Is reverse time dilation posssible?

[JesseM]

I agree with JesseM: the Casimir Effect proofs the possibility of less energy density than in a vacuum. However, the calculation of the vacuum energy density is not exactly a glorious chapter of modern physics. So I´d say it´s absolutely correct to say that there is no proof for negative energy.

John Baez has a good discussion of the problems with calculating the vacuum energy density here. He points out that although physicists have failed pretty badly in calculating the vacuum energy from quantum principles, an upper limit can be placed on it from cosmological observations, around 10^-9 joules per cubic meter. So I wonder, can the energy density between plates in the casimir effect go below the vacuum energy density by significantly more than 10^-9 joules per cubic meter? Does anyone know the equation for calculating the energy density between plates in terms of their size and separation? I found a bunch of pages that give an equation for the force (see here, for example), but I'm not sure how to translate this into energy density.

 

[pervect]

John Baez has a good discussion of the problems with calculating the vacuum energy density here.

Thanks for the reference. I am experiencing some sign confusion here, so I did some more reading, specifically

http://arxiv.org/PS_cache/gr-qc/pdf/0001/0001099.pdf

(3) If you believe the recent observational data
regarding the accelerating universe, then the SEC [strong energy condition] is violated on cosmological scales right now!

As nearly as I can figure out, the cosmological constant represents a positive energy density T_00 and (in a locally Minkowskian coordiante system) a negative pressure T_11 = T_22 = T_33 = -T_00.

[Any references otherwise from respectable sources will be gratefully accepted.]

In spite of the positivity of the actual energy density T_00 assigned to the cosmological constant $\Lambda$, the net effect is one of "effective" anti-gravity. Thus the over-simplification on my part is in thinking/describing that the strong energy condition is one of positive energy density - the energy density due to our positive cosmological constant is technically positive, but it causes the universal expansion to accelerate, not slow down, so it "acts" a lot like negative mass, and it meets the technical defintion of exotic matter in that it violates the strong energy condition.

[add]
Another way of thinking about it - in flat space-time, gravity couples to $\rho+3P$, so the negative pressure terms do give an effectively "negative" mass in spite of the positve density term

 

[LodeRunner]

I agree with JesseM: the Casimir Effect proofs the possibility of less energy density than in a vacuum. However, the calculation of the vacuum energy density is not exactly a glorious chapter of modern physics. So I´d say it´s absolutely correct to say that there is no proof for negative energy.

Since we apparently live in an "open" universe which will keep on expanding to infinity, doesn't this violate conservation of energy in a rather big way?

Consider a thought experiment; assume there is a way to harness zero-point energy. Again, you would be violating conservation of energy because no matter how much energy you draw off, the Heisenberg Uncertainty Principle forbids us from ever having an energy state of exactly zero.

The Casimir Effect shows us that energy can indeed be harnessed for work (pressing two metal plates together), yet at the same time QM says that we can never use all of the energy (indeed, assuming zero point fluctuations are *just* powerful enough to prevent empty space from violating the Uncertainty Principle, we shouldn't be able to use any of it at all.)

In short, I don't see how we can explain away this creation of energy, unless we also allow the creation of negative energy.

 

[JesseM]

Since we apparently live in an "open" universe which will keep on expanding to infinity, doesn't this violate conservation of energy in a rather big way?

Ich wasn't talking about conservation of energy, he/she was talking about negative energy, a separate issue. Energy is not conserved globally in general relativity, see here.

The Casimir Effect shows us that energy can indeed be harnessed for work (pressing two metal plates together)

What do you mean by "harnessed", exactly? I don't know how you'd use the two plates to do work, and even if you could, I'd bet that you couldn't get out more energy than you put in...I did some quick googling and found there was a proposal by physicist Robert Forward to extract energy from the vacuum, but this page says:

13. J. Maclay, "Unusual properties of conductive rectangular cavities in the zero point electromagnetic field: Resolving Forward's Casimir energy extraction cycle paradox," PROCEEDINGS of STAIF-99 (Space Technology and Applications International Forum-1999, Albuquerque, NM, January, 1999), edited by M.S. El-Genk, AIP Conference Proceedings 458, American Institute of Physics, New York 1999. Published in hardcopy and CD-ROM by AIP.

This article presents the explanation of why Bob Forwards idea for extracting energy from the vacuum does not work. Usually we think that if the energy density in a box is greater than the energy density outside the box, that there will be an outward force on the walls of the box, which is what Bob’s idea was based on. However, for vacuum energy this it not true!

 

[LodeRunner]

Negative energy is related to conservation of energy because mathematically, one should be able to create energy out of nothing if one also creates an equal amount of negative energy.

My point, which you may or may not have refuted (unfortunately I have very little formal education, so many of the details of that link were lost on me), was that assuming all of empty space has a positive energy value would violate conservation of energy, whereas assuming that the value could go negative would not.

What do you mean by "harnessed", exactly? I don't know how you'd use the two plates to do work, and even if you could, I'd bet that you couldn't get out more energy than you put in...I did some quick googling and found there was a proposal by physicist Robert Forward to extract energy from the vacuum, but this page says:

It's not a matter of extracting useful amounts of power; it's a matter of reducing the amount of zero-point energy in any region of space. If we accept the QM explanation, then it seems as though zero-point energy should be just strong enough to avoid breaking the Uncertainty Principle by allowing us to be too certain about the speed/position/value of a certain point in space.

My point is that if we reduce the amount of vacuum energy by any means, such as using it to push two small metal plates together (converting it into macroscopic kinetic energy), then that energy MUST be replenished somehow in order to prevent over-certainty. My assumption was that this was done by creating negative energy.
Perhaps there is another way it can be replenished, or perhaps the problem is altogether moot if GR and/or QM does allow for creation of energy in some circumstances.

Or perhaps I just don't know what I'm talking about 'cause I'm just a high school dropout wannabe writer who reads up on physics in his spare time...

 

[JesseM]

Negative energy is related to conservation of energy because mathematically, one should be able to create energy out of nothing if one also creates an equal amount of negative energy.

But if you're creating an equal amount of negative energy, then you aren't actually violating conservation of energy, right?

My point, which you may or may not have refuted (unfortunately I have very little formal education, so many of the details of that link were lost on me), was that assuming all of empty space has a positive energy value would violate conservation of energy

Why would a positive vacuum energy violate conservation of energy?

It's not a matter of extracting useful amounts of power; it's a matter of reducing the amount of zero-point energy in any region of space.

Who said anything about useful? What I'm saying is that I don't think you can extract any energy from the vacuum using the casimir effect, no matter how tiny. I don't understand what connection you're making between reducing the energy density of a given region of empty space and extracting energy to do work from it.

If we accept the QM explanation, then it seems as though zero-point energy should be just strong enough to avoid breaking the Uncertainty Principle by allowing us to be too certain about the speed/position/value of a certain point in space.

Huh? How would zero-point energy break the uncertainty principle if it were higher? What does it even mean to measure the speed/position of a "point in space", as opposed to a particle?

My point is that if we reduce the amount of vacuum energy by any means, such as using it to push two small metal plates together (converting it into macroscopic kinetic energy), then that energy MUST be replenished somehow in order to prevent over-certainty. My assumption was that this was done by creating negative energy.

