Freethought & Rationalism Archive

cfgauss · 01-21-2003, 05:14 PM

Jesse:

There's no precedent for negative energy. Also, we have to be careful about what negative energy is. For example, on the surface of the Earth an object has zero gravitational potential energy; I take it into a hole one meter below the ground and all of the sudden it's gravitational potential energy is negative! Of course, this is simply because of our definition of position on a coordinate axis!

And IIRC, you need negative energy, imaginary mass, and(?) exotic matter to have a wormhole. They're, as far as we can see, entirely unstable. Even if we had these things, it would probably collapse by the disruptions caused by anything entering it. Also, even if you didn't, you'd still need "imaginary time" to exist, because that's what the wormhole is "in"!

These things are logically inconsistent! They aren't defined! They have no meaning!

Any kind of preferred frame would have drastic consequences for SR and GR. They would most likely not work in the exotic conditions in which they are shown to work. And I know of no theory that postulates a preferred frame whose results are exactly like SR and GR except in a few *highly* exotic situations, and I seriously doubt that one exists!
b
And, looking at that website (gmu), neutrinos have been shown *not* to be tachyons! There's also rather little content there. 20 Helens agree that there's no such thing as tachyons.

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

Dark Jedi · 01-21-2003, 06:11 PM

Quote:

Originally posted by Undercurrent
If you're imagining a discrete particle taking a sinusoidal path in space, then that's not right.

A better, but still flawed, analogy would be to imagine a particle moving in a straight line, but with sinusoidally varying colour while it moves along. The dimension(s) in which photons oscillate are not spacial, therefore there is no "longer path" that the photon is actually taking, and hence no breaking of the light barrier.

Aww, come on. You gonna let facts get in the way of a good hypothesis?

I figured I was missing something in that thought process.

cfgauss · 01-21-2003, 06:37 PM

You should check out the quantum mechanical description of particles. You can find good descriptions on lots of sites if you google for it or something.

Jesse · 01-21-2003, 06:45 PM

cfgauss:
There's no precedent for negative energy.

Yes there is. Here is Kip Thorne on "exotic matter" with negative energy:

Quote:

Then, in 1974, came a great surprise: Hawking inferred as a byproduct of his discovery of black-hole evaporation (Chapter 12) that vacuum fluctuations near a hole's horizon are exotic: They have negative average energy density as seen by outgoing light beams near the hole's horizon. In fact, it is this exotic property of the vacuum fluctuations that permits the hole's horizon to shrink as the hole evaporates, in violation of Hawking's area-increase theorem. Because exotic material is so important for physics, I shall explain this in greater detail:

Recall the origin and nature of vacuum fluctuations, as discussed in Box 12.4: When one tries to remove all electric and magnetic fields from some region of space, that is, when one tries to create a perfect vacuum, there always remain a plethora of random, unpredictable electromagnetic oscillations--oscillations caused by a tug-of-war between the fields in adjacent regions of space. The fields "here" borrow energy from the fields "there," leaving the fields there with a deficit of energy, that is, leaving them momentarily with negative energy. The fields there then quickly grab the energy back and with it a little excess, driving their energy momentarily positive, and so it goes, onward and onward.

Under normal circumstances on Earth, the average energy of these vacuum fluctuations is zero. They spend equal amounts of time with energy deficits and energy excesses, and the average of deficit and excess vanishes. Not so near the horizon of an evaporating black hole, Hawking's 1974 calculations suggested. Near a horizon the average energy must be negative, at least as measured by light beams, which means that the vacuum fluctuations are exotic.

...

Under what other circumstances will vacuum fluctuations be exotic? Can they ever be exotic inside a wormhole, and thereby hold the wormhole open? This was the central thrust of the research effort triggered by Page's noticing that the only way to hold any wormhole open is with exotic material.

The answer has not come easily, and it is not entirely in hand. Gunnar Klinkhammer (a student of mine) has proved that in flat spacetime, that is, far from all gravitating objects, vacuum fluctuations can never be exotic--they can never have a negative average energy density as measured by light beams. On the other hand, Robert Wald (a former student of Wheeler's) and Ulvi Yurtsever (a former student of mine) have proved that in curved spacetime, under a very wide variety of circumstances, the curvature distorts the vacuum fluctuations and thereby makes them exotic.

