Freethought & Rationalism Archive

Jesse · 01-22-2003, 10:26 PM

Well, here are some comments by John Baez from sci.physics.research which suggest that our current version of quantum field theory does indeed forbid sending information faster-than-light through quantum teleportation:

Quote:

In article <remove.haberg-1108000008330001@du136-226.ppp.su-anst.tninet.se>,
Hans Aberg <remove.haberg@matematik.su.se> wrote:

>>Aaron Bergman wrote:
...
>>>No information is transmitted FTL in quantum teleportation. Isn't
>>>there some proof out there that no information can be sent FTL in
>>>Quantum Mechanics?
>>
>>Not in quantum mechanics as such --
>>just take a nonrelativistic example to see this.
>>You want to say there's been such a proof for quantum field theory.
>>The problem with this, of course,
>>is that QFT is so wishy washy mathematically.
>>You can take proposed axioms for QFT
>>and then, yes, we have such a theorem.
>>This puts your theorem on a par with
>>PCT, spin and statistics, and all that.
>>Whether any specific field theories obey it is another question,
>>since nobody has proved any of the realistic ones satisfy the axioms.
>>But that's the way things are in QFT, I guess.
>
>This is the point I wanted to bring out: If there was an animal such a
>relativistic Schrodinger equation, then one could use that to decide such
>things whether information can travel FLT, in teleportation or otherwise.
>
>But in the absence of such a relativistic Schrodinger equation, one only
>has QFT, and they do not have the capacity of deciding that, for the
>reasons indicated above.

There actually is a relativistic Schrodinger equation: it's just

i d psi / dt = H psi

where

H = sqrt(p^2 + m^2)

This equation is equivalent to the real Klein-Gordon equation when you do
things right. (For a long time people were confused about this, because
they were doing things wrong. Lots of people are still confused about it.)

And this equation prohibits faster-than-light transfer of information.

from http://www.lns.cornell.edu/spr/2000-08/msg0027320.html
in respose to http://www.lns.cornell.edu/spr/2000-08/msg0027299.html

and:

Quote:

In article <remove.haberg-0308002049020001@du136-226.ppp.su-anst.tninet.se>,
Hans Aberg <remove.haberg@matematik.su.se> wrote:

>In article <slrn8o7i5q.f9k.abergman@tree1.Stanford.EDU>,
>abergman@princeton.edu wrote:

>>No information is transmitted FTL in quantum teleportation. Isn't
>>there some proof out there that no information can be sent FTL in
>>Quantum Mechanics?

Not in Quantum Mechanics per se, but certainly in Relativistic
Quantum Mechanics, and also Relativistic Quantum Field Theory.

In fact, causality is built right into the axioms of relativistic
quantum field theory, as I explained quite recently on this newsgroup.

The only catch, as Toby Bartels notes, is that it's usually quite
hard to construct a quantum field theory with sufficient rigor to
show that it satisfies all the axioms! However, this has been done
for a large class of quantum field theories in 2- and 3-dimensional
spacetime. There is currently a $1,000,000 prize awaiting anyone
who can do it for Yang-Mills theory in 4-dimensional spacetime.

>Theorems that specify that no information can be transported faster than
>light I think assume that information must be carried via energy.

The causality axioms do NOT rely on the assumption that information
must be carried by energy. They simply say [in a mathematical way]
that no difference in the situation here and now can cause a difference
in the situation there and then if one has to go faster than light to
get from the point "here and now" to the point "there and then".

from http://www.lns.cornell.edu/spr/2000-08/msg0027237.html
in response to http://www.lns.cornell.edu/spr/2000-08/msg0027129.html

The entire thread can be found here:

http://www.lns.cornell.edu/spr/2000-...s.html#0027299

(Incidentally, the thread also features a discussion of why experiments like the one in the first post on this thread do not violate relativity and do not allow information to be transmitted faster-than-light; see this post by Greg Egan, for example.)

Friar Bellows · 01-22-2003, 11:43 PM

Quote:

Originally posted by cfgauss
Friar Bellows:
This explains it pretty well.
http://einstein.stanford.edu/gen_int...ty/a10854.html

Yes, I get it now, thanks.

cfgauss · 01-23-2003, 12:30 PM

Coolness. You should also check out http://www.desy.de/pub/www/projects/Physics/, it's got some pretty good stuff on cosmology (but all the stuff there is great), IIRC.

Answerer · 01-24-2003, 12:47 AM

Quote:

Originally posted by cfgauss
"Quantum entanglement could just be another type of superluminal influence like quantum randomness"
Uhm, there's no superluminal anything in QM!
http://www.desy.de/pub/www/projects/...nequality.html

Come on cfgauss, its either QM talks about non-locality or Einstein is right and QM is very wrong(which is apparently not the case).

"

Quote:

Originally posted by cfgauss
"Anyway, Cf is a hard-core defender of the speed of light, you will have great problems convincing him otherwise."
Yes, I am a physicist!

Yeah, so is some of us here.

Answerer · 01-24-2003, 02:07 AM

Well Jesse, again, I'm only refering to the quantum channel of the quantum entanglement. Hope you understand.

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01-23-2003, 12:30 PM	#33
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	Coolness. You should also check out http://www.desy.de/pub/www/projects/Physics/, it's got some pretty good stuff on cosmology (but all the stuff there is great), IIRC.

01-24-2003, 02:07 AM	#35
Answerer Veteran Member Join Date: Feb 2002 Location: Singapore Posts: 3,956	Well Jesse, again, I'm only refering to the quantum channel of the quantum entanglement. Hope you understand.

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