X-Message-Number: 16299
Date: Sat, 19 May 2001 04:41:29 EDT
Subject: Re: CryoNet #16293 time machine

John de Rivaz said:

> Suppose instead of cryonics you could simply put ill people into a physical
> device that sends them to the future? Many people would say that that is
> just as (un)likely to work.
> But an article in this weeks New Scientist
>  http://www.newscientist.com/features/features.jsp?id=ns22911
> points at work that may be worth following - and it still does involve
> extreme cold.
> It is much better than previous proposals for time machines - no rotating
> black holes or infinitely long cylinders. Important features are ...

I am cautious about time machine because of a
thermodynamical problem: If you go back in time,
you can decrease the entropy in a closed system, that
is, you can cheat with the thermodynamics second
law and build a so called a perpetual motion
machine of the second kind.

If you think of the time axis as a symetry, you may
figure it out as a group symetry. Such groups may have
a local representation or a global one. For example the
U(1) group when local defines the electromagnetism
and when global the Special theory of Relativity.
Time travel is a global problem and light is a
product of the electromagnetic theory, that is a local
"object". There is no way to get a global effect with
a local symetry. On the other hand, you can pick up
a local effec and look at the corresponding one
in a global group representation, this is what
Einstein did when starting from the Maxwell
symetry laws he built the Special Relativity.

If a rotating light beam displays a time travel-
like behavior, then we must look at the corresponding 
global sysmetry to have a real time machine.

Unfortunately, it seems that the global system is
a rotating black hole, a so called Reisner-Nordstrom
object, not something you can put in a laboratory.
This is not a chance effect that you collide with
black hole when looking at time travel. As I have
said before, time travel has a big problem with
the thermodynamics second law. To overcome
this,you have to reverse entropy on a global scale
 and to do that, you must reverse it at black hole
surface. Horizon black holes contain more than
99.999999999999999... percent of the global
entropy and even the number above is a gross
understatement of the true part.

To have an idea of the problem, think about a 
train moving at full speed and you try to reverse it
using the air drag produced by a paper fan.

The only hope in that domain would be in the
Black Magic domain: BM may produce a linearized
quantum  field and that object works as a gravitational
field up to a BH horizon. See for example:
gr-qc/0104001 (Analog gravity from linearized field
theories) on the ArXiv server at: http://xxx.lanl.gov

The best you can hope in the science domain, is a
simulation of global symetry with an entangled
photon pair.

At time t=0 you produce an entangled photon pair.
You split photon a) along two channels, one ending
with a left polarized befor a detector, the other with
a right one. The photon b) is sent to a far away space craft
or a slow light channel in the laboratory.
At time t=1 you capture it in a detector with a L polarizer
for example.
The initial photon pair was not polarized so, if b) is forced
in the L state, a) must be in the R state and will be
received in the R channel.
So, depending on the choice made at t=1 on photon b)
the photon a) will it the Ror L detector at time t=0.
Because t=0 is before t=1, you have sent an information
back in time. The effect was noted first by John A.
Weehler many years ago.

The paradox comes from the neglect of relativistic effects:
Photons move at the limit speed of the universe an for them
time is infinitely expanded. Emission, travel and reception
all take place in a single instant. For a photon pair, there
is one instant for what we look as two objects. In fact,
there is one object, the photon pair, with two components.
The fact that we detect separately each component at
a different epoch in our time frame do nothing for the
photons' time.

Yvan Bozzonetti.

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