# Are photons affected differently by gravity?

Nethral
I haven't found any source confirming my thoughts so I guess I'm wrong, but I was hoping someone could explain it to me :) This is how I'm thinking...

Mass <=> energy, and therefore you could say that energy is also affected by gravity (light, for example). Then more energetic light should be more affected by gravity than less energetic light, like in the way a heavier object is more affected by the earth than a lighter object.

Or am I wrong? For example, is it wrong to say that "energy is affected by gravity"? I guess the common explanation is that light just follows a straight path through universe, thereby being "affected" by bent space-time/gravity...

Hope someone can explain this to me. Thanks in advance!

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Then more energetic light should be more affected by gravity than less energetic light, like in the way a heavier object is more affected by the earth than a lighter object.

There's nothing wrong with this.

Example #1: A 1-joule flash of light passes by the earth.

Example #2: A 2-joule flash of light passes by the earth, at the same distance of closest approach.

The change in the earth's momentum is twice as much in example #2. An observer who sees this change in momentum will infer that the earth was subjected to twice as much force.

I guess the common explanation is that light just follows a straight path through universe, thereby being "affected" by bent space-time/gravity..

This is just a different way of describing the same thing.

Nethral
Thank you, bcrowell!

I kind of understand your examples but also kind of... not. For example, I have learned that photons are affected by gravity by gravitational redshift and bending. Would then blue light be more bent than red light? Or would blue light have a greater gravitational redshift/energy loss?

Staff Emeritus
Gold Member
I kind of understand your examples but also kind of... not. For example, I have learned that photons are affected by gravity by gravitational redshift and bending. Would then blue light be more bent than red light?
No. General relativity is a geometrical theory. Think of the axiom in Euclidean geometry that says that a line segment can always be extended in a unique way. The same is true in GR. In GR, the "lines" are geodesics, which are interpreted as the world-lines of particles that aren't subjected to any nongravitational forces. Since a geodesic is uniquely determined by any initial segment, you can't have different geodesics for red and blue light.

Or would blue light have a greater gravitational redshift/energy loss?

No. The fractional energy change $\Delta E/E$ is the same regardless of initial color. One way to see this is that you can interpret the frequency change as a time dilation effect, and time dilation is universal in the sense that it doesn't matter what clock you use to measure it.

Altabeh
Thank you, bcrowell!

Would then blue light be more bent than red light? Or would blue light have a greater gravitational redshift/energy loss?

Energy of photon does not have any interference with the bending of its trajectory in a strong gravitational field. This is because the geodesic equations have no inclusion of mass, but only the components of the proper 4-velocity, metric and its first derivatives.

AB

cragar
would blue photons creates a stronger gravitational field then red photons ?

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would blue photons creates a stronger gravitational field then red photons ?

Yes, for a fixed number of photons, because the energy would be greater.

utesfan100
Thank you, bcrowell!

I kind of understand your examples but also kind of... not. For example, I have learned that photons are affected by gravity by gravitational redshift and bending. Would then blue light be more bent than red light? Or would blue light have a greater gravitational redshift/energy loss?

Suppose we have a "purple" photon make of a red photon at 700 hertz and a blue photon at 400 hertz moving at the same spot in the same direction.

Would this pair of photons stay coherent in a gravitational lens? The observation of star light shows no angular defect due to gravitational lenses, so experimentally the answer is not as far as we can measure.

The force of gravity should be higher for the blue photon than for the red photon by a factor of 7/4.

The momentum for the blue photon will also be 7/4 that of the red photon. Force is dP/dt.

The bending is caused by the force in the direction perpendicular to the motion. This is P*sin(angle of approach). Since gravitational force is proportional to momentum for a photon its energy/frequency does not alter its path. This is like the mass not changing the path of an object in classical mechanics, as the mass of the orbiting object cancels out of the equations of motion.

Altabeh
Yes, for a fixed number of photons, because the energy would be greater.

How are photons able to produce gravitational field with a vanishing rest-mass? Let alone the energy of the field. The gravitational correction to the Minkowski spacetime in the case of a photon is zero due to its zero mass!

AB

Frame Dragger
I feel as though this is yet another place where The Parable of The Apple from MTW can do some good for the OP. Granted, it's a bit of a step back, but I think we may have lost the OP there.

