# Does gravity impact the speed of light?

• I
As a neophyte when it comes to the relativity solutions I have been surfing the web. I came across something in Science Forums.

I have now become interested in what gravity does to light. My understanding of the post above is that the reference frame for light speed depends on gravity. Is there any merit to this?

Cheers

phinds
Gold Member
2021 Award
As a neophyte when it comes to the relativity solutions I have been surfing the web. I came across something in Science Forums.

I have now become interested in what gravity does to light. My understanding of the post above is that the reference frame for light speed depends on gravity. Is there any merit to this?

Cheers
No, EVERY reference frame is a "reference frame for the speed of light" since c is the same in all reference frames.

EDIT: just to be clear, as pointed out below by ibix, light itself HAS no frame of reference. My statement was that every inertial frame of reference measure the same c

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Ibix
It's nonsense. As is the phrase "reference frame for light speed".

The speed of light in vacuum is an invariant. Any local measure will always give you the same value, and light will always win a race.

You are going to need to understand Special Relativity before you try to learn General Relativity

PeroK
Homework Helper
Gold Member
2021 Award
As a neophyte when it comes to the relativity solutions I have been surfing the web. I came across something in Science Forums.

I have now become interested in what gravity does to light. My understanding of the post above is that the reference frame for light speed depends on gravity. Is there any merit to this?

Cheers

The moderators' comments on that forum should be enough to convince you that it is crackpottery and not physics.

For everybody light speed in vacuum is constant c in his place, HERE. Gravity would change light speed at out of his place, OVERTHERE.

phinds
Gold Member
2021 Award
For everybody light speed in vacuum is constant c in his place, HERE. Gravity would change light speed at out of his place, OVERTHERE.
What on Earth are you talking about ???

pervect
Staff Emeritus
As a neophyte when it comes to the relativity solutions I have been surfing the web. I came across something in Science Forums.

I have now become interested in what gravity does to light. My understanding of the post above is that the reference frame for light speed depends on gravity. Is there any merit to this?

Cheers

The above link seems to be someone's personal theory posted to another physics discussion site, rather than any reference to a textbook or a peer reviewed paper. The moderators at that other science site had had a similar reaction to the moderators at our site when what appears to be a speculative personal theory is posted. They ask for references. Acceptable references were not provided, and the thread was closed.

However, some useful remarks about gravity and the speed of light can be made.

The SI defintion of the meter is https://physics.nist.gov/cuu/Units/meter.html

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

While this is a website reference, the website in question is the national institute of standards, NIST, so it's a reasonably high quality reference. Thus if one uses a standard meter stick, in a vacuum, light will always take 1 / 299792458 of a second to transverse said meter stick, regardless of "gravity".

Gravity was not specifically discussed, alas, but the definition of the meter makes no mention of gravity. In general, the meter is usually defined in the above matter unless otherwise stated. If it is defined in this manner, it's independent of gravity or (more to the point) gravitational potential.

Basically, nowadays the speed of light is used as a reference, because we are confident it doesn't change. It's no longer measured. In a historical context, before the current defintion of the meter was adopted, the speed of light was measured, in terms of the standards used at the time, based on copies of a physical artefact, a prototype meter bar. Confidence in the constancy of the speed of light eventually became so high that nowadays we use it as part of our reference standard for distances.

There are situations where "the speed of light" is expressed using coordinates that are not evenly spaced as measured by standard rulers , such as Schwarzschild coordiantes. I don't think it's really a good idea to talk about the rate of change of a coordinate as a "speed", but sometimes people do it anyway. So be warned, if you're doing something other than measuring the speed of light in a way that can be referenced to standarrd meter sticks and standard clocks, the above remarks may not apply.

There may be other rare instances where the SI definitions are not used, in such cases similar cautions would apply.

Dale
What on Earth are you talking about ???

A man living in ground floor of a building measures speed of light just emitted from laser pointer in his room to be c using his measure and watch at hand.
A woman living in higher floor of the same building measures speed of light just emitted from laser pointer in her room to be c using her measure and watch at hand.
The man observes speed of HER light to be more than c using her measurement and his proper time.
The woman measures speed of HIS light to be less than c using his measurement and her proper time.

phinds
Gold Member
2021 Award
A man living in ground floor of a building measures speed of light just emitted from laser pointer in his room to be c using his measure and watch at hand.
A woman living in higher floor of the same building measures speed of light just emitted from laser pointer in her room to be c using her measure and watch at hand.
The man observes speed of HER light to be more than c using her measurement and his proper time.
The woman measures speed of HIS light to be less than c using his measurement and her proper time.
OK, that's just weird. Why in the world would anyone mix up measurements like that to measure the speed of light? In any case your scenario is utterly irrelevant to the constancy of c and the fact that its speed is not affected by gravity (which is the subject of this thread)

