Two Questions about Black Holes

[Ziggurat]

So if light has matter (photons)

No, it doesn't have matter, it has relativistic mass. But relativistic mass is just another term for energy. Light has zero invariant mass, which is what physicists generally mean by "mass".

and light is an electro-magnetic phenomenon, then do radio waves also have...radons? no, I guess photons too.

Correct: they are photons.

Just at such a low rate that we can't see them.

No. The way our vision works, our eyes are only sensitive to certain frequency ranges. You can boost the intensity of radio waves all you want to (and hence the number of photons), but if they're at too low a frequency for our eyes, they cannot be detected.

Perhaps they are visible in the low infra red, at a very low 'volume'?

No they're well below the frequency of infrared:

Lowest in that continuum would be gravity? So, then I guess gravity would have photons too?

No, gravity is a different force than electromagnetism, so it would have a different carrier particle (gravitons) and a completely independent spectrum. Just like the audio spectrum is completely independent of the electromagnetic spectrum. Most gravity waves are expected to be very low frequency, that much is true, but that's a different matter.

 

[Buckaroo]

Yes, it's just the nature of the particle. It's as intrinsic a part of being a photon as moving at less than the speed of light is a part of being an electron or proton.

I'd add that if I recall my EM courses correctly, the whole photonscanonlymoveatthespeedoflightlesttheyceasetoexist thing falls right out of Maxwell's equations, so you don't even need to consider the photon picture -- it follows from the wave approach, too. I thought it was pretty neat when we derived that result.

 

[joobz]

No, it doesn't have matter, it has relativistic mass. But relativistic mass is just another term for energy. Light has zero invariant mass, which is what physicists generally mean by "mass".



Correct: they are photons.



No. The way our vision works, our eyes are only sensitive to certain frequency ranges. You can boost the intensity of radio waves all you want to (and hence the number of photons), but if they're at too low a frequency for our eyes, they cannot be detected.



No they're well below the frequency of infrared:
http://upload.wikimedia.org/wikiped...ectrum.png/377px-Electromagnetic-Spectrum.png



No, gravity is a different force than electromagnetism, so it would have a different carrier particle (gravitons) and a completely independent spectrum. Just like the audio spectrum is completely independent of the electromagnetic spectrum. Most gravity waves are expected to be very low frequency, that much is true, but that's a different matter.

I've never considered the concept of gravitons (never had to with my work...). If they turn out to be true, what would the physical implications of a gravitational frequency spectrum? And how does it fit into the mass/energy conservation laws?

BTW: This has been a very interesting thread. Keep up the good work Ziggaruat, Schneibster!

 

[Schneibster]

So if light has matter (photons)

No, not matter- mass. Just as you can think of "charge" as "that which is affected by an electromagnetic field," you can think of "mass" as "that which is affected by a gravity field." It's "gravity charge," if you will.

Zig made this point one way; I'll make it another, and between them, you should get a good idea. Here goes:

When something is moving, it has energy. You know this; throwing a rock at a window and breaking it tells you so. But the rock also has rest mass; when you pick it up, you can feel that rest mass. All of a photon's energy is that energy of motion; it doesn't have anything else. If it stops moving, it stops existing.

But photons only move at the speed of light; how, then, you may ask, can there be photons with different amounts of energy? The answer is, photons only move at one speed, but they have different wavelengths. The shorter the wavelength, the more energy they have.

Finally, you may ask, I've spoken here of a photon's energy; what about its mass? Remember, E=mc^2; energy equals mass times a big number. Energy and mass are the same kind of thing. And we know that because nuclear weapons work.

HTH.

and light is an electro-magnetic phenomenon,

Yes, it is.

then do radio waves also have...radons? no, I guess photons too. Just at such a low rate that we can't see them.

More or less. "Low rate" = "low frequency" = "long wavelength."

Perhaps they are visible in the low infra red, at a very low 'volume'?

Zig's spectrum shows this nicely. Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio- they're all the same "kind of thing." That thing is electromagnetic radiation. And they all move at the speed of light.

Lowest in that continuum would be gravity?