@Nethral: From Misner, Thorne, and Wheeler's "Gravitiation:
MTW said:
Once upon a time a student lay in a garden under an apple tree
reflecting on the difference between Einstein's and Newton's views
about gravity. He was startled by the fall of an apple nearby. As he
looked at the apple, he noticed ants beginning to run along its
surface. His curiosity aroused, he thought to investigate the
principles of navigation followed by an ant. With his magnifying
glass, he noticed one track carefully, and, taking his knife, made a
cut in the apple skin one mm above the track and another cut one mm
below it. He peeled off the resulting little highway of skin and laid
it out on the face of his book. The track ran as straight as a laser
beam along this highway. No more economical path could the ant have
found to cover the ten cm from start to end of that strip of skin. Any
zigs and zags or even any smooth bend in the path on its way along the
apple peel from starting point to end point would have increased its
length.

"What a beautiful geodesic," the student commented.

His eye fell on two ants starting off from a common point P in
slightly different directions. Their routes happened to carry them
through the region of the dimple at the top of the apple, one on each
side of it. Each ant conscientiously pursured his geodesic. Each went
as straight on his strip of appleskin as he possibly could. Yet
because of the curvature of the dimple itself, the two tracks not only
crossed but emerged in very different directions.

"What happier illustration of Einstein's geometric theory of gravity

murmured the student.

"The ants move as if they were attracted by the apple stem. One might
have believed in a Newtonian force at a distance along his track. This
is surely Einstein's concept that all physics takes place by 'local
action'. What a difference from Newton's 'action at a distance' view
of physics! Now I understand better what this book means"

@Altabeh: They may have no (or miniscule) 'rest-mass', but energy/stress/momentum deforms spacetime. I don't see why that would be different for light; we already know it's SUBJECT to gravitational fields, and I was under the impression that was always (excepting theoretical constructs) a mutual effect. Light still follows a geodesic informed and deformed by local geometry, or so it seems, and if we accept photons as discrete quanta of energy, then the SET has to kick in.

Yes, for a fixed number of photons, because the energy would be greater.

How? Classically, the electromagnetic field should curve spacetime as a solution of Einstein-Maxwell equations. But does frequency enter the classical energy-momentum tensor? Photon energy being proportional to frequency seems to be from quantum field theory. But in QFT on curved spacetime, the field doesn't contribute to spacetime curvature.

Staff Emeritus
Gold Member
bcrowell said:
Yes, for a fixed number of photons, because the energy would be greater.
How? Classically, the electromagnetic field should curve spacetime as a solution of Einstein-Maxwell equations. But does frequency enter the classical energy-momentum tensor?
No, frequency doesn't. But for a fixed number of photons, the energy is proportional to the frequency.

Frame Dragger
@bcrowell: Do you know of any good papers on this? I'd love to read some experimental and theoretical thinking on this.

Staff Emeritus
Gold Member
@bcrowell: Do you know of any good papers on this? I'd love to read some experimental and theoretical thinking on this.

Isn't this all pretty standard textbook stuff?

No, frequency doesn't. But for a fixed number of photons, the energy is proportional to the frequency.

What does the formalism look like? Is it QED + Donoghue's GR as an effective QFT?

Staff Emeritus
Gold Member
What does the formalism look like? Is it QED + Donoghue's GR as an effective QFT?

I'm claiming that there's nothing in any of this beyond freshman relativity. If you keep the number of photons constant while increasing the frequency, you get more mass-energy. A greater mass-energy means proportionately stronger gravitational interactions.

Frame Dragger
I'm claiming that there's nothing in any of this beyond freshman relativity. If you keep the number of photons constant while increasing the frequency, you get more mass-energy. A greater mass-energy means proportionately stronger gravitational interactions.

Ah, that's not in line with my (albiet limited) understanding of QED, nor is it "textbook".

So, if Bob sends a radio signal to Alice, and Alice and Bob are converging (so, blue-shift), its participation in the gravitational interaction becomes stronger as it shifts from radio, through visible, into UV and beyond?

Staff Emeritus
Gold Member
So, if Bob sends a radio signal to Alice, and Alice and Bob are converging (so, blue-shift), its participation in the gravitational interaction becomes stronger as it shifts from radio, through visible, into UV and beyond?

This seems like a much less conceptually straightforward case to me. In your example, you have a whole bunch of other factors that are changing, not just the energy of the radio beam.

