# Energy and Gravity

1. Jul 18, 2008

### Peaceful

Gravity is the mutual attraction between matter right?
I heard photons are light particles, so would that mean theres an attraction between photons and photons or photons and regular particles. Since matter is a concentrated form of energy and light is energy wouldn't that mean there would be a mutual attraction between energy(in this case light). Would there also be a attraction between energy and matter.

BTW I'm just some curious 14 year old so don't laugh at me :(
Forgive me for making a lot of typos.

2. Jul 19, 2008

Yes gravity is the attraction between matter. In Newton's equatyion for gravitational attraction you need mass to create a force. Photons how ever have no mass so no force goes between them. Now to avoid confusion I'll explain about E = mc^2, this confused me alot. E= mc^2 is only for rest mass, photons rest mass is 0, since they are always moving we can't use this equation.

3. Jul 19, 2008

### pallidin

Here's some info I found... not sure if it helps your understanding.

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"For most average objects, momentum is truly mass x velocity. When motion
gets close to the speed of light, we find that the momentum relation p=mv is
only an approximation. It is only correct when speed (v) is much smaller
than the speed of light (c). The relation that works for all speeds is E^2
= p^2c^2 + m^2c^4. It is much less convenient to use, and doesn't help
figure anything out until you reach speeds of perhaps thirty million meters
per second. For a particle with no mass, the relation reduces to E=pc.
This works for a photon. For very small speeds, the system reduces to
E=mc^2 + (1/2)mv^2, and p=mv. This leads to relations with kinetic energy
and momentum: much more convenient to work with and just as accurate until
you reach speeds close to the speed of light.

As for magnetic field, there is no reason why it should behave like gravity.
For one thing, the strength of magnetic FORCE depends on the speed of the
particle being pushed or pulled by the field. Also, unlike gravity,
magnetic force pushes sideways, perpendicular to the field direction.
Gravitational force is just gravitational field multiplied by the mass being
pushed or pulled. Electric force is just electric field multiplied by the
charge being pushed or pulled. Magnetic force depends on the charge, speed,
AND direction of the charge being pushed or pulled, as well as the strength
of the field. It is a very different kind of force.

As for "carrying" the field, it is known that photons of light transmit both
electric and magnetic force. In fact, light is waves made of oscillating
electric and magnetic fields."

Dr. Ken Mellendorf
Illinois Central College

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