No, for two reasons. First, electromagnetic radiation is a different "kind of thing" than the electric field; they are associated, but not the same. In a similar way, gravity is a field; there is an associated radiation, gravity waves. We haven't unambiguously detected any gravity radiation yet, but relativity says it's there, and we are running experiments to try to detect it. Google up LIGO, the Laser Interferometer Gravity Observatory experiment. Second, the electromagnetic field and the gravity field (and electromagnetic radiation and gravity radiation too) are different "kinds of things." Gravity is a deformation of space and time; electromagnetism is not (or at least, not a deformation of the four dimensions we're used to; there is a theory out there called "string theory" that proposes that electromagnetism is a deformation of a dimension, just not one we can see in the ordinary way; ask more questions if you're curious).

So, then I guess gravity would have photons too?

Well, sort of. If gravity really does make radiation (and we're pretty sure it does, even though we haven't detected any yet), then that's a hint that it might have its own sort of "photons," called "gravitons." But gravitons would have to be different from photons in several important ways, and we haven't even detected the radiation they build up into, much less gravitons themselves; in fact, we haven't even built a consistent theory that makes good predictions of what gravitons would be like. "String theory," "Loop Quantum Gravity," and "twistors" are three incomplete theories that might describe how gravitons are; none of them makes predictions yet that we can test. So we're still looking into that. But your analogy is at least partially apt, and it's the reason physicists think that eventually, we will find gravitons, and we will find a theory that describes them.

Since we know of four forces, and have quantum theories that describe three of them, if we find a quantum theory that describes gravity, we'll then have the pieces that should let us make a single overarching theory that combines the theories of all four forces into one. This overarching theory would then be a "theory of everything," or "TOE." Einstein thought that there had to be a theory that unified gravity and electromagnetism; it was called a "unified theory." He looked for it for many years, but never found it. While he was looking, we discovered two more forces, and it turned out to be easier to understand these two other forces than it has been to understand gravity. It could be that there is no quantum theory that describes gravity; but most physicists don't believe that. So they keep looking for that unified theory, that theory of everything. That's a great deal of what's going on in physics right now.

 

[Schneibster]

I'm not sure why Schneibster is so attached to relativistic mass, but there's really no need for it. It's a completely superfluous and redundant concept (it's exactly the same thing as total energy, except for a scale factor). You can do everything correctly considering relativistic mass, so he's not wrong, but there's really no point. Which is why, for the most part, it's been abandoned. Rest mass, or invariant mass, is a more fundamental concept, and that is what physicists generally mean when they use "mass" without any qualifiers. And photons have zero rest mass. So I think you're better off continuing to think of photons as massless, and yes, massless particles not only can travel at c, they can only travel at c.

There actually is a good reason. If you're teaching people to do physics, then you're right, better off to avoid relativistic mass. But if you're just helping people who have no interest in doing physics, just understand physics, then it's easier. They'll never learn the math, they don't care. They just want to understand how things will happen, and why they happen, without having to get into the math; and that means they have to be able to relate what you tell them to things they encounter in everyday life. Hell, it's difficult enough explaining what momentum is- why make it harder than it has to be?

Anyway, it's mostly a stylistic difference. I know, of course, that it's not absolutely accurate to conflate relativistic mass with rest mass- but it gets the concept across. You do it your way, I'll do it mine, and between them, people will generally understand both the way that physicists think of it, and the way it relates to what they're used to thinking about. If you weren't here, I'd have to do it all myself, so thanks.

 

[Ziggurat]

I've never considered the concept of gravitons (never had to with my work...). If they turn out to be true, what would the physical implications of a gravitational frequency spectrum?

Gravity waves, even unquantized, don't seem to be terribly important. Hell, we haven't even managed to detect them (although that's not totally unexpected - we think they should be hard to detect). Where you really want to know about whether or not gravitons exist is at the super-small and super-dense scale - in other words, the sort of scales you only get around black hole singularities, the very early stages of the universe, or possibly (if we can ever make them big enough) ultra-high-energy particle colliders. Outside those sorts of contexts, you'll probably never see any hint of an effect. Which is why we don't really know how to handle that stuff, because we don't have any experimental evidence to work off of.

And how does it fit into the mass/energy conservation laws?