I'm not following at all. Are you basing your ideas about how the uncertainty principle relates to zero-point energy on something you've read somewhere, or is this your own idea? If it's something you've read, maybe you could provide a link or a reference so I could better understand your argument.

 

[LodeRunner]

I do not know where I first read about the connection of the Casimir Force to the Uncertainty Principle, but a quick wikipedia lookup yields:

In a quantum mechanical system such as the particle in a box or the quantum harmonic oscillator, the lowest possible energy is called the zero-point energy. According to classical physics, the kinetic energy of a particle in a box or the kinetic energy of the harmonic oscillator may be zero if the velocity is zero. Quantum mechanics with its uncertainty principle implies that if the velocity is measured with certainty to be exactly zero, the uncertainty of the position must be infinite. This either violates the condition that the particle remain in the box, or it brings a new potential energy in the case of the harmonic oscillator. To avoid this paradox, quantum mechanics dictates that the minimal velocity is never equal to zero, and hence the minimal energy is never equal to zero.

So it appears as though empty space in QM is subject to the same Uncertainty limitations as particles.

But if you're creating an equal amount of negative energy, then you aren't actually violating conservation of energy, right?

This is exactly my point. Creation of positive and negative energy (or matter) in equal amounts does not violate conservation. Creation of positive-only energy, OTOH, does seem to violate conservation, and your explanation of vacuum energy (that all of empty space has some positive energy but never any negative energy) would seem to fall into this category.

Why would a positive vacuum energy violate conservation of energy?

It wouldn't, provided you could prove that the total vacuum energy in the universe never changed, or that enough negative energy is created to counterbalance the positive.

Who said anything about useful? What I'm saying is that I don't think you can extract any energy from the vacuum using the casimir effect, no matter how tiny. I don't understand what connection you're making between reducing the energy density of a given region of empty space and extracting energy to do work from it.

Well, I thought you said something about always having to "put in" more energy than you'd get out of vacuum energy. My point wasn't getting free energy out of vacuum energy; it was getting ANY energy out of it at all.

If one made microscopic and very light metal plates and put a small amount of pressure on them, not quite enough to move them on its own, but enough to move them after vacuum energy has exterted its influence, then wouldn't that be a case of someone turning vacuum energy into kinetic energy? Wouldn't that mean that we must have LESS vacuum energy than we started with?

My contention is as follows:

The Casimir Effect is a direct consequence of the Uncertainty Principle, and it proves we can convert vacuum energy into another form (kinetic.) BUT, the universe will not allow the reduction of vacuum energy in this manner because this would violate the Uncertainty Principle.

So, if the universe will not allow this reduction, then it must replace the energy lost to maintain Uncertainty. Empty space does not need to have an inherent energy; it just needs to have a non-zero value to preserve Uncertainty. This non-zero value does not need to be positive, so I believe that the most elegant explanation that preserves conservation of energy is to say that zero-point fluctuations include both positive and negative values.

I hope this will clear things up...

 

[JesseM]

So it appears as though empty space in QM is subject to the same Uncertainty limitations as particles.

I still don't understand, that wikipedia quote was talking about a particle in a box, not empty space. I think the zero-point energy is related to uncertainty in the sense that the quantum fields in quantum field theory are in some sense subject to the uncertainty principle just like particles, but I don't think this has anything to do with measuring the position/speed of a point in space, which I don't think is even a meaningful concept.

This is exactly my point. Creation of positive and negative energy (or matter) in equal amounts does not violate conservation. Creation of positive-only energy, OTOH, does seem to violate conservation, and your explanation of vacuum energy (that all of empty space has some positive energy but never any negative energy) would seem to fall into this category.

But this is just a static amount of energy associated with the vacuum, it isn't the "creation" of positive energy where none was previously.

It wouldn't, provided you could prove that the total vacuum energy in the universe never changed[/url] I think the vacuum energy is thought to have decreased at the end of the inflationary era, but I don't understand the details enough to say whether energy was conserved in this process...if the vacuum energy decreases, perhaps it would be made up for by the creation of a large number of energetic particles, so that the total energy of space + particles remains constant.

Well, I thought you said something about always having to "put in" more energy than you'd get out of vacuum energy. My point wasn't getting free energy out of vacuum energy; it was getting ANY energy out of it at all.
If one made microscopic and very light metal plates and put a small amount of pressure on them, not quite enough to move them on its own, but enough to move them after vacuum energy has exterted its influence then wouldn't that be a case of someone turning vacuum energy into kinetic energy? Wouldn't that mean that we must have LESS vacuum energy than we started with?

I don't know, I don't understand the casimir effect or quantum field theory well enough to address this. You may be right that the increase in kinetic energy of the plates as they are pulled together is balanced by the decreased vacuum energy between them.

The Casimir Effect is a direct consequence of the Uncertainty Principle, and it proves we can convert vacuum energy into another form (kinetic.) BUT, the universe will not allow the reduction of vacuum energy in this manner because this would violate the Uncertainty Principle.

I still don't follow your reasoning. Why would it violate the uncertainty principle? From what I've read, the uncertainty principle's relation to vacuum energy has something to do with the different possible modes of the quantum field carrying energy and momentum. But by placing two plates near each other, you are reducing the number of allowable modes between the plates, because wavelengths that don't divide evenly into the spacing between the plates interfere with themselves. I think each individual mode that's present is still subject to the uncertainty principle in the same way as usual, but some modes are simply no longer present between the plates that are present in empty space, and this has something to do with why the vacuum energy is lower between the plates.

 

[LodeRunner]

To avoid this paradox, quantum mechanics dictates that the minimal velocity is never equal to zero, and hence the minimal energy is never equal to zero.

(emphasis mine.)

This means that empty space cannot exist, because empty space=0 energy. This is why we have "quantum foam" or zero-point fluctuations instead. And the Casimir Effect is a direct test of the existence these fluctuations. That is the relationship between the Casimir Effect and Uncertainty--zero-point fluctuations are a consequence of Uncertainty, and the Casimir Effect is a consequence of zero-point fluctuations.

Vacuum energy is present to preserve Uncertainty, and to convert it to another form (in essence taking it away from the vacuum) would allow us to be too certain about the value of a particular point in spacetime.

I can try to find some more links for you, but I'm a little pressed for time. I'm not saying anything that I haven't read in several pop science books (Hawking, etc.) I suppose we could take this to the QM forum and see if we can find a consensus there.

You may be right that the increase in kinetic energy of the plates as they are pulled together is balanced by the decreased vacuum energy between them.

Where else is the kinetic energy coming from, if not a decrease in vacuum energy? Again, this is a Conservation problem. Assuming I'm not horribly wrong about Uncertainty applying to empty space, any vacuum energy converted to another form must be replaced or else we lose that Uncertainty. The idea of negative energy let's us seemlessly replace this lost energy, and no net energy will have been created--thus both Conservation and Uncertainty are preserved.

 

[JesseM]

(emphasis mine.)
This means that empty space cannot exist, because empty space=0 energy. This is why we have "quantum foam" or zero-point fluctuations instead. And the Casimir Effect is a direct test of the existence these fluctuations. That is the relationship between the Casimir Effect and Uncertainty--zero-point fluctuations are a consequence of Uncertainty, and the Casimir Effect is a consequence of zero-point fluctuations.

