File name:BLACKHOL.TXT Firda BBS
Last revision: 07.02.95 blt9c15
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T I M E A F T E R T I M E
(Once more the black-hole time machine)
The journal Physical Review Letters is not generally considered
light reading, but a recent paper by Michael S.Morris, Kip S.
Thorne and Ulvi Yurtsever of the California Institute of
Technology seems designed to produce a few smiles. The authors
propose to use a black hole as a time machine.
The idea is actually not new. Since the early decades of this
century it has been known that a black hole is one end of a
"wormhole" that connects two different regions of space-time.
Unfortunately such passageways do not make a useful rapid-transit
system. In a non-rotating black hole the wormhole contains a
singularity, or point of infinite gravitational and tidal forces,
that destroys any would-be commuter before he or she gets through
the turnstile.
In both rotating and electrically charged black holes the
wormhole does not encounter a singularity. This fact led to much
speculation in the 1960's that a cosmonaut might travel through
the black hole and emerge in a different region of spacetime.
Within a decade, however, investigators had shown that the
wormhole in a charged or rotating black hole is unstable; any
attempt to traverse or send a signal through it causes the
passage to collapse into a singularity with the above-mentioned
results.
The Caltech report is a clever attempt to solve the problem
for would-be spacetime travelers. The authors suggest some
advanced civilization might first "plausibly" extract a wormhole
from quantum foam -a state of spacetime that might exist where
the curvature is so high that relativity becomes wedded to
quantum mechanics and wormholes are created. Such regions would
have dimensions of about 10 to minus the 33'd power of a
centimeter. Once having extracted the wormhole by techniques
still to be developed, the civilization would enlarge it to a
size suitable for transportation of people.
The key to the Caltech proposal is to place one perfectly
conducting charged electrical plate on each side of the wormhole
throat. By a phenomenon known as the Casimir effect, the plates
cause a violation of the "weak energy condition." All normal
matter obeys the weak energy condition which can be thought of
as the requirement that the average energy density of the
material be non-negative -that is, zero or positive. It is the
weak energy condition that causes singularities to form in black
holes. if it is violated, a singularity need not be inevitable.
Once the wormhole is in place, it can then be converted into a
time machine as follows: one end is accelerated away from the
other "gravitationally or electrically" to nearly the speed of
light and brought back again. According to relativity, the moving
mouth "ages" less than the stationary mouth; it is easy to show
that a voyager traversing the wormhole from the moving end to the
stationary end will consequently travel back in time.
The Caltech time machine apparently has an advantage over its
predecessors in that the wormhole seems to be stable.
Nevertheless, the authors seem reluctant to state this
categorically. Moreover, in order for the Casimir effect to work,
the plates might have to be placed closer together than the
radius of an electron, and this could be forbidden. In any case,
it will be a few years before the machine appears in the
Hammacher Schlemmer catalogue.
-Tony Rothman. (Scientific American, January 1989, p 14)
MORE ABOUT WORMHOLE VOYAGES:
============================
Some theorists among the astrophysicists and quantum
physicists continue to play with the idea of making time
voyages through black holes.
Certainly most scientists consider this train of thought little more
than a matemathical pastime, - perhaps even a laughing matter, and one
that is exceedingly unlikely to ever become of any practical use.
Still, since even the slightest such theoretical possibility is of
considerable philosophical interest, and since the idea has been used so
much in Sci.Fi. literature, I shall continue to add short reviews to
this file.
The current text was cut from New Scientist, vol 131, issue 1785, p 27.
Ottar Sande, SysOp Firda BBS
WORMHOLES OPEN A GATEWAY TO OTHER WORLDS
========================================
Black holes might really provide a gateway to other
universes through tunnels in space-time, according to Amos Ori
of the California Institute of Technology. He says that
quantum effects may make it possible to "tunnel" through the
surface surrounding a black hole and emerge into another
region of space and time.
Physicists have known for more than 50 years that the
mathematical description of a black hole in ordinary "flat"
space actually portrays the hole as a kind of a tunnel, or
bridge, connecting two separate regions of flat space.
