Gravitational force on light?

[RoboticAnimals]

I am a complete novice when it comes to any kind of topic relating to Physical Science- however, recently I came across an idea I couldn't seem to fully grasp. Perhaps some people on this message board can help me out.


I have often heard the statement "black holes are so powerful, in fact, that even light can't escape their gravitational pull!"

If I am understanding this correctly, that means that light is being effected by gravity. In order for this to be true, doesn't that mean that light must have some kind of physical mass? After all, how could the equation "Weight = Force * Mass" make sense if the Mass was zero?

My question is simply "do the particles that make up light have mass?" If they do not, how does gravity effect light?

I also have come to understand that the direction in which an object in space is traveling can be determined by a possible red or blue shift because of a kind of 'doppler effect' with light waves. Since light is effected by the extreme gravitational forces of a black hole, then surely light can be slightly effected by the weaker gravitational forces of things like planets or stars near Earth. Are light waves not slightly effected by these other objects before they reach earth? And in that sense, are we getting possibly distorted readings by the time light reaches Earth?

 

[no one in particular]

In order for this to be true, doesn't that mean that light must have some kind of physical mass?

Well, if photons have mass, it certainly is not very much. Earlier this year there was a new upper limit placed on photon mass. As reported by Physics News Update:

A new limit on photon mass, less than 10^-51 grams or 7 x 10^-19 electron volts, has been established by an experiment in which light is aimed at a sensitive torsion balance; if light had mass, the rotating balance would suffer an additional tiny torque. This represents a 20-fold improvement over previous limits on photon mass.

And;

Photon mass is expected to be zero by most physicists, but this is an assumption which must be checked experimentally. A nonzero mass would make trouble for special relativity, Maxwell's equations, and for Coulomb's inverse-square law for electrical attraction.

edited because I forgot my "if"

 

[espritch]

I an not a physicist, but this is how I understand it (take with heavy grain of salt):

In general relativity, gravity curves space. Photons traveling through a gravity well follow this curvature and consequently their path is curved rather than straight. So they are effected by gravity.

There are two kinds of mass, rest mass and relativistic mass (due to motion). Photons have zero rest mass but a non zero relativistic mass.

 

[davefoc]

I am a little confused by this discussion.

I had never heard that there was any mainstream physics thought that a photon has a rest mass. There is no question that light has inertial mass. When it runs into things it pushes on them. This is the idea behind solar sails and is accepted as experimental and theoretical fact.

In a recent thread CurtC suggested that he thought light might have a rest mass and S. Cat said this about that:

light behaves exactly the way we would expect it to if the photon were a particle with very, very small rest-mass. In fact, as CurtC suggested, we cannot even be sure this is not the case. We can only say that the photon's rest mass is small enough that if it is not zero, we can't tell the difference (yet).

I didn't understand exactly what S. Cat meant by that since S. Cat also explained in the same thread that strictly speaking light didn't have a rest mass.

I don't understand how the experiment described in the link could have been used to determine the rest mass of light. If light strikes a very sensitive torsion balance I would expect the torsion balance to twist because of the inertial mass. I don't know how one would separate out a rest mass component from the result.

In addition to the above quotes, S. Cat had a little bit more to say about the issue of the rest mass of light in this thread:

http://www.randi.org/vbulletin/showthread.php?s=&threadid=27772&perpage=40&pagenumber=1

One question that I asked in that thread but didn't get an answer to was:

If I were a photon with a small rest mass and I was just entering a vacuum after traveling through a piece of glass would I need a little shot of energy to cause me to accelerate up to vacuum speed?

Any thoughts on this same question if I was a neutrino?

This is probably way too simplistic view of this, but it seems to me if a particle has a rest mass then it takes energy to cause it to accelerate so where would the energy come from to accelerate a photon as it exited a piece of glass and accelerated up to vacuum speed.

 

[Yahweh]

"Weight = Force * Mass"

Your equation is a bit off, it should look like this:
Weight = Mass * g, where g is the acceleration of Gravity

Here's a source on Newton's Law of Universal Gravitation: http://www.glenbrook.k12.il.us/gbssci/phys/Class/circles/u6l3c.html

 

[Yahweh]

One question that I asked in that thread but didn't get an answer to was:

--------------------------------------------------
If I were a photon with a small rest mass and I was just entering a vacuum after traveling through a piece of glass would I need a little shot of energy to cause me to accelerate up to vacuum speed?

