A brief history of time 10

 Chapter 10

The wormholes and time travel

Observations of the microwave background and of the abundances of the light elements in universe indicate that the early universe did not have the kind of curvature required to allow time travel.

If you can travel faster than light, the theory of relativity implies you can also travel back in time. There is a problem with breaking the speed-of-light barrier.

There were experiments in Fermilab or CERN (European Centre for Nuclear Research) which showed that we can accelerate particles to 99.99 percent of the speed light, but however much power we feed in we can’t get them beyond the speed-of-light barrier. Similarly with spaceships: no matter how much rocket power they have, they can’t accelerate beyond the speed of light.

That might seem to rule out both rapid space travel and travel back in time.

However, there is a possible way out. If there was a shortcut between A and B. One way to do this would be to create a wormhole between A and B. This is a thin tube of space-time which can connect two nearly flat regions far apart. So if one could travel faster than light and through a wormhole from Earth to our nearest star A Centauri and then come back to Earth also through a wormhole, he would travel into past!

In 1935, Einstein and Nathan Rosen called these “bridges”. An advanced civilization could keep a wormhole open!

So what one needs to allow travel into the past is matter with negative energy density. The quantum theory allows the energy to be negative in some places, provided that this is made up for by positive energy densities in other places, so that the total energy remains positive.

We have experimental evidence that space can be curved in the way necessary to allow time travel. One might hope therefore that as we advance in science and technology, we would eventually manage to build a time machine.

Why hasn’t anyone come back from the future and told us how to do it?

Of course someone might claim that sightings of UFOs are evidence that we have been visited either from aliens or by people from the future.

But why do they reveal themselves to those who are not regarded as reliable witnesses?

The radiation by black holes shows that quantum theory allows travel back in time on a microscopic scale

The possibility of time travel remains open.