Photons and black holes

1. Apr 6, 2005

mee

Does a black hole bend the space that photons travel in so that they are bent towards it? How would gravity attract a photon if they have no actual regular mass? Also, are neutrinos attracted to black holes and sucked in (theoretically?)

2. Apr 6, 2005

mathman

Although they have no rest mass, photons have energy so behave as if they have mass. All things (photons, neutrinos, whatever) are affected by gravity, whether or not the source of the gravity is a black hole or any object with mass. One of the most famous early tests of general relativity was the bending of light by the sun, observed during a solar eclipse in 1919.

3. Apr 6, 2005

mee

Does this mean that neutrinos could be orbiting within a large enough mass?

4. Apr 7, 2005

mathman

Please clarify - what do you mean by "within"? It is very likely that there are neutrinos orbiting the sun.

5. Apr 12, 2005

mee

Thanks you for your reply, I meant within by which a neutrino can pass through mass without likleyhood of collision so a neutrino in an eccentric orbit could pass through the sun on part of its orbit perhaps.

6. Apr 12, 2005

mee

In that case, could there be enough neutrinos orbiting the galaxy to account for some "dark" matter?

7. Apr 12, 2005

Symbreak

I've asked myself the same question sometimes... and although photons have an amount of energy, comparable to a mass, it still does not explain things. Light always travels at the same speed, c, so why is it apparently effected by the escape velocity, v, of a gravitational field?

Maybe someone could also clarify the summarisation of General Relativity - i.e the aim was to say that all motion, including gravitational fields, is essentially equivalent, and there are no changes in the laws of physics from one form of motion to another. So... why the apparent change in direction of a light ray? How can the gravitational field slow down a particle of light - or anything else for that matter?

8. Apr 14, 2005

Ich

There is no gravitytional field in GR. Light always runs as fast and straight as possible. ItÂ´s just that "straight" could be quite convoluted in curved space-time.

9. Apr 14, 2005

A_I_

i know "neutrino" is a sort of energy? i a mtotally confused!!

10. Apr 14, 2005

eNathan

Photons only have mass by virtue of its movement. There are 2 types of mass. Relativistic mass, and ress mass. Photons have 0 mass when they are at rest (which they cannot be, v = c for photons). But photons have energy, which is "equivalent" to mass. That is why when you near the speed of light, your mass increases. Because all that really happens is your Kinetic Energy increases, and because Kinetic Energy has mass, your mass increases

Einstein didn't belive in black holes tho :uhh:

11. Apr 14, 2005

Meir Achuz

12. Apr 14, 2005

Dragongod

The problem isn't whether photons interact with gravity or not. The problem is that in GR Einstein never intended for photons to interact with gravity. Einstein never addressed the issue of gravity. According to Einstein nothing can go faster than the speed of light. The problem is that GR is in contradiction with the rest of physics. Gravity does exist. This proves that gravity does go faster than the speed of light because it is the gravitational pull by black holes that can suck in even light. Also we can feel the force of gravity by black holes therefore showing that gravity or gravitons are faster than the speed of light or else gravity itself would get sucked into the black holes as light(photons) does, which it doesn't.

13. Apr 14, 2005

pervect

Staff Emeritus
14. Apr 14, 2005

Dragongod

"If things like gravity correspond to the exchange of 'particles' like gravitons, how can they get out of the event horizon to do their job?

Gravitons don't exist in general relativity, because GR is not a quantum theory. They might be part of a theory of quantum gravity when it is completely developed, but even then it might not be best to describe gravitational attraction as produced by virtual gravitons."
---this directly from the second article link even though I was wrong about Einstein addressing gravity and combining it with special relativity, his theory is like I said in contradiction with the rest of physics. Nowhere, as you can see, as directly stated by the article, does it ever explain how gravity can escape the event horizon! People still aren't even sure if gravity exist by gravitons or if it goes faster than the speed of light. According to Einstein, gravity doesn't go faster than the speed of light but that wouldn't be consistant with reality, or the rest of physics, because we can still feel the gravitational force of a black hole. Therefore it contradicts the rest of physics, or at least the part of physics, that deals with feeling the effects of a black hole.

Last edited: Apr 14, 2005
15. Apr 14, 2005

Dragongod

"To begin with, the speed of gravity has not been measured directly in the laboratory--the gravitational interaction is too weak, and such an experiment is beyond present technological capabilities." -this shows that no one knows exactly what gravity is yet.

"In the simple Newtonian model, gravity propagates instantaneously: the force exerted by a massive object points directly toward that object's present position. For example, even though the Sun is 500 light seconds from the Earth, Newtonian gravity describes a force on Earth directed towards the Sun's position "now," not its position 500 seconds ago. Putting a "light travel delay" (technically called "retardation") into Newtonian gravity would make orbits unstable, leading to predictions that clearly contradict Solar System observations."

"In general relativity, on the other hand, gravity propagates at the speed of light; that is, the motion of a massive object creates a distortion in the curvature of spacetime that moves outward at light speed. This might seem to contradict the Solar System observations described above, but remember that general relativity is conceptually very different from Newtonian gravity, so a direct comparison is not so simple."

The article states, thought not very bluntly, GR is in contradiction with Newtonian law. This proves what I was saying about GR contradicting the rest of physics, except that I should have refrased the "rest of physics" to "Newtonian Law." lol my bad but it still contradicts aspects of physics i.e. newtonian law.

16. Apr 14, 2005

Dragongod

"While current observations do not yet provide a direct model-independent measurement of the speed of gravity, a test within the framework of general relativity can be made by observing the binary pulsar PSR 1913+16. The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation. "
-----this basically states that they are ASSUMING that the loss of energy is due to radiation escaping by gravity. They in know way know that it is definately escaping due to gravity. They are merely assuming. Therefore it doesn't necessaritly prove what they say it does: "The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%." This experiment is inherently flawed because they are assuming the radiation is escaping because of gravity which is not necessarily true. The only way to truely measure if gravity is faster than light would be to see if we feel the gravitational force of a black hole. If we do, then gravity is as fast as present time (instantaneous). If we don't, then gravity isn't as fast or just as fast as the speed of light!