Does gravity REALLY pull on light? Really?

In summary: The local speed is always the same, but the global speed is different near a massive object.The part of the ray closer to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray.No, time only runs slower on the ray as a whole. Time doesn't run slower for different parts of the ray.
  • #1
1MileCrash
1,342
41
Okay, so here's the though process that led me to ask this question.

We know that the speed of light through a vacuum is constant.

Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent. Gravity pulls on the light, apparently. But how can this be? For this to be so, that curved beam of light must be traveling slower on the end closer to the Earth (the more curved portion) and faster on the end away from the Earth (the less curved part). General relativity does say that the speed of light decreases near large masses, but is this really so?

What if the "bending" of the light had nothing to do with a pulling force of gravity? What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray.

Just some thoughts, have at it.
 
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  • #2
1MileCrash said:
We know that the speed of light through a vacuum is constant.
true

Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent.
Yes - from your point of view. From the light's point of view it's going in a straight line but space is bent.

Gravity pulls on the light, apparently. But how can this be?
Actually gravity pulls on space

For this to be so, that curved beam of light must be traveling slower on the end closer to the Earth (the more curved portion) and faster on the end away from the Earth (the less curved part).
Nope

The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray.
A photon is going at the speed of light time runs about as slow as it's possible to run. A photon doesn't experience time passing.
 
  • #3
NobodySpecial said:
A photon is going at the speed of light time runs about as slow as it's possible to run. A photon doesn't experience time passing.

Yes, you're right, I worded that horribly.

I didn't mean "experiences" as in from the photon's point of view, I meant what an outside observer sees it experiencing outside the zone of the time dilation.

Actually gravity pulls on space

Yes, and the space pushes on us, regardless, gravity exerts of a force on light, be it through pulling on space, etc. This is not important right now.

Nope

Could you elaborate? I'm trying to understand but according to my brain, a curved light beam has to travel slower on the bottom end and faster on the top end to arrive in one piece.
 
  • #4
1MileCrash said:
Could you elaborate? I'm trying to understand but according to my brain, a curved light beam has to travel slower on the bottom end and faster on the top end to arrive in one piece.
No one says a beam has to arrive in one piece. Photons on side of a wide beam of light don't know anything about photons on the other side.
Light traveling through your camera lens doesn't all arrive at the same time
 
  • #5
NobodySpecial said:
No one says a beam has to arrive in one piece. Photons on side of a wide beam of light don't know anything about photons on the other side.

Then that simply means that the beam is not bent at all..
 
  • #6
1MileCrash said:
Then that simply means that the beam is not bent at all..
No, it doesn't, it means the beam spreads out as it curves.
 
  • #7
HallsofIvy said:
No, it doesn't, it means the beam spreads out as it curves.

Okay, I'm not sure I'm understanding.

If I were to shine a perfect circle ray of light near earth, it would bend with Earth until it passed it, right? If I interrupted its path with a big, white poster (or something) and the light hit it, what shape would we see on the poster?
 
  • #8
1MileCrash said:
Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent. Gravity pulls on the light, apparently. But how can this be? For this to be so, that curved beam of light must be traveling slower on the end closer to the Earth (the more curved portion) and faster on the end away from the Earth (the less curved part). General relativity does say that the speed of light decreases near large masses, but is this really so?
You have to distinguish between the local speed in a very small patch of spacetime where the effects of curvature are negligible, and the coordinate speed in some coordinate system covering a large region of curved spacetime. In the first case the equivalence principle says you can construct a "local inertial frame" in that patch and that light will always have a speed of c in that local frame, but in the second case your coordinate system cannot qualify as "inertial" so the coordinate speed of light may differ from c (even in special relativity where spacetime is flat, the coordinate speed of light may differ from c if you use a non-inertial frame).
 
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  • #9
You would probably "observe" the same shape of light...depending on the accuracy of your experimental measurement. But there would be a tiny,tiny difference between the bending of the light nearer Earth than farther away...because space is bent slightly more closer to earth...so the image is actually distorted slightly from its original shape.

I think that's what Halls of Ivy meant.

"Photons on (one) side of a wide beam of light don't know anything about photons on the other side."

Actually, they do. Since all photons have energy there is a tiny gravitational effect between them, but it's dwarfed by the curvature of space from the object causing the beam of light to bend and so can be ignored for practical purposes.