Einstein carried out some of the pioneering calculations along
these lines.
For many years, theorists regarded these space-time tunnels
as simply a quirk of the mathematics, with no physical
significance, rather like the imaginary root of a quadratic
equation.
Recently, however, several theorists have become intrigued
by the possibility that such tunnels through the space-time
might exist. On a sub-microscopic scale, such "wormholes"
could explain why the constants of nature have the values they
have. Some theorists have even suggested that "macroscopic"
wormholes, big enough to travel through, are allowed by
physics. (See "Wormholes, time travel and quantum gravity,"
New Scientist, 28 April 1990).
The snag with these speculations is the so-called blue
sheet problem. The boundary , or horizon, of a black hole is a
surface where the pull of gravity is so strong that nothing,
not even light, can escape. One way of interpreting this is to
say that light leaving the horizon is infinitely "red
shifted", to zero energy. But that means that light falling
*on* to the horizon from outside must be infinitely *blue*
shifted, piling up to form an energetic *blue sheet* around
the hole.
Theorists used to think that the blue sheet would be an
impenetrable barrier. But Ori has shown that extended objects
that approach this barrier need not necessarily be destroyed
by it (Physical Review Letters, vol 67, p 789).
The key question is whether tidal forces are strong enough
to tear apart an object near the critical horizon. According
to Ori's calculations, they are not. The horizon does provide
a barrier between two regions of flat space-time, yet it can
be approached as close as you like.
This raises the possibility of crossing the barrier by
another kind of tunnelling - quantum tunnelling. Quantum
theory allows a particle, such as an electron, to tunnel
through an energy barrier. On one side of the barrier, says
Ori, we have an ordinary electron, more or less describable in
terms of classical physics. on the other side of the barrier
we also have a classical electron. But the electron tunnels
through the barrier by behaving as a quantum wave. This is the
process that explains, for example, how alpha particles escape
from atomic nuclei.
If and when we have a satisfactory quantum theory of
gravity, says Ori, it may show us how material objects can
penetrate the black hole barrier in similar fashion, entering
into the other region of space-time beyond the black hole. It
may be that "the black hole is a tunnel to other universes",
he says.
In 1985, Kip Thorne, Michael Morris and Ulvy Yurtsever at
the California Institute of Technology in Pasadena
investigated the possibility of interstellar space-travel
through wormholes. However, they concluded, after solving
Einstein's field equations of gravity, that any traversible
holes would have to be made of "exotic" matter which exhibited
some pretty peculiar properties, such as "negative pressure".
Certainly, physicists were not aware of the existence of such
matter. But it was not clear to Thorne and his colleagues
whether their calculations ruled out the existence of
wormholes in the universe or not.
(New Scientist, Sept. 7 1991, p.27)
****************
IS "POSSIBLE" THE SAME AS "TRUE"?
So, according to this article, purely mathemathical logic
does not *necessarily* rule out the the possibility that
wormholes may exist, and maybe further mathematical logic does
not absolutely forbid a particle like an electron from
tunnelling through such a wormhole, - if one should happen to
exist.
Whether everything that is not contrary to the logic of matematicians
*must* exist is another matter altogether. Some physicists appear to
think that possible in these contexts equals true, but they also admit
that it is a philosophical view rather than a physical necessity.
Ottar Sande
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News about time travel theory, December 1993:
-----------------------------------------------
Most physicists consider time travel a dubious notion at best, but one
small group, including Stephen Hawking of the university of Cambridge,
has always claimed that it is impossible in principle. Now Amos Ori of
the Technion-Israel Institute of Technology in Haifa has found a flaw in
Hawking's argument. He maintains that the possibility of time travel is
not ruled out by the laws of physics.
This is the latest twist in a story that began in the late 1980s, when
Kip Thorne and colleagues at the California Institute of Technology in
Pasadena suggested that although construction of a time machine might
pose considerable practical difficulties, there is nothing in the laws
of physics as understood at present to make such a device an
impossibility.
