Horizon Radiation

SPEAKER_1 (0:00)

College football is back, so Hilton called on me, the superstition concierge, to make your fan rituals a reality. Need a room to match your lucky number? We got you. Want to make sure our team doesn't wash your lucky jersey? Ooh, that smells lucky.

Hilton's unmatched hospitality can keep up with any superstition. Even a marching band wake up call at 555 in 55 seconds. Hit it! When you need a team that will do whatever it takes on game day, it matters where you stay. Hilton, for the stay.

SPEAKER_3 (0:31)

When you need to build up your team to handle the growing chaos at work, use Indeed Sponsored Jobs. It gives your job post the boost it needs to be seen, and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a $75 sponsored job credit at indeed.com/podcast. That's indeed.com/podcast. Terms and conditions apply. Need a hiring hero? This is a job for Indeed-sponsored jobs.

Matt O'Dowd (1:03)

The most successful theory in physics combines the weirdness of quantum mechanics with, well, the weirdness of special relativity to give quantum field theory. This theory tells us that particles can be created and destroyed during interactions. Even so, every observer agrees on whether a particle exists or not, right?

Yeah, about that.

Both theories of relativity, special and general, tell us that many things are observer dependent. Different observers might disagree about speeds, lengths, or times. But the laws of physics should be the same for everyone. And for two observers with very different but constant speeds, inertial observers, the vacuum itself should appear the same.

But relativity can throw some weirder things into the mix. Specifically, the idea of a horizon. For example, the event horizon of a black hole, out of which no information can travel. Or the cosmological horizon that limits the observable universe. And a strange type of apparent event horizon even appears when we accelerate.

Generally speaking, a horizon is a boundary in space-time from beyond which no influence can pass. It limits an observer's causal connection to part of the universe. As weird as these space-times with horizons may be, the statement from before holds the fundamental laws of physics shouldn't change if we go near a black hole or if we start accelerating. But enforcing this isn't automatic and something has to give. As it turns out, what gives is the nature of the vacuum. And in fact, the notion of what a particle is becomes observer dependent. In coming episodes, we'll be delving into Hawking radiation and the Unruh effect, both of which are very similar and are results of this observer dependent vacuum. There are a couple of compelling but inaccurate explanations of Hawking radiation around. And here at space-time, we want to be as simple as we can, but no simpler. So we're going to take this episode to lay the final stepping stone we need to examine these effects in detail and accurately. To get at this idea of observer dependent particles and vacua, we're going to need some quantum field theory and we're going to need to draw heavily on this recent episode. In QFT, we think about each particle type as having its own quantum field that exists at all locations in space. If the field vibrates with a single quantum of energy, we see a particle. That oscillation can be distributed over some region of space, representing the possible positions of the particle. The properties of the particles are encoded in the properties of the fields. The laws of physics, as we know them, are the rules defining how particles interact. For the laws of physics to be consistent, the fundamental properties of these fields must be the same for all observers. Imagine, I fire a pair of photons which annihilate to produce an electron-positron pair. All observers, be they floating in empty space or accelerating or orbiting a black hole, should agree on the basic result of that interaction. Two photons in, one electron, one positron out. That means everyone has to agree on the fundamental nature of the quantum fields that describe these particles and the way they interact. There's no conflict for constant-speed inertial observers. In fact, quantum field theory is what we call Lorentz invariant. Special relativity is built in, so the equations describing the interaction transform cleanly between inertial reference frames in a way that leaves the laws of physics and the nature of the vacuum intact. But when an observer who sees a horizon tries to write down these equations, in order to preserve the laws of physics, they find they have to redefine the nature of the vacuum itself. To see how this happens, we need to think about how particles, interactions, and vacuums are described in quantum field theory.