Is Universe Unitary or Non-Unitary at the Planck Scale?

[marcus]

Over in sci.physics.research there's currently a discussion about
"why are QM and GR incompatible?"

well that's a big question these days----how should gravity (that is General Relativity) be quantized? why has it proven so difficult for the past 70 years when everything else has been successfully quantized? what changes have to be made in Quantum Theory rituals and/or General Rel in order to make them compatible?

So lots of people spoke up and you can go read the thread if you want. But there was one part of one person's post that I want to quote. He points out a big issue between QM and GR and a big split in how people think gravity should be quantized.

Is or isn't time-evolution working by unitary operators? Is time-evolution unitary or non-unitary? Here is part of Peter Shor's recent post

-----exerpt from Peter Shor post on SPR----


...Another fundamental difficulty of unifying quantum mechanics and gravity
is that quantum mechanics has as one of its most fundamental assumptions
that the universe is unitary, so no information is ever fundamentally lost.
General relativity seems to say that when you toss something in a black
hole, the only information about that something that survives is its
mass, its charge, and its angular momentum (and of course, any classical
records that mention it). And it says this pretty convincingly, so,
barring Hawking's lecture next week, nobody has come up with a convincing
mechanism for getting information out of a black hole, a necessary
prerequisite for reconciling GR with QM.

String theorists are absolutely convinced that the universe is unitary,
but none of them has been able to convince me that it's impossible for
the universe to be non-unitary at the Planck scale and still look very,
very close to unitary at experimental scales. A couple of them have
tried to, but these attempts generally involve a lot of waving of hands
and words like "in the generic case," and arguments along the same lines
would seem to imply that quantum error correction is impossible, and
that's something I know is wrong.

I'll be very interested to hear reports of Hawking's lecture next week.


Peter Shor
-------end quote----
https://www.physicsforums.com/showthread.php?p=258460#post258460

 

[Brad_Ad23]

I concur, Dr. Hawking's presentation will hopefully be what it seems to be, and not something less.

On the issue of unity vs. non-unity, do you even think we will be able to experimentally verify such things? I'm inclined to say probably not, but I could be wrong.

 

[marcus]

for me there is a special interest in this because Jorge Pullin and friends have just given what I think is a clear elegant argument that realistic clock-time evolution MUST have a very tiny non-unitariness.
that is, the fundamental unitariness assumption of QM is unrealistic on QM's own terms

they have shown that unless you believe in God the basic assumption of QM is inconsistent with QM.

because unless there is a deterministic classical ideal unquantized God sitting on His throne out there with a classical ideal Rolex on His wrist, and that is absurd, unless there is that perfect Rolex somewhere the only time you can talk about is the time told by real observable clocks

and Jorge Pullin and friends have demonstrated elegantly that with the best possible clocks, which happen to be the ringing vibrations of black holes (beautifully enough using Wigner argument and thought experiment there is no possible better clock)

that using the best possible clocks, there is a very very tiny non-unitariness in time evolution of anything

and the slow very slow loss of information that this entails is just enough to dispose of the celebrated "Black Hole Information Paradox"
because the information that fell into the hole and was lost by the time the hole evaporates would have died anyway

 

[marcus]

In a way Peter Shor's mention of Hawking at the end of his post is an (unintended) red herring.

Hawking is not the point of his post. the main point is here:

"...String theorists are absolutely convinced that the universe is unitary,
but none of them has been able to convince me that it's impossible for
the universe to be non-unitary at the Planck scale and still look very,
very close to unitary at experimental scales..."

Probably without knowing anything about the Pullin et al papers, Peter Shor is saying the same thing as they do. Here are links to Pullin papers

http://arxiv.org/hep-th/0406260
"Realistic clocks, universal decoherence and the black hole information paradox"

http://arxiv.org/hep-th/0405183
"No black hole information puzzle in a relational world"

 

[marcus]

What has emerged, I guess you could say, is a theoretical inconsistency in Quantum Theory.

evolution in quantum theory cannot be according to a classical time variable T because such a variable would not be quantum

when the classical ideal-time variable T is replaced by a realistic quantum clock-time, with the best conceivable clock one can make with real matter, then a gradual non-unitariness or "decoherence" creeps in
a leaking or seeping of pure to mixed states at a certain rate which they are able to calculate.
a kind of drooling dribbling seepage-----and it is the arrow of time.