Vacuum energy is present to preserve Uncertainty, and to convert it to another form (in essence taking it away from the vacuum) would allow us to be too certain about the value of a particular point in spacetime.

Ok, but you're not telling me what entity it is that you think is subject to uncertainty here--you said earlier that it was the position/speed of "points in space" which was uncertain, I still think that doesn't make any sense. What I've read in a few places is that different "modes" of the quantum fields which fill empty space carry momentum/energy and are therefore subject to uncertainty, and this is what's responsible for vacuum energy. In between the plates in the Casimir effect, there are fewer possible modes than in free space, therefore the minimum energy is less, without any violation of the uncertainty principle. Are you disagreeing with me about this?

Where else is the kinetic energy coming from, if not a decrease in vacuum energy?

I'd guess you're probably right about this, but again, I don't have enough knowledge of this subject to say for sure. It's conceivable that it would instead be due to a decrease in some form of potential energy, for example.

Assuming I'm not horribly wrong about Uncertainty applying to empty space, any vacuum energy converted to another form must be replaced or else we lose that Uncertainty. The idea of negative energy let's us seemlessly replace this lost energy, and no net energy will have been created--thus both Conservation and Uncertainty are preserved.

Just to be clear, do you think this is the opinion of mainstream physicists, or are you expressing your own ideas here? Like I said before, I don't think mainstream physicists would say there is any problem with the uncertainty principle being violated in the Casimir effect, in fact I think the lower energy density between the plates can be calculated from the uncertainty principle in exactly the same way that the energy of the ordinary vacuum is calculated from it (by looking at the minimum energy of all the allowable modes in each case, with more modes being allowed in empty space than between the plates).

 

[LodeRunner]

I don't want anyone to get the impression that I'm pulling stuff out of my ass, so:

Mainstream physicists have said that the Casimir Effect arises as a consequence of Uncertainty, not that it violates it. Mainstream physicists have also brought up the possibility of negative energy or negative mass, and I have heard them use the Casimir Effect as one such example.

I have made the (possibly wrong) leap of logic to connect these two ideas and show why negative energy may be necessary in order to preserve Conservation while simultaneously preserving Uncertainty. I have not heard this idea from any source; it is just the result of me trying to make several different mainstream ideas fit together.

My claim that a reduction in energy would result in Uncertainty breaking is my own. Since I have always heard the Casimir Effect explained in terms of the Uncertainty Principle, I assume that the fluctuations are just strong enough to preserve Uncertainty. It may be that there is a margin there, and that it is possible to convert some vacuum energy into other forms but not enough to break Uncertainty, but because of the way it was explained to me, I have no reason to believe that such an excess of vacuum energy exists.

My claim that an increase in vacuum energy (e.g. to replace that loss through the Casimir Effect) violates Conservation is also my own, though I think it is a fairly straightforward assumption.

Ok, but you're not telling me what entity it is that you think is subject to uncertainty here--you said earlier that it was the position/speed of "points in space" which was uncertain, I still think that doesn't make any sense. What I've read in a few places is that different "modes" of the quantum fields which fill empty space carry momentum/energy and are therefore subject to uncertainty, and this is what's responsible for vacuum energy. In between the plates in the Casimir effect, there are fewer possible modes than in free space, therefore the minimum energy is less, without any violation of the uncertainty principle. Are you disagreeing with me about this?

Interesting. I have never heard this explanation before. I have always read that it is the appearence and disappearence of virtual particles, the kind that are responsible for Hawking Radiation (one particle of the particle/antiparticle virtual pair appears just barely within the event horizon of a black hole and the other one appears just barely outside of it. The one outside escapes to freedom and becomes "real" whereas the other one falls into the black hole and reduces its mass through QM weirdness that I do not fully understand.) It was my understanding that all of empty space was composed of these virtual particle pairs, constantly appearing and annihilating one another--the quantum "foam". They have no mass and usually have no effect, except that Uncertainty commands that they MUST have a mass or energy value, at least sometimes.

And if all of my assumptions are correct, then they must also have an negative mass (or negative energy), at least sometimes.

Anyway, I really must be getting to bed now. It's been great debating with you... will come back again and check tonight or Friday night.

 

[selfAdjoint]

What are uncertain in quantum mechanics are complementary pairs of observables. There are several of these; the easiest to prove from Schroedinger's (or any other quantum) equation is momentum and position. The more accurately you know momentum the more uncertain you must be about position, and vice versa. If you multiply the uncertainties together you get Planck's constant, so if you were to plot the uncertainty on axes of position for x and momentum for y, the uncertainty curve would be a hyperbola with those two axes as asymptotes.

Another set of complementary observables is spin around different axes. If you measure the spin around the z-axis, thus reducung its uncertainty to zero, the spins around the x and y-axes become completely uncertain. This is the physics behind the repeated Stern-Gerlach experiments described by Feynmann.

The case of energy and time is a little different. It turns on they behave just like complementary observables, but because energy is an observable but time is not, the proof is different.

I don't see any need for negative energy to balance conservation of energy in quantum mechanics. energy is conserved up to observation in quantum mechanics. And just as there is no preferred frame in relativity from which you can see "what really happens", so there is no opportunity in quantum mechanics to see the unobservable. For more on this, check out the threads on the quantum forum about hidden variables and Bell's theorem.

 

[RandallB]

some popularized musings by Fenyman - don't appear to me to actually lead to any experimental predictions.
I could be wrong, perhaps RandallB could cite some papers.

I wasn’t referring to his musings; Fenyman’s Diagrams have lead to improvements, discoveries, and understands in working out and planning partial collisions.
Work through some of the mildly complicated ones, you don’t need a cited papers to find them. Once you understand them & where they are useful, the hard part is to imagine a way to workout the issues without backwards time. Maybe with a MWI/HUP concept but I’d find that hard to implement usefully as the diagrams already have been.

As to the Casimir force—“ it is an actual observed force that is predicted and explained by QM as negative energy.” As it’s only explanation!
Therefore by using that explanation:
“we KNOW exotic energy & matter exists, period”.
Therefore by implication we know:
Negative gravitation even Wormholes must be real!
This is great! with ‘proof’ like this we should be able to show that HUP and thus QM is just as exclusively correct as that theory claims to be!
– except that was the assumption that started all these 'proofs'!

The point is; mind numbing "circular logic" like this may have some use in science (I’m not sure where), but why use something that hard to follow in fiction, when you already know that the cheesy ‘Flux Capacitor’ style is proven as acceptable anyway.

Now if this thread has turned to finding what’s ‘correct’, I don’t think Casimir will do. Entanglement is the only recognized non-circular ‘proof’ for QM/HUP and we already have two or three active threads debating that issue.

Accepting backwards time as something real that needs to be figured out is just one thing.
QM – Casimir – wormhole – HUP is a spaghetti bowl. I’ll take Occam as my guide.