Any thoughts on this same question if I was a neutrino?
--------------------------------------------------

This is probably way too simplistic view of this, but it seems to me if a particle has a rest mass then it takes energy to cause it to accelerate so where would the energy come from to accelerate a photon as it exited a piece of glass and accelerated up to vacuum speed.

(Keep in mind, I'm about to pull an answer out of my ass using the limited knowledge I have of the subject, so dont quote me on it... I'm bound to be corrected if I say something inaccurate...)

When a photon is absorbed by another atom, it "excites" the atom's electrons. The electron breifly moves to the next higher orbital than back releasing another photon.

The molecular properties of glass allow this cascading process to easily occur giving glass its transparency. The process occurs virtually at the speed of light, which is why light travels at a slower velocity through a medium. Photons are being continually absorbed and re-emitted, every photon is emitted travelling at a velocity of c. That's why the photon doesnt need a jump start when it passes through (which isnt the appropriate word, by the way) glass into a vacuum.

Neutrinos rarely interact with matter. In actuality, if a neutrino and a photon are created at the same time in the center of the sun, the neutrino would make it to earth before the photon although they travel at or very near the same velocity.

 

[CurtC]

dovafoc wrote:
In a recent thread CurtC suggested that he thought light might have a rest mass...

Let me just clarify by saying that I thought I was making it known that I just made this up at that very moment, just to see what kind of wild idea I could come up with. My idea was that maybe light travels just slightly slower than the universe's speed limit, inferring a small but nonzero rest mass for the photon. I'm not smart enought to say why this can't be (other than violating the idea of relativity of light).

 

[Ziggurat]

Let me just clarify by saying that I thought I was making it known that I just made this up at that very moment, just to see what kind of wild idea I could come up with. My idea was that maybe light travels just slightly slower than the universe's speed limit, inferring a small but nonzero rest mass for the photon. I'm not smart enought to say why this can't be (other than violating the idea of relativity of light).

That might work if the mass were small enough, but small enough really would be amazingly small. I don't think there's much reason to believe it has non-zero mass (in other words, I don't think that would help explain anything that doesn't make sense assuming zero photon mass), but at least experimentally there isn't any way to distinguish between zero and arbitrarily small - you always run into the question of whether it's maybe just too small to measure. But for simplicity, it's fairly safe to assume that the mass is zero.

 

[Stimpson J. Cat]

Robotic,

I also have come to understand that the direction in which an object in space is traveling can be determined by a possible red or blue shift because of a kind of 'doppler effect' with light waves. Since light is effected by the extreme gravitational forces of a black hole, then surely light can be slightly effected by the weaker gravitational forces of things like planets or stars near Earth. Are light waves not slightly effected by these other objects before they reach earth? And in that sense, are we getting possibly distorted readings by the time light reaches Earth?

Absolutely. These effects are quite measurable.


davefoc,

In a recent thread CurtC suggested that he thought light might have a rest mass and S. Cat said this about that:

quote:
--------------------------------------------------------------------------------
light behaves exactly the way we would expect it to if the photon were a particle with very, very small rest-mass. In fact, as CurtC suggested, we cannot even be sure this is not the case. We can only say that the photon's rest mass is small enough that if it is not zero, we can't tell the difference (yet).
--------------------------------------------------------------------------------

I didn't understand exactly what S. Cat meant by that since S. Cat also explained in the same thread that strictly speaking light didn't have a rest mass.

I can see how that may have been a bit confusing. Let me put it this way. There are two possibilities:

1) Light moves at just slightly under c, and has a very small rest mass. As NOIP pointed out, if this is true, then the rest mass of the photon is extraordinarily small.

2) Light moves exactly at the speed of light. If this is the case, then strictly speaking, it has no rest mass at all, since it can never be at rest. Saying its rest mass is zero is basically just a mathematical convenience.

I don't understand how the experiment described in the link could have been used to determine the rest mass of light. If light strikes a very sensitive torsion balance I would expect the torsion balance to twist because of the inertial mass. I don't know how one would separate out a rest mass component from the result.

The specific amount it twists should be slightly different for the two cases. Essentially the experiment is measuring the ratio of momentum to total energy. For a particle moving at c, that ratio is 1/c. For any particle with a non-zero rest mass, that ratio will be slightly lower.


Yahweh,

One question that I asked in that thread but didn't get an answer to was:

--------------------------------------------------
If I were a photon with a small rest mass and I was just entering a vacuum after traveling through a piece of glass would I need a little shot of energy to cause me to accelerate up to vacuum speed?

