# The Naked Scientists Forum

### Author Topic: Questions.  (Read 1877 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### Questions.
« on: 03/06/2005 22:29:39 »
Photons are generally accepted as having a rest mass of zero. However, since they are never actually "at rest", does that mean they do have a true mass? Similarly, do photons ever exhibit gravitational fields? Are mass and energy the exact same thing, or two seperate things that are merely interchangable?

A further photon question regarding gravity: Imagine that you have a sphere that is mirrored on the inside, and the sphere contains light. The mirrored interior is a "perfect" mirror, that reflects all light without absorption, so the light inside bounces around. Since light is composed of bosons, this sphere can be filled with as much light as you want, since bosons can occupy the same space as other bosons. The light inside this sphere obviously contains a certain level of energy. If there is enough energy in this light, will the sphere collapse into a blackhole? Let's say that the sphere has a radius that is smaller than the Schwarzchild Radius of the mass equivalent of the energy of the light inside of it. Does it collapse into a blackhole? Is this light-filled sphere any more massive than the same sphere would be if it was empty?

#### gsmollin

• Hero Member
• Posts: 749
##### Re: Questions.
« Reply #1 on: 04/06/2005 05:57:20 »
Photons are generally accepted as having a rest mass of zero. However, since they are never actually "at rest", does that mean they do have a true mass?

Photons have a moving mass given by m=E/c^2. Their rest mass is zero.

Similarly, do photons ever exhibit gravitational fields?

Yes. This was the first test of general relativity: The bending of a ray of starlight as it passed near the sun during a solar eclipse. It demonstrated that light has mass and a gravitational field always accompanies a mass.

Are mass and energy the exact same thing, or two seperate things that are merely interchangable?

They are separate things. They can be interchanged under the right conditions, but not spontaneously.

A further photon question regarding gravity: Imagine that you have a sphere that is mirrored on the inside, and the sphere contains light. The mirrored interior is a "perfect" mirror, that reflects all light without absorption, so the light inside bounces around. Since light is composed of bosons, this sphere can be filled with as much light as you want, since bosons can occupy the same space as other bosons. The light inside this sphere obviously contains a certain level of energy. If there is enough energy in this light, will the sphere collapse into a blackhole?

Classically, yes. Quantum mechanics says maybe. It's one scenario for the generation of primordial black holes. These conditions existed a small fraction of a second after the big bang.

Let's say that the sphere has a radius that is smaller than the Schwarzchild Radius of the mass equivalent of the energy of the light inside of it. Does it collapse into a blackhole? Is this light-filled sphere any more massive than the same sphere would be if it was empty?

Yes.
« Last Edit: 04/06/2005 05:59:02 by gsmollin »

#### The Naked Scientists Forum

##### Re: Questions.
« Reply #1 on: 04/06/2005 05:57:20 »