SO, for me my money’s on backwards time a la Fenyman’s Diagrams is shown to be real and not a trick before Negative Gravitation is. In fact I’d bet that a Theory proving so would also show QM/HUP to wormholes as wrong.
Since I seem to be in a minority on that at least I should get good odds, do ya’ suppose they have a line in Vegas?
RB

 

[JesseM]

Now if this thread has turned to finding what’s ‘correct’, I don’t think Casimir will do. Entanglement is the only recognized non-circular ‘proof’ for QM/HUP and we already have two or three active threads debating that issue.

You are doubting QM/HUP?? These fall into the realm of very well-established physics. This board has a rule against trying to call into question established physics, so I wonder which active threads you're talking about.

SO, for me my money’s on backwards time a la Fenyman’s Diagrams is shown to be real and not a trick before Negative Gravitation is. In fact I’d bet that a Theory proving so would also show QM/HUP to wormholes as wrong.

What does "shown to be real" mean here? What possible test could distinguish backwards-time-as-something-real from backwards-time-as-a-useful-mathematical-trick, since it's clear that quantum field theory does respect causality and thus cannot be used to actually send messages backwards in time? I still think you're confusing physics with philosophy--unless a particular idea leads to testable consequences, it ain't physics.

 

[JesseM]

I don't want anyone to get the impression that I'm pulling stuff out of my ass, so:

Mainstream physicists have said that the Casimir Effect arises as a consequence of Uncertainty, not that it violates it. Mainstream physicists have also brought up the possibility of negative energy or negative mass, and I have heard them use the Casimir Effect as one such example.

Agreed (although by 'negative energy' I think they just mean something like 'energy lower than that of the vacuum')

My claim that a reduction in energy would result in Uncertainty breaking is my own. Since I have always heard the Casimir Effect explained in terms of the Uncertainty Principle, I assume that the fluctuations are just strong enough to preserve Uncertainty. It may be that there is a margin there, and that it is possible to convert some vacuum energy into other forms but not enough to break Uncertainty, but because of the way it was explained to me, I have no reason to believe that such an excess of vacuum energy exists.

I don't understand what you mean by "the fluctuations are just strong enough to preserve uncertainty", or why converting vacuum energy into other forms (like the kinetic energy of the plates) would "break" uncertainty.

My claim that an increase in vacuum energy (e.g. to replace that loss through the Casimir Effect) violates Conservation is also my own, though I think it is a fairly straightforward assumption.

I don't understand this either. In the Casimir effect, the vacuum energy between the plates decreases, it doesn't increase. Are you saying the vacuum energy would increase in some other region to compensate? I'm pretty sure that is not the case...like you suggested earlier, energy conservation may just have to do with the fact that the kinetic energy of the plates balances the decrease in vacuum energy in the space between them, or it may have something to do with creation/destruction of energetic particles, I'm not sure.

Interesting. I have never heard this explanation before. I have always read that it is the appearence and disappearence of virtual particles, the kind that are responsible for Hawking Radiation (one particle of the particle/antiparticle virtual pair appears just barely within the event horizon of a black hole and the other one appears just barely outside of it. The one outside escapes to freedom and becomes "real" whereas the other one falls into the black hole and reduces its mass through QM weirdness that I do not fully understand.) It was my understanding that all of empty space was composed of these virtual particle pairs, constantly appearing and annihilating one another--the quantum "foam". They have no mass and usually have no effect, except that Uncertainty commands that they MUST have a mass or energy value, at least sometimes.

Sure, in quantum field theory I think it's true that to understand the properties of empty space you have to do a sum of Feynman diagrams in which virtual particles appear out of nothing and then annihilate each other. But what's your understanding of how this would explain the Casimir effect? Presumably the virtual particles would have to be behaving differently between the plates than they do in empty space, but why? Reading a little more, I think the usual explanation in terms of virtual particles is probably equivalent to what I was saying about the different "modes" of the quantum field--virtual particles have wavelengths just like modes, and in both cases only wavelengths that divide evenly into the separation between plates can avoid cancelling themselves out, so only these wavelengths contribute to the properties of the vacuum between the plates. See [URL='https://www.physicsforums.com/insights/author/john-baez/']John Baez's page on the Casimir effect[/url], for example:

The Casimir effect is a small attractive force which acts between two close parallel uncharged conducting plates. It is due to quantum vacuum fluctuations of the electromagnetic field.

The effect was predicted by the Dutch physicist Hendrick Casimir in 1948. According to the quantum theory, the vacuum contains virtual particles which are in a continuous state of fluctuation (see physics FAQ article on virtual particles). Casimir realized that between two plates, only those virtual photons whose wavelengths fit a whole number of times into the gap should be counted when calculating the vacuum energy. The energy density decreases as the plates are moved closer which implies there is a small force drawing them together.

And here's an article from "Physics World" which gives a longer explanation:

Understanding the Casimir force

Although the Casimir force seems completely counterintuitive, it is actually well understood. In the old days of classical mechanics the idea of a vacuum was simple. The vacuum was what remained if you emptied a container of all its particles and lowered the temperature down to absolute zero. The arrival of quantum mechanics, however, completely changed our notion of a vacuum. All fields - in particular electromagnetic fields - have fluctuations. In other words at any given moment their actual value varies around a constant, mean value. Even a perfect vacuum at absolute zero has fluctuating fields known as "vacuum fluctuations", the mean energy of which corresponds to half the energy of a photon.

However, vacuum fluctuations are not some abstraction of a physicist's mind. They have observable consequences that can be directly visualized in experiments on a microscopic scale. For example, an atom in an excited state will not remain there infinitely long, but will return to its ground state by spontaneously emitting a photon. This phenomenon is a consequence of vacuum fluctuations. Imagine trying to hold a pencil upright on the end of your finger. It will stay there if your hand is perfectly stable and nothing perturbs the equilibrium. But the slightest perturbation will make the pencil fall into a more stable equilibrium position. Similarly, vacuum fluctuations cause an excited atom to fall into its ground state.

The Casimir force is the most famous mechanical effect of vacuum fluctuations. Consider the gap between two plane mirrors as a cavity (figure 1). All electromagnetic fields have a characteristic "spectrum" containing many different frequencies. In a free vacuum all of the frequencies are of equal importance. But inside a cavity, where the field is reflected back and forth between the mirrors, the situation is different. The field is amplified if integer multiples of half a wavelength can fit exactly inside the cavity. This wavelength corresponds to a "cavity resonance". At other wavelengths, in contrast, the field is suppressed. Vacuum fluctuations are suppressed or enhanced depending on whether their frequency corresponds to a cavity resonance or not.

An important physical quantity when discussing the Casimir force is the "field radiation pressure". Every field - even the vacuum field - carries energy. As all electromagnetic fields can propagate in space they also exert pressure on surfaces, just as a flowing river pushes on a floodgate. This radiation pressure increases with the energy - and hence the frequency - of the electromagnetic field. At a cavity-resonance frequency the radiation pressure inside the cavity is stronger than outside and the mirrors are therefore pushed apart. Out of resonance, in contrast, the radiation pressure inside the cavity is smaller than outside and the mirrors are drawn towards each other.

It turns out that, on balance, the attractive components have a slightly stronger impact than the repulsive ones. For two perfect, plane, parallel mirrors the Casimir force is therefore attractive and the mirrors are pulled together. The force, F, is proportional to the cross-sectional area, A, of the mirrors and increases 16-fold every time the distance, d, between the mirrors is halved: F ~ A/d^4. Apart from these geometrical quantities the force depends only on fundamental values - Planck's constant and the speed of light.

