Insurmountable problems?

There have been a number of objections to the wormhole time
machine plan. All have been based on serious calculations which
have shown one or other of the steps to be an obstacle thatwecould
never overcome. The most serious of these has been that, even if a
traversable wormhole could be built, and a time shift induced, the
last step of bringing the two mouths close together—which you
might have thought would be the easiest—would in fact cause the
wormhole to be destroyed. It is expected—though no one is sure
yet—that as soon as the wormhole becomes a time machine, light
which has travelled through it will be able to get back, through
normal space, to the mouth it entered before it entered. It will then
be able to enter along with its original version, thus doubling up
its energy. But if twice as much radiation goes through and can get
back to the entrance before it enters then it should be four times as
intense, and so on. In fact, calculations show that the instant the
two mouths (with their different time frames) are brought close
enough together for a little of the light leaving the exit mouth to
get back and go through the entrance mouth before it went in, an
infinite amount of light will instantly have built up by flowing
through the wormhole and will either collapse the throat of the
wormhole or cause its two mouths to explode in a burst of energy.
This light (or electromagnetic radiation) will always be a problem
as it is produced by the vacuum itself, and is therefore referred to
as vacuum fluctuations.is shown in figure 9.6 might work.
Stephen Hawking is convinced that nature forbids time loops
and time travel. He has come up with what is known as the
chronology protection conjecture which states simply that time
travel into the past should not be allowed in physics. This has
yet to be proved mathematically but so far has equally not been disproved. If the chronology protection conjecture does turn out to
be a law of nature then time travelwouldbe ruled out for good. The
Roman configuration of wormholes (whether with two or many)
also appears to have problems, with the mathematics not giving a
definitive, or even sensible, answer.
Latest results indicate that an even earlier stumbling block is
the exotic matter needed to keep a traversable wormhole open in
the first place. Anumberof researchers claim that their calculations
rule out the possibility of ever getting enough exotic matter to hold
open any wormhole bigger than a quantum one.
So what arewe to believe? Can wormholes ever be built? Can
they form time machines? Can time loops be formed at all in the
Universe? You will have gathered from everything I have said in
the last few chapters that the jury is still out on all these questions.
We simply do not know for sure yet. In fact, apart fromthe obvious
problems of how to keep a wormhole open and how to transport
it, there is a more basic theoretical stumbling block.
Quantum mechanics describes the behaviour of the world of
the very small, whereas most phenomena that are described within
the framework of general relativity tend to involve immense
expanses, leviathan masses and titanic forces. Stars, black
holes, galaxies, even the whole Universe, all rely upon Einstein’s
description of gravity and how it affects space and time. They
are far removed from the microscopic quantum world. There
are, however, a number of processes, such as Hawking radiation
from the surface of black holes, which can only be understood if
quantum mechanics is incorporated in the explanation. But such
a successful use of both general relativity and quantum mechanics
to explain the same phenomenon is rare. It is only achieved by
artificially grafting the quantum rules on top of general relativity
in an approximate way. The bottom line is that general relativity
and quantum mechanics are incompatible. A symbiosis between
these two successful descriptions of reality will only be achieved if
they can be merged into one unified scheme; an all-encompassing
theory of quantum gravity.
Until we find such a theory we will not be able to definitively
answer the question of whether or not Hawking’s chronology protection conjecture is really a law of nature, thus forbidding time
travel. I will end this chapter with a quote from Frank Tipler, the
physicist who published the first serious paper on how to build a
time machine. Three years after that work, in 1977, he published a
longer article in which he examined more carefully the likelihood
of his rotating cylinder time machine ever being realized. He
ended the article by borrowing a quote fromthe astronomer Simon
Newcomb who had written a number of papers at the turn of the
century maintaining the impossibility of heavier-than-air flying
machines. Tipler felt that it applied equally well to time machines:
“The demonstration that no possible combination of known
substances, known forms of machinery, and known forms of force
can be united in a practicable machine by which men shall [travel
back in time], seems to the writer as complete as it is possible for the
demonstration of any physical fact to be.”
I don’t need to remind you how Orville and Wilbur Wright
proved Newcomb wrong about heavier-than-air flying machines
just a few years afterwards.
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