Is a wormhole that is trying to pinch off such a circumstance? Can the curvature of the wormhole, by distorting the vacuum fluctuations, make them exotic and enable them to hold the wormhole open? We still do not know, as this book goes to press.

Black Holes and Time Warps, pp. 491-492

Actually, the section where he says it is impossible to have vacuum fluctuations with negative energy in flat spacetime confuses me a bit, since I have read in a number of other sources that the Casimir effect involves negative energy densities. Here is Paul Davies on the subject:

Quote:

So how do you create negative energy? A simple method was discovered by the Dutch physicist Hendrik Casimir in 1948. This is what you do. Take two sheets of metal and place them close to each other, face-to-face. Secure them so they don�t move. Now enclose the entire system in a large, thick metal box from which all other material (including gases and electrically charged and neutral particles) has been removed, and cool to absolute zero (-273 degrees Celsius). The slab of empty space between the plates now contains negative energy.

The Casimir effect is a phenomenon of the quantum vacuum. Strictly, I should not cite it as an example of exotic matter, because it refers to a state of empty space. But this is a terminological quibble: the distinctions between field excitations, matter, and emptiness are very blurred in quantum physics.

This is why the negative Casimir energy arises. The apparently empty space between the plates is not a complete vacuum, but populated by a seething mass of virtual photons. Like their real counterparts, virtual photons rebound from the metal plates. Being sandwiched between the plates, they are not free to move in any direction, so this restriction effectes the variety of virtual photons that can inhabit the interplate region compared with the space outside the two plates. As a result, the total energy being "borrowed" (via Heisenberg�s uncertainty principle) in the interplate region is a bit less than it would have been without the plates. If we agree that the apparently empty space without any plates has exactly zero energy, then the region between the plates must have negative energy. The negative energy manifests itself by producing a tiny force of attraction between the plates.

How to Build a Time Machine, pp. 80-81.

Davies goes on to mention that the Casimir effect has actually been verified experimentally a number of times. Here is a section of a book by physicist J. Richard Gott that mentions the possibility of using the Casimir effect to hold open a wormhole:

Quote:

Negative-energy-density stuff is pretty special. It opens the way to solutions in general relativity ranging from wormholes to warpdrives. Indeed, Morris, Thorne, and Yurtsever have designed a wormhole using the Casimir effect to keep the tunnel propped open. To work, such a wormhole tunnel must have a circumference of 600 million miles. Each wormhole mouth would be covered by an electrically charged spherical Casimir plate. The plates would be separated by only 10^-10 centimeters across a short wormhole tunnel connecting the two mouths. (This conforms to a limit found by L. H. Ford and Thomas A. Roman that, in such wormhole solutions, the negative-energy-density stuff must be confined to a very narrow layer in the wormhole tunnel.) Building this wormhole would be a nontrivial engineering challenge, to say the least. The total mass involved is 200 million times that of the Sun. Astronauts wishing to pass through the wormhole would have to avoid being fried by blueshifted radiation falling onto the plates and would have to open trapdoors in each plate to get through. Not easy�but the Casimir vacuum at least creates the possibility.

Time Travel in Einstein�s Universe, p. 136

cfgauss:
Also, we have to be careful about what negative energy is. For example, on the surface of the Earth an object has zero gravitational potential energy; I take it into a hole one meter below the ground and all of the sudden it's gravitational potential energy is negative! Of course, this is simply because of our definition of position on a coordinate axis!

That isn�t what physicists would call a "negative energy density", though. The effects described above are.

cfgauss:
And IIRC, you need negative energy, imaginary mass, and(?) exotic matter to have a wormhole.

