**Ask a monkey a physics question thread**

[OngBonga]

While the energy is conserved, mass is not conserved. While the total energy of the resulting photons is equal to the total energy of the initial particles, it is no longer expressed in terms of mass. If no mass, no gravity.

I still can't accept this. I feel like energy is being lost in the form of gravitational potential energy or whatever. I feel like this is a quirk of our limitations on understanding.

idk, once we thought that gravity was a force, carried by a particle, but now we understand it as a consequence of warped spacetime. Maybe the photon plays a similar role... maybe electromagentism isn't a force, maybe it's a consequence and we're just doing it wrong.

It seems logical to me that, if photons are massless but have an energy value, anything emitting photons is losing mass, and therefore gravity.

I feel like if a photon is a particle, then it must have a non-zero mass, just perhaps a mass so small as to be immeasurable. Otherwise, I'm inclined to think the fate of the universe is graviational decay, as matter slowly decays into photons.

Can photons turn into matter? A quick google search seems to imply this is theoretically possible. If two photons collide, then we have an electron and a positron. That's two particles with mass that interact with one another graviationally, created from two particles that apparently don't interact with one another gravitationally.

That "missing" gravity must exist somewhere in some form, imo. Maybe in the case of the photon, gravity is turned into electromagnetism. Maybe our missing "mass" is how we can prove gravity and electromangetism are one and the same.

Well this is a nice start to my sunday. I'm still in bed with a spliff, talking shit about the most complex of subjects. All that's missing is tea...

[MadMojoMonkey]

I still can't accept this. I feel like energy is being lost in the form of gravitational potential energy or whatever. I feel like this is a quirk of our limitations on understanding.

idk, once we thought that gravity was a force, carried by a particle, but now we understand it as a consequence of warped spacetime. Maybe the photon plays a similar role... maybe electromagentism isn't a force, maybe it's a consequence and we're just doing it wrong.

Of course, this is always possible - expected, even. However, we're at the point of refinement. What we've figured out is demonstrably repeatable, and makes precise predictions.

We're trusting the theories and looking for things that seem impossible, and we're actually seeing those things.
The theories are at least robust enough to trust in this regard. New theories must agree with these theories where they describe the same things. If a new theory says our old data is wrong, then that's a bad sign for the new theory. The data is there.

It seems logical to me that, if photons are massless but have an energy value, anything emitting photons is losing mass, and therefore gravity.

Yes.

I feel like if a photon is a particle, then it must have a non-zero mass, just perhaps a mass so small as to be immeasurable. Otherwise, I'm inclined to think the fate of the universe is graviational decay, as matter slowly decays into photons.

This argument is lost on me. What evidence do you have that this is not the eventual fate of the universe?

Can photons turn into matter? A quick google search seems to imply this is theoretically possible.

Yes.
It's not theoretical. The 1948 Nobel Prize in Physics was awarded for demonstrating pair production.
It is well known and studied.

If two photons collide, then we have an electron and a positron. That's two particles with mass that interact with one another graviationally, created from two particles that apparently don't interact with one another gravitationally.

Well, photons don't really collide. They are Bosons. Bosons tend to pull each other into sharing the same state at the same time. This effectively means that they pass right through each other if they are on a collision course.

A single photon in an electromagnetic field can experience pair production.

The pair production doesn't have to be electron-positron, but it is always particle-antiparticle. All the conservation laws are followed. I.e. conservation of charge means if one of them has +1 charge, then the other must have -1 charge.

Also, the conservation of energy means that the photon which annihilates must have at least some minimum threshold of energy. This minimum threshold is equal to mc^2, where m is the rest mass of the particle-antiparticle pair. Any energy above this threshold is transformed into kinetic energy of the created particles.

The rest mass of an electron or positron is ~0.511 MeV. So any photon with less than (2 x 0.511 MeV = ) 1.02 MeV cannot undergo pair production into an electron-positron pair.

That "missing" gravity must exist somewhere in some form, imo. Maybe in the case of the photon, gravity is turned into electromagnetism. Maybe our missing "mass" is how we can prove gravity and electromangetism are one and the same.

The conservation of mass-energy is stronger than either the conservation of mass or the conservation of energy.

The notion that energy changes forms is not foreign to you. Just get comfortable with the fact that mass is another form of energy. While energy is never created or destroyed, it changes forms all the time. Furthermore, relativity says that the form that energy expresses itself may not even be the same in all reference frames, but the total energy is the same.

Well this is a nice start to my sunday. I'm still in bed with a spliff, talking shit about the most complex of subjects. All that's missing is tea...

I'll hook you up