# Light and black hole

1. Apr 10, 2014

### harshit

Since light has no MASS............ Then how does a black hole absorb light?

2. Apr 10, 2014

### Staff: Mentor

3. Apr 10, 2014

### Matterwave

Light has no mass, as far as we know. Mass only makes sense in the sense of rest mass. Statements such as "light does have mass" are not helpful (nor accurate), in my opinion, especially when talking to someone who is not well versed in physics.

The reason light can be attracted to gravity is that gravity is not a force (F=Gm1m2/r^2) as Newton postulated, but it's the curvature in spacetime. As such, EVERYTHING behaves in gravity the same way (the equivalence principle), even things without mass.

4. Apr 11, 2014

### Drakkith

Staff Emeritus
To fully understand how a black hole absorbs light you would need to learn about General Relativity, which is our most accurate theory regarding gravity.

Try starting at the following link and then searching for more information on GR as needed: http://en.wikipedia.org/wiki/Introduction_to_general_relativity

The very short answer, which will probably confuse you at the moment, is that once light passes beyond the event horizon of a black hole there are no paths through spacetime that lead away from the black hole. All paths through spacetime lead towards the center.

5. Apr 11, 2014

### Mordred

I disagree with this sentiment, your implying that people are unable to understand the distinction and we should answer with incomplete answers.

the mass of a photon is
1*10−18eV/c2

the rest mass is 0

6. Apr 11, 2014

### Drakkith

Staff Emeritus
That is not the mass of a photon. 1x10-18 is the upper limit on any possible photon mass. In other words, if the photon does have mass, it's going to be that value or less.

7. Apr 11, 2014

### Staff: Mentor

Welcome to PF!

This question does not compute. Consider this similar question:
Since light has not mass, then how does a chair absorb light?

The premise and the question have nothing to do with each other.

8. Apr 11, 2014

### Mordred

your right I was in a hurry, wife was rushing me to help her clean lol. So I grabbed the wiki value

9. Apr 20, 2014

### CharlesMichael

Matter is viewed in two forms, mass and energy, with something is 100% energy, it's form has no mass. That is not to say that it can not be converted to mass, on Earth, photosynthesis traps energy and the plant acquires mass. Both mass and energy are effected by gravity and mass can be thought of a a greater potential energy than radiation energy itself because it's energy is condensed in the form of mass. The function of gravitation aligns both mass and energy, energy is the least effected but still is effected.

10. Apr 20, 2014

### Matterwave

I've seen some treatments of "effective inertial mass" or "effective gravitational mass" of a photon in some physics textbooks, which are, in the end, inevitably just mathematical tricks to making some equations work for light as they do for massive particles. They have done nothing to help my comprehension of the subject, they just make me confused when I read them. That is my opinion. I don't think statements such as "light has mass" are very useful at all, especially if you don't specify that what you're really saying is that "under certain, limited, circumstances, we can make certain equations for massive particles work for photons if we give them some effective masses".

11. Apr 20, 2014

### Drakkith

Staff Emeritus
A couple of things.

First, it's not that gravity affects mass and energy, it's that the presence of mass and energy warp spacetime and cause the effect we call gravity.

Second, saying that matter is viewed in two forms is misleading. There is no sliding scale from 100% mass/matter to 100% energy. Elementary particles have mass and when bound into atoms and molecules a system of particles loses some of that mass as energy, either in the form of radiation, kinetic energy, or heat. But this mass is only a property of the particles and the systems they occupy, it is not a "form" that they exist in. Electrons don't exist "as energy" sometimes and "as mass" at other times.

Third, I just want to make sure people understand that it isn't just that energy is converted to mass or vice versa, but also that when a quantity of energy leaves a system an equivalent amount of mass does so as well. Conversely, when energy enters a system, that system is now more massive. There doesn't have to be a conversion of what is known as rest mass into energy, though that can and does happen in chemical and nuclear reactions.

I'm not sure what this means. I'm guessing it has something to do with how gravity affects light? If so, remember that energy and light are NOT the same thing. 1 joule of energy carried by a warm object acts differently than 1 joule of energy carried by photons.

12. Apr 20, 2014

### CharlesMichael

If you have to cut your lawn every week, you basically understand that sunlight is trapped by the process of photosynthesis and creates mass with addition to air and soil and water. A trapped photon adds to mass, whether it is trapped in a galaxy or a blade of grass.
All forces required to maintaining equilibrium require an expenditure of energy period! So stating that mass warps space time and the warpage of space time creates gravity is confusing because it seems to create a static field and a static process. That is a fundamental violation of forward time and space. If space is finite and expanding it has to come from something to expand. If on the other hand mass creates space-time ( the gravitational wave as a function

13. Apr 20, 2014

### CharlesMichael

If you have to cut your lawn every week, you basically understand that sunlight is trapped by the process of photosynthesis and creates mass with addition to air and soil and water. A trapped photon adds to mass, whether it is trapped in a galaxy or a blade of grass.
All forces required to maintaining equilibrium require an expenditure of energy period! So stating that mass warps space time and the warpage of space time creates gravity is confusing because it seems to create a static field and a static process. That is a fundamental violation of forward time and space. If space is finite and expanding it has to come from something to expand. If on the other hand mass creates space-time ( the gravitational wave as a function of surface decay of mass, and since it is known that identical low energy waves generate from objects in a medium create combine into constructive interference waves and the resulting kickback brings the objects together, there is a better explanation. Mass gives off a gravitational wave creating space-time and the wave interaction creates the back action of gravitation.
If you have ever jumped on a trampoline, when two people jump together they create one large depression as they jump down and the result when they rebound back up is that they come together and collide. This is the fundamental way the force of gravity works, it is a back reaction to wavefront collision and reformation. This explains how gravity can act as a force and use up energy just as you can push a battery dead car but you also need to expend energy to do that. All forces need to use energy up.
Also light or visible light is a sliver in the spectrum of all energy

Last edited: Apr 20, 2014
14. Apr 20, 2014

### Drakkith

Staff Emeritus
I'm not arguing against this. The energy given up by the photon is taken by the grass and the grass will be more massive until it gets rid of that energy somehow.

This is incorrect. A book sitting on a desk has no work done on it and no energy is expended to keep the normal force balanced with the gravitational force.

No, it's the actual reason for gravity. The stress-energy tensor in GR determines how spacetime is curved. The more energy (which includes the mass of objects and systems of objects) or stress, the more spacetime is curved. The path that objects follow through this curved spacetime causes them to move towards each other in the spatial dimension, which manifests as the "force of gravity".

No, it does not. Space is not a "thing" in the usual meaning. It is framework, it is geometry, it is not a substance that requires more to be made.

I don't know where you're getting this from but it's utterly wrong. Mass adds to the energy part of the stress-energy tensor, which is how we calculate curved spacetime. It does not create anything.

No, this is not how gravity works at all. Do some reading in the General Relativity forum and I'm sure you can find plenty of good posts there about it.

Fundamental forces of nature do not require an expenditure of energy to function. No energy is expended in keeping an electron in a hydrogen atom, just like no energy is expended keeping me in my chair right now.

Remember what energy is. It is the ability to perform work. Work is defined as a force applied over a distance. Applying a force on an object that does not move does not generate work on that object.

15. Apr 21, 2014

### Staff: Mentor

Thread closed temporarily for Moderation...

EDIT - Thread re-opened.

Last edited: Apr 21, 2014