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Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it

by p1mrx 3y ago

Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?

credit_guy 3y ago
Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.

by p1mrx 3y ago

Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?

credit_guy 3y ago

Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.