Is light speed really constant?

[Erez Lerner]

Big masses like planets and stars have gravitational fields.
Gravitational fields curve space around them.
So in theory a particle having mass moving at fixed speed, from its own perspective, will accelerate when moving closer to such a planet. It'll be moving at fixed speed in a curved space. This is similar to having a car move "straight" on a curved road (although I'm talking about moving toward and not beside).
Light is also affected by such a mass. We know this because light curves away (changes direction) when passing close to such a mass.
If light is affected, then it should also be affected when moving directly towards the mass.
So it should be that although from the single photon perspective it's moving in constant speed (light speed), from an observer's perspective it's actually moving faster as it nears the planet, hence light changes its speed.
Is this true?
If not, what's the explanation?

 

[jerromyjon]

If not, what's the explanation?

Its speed is fixed by the laws of spacetime, so when acceleration is applied, there is a Doppler shift to add the energy to its frequency.

 

[Erez Lerner]

Its speed is fixed by the laws of spacetime, so when acceleration is applied, there is a Doppler shift to add the energy to its frequency.

Sorry. I don't understand.
Can you elaborate with an example, or say it in layman language?

 

[jerromyjon]

Photons (the individual "chunks" of light) have a very simple equation for their energy: Since we are talking about them moving at their maximum speed it makes it simple to think about their energy being constant as they fly through empty space (vacuum). If it heads towards the sun, it can't speed up, its already going as fast as it can! But it "feels" the pull of the Sun growing as it gets closer, so it must add energy. This force can't make whole chunk of waves move any faster through space, so it makes the frequency of the waves move faster.

 

[ZapperZ]

Light is also affected by such a mass. We know this because light curves away (changes direction) when passing close to such a mass.
If light is affected, then it should also be affected when moving directly towards the mass.

Please read our FAQ.

So it should be that although from the single photon perspective it's moving in constant speed (light speed), from an observer's perspective it's actually moving faster as it nears the planet, hence light changes its speed.
Is this true?

Single-photon perspective is a fallacy. There is no physics that works at the "single photon perspective", because that violates Special Relativity (are you proposing to violate such a thing?). Again, read our FAQ.

Zz.

 

[jerromyjon]

Single-photon perspective is a fallacy

So my description is wrong? Or is it just the wrong way to visualize it?

 

[Erez Lerner]

Jerryomyjon,
This is understood.
What you said does not contradict what I said but rather aligns with it.
1. The particle needs not change speed from its own perspective. It can be thought of as a viewpoint thing.
2. If it "adds energy" then it must add mass, because mass is condensed energy. That's problematic because that would slow it down considerably and it would no longer travel at the speed of light.
3. If you suggest Doppler effect, then that means adjacent particles must travel at different speeds (faster and faster as one gets closer to the big mass).

So the original question remains.

 

[jerromyjon]

1. The particle needs not change speed from its own perspective. It can be thought of as a viewpoint thing.

No. It can't. It's a global relativistic constraint. The viewpoint only rationalizes why relative observations differ.

 

[Erez Lerner]

Please read our FAQ.
Single-photon perspective is a fallacy. There is no physics that works at the "single photon perspective", because that violates Special Relativity (are you proposing to violate such a thing?). Again, read our FAQ.

Zz.

Can you supply a link to a specific topic in the FAQ? it's quite elaborate.

 

[ZapperZ]

Can you supply a link to a specific topic in the FAQ? it's quite elaborate.

Didn't I linked to TWO specific topics there?

One explains what it means that light is "affected" by gravity. It isn't light, it is the spectime "geodesic"!

The second explains why you can't "work" at the speed of light and still use the physics that we know of, because that will violate Special Relativity.

The whole starting premise of this topic is faulty, i.e. you need to learn basic Special Relativity first, such as starting with its postulates. And please refrain from using the phrase that light "condenses" into matter. That is false!

Zz.

 

[Erez Lerner]

No. It can't.

That's the religious reply.
ZapperZ suggested that my whole viewpoint is wrong and wondered if I want to violate special relativity.
I'm just wondering about this topic.
If that turns out to be a breakthrough in physics - it's a bonus

 

[jerromyjon]

If that turns out to be a breakthrough in physics - it's a bonus

All you are achieving is broken physics.

 

[ZapperZ]

That's the religious reply.
ZapperZ suggested that my whole viewpoint is wrong and wondered if I want to violate special relativity.
I'm just wondering about this topic.
If that turns out to be a breakthrough in physics - it's a bonus

But there's a strong likelihood that this will turn out to be crackpottery. I know where I'm betting my money on.