The original example seems very simple and straightforward to me. If you want a slightly more detailed version of the argument in #16, here you go. In the limit of weak fields, GR is to an excellent approximation a linear theory, so that we can think in terms of superposing the gravitational field of a ray of light on top of some background field. The light's field is some (non-Schwarzschild) metric, and its $g-\eta$ superposes with the $g-\eta$ of the background field, where $\eta$ is the Minkowski metric. The light beam's $g-\eta$ is (in the limit of weak-field GR) linearly proportional to the stress-energy tensor, and the stress-energy tensor is in turn linearly proportional to the energy of the beam of light.

Frame Dragger
This seems like a much less conceptually straightforward case to me. In your example, you have a whole bunch of other factors that are changing, not just the energy of the radio beam.

The original example seems very simple and straightforward to me. If you want a slightly more detailed version of the argument in #16, here you go. In the limit of weak fields, GR is to an excellent approximation a linear theory, so that we can think in terms of superposing the gravitational field of a ray of light on top of some background field. The light's field is some (non-Schwarzschild) metric, and its $g-\eta$ superposes with the $g-\eta$ of the background field, where $\eta$ is the Minkowski metric. The light beam's $g-\eta$ is (in the limit of weak-field GR) linearly proportional to the stress-energy tensor, and the stress-energy tensor is in turn linearly proportional to the energy of the beam of light.

Well, that does seem very straightforward and "freshman". Thanks bcrowell, sometimes it's tough sorting out the Classical from the Semi-Classical from the Quantum, etc... etc...

yuiop
Mass <=> energy, and therefore you could say that energy is also affected by gravity (light, for example). Then more energetic light should be more affected by gravity than less energetic light, like in the way a heavier object is more affected by the earth than a lighter object.

It is probably better to think of it as the Earth being more affected (accelerated) by the heavier object than the lighter object. The heavier object on the other hand is affected exactly the same as the lighter object by the Earth and both are accelerated towards the Earth at exactly the same rate. The rate that a particle is accelerated towards the Earth is independent of its rest mass and so even a particle with zero rest mass (such as a photon) is accelerated like any other particle.

Suppose we have a "purple" photon make of a red photon at 700 hertz and a blue photon at 400 hertz moving at the same spot in the same direction.

Would this pair of photons stay coherent in a gravitational lens? The observation of star light shows no angular defect due to gravitational lenses, so experimentally the answer is not as far as we can measure.

The amount of gravitational deflection is independent of the rest mass of the test particle but it does depend on the horizontal velocity of the particle. All photons have the same velocity, so we can confidently say the red photon and the blue photon will deflect to exactly the same extent and so there is no chromatic or prismatic aberration in a gravitational lens unlike an uncorrected glass lens and so no galactic rainbows due to gravitational lensing.

Frame Dragger
Galactic Rainbows... would be amazing! Darned laws of physics! I'm claiming that there's nothing in any of this beyond freshman relativity. If you keep the number of photons constant while increasing the frequency, you get more mass-energy. A greater mass-energy means proportionately stronger gravitational interactions.

So you would say that light rays of different frequencies don't travel differently (say in gravitational lensing) because the classical state does not have a definite number of photons?

Frame Dragger
So you would say that light rays of different frequencies don't travel differently (say in gravitational lensing) because the classical state does not have a definite number of photons?

I can't tell if you're asking a question from lack of knowledge... because the feeling I keep getting here is, "It's A TRAP!!" cragar
It is probably better to think of it as the Earth being more affected (accelerated) by the heavier object than the lighter object. The heavier object on the other hand is affected exactly the same as the lighter object by the Earth and both are accelerated towards the Earth at exactly the same rate.

I mean this is probably crazy to bring this up , But wouldn't a heavier object distort space more so the force between that object and the earth would be a little greater , I mean the effect would be very small and negligible , but I am just wondering , There was like a thread a month ago about , shouldn't heavier objects fall faster based on this idea. I mean this would obviously make sense with 2 large bodies coming together.

Frame Dragger
I mean this is probably crazy to bring this up , But wouldn't a heavier object distort space more so the force between that object and the earth would be a little greater , I mean the effect would be very small and negligible , but I am just wondering , There was like a thread a month ago about , shouldn't heavier objects fall faster based on this idea. I mean this would obviously make sense with 2 large bodies coming together.