You need exotic matter in order to have negative energy, and anything that does have negative energy (like the vacuum in certain circumstances) is defined as "exotic matter". So they�re not two separate things. As for imaginary mass, I�ve never heard that you need that�do you have a reference for this?

cfgauss:
They're, as far as we can see, entirely unstable. Even if we had these things, it would probably collapse by the disruptions caused by anything entering it.

This sounds like a practical concern, but I�m not interested in practicality here, just the question of what is allowed by the laws of physics in principle.

cfgauss:
Also, even if you didn't, you'd still need "imaginary time" to exist, because that's what the wormhole is "in"!

I have not heard that wormholes require imaginary time, although it�s quite possible. Again, do you have a reference?

I�m not that familiar with how physicists use the term "imaginary time", but it seems to me there may be two different meanings. One, if you try to calculate the time dilation experienced by a tachyon, you get the odd result that the time elapsed is imaginary�but I don�t think this would be an issue in the case of wormoles, since an object travelling through a wormhole does not locally exceed the speed of light. The other notion of "imaginary time" comes from the formula for calculating proper time:

- dt^2 + dx^2 + dy^2 + dz^2

This almost looks like the pythagorean formula for distance in space, but not quite because the sign of the time dimension is different from the sign of the spatial dimensions. But if you turn the time into an "imaginary" spatial dimension it works out nicely:

+ (idt)^2 + dx^2 + y^2 + z^2

in special relativity this trick would not change the results of any calculations, but treating time as an imaginary spatial dimension seems to yield some nontrivial results when you bring in quantum physics; see this article by John Baez, for example:

http://math.ucr.edu/home/baez/gr/oz3.html

Quote:

Oz adds: "It's kind of interesting that considering energy as momentum flow in the time direction, we can dispense mentally with both mass and energy. We only need to consider momentum flow to describe space curvature, and everything else. Indeed it would be 'nice' to remove that nasty -1 in our metric and make it +1 which I suspect would make us view the momentum flow in the time direction as something slightly different. Has anybody done this, and how would you view momentum flow in the time direction if it had a metric of (+,+,+,+)? (I.e. still modeling the real world)."

The wizard nods. "A chap named Hawking did that once. He called this trick `imaginary time' because (it)2 = -t2, so you can get rid of the minus sign in the metric by making the substitution t -> it. In the world of imaginary time, time is no different from space."

"Why did he do this? Well, he was wondering about the question: `what happened right at the moment of the big bang, or before?' Of course, classically this question doesn't make sense at all. But what about when you take quantum gravity into account? That's what Hawking was wondering about."

"Unfortunately, to answer this question, Hawking had to go way back into that back room where we keep the mathematical machinery. [The wizard gestures with his staff to the curtain, which looks blacker than ever, shadows seeping from it and filling the room. Oz suddenly notices it has grown very late and is dark outside.] Way, way back where they keep the REALLY scary mathematics, stuff you wouldn't believe. And unfortunately to understand his answer, you'd have to go in there too. Because, you see, he never came out!"

So, which meaning of imaginary time were you using when you said that wormholes require imaginary time? If it�s the same sort of "imaginary time" that Hawking uses in his cosmological no-boundary theory, would you accuse Hawking of logical inconsistency as well?

cfgauss:
These things are logically inconsistent! They aren't defined! They have no meaning!

A lot of prominent physicists seem to disagree with these emphatic assertions of yours. Care to give a substantive argument as to why you are right and they are wrong?

cfguass:
Any kind of preferred frame would have drastic consequences for SR and GR.

Can you name some?

cfgauss:
They would most likely not work in the exotic conditions in which they are shown to work. And I know of no theory that postulates a preferred frame whose results are exactly like SR and GR except in a few *highly* exotic situations, and I seriously doubt that one exists!

Perhaps, but even if such a theory would be highly inelegant it can�t be completely ruled out. Again, I agree that it doesn�t seem very likely.

ajm51987 · 01-21-2003, 07:43 PM

Geez guys. Don't you people use improbility drive yet?

Ted Hoffman · 01-22-2003, 05:45 AM

go easy on the boy he is only a child.

cfgauss · 01-22-2003, 04:42 PM

ajm51987:
Improbability drive!