Zz.

 

[jerromyjon]

But there's a strong likelihood that this will turn out to be crackpottery.

All you are achieving is broken physics.

Agreed.

 

[alw34]

1. The particle needs not change speed from its own perspective. It can be thought of as a viewpoint thing.
2. If it "adds energy" then it must add mass, because mass is condensed energy. That's problematic because that would slow it down considerably and it would no longer travel at the speed of light.
3. If you suggest Doppler effect, then that means adjacent particles must travel at different speeds (faster and faster as one gets closer to the big mass).

None of this is correct.

 

[Erez Lerner]

I've now read more about special relativity.
The point is that fixed light speed in an axiom (meaning assumed and not proven or witnessed). Wikipedia also says there are experiments proving light speed in fixed.
The question is do you know of an experiment (can you supply a link) that checks what I asked above:
Light traveling toward a big mass?

 

[Ibix]

The GPS system relies on radio signals between satellites and the ground. It includes relativistic maths. It works. Ergo the behaviour of light in a gravitational field is as per relativity.

 

[Jonathan Scott]

The speed of light is always the same locally, as according to Special Relativity. On a larger scale, gravity changes the shape of space-time, effectively making it a fraction "denser" closer to massive objects. If one tries to map space-time using a flat coordinate system for a region affected by gravity, then relative to that coordinate system the speed of light appears to vary slightly. However, at any local point the speed of light has its usual value.

A static gravitational field does not affect the frequency of a beam of light. However, observers at different gravitational potentials will see the frequency of the beam to be slightly different because their clocks will be running at slightly different rates.

 

[jerromyjon]

Light traveling toward a big mass?

Gravitational lensing

 

[jerromyjon]

However, observers at different gravitational potentials will see the frequency of the beam to be slightly different because their clocks will be running at slightly different rates.

Thank you, wonderful tidbit I was neglecting...

 

[alw34]

Here is an introductory explanation that someone posted in another discussion in these forums

It explains what zz implied when he posted:

"Single-photon perspective is a fallacy...and...

There is no physics that works at the "single photon perspective",

http://www.newyorker.com/tech/elements/the-space-doctors-big-idea-einstein-general-relativity

" ...The numbers said that the wave moved through space a certain distance every second. (The distance is about seven times around Earth.) They didn’t say what was sitting still. They just said a certain distance every second.

It took people a while to realize what a huge problem this was. The numbers said that everyone will see light going that same distance every second, but what happens if you go really fast in the same direction as the light? If someone drove next to a light wave in a really fast car, wouldn’t they see the light going past them slowly? The numbers said no—they would see the light going past them just as fast as if they were standing still... And the numbers said that no matter how fast you move, light moves past you {Locally} at a certain distance every second {the same fixed velocity}.

In other words, no matter how fast you go, light still passes you locally at the same old velocity, "c". You cannot catch up to light,never...you can never get to velocity 'c' locally.

 

[CuriousFamily]

When you say that "you cannot catch up light, never...you can never get to velocity 'c' locally," what does that mean? Does "you" refer only to human observers? If two photons are traveling next to each other in the same direction, and they approach and pass a photon traveling in the opposite direction, I don't understand how each photon can be said to move "past" the other photons at the same fixed velocity.

 

[Ibix]

When you say that "you cannot catch up light, never...you can never get to velocity 'c' locally," what does that mean? Does "you" refer only to human observers? If two photons are traveling next to each other in the same direction, and they approach and pass a photon traveling in the opposite direction, I don't understand how each photon can be said to move "past" the other photons at the same fixed velocity.

Whatever you do to it, something with mass (that includes humans, even supermodels) will never be able to travel at the speed of light. Furthermore, it will always measure a pulse of light passing it at c, however much you accelerated it.

To you, or any other observer, you just have two photons going in one direction at speed c and one in the opposite direction at speed c. That's all there is to say about your scenario from that perspective. I think you are asking about the photon's perspective. Don't. Any attempt to describe a "photon's perspective" is self contradictory, since it would require a description in which light is stationary. However, one of the postulates of relativity is that light is always traveling at c. Photons don't have a perspective. Trying to reason as if they did is where the OP in this thread is going wrong.

 

[ZapperZ]

When you say that "you cannot catch up light, never...you can never get to velocity 'c' locally," what does that mean? Does "you" refer only to human observers? If two photons are traveling next to each other in the same direction, and they approach and pass a photon traveling in the opposite direction, I don't understand how each photon can be said to move "past" the other photons at the same fixed velocity.

The problem here is that you think you CAN be in a photon's perspective.