"What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve."

In a way you can say that...but the point is curvature of spacetime, gravitational potential, and time dilation all happen concurrently. Einstein's work clearly reflects that space and time are an integrated entity...(even Einstein did not initially realize that) so the change sometimes is due more to one than the other. It's more accurate to say that spacetime is curved,or warped, rather than referring to either space or time as a separate entity.
 
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  • #10
Probably the most powerful way of thinking of gravity is to think of it as curved space-time. And the philosophical consequence of this point of view is that it doesn't "really" pull on anything. Note that any talk about what "really" happens is philosophical, and that it is generally possible to explain any particular phenomenon in several different ways. So there isn't "really" any single answer to questions about what "really" happens, it's usually possible to provide several, different explanations of what "really" happens, all of which match up perfectly with experiment. As far as science goes, all of these different explanations are "equally good", so there isn't any way to choose among them.

It takes a bit of math to fully appreciate how curved space-time leads to the appearance of a force, however. One can actually draw simple pictures that illustrate this, but it's difficult to communicate, as the exacting technical wording in terms of geodesic's appearing to accelerate away from each other tends to be unfamiliar, and every-day attempts to describe the phenomenon are a bit vague due to the imprecision of everyday language.

One alternative is to look at the unified whole presented by General Relativity of gravity as curved-space time, and to break it up into pieces. One can think of one particular piece of the unified whole is a "force", and another piece of the whole as "gravitational time dilation", and yet another piece as "curved space".

http://www.eftaylor.com/pub/chapter2.pdf has some information about how curvature in General Relativity really works for those that might be interested.
 
  • #11
to the op:
gravitational time dilation will indeed cause light to curve.
in fact, when Dr Einstein first calculated the bending of light near the sun that is all he used.
Unfortunately the result was only half of what was actually observed so
he went back and included a factor for the stretching of space and that doubled the end result.​
 
  • #12
granpa said:
to the op:
gravitational time dilation will indeed cause light to curve.
in fact, when Dr Einstein first calculated the bending of light near the sun that is all he used.
Unfortunately the result was only half of what was actually observed so
he went back and included a factor for the stretching of space and that doubled the end result.​

Interesting!
 
  • #13
1MileCrash said:
Okay, I'm not sure I'm understanding.

If I were to shine a perfect circle ray of light near earth, it would bend with Earth until it passed it, right? If I interrupted its path with a big, white poster (or something) and the light hit it, what shape would we see on the poster?
An ellipse with major axis through the center of the earth.
 
  • #14
HallsofIvy said:
An ellipse with major axis through the center of the earth.
Why "through the center of the Earth"? The thought-experiment was just that the initially circular beam was shined "near" the Earth.
 
  • #15
1MileCrash said:
Okay, so here's the though process that led me to ask this question.

We know that the speed of light through a vacuum is constant.

Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent. Gravity pulls on the light, apparently. But how can this be? For this to be so, that curved beam of light must be traveling slower on the end closer to the Earth (the more curved portion) and faster on the end away from the Earth (the less curved part). General relativity does say that the speed of light decreases near large masses, but is this really so?
Yes, it is really so. The speed of light near a large mass is slower relative to the speed of light further away from the large mass according to the coordinate calculations of a distant observer.

1MileCrash said:
What if the "bending" of the light had nothing to do with a pulling force of gravity? What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray.

Just some thoughts, have at it.
I understand what you are getting at here and I understand that you are not talking about the "point of view" of a photon. However, can you justify that light lower down moving slower than light higher up brings about curvature of the light path by itself? For example let us say we two parallel laser beams in a vacuum. One beam has a cube of glass in its path with its incident surface orthogonal to the beam. The beam passing through the glass slows down (due to the refractive index of glass) while the other beam continues at c. The two rays remain parallel to each other at all times. Different velocities do no automatically bring about curvature of the path. Now in high school physics they teach that a beam of light bends as it passes through a prism at an oblique angle to the incident surface, due to the part of the beam that hits the incident surface first slowing down, but personally I think that is a bad explanation. To me, the slowing down is a side effect rather than a cause of bending. One interesting aspect is that the path followed by a ray of light is the fastest possible route between the emitter and the receiver (sometimes called the principle of least action?) and this is might be a more productive avenue to explore?
 
