# Gravitational redshift

Is the change in frequency an actual change or is it like the Doppler effect????
Does the speed of light also change in a g-field????

Staff Emeritus
2021 Award
The Doppler effect is an actual change in frequency.

A.T.
Is the change in frequency an actual change or is it like the Doppler effect?
What is an "actual change", and why isn't Doppler one?
Does the speed of light also change in a g-field????
Depends on how you measure it.

Is the change in frequency an actual change or is it like the Doppler effect????
Does the speed of light also change in a g-field????
This is the wrong sub forum for your questions... and as noticed, your questions are fuzzy -but that's quite normal if you don't understand it. :) I'll thus try to answer what you may be asking.

1. Gravitational redshift can only be a "change" if something is changing, for example if an atomic clock moves away from Earth. And if with Doppler effect you mean the effect that you measure by reflecting light from a moving mirror, then the gravitational redshift is still an "actual change" that is distinct from that Doppler effect. This has been verified with an experiment known as "Gravity probe A", in which a rocket with atomic clock (a Maser) was launched upwards and allowed to free-fall back to Earth. The rocket had the electronic equivalent of a mirror which reflected a signal from the ground station, so that the ordinary Doppler signal could be subtracted from the signal by the Maser. - http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.45.2081

2. A "local" measurement should always give the standard speed of light. But if you measure the speed of light "globally" then you find that the speed is affected by a g-field, and this allows light to bend. Einstein used the Huygens construction to calculate the bending of light when it passes near the Sun. As an illustration, when you row a boat and brake with your peddle on the right side, this will steer the boat to the right; a better illustration, if you know it, is how a Segway changes its course. See pages 198, 199 of the English version of https://web.archive.org/web/20080306023634/http://www.alberteinstein.info/gallery/gtext3.html
Another way of measuring the effect of g-fields on light is called Shapiro time delay, you can "google" it. :)

Last edited:
The Doppler effect is an actual change in frequency.
Doppler effect is an apparent change in the frequency of the sound source due to relative motion between it and the observer.

Dale
Mentor
2021 Award
I don't know of any experiment which could distinguish between an actual and an apparent change in this context. It is a distinction without a difference.

Staff Emeritus
2021 Award
I would argue it is a real change. You measure the frequency, and that's what you see. Yes, it's not the source frequency. But it's still the frequency. If someone fills a bucket and spills 1/3 of it before giving it to you, the real volume of water is 2/3 of a bucket. It's not an "apparent change". That's how much is really there.

Here's a little story:

There's a bakery in a valley making 100 buns a day. Valley people buy 100 buns a day.

Then the bakery is moved to the top of a mountain. Now bakery makes 101 buns a day. After million days bakery has million unsold buns.

The point of the story is that the extra buns are real buns. The reason for extra real buns is real extra swiftness of the staff.

harrylin
PeterDonis
Mentor
The reason for extra real buns is real extra swiftness of the staff.

Relative to the people who are buying the buns. Move all the valley people who buy buns to the top of the mountain, and they will buy 101 buns a day instead of 100 because their metabolism is faster.

And even then, the reason the number of buns per day changes is that the "day" is determined by something external to the whole system (the Sun), so it can be viewed as staying the same while the swiftness and metabolism of the people changes. If the bakery and the people buying buns were inside a rocket that was hovering deep in the gravity well of a black hole, their "day", as judged by their own clocks, would not change at all relative to their bun production and consumption; they would be baking and consuming 100 buns a day, according to their own clocks, and would have no knowledge that, to someone far outside the gravity well, they were only producing and consuming a fraction of a bun per year, or decade, or whatever (unless they communicated with the person far outside the gravity well).

The point of all this is that getting fixated on what changes are "real" and what changes are only "apparent" is, IMO, a rabbit hole with no bottom. Different observers on different worldlines through spacetime observe different things. It's possible to construct a viewpoint in which the different observations are due to "real changes", and it's possible to construct a viewpoint in which the different observations are only due to "apparent changes", or "relative changes", and nothing "real" changed. Since both interpretations can be made consistent with the actual observable data, which one is "true" is not, IMO, a question of physics.

A.T.
Then the bakery is moved to the top of a mountain.

If the people have to climb a mountain every day, wouldn't they need to eat more buns?

phinds
Gold Member
2021 Award
I don't know about you guys but I wouldn't climb no damn mountain for a lousy bun. Pizza maybe but buns? ... fugedaboutdit.