Think about it. Special Relativity, which is what all of this is based on, says that the speed of light is the SAME in ALL inertial reference frame. PERIOD!

This means that if you can imagine yourself being with the photon, then SR is no longer valid, and you will have to invent a whole NEW PHYSICS to do your description. You can't use any of the existing ones. It also means that the concept of "observers" and "observing" will have to be redefined and reinvented for that reference frame. All of SR's equations and descriptions can't be used, because there were never meant to be in a frame where the speed of light is zero in that frame.

Now, you may want to say that in that case, we need to invent new ones. But that's a different topic entirely, and one that will require someone to first publish those new set of rules of physics in a peer-reviewed journals BEFORE they can be applied here in this forum. Till that happens, all we have, and the BEST we have, are SR/GR. If you want to follows SR/GR's rules, then being in the frame of a photon doesn't work.

Zz.

 

[Erez Lerner]

then relative to that coordinate system the speed of light appears to vary slightly

When you say vary slightly, do you mean up to and never past the limit of c?
Or do you mean it can appear to pass that limit? i.e. appear to travel faster then light speed?

 

[Ibix]

Light apears to travel slower in the circumstances Jonathan Scott is talking about. Google "Shapiro delay".

 

[Jonathan Scott]

When you say vary slightly, do you mean up to and never past the limit of c?
Or do you mean it can appear to pass that limit? i.e. appear to travel faster then light speed?

The variation is relative to the coordinate system, so it depends what choices were made in setting up the coordinate system to map the region. The general rule for the usual types of coordinate system used in this case is that the speed of light relative to the coordinate system gets slower in a deeper gravitational potential, by a factor of approximately (1-2Gm/rc²) near a central mass m at distance r. The conventional choice is that the scale of the coordinate system is chosen to match local space "at infinity", so the effective speed relative to the map is a little slower than the standard speed everywhere.

 

[Erez Lerner]

Light apears to travel slower in the circumstances Jonathan Scott is talking about. Google "Shapiro delay".

Excellent. Shapiro delay is the answer I was looking for.
It only says that light slows down rather than speed up.
Thanks.
Further, a new physics for "photon perspective" is long overdue if you ask me. I wish someone would invent it.
We'll never build light-speed space ships without it ;)

 

[weirdoguy]

I wish someone would invent it.

No one can invent it because it contradicts special relativity.

 

[ZapperZ]

Further, a new physics for "photon perspective" is long overdue if you ask me.

I don't believe anyone did.

Zz.

 

[Nugatory]

Light apears to travel slower in the circumstances Jonathan Scott is talking about. Google "Shapiro delay".

Excellent. Shapiro delay is the answer I was looking for.
It only says that light slows down rather than speed up.

Do note that Ibix very carefully said "appears to travel slower", not that it does slow down. At every point on its path the flash of light is moving at speed c, so the Shapiro effect can be understood as the distance from Venus to Earth being lengthened by the effect of the sun's gravity. The light signal takes longer to get back to us because it is traveling farther than we think.

Further, a new physics for "photon perspective" is long overdue if you ask me. I wish someone would invent it.

However, the universe isn't asking you, it's telling you...
Physics is about observing and understanding the way the universe we live in works, and in that universe the phrase "photon perspective" makes about as much sense as "older brother of an only child".

We'll never build light-speed space ships without it ;)

You might want to try googling for "Alcubierre drive". It's possible, even likely, that no such thing can be built, but it's still a strong hint that you shouldn't give up on the physics we have until you understand it.

 

[alw34]

Special Relativity, which is what all of this is based on, says that the speed of light is the SAME in ALL inertial reference frame. PERIOD!

It IS a 'crazy' idea that Einstein had. Takes a while to get used to it. Takes some thinking. It was a surprise, not expected. While it appears that at everyday low speeds, space and time are constants, Einstein realized they are not. He realized that, contrary to everyday perceptions, it is the speed of light that is a universal constant [locally]. And he was just getting started! Then it took him ten years to formulate general relativity, his theory of gravity.

 

[jerromyjon]

However, the universe isn't asking you, it's telling you...

Everyday we see the world around us. These days we have so much access to information in so many interactive medium there is bound to be sensory overload. That is when you are flooded with so many possibilities you can't lock onto the most promising choice, so you are forced to just jump in a nearly random direction and run with it. I've been there. You really have to back up and digest small pieces and really nourish your mind with strong understanding of basic concepts. Then when you try to put the whole picture together you see all the pieces coming together and you see what makes sense and what can't possibly work because it breaks other concepts you already know all too well...