Any thoughts on this same question if I was a neutrino?
--------------------------------------------------

This is probably way too simplistic view of this, but it seems to me if a particle has a rest mass then it takes energy to cause it to accelerate so where would the energy come from to accelerate a photon as it exited a piece of glass and accelerated up to vacuum speed.
--------------------------------------------------------------------------------


(Keep in mind, I'm about to pull an answer out of my ass using the limited knowledge I have of the subject, so dont quote me on it... I'm bound to be corrected if I say something inaccurate...)

When a photon is absorbed by another atom, it "excites" the atom's electrons. The electron breifly moves to the next higher orbital than back releasing another photon.

The molecular properties of glass allow this cascading process to easily occur giving glass its transparency. The process occurs virtually at the speed of light, which is why light travels at a slower velocity through a medium. Photons are being continually absorbed and re-emitted, every photon is emitted travelling at a velocity of c. That's why the photon doesnt need a jump start when it passes through (which isnt the appropriate word, by the way) glass into a vacuum.

Exactly. Well done.

Neutrinos rarely interact with matter. In actuality, if a neutrino and a photon are created at the same time in the center of the sun, the neutrino would make it to earth before the photon although they travel at or very near the same velocity.

Well, from what I understand, it has been recently shown that neutrinos do, in fact, have a small rest mass, and therefore travel at slightly less than c. So this may or may not be the case. Of course, I am also pretty sure that the photons generated at the center of the sun never actually make it out. They are all absorbed, but are not re-emitted at the same frequency. The optical properties of super-hot plasma are, not surprisingly, somewhat different than those of ordinary molecular matter.


Ziggurat,

Let me just clarify by saying that I thought I was making it known that I just made this up at that very moment, just to see what kind of wild idea I could come up with. My idea was that maybe light travels just slightly slower than the universe's speed limit, inferring a small but nonzero rest mass for the photon. I'm not smart enought to say why this can't be (other than violating the idea of relativity of light).
--------------------------------------------------------------------------------

That might work if the mass were small enough, but small enough really would be amazingly small. I don't think there's much reason to believe it has non-zero mass (in other words, I don't think that would help explain anything that doesn't make sense assuming zero photon mass), but at least experimentally there isn't any way to distinguish between zero and arbitrarily small - you always run into the question of whether it's maybe just too small to measure. But for simplicity, it's fairly safe to assume that the mass is zero.

There are some reasons, from a symmetry point of view, to think they might. Time will tell.


Dr. Stupid

 

[Matabiri]

Well, from what I understand, it has been recently shown that neutrinos do, in fact, have a small rest mass, and therefore travel at slightly less than c. So this may or may not be the case. Of course, I am also pretty sure that the photons generated at the center of the sun never actually make it out. They are all absorbed, but are not re-emitted at the same frequency. The optical properties of super-hot plasma are, not surprisingly, somewhat different than those of ordinary molecular matter.

Correct. It takes timescales on the order of years (might be hundreds or even thousands of years, but it's been a while since I've read my notes) for light to make it out of the core of the sun, because the photons are absorbed and re-emitted so many times.

Rich

 

[exarch]

I believe the basis of Einsteins general theory of relativity is that the speed of light is constant and as a result, the other factors in the equasion become variable. As such, there is no point in talking about a rest mass for photons, as they are assumed to always be traveling at the speed of light. Correct?

Still, I am in favor of the idea that photons DO have mass, even if it is almost zero, which might also imply that the speed of light is not the upper speed limit, and explain why some particles can move faster than light (since they are perhaps even lighter than photons)?

The part I've personally always found the hardest to grasp is the idea that the speed of light from your point of view is constant, no matter how fast you are moving yourself.

 

[Stimpson J. Cat]

Exarch,

I believe the basis of Einsteins general theory of relativity is that the speed of light is constant and as a result, the other factors in the equasion become variable. As such, there is no point in talking about a rest mass for photons, as they are assumed to always be traveling at the speed of light. Correct?

Not exactly. The basis for special relativity was originally that Maxwell's equations should be invariant under all inertial frames (general relativity comes into play when you add gravity).

Maxwell's equations implicitly assume that light does not have any "rest mass" as such. But those equations were empirically derived, so all that really means is that if light does have rest mass, then it was too small to be measured at that time.

Anyway, there is nothing really special about light. The theory is that any particle with no rest mass should move at the "maximum speed". We call that the speed of light, because even if light does have some tiny rest mass, it is moving so close to the maximum speed that we can't measure the difference.

