# Does light have gravity?

1. Jul 19, 2010

### Sci1

I learned that when matter is transformed into energy in a fusion reaction that the gravity associated with the matter that has transformed into energy is not lost. Electromagnetic energy has gravity and is not just a property of mass. I was given this answer on a question about black holes that are very hot (trillions of degrees?) inside, transforming matter into energy passed the event horizon, but not losing any gravity in the process.

How does a massless electromagnetic wave generate the property of gravity?

Electromagnetic waves are two interacting fields. Light differs in energy based upon wavelength. The shorter the wave length, the higher the energy, and, if I understand, the greater the gravity. If gravity can be simplified to a property of two mutually supporting fields moving through a space then can gravity become a property of how much space a quantity travels through from point to point? A high energy light wave moves straighter through space than lower energy light wave.

2. Jul 19, 2010

### fatra2

Cheers

3. Jul 19, 2010

### cometraza

Nice reply. To the point and lucid.

4. Jul 20, 2010

### Chalnoth

Correct. Light does gravitate. You can sort of think of the gravity field produced by light waves as being the gravity field of a normal particle in the limit of extremely high velocity motion.

This doesn't sound coherent to me. Nobody knows what black holes are like inside the event horizon. That area is likely governed by quantum gravity, which we just don't yet understand sufficiently. Astrophysical black holes, however, tend to be extraordinarily cold, and thus radiate at a very low rate. But they do lose energy as they radiate, and shrink as a result, albeit extraordinarily slowly (technically any object which radiates loses energy and thus shrinks, but typically the rest mass of the object is so large that the energy loss is negligible).

In General Relativity, gravity doesn't couple to mass. Rather, it couples to energy, momentum, pressure, and stresses. The reason why we think of gravity as coupled to mass is just that in our low-velocity world, the mass energy of the particles around us completely swamps these other parameters. For a photon this isn't the case, but it still gravitates just the same (it has energy, momentum, and pressure).

This isn't actually a valid conclusion. While it is true if you take a massive object and move it at higher energy (which requires a higher velocity), it will take a straighter path through a gravitational field, light does not behave in this way. All light waves, no matter their wavelength, take the exact same path through a gravitational field. This is because they are all moving at the same velocity.

5. Jul 20, 2010