Finally, this site talks about the connection to the uncertainty principle:

The basis of zero-point energy is the Heisenberg uncertainty principle, one of the fundamental laws of quantum physics. According to this principle, the more precisely one measures the position of a moving particle, such as an electron, the less exact the best possible measurement of momentum (mass times velocity) will be, and vice versa. The least possible uncertainty of position times momentum is specified by Planck's constant, h. A parallel uncertainty exists between measurements involving time and energy. This minimum uncertainty is not due to any correctable flaws in measurement, but rather reflects an intrinsic quantum fuzziness in the very nature of energy and matter.

A useful calculational tool in physics is the ideal harmonic oscillator: a hypothetical mass on a perfect spring moving back and forth. The Heisenberg uncertainty principle dictates that such an ideal harmonic oscillator -- one small enough to be subject to quantum laws -- can never come entirely to rest, since that would be a state of exactly zero energy, which is forbidden. In this case the average minimum energy is one-half h times the frequency, hf/2.

Radio waves, light, X-rays, and gamma rays are all forms of electromagnetic radiation. Classically, electromagnetic radiation can be pictured as waves flowing through space at the speed of light. The waves are not waves of anything substantive, but are in fact ripples in a state of a field. These waves do carry energy, and each wave has a specific direction, frequency and polarization state. This is called a "propagating mode of the electromagnetic field."

Each mode is subject to the Heisenberg uncertainty principle. To understand the meaning of this, the theory of electromagnetic radiation is quantized by treating each mode as an equivalent harmonic oscillator. From this analogy, every mode of the field must have hf/2 as its average minimum energy. That is a tiny amount of energy, but the number of modes is enormous, and indeed increases as the square of the frequency. The product of the tiny energy per mode times the huge spatial density of modes yields a very high theoretical energy density per cubic centimeter.

From this line of reasoning, quantum physics predicts that all of space must be filled with electromagnetic zero-point fluctuations (also called the zero-point field) creating a universal sea of zero-point energy.

So again, I think that somehow the uncertainty principle can be used to find each individual wavelength's contribution to the vacuum energy, and the explanation for the lower energy between the plates is just that fewer wavelengths are allowed. I may be misunderstanding since I'm certainly no expert on quantum field theory, but that's the impression I get from articles like these.

 

[hellfire]

So I wonder, can the energy density between plates in the casimir effect go below the vacuum energy density by significantly more than 10^-9 joules per cubic meter? Does anyone know the equation for calculating the energy density between plates in terms of their size and separation? I found a bunch of pages that give an equation for the force (see here, for example), but I'm not sure how to translate this into energy density.

The force is inversely proportional to the fourth power of the distance between the Casimir plates:

$$F \sim \frac{hc}{d^4}$$

You can consider the energy as a negative work done by the vacuum (W ~ F d):

$$\frac{F}{A} = \frac{E}{V} = \rho_{casimir} \sim - \frac{hc}{d^4}$$

The cosmological term is about:

$$\rho_{\Lambda} \sim 10^{-120}$$

In Planck units. Between the Casimir plates one has:

$$\rho \sim \rho_{\Lambda} - \frac{hc}{d^4}$$

$$\rho \sim 10^{-120} - \frac{1}{d^4}$$

Therefore d must be less than 10³⁰ Planck lengths (10^-5 meters) to have a negative energy density for gravitational purposes. With this estimation one would conclude that every Casimir experiment deals with negative energy densities, as usual distances are about 1 micrometer.

 

[pervect]

I wasn’t referring to his musings; Fenyman’s Diagrams have lead to improvements, discoveries, and understands in working out and planning partial collisions.
Work through some of the mildly complicated ones, you don’t need a cited papers to find them. Once you understand them & where they are useful, the hard part is to imagine a way to workout the issues without backwards time. Maybe with a MWI/HUP concept but I’d find that hard to implement usefully as the diagrams already have been.

I don't do a lot with Feynman diagrams, so it's possible I'm missing something. However, if we consider a typical case, that of pair production, it doesn't seem to me that the "backwards in time" interpretation really adds anything to the diagrams. You have a pair of photons in, and you get an electron and positron out. While you can think of the positron as an electron going backwards in time, this is a matter of interpretation that doesn't seem essential - as far as I can tell it doesn't even make any experimental preditctions, it's really just as easy to consider a positron as a positron moving forwards in time than as an electron moving backwards.

As to the Casimir force—“ it is an actual observed force that is predicted and explained by QM as negative energy.” As it’s only explanation!
Therefore by using that explanation:
“we KNOW exotic energy & matter exists, period”.

Yes, because of the defintion of exotic matter, as matter that violates one of the assorted energy conditions in GR. The Wikipedia article is a little vague about which particular energy condition needs to be violated for one to have exotic matter - I think it's the strong energy condition, but my memory might be off, or the term "exotic matter" might have mutated into terminal vagueness while I wasn't looking.

Therefore by implication we know:
Negative gravitation even Wormholes must be real!

No, we don't know that wormholes exist for a fact. We know that some forms of exotic matter exist for a fact (we've seen lots of examples) - but we do not know for sure that wormholes do.

For the purposes of science fiction, though, wormholes are certainly a respectable topic, one which has some basis in serious physics, even if we do not at the current time have proof of their existence.

 

[LodeRunner]

The case of energy and time is a little different. It turns on they behave just like complementary observables, but because energy is an observable but time is not, the proof is different.

I don't see any need for negative energy to balance conservation of energy in quantum mechanics. energy is conserved up to observation in quantum mechanics. And just as there is no preferred frame in relativity from which you can see "what really happens", so there is no opportunity in quantum mechanics to see the unobservable.

Eh, I really don't see how this addresses any of my statements one way or the other. Do you agree with what I have read and asserted, that the Uncertainty principle regarding energy (wikipedia implies that is it energy vs. position, though I suppose it could be energy vs. time) says that we cannot know the precise value of a point in space, and that this gives rise to "virtual particles" with a mass or energy hovering around zero and the Casimir Effect?

I think that these things are fairly well supported by mainstream physics. Given that, I don't see how you can say that energy is conserved up to observation in QM. Casimir shows the extraction of kinetic energy from literally nothing, thus showing that "nothing" can contain positive energy. If that "nothing" doesn't also (at times) contain negative energy, then I don't see how you can say that energy is conserved.

 

[LodeRunner]

RandallB:

1. I don't care about wormholes at all. They may or may not exist, but they have no place in my ideas or assertions. Neither does backwards time travel.
2. I aim to be writing more of a "hard" sci fi story than pop sci fi.
3. I don't care if Casimir doesn't prove QM. I wasn't trying to prove QM; I was showing a consequence of it.
4. I don't really understand your objections to my argument, except that you seem to think it needlessly complicates things.

5. I *was* going to say Fenyman's time traveling electrons/positrons have nothing to do with what I'm saying, but I'm not sure anymore.

As I recall, Hawking radiation can be represented as one of two things: one particle of a virtual particle pair is trapped by a black hole while another escapes to freedom, OR a particle leaves a black hole by traveling back through time. Hawking himself said that both ways of looking at the phenomina were equally valid, and that either way satisfied the requirements of the second law of thermodynamics.

