Up until now, it was thought that only light could cross a vacuum. A new experiment suggests that heat can do so as well under very specific conditions.
Light is not the only thing that can be mentally chopped up into discrete packages of energy. Vibrational energy can also be diced into little packages called phonons. Phonons behave in many of the same ways that photons do. For example, certain phonons can only be absorbed by certain materials. If the phonons have the wrong amount of energy for the material they come in contact with, they pass through the way x-rays pass through soft tissue.
Air Force researchers recently added a new trait that photons and phonons have in common. Phonons can ‘tunnel' through barriers.
Quantum tunneling has long been studied with photons. Because they are both particles and clouds of probability, they can ‘tunnel' through barriers. As a particle, a photon can move up to a barrier – close enough that its ‘cloud of probability' falls on the other side. Since walls can't limit abstract ideas such as probability functions, the photon just is at the other side of the barrier. This is quantum tunneling. It's one of the reasons our sun shines. It's been calculated that the heat at the center of the sun is not enough to induce fusion. Quantum tunneling is responsible for the hydrogen atoms in the sun moving through the barriers which separate them and fusing.
One of the most effective barriers for heat has always been a vacuum. Heat needs to radiate – one particle pushes another particle, which pushes a particle, and so on. Each push makes each particle move a little faster and so the head spreads. A vacuum has no particles to push, and so heat can't move through it.
Apparently, it can tunnel, though. Electric fields between two pieces of metal, separated by a nanometer of vacuum, allow vibrational energy to be transferred from one piece to the other. This is only a tiny step, of course, but for something that was previously not thought to be possible, every step is significant.