 

[CuriousFamily]

ZapperZ: I think I understand what you're saying with respect to SR's equations not being meant to be used in a frame in which the speed of light is zero. Forgive my ignorance if I don't. Is there a particular speed at which the equations no longer apply/work? For example, if imagining myself (or another observer) traveling with (or at the speed of) the hypothetical photons is not "proper," is it similarly "improper" to imagine myself traveling with (or at the speed of) neutrinos? The speed of the space shuttle? The speed of sound?

Am I correct in thinking that the equations would apply/work for an observer traveling at any speed less than the speed of light because, according to the most commonly-accepted theories, space and time would contract and dilate, respectively, in such a manner that the photons (or flashes of light) would appear to the observer to be traveling at a velocity of c?

Lastly, am I mistaken in thinking that one of the implications of Einstein's GR is that the speed of light waves emitted from a distant star will vary as the waves encounter massive objects (and their gravitational fields) along the way, but that that particular implication has largely been dismissed by most modern physicists?

 

[ZapperZ]

ZapperZ: I think I understand what you're saying with respect to SR's equations not being meant to be used in a frame in which the speed of light is zero. Forgive my ignorance if I don't. Is there a particular speed at which the equations no longer apply/work? For example, if imagining myself (or another observer) traveling with (or at the speed of) the hypothetical photons is not "proper," is it similarly "improper" to imagine myself traveling with (or at the speed of) neutrinos? The speed of the space shuttle? The speed of sound?

In ALL of those frames, except for the frame of reference of a photon, "c" is still a constant. That is one of the fundamental postulates of SR, that the speed of light is the same in an inertial reference frame.

If you are in a reference frame of a photon, then you are saying that "c" is zero, since you are moving with the photon. Then I put it to you that the concept of "moving" is now suspect. How do you know that you are "moving"? How would you measure such a thing, assuming that you have a "measuring device" that can do that. The concept of time, space, and a "measurement" no longer have the same meaning as what you THINK you know, because all of them are based on an inherent assumption and condition that abide by SR's rules. You think you can detect and adopt the same ideas, but you can't! ALL the rules, and I mean ALL, of them have to be reformulated, and we don't know what those are, or if that is even possible.

Lastly, am I mistaken in thinking that one of the implications of Einstein's GR is that the speed of light waves emitted from a distant star will vary as the waves encounter massive objects (and their gravitational fields) along the way, but that that particular implication has largely been dismissed by most modern physicists?

I have no idea what you are saying. Gravitational lensing is well known, but that has nothing to do with variation in the speed of light.

Zz.

 

[alw34]

Lastly, am I mistaken in thinking that one of the implications of Einstein's GR is that the speed of light waves emitted from a distant star will vary as the waves encounter massive objects (and their gravitational fields) along the way, but that that particular implication has largely been dismissed by most modern physicists?

The DIRECTION of light from open free space is changed by a massive, that is, gravitating body, not the local speed.

As you maintain 60 mph down a highway and drive through a curve,for example, your speed doesn't change as you continue to move over the local road at the same constant speed. But someone watching you from a distance might see your difference in distance traveled, thinking you were still moving in a straight line without any acceleration, and think "that car has slowed down a bit".

 

[Nugatory]

I think I understand what you're saying with respect to SR's equations not being meant to be used in a frame in which the speed of light is zero. Forgive my ignorance if I don't. Is there a particular speed at which the equations no longer apply/work? For example, if imagining myself (or another observer) traveling with (or at the speed of) the hypothetical photons is not "proper," is it similarly "improper" to imagine myself traveling with (or at the speed of) neutrinos? The speed of the space shuttle? The speed of sound?

The equations work for converting frame-dependent quantities (lengths, times, speeds, kinetic energies, momenta) from one frame to another as long as the relative speed between the two frames is less than (not less than or equal to) $c$. If one of the frames is the one in which you are at rest and you're working with an object that is at rest in the other frame, then the object's speed relative to you may be anything up to but not including $c$.

One consequence of the assumption that the speed of light is the same in all frames is that the relative speed between any two frames must be less than or equal to c. Thus, when we use that assumption to derive the various equations of special relativity, we are automatically bringing along the additional conclusion that there are no frames with relative speeds greater than or equal to $c$.

Am I correct in thinking that the equations would apply/work for an observer traveling at any speed less than the speed of light because, according to the most commonly-accepted theories, space and time would contract and dilate, respectively, in such a manner that the photons (or flashes of light) would appear to the observer to be traveling at a velocity of c?

