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Comment by withinboredom | Hacker News Reader

I always wonder if this "missing mass" is just light. As in it is so close to zero mass to be undetectable at a normal scale, but at galaxy scale, it actually has mass.

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General Relativity has https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor which is the term that generates the gravitational field. It includes the energy of photons.

I've wondered similarly, but I don't know why it would need mass. That just complicated things. It has energy, and we have an equation that tells us how much equivalent mass that would be. It's not missing mass, it's energy that's the cause of the extra gravity.

I also assume this has been accounted for though

How do you square your idea with the other substantial implications of photons having mass?

Photons bend spacetime even without having mass.

Bingo, does energy not affect spacetime the same as mass? I always assumed it did. My assumption was this was already worked into the calculations and not enough to account for dark matter.

If photons had a very small but non-0 mass, it would simply mean that they actually move slightly slower than c, but it wouldn't otherwise have any significant implications. There are in fact experiments that are trying to check whether photons have a mass we can detect.

Also, just as a note, I'm using the term "mass" for what used to be called "rest mass" (as opposed to "relativistic mass"). Even if their "rest mass" is truly exactly 0, and their speed is exactly c, they still have energy and momentum, and thus they have mass in general relativity, but that is a different discussion.

Wouldn't that mean that light also has infinite energy?

This is the basis of the "ultraviolet catastrophe" and lead to the discovery of quantization of radiation. The "catastrophe" being that as classical laws for light implied a potentially unbounded amount of energy as wavelength decreased.

https://en.m.wikipedia.org/wiki/Ultraviolet_catastrophe

https://youtu.be/rCfPQLVzus4

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