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Khashishi
A man living in ground floor of a building measures speed of light just emitted from laser pointer in his room to be c using his measure and watch at hand.
A woman living in higher floor of the same building measures speed of light just emitted from laser pointer in her room to be c using her measure and watch at hand.
The man observes speed of HER light to be more than c using her measurement and his proper time.
The woman measures speed of HIS light to be less than c using his measurement and her proper time.
There are huge caveats to this statement. As pervect noted, it is probably not a good idea to call a rate of change in coordinates a speed. This depends on a particular choice of coordinates, which really needs to be precisely stated when you say "her measurement" and "his measurement".

Your coordinates are not conventional. Normally, we would not regard the speed of light in vacuum as changing due to gravity--rather we would say there is gravitational time dilation. Light is still traveling at the same speed, but less time has passed deeper in the gravitational well.

Locally, the speed of light in a vacuum is constant, because it is a result of the geometry of space-time. Nonlocally, it if well-defined only for flat space-time.

I remember once trying to calculate it for small departures from flatness. I did it in this way:

Take points A, B, and C. Make AB a spacelike geodesics, BC a timelike one, and AC a null one. Also make (AB).(BC) = 0 (a right angle). The measured value of c is thus |AB|/|BC|. I found it to be

1 + (Riemann tensor).(combination of (AB) and (BC))

This is in the limit of (Riemann tensor) * (distance/time traversed)2 << 1. When it is >~ 1, it varies much more.

Okay, so nobody answered my question about the effect of gravity on the speed of light. I got some “retweet” answers.

What experiments have been done to study the influence of gravity on the speed of light?

Please, no mathemagical equations. I also request no dismissive remarks. Those come from fake physicists. Let’s be civil. If you don’t have an answer, please remain silent.

Much appreciated.

The above link seems to be someone's personal theory posted to another physics discussion site, rather than any reference to a textbook or a peer reviewed paper. The moderators at that other science site had had a similar reaction to the moderators at our site when what appears to be a speculative personal theory is posted. They ask for references. Acceptable references were not provided, and the thread was closed.

However, some useful remarks about gravity and the speed of light can be made.

The SI defintion of the meter is https://physics.nist.gov/cuu/Units/meter.html

While this is a website reference, the website in question is the national institute of standards, NIST, so it's a reasonably high quality reference. Thus if one uses a standard meter stick, in a vacuum, light will always take 1 / 299792458 of a second to transverse said meter stick, regardless of "gravity".

Gravity was not specifically discussed, alas, but the definition of the meter makes no mention of gravity. In general, the meter is usually defined in the above matter unless otherwise stated. If it is defined in this manner, it's independent of gravity or (more to the point) gravitational potential.

Basically, nowadays the speed of light is used as a reference, because we are confident it doesn't change. It's no longer measured. In a historical context, before the current defintion of the meter was adopted, the speed of light was measured, in terms of the standards used at the time, based on copies of a physical artefact, a prototype meter bar. Confidence in the constancy of the speed of light eventually became so high that nowadays we use it as part of our reference standard for distances.

There are situations where "the speed of light" is expressed using coordinates that are not evenly spaced as measured by standard rulers , such as Schwarzschild coordiantes. I don't think it's really a good idea to talk about the rate of change of a coordinate as a "speed", but sometimes people do it anyway. So be warned, if you're doing something other than measuring the speed of light in a way that can be referenced to standarrd meter sticks and standard clocks, the above remarks may not apply.

There may be other rare instances where the SI definitions are not used, in such cases similar cautions would apply.

Using the proposed speed of light to define a meter is completely circular science. It is simply a definition, and serves no value except to use as and argument by the misled relativity buffs.

Wave propagation requires a medium (duh). The particle theory breaks the circular reasoning of special relativity.

Is it perhaps time to investigate outside of Einstein’s assumptions? Just asking.

No, EVERY reference frame is a "reference frame for the speed of light" since c is the same in all reference frames.

EDIT: just to be clear, as pointed out below by ibix, light itself HAS no frame of reference. My statement was that every inertial frame of reference measure the same c

Einstein’s first assumption in special relativity was that one regarding frames of reference. Too bad he did not provide any citations in that paper, and that most physicists at that time disagreed with it. Thus, no Nobel (or should I call it the Higgs) award.

