Season 17, Episode 63: What is The Chronology Protection Conjecture?

Hey StarTalkians! Season 17, episode 63 features Neil and special co-host, astrophysicist Janna Levin, talking about the new StarTalk series “In Theory” and touching on a range of cosmic issues. In the discussion, they briefly mention the so-called “chronology protection conjecture” of Stephen Hawking:

In Theory with Janna Levin - StarTalk Radio

(from 32:20)

But the discussion is more focused on the nature of conjecture in physics, and it left me thinking: what really is the chronology protection conjecture?

Bending Light Cones and Time Travel

According to Einstein’s theory of relativity, the speed of light is the ultimate speed limit of the universe. If you’re at some location at time t(1), by time t(2), you can’t possibly have moved further than a photon would have been able to. This sets boundaries on the possible places you could get to, and therefore, the things you could have a causal effect on.

These boundaries are expressed as a “light cone”. Every event has a light cone in its past – the possible events that could have led to it – and one off in its future – the things it could possibly effect.

However, just like gravity warps spacetime, certain spacetime arrangements can bend your lightcone too. Imagine being close to a black hole – even if you don’t fall in, the intense gravity certainly limits how far you can possibly move.  

People have solved Einstein’s equations to show spacetimes that bend lightcones into a big loop, called closed timelike curves. If you went around such a loop, you’d end up back where and when you started the loop.

The Wormhole Time Machine

One example is a wormhole time machine, as described by Kip Thorne. If you sent one end of a wormhole moving at close to the speed of light, it would experience less time than the other end, a little like the “twin paradox”. Going through the wormhole and out of the moving end would send you back to this earlier time. But surely that’s not possible, right?

The Chronology Protection Conjecture

That’s where Hawking’s paper comes in. Although it’s technically a “conjecture” because it can’t be truly settled without a theory of quantum gravity, the paper’s argument is detailed and complex. Hawking argues that establishing such a system would lead to a build up of quantum vacuum fluctuations, to the point where the wormhole would be destroyed before it could ever be used.