# Photons and black holes

1. Dec 1, 2008

### Pjpic

As I understand (?) it, if you want to observe something too small, the photons needed will have to be so energetic that a black hole will form.

The question I have is: why would photons form a black hole, I thought photons were massless?

2. Dec 1, 2008

### benbrooks

This probably isn't the correct explanation as to why black holes would form, but if you get anything moving fast enough it's mass will increase, including photons.

3. Dec 1, 2008

### Riogho

Photons never increase in speed. They are always traveling at c.

4. Dec 2, 2008

### munky99999

http://www.hno.harvard.edu/gazette/1999/02.18/light.html" [Broken]

Last edited by a moderator: May 3, 2017
5. Dec 2, 2008

### Riogho

ALWAYS.

Those photons are absorbed and re-emitted. In between absorption and re-emission they are traveling at c. Always.

6. Dec 2, 2008

### Pjpic

Back to the subject - how do photons create black holes if they have no mass? Do they have to be absorbed?

7. Dec 2, 2008

### Riogho

You have to remember that the word mass ALWAYS means energy, but the word energy does not always mean mass.

In this case, you can have gravitational forces because of the energy of the photons, gravity is a product of mass, which is a product of energy.

If you had one photon shooting by, or even a hundred photons shooting by in the same direction, they would not form a black hole. However, if you had a bunch of radiation all focused onto one spot, then you could, theoretically form a black hole. The Schwarzchild radius of the black hole would be the size of the wavelength of radiation you were using.

8. Dec 2, 2008

9. Dec 2, 2008

### Pjpic

Does this mean that photons always generate gravition forces, that photons sometimes generate gravity, that within a certain radius the photons somehow change, or aren't I getting this at all?

10. Dec 2, 2008

### Riogho

The reason that is, is because if you were to take the photon traveling in a single direction along a coordinate plane at high speeds the energy and momentum would tend to zero.

However, if you had two beams (or more), traveling opposite of each other than you will get a gravitational effect because the energy of the system is positive.