# Light, Mass and Black Holes.

1. Dec 6, 2006

### Alistair

Ok i'm under the impression that gravity only effects matter. therefore, because light is effected by black 'holes' it must have some form of matter that it is formed from. Einstein theorised that as an object gets closser to the speed of light, it will take on 'infinite mass'. and going by this it woudl take an infinite amount of energy to get an object to the speed of light. thus light contains an infinite amount of energy. (please explain this to me because i am having trouble with the whole thing.)

2. Dec 6, 2006

### paaghi

Pooya Afaghi

The photon has only relativity mass that depends on photon wavelength.
One important thing is that photon has NOT any mass in speeds less than c (~3*10^8 m/s) and we don't have static photon.
Note1: Today we say "photon has momentum" instead of "photon has mass"
Note2: Light is the most mysterious event in physics!

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Pooya Afaghi

3. Dec 6, 2006

### Jheriko

What is important is that the photon has zero rest mass. Unlike massive particles, massless particles are constrained to move at the speed of light.

In some sense it does have a mass from its energy, but this is not the rest mass, which is what restricts an object from moving at the speed of light.

Further more light can not accelerate in the usual sense, from its own perspective it is always moving at the speed of light, even when other relativistic effects make it appear otherwise. i.e. When light curves through space it is still moving at the speed of light.

4. Dec 6, 2006

### bernhard.rothenstein

do we say today that the photon has momentum and energy which depends on its frequency?
sine ira et studio

5. Dec 6, 2006

### HallsofIvy

Staff Emeritus
Whether that is true or not depends on what you mean by "matter"! Do you consider photons to be matter?
No, he didn't. He showed that if an object has non-zero rest mass, then it's mass increases without bound as its speed nears the speed of light. Your statement is different in that it doesn't have the provision "has non-zero rest mass" and implies that the mass is infinite for some speed "close" to the speed of light. That's incorrect.

Light is "matter" (photons) that have zero rest mass so "mass increasing without bound" and "taking an infinite amount of energy" do not apply.

6. Dec 6, 2006

### kesh

it is rather. from it's own frame of reference, it doesn't even exist lol.

7. Dec 6, 2006

### Jheriko

I thought $$E=hf=pc$$, or something to that effect ?

8. Dec 6, 2006

### korican

Also since photons do not have rest mass they are not affected by gravity. What is affected by gravity is space time. Gravitational fields bend space-time and create dents. Those are the things that photons fall into. A black hole simply has some much gravity that it bends space so much that it raps around it self, thus light follows that path and doesn't "escape". The statement the gravitational force of a black hole is so great that light can not escape, doesn't mean that light is being pulled in directly due to the force of gravity, but it follows the bent path that is created by a black hole.

Last edited: Dec 6, 2006
9. Dec 6, 2006

### Chris Hillman

The EM field acts as a source of gravitional field

Hi, Alistair,

You've gotten some good information from HallsofIvy, but I'd add two pedantic quibbles:

1. When you referred to "it would take an infinite amount of energy to get an object to the speed of light", I think you were referring to trying to accelerate some particle with positive rest mass such as a neutron to the speed of light, and then your understanding of that point is correct--- but as HallsofIvy said, photons have zero rest mass, so at this point in your post, you suddently started talking about something different.

2. In the gtr literature, usually photons --- or more properly, since gtr is a classical field theory, "electromagnetic radiation"--- is not called "matter". Usually this term is reserved for something like a dust (pressure-free perfect fluid) or general fluid or elastic solid, e.g. in an idealized model of a stellar interior.

In addition, one should point out that in gtr, the electromagnetic field carries field energy (because it does according to Maxwell's theory of EM, which carries over with minimal changes to gtr), and this field energy gravitates. Specifically, it contributes to the stress-energy tensor $$T^{ab}$$ which serves as the source of the gravitational field in gtr. If the only nongravitational mass-energy present is a "null" EM field (an EM wave), with respect to an "adapted frame field", this contribution looks like this:
$$T^{\hat{a}\hat{b}} = \epsilon \; \left[ \begin{matrix}1 & 1 & 0 & 0 \\ 1 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 &0 & 0 \end{matrix} \right]$$
If the only nongravitational mass-energy present is a "non-null" EM field (e.g. outside a charged object), this contribution looks like this:
$$T^{\hat{a}\hat{b}} = \epsilon \; \left[ \begin{matrix}1 & 0 & 0 & 0 \\ 0 & -1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 &0 & 1 \end{matrix} \right]$$
"Null dust", which you can think of as something like a "pressureless fluid composed of massless particles", hence the name, and which is used to model "incoherent massless radiation" generally, contributes the same way as a null EM field, except that in this case we do not insist upon providing an explict solution of Maxwell's equations (on our curved spacetime).

What this means is that an EM field $$does$$ contribute to the curvature of spacetime and therefore it can indirectly affect the motion of test particles (if you prefer, the gravitational attraction of the EM field energy can alter their motion) and indeed of "light rays".

See http://en.wikipedia.org/w/index.php?title=Exact_solutions_in_general_relativity&oldid=45119659 and http://en.wikipedia.org/w/index.php?title=Electrovacuum_solution&oldid=43582933 with the usual cautions about the instability of Wikipedia. (I have linked here to specific versions of two WP articles which I myself edited and therefore have confidence in; the "canonical forms" given above differ inessentially from those given in the preferred version of the second article.)

Regarding light rays, interestingly enough, it turns out that according to gtr, two parallel light rays traveling in the same direction do not attract one another. But two parallel light rays traveling in opposite directions will attract one another. In practice this effect is too small to be measureable in the laboratory, however.

I take it that kesh wasn't being entirely serious, but someone should point out that even in str, the notion of "an observer traveling at the speed of light" doesn't make much sense, and in particular, is NOT associated with any "Lorentz frame". This is related to a phenomenon noted above: in str (and thus also in gtr), it is impossible to accelerated matter with positive mass to the speed of light.

Well, I don't think korican should have put it that way, since this isn't true as stated, as korican admits in the next sentence. (After substituting "light ray" or "laser pulse" or "EM radiation" or "massless radiation" or "lightlike signal" for "photon", see "light bending" in any gtr textbook.). In addition to the fact that the gravitational field of a neutral object affecting the propagation of light, as I mentioned above, the EM field itself possesses field energy and momentum and therefore acts as a source of the gravitational field, so its gravitational effects will indirectly affect the motion even of uncharged matter.

Chris Hillman

Last edited: Dec 6, 2006
10. Dec 7, 2006

### kesh

no, i wasn't being serious. i was just doodling in an unscientific way with the argument that because "from the point of view of the photon" is meaningless it follows that photons' existence is of a more tenuous sort to those things that might have "subjective existence", whatever that means.

i should spend time in the philosophy forum

11. Dec 7, 2006

### bernhard.rothenstein

Do you consider that the high frequency at which the problem appears among the threads is due to the fact that according to a new trend in teaching special relativity relativistic mass has become nomina odiosa? My impression is that such a link exists.

12. Dec 7, 2006

### paaghi

to: bernhard.rothenstein >> Yes,

p=mc >> p=(mc^2)/c >> p=E/c >> p=hf/c >> p=h/w

(E=hf, f=c/w, E=mc^2)
(p: momentum, w: wavelength)

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Pooya Afaghi

13. Dec 7, 2006

### paaghi

[In some holy books it's a famous statement: "God is light of earth and skies"]

You spoke about photon while we don't know about nature of light yet.
In some cases we say light is photon and some others we say light is wave.

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Pooya Afaghi