Possibly the same way photons do: they could carry momentum and energy.

 

[Crossbow]

Light consists of photons which have mass. Mass is subject to the forces of gravity.

Ummm,

I think you need to double-check that point!

While it is true that Photons have Momentum,

however,

Photons do not have Mass.

But,

Photons are indeed subject to Gravity.

 

[Schneibster]

I've never considered the concept of gravitons (never had to with my work...). If they turn out to be true, what would the physical implications of a gravitational frequency spectrum?

Well, first of all, gravity radiation would be a series of distortions of spacetime radiating out from the source. You could make them by moving a body back and forth; or by accelerating or decelerating a body, just the same way as if you move a charge back and forth, or accelerate or decelerate a charge, it makes electromagnetic waves. But there's a big difference between the electromagnetic force and the gravity force: the electromagnetic force is much, much stronger. So gravity is very weak. And that makes it very hard to detect. We're having a lot of trouble separating out gravity waves from, say, the vibrations of a truck driving by on the freeway a mile away, or little seismic shifts in the Earth's surface, or someone walking down a hallway fifty feet away, and so forth.

And how does it fit into the mass/energy conservation laws?

Pretty much the same way that light does. To get a little way into a thing called quantum field theory, the idea behind the quantum field theory of electromagnetism is, photons actually transmit momentum changes between charged particles. In other words, when a matter particle like an electron or proton absorbs a photon, that changes its momentum. Physicists say that photons have momentum; when they are absorbed, since momentum is conserved, whatever absorbs them adds their momentum to its own. It's because of this that I often avoid talking about the difference between relativistic mass and rest mass; it makes it easier to understand how a photon can have momentum if you say it has mass, because everything else people see that has momentum has mass.

Presumably, when ( Yes, I believe it too) we find a quantum field theory of gravity, we'll account for the momentum exchange between two pieces of matter that are interacting by gravity, that is, by exchanging gravitons, in much the same way we do for photons. That's all speculative, though; we haven't seen any gravity radiation yet, much less any gravitons.

From the field theory of gravity (which is what is derived from general relativity, it's a set of ten field equations of much the same form as Maxwell's four equations for the electromagnetic force, but describing gravity rather than electromagnetism), we would conclude that just as an electromagnetic field can change the momentum of two charged objects that interact, the gravity field can change the momentum of two massive (that is, possessing a gravity charge) objects that interact, and the form of the equations in both cases is similar. So from that point of view, the point of view of a field theory, EM and gravity are much the same, except gravity is weaker, and the gravity charge and the electric charge are of different kinds. Presumably, when we talk about gravity radiation, it should change the momentum of massive (I don't mean huge, I just mean possessing mass) objects in much the same way that EM radiation changes the momentum of charged objects. And energy is conserved in both those interactions.

BTW: This has been a very interesting thread. Keep up the good work Ziggaruat, Schneibster!

Thanks, joobs.

 

[IXP]

Another, perhaps odd, question: what is it exactly about the nature of a photon which allows it to only exist at the speed of light? Or is it simply that's the nature of the particle?

Think of a wave on the water. Can the wave exist without moving at a specific speed? Photons are waves of EM field, which move at a speed defined by Maxwell's equations. It was the fact that these equations predicted a wave moving at the speed of light that gave the first clue to the nature of light.

IXP

 

[joobz]

Pretty much the same way that light does. To get a little way into a thing called quantum field theory, the idea behind the quantum field theory of electromagnetism is, photons actually transmit momentum changes between charged particles. In other words, when a matter particle like an electron or proton absorbs a photon, that changes its momentum. Physicists say that photons have momentum; when they are absorbed, since momentum is conserved, whatever absorbs them adds their momentum to its own. It's because of this that I often avoid talking about the difference between relativistic mass and rest mass; it makes it easier to understand how a photon can have momentum if you say it has mass, because everything else people see that has momentum has mass.

Interesting, so when we describe Raman scattering and the resultant stokes shift (I'm ignoring anti-stokes here for simplicity), we really are talking about an inelastic collision and therefore partial momentum transfer? I always thought of this as just an abstraction of reality and just calculated everything by energy transfer effects.