You have to include relativity of simultaneity in there with time dilation and length contraction, but if you do, the answer is "yes". Try using the relativistic velocity addition law to answer the question "A flash of light is moving at speed $c$ relative to you, and you are moving at speed $v$ relative to me - what is the speed of the flash relative to me?" and you'll see how this works.

A historical note - this fact about the behavior of light was first observed in 1851, a decade before anyone had any idea what light was and more than a half-century before Einstein developed SR.

Lastly, am I mistaken in thinking that one of the implications of Einstein's GR is that the speed of light waves emitted from a distant star will vary as the waves encounter massive objects (and their gravitational fields) along the way, but that that particular implication has largely been dismissed by most modern physicists?

You are mistaken about that. The gravitational effects will change the direction the light is moving so that it ends up taking a longer path than it would if the massive objects were not there. It takes longer to get to us not because it's moving more slowly - it's still moving at speed $c$ - but because it had a longer distance to travel.
(Be warned that there are some subtleties involved in defining distance and speed in a curved spacetime and I'm glossing over them. You will have to nail down your understanding of special relativity and flat spacetime before you're ready to take on these subtleties).

 

[DrStupid]

Gravitational lensing is well known, but that has nothing to do with variation in the speed of light.

That depends on the coordinate system. Shapiro delay has already been mentioned above.

 

[CuriousFamily]

In ALL of those frames, except for the frame of reference of a photon, "c" is still a constant. That is one of the fundamental postulates of SR, that the speed of light is the same in an inertial reference frame.

If you are in a reference frame of a photon, then you are saying that "c" is zero, since you are moving with the photon. Then I put it to you that the concept of "moving" is now suspect. How do you know that you are "moving"? How would you measure such a thing, assuming that you have a "measuring device" that can do that. The concept of time, space, and a "measurement" no longer have the same meaning as what you THINK you know, because all of them are based on an inherent assumption and condition that abide by SR's rules. You think you can detect and adopt the same ideas, but you can't! ALL the rules, and I mean ALL, of them have to be reformulated, and we don't know what those are, or if that is even possible.
I have no idea what you are saying. Gravitational lensing is well known, but that has nothing to do with variation in the speed of light.

Zz.

Thank you for your response. Your response to my questions regarding the photons was particularly helpful. With respect to your last paragraph, I was asking whether I was mistaken in thinking that Einstein's equations predict that the speed of light will vary as the light travels through different gravitational "environments." My understanding is that Einstein predicted that the speed of light would vary in particular circumstances, but that his prediction is believed by many or most modern physicists to be inaccurate. Am I mistaken?

 

[CuriousFamily]

The DIRECTION of light from open free space is changed by a massive, that is, gravitating body, not the local speed.

As you maintain 60 mph down a highway and drive through a curve,for example, your speed doesn't change as you continue to move over the local road at the same constant speed. But someone watching you from a distance might see your difference in distance traveled, thinking you were still moving in a straight line without any acceleration, and think "that car has slowed down a bit".

I understand that what you wrote in your first sentence is now the commonly-accepted position. Am I correct, though, that Einstein's position contradicted what is now the commonly-accepted position?

 

[Nugatory]

I understand that what you wrote in your first sentence is now the commonly-accepted position. Am I correct, though, that Einstein's position contradicted what is now the commonly-accepted position?

No. I mentioned above that there are some subtleties in defining speed and distance in a curved spacetime... But today's understanding of the speed of light is also Einstein's - and it was his idea.

 

[Jonathan Scott]

There are two points of view involved here:

Locally, the speed of light in vacuum is always the same everywhere.

Over a larger region where gravity is involved, it is not possible to map space-time using a coordinate system which always matches local space-time, in the same way that it is not possible for a flat map to describe a large area of the Earth accurately at the same scale factor. Relative to the map, the speed of light therefore appears to vary slightly, so in a deeper gravitational potential it appears to be moving in a similar way to moving through a medium with a refractive index slightly greater than 1, and may be deflected accordingly.

As far as I know, Einstein not only agreed with this position, but he was the first to explain it clearly.

 

[DrStupid]

As far as I know, Einstein not only agreed with this position, but he was the first to explain it clearly.

Yes, he explained it at the last two pages of his original paper about GR:

http://myweb.rz.uni-augsburg.de/~eckern/adp/history/einstein-papers/1916_49_769-822.pdf

He calculated the light deflection with the Shapiro delay and the Huygens–Fresnel principle.

 

[CuriousFamily]

Thank you all. Again, your responses were helpful. I have another question (about "tired light"), but I'm going to search the forum for similar questions before I go down that rabbit hole.