Jesse:
That's not quite what I meant! It should have read more along the lines of "there's no precedent for negative energy like that." For example, I can talk about magnetic fields around 10^123 gauss, but there's not been anything known to exist around 10^50-somethingth. I should've been more clear!

The Casimir effect is really cool, though! It'll actually pull the two plates together! Unfortunately, it is so insanely small that it could probably never be used for anything. Though I believe that it has been measured.

"You need exotic matter in order to have negative energy"
But aren't they considered as separate? A magnetic field is energy, a ball is matter. I had assumed that when they say "exotic matter" they actually mean *matter* whose energy content is negative, not just some energy field.

"This sounds like a practical concern, but I�m not interested in practicality here, just the question of what is allowed by the laws of physics in principle."
When you get to some areas, these concepts can be the same thing. If I can't do something, is it necessarily meaningful to talk about? For example, let's say the universe has 1 "unit" of energy in it. Is it meaningful to talk about an object with 1.1 "units" of kinetic energy then? Not necessarily.

"I have not heard that wormholes require imaginary time, although it�s quite possible. Again, do you have a reference?"
IIRC, Wheeler talked about this. The tunnel part of the wormhole exists in "imaginary time." Though I can't recall where I've seen that in print, I know it was an idea he came up with when advising for the Novel Contact.

"A lot of prominent physicists seem to disagree with these emphatic assertions of yours. Care to give a substantive argument as to why you are right and they are wrong?"
They aren't measured or anything, they exist strictly as mathematical entities. This is great to come up with ideas, but eventually we need observation. This is the same reason lots of astrophysical ideas aren't bought by everyone. They're nice ideas, but they don't do anything. Until there's some reason to do these things other than for fun, they're nothing more than pet theories! And they don't disagree with that statement, the only difference is that they believe that it may meaningfully exist, while people like me are more skeptical.

"Perhaps, but even if such a theory would be highly inelegant it can�t be completely ruled out. Again, I agree that it doesn�t seem very likely"
For some reason, physics always turns out to be very nice. Theories that you have to wield like a giant club just don't do much!

cfgauss · 01-22-2003, 04:45 PM

Oh, also, with regards to the using (it)^2 to make things work cool, I wonder about it's significance! I could say the same thing for a lot of things! For example, if an economist discovers that satisfaction^2 - income^2 = constant for a family (yes, they really do quantitatively measure satisfaction!), is it meaningful to change that to: satisfaction^2 + (i income)^2 = const? Well, what's imaginary income?! Although one may exist, I've never seen a good *physical* justification for this.

Oh, also also, the minus sign is because SR plays in Lorentzian (sp?) geometry instead of Euclidian!

Answerer · 01-22-2003, 05:58 PM

Quote:

Originally posted by Jesse
Answerer, I believe the people in the articles you cite are hoping for some kind of new physics which will allow faster-than-light communication using entanglement (for example, if quantum randomness was actually based on some pseudorandom deterministic law it might be possible)...I think it's been proven pretty definitively that quantum entanglement cannot be used to transmit information faster-than-light given the current known laws of physics.

Hi Jesse, quantum teleportation is divided into two channels, the classical and quantum parts. I'm actually refering to the quantum channel, not classical one(although I know they can't be separated). Quantum entanglement could just be another type of superluminal influence like quantum randomness(leave out the classical portion for a while).

Anyway, Cf is a hard-core defender of the speed of light, you will have great problems convincing him otherwise.

:banghead: :banghead: :banghead:

cfgauss · 01-22-2003, 10:18 PM

"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

"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!

