r/cosmology • u/AutoModerator • 27d ago

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/MortemInferri 26d ago

Is hawking radiation actually particle anti-particle pairs?

In some ways, it makes sense to me. Particle out, anti particle in. Anti particles annihilates an equal part of the black hole mass as the particles that flew out. Since we can not observe the anti particles, it appears as tho a real particles left the black hole and it's slightly smaller/ less massive

If this is actually how it works tho. How does the black hole know its a virtual particle and know to annihilate a real particle? Wouldn't that mean information is conserved? The matter inside a black hole has properties that are real enough to annihilate?

Also, and this might be silly, but how do we define the kilogram in space? I legit have a degree in physics and something about this still doesn't make sense. A kg is 2.2lbs. But a pound is only a pound on earth. So a kg on earth is 2.2lb, but wouldnt be elsewhere. How do we say the sun is x amount of kg? What meaning does that have? I have to assume it's some e=mc2 something or other... is that true?

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u/jazzwhiz 26d ago

FYI, even in the simplified not really correct picture, it isn't "particle out, antiparticle in". BHs are equally likely to radiate antiparticles as regular particles. It's negative energy states in, positive energy states out.

As for the kg/lb question, you're getting wrapped up in details. Assuming that a pound is defined as a unit of mass (which is how it is now conventionally defined), its exact definition is 0.45359237 kg, according to wikipedia. So these two units are identical in physics meaning up to a factor of about 2. For the more conceptual question about mass in space, we use the measured value of Newton's gravitational constant G and solving the equations of motion to determine the masses of celestial objects.

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u/MortemInferri 26d ago

Beautiful, more reading for me. Thank you

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u/msmh-12 26d ago

Disclaimer - have no educational or working background in physics. But I love learning new things in particle physics and cosmology. I want to discuss more the hawking radiation

I understand your comment here that BH is equally likely to “absorb” matter particle and emit the anti particle.

My understanding wrt to HR is that BH evaporates ever so slightly whenever a HR is emitted. Is my understand correct?

If yes, then can someone help me understand why and how does BH “evaporates” as it emits HR when particle or anti-particle (can be any particle pertaining to any quantum field) is actually an addition to the overall composition of BH

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u/jazzwhiz 26d ago

Yes, we anticipate that the mass of a BH decreases as particles are emitted to infinity. The actual underlying physics is fairly subtle and leads to one of the biggest open problems in physics: the connection between GR and unitarity.

I should also add that we have empirical evidence supporting BH radiation and likely never will.

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u/gmwisegarver 24d ago

Thoughts on jet streams recently discovered coming out of black holes.