So it is turning around our understanding of the BH info paradox
Instead of saying "What is wrong with Black Holes?"
we can say "What is wrong with quantum theory's time-evolution?"

People (like for example recently Stephen Hawking but also many others) have been addressing the paradox and saying "What can be wrong with our understanding of black holes? How can we fix our model of black holes so they will not make information slowly evaporate to nothing?"

But now other people (like for example Peter Shor and also Jorge Pullin et al) are saying "Why couldn't there be a very slow fading out of information anyway? at a rate too slow to have been noticed." And this could be a theoretical necessity at a fundamental quantum level, just to get QM right.

But one can suspect that getting QM right would also help make QM and GR compatible.

I hope I am correctly understanding the gist of Peter Shor's post and of those research papers. If I understand this right it is likely to be a violent bone of contention involving the Quantum Gravity enterprise. there will be those who think that QM should retain its Golden Rolex platonic ideal Time. And there will be those who want God to wear a watch made of real matter---something much less expensive I would guess.

 

[kurious]

How does anyone know a black hole is a pure state in the first place?
There is no experimental evidence for this.The information loss problem assumes that information only escapes in Hawking radiation.But gravitational force carriers such as gravitons might carry information out of the hole - there would only need to be one graviton per hole particle.

 

[jeff]

...how should gravity (that is General Relativity) be quantized? why has it proven so difficult for the past 70 years when everything else has been successfully quantized?

It's been so difficult because we we're trying to get a sensible quantum gravity theory by quantizing GR directly without realizing that this approach can never work.

 

[marcus]

How does anyone know a black hole is a pure state in the first place?
.

I don't think that is part of it, kurious.

one does not have to assume that in order to state the paradox.

I never heard anyone say anything about the black hole in the paradox being in a pure state. not that I can remember. Maybe this is a misunderstanding on my part and somebody else can clear it up.



bTW one thing I really like about the two Jorge Pullin papers is that the black hole of the paradox is extremely secondary.

What they focus on is the CLOCK not the black hole.
It can be a clock that is being used to time anything you want. it does not have to be timing the evaporation of a black hole into which some pure-state information has been dropped.
It can be timing anything.

they are asking basic questions about a real made-of-matter clock.
How accurate can it be?
How long can it run for?

I would be delighted if you have a look at the two papers. they are short and I think you wouldn't regret seeing the different approach they take

 

[Jeebus]

It's been so difficult because we we're trying to get a sensible quantum gravity theory by quantizing GR directly without realizing that this approach can never work.

What approach might work, if one?

 

[marcus]

Jeff:
"It's been so difficult because we we're trying to get a sensible quantum gravity theory by quantizing GR directly without realizing that this approach can never work."


I believe Jeff is expressing the standard String theory view
which is the majority view (among high energy physicists)

if QM and GR are incompatible it must be the fault of GR
so change GR
(dont try to directly quantize it)

quantize something else of which GR is only an approximation
so you get a quantum theory of something "more fundamental"
than GR

My guess is that it is this majority view which is being expressed
so I want to say yes it is the prevailing or at least a very common one.
(which does not mean that I concur)

------
one small trouble with what jeff says is the word "we"
"we" here means a minority of theoretical physicists (LQG, spinfoams, simplex gravity, path integral) who actually are trying to directly quantize GR.

string theory does not try to quantize GR directly

so a way to expand on what he is saying, if I understand, is more like this:
What WE (string theorists) have been doing for 20 or 25 years is the right approach and the approach which will eventually succeed. What WE do is not quantize GR directly but something else
What THEY (some Loop and other recently emerged theorists) are trying is doomed to failure because it is trying to quantize GR directly and WE know that is impossible.

it is a legitimate viewpoint, and indeed a widespread one, but
the pronouns could be a little more exact.

 

[jeff]

...[jeff's use of] pronouns could be a little more exact.

That hurt. ()

Seriously though, despite my ad nauseum bashing of nonstringy theories, I do sympathize with guys like peter woit in the sense that if I could ban all nonstringy research, I certainly wouldn't do it. But let's just keep this between us PF members, so don't you guys go telling anyone, okay?

 

[marcus]

That hurt. ()

Seriously though, despite my ad nauseum bashing of nonstringy theories, I do sympathize with guys like peter woit in the sense that if I could ban all nonstringy research, I certainly wouldn't do it. But let's just keep this between us PF members, so don't you guys go telling anyone, okay?

YOUR SECRET IS SAFE WITH ME, jeff