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01-21-2003, 05:14 PM	#21
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	Jesse: There's no precedent for negative energy. Also, we have to be careful about what negative energy is. For example, on the surface of the Earth an object has zero gravitational potential energy; I take it into a hole one meter below the ground and all of the sudden it's gravitational potential energy is negative! Of course, this is simply because of our definition of position on a coordinate axis! And IIRC, you need negative energy, imaginary mass, and(?) exotic matter to have a wormhole. They're, as far as we can see, entirely unstable. Even if we had these things, it would probably collapse by the disruptions caused by anything entering it. Also, even if you didn't, you'd still need "imaginary time" to exist, because that's what the wormhole is "in"! These things are logically inconsistent! They aren't defined! They have no meaning! Any kind of preferred frame would have drastic consequences for SR and GR. They would most likely not work in the exotic conditions in which they are shown to work. And I know of no theory that postulates a preferred frame whose results are exactly like SR and GR except in a few highly exotic situations, and I seriously doubt that one exists! b And, looking at that website (gmu), neutrinos have been shown not to be tachyons! There's also rather little content there. 20 Helens agree that there's no such thing as tachyons. Friar Bellows: This explains it pretty well. http://einstein.stanford.edu/gen_int...ty/a10854.html

01-21-2003, 06:37 PM	#23
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	You should check out the quantum mechanical description of particles. You can find good descriptions on lots of sites if you google for it or something.

01-21-2003, 07:43 PM	#25
ajm51987 Regular Member Join Date: May 2002 Location: Seattle, USA Posts: 245	Geez guys. Don't you people use improbility drive yet?

01-22-2003, 05:45 AM	#26
Ted Hoffman Veteran Member Join Date: Feb 2001 Location: "" Posts: 3,863	go easy on the boy he is only a child.

01-22-2003, 04:42 PM	#27
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	ajm51987: Improbability drive! Jesse: That's not quite what I meant! It should have read more along the lines of "there's no precedent for negative energy like that." For example, I can talk about magnetic fields around 10^123 gauss, but there's not been anything known to exist around 10^50-somethingth. I should've been more clear! The Casimir effect is really cool, though! It'll actually pull the two plates together! Unfortunately, it is so insanely small that it could probably never be used for anything. Though I believe that it has been measured. "You need exotic matter in order to have negative energy" But aren't they considered as separate? A magnetic field is energy, a ball is matter. I had assumed that when they say "exotic matter" they actually mean matter whose energy content is negative, not just some energy field. "This sounds like a practical concern, but I�m not interested in practicality here, just the question of what is allowed by the laws of physics in principle." When you get to some areas, these concepts can be the same thing. If I can't do something, is it necessarily meaningful to talk about? For example, let's say the universe has 1 "unit" of energy in it. Is it meaningful to talk about an object with 1.1 "units" of kinetic energy then? Not necessarily. "I have not heard that wormholes require imaginary time, although it�s quite possible. Again, do you have a reference?" IIRC, Wheeler talked about this. The tunnel part of the wormhole exists in "imaginary time." Though I can't recall where I've seen that in print, I know it was an idea he came up with when advising for the Novel Contact. "A lot of prominent physicists seem to disagree with these emphatic assertions of yours. Care to give a substantive argument as to why you are right and they are wrong?" They aren't measured or anything, they exist strictly as mathematical entities. This is great to come up with ideas, but eventually we need observation. This is the same reason lots of astrophysical ideas aren't bought by everyone. They're nice ideas, but they don't do anything. Until there's some reason to do these things other than for fun, they're nothing more than pet theories! And they don't disagree with that statement, the only difference is that they believe that it may meaningfully exist, while people like me are more skeptical. "Perhaps, but even if such a theory would be highly inelegant it can�t be completely ruled out. Again, I agree that it doesn�t seem very likely" For some reason, physics always turns out to be very nice. Theories that you have to wield like a giant club just don't do much!

01-22-2003, 04:45 PM	#28
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	Oh, also, with regards to the using (it)^2 to make things work cool, I wonder about it's significance! I could say the same thing for a lot of things! For example, if an economist discovers that satisfaction^2 - income^2 = constant for a family (yes, they really do quantitatively measure satisfaction!), is it meaningful to change that to: satisfaction^2 + (i income)^2 = const? Well, what's imaginary income?! Although one may exist, I've never seen a good physical justification for this. Oh, also also, the minus sign is because SR plays in Lorentzian (sp?) geometry instead of Euclidian!

01-22-2003, 10:18 PM	#30
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	"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 "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!

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