  • #17
granpa said:
to the op:
gravitational time dilation will indeed cause light to curve.
in fact, when Dr Einstein first calculated the bending of light near the sun that is all he used.
Unfortunately the result was only half of what was actually observed so
he went back and included a factor for the stretching of space and that doubled the end result.​

Johann Georg von Soldner did some calculations in 1801 about light bending, were they very close to Einstein?
 
  • #18
I don't know anything about that
I can only quote wikipedia
http://en.wikipedia.org/wiki/Johann_Georg_von_Soldner
Albert Einstein calculated and published a value for the amount of gravitational light-bending in light skimming the Sun in 1911,
leading Phillipp Lenard to accuse Einstein of plagiarising Soldner's result.
Lenard's accusation against Einstein is usually considered to have been at least partly motivated by
Lenard's Nazi sympathies and his enthusiasm for the Deutsche Physik movement.

At the time, Einstein may well have been genuinely unaware of Soldner's work, or
he may have considered his own calculations to be independent and free-standing, requiring no references to earlier research.
Einstein's 1911 calculation was based on the idea of gravitational time dilation.

In any case, Einstein's subsequent 1915 general theory of relativity quickly argued that all these calculations had been incomplete,
and that the "classic" Newtonian arguments, combined with light-bending effects due to gravitational time dilation,
gave a combined prediction that was twice as high as the earlier predictions
 
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  • #19
granpa said:
I don't know anything about that
I can only quote wikipedia
http://en.wikipedia.org/wiki/Johann_Georg_von_Soldner

It's fascinating that Soldner calculations came 100 years before Einstein, but he couldn't challenge any astronomer to get some pictures during a solar eclipse...
 
  • #20
myuncle said:
It's fascinating that Soldner calculations came 100 years before Einstein, but he couldn't challenge any astronomer to get some pictures during a solar eclipse...

Newton himself considered light to consist of particles but at that time they could assign a figure to the mass of a photon. However, the downward acceleration of a small particle due to gravity is independent of its mass (Galileo), so anyone could work out how much the bending of light would be due to Newtonian considerations would be without any regard to whether the photon had mass or not. This, I assume is what Soldner did, simply work out the Newtonian downward acceleration of a particle moving at the speed of light and work out how much a light ray would bend as a result. There would have been no strong urge to test Soldner's calculations as at that time no one really thought there were any problems with Newtonian physics. When Einstein predicted that the bending of light ray would be twice what Newtonian physics (or Soldner) predicted at time when there there was still some uncertainty in people's minds as to whether Newtonian physics or Relativity was a better reflection of reality, then there would have a lot of interest. For some reason, vast amounts of research funds are still spent on testing the limits of Relativity predictions, presumably because people are waiting for the next theory to supercede relativity as relativity superseded Newtonian physics. I assume that because I find it hard to believe any serious scientists still think Newtonian physics is more accurate than relativity.
 
  • #21
No - while it is true that Einstein changed his prediction in 1916, that was because he didn't have the full theory of general relativity until 1915.

The first experimental tests were done in 1919, 3 years after his 1916 prediction, by Eddington. So it wasn't a case of him changing his predictions to match the experimental results at all.

There was immediate excitement about the results, followed by delayed controversy (in which politics probably played some part). After the results were confirmed by a more accurate experiment in 1922 by Lick, who was IIRC an opponent of the theory and in favor of a different one, the controversy was mostly ended.

The paper you quote by Poor strikes me as having a "cranky" feel to it. For instance, the derivation of the deflection of light was published in the journals. Poor's complaint that said derviation wasn't published in popularizations of the time seems distinctly odd, and his wording is such to invite the unwary reader to believe that there is something suspect about the derivation. One can nowadays find the derivation in various forms in standard textbooks, and there's nothing at all mysterious, ambiguous, or philosophical about it.
 
  • #22
I notice that you didnt say that he was actually wrong about anything.
whether he was 'cranky' or not should not be relevant.

He was a professional astronomer.
wikipedia says
Charles Lane Poor (January 18, 1866 – September 27, 1951)[1][2] was born in Hackensack, New Jersey, the son of Edward Erie Poor. He graduated from the City College of New York and received a Ph.D. in 1892 from Johns Hopkins University. Poor became an American astronomer and professor of celestial mechanics at Columbia University from 1903 to 1944, when he was named Professor Emeritus.
 