Is it possible that you are using one assumption to “prove” another assumption?

Cheers

phinds
Gold Member
2021 Award
Is it possible that you are using one assumption to “prove” another assumption?
what do you consider the "other assumption"? If it's that there is no frame of reference for light, that's not an assumption, it's a direct and obvious result of c being the same in all inertial reference frames which is itself not an assumption, it's a postulate and since it leads to conclusive experimental evidence of its correctness, I'd say it's a damn good one.

what do you consider the "other assumption"? If it's that there is no frame of reference for light, that's not an assumption, it's a direct and obvious result of c being the same in all inertial reference frames which is itself not an assumption, it's a postulate and since it leads to conclusive experimental evidence of its correctness, I'd say it's a damn good one.

It is my understanding that a postulate IS an assumption. At least that is what the dictionaries tell me. It is a starting point for an argument. There is nothing that creates a postulate except our imagination. Having said that, I believe the human imagination to be grand and has physical properties that far exceed Einstein’s assumptions (postulates)

Cheers

phinds
Gold Member
2021 Award
It is my understanding that a postulate IS an assumption. At least that is what the dictionaries tell me. It is a starting point for an argument. There is nothing that creates a postulate except our imagination.
"Postulate" is a formal term in physics and math. It is more rigorous than "assumption".

Having said that, I believe the human imagination to be grand and has physical properties that far exceed Einstein’s assumptions (postulates)
I have NO idea what that means but it sounds a lot more like philosophy than physics.

I hate to disagree. However a postulate is an assumption by definition. Where did Einstein get his frame of reference idea. Do you have a paper that presents that before Einstein? Again, Einstein had no citations in his philosophical musings.

Read about the History of all this. Most established physicists did not take his paper seriously. I wonder why? A thought experiment is a philosophy. Again, by definition.

Cheers

phinds
Gold Member
2021 Award
I hate to disagree. However a postulate is an assumption by definition.
I think you are right as far as standard English goes but math and physics don't always use words quite the same way as normal English (and sometimes they use them in quite different ways). It is still true that in math and physics, "postulate" is a formal term and "assumption" is not. No one would say "Einsteins assumptions of Special Relativity".

PeterDonis
Mentor
What experiments have been done to study the influence of gravity on the speed of light?

The problem with this question is that you think "the influence of gravity on the speed of light" has a definite meaning, but it doesn't. It could mean any of at least three related things:

(1) If we set up spherical coordinates centered on a gravitating body like the Earth, and measure the rate of change of the space coordinates of a light ray with respect to the time coordinate, we will find that this rate depends on our height above the gravitating body. This is sometimes described as the "speed of light" being affected by gravity, but this "speed" is a coordinate speed, a mathematical calculation, not a speed that anyone directly measures.

(2) If we set up two observers at different altitudes above a gravitating body like the Earth, and have them exchange light signals, and each of them measures the round-trip travel time of those signals by their own clocks, we will find that the two times are different. Since it seems evident that the distance covered by the light signals doesn't change from one observer to the other, this phenomenon is sometimes described as the "speed of light" being affected by gravity. However, this "speed" is again a mathematical calculation.

(3) If we set up an apparatus to measure the speed of light at our location--for example, a light source/detector, a mirror, and a clock--and make measurements with it as we travel around to various different locations in the gravitational field of a body like the Earth, we will find that the apparatus always gives us the same answer. (The numerical value of this answer will depend on the units we adopt, but it will be the same at every location that we explore anywhere in the universe.) This direct measurement of the speed of light is what physicists are referring to when they say that the speed of light is the same everywhere (and is not affected by gravity).

PeterDonis
Mentor
Where did Einstein get his frame of reference idea. Do you have a paper that presents that before Einstein? Again, Einstein had no citations in his philosophical musings.

Read about the History of all this. Most established physicists did not take his paper seriously. I wonder why? A thought experiment is a philosophy. Again, by definition.

All of this is off topic here. Relativity (both special and general) has been confirmed by many, many experiments. Where Einstein originally got the ideas from, or what physicists thought of them at the time, is irrelevant to the actual physics involved.

All of this is off topic here. Relativity (both special and general) has been confirmed by many, many experiments. Where Einstein originally got the ideas from, or what physicists thought of them at the time, is irrelevant to the actual physics involved.

I personally did not find it irrelevant. You left out the context of my answer. All of my answers and questions are contextually based. Pulling out a single statement confuses the matter.