Still, I am in favor of the idea that photons DO have mass, even if it is almost zero, which might also imply that the speed of light is not the upper speed limit, and explain why some particles can move faster than light (since they are perhaps even lighter than photons)?

How light they are has nothing to do with it. The maximum speed is there regardless of whether light has rest mass or not. If it does, then what that means is that photons with very very low energy will be moving slightly slower than ones with very high energy. In the limit of energy going to infinity, any particle, no matter what its rest mass is, approaches this maximum speed.

But the relation between energy and speed is highly nonlinear. When the total energy of the particle is large compared to the rest energy, the speed will already be very very close to the maximum speed. So, radio waves may move slightly slower than gamma rays, but both already move so close to the maximum speed that we can't tell the difference. Likewise, you could, in principle, speed up an electron so that it is moving faster than a radio photon, but not as fast as a gamma photon, and so on.

The part I've personally always found the hardest to grasp is the idea that the speed of light from your point of view is constant, no matter how fast you are moving yourself.

Light having a small rest mass doesn't really alter this. It just means that it is not the speed of light which is truly constant, but instead the maximum speed, which the speed of light is very very close to.

An extremely high energy electron will appear to be moving at the same speed from many different inertial frames, too. Whether or not light has rest mass, the fact remains that velocities simply don't add in reality the way they do in classical mechanics.


Dr. Stupid

 

[LuxFerum]

When a photon is absorbed by another atom, it "excites" the atom's electrons. The electron breifly moves to the next higher orbital than back releasing another photon.

The molecular properties of glass allow this cascading process to easily occur giving glass its transparency. The process occurs virtually at the speed of light, which is why light travels at a slower velocity through a medium. Photons are being continually absorbed and re-emitted, every photon is emitted travelling at a velocity of c. That's why the photon doesnt need a jump start when it passes through (which isnt the appropriate word, by the way) glass into a vacuum.

I'm not buying this.

How do expect that the emitted photon will have the same direction that the original one?

This only happens in Stimulated Emission or lasers.

Originally by wolfram.com
When a photon (electromagnetic wave) impinges on a particle with an excited energy state above some other state, it can stimulate the emission of an additional photon of exactly the same polarization, direction of propagation, and frequency. This is the principle used in lasers and masers.

 

[Stimpson J. Cat]

LuxFerum,

When a photon is absorbed by another atom, it "excites" the atom's electrons. The electron breifly moves to the next higher orbital than back releasing another photon.

The molecular properties of glass allow this cascading process to easily occur giving glass its transparency. The process occurs virtually at the speed of light, which is why light travels at a slower velocity through a medium. Photons are being continually absorbed and re-emitted, every photon is emitted travelling at a velocity of c. That's why the photon doesnt need a jump start when it passes through (which isnt the appropriate word, by the way) glass into a vacuum.
--------------------------------------------------------------------------------

I'm not buying this.

How do expect that the emitted photon will have the same direction that the original one?

It is easiest to see how this works by looking at classical electrodynamics. This is also sufficient, since on the scale being discussed, the quantized nature of light is not very important.

So what you have got is an oscillating electromagnetic field. This causes the electric charges in the glass to oscillate, which in turn generates another oscillating electromagnetic field. The phase difference depends on the mechanical properties of the matter involved, and results in the ordinary optical properties we are familiar with (reflection, refraction, absorption, and reduced speed of propagation).

Quantum mechanically speaking, this corresponds to the photons being absorbed by the matter, and then re-emitted, as Yahweh said. Nothing causes the emitted photons to have the same direction as the original ones. Instead, you can look at it as a scattering problem. The scattered photons interfere with each other, and with the incoming photons. The result of that interference is, as it must be in cases where the wavelengths are large compared to the structures the light is scattering off of, approximately equivalent to the results expected from classical electrodynamics.

This only happens in Stimulated Emission or lasers.

That is slightly different. In that case, the atoms are already in their excited state, and the incoming photon is not absorbed at all, but instead stimulates the emission of another identical photon. In our case, the atoms are in the ground state, and absorb the photon, only to re-emit it a short time later.


Dr. Stupid

 

[TeaBag420]

To return to the original question, it's "affected", not "effected".

 

[davefoc]

So I take from this that the slowing of light in a transparent medium is actually caused by the absorption and retransmission of photons and not by any change in the permeability and permittivity of the space between the atoms.