  • #23
Light can also be bent through curvatures in space-time. The light that crosses a curvature will deviate from its original path.
 
  • #24
granpa said:
I notice that you didnt say that he was actually wrong about anything.
whether he was 'cranky' or not should not be relevant.

He was a professional astronomer.
wikipedia says
Charles Lane Poor (January 18, 1866 – September 27, 1951)[1][2] was born in Hackensack, New Jersey, the son of Edward Erie Poor. He graduated from the City College of New York and received a Ph.D. in 1892 from Johns Hopkins University. Poor became an American astronomer and professor of celestial mechanics at Columbia University from 1903 to 1944, when he was named Professor Emeritus.

My reading of the paper is that what he says is accurate, but more or less pointless. The pointlessness of what he is saying is what gives the paper a "cranky" feel to me.

Let me take one specific section that drew my attention.

by obtaining results which "are in exact accord with the requirments of Einstein's theory.

But just what the requirements of the theory really are, and how they result from the theory, neither Einstein or any of his followers has explained in simple, understandable language.

Now, if you take this literally, there's nothing the author said that's factually incorrect. It's only when you assume some implied meaning, some purpose for the author writing what he wrote, that there is any sort of issue.

What he actually says in the above paragraph that Einstein's theory is hard to understand. One might even add in the implication, though not directly stated "it's hard to understand even for a professional astronomer such as myself". This should not be a huge surprise. It's not an easy theory to understand.

The not-directly stated implication is that the theory is not well defined. This unstated implication is false. While one might have a certain amount of sympathy for astronomers faced with such a radical new theory, the fact that they didn't immediately understand it isn't really worthy of publication.

Of course if the author had written

"I don't understand this newfangled relativity theory, and by golly, I don't trust it."

the pointlessness of what he was saying would probably have been obvious enough that it wouldn't have passed peer review.

Nowadays, the calculation of light deflection is something that every graduate student in the field of astronomy is exposed to. It's still more work to do the calculation than one would want to put into a PF post, but there is nothing "fuzzy" about it. It's basically just a matter of writing down the metric, and solving the geodesic equations (second order differential equations) to find the path that light takes.

The geodesic equations are written out for instance in http://nedwww.ipac.caltech.edu/level5/March01/Carroll3/Carroll3.html in equation 3.47. It's just a matter of solving these equations with the proper initial conditions to find the particular path that any given light beam takes, and "just" a matter of selecting the correct initial conditions that correspond to the experiment to see whether or not the theory works.

Some of the symbols used might be unfamiliar, but the Christoffel symbols [itex]\Gamma[/itex] are well defined in terms of partial derivatives of the metric in earlier sections of Caroll's paper, and have standard and well-defined meanings.

So - the point is that Einstein has a theory, a theory that predicts specific differential equations that should predict the path of light. A detailed analysis of these equations show that light should be deflected twice as much as previous theories would predict - and when the experiment was actually performed, which was 3 years (for the very first results) after Einstein published his prediction, the results came out in agreement with Einstein's theory. Also, there was some controversy about the result, better results didn't come out to confirm the initial results for another 3 years, i.e. six years after the publication of GR. Since then, even more accurate tests have been done. All of the tests are consistent with the predictions of General Relativity, along with other tests such as the perihelion advance of Mercury.
 
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  • #25
yuiop said:
Yes, it is really so. The speed of light near a large mass is slower relative to the speed of light further away from the large mass according to the coordinate calculations of a distant observer.

I understand what you are getting at here and I understand that you are not talking about the "point of view" of a photon. However, can you justify that light lower down moving slower than light higher up brings about curvature of the light path by itself? For example let us say we two parallel laser beams in a vacuum. One beam has a cube of glass in its path with its incident surface orthogonal to the beam. The beam passing through the glass slows down (due to the refractive index of glass) while the other beam continues at c. The two rays remain parallel to each other at all times. Different velocities do no automatically bring about curvature of the path. Now in high school physics they teach that a beam of light bends as it passes through a prism at an oblique angle to the incident surface, due to the part of the beam that hits the incident surface first slowing down, but personally I think that is a bad explanation. To me, the slowing down is a side effect rather than a cause of bending. One interesting aspect is that the path followed by a ray of light is the fastest possible route between the emitter and the receiver (sometimes called the principle of least action?) and this is might be a more productive avenue to explore?

