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Comment by starbeast | Hacker News Reader

Don't you have two things that can generate gravitational fields though? As distortions in spacetime can be caused by either mass or the momentum of massless particles, such as photons. Or do I have that wrong and photons merely follow the distortions and not cause any of their own? As presumably, you could have a negative gravitational field from a photon traveling backward in time, without having any issues with changes in inertial mass.

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> Or do I have that wrong and photons merely follow the distortions and not cause any of their own?

You have that wrong in the momentum of massless particle has mass, independently of its carrier. Massless particles do not actually exist, what exist are particles with zero "mass at rest" and that are never at rest.

Ahh, I was getting a bit confused from photons distorting spacetime off their wavelength and plank's constant, given they also apparently have no mass. Thanks for that. I have to go and re-read a lot of stuff now though. ;)

You may like to make a quick read of this:

https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...

Besides it being a basic relation for relativity, there's also an entire "parallel" quantum mechanics for particles that have too much energy at the momentum component based on it.

You're talking about relativistic mass, but that isn't a commonly used concept anymore. Today it is indeed the norm to think of "mass" as being the mass at rest. Massless particles are really massless and have momentum without having mass.

In which case, we seem back to having two things distorting spacetime. Could mass just be the momentum of massless particles going in really small circles such that you can pack them together and call them matter?

Space is distorted by the stress-energy tensor field. Mass is only a component of that.

If you use "mass" to mean the rest mass of something. If you use it meaning the relativistic mass, it's the entire value, not a component.

Yes, using it as the relativistic mass leads to confusing papers and equations, so it's better avoided. Yet, the press does that, what makes everything very confusing for lay people. I should have clarified that the OP usage of the term wasn't optimal (although common on the media he sees), but this following discussion about word definitions seems to be completely missing the point.

What you described sounds like a geon.

https://en.wikipedia.org/wiki/Geon_(physics)

Oh, cool, yep. I didn't know that's what they are. I have basically been wondering if all matter is variations of those.

Energy distorts spacetime. Mass only distorts spacetime because it is a form of energy. Light is also a form of energy.

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