Does this explanation for the reduction in the speed of light apply to the Bose-Einstein condensate experiments where light is slowed to phenomenally slow speeds?

http://www.fortunecity.com/emachines/e11/86/bose.html

 

[LuxFerum]

Quantum mechanically speaking, this corresponds to the photons being absorbed by the matter, and then re-emitted, as Yahweh said. Nothing causes the emitted photons to have the same direction as the original ones. Instead, you can look at it as a scattering problem. The scattered photons interfere with each other, and with the incoming photons. The result of that interference is, as it must be in cases where the wavelengths are large compared to the structures the light is scattering off of, approximately equivalent to the results expected from classical electrodynamics.

I'm still not geting it.

How do you expect that all the photons will hit some electron in the glass?I think that is easier to hit bullets with bullets than all the photons hiting the electron.

And as far as I know, the process of exciting an atoms is not that easy. Some frequencies will never exciting some atoms, no matter how powerful they are. So it does not explain how some things are transparent for some frequencies and not for others.

 

[Stimpson J. Cat]

davefoc

So I take from this that the slowing of light in a transparent medium is actually caused by the absorption and retransmission of photons and not by any change in the permeability and permittivity of the space between the atoms.

Yes. The introductory texts on this issue can be somewhat confusing, because they usually present it from the classical point of view first. You see, when it comes to ordinary light passing through a dielectric, like glass, the size of the atoms is small enough compared to the wavelength of light that the whole thing can be treated classically. Instead of describing it in terms of individual photons scattering off of individual atoms, and then doing statistical mechanics to get the macroscopic results, they simply treat the material as a continuous homogeneous and isotropic "field", and treat the light as a wave. Thus the entire process can, in a sense, be averaged out into a new value of permitivity and permeability for the material.

Does this explanation for the reduction in the speed of light apply to the Bose-Einstein condensate experiments where light is slowed to phenomenally slow speeds?

Not completely. It is still a process of the photons interacting with the material, but it is no longer as simple as the ordinary case. For one thing, things can no longer be treated classically at all.


LuxFerum,

Quantum mechanically speaking, this corresponds to the photons being absorbed by the matter, and then re-emitted, as Yahweh said. Nothing causes the emitted photons to have the same direction as the original ones. Instead, you can look at it as a scattering problem. The scattered photons interfere with each other, and with the incoming photons. The result of that interference is, as it must be in cases where the wavelengths are large compared to the structures the light is scattering off of, approximately equivalent to the results expected from classical electrodynamics.
--------------------------------------------------------------------------------

I'm still not geting it.

How do you expect that all the photons will hit some electron in the glass?I think that is easier to hit bullets with bullets than all the photons hiting the electron.

You can't think of it as photons hitting electrons. The wavelength of visible light is larger than the atoms are.

And as far as I know, the process of exciting an atoms is not that easy. Some frequencies will never exciting some atoms, no matter how powerful they are. So it does not explain how some things are transparent for some frequencies and not for others.

It isn't that simple. But when you are talking about molecular matter (complex molecular structures), rather than individual atoms (say a monatomic gas), there are many many excitable modes, many of which have very broad response functions. Remember that we are not just talking about the excited states of the electrons in the individual atoms, but also the vibrational modes of the crystalline structure, and in the case of fluids, the translational and rotational modes as well.

The spectral response of a dialectric material will tend to be a continuous function, similar to that of an ideal blackbody, with little sharp peaks and irregularities that correspond to the more pronounced of the discrete modes.

For metals, things are even more complicated, because the free electrons tend to do most of the interaction. That is why metals tend to be highly reflective (when polished), and extremely opaque.


Dr. Stupid

 

[RoboticAnimals]

Re: Re: Gravitational force on light?

Your equation is a bit off, it should look like this:
Weight = Mass * g, where g is the acceleration of Gravity

oh okay, thanks.

I was confused about that unbelievably complicated 8th grade physics formula. Thanks for clearing that up.

 

[davefoc]

One of the things that give S. Cat a lot of credibiltiy with me is that he tends to say things a lot like what I've read before. Unfortunately I often don't understand them any better after S. Cat has said them then I did when I have read them elsewhere.

A case in point is this:

For metals, things are even more complicated, because the free electrons tend to do most of the interaction. That is why metals tend to be highly reflective (when polished), and extremely opaque.

Why is it that photons interact mostly with the electrons in metals. I get that the electrons are shared and loosely bonded and are readily available for conducting electricity. So how does this lead to opacity?

One of the more amazing facts to me is that a mil or so of metal is more opaque than billions of light years of space. I wonder if S. Cat might have some thoughts on this?