Not sure what this means for my ideas, but it could mean that Fenyman's time traveling particles are just another way of modelling the phenomina that arises from virtual particle pairs, which are integral to my ideas.

Anyway, GF wants to go on a moonlit picknick, so I'll have to reply to others later.

Sexy GF>physics.

 

[LodeRunner]

JesseM:

(ok, sexy girlfriend can wait 5 minutes damnit)

The central point I'm trying to make it that vacuum energy is necessary to preserve Uncertainty. If energy is decreased, then we may be too certain about the energy/mass value of virtual particles (I'm not sure which other value we're comparing this too; I've seen "position" and "time" proposed.) By changing vacuum energy into macroscopic kinetic energy, we can be more sure about the virtual particles because they have less energy (they are closer to zero.)

If the purpose of vacuum energy is to counterbalance impossible certainty of knowing that energy = 0 in a certain region of space, then reducing that energy through any means would seem to violate Uncertainty, and magically replacing it without any negative energy would seem to violate Conservation.

ttyl

 

[JesseM]

I think that these things are fairly well supported by mainstream physics. Given that, I don't see how you can say that energy is conserved up to observation in QM. Casimir shows the extraction of kinetic energy from literally nothing, thus showing that "nothing" can contain positive energy. If that "nothing" doesn't also (at times) contain negative energy, then I don't see how you can say that energy is conserved.

Did you read my last post to you? The whole point of vacuum energy is that empty space is not a pure nothing, that it contains field modes or virtual particles that contribute to some nonzero value of energy in space which contains no real particles, and the lower energy between plates in the Casimir effect is explained in terms of fewer wavelengths of the modes/virtual particles being allowed. I don't see why this in itself would require negative energy to insure conservation of energy, although hellfire's answer to my earlier question shows that it is apparently possible to get the energy between the plates to be negative if they are close enough (based on observational evidence which places an upper limit on the value of the normal vacuum energy, and based on the fact that the difference in vacuum energy between the space inside the plates and ordinary space can exceed this limit).

 

[pervect]

Energy vs time are the proper complementary variables, as Self-Adjoint says - in fact, time symmetry always gives rise to a conserved energy.

Energy *is* conserved in quantum mechanics up to observation - the Casimir force does not allow one to extract kinetic energy from nothing. The Casimir force can act as a battery, but not a particularly good one - it can't act as a perpetual motion machine.

Energy is not necessarily conserved in General Relativity - I think someone has already mentioned the sci.physics.faq on energy in GR. For completeness I'll give the URL anyway

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

Energy is conserved in GR when one has a static space-time (which generates a symmetry in time which as I mentioned earlier gives rise to a conserved energy) - it is also consered when one has an asymptotically flat space-time. The universe, according to current cosmological theories, is neither asymptotically flat nor static.

The lack of energy conservation for a general metric in GR does not AFAIK cause "perpetual motion". The issue arises only on cosmological scales, not in small volumes (like, for instance, the solar system). On the cosmological scale, for instance, one can take the energy density of the universe and multiply it by the volume of the universe in the CMB frame to get a number. This number, assuming standard GR (and not for instance some published but non-mainstream theory like Garth's SCC) is a decreasing number, not an increasing number.

[add]
I should add that GR's "problem" with energy conservation is independent of the cosmological constant. One finds non-conservation of the above number energy density*volume (which strictly speaking isn't defined as energy) even when the cosmological constant is zero.

 

[RandallB]

RandallB:
1. I don't care about wormholes at all. They may or may not exist, but they have no place in my ideas or assertions. Neither does backwards time travel.
2. I aim to be writing more of a "hard" sci fi story than pop sci fi.
4. I don't really understand your objections to my argument, except that you seem to think it needlessly complicates things.

virtual particle pairs, which are integral to my ideas.

Sorry I’d taken your interest in “reverse time dilation” as a need to get ‘backwards time travel’ into your story line.
Which I can see no “"hard sci fi story” foundation to base that on, only ‘pop sci fi’. In the pop sci fi area many of the unproven and currently “popular” ideas in physics (MWI etc.) would fit just fine as the “pop” in pop sci fi.

For moving forward in time bunches of “hard” approaches that could work.

For Backwards – I’m only suggesting any approach is ultimately going to be “soft”. So why go to a lot of effort to make it look “hard”; just pick a write-able pop sci soft approach of your own and make it an involving part of story.

So I guess I’m not getting what the point “reverse time dilation” has in a story line if not for another back in time angle.
---Something to do with VPs?

 

[JesseM]

Which I can see no “"hard sci fi story” foundation to base that on, only ‘pop sci fi’. In the pop sci fi area many of the unproven and currently “popular” ideas in physics (MWI etc.) would fit just fine as the “pop” in pop sci fi.

Wormholes are not "pop sci fi", they are perfectly valid solutions to the equations of general relativity. Many serious physicists, such as Kip Thorne, write papers on them. Black holes were also 'discovered' as a solution to the equations of GR long before there was any observational evidence for them (of course, it was known that there were natural processes that could probably form black holes, whereas physicists don't know of any natural process that could form a wormhole).

 

[LodeRunner]

Sorry I was gone for a while. I caught the flu pretty bad...

Did you read my last post to you? The whole point of vacuum energy is that empty space is not a pure nothing, that it contains field modes or virtual particles that contribute to some nonzero value of energy in space which contains no real particles, and the lower energy between plates in the Casimir effect is explained in terms of fewer wavelengths of the modes/virtual particles being allowed. I don't see why this in itself would require negative energy to insure conservation of energy, although hellfire's answer to my earlier question shows that it is apparently possible to get the energy between the plates to be negative if they are close enough (based on observational evidence which places an upper limit on the value of the normal vacuum energy, and based on the fact that the difference in vacuum energy between the space inside the plates and ordinary space can exceed this limit).

My point was that there is also a lower limit on vacuum energy as dictated by Uncertainty. This is the key to my assertion, and if I'm wrong on this point then "less energy than the surrounding space, but still positive energy" is perfectly valid and in fact much more elegant than introducing negative energy. However, I have always understood that virtual particles were a direct result of Uncertainty (please, if I'm wrong on this point, correct me!) This implies that there is a lower limit to vacuum energy, and it seems to me that this limit should be very close or equal to the average vacuum energy density. (Unless there is some other "reason" for vacuum energy that I am not aware of.)

If there is a lower limit, and there is a way to convert this energy into kinetic energy via the Casimir effect, then it seems to me that we have a conservation of energy problem on our hands. At this point, this is all purely QM so it doesn't necessarily matter if energy must be conserved in GR.

Again, if the kinetic energy of the two metal plates is not extracted from vacuum energy, where is it coming from? And if the vacuum energy lost is not replaced, how is Uncertainty preserved? And if the vacuum energy lost <i>is</i> replaced, where does the energy come from--how could it be replaced without violating conservation of energy <i>and</i> without introducing negative energy?

