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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
"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
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