in high pressure. One signal goes from low pressure to high and the other goes from high to low. Comparable to sending signals between a large mass and a higher altitude. How is this essentially different from your problem other than the lack of time dilation???? The critical factor in both cases is the difference in potential/pressure at the locations. As you just pointed out localized areas that are passed through in between aren't important. Do you see there would be no frequency shift in this situation??? 


#36
Apr1512, 11:36 PM

P: 26

Yes, more accurately, a signal emitted in a LARGE high pressure area and then received in a SMALLER, lower pressure area, would be the same scenario. Wouldn't there be a frequency shift in the above scenario? In this case, "redshifted"? Also, aren't small amounts of energy lost into the medium as waves travel from one medium in to another, and from traveling from dense to denser areas of the same type of medium? ===== The mechanics of how this scenario would operate might be different for photons traveling through curved spacetime, and for molecules vibrating in air, though. Quantum mechanics is needed to describe what happens to photons as they travel through curved spacetime. 


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