3 ms·

> if ever a matter and antimatter particle come into contact, they will annihilate each other in a burst of energy. > To complicate things, some particles, suc

by brabel 5y ago

> if ever a matter and antimatter particle come into contact, they will annihilate each other in a burst of energy. > To complicate things, some particles, such as photons, are actually their own antiparticles. I believe "a burst of energy" means a bunch of photons? So, we still end up with the same amount of energy as before, but all particles are now photons? Like in the heat death of the universe?

leephillips 5y ago
I don’t know about the heat death of the universe, but yes, the mass-energy is conserved, and the energy that comes out is in the form of photons.
- arbitrage 5y ago
  mass-energy is only conserved in static spaces. since the universe is expanding, it doesn't necessarily hold all the time.
  - leephillips 5y ago
    Do you think that the cosmic expansion of spacetime is remotely relevant to the experiment that we are talking about?
Filligree 5y ago
Nah, it's symmetric. An electron and a positron can annihilate, producing a pair of photons. Or two photons can annihilate, producing an electron and a positron. Or... Well, in fact the symmetry works for any rotation, not just 180 degrees. An electron and a photon can "annihilate", producing an electron* and a photon going the other way. This is also known as "A photon bouncing off a mirror". *: The careful reader will note there's no positron in this one. That's because positrons are what happen when an electron hits a photon so hard it bounces backwards in time -- that is, it's an electron going backwards in time. For one definition of 'time', at least. To make the mirror version work, the photon needs to have lower energy than would be involved with matter annihilation. Otherwise your gamma-ray photon will tear through the mirror instead of bouncing back, probably making a nice hole. This doesn't mean the rotation doesn't work, it's just that what I was describing wasn't really a "mirror" as commonly considered. But also, because momentum (or, more usefully, rapidity) is defined in terms of the angle the particle makes with time, rotations other than 180 degrees don't conserve momentum. So it's not a gamma-ray photon anymore.** **: Photons, obviously, have a fixed 'angle with relation to time'. You can take that as referring to the angle their waves make with time, instead.

by brabel 5y ago

leephillips 5y ago

I don’t know about the heat death of the universe, but yes, the mass-energy is conserved, and the energy that comes out is in the form of photons.

arbitrage 5y ago
mass-energy is only conserved in static spaces. since the universe is expanding, it doesn't necessarily hold all the time.
- leephillips 5y ago
  Do you think that the cosmic expansion of spacetime is remotely relevant to the experiment that we are talking about?

Filligree 5y ago

Nah, it's symmetric. An electron and a positron can annihilate, producing a pair of photons. Or two photons can annihilate, producing an electron and a positron. Or... Well, in fact the symmetry works for any rotation, not just 180 degrees. An electron and a photon can "annihilate", producing an electron* and a photon going the other way. This is also known as "A photon bouncing off a mirror". *: The careful reader will note there's no positron in this one. That's because positrons are what happen when an electron hits a photon so hard it bounces backwards in time -- that is, it's an electron going backwards in time. For one definition of 'time', at least. To make the mirror version work, the photon needs to have lower energy than would be involved with matter annihilation. Otherwise your gamma-ray photon will tear through the mirror instead of bouncing back, probably making a nice hole. This doesn't mean the rotation doesn't work, it's just that what I was describing wasn't really a "mirror" as commonly considered. But also, because momentum (or, more usefully, rapidity) is defined in terms of the angle the particle makes with time, rotations other than 180 degrees don't conserve momentum. So it's not a gamma-ray photon anymore.** **: Photons, obviously, have a fixed 'angle with relation to time'. You can take that as referring to the angle their waves make with time, instead.