Making a wormhole just got easier...

...but it's no simple matter.

2 June 2003

Philip Ball

A bit of 'exotic matter' and you could be going anywhere.

Good news for time travellers - it just got cheaper. The amount material needed to build a window through time is infinitesimally small, new research shows.

To travel through time, all you need to do is open a wormhole in space-time and step through it. And to do that you need a magic ingredient called 'exotic matter', which is repelled rather than attracted by gravity.

The hitch is that no one has the remotest idea how to make exotic matter. But don't despair, say Matt Visser, of the Victoria University of Wellington in New Zealand, and his colleagues. They have shown that when we do figure out how to make the stuff, we won't need very much of it1.

As Star Trek: Deep Space 9, Quantum Leap and Stargate have taught us, wormholes are the preferred mode of transport for today's fashionable time-traveller. These hypothetical tunnels connect distant parts of space-time, the fabric of our Universe. And despite the philosophical havoc that wormholes wreak with notions of causality, Einstein's theory of general relativity - which describes space-time - allows them to exist.

Six years ago, Visser and his colleague David Hochberg showed that in order to stay open, wormholes need exotic matter. It's weird stuff, however - it can be considered to have negative energy, meaning that it has even less than empty space. It's the same as saying that it experiences gravity as a repulsive force, and physicists have never encountered anything of the sort.

So they imagine it. They key to exotic matter lies in quantum fluctuations, which give empty space a kind of fizziness. Quantum theory says that subatomic particles and their corresponding antiparticles are continually popping in and out of existence in the vacuum of empty space. Exotic matter might arise by suppressing this fizz, or as a physicist would say, by violating the averaged null energy condition (ANEC).

If this were to happen, quantum effects could give rise to tiny amounts of exotic matter. But how much is needed to sustain a wormhole?

That is what Visser and colleagues have now calculated. They find that, if the wormhole is designed carefully, "the total quantity of ANEC-violating matter can be made infinitesimally small". This makes a wormhole considerably easier to create.

Back to the future again

It's not the first time that traversable wormholes have been pulled out of a pit of implausibility. In the 1980s the British astrophysicist Stephen Hawking conjectured that even if you could make a wormhole stabilized by exotic matter, you couldn't go through it to travel in time and space, because even a single particle would destabilize it.

This became known as the Chronology Protection Conjecture. It was a relief for philosophers who were trying to protect the notion of causality. The paradox they envisioned - immortalised in the movie Back to the Future - was that if wormholes could exist it would theoretically be possible to go back in time and prevent your parents from meeting. This would prevent your own existence, and therefore your ability to go back in time.

But physicists subsequently thought of a way around this problem - there are, for example, 'time loops' threading through a wormhole along which backwards time travel is possible, but without its being able to alter the future.

Sadly, an infinitely small amount of exotic matter is not the same as none at all. So until someone figures out how to get hold of it (not to mention how to open up a wormhole in the first place), you can forget about trips to the Jurassic era - or your parents' first date.

  1. Visser, M., Kar, S. & Dadhich, N. Traversable wormholes with arbitrarily small energy condition violations. Physical Review Letters, 90, 201102, (2003).

Nature News Service / Macmillan Magazines Ltd 2003