PeterDonis
pervect
Staff Emeritus
Doppler effect is an apparent change in the frequency of the sound source due to relative motion between it and the observer.

Color me confused. Why the sudden switch from light to sound? How do you suggest we go about distinguishing an "apparent change" from a "real change"?

Taking my best guess, are you perhaps assuming there is some "absolute time", and some "absolute state of rest", and calling the frequency measured in the presumed "absolute state of rest frame" the "real frequency"?

I'm afraid I have some bad news for you if you're looking for an answer in relativity consistent with the notions of "absolute time" or "absolute rest". But I'm not really sure I understand your question or its motivation yet, so I'll defer giving you the bad news until I am sure it is applicable to whatever it was you were trying to ask.

Relative to the people who are buying the buns. Move all the valley people who buy buns to the top of the mountain, and they will buy 101 buns a day instead of 100 because their metabolism is faster.

And even then, the reason the number of buns per day changes is that the "day" is determined by something external to the whole system (the Sun), so it can be viewed as staying the same while the swiftness and metabolism of the people changes. If the bakery and the people buying buns were inside a rocket that was hovering deep in the gravity well of a black hole, their "day", as judged by their own clocks, would not change at all relative to their bun production and consumption; they would be baking and consuming 100 buns a day, according to their own clocks, and would have no knowledge that, to someone far outside the gravity well, they were only producing and consuming a fraction of a bun per year, or decade, or whatever (unless they communicated with the person far outside the gravity well).

The point of all this is that getting fixated on what changes are "real" and what changes are only "apparent" is, IMO, a rabbit hole with no bottom. [..]
Of course, in some specific cases it is difficult or even impossible to say with certainty what "really" changed. However, unwittingly you just explained (although perhaps not elaborately enough) how one can often in general distinguish changes in observed objects from changes in the used systems of observation. In physics experiments it's a rather standard operating procedure to induce changes at will at specified instances so that other influences can be filtered out. Such discovered laws of physics can give the kind of generic answers that the OP may be looking for.

If you take pictures on the street and you move your camera relative to the street, the houses in your observation will rotate; and every time you stop moving, the rotation will stop as well. It's certainly part of physics teaching to clarify that one cannot physically rotate houses at a distance by such means, in case a student would believe such a thing.

However, the OP has not commented on the replies yet; so we cannot know for sure, as perfect also noticed, what kind of answers the OP really hopes to find. Let's not waste our time and energy for nothing. ;)

Last edited:
Here's a little story:

There's a bakery in a valley making 100 buns a day. Valley people buy 100 buns a day.

Then the bakery is moved to the top of a mountain. Now bakery makes 101 buns a day. After million days bakery has million unsold buns.

The point of the story is that the extra buns are real buns. The reason for extra real buns is real extra swiftness of the staff.
The reason for the extra buns is there is less gravitational 'drag' at higher elevation. The swiftness is due to less drag. Yes?

Relative to the people who are buying the buns. Move all the valley people who buy buns to the top of the mountain, and they will buy 101 buns a day instead of 100 because their metabolism is faster.

And even then, the reason the number of buns per day changes is that the "day" is determined by something external to the whole system (the Sun), so it can be viewed as staying the same while the swiftness and metabolism of the people changes. If the bakery and the people buying buns were inside a rocket that was hovering deep in the gravity well of a black hole, their "day", as judged by their own clocks, would not change at all relative to their bun production and consumption; they would be baking and consuming 100 buns a day, according to their own clocks, and would have no knowledge that, to someone far outside the gravity well, they were only producing and consuming a fraction of a bun per year, or decade, or whatever (unless they communicated with the person far outside the gravity well).

The point of all this is that getting fixated on what changes are "real" and what changes are only "apparent" is, IMO, a rabbit hole with no bottom. Different observers on different worldlines through spacetime observe different things. It's possible to construct a viewpoint in which the different observations are due to "real changes", and it's possible to construct a viewpoint in which the different observations are only due to "apparent changes", or "relative changes", and nothing "real" changed. Since both interpretations can be made consistent with the actual observable data, which one is "true" is not, IMO, a question of physics.
Don't you really need a third observer to arbitrate between two other measurers. E.g. Fr A and Fr B are arguing results from each's perspective. Obtain a result from the 3rd observer as to the exact differences between those two frames. I advocate a Universal Reference Point to be the arbitrator. A space station at a fixed place between 3 nearby galaxies. That would be the "local" arbitrator. Of course, there is no absolute answer. All is interpretation as Nietzsche said. gdc

PeterDonis
Mentor
Don't you really need a third observer to arbitrate between two other measurers.