Re: RandallB

I should have used the phrase "Negative Time Dilation" instead of "Reverse Time Dilation" to avoid connotations of backwards time travel. Negative or reverse time dilation refers to speeding up the passage of time in a particular frame of reference. Normal time dilation shows how the passage of time in one frame of reference may be slowed down by the addition of gravitational fields or velocity (as compared to other frames of reference.) As far as I know there is no such thing as "negative velocity", but many mainstream physicists have speculated about the existence of http://en.wikipedia.org/wiki/Exotic_matter" , which it seems would exhibit negative gravity (and thus negative time dilation.)

Even if the Casimir Effect does not support negative energy (and I'm sure my logic regarding this is far from bulletproof), exotic matter is still a perfectly valid theoretical concept, just as black holes were before they were (indirectly) observed. "Science fiction" in which the characters use technology that we already have isn't really sci-fi at all. Surely you can see the difference between using mathematically proven properties of a hypothetical material that many mainstream physicists have discussed vs. inventing, say, a flux capacitor and ignoring the principles of Newtonian and Special Relativity ("88 miles per hour!"... as measured from what frame of reference now?)

 

[JesseM]

My point was that there is also a lower limit on vacuum energy as dictated by Uncertainty.

The quotes I provided earlier seem to say that uncertainty just allows you to determine the energy contributed by each wavelength of virtual particle/quantum field mode, and putting two plates next to each other eliminates some wavelengths, thus the energy is lower. This would seem to imply that the lower limit would be if you could eliminate every single wavelength, although I don't really understand why this would give a lower limit less than zero as hellfire's post seems to say--I could be understanding this wrong, maybe the total vacuum energy is not just a sum/integral of the energies of each wavelength. Does anyone know the answer?

Anyway, is this the sort of thing you're talking about when you say there's a lower limit on vacuum energy dictated by uncertainty, or do you have some different explanation in mind? Like I said before, I'm pretty sure that talking about the uncertainty in the position of a "point in space" is meaningless.

However, I have always understood that virtual particles were a direct result of Uncertainty (please, if I'm wrong on this point, correct me!)

I think there's a certain sense that that might be true, but I don't know if it's really that simple--after all, in ordinary nonrelativistic quantum mechanics the uncertainty principle still holds but there is no such thing as virtual particles, virtual particles only come in when you try to create a relativistic theory of quantum fields (like the electromagnetic field), as I understand it. Also, there's some subtlety over whether virtual particles should even be thought of as "real" or if they're more just like terms in a mathematical series used to calculate the probability of different outcomes--check out the sections on virtual particles in this FAQ (sections S3a - S3f), although I'm not sure all physicists would agree with the author's arguments there.

This implies that there is a lower limit to vacuum energy, and it seems to me that this limit should be very close or equal to the average vacuum energy density. (Unless there is some other "reason" for vacuum energy that I am not aware of.)

I'm still not following your reasoning--even if we accept that the statement "virtual particles are a direct result of Uncertainty", how does this lead you to conclude a lower limit on vacuum energy, and why do you think it should be equal to the average? Also, when you say "the average", do you acknowledge that the average vacuum energy between the parallel plates in the Casimir effect is different then the average vacuum energy in empty space, or are you assuming there can be only one average everywhere?

Again, if the kinetic energy of the two metal plates is not extracted from vacuum energy, where is it coming from?

Well, I agreed that this is the most likely explanation for where the kinetic energy of the plates is, although I can't be sure since I don't know much about this subject--it's conceivable there could be some form of potential energy that decreases as the plates get closer together. But assuming this is the correct explanation, then presumably the vacuum energy between the plates decreases by the same amount the kinetic energy of the plates increases, so energy is conserved.

And if the vacuum energy lost is not replaced, how is Uncertainty preserved?

Again, this is the part of your argument I don't get. Why couldn't the uncertainty principle itself dictate that the vacuum energy between the plates be lower than the vacuum energy in empty space?

 

[RandallB]

I should have used the phrase "Negative Time Dilation" instead of "Reverse Time Dilation" to avoid connotations of backwards time travel. Negative or reverse time dilation refers to speeding up the passage of time in a particular frame of reference.

But if your using "Negative or reverse time dilation" to speeding up time you have that already with simple SR. When “The Twin” returns home he sees a now much older twin. He has not experienced his own time slowing down how could he, time in his frame has remained normal. He has observed time in the reference frame of his twin dramatically speed up as she has grown much older than he. No need for anything more exotic to do that, already a very real part of reality.

So the question is – what are you looking for?

Do you have some exotic paradoxical affect in your sci-fi story line you would like to support though some supportable even if unproven current theory.
OR
Are you just casting about looking for a unique supportable exotic idea you can use to start a story line from.

Either way if you’re clear about what you’re looking for I’m sure you’ll get some creative feedback that is a little more on point.

 

[pervect]

But if your using "Negative or reverse time dilation" to speeding up time you have that already with simple SR. When “The Twin” returns home he sees a now much older twin.

No - you don't. Suppose you have an important job to do (saving the Earth, perhaps), and an impossible schedule - you can do the job, given enough time, but there isn't enough time.

If you get on a spaceship and fly around at near-light velocities, it will be even harder to make your scehdule. The absolute best you can do is to stand still.

If you had the ability to create a negative-mass Schwarzschild solution somehow, this could (in theory) help you meet that schedule.

 

[RandallB]

No - you don't. Suppose you have an important job to do (saving the Earth, perhaps), and an impossible schedule - you can do the job, given enough time, but there isn't enough time.
If you get on a spaceship and fly around at near-light velocities, it will be even harder to make your scehdule. The absolute best you can do is to stand still.

What do you mean “No – you don’t” ?
LodeRunner doesn’t need “backwards” Time; just fast time and your idea gives an example of how to use fast time in SR if you think clearly.

Emperor Ming the Merciless (need to go back in time to find that one) has given you the impossible task of (saving the Earth) by decoding the key number sequence required to safely disarm the bomb that will destroy the Earth and everything within a full light year. Easy to solve and confirm the correct key for the one and only input that can disarm it safely; except the complete solution requires 9 days and the bomb is set for 5 days! As you said taking your laptop with on your near light speed ship is no good. But sending your twin sister on the ship WITH THE BOMB will work. Unable to get a light year away in 5 days, she returns to Earth with the 5 days about to expire. And of course she observes from her (and the bomb’s) reference that you indeed have been ‘running on fast time’ and 10 days older you completed the solution yesterday, thus saving the world.
I’d give you the girl in the story but she’s your sister – at least she can legitimately lie about her age.

But on the topic of Backwards Time –
I don’t understand what people expect.
You roll a ball across a table it falls to the floor and rolls across the floor.
Now you transfer this experimental frame into an environmental frame where time runs backwards. What – the ball is to roll backwards on the floor jump up on the table and run backwards some more?? By what means of memory would it know to do that and not just fly up to the ceiling??
But then why would it ever go up just because negative gravity was created?? Isn’t it now a negative mass and the source of negative gravity is either a negative mass or acceleration? That means it would still fall down the stairs the ball is rolling towards.
Same deal with clocks, if you get time to move backwards a clock cannot tell. Be it a pendulum, spring, of oscillating quartz based clock. They are all designed to measure the change in time not the direction of time. The clock would be no more able to turn its gears backwards within their design, than it would be able to “keep time” with a cock on Earth as it traveled at near light speed. Only make calculated adjustments to what observations can be made of other reference frames. Change + or - will still only increase the measurement of time passing, and will not find a way to reconstruct the past, by ‘going backwards into the past’. Other than wishful thinking, science just isn’t going to work otherwise.