You're asking why lead and feathers fall at the same rate in a vacuum, essentially?

There was an example in another thread of dropping a particle from a given hight through the earth... an its rebound (oscillation) and so forth. I'm going to see if I can find that, and link it here, because it contains a better answer to your question than I'm likely to give.

The simple answer is that Earth always exerts a "steady" um... "pull" or pseudoforce, etc.. lets say "pull" of 9.8m/s^2. I could drop a planet from freefall, and it would fall at the same rate. A feather, or lead, or a melon, etc... all are following a geodesic determined by Earth's mass. A feather DOES have a gravitational field, and as you say it would exert some minute "force".

To use a bigger object, think of these nutty 2012 people, ok? They think some massive body is going to pass by Earth, and disrupt our orbit around Sol. In that scenario, BOTH bodies are falling towards each other (neglecting the momentum of the killer body), hence the idiotic concern that earth could be perturbed from its orbit (more than it is over time naturally). Of course, if something really massive DID make a pass, especially something far more massive than Earth, and CLOSE, we'd move. You'd notice it. :rofl:

Of course, finally... think of the moon. It's gravity effects Earth, and we sure as hell effect the moon (quakes, orbit, etc). If we took the two bodies at rest, and released them, they would fall towards each other. If the moon posessed a greater mass, the Earth would fall faster towards the moon. HOWEVER... the moon would fall no faster than 9.8m/s^2. The net "attraction" would appear to a human observer, to be a combination of the two. That's all orbit really is... falling past each other constantly.

So... the simplest answer is: Lead causes the earth to fall faster towards IT, but not visa versa. Each mass exerts constant gravity based on conditions, and that field appears to extend over an infinite distance, and always be additive. Gravity is odd. :tongue:

Note: I am ignoring a lot here for the sake of simplicity, but I fet I should give SOME answer.

cragar

I can't tell if you're asking a question from lack of knowledge... because the feeling I keep getting here is, "It's A TRAP!!" No, definitely not, but I appreciate the compliment I've long known about this heuristic, but I don't know how it can be made solid. The formalisms I know are:

1) classical GR + geodesic equation
2) classical GR + equation of state
3) QFT on curved spacetime
4) GR + electrodynamics as effective quantum field theory

(1) is a ray limit of (2), (2) and (3) are respectively classical and semiclassical limits of (4). In (1) and (3) the field does not modify spacetime, so presumably we should use either (2) or (4).

For (2), maybe a solution of the Einstein-Maxwell equations might work?
http://arxiv.org/abs/gr-qc/9704043
http://arxiv.org/abs/0808.0997

yuiop
I mean this is probably crazy to bring this up , But wouldn't a heavier object distort space more so the force between that object and the earth would be a little greater , I mean the effect would be very small and negligible , but I am just wondering , There was like a thread a month ago about , shouldn't heavier objects fall faster based on this idea. I mean this would obviously make sense with 2 large bodies coming together.

Hi Cragar,

Lets look at the Newtonian equations for gravity which is adequate for the masses and velocities involved in dropping small objects on the surface of the Earth. The force of gravity is given by:

$$\frac{GMm}{r^2}$$

where M in this case is the mass of the Earth and m is the mass of the small object.

The acceleration of the small object towards the Earth is given by:

$$\frac{GM}{r^2}$$

Note that the mass of the small object in absent in this term, so the acceleration of the small object towards the Earth is independent of the mass of the small object and so the acceleration of all objects dropped towards the Earth is identical.

The acceleration of the Earth towards the small object is:

$$\frac{Gm}{r^2}$$

From the above it can be seen that the acceleration of the Earth towards the small object IS dependent on the mass of the small object. Now if we have a lead weight of mass m1, the combined acceleration (acceleration of the lead weight towards the Earth AND acceleration of the Earth towards the lead weight) is:

$$a_1 = \frac{GM}{r^2}+ \frac{Gm_1}{r^2} = \frac{G(M +m_1)}{r^2}$$

and the combined acceleration of a bag of feathers (of mass m2) and the Earth towards each other is:

$$a_2 = \frac{GM}{r^2}+ \frac{Gm_2}{r^2} = \frac{G(M +m_2)}{r^2}$$

Now a1 is slightly different from a2 and this means that if a lead weight and a bag of feathers were dropped one at a time, the time for the lead weight to fall would be slightly less than the time for the bag of feathers to fall.