Sometimes that works; but in general, the third observer is just as constrained by his own point of view as the first two.

I advocate a Universal Reference Point to be the arbitrator.

But as you yourself note later in your post...

Of course, there is no absolute answer.

...there is no such thing.

PeterDonis
Mentor
The reason for the extra buns is there is less gravitational 'drag' at higher elevation.

What is "gravitational drag"?

stevendaryl
Staff Emeritus
Here's a little story:

There's a bakery in a valley making 100 buns a day. Valley people buy 100 buns a day.

Then the bakery is moved to the top of a mountain. Now bakery makes 101 buns a day. After million days bakery has million unsold buns.

The point of the story is that the extra buns are real buns. The reason for extra real buns is real extra swiftness of the staff.

I'm a little uncomfortable with this description, because it makes it seem that there is an objective, coordinate-independent notion of clock rates.

Being a little more precise, what's true is this:

In a spacetime diagram, we can identify 4 events:

$e_1$: The start of one day at the bottom of the mountain.
$e_2$: The start of that day at the top of the mount.
$e_3$: The end of the day at the bottom of the mountain.
$e_4$: The end of the day at the top of the mountain.

It's an objective fact that a clock at the top of the mountain advances 1% farther between events $e_2$ and $e_4$ than a clock at the bottom of the mountain advances between $e_1$ and $e_3$. What's relative, rather than objective, is the claim, necessary to make a case for the mountaintop clock running faster, that $e_1$ and $e_2$ are simultaneous, and $e_3$ and $e_4$ are simultaneous. They are simultaneous in the noninertial frame in which the clocks are at rest and the gravitational field is time-independent. But not in all frames.

Here's a Euclidean analogy: You have a flat region on Earth. There are roads going in all different directions, some straight and some winding. Each road has associated roadside markers on the side of road with consecutive integers on them: marker #1, marker #2, etc. It's not clear whether the markers have a standard separation (say, every 10 meters) or whether different roads use different separations. Let me use the phrase "marker rate" to mean the number of markers per kilometer.

So let's suppose that there are two roads that are parallel--they stay the same distance apart. What you notice is that where Road A has marker number 1, you can look to the closest marker on Road B, and it's marker number 2. Where Road A has marker number 2, you can look to the closest marker on Road B, and it's marker #4. Etc. So clearly, the "marker rate" of Road B is twice that of Road A. Right?

No. What if in reality, both roads are circular, with the same center. Road A is a circle of radius 1 kilometer, and Road B is a circle of radius 2 kilometers. They actually have exactly the same "marker rate" of one marker every 10 meters. It's just that traveling through the same angle on Road B gets you twice as far as traveling through that same angle on Road A.

Time dilation in a weak gravitational field is analogous to traveling on circular roads. The analogy of "number of markers between two points" is "elapsed time between two events". The analogy of "angle traveled" is coordinate time. The analogy of "radius of the circle" is "height of the clock". Different clocks travel different amounts of elapsed time for the same amount of coordinate time in the same way that different roads pass different numbers of markers for the same amount of angular distance traveled.

What is "gravitational drag"?
Gravity is a field phenomenon as all other things are. Once mass is deposited in the field it has inertia. I guess that is drag. I use this view to get away from paradoxes in SRT and GRT. Gravity is not a pull or a push. It is the self attraction of mass due to field requirements. Matter and anti-matter are both in the same location in space. Both are field properties. These ideas do not change anything, but provide a better model for explaining physics.

stevendaryl
Staff Emeritus
Gravity is a field phenomenon as all other things are. Once mass is deposited in the field it has inertia. I guess that is drag.

That's a misleading term. Usually "viscous drag" is used to mean a dissipative force, which lowers the energy of a moving object. Gravity is not a dissipative force.

Sometimes that works; but in general, the third observer is just as constrained by his own point of view as the first two.

But as you yourself note later in your post...

...there is no such thing.
I use the so-called universal reference to clarify concepts of the SOL. The third observer or the Universal observer would give a better estimate of the velocity of light as it is coming from another frame of reference or two. It would get rid of the paradoxes when just using two observers or space ships and the light signals they might send.

stevendaryl
Staff Emeritus