That a gradient in speed has an optical effect you can see on a hot day, when far objects near the horizon are deformed. And the gradient in speed of light is indeed the cause of light bending according to GRT: the school physics is based on the Huygens construction which Einstein used for his calculation[1] of light bending. About the principle of least action, I recently read somewhere (sorry I don't recall where, perhaps it was actually on this forum) that it can be derived from Huygens.

Thus Einstein explained in 1916 (1920 translation, and here Geschwindigkeit simply means speed):

"our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo [..] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position."
- http://www.bartleby.com/173/22.html

1. http://www.Alberteinstein.info/gallery/gtext3.html [Broken] (light bending is near the end)
 
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  • #26
Acknowledging that massive objects warp space and our apparent observation of light is perturbed, how can we draw any conclusions about stars that are biilions of light years from us? If one had a model of the universe and could predict observations as a function of time from our vantage point in space, would not light from a very distant object follow some tortuous path through space time, making any conclusions of our apparent observations highly suspect?
 
  • #27
drfwl said:
Acknowledging that massive objects warp space and our apparent observation of light is perturbed, how can we draw any conclusions about stars that are biilions of light years from us? If one had a model of the universe and could predict observations as a function of time from our vantage point in space, would not light from a very distant object follow some tortuous path through space time, making any conclusions of our apparent observations highly suspect?

The curvature of space-time does complicate things - for instance, one needs information about the particular model/map of the universe to compute the behavior of apparent luminosity, which is one of the astronomical distance scales.

But it's not fundamentally impossible to match up the models we have of curved space time with observations, it's just more work.

And we have a fairly standard model, the concordance model called lambda-CDM, that provides good predictions and matches with observation.
 
  • #28
1MileCrash said:
Okay, so here's the though process that led me to ask this question.

We know that the speed of light through a vacuum is constant.

Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent. Gravity pulls on the light, apparently. But how can this be? For this to be so, that curved beam of light must be traveling slower on the end closer to the Earth (the more curved portion) and faster on the end away from the Earth (the less curved part). General relativity does say that the speed of light decreases near large masses, but is this really so?

What if the "bending" of the light had nothing to do with a pulling force of gravity? What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray.

Just some thoughts, have at it.

It may be useful to reply to this original post, although others have already answered most of it.

Locally measured the ray is always traveling at c, but as seen from far away the ray is slowed down near to the mass; consequently a light clock would be slowed down, and this slow-down effect is general ("time differs along the ray", as you say). What you see as alternative ideas are in fact the same idea put in different words. Einstein's prediction is based on the conclusion that the speed of light as measured from far away, is affected by the presence of the sun; he calculated the bending of light that results from the gradient in speed based on the Huygens principle which is also used in optics - "gravitational lensing".

Thus gravitational time dilation does play a role as you think, and according to GRT there is indeed no literal "pulling" on the light. However, the bending is not only related to gravitational time dilation, but it is also related to gravitational length contraction in radial direction. Gravitational time dilation alone yields the same deflection as from Newton's theory, which is half that of GRT.
 

1. How does gravity affect light?

Gravity affects light by bending its path as it travels through space. This is known as gravitational lensing and is a result of the curvature of space-time caused by massive objects like stars or black holes.

2. Does gravity pull on light with the same force as it does on matter?

Yes, gravity pulls on light with the same force as it does on matter. According to Einstein's theory of general relativity, gravity is the curvature of space-time caused by the presence of mass or energy. Since light has energy, it is also affected by gravity in the same way as matter.

3. Can light escape from a black hole due to gravity?

No, light cannot escape from a black hole due to its strong gravitational pull. As light travels towards a black hole, its path is bent more and more until it reaches a point of no return, known as the event horizon. Beyond this point, the gravitational pull is so strong that even light cannot escape.

4. Does gravity affect all types of light equally?

Yes, gravity affects all types of light equally. This is because gravity is a result of the curvature of space-time, which is independent of the type or wavelength of light. However, the amount of bending of light may vary depending on its energy and direction of travel.

5. Can gravity be used to bend light for practical purposes?

Yes, the phenomenon of gravitational lensing can be used to bend light for practical purposes. This is commonly used in astronomy to study distant objects that would otherwise be too faint to observe. It can also be used in gravitational wave detectors, which use the bending of light to detect the ripples in space-time caused by massive objects like black holes.

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