Einstein's equations and the enigma of wormholes | Janna Levin

• Once you start accepting that spacetime can curve or it can stretch, it can contract, time can warp—you can play any game you want. One very simple thing to play around with is the idea of a wormhole. If you can start to manipulate space, wouldn't you wanna manipulate it to make a bridge between here and some far-off point? And then what you can ask is, "How would this be possible? How would I find matter and energy to do this?"

And the Einstein equations will tell you these are the features of the matter and the energy you will need to make that bridge. The problem is that the forms of matter and energy that are required to build them and keep them open, they want to collapse. So I think the argument would be wormholes are mathematically, perfectly feasible. Physically, they might be impossible because there might never be the forms of matter and energy that are required to build them.

Quantum wormholes are still under discussion. This stuff's so hard, man. Okay, let's try something: All of matter really is just a list of descriptions of a short number of characteristics. To be an electron means to have that mass, that spin, and that charge—and that's all it means. There's no electron that's slightly heavier. There's no electron that's spinning a little slower or has a different charge. That is all that they are is that list. And we call that "the information."

There's also information in how things relate to each other, and we believe that all of the information in the Universe has to be preserved. It can't be lost. If that electron runs into the singularity and its quantum numbers are gone, that is information falling out of the Universe, disappearing. Now, the laws of physics really are predicated on the idea that that can never happen because if that can happen, then the laws are meaningless.

The discovery of black holes did not lead to a big emotional crisis about the loss of information because of this idea that it was always hidden behind the event horizon. And that one-way window meant that we would be completely ignorant of the loss of the electron and the loss of the electron's quantum numbers and it floating out of existence. We're protected from that so completely, that people were willing to kind of just shrug their shoulders and call it a conjecture of a protection that we would never run into any problems in the reality of the world, this side of the black hole.

I always thought that was a bit glib, but that was an attitude people were willing to take, at least until Stephen Hawking. He begins to ask about the quantum properties around black holes. In quantum mechanics, we believe that there is, in empty space, still quantum activity—and that's what we think the vacuum is. The vacuum kind of churns and froths with this potential. The vacuum doesn't allow anything to happen.

There are rules that things always have to come in pairs; and they have to come in pairs so that they don't disturb the nature of the vacuum. Think of an analogy with color: Imagine you have a color green, and that is the property. You might be able to create a little blue droplet of paint out of this color green if you have the exact yellow pair, such that in combination they make that green—and that has all the properties of the vacuum.

What Hawking does that's so unusual is he realizes that if I have a pair, the black hole has the capacity which no other thing in the Universe has, which is it can steal one and leave the other completely alone. It can hide one of those droplets behind the event horizon. Its pair, which is left behind, let's say it's the yellow droplet, can't go back into the vacuum 'cause it no longer has the right properties. It needs its pair to disappear again, in our analogy to become green again. It can't, by itself, be green.

And so that little particle in Hawking's description, he realizes this particle can emanate from just outside the black hole. It's not coming from inside. It's coming from the nothingness, the quantum energy of the vacuum. And it can escape. And if I'm very far, I will receive this y...