A bridge to another world

The idea of wormholes dates almost as far back as general relativity
itself. Remember from Chapter 4 that Karl Schwarzschild was the first to realize that Einstein’s equations of general relativity
predicted the existence of black holes. More specifically, his black
hole contained a singularity at its centre; a point of infinite density
where time itself came to an end. At the singularity, all the known
laws of physics break down. This troubled Einstein. He didn’t
like these holes in spacetime and it was not enough for him that
they were shielded from the outside world by event horizons. For
him it was not simply a case of ‘out of sight, out of mind’.
In 1935, Einstein published a paper with his collaborator Nathen
Rosen in which they attempted to prove that Schwarzschild
singularities did not exist. By using a mathematical trick known
as a co-ordinate transformation, they were able to rewrite
Schwarzschild’s mathematical solution so that it did not contain
a point where space and time stopped. The alternative, however,
was just as strange. They showed that the singularity became a
bridge connecting our Universe with . . . a parallel universe! This
is not the sort of parallel universe that would have split off from
ours as a result of quantum mechanics as I described in the last
chapter. This link between the two universes became known as
the Einstein–Rosen bridge. It was, for Einstein, a purely theoretical
exercise in geometry in which two spacetimes would be joined
together. He did not believe that such a bridge really existed, any
more than he believed singularities really existed. It was just an
oddity of the mathematics of general relativity.
Such bridges between different worlds were not new even
then. The mathematicians of the nineteenth century were very
keen on curved space and higher dimensions. In fact, exactly half
a century before Einstein published his work on general relativity,
an English mathematician by the name of Charles Dodgson wrote
a children’s book on the subject of higher dimensional geometry
and parallel universes. Under the pen name of Lewis Carroll, he
wrote Alice’s Adventures in Wonderland in 1865. We are all familiar
with the bit when Alice chases the white rabbit down an Einstein–
Rosen bridge into another universe. I believe it is referred to in
the book as a rabbit hole, but it means the same thing. The reason
such strange things could happen in Wonderland was because
the laws of physics were different in that universe. Of course Dodgson was unaware what sort of mechanism could cause such
a tunnel to join our world with another. Remember this was before
relativity, quantum mechanics and modern cosmology. The story
was based solely on geometrical ideas about how space could be
curved and how two spaces could link together in some higher
dimensional hyperspace. What Einstein showed fifty years later
was that such curvature of space occurs wherever there is a strong
enough concentration of mass (or energy since it is equivalent to
mass). His theory of gravity (general relativity that is) provided
the physical basis for such tunnels to other worlds even though
they were no more likely to exist in reality.
In one of Dodgson’s very last works, Sylvie and Bruno,
which was published in 1890, we nevertheless find that he (and
presumably therefore other mathematicians at that time) was also
thinking about shortcuts within the same universe. In that story,
Fairyland and Outland are a thousand miles apart but are linked by
a ‘Royal Road’ which could take you from one to the other almost
instantly. He also describes time travel, changing clock rates and
the reversal of time.
Back to the 1930s and the reason no one was too excited about
the Einstein–Rosen bridge was that, unlike the rabbit hole in Alice’s
Adventures in Wonderland, it could never be used as a practical
means of getting to another universe. One way to think about
how an Einstein–Rosen bridge could form would be to imagine
a singularity in our Universe attaching itself to a singularity in
the parallel universe. So could this be what would happen if we
were to fall into a black hole? Think of black holes as a bit like the
afterlife. Nobody really knows what awaits them when they die
and, in the same way, we cannot be sure what will happen to us
when we jump into a black hole until we actually do. Even then
we are unable to relay the news back to those waiting outside the
event horizon. As a scientist I would like to think that we know a
little bit more about black holes than the afterlife since at least the
former obey mathematical equations!
So what is wrong with the Einstein–Rosen bridge as a means
of getting across to another universe? Well, to begin with there is
the event horizon. Once you jump into a black hole you cannot come back out again. Of course, in order to come out the other
side, the black hole you jump into would need to be hooked up
to a white hole. Remember that this is the opposite of a black
hole from which matter will emerge rather than fall into. White
holes must therefore be surrounded by the opposite of an event
horizon, something known as an antihorizon, which would allow
one-way traffic out and never in. Unfortunately, antihorizons are
very unstable and get converted to normal horizons in a matter
of seconds after forming. So, having passed through the event
horizon of the original black hole you would find that there is a
second event horizon blocking your exit at the other end. Imagine
a prisoner in a locked cell who discovers a tunnel under his bed.
It leads underground for a few metres only to come out the other
end inside an adjacent locked cell.
The major problem with the Einstein–Rosen bridge is that the
whole thing is highly unstable. The connectionwouldonly survive
for a fraction of a second before pinching off. In fact, so short is the
lifetime of the bridge that not even light travels fast enough to get
through. So if you were ever to jump into a black hole in the hope
of getting across, you would always get caught in the singularity,
and having one’s body squeezed down to a size much smaller than
an atom is never very desirable.
All this is assuming you weren’t ripped apart by the tidal
forces of gravity before you reached the singularity. The black
hole would have to be a supermassive one for you to even survive
going through the horizon. All in all, Einstein–Rosen bridges could
never become a means for visiting a neighbouring universe, and
therefore remained just a theoretical curiosity for many years.