Other researchers tried to find flaws in Thorne's argument and, in
particular, pointed to problems in satisfying a requirement known as
the "weak energy condition". This condition maintains that any real
observer can only measure positive rather than negative energy, ruling
out the types of theoretical time machines that involve travelling
through black holes held open by exotic material with negative energy.
There are also problems with time machines that involve so-called
singularities, points where space and time are crushed out of existence
and the laws of physics break down. But within the framework of
Einstein's general thery of relativity, Ori has found mathematical
descriptions of space-time which loop back on themselves in time, but in
which no singularity appears early enough to interfere with time travel
and the weak energy condition is satisfied. "At present, one should not
completely rule out the possibility of constructing a time machine from
materials with positive energy densities," says Ori.
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February 1995
Time travel: it's all done with smoke and mirrors
TIME-travel enthusiasts can breathe more easily. A Chinese physicist has
thought of a way to resolve a serious flaw in the design of the most
popular type of proposed time machine.
The kind of machine involved is a "wormhole" through space-time,
invented by Kip Thorne and colleagues of California Institute of
Technology in the late 1980s. This consists of a pair of connected black
holes, creating a tunnel held open by some form of exotic matter. The
tunnel's "mouths" could, in theory, open in different times.
Physicists have argued, however, that such time machines would
eventually be destroyed by a catastrophic build-up of energy caused by
radiation looping around within the wormhole, doubling its strength with
each journey through time.
Now, Li-Xin Li of the Chinese Centre of Advanced Science and
Technology in Beijing has calculated that it is possible to avoid this
energy build-up using nothing more sophisticated than a mirror.
Many scientists are horrified by the implications of time travel -
not least the paradoxical possibility of going back in time and
inadvertantly causing the death of your parents before you were
conceived. Some physicists have even proposed a law of nature to forbi
time travel, dubbed "the chronology protection conjecture".
And Stephen Hawking of Cambridge University has come up with a
practical explanation of why a wormhole time machine cannot work.
Radiation entering one mouth and travelling down the tunnel will not
escape harmlessly from the other end, but will in effect be "copied"
into the other time by converting some of the wormhole's own mass into
energy. Over time, radiation looping around inside the wormhole will
keep doubling its strength, consuming the wormhole until it "boils"
away.
Li's solution to this problem is disarmingly simple, but is backed up
by calculations based on Einstein's general theory of relativity. A
working time machine might require two wormhole mouths each about 10
kilometres in diameter. All you need, says Li, is a perfectly reflecting
sphere, about the same size as each mouth, between the two and exactly
on a line joining their centres. The wormhole mouths would then be moved
close to, but not touching, the mirror. (Physical Review D, vol 50 p
R6037).
Any radiation leaking from the wormhole mouth into the tunnel itself,
says Li, will be reflected away harmlessly into the universe at large.
The mirror would have to be perfect, reflecting not only everyday
radiation, but more exotic forms such as gravitational waves - ripples
in the fabric of spacetime caused by the acceleration of large masses.
But any civilisation advanced enough to build the wormhole in the first
place, Li argues, ought to be able to build such a mirror.
Whatever the practicalities, however, the fact remains that there are
situations, described by Einstein's equations, under which stable time
machines can exist. It seems that physicians who hate the idea are just
going to have to learn to live with it.
John Cribbin
(New Scientist, 4 February 1995, p 14)
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-Kip Thorne, Michael Morris and Ulvy Yurtsever, 1985,
California Inst. Tech, Pasadena.
-Li-Xin Li of the Chinese Centre of Advanced Science
and Techn. Beijing.
-Physical Review Letters, vol 67, p 789
-Physical Review Letters, vol 71, p 2517
-New Scientist, 28 apr. 1990
-New Scientist, Sept. 7 1991, p.27
-New Scientist 25 Dec 1993 / 1 Jan 1994, p 14.
-New Scientist, 4 February 1995, p 14
-Scientific American, January 1989, p 14
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