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[Phys-l] A theory of quantum gravity based on quantum computation


An except in case there is any interest in this topic. From another list.


In a message dated 4/9/2007 1:08:33 A.M. Eastern Daylight Time,
_999999PTUSNET.COM.AU_ (mailto:999999@OPTUSNET.COM.AU) writes:

From: "Bob Zannelli" <RBZannelli@AOL.COM>
99999@OPTUSNET.COM.AU writes:

From: "Bob Zannelli" <RBZannelli@AOL.COM>

But if Kiefer and others are correct it's the beginning of inflation
that gravity Decoheres and time emerges.

This makes little sense. Decoherence is a process in time: if time
is not defined, then one cannot progress from a coherent state to
one which has decohered--that possibility is an intrinsically temporal
concept. Decoherence might possibly be used to define an
arrow of time, but it cannot, logically, be used to define time
itself, emergent or not.


This is of course the problem of time in QG. I think the way to
view this in terms of an evolution in both time directions , a
superposition of time CPT inversed Universes with the
environment selecting one time directions. This process
somehow is responsible for the positivity requirement of the
Decoherence functional, why we don't see advanced waves.

In this sense , time doesn't exist for the superposition but
emerges due to the phase decoherence induced by the
environment.

This does not account for the 'emergence' of time. As I said,
decoherence is a dynamical process that takes place *in* time.
These dynamical processes might be used to define a physical
clock, but the parameter 't' must already be present in the
theory--one is formalizing an existent property, one is not
finding something that emerges from a situation in which
it did not previously exist.>.

))))))))))))))

BZ

Right. But the arrow of time which is defined as irreversibility is due to
Decoherence and this is the key. For pure quantum states , which is what the
Universe is viewed as a closed system dS/dt=0 so you have no time evolution.
It the measurement process which causes dS/dt>0 giving us time evolution. But
you are correct we never really lose time, , we just submerge it in a
superposition state. A while back you linked an excellent paper on this. In this
paper the authors pointed out that decoherence can proceeds in both time
directions. But we can only see one time direction with our frog eyes. But our bird
eyes surveying the whole scene sees a static state.

Let me give you a very simple example of what I am saying. Imagine time
represented by the spin of a photon. Here we can have two possible directions ,
like time which can of course also have two directions. Now take a linear
polarized photon. This photon is a superposition of left and right circular
polarization states. Of itself it has no intrinsic circular polarization. In a
sense it is spin less. But it does have a probability amplitude to exhibit
left or right circular polarization given a particular measurement. I am saying
that quantum gravity is like this simple analogy. The pure quantum state at
origin is a superposition of CPT inverses. It has no intrinsic time in the
same sense the linear polarized photon has no intrinsic circular polarization.

However, just as all photons have spin, ( we never really lose spin )
from the frog perspective we never really lose time. Only from the Bird
perspective do we lose time.

)))))))))))))))))




This is illustrated by the ABL formalism in "ordinary" QT. Of
course in the ABL formalism time doesn't emerge,

That is where the problem lies.

but the assumption would be that for the decoherence of
gravity it does.

How do you persuade me that this is anything more than just
wishful thinking?

))))))))))))))))

BZ

Bruce this a loaded question. All I can say is that I think this way of
looking at time and quantum gravity is suggested by the no boundary proposal and
the Wheeler De Witt equation which have no time parameter. Somehow time must
emerge from this quantum state assuming the correctness of this equation and
the HH model.

)))))))))))))))




Of course there is no such quantum gravity theory but the
various attempts to describe the Universe quantum
mechanically suggest that this may be true.

Claus Kiefer writes

" As is well known , one can recover from the Wheeler-Dewitt
equation in the semi classical limit a functional Schrodinger
equation for non gravitational degrees of freedom. The
corresponding time parameter is defined through the slowly
evolving gravitational variables. ( in the semi classical regime,
my note) , typically the expansion of the Universe.

He is describing a clock to measure the parameter 't', he
is not describing the emergence of 't' from 'non-t'.

))))))))))))))))))

BZ

Yes I think this is what I am saying too. Just like I would be saying for
the linear polarized photon state above we need a measurement process to
observe circular polarization.

)))))))))))))))))



An important ingredient is decoherence of relevant variables
(such as the volume of the Universe) by irrelevant variables
(such as small density fluctuations) otherwise one would
encounter superpositions of macroscopically different Universes.
Decoherence starts with inflation, before inflation, the Universe
is timeless and there is no classical evolution. In fact due to the
unavoidable quantum entanglement between matter and
gravity, mutual decoherence arises.


This does not solve the problems discussed above.

))))))))))

BZ

Well consider this in light of my photon example.



))))))))))




" Quantum Cosmology and the Arrow of Time" Brazilian Journal
of Physics.



Another way of putting this is that decoherence is a dynamical
process, involving interactions according to some theory. Any
theory in which this can be described must already involve a
well-defined time variable. Emergentist theories of time have
a real problem to overcome.


I agree. How to have a dynamic process generate the emergence
of time is almost a Zen Koan. But what I am saying is that the
solution may be to realize that the static structure of the Universe
at origin, its lack of apparent time evolution is due to the
superposition of CPT reversed Universes, a superposition
which undergo decoherence as gravity becomes classical.

But you are still talking about 'becomes classical' as part of
your description of a timeless state. This does not make sense.
You really do assume that time is actually there all the time (as
it were), and you are merely uncovering this as the dynamical
evolution unfolds. This is not an emergentist theory of time.

)))))))))))))))

BZ

Yes I do, in the same sense that circular polarization is always present in
the linear polarized photon state.

))))))))))))))



The ontological status of the CPT reversed Universe is the
same as the path of an electron through a double slit
experiment in hole A when the electron has been
measured going through hole B.

If the electron has been measured going through hole B,
you can be 100% sure that it did not go through hole A,
so that path has no reality. Neither does your CPT-reversed
universe. This whole notion of timelessness as the superposition
of time reversed states is just confused.

))))))))))))))))

BZ

This is exactly why I used this example. My point is that arguing about the
reality of the time reversed Universe is like arguing about the existence of a
Universe where the electron went though hole A with regard to the
experiment that measured which hole the electron went through in my example.


Bob Zannelli








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