Now if the lead weight and the bag of feathers are dropped at exactly the same time, the acceleration of the Earth towards the combined mass of the lead and feathers is:

$$\frac{G(m_1+m_2)}{r^2}$$

and the acceleration of the lead weight towards the Earth, combined with acceleration of the Earth towards both small masses is:

$$a_3 = \frac{GM}{r^2} + \frac{G(m_1+m_2)}{r^2} = \frac{G(M +m_1+m_2)}{r^2}$$

and the acceleration of the feathers towards the Earth, combined with acceleration of the Earth towards both small masses is:

$$a_4 = \frac{GM}{r^2} + \frac{G(m_1+m_2)}{r^2} = \frac{G(M +m_1+m_2)}{r^2}$$

Now a3 and a4 are identical, which means that if the lead and the feathers are dropped at exactly the same time, then they will hit the floor at exactly the same time.

Take home messages:
Two objects of different masses fall at different rates if dropped one at a time.
Two objects of different masses fall at the same rate if dropped at the same time.
The acceleration, rather than the force, is the important thing to consider when comparing falling rates.
The gravitational field of a object does not act on itself, where falling is concerned.

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Frame Dragger
No, definitely not, but I appreciate the compliment I've long known about this heuristic, but I don't know how it can be made solid. The formalisms I know are:

1) classical GR + geodesic equation
2) classical GR + equation of state
3) QFT on curved spacetime
4) GR + electrodynamics as effective quantum field theory

(1) is a ray limit of (2), (2) and (3) are respectively classical and semiclassical limits of (4). In (1) and (3) the field does not modify spacetime, so presumably we should use either (2) or (4).

For (2), maybe a solution of the Einstein-Maxwell equations might work?
http://arxiv.org/abs/gr-qc/9704043
http://arxiv.org/abs/0808.0997

Hmmm... for your question re: #2... it SEEMS like it should work, but I really lack the expertise. As much as it pains me, I believe this is an area with which Conway is intimately familiar, even if we disagree on his theoretical outcroppings of it (specifically a solution for #4).

Staff Emeritus
Gold Member
So you would say that light rays of different frequencies don't travel differently (say in gravitational lensing) because the classical state does not have a definite number of photons?

I don't think you need any quantum-mechanical ideas at all in order to understand why the vacuum is nondispersive. Does my explanation in #4 work for you?

Staff Emeritus
Gold Member
I mean this is probably crazy to bring this up , But wouldn't a heavier object distort space more so the force between that object and the earth would be a little greater , I mean the effect would be very small and negligible , but I am just wondering , There was like a thread a month ago about , shouldn't heavier objects fall faster based on this idea. I mean this would obviously make sense with 2 large bodies coming together.

GR is a nonlinear theory.

In the limit where we consider a test particle of very small mass, the test particle moves along a geodesic. This is required by the correspondence principle, since universality of trajectories for low-mass objects has already been observed in Eotvos experiments and verified in the context of Newton's laws and Newton's law of gravity.

When you're no longer in the limit of a low-mass object, it's no longer true that the trajectory is a geodesic; for instance, you can get effects like gravitational radiation.

Frame Dragger
I don't think you need any quantum-mechanical ideas at all in order to understand why the vacuum is nondispersive. Does my explanation in #4 work for you?

There really is no luminiferous aether is there papa?! I don't think you need any quantum-mechanical ideas at all in order to understand why the vacuum is nondispersive. Does my explanation in #4 work for you?

No, because if light of different wavelengths bend spacetime differently, then the curvature would be different, so the geodesics would be different.

yuiop
No, because if light of different wavelengths bend spacetime differently, then the curvature would be different, so the geodesics would be different.

The equivalence principle tells you that the paths of light with different wavelengths in a gravitational field must be exactly the same.

In the falling elevator thought experiment, two horizontal lazer beams of different wavelength, must appear to follow the same straight path as they would in a "stationary" elevator in flat space.

In an accelerating rocket in flat space thought experiment, two lazer beams of different wavelength directed orthogonally to the acceleration of the rocket must follow the same path, because they have no reason to do anything else in flat space. In this case both beams must be intercepted by the rear of the accelerating rocket at exactly the same time so they must appear to fall at exactly the same rate in the accelerating rocket and by the EP they must fall at the same rate in a gravitational field.