 

[JesseM]

What do you mean “No – you don’t” ?
LodeRunner doesn’t need “backwards” Time; just fast time and your idea gives an example of how to use fast time in SR if you think clearly.
Emperor Ming the Merciless (need to go back in time to find that one) has given you the impossible task of (saving the Earth) by decoding the key number sequence required to safely disarm the bomb that will destroy the Earth and everything within a full light year. Easy to solve and confirm the correct key for the one and only input that can disarm it safely; except the complete solution requires 9 days and the bomb is set for 5 days! As you said taking your laptop with on your near light speed ship is no good. But sending your twin sister on the ship WITH THE BOMB will work. Unable to get a light year away in 5 days, she returns to Earth with the 5 days about to expire. And of course she observes from her (and the bomb’s) reference that you indeed have been ‘running on fast time’ and 10 days older you completed the solution yesterday, thus saving the world.

OK, now suppose the bomb itself has an acceleration detector, so any attempt to expose it to any significant G-force will cause it to explode. Now you can't send your sister away and then back, because the turnaround will cause the bomb to explode. But if you can fly away to that negative-mass object and camp out near it for a while, you'll have time to solve the code and then return before the bomb's time has run out.

But on the topic of Backwards Time –
I don’t understand what people expect.
You roll a ball across a table it falls to the floor and rolls across the floor.
Now you transfer this experimental frame into an environmental frame where time runs backwards. What – the ball is to roll backwards on the floor jump up on the table and run backwards some more?? By what means of memory would it know to do that and not just fly up to the ceiling??

What do you mean by backwards time? No one is saying that there would be regions where your consciousness would be running forwards and external time would seem to be running backwards--if you travel through a wormhole, for example, I assume the second law of thermodynamics would still work fine while you are going through the wormhole, and when you exit the other side time will still be running forwards, but you'll just be in the past light cone of the time and place you entered the wormhole. This isn't because the past has been "recreated" by means of some sort of "memory", it's you who has traveled in a path that's taken you to a region of spacetime that's in the past of the one you departed. Here's how I put it on another thread a while back:

I hardly expect a controlled repositioning of all constituents of the Universe. It's self-contradictory.

That is not how time travel would work in general relativity. In both special and general relativity, you have to get rid of the idea of a single universal present, since these theories say that different observers have different views of whether two different events happened "at the same time" or not, and each observer's reference frame is equally valid. So instead you have to think of a single static 4-dimensional "spacetime" which contains the entire history of the universe; traveling back in time in this context means that an object's "worldline" curves back on itself and revisits a region of spacetime it already crossed through before.

Think of a block of solid ice with various 1-dimensional strings embedded in it--if you cross-section this block, you will see a collection of 0-dimensional points (the strings in cross-section) arranged in various positions on a 2-dimensional surface, and if you take pictures of successive cross-sections and arrange them into a movie, you will see the points moving around continuously relative to one another (in terms of this metaphor, the idea that there is no single universal present means you have a choice of what angle to slice the ice when you make your series of cross-sections). You shouldn't think of time travel as the points returning to precisely the same configuration they had been in at an earlier frame of the movie; instead, you should just imagine one of the strings curving around into a loop within the 3-dimensional block, what in general relativity is known as a "closed timelike curve".

 

[pervect]

What do you mean “No – you don’t” ?
LodeRunner doesn’t need “backwards” Time; just fast time and your idea gives an example of how to use fast time in SR if you think clearly.
Emperor Ming the Merciless (need to go back in time to find that one) has given you the impossible task of (saving the Earth) by decoding the key number sequence required to safely disarm the bomb that will destroy the Earth and everything within a full light year. Easy to solve and confirm the correct key for the one and only input that can disarm it safely; except the complete solution requires 9 days and the bomb is set for 5 days! As you said taking your laptop with on your near light speed ship is no good. But sending your twin sister on the ship WITH THE BOMB will work.

This won't work if the bomb can't be moved, as JesseM points out. If the Earth is being approached by an invading fleet, and the inventor needs more time to perfect his "kill-o-zap" death ray, this approach also fails.

If one has the capability of moving the enitire Earth, and if one can also get through the paperwork to get this process approved :-), and if the Earth can sustain the necessary accelration, what you say might have a certain amount of merit.

If one assumes that moving the entire Earth to relativistic velocities is not feasible (or if humanity has spread out to the galaxy, and one assumes that moving the entire galaxy at relativistic velocities is not feasible), the distinction between relativistic time dilation and its inverse becomes meaningful.

 

[RandallB]

This won't work if the bomb can't be moved, as JesseM points out.

What’s that got to do with it!
– obviously the bomb can be moved as I defined it to demonstrate the SR can produce “Fast Time”, making up an extra requirement doesn’t help demonstrate how SR works.
With no bogus extra requirement to insist that the entire Earth be accelerated– as you point out:

what you say might have a certain amount of merit.

Of course it has merit, Except I don’t see any “certain amount” of limitation to it – SR simply enough when applied correctly can produce “fast time” effects requested, in fact you cannot do the ‘the twins’ without producing it from somebody’s view.

 

[RandallB]

But if you can fly away to that negative-mass object and camp out near it for a while, you'll have time to solve the code and then return before the bomb's time has run out.

But only if this “weird space” moves you into to a past history of Earth’s reference frame while your time is changing long enough to solve the problem thus giving enough relativistic travel time to get back to Earth before the 5 days are up on Earth time.

What do you mean by backwards time? No one is saying that there would be regions where your consciousness would be running forwards and external time would seem to be running backwards--

Of course someone is – you just did so in the above!

 

[JesseM]

But only if this “weird space” moves you into to a past history of Earth’s reference frame while your time is changing long enough to solve the problem thus giving enough relativistic travel time to get back to Earth before the 5 days are up on Earth time.

Huh? You don't need to go backwards in time, you just need to have time pass at a different rate for you than on earth. If you can spend 20 days near the negative mass and then return to Earth and find only 5 days have passed there, you were never moving backwards in time relative to the earth, it's just that for every 4 seconds that pass for you only 1 second passes on earth.

 

[RandallB]

Huh? You don't need to go backwards in time, you just need to have time pass at a different rate for you than on earth. If you can spend 20 days near the negative mass and then return to Earth and find only 5 days have passed there, you were never moving backwards in time relative to the earth, it's just that for every 4 seconds that pass for you only 1 second passes on earth.

No problem I think we both agree that the fast time affect can be accomplished be speed SR or correct portioning in greatly different gravitational fields (GR) close enough that moving between those positions would be useful.

We just disagree on the implications of exotic matter, negative matter. To me they imply “backwards time”. It’s just one reason but not the only reason one that I don’t believe the exotic / negative matter idea is valid or useful, even theoretically. And that no “wormhole” of any size or type will ever be found to validate the concept.

So for me the idea that exotic matter, negative matter means “backwards time” is fine because none of them are any more real than a “flux capacitor” – and in this case just as unneeded. Since the writer only needs fast time which can be accomplished using SR or GR as we’ve shown.