Also, if photons can be scattered, does this mean that gravitons may also have Rayleigh and Raman scattering behavior as well? (pretending for a moment that we could detect such things).

 

[Ziggurat]

Interesting, so when we describe Raman scattering and the resultant stokes shift (I'm ignoring anti-stokes here for simplicity), we really are talking about an inelastic collision and therefore partial momentum transfer?

Basically all collisions involve a momentum transfer - inelasticity is about whether or not there's an energy transfer as well. And for a given momentum transfer, there may or may not be an energy transfer as well.

But Raman is generally considered zero momentum transfer because of the huge difference in the energy-momentum dispersions for photons and for electrons and phonons. If you plot out an electron dispersion relation for a solid, for example, and then try to overlay the dispersion curve for a photon, it'll be so steep that it might as well be a vertical line. So at the energy scales of interest for Raman scattering, the momentum of the photons isn't zero, but it's very small compared to the relevant momentum scales for electrons and phonons, so you can basically ignore it.

 

[arthwollipot]

BTW: This has been a very interesting thread. Keep up the good work Ziggaruat, Schneibster!

I agree. For me, a good thread to lurk in.

 

[Yllanes]

I'll wait to hear what some of the other folks on this forum who have experience in physics have to say before I go any further. I'd be curious to know whether the field equations of gravity from GR include a self-interaction, and whether that means that a gravity field should interact, either with itself or another field. The comments on this (assuming anyone with detailed knowledge in this area chooses to comment) should be interesting.

I'm not sure if this is what you meant, but as gravity couples to energy-momentum, gravitons would interact with every kind of particle, including other gravitons. In contrast, in QED there is no Feynman vertex where photons interact with one another (if you want, there is no Feynman diagram where photons exchange a virtual photon).

Anything that has energy adds to the curvature of spacetime.

 

[Schneibster]

Interesting, so when we describe Raman scattering and the resultant stokes shift (I'm ignoring anti-stokes here for simplicity), we really are talking about an inelastic collision and therefore partial momentum transfer? I always thought of this as just an abstraction of reality and just calculated everything by energy transfer effects.

No, according to what I've been able to dig up, it really is a momentum transfer. The molecules apparently are capable of various modes of vibration, including rotational, bending, and stretching vibrations. What happens is, the momentum of the photons excites the molecules into one of these vibrational modes; the modes are unstable, and decay and emit a photon, but some of the energy remains in the molecule, to be dissipated as heat (a very small amount; unmeasurable according to the source I found). That's the Stokes shift. Occasionally, one of the molecules that is excited in this manner absorbs a photon too; when that happens, the emitted photon when the vibrational mode decays gets not only the energy of the incoming photon, but the excitation energy as well. That's the anti-Stokes shift.

There's also a much more explicitly quantum mechanical description of the phenomenon, and a field mechanical description that involves the dipole moment of the molecule. But the above gets the idea across. If I understand correctly, the energy transfer effects you are talking about are the quantum mechanical description.

Worth mentioning that this is elastic scattering; the inelastic mode is Rayleigh scattering. Rayleigh scattering is, of course, much more common; only about 1 scattering event in 10,000 or more is a Raman scattering event. Rayleigh scattering is responsible for the sky being blue.

Also, if photons can be scattered, does this mean that gravitons may also have Rayleigh and Raman scattering behavior as well? (pretending for a moment that we could detect such things).

I don't think gravitons would be energetic enough to excite the Raman scattering modes of molecules. Depending on their wavelength, they probably would be subject to Rayleigh scattering from properly sized objects.

 

[Schneibster]

I'm not sure if this is what you meant, but as gravity couples to energy-momentum, gravitons would interact with every kind of particle, including other gravitons. In contrast, in QED there is no Feynman vertex where photons interact with one another (if you want, there is no Feynman diagram where photons exchange a virtual photon).

Anything that has energy adds to the curvature of spacetime.

Yes, actually, that's exactly what I had in mind. You know, I realized reading this that that is most likely the problem with quantum gravity. In QED, they only had to figure out how to deal with a charge's self-interaction; but in quantum gravity, there would also be a self-interaction of the field. Very cool; thanks, Yllanes.