ALICE and Bob are having an argument. They are looking at the same space, but while Alice sees only emptiness Bob sees lots of particles. Alice whips out a thermometer which registers zero temperature, but Bob’s registers heat from the motion of the particles. Who is right? They both are. If you’ve been thinking that a particle’s existence is independent of who sees it, time to think again. Existence, it seems, is all in the eye of the beholder.
Is this so very strange? We already know from quantum theory that something can be either a particle or a wave depending on who is looking. And we know from relativity that an object’s length or speed might look different to different observers. But in each case, everyone agrees that something is there. Alice and Bob can’t even agree on that. In one case particles exist, in the other they don’t. What is going on?
The particles Bob sees are Rindler particles, named after Wolfgang Rindler, a physicist at the University of Texas at Dallas who, with others, discovered the effect. Most of the particles are photons or neutrinos, but quarks and others are also in the mix.
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Bob sees them because there is a crucial difference between him and Alice. Bob is accelerating, which means that light from certain places in the universe can never reach him as long as he keeps accelerating. The boundary between these inaccessible places and everywhere else is called an event horizon. Alice, on the other hand, is moving along at a constant speed, so light from everywhere in the universe will always reach her, given enough time. For her there is no event horizon.
“If you think that a particle’s existence is independent of who sees it, think again”
Why then does an event horizon create Rindler particles? For the same reason a black hole gives off radiation. A black hole also has an event horizon, the point of no return for light trying to escape. This horizon radiates because the universe is filled with pairs of particles that are popping into and out of existence. Usually they annihilate each other, but if one particle crosses an event horizon into a black hole, it cannot escape. The other, however, can. It moves away, making the black hole appear to radiate. Exactly the same process occurs for Bob, even though it is just the result of his perspective. But if Bob stops accelerating, the horizon and the particles disappear.
Rindler particles and black hole radiation have yet to be seen. But that doesn’t stop them packing a powerful philosophical punch. The inescapable conclusion is that particles have no independent existence but are defined by how an observer’s motion relates to the geometry of the universe.