The point was: Did Einstein make an assumption concerning the relative concordance regarding independent fields of reference?

As an example, if a GPS satellite is used as the field of reference, wouldn’t the Earth’s clock be slower? I am sure I am missing something, and I am here to learn. Seriously. I do not like confrontation and humiliating replies. I want to get down to the science itself.

Best regards.

berkeman
Mentor

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PeterDonis
Mentor
I personally did not find it irrelevant.

Then you apparently do not grasp the distinction between physics and the history of physics.

What assumptions Einstein made, why he made them, and what other physicists thought of them, are all history of physics. They are irrelevant to whether the physics itself is correct or not. A physical theory stands or falls on its own merits--whether it makes predictions that match experimental data--regardless of anyone's assumptions or opinions.

I am here to learn

Then you need to stop asking about what assumptions Einstein or anyone else made, and stop making irrelevant comments about philosophical musings or what other physicists thought about relativity at the time it was first published, or why Einstein didn't get a Nobel prize for it. You need to focus on the physics and only the physics.

I do not like confrontation and humiliating replies. I want to get down to the science itself.

Then you need to spend some time learning what special and general relativity actually say, and how strong their experimental confirmation is. You do not appear to grasp either of these points.

For a start, go back and read my post #20. If you understand what I'm saying there, we can move forward to discuss some more complicated cases. If you have questions about what I say there, ask them.

PeterDonis
Mentor
that is a semantic game of course. It reinforces the priesthood of scientists. Humility is not one of their strong points. :-)

You have received a warning for this post. This kind of statement has no place in discussion here, particularly for someone who claims to be here to learn and to want to get down to the science itself. Please take heed.

Dale
Clifford Will's paper lists these tests:
• The weak equivalence principle -- nongravitational composition independence
• Local Lorentz invariance -- is the speed of light in a vacuum independent of direction, source motion, and observer motion? Is it also equal to the limiting speed of massive particles?
• Local position invariance
• Gravitational redshift
• Variation of elementary-particle constants over the Universe's lifetime: electromagnetic fine-structure constant, low-energy weak-interaction constant, electron-proton mass ratio
• PPN parameter (gamma): (space distortion) / (time distortion) -- the lowest-order time distortion gives the Newtonian gravitational potential
• Deflection of visible light and radio waves
• PPN parameter (beta): first nonlinear part of time distortion
• Extra pericenter precession, notably of planet Mercury
• Gravitational self-energy having different gravity than nongravitational material: violation of the strong equivalence principle
• Preferred-frame and preferred-location effects
• Variations in the Newtonian gravitational constant over the Universe's lifetime
• Gravitational-wave energy loss from binary-pulsar systems
General relativity has been remarkably successful with surviving alternatives like Generalized Brans-Dicke having parameters that can make them arbitrarily close to GR.

Since his paper has been the detection of gravitational waves from events that are presumably the coalescence of black holes and neutron stars.

The black-hole ones fit the shape expected from GR: an inspiral followed by ringing of the combined black hole. No electromagnetic correlates were detected for any of those events, however.

The neutron-star one, GW170817, was accompanied by a flash of gamma rays that arrived at some satellites' gamma-ray detectors about 1.7 seconds after the arrival of the gravitational waves at the LIGO and VIRGO G-wave detectors. A "kilonova" was observed a few hours later in NGC 4993, a galaxy in the line of sight, confirming the identification of the source as coalescing neutron stars. The distance traveled by these signals is about 4.41 megaparsecs (144 million light years). The G-wave/gamma-ray time difference is thus about 4*10^(-16) times the travel time. The gravitational potentials along the way vary by about 10^(-6), thus making a strong constraint on variation induced by that source.

[1710.06427] Constraints from the Time Lag between Gravitational Waves and Gamma Rays: Implications of GW 170817 and GRB 170817A at arxiv -- notes what I have noted, and does a more precise calculation of gravitational-potential delays. It does the calculation for our Galaxy, though apparently not for NGC 4993 or for intergalactic space.

To lowest order, the gravitational time delay is
$$\Delta t = - (1 + \gamma) \int V \, ds$$
for PPN parameter ##\gamma## = (space distortion) / (time distortion) (GR value: 1), gravitational potential V = - GM/r for point source with mass M and distance r, and distance s is the space distance traveled. So if ##\gamma## for photons and for G-waves differs, then that difference must be very tiny. That paper's upper limit of at most 10^(-6) was calculated using only our Galaxy's gravitational potential, and is likely a gross overestimate.