# Is reverse time dilation posssible?

#### hellfire

JesseM said:
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):

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

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

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.

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#### pervect

Staff Emeritus
RandallB said:
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.

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#### 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 priciple 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

LodeRunner said:
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

Staff Emeritus
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.

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.

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#### RandallB

LodeRunner said:
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

RandallB said:
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...

JesseM said:
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" [Broken], 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?)

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#### JesseM

LodeRunner said:
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.
LodeRunner said:
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.
LodeRunner said:
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?
LodeRunner said:
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.
LodeRunner said:
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

LodeRunner said:
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

Staff Emeritus
RandallB said:
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

pervect said:
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

RandallB said:
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.
RandallB said:
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 travelled 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:
JesseM said:
CeeAnne said:
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; travelling 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

Staff Emeritus
RandallB said:
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

pervect said:
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

JesseM said:
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

RandallB said:
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.

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#### RandallB

JesseM said:
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.

#### pervect

Staff Emeritus
RandallB said:
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: 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.

SR cannot speed up somone significantly with respect to the bulk of humanity, unless the bulk of humanity can somehow be accelerated to relativistic velocities.

Speeding up the bulk of humanity is highly impractical, among other problems, such as the political issues of gaining its consent to the process.

The preferred frame of reference here is created not by physics, but by the choice of most people of where to live.

Note that a _very_ small speedup can be obtained by the fact that the bulk of humanity lives at the bottom of a gravity well. This is a GR effect, and is extremely tiny - it would not be useful for the stated plot purposes. Any large speedup with respect to the bulk of humanity is going to require some other mechanism (like the exotic matter hollow sphere).

#### JesseM

RandallB said:
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”.
If you're saying you could go backwards in time just by getting near some exotic matter, then I'm pretty sure you're wrong. Remember, any time physicists talk about travelling backwards in time they're doing it in the context of what's allowed under general relativity, and although it's true that exotic matter plays an integral role in certain spacetimes where time travel is possible like ones involving wormholes, that doesn't mean that GR predicts that just having a clump of exotic matter somewhere would allow you to travel back in time.
RandallB said:
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.
Do you believe in features of black holes that GR predicts but that there is zero experimental evidence for, like the idea that they have "event horizons" beyond which even light cannot escape? If so, why do you think one unobserved prediction of GR is more likely than another?
RandallB said:
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”
As hellfire said in answer to a question of mine earlier, the amount by which the vacuum energy between plates in the casimir effect can be less than the regular vacuum energy is significantly more than cosmological evidence suggests the regular vacuum energy could be above zero, which is good reason to believe that negative energy in the GR sense is possible.

#### daytripper

JesseM said:
I dunno if LodeRunner was talking about going backwards in time, I thought he/she was asking if clocks would speed up relative to distant observers when they got close to a negative-mass object, the way they slow down relative to distant observers when they get near a positive-mass object. If I'm understanding pervect's answer correctly, the answer is yes.
I'm no expert and I haven't read this entire thread so excuse me if I'm repeating an answer or making an ass of myself, but it seems that if you had negative-mass that repelled objects, rather than attract them, you would have space warping out instead of warping in, pushing instead of pulling. You're still constantly accelerating due to the warp in spacetime, reguardless of the direction. I'd say the gravitational time dialation would be the same in this repulsive field as in a planet's attractive field. Maybe I'm in over my head on this one but it seems correct, if this is wrong, could someone explain to me why?

#### JesseM

daytripper said:
I'm no expert and I haven't read this entire thread so excuse me if I'm repeating an answer or making an ass of myself, but it seems that if you had negative-mass that repelled objects, rather than attract them, you would have space warping out instead of warping in, pushing instead of pulling.
I don't know if "warping out" as opposed to "warping in" is really meaningful here--see my post #21 to nemosum:
JesseM said:
nemosum said:
I think Mortimer has a point. Picture space-time as the classic rubber sheet. if you drop a bowling ball in the middle the sheet will bend down, representing gravity caused by a large body of mass. Likewise we can picture this body of imaginary mass as a bowling ball being shoved up from underneath the sheet, resulting in everything rolling off of it, or "anti-gravity". But either way, the sheet bending down or up, it will take longer for a lightbeam to traverse that area of space due to the warping of the space-time fabric.
I think you're taking the rubber-sheet metaphor too literally--the orientation of the curved sheet in the higher-dimensional embedding space (a depression pointing 'down' vs. a bump pointing 'up') isn't relevant, it's only the curvature that matters. Also, although on an ordinary curved 2D surface a geodesic would be the shortest path between two points on the surface, in GR a geodesic is the path through curved spacetime with the greatest proper time (time as measured by a clock that travels along the path), which is not captured by the rubber-sheet metaphor. I don't know the details, but I assume when you do the math, negative energy would curve spacetime in a different way from positive energy...stable wormholes are said to require negative energy to hold them open, which wouldn't make sense if negative energy and positive energy curved spacetime in just the same way.
daytripper said:
You're still constantly accelerating due to the warp in spacetime, reguardless of the direction. I'd say the gravitational time dialation would be the same in this repulsive field as in a planet's attractive field. Maybe I'm in over my head on this one but it seems correct, if this is wrong, could someone explain to me why?
Presumably you have to solve the Einstein field equations using the assumption of a negative energy density to solve this rigorously. I'm not too knowledgeable about GR, but pervect is, and he's said on this thread that negative mass/energy would indeed cause this sort of "reverse time dilation" (ie time running faster instead of slower for observers near the negative mass). I dunno if there's any nontechnical way of understanding why this is true--pervect, any thoughts?

#### daytripper

JesseM said:
I don't know if "warping out" as opposed to "warping in" is really meaningful here--see my post #21 to nemosum: Presumably you have to solve the Einstein field equations using the assumption of a negative energy density to solve this rigorously. I'm not too knowledgeable about GR, but pervect is, and he's said on this thread that negative mass/energy would indeed cause this sort of "reverse time dilation" (ie time running faster instead of slower for observers near the negative mass). I dunno if there's any nontechnical way of understanding why this is true--pervect, any thoughts?
I suppose if pervect says so, it would be so. I don't understand why it would, but i'm in no position to disagree. haha. i just figured that "reverse time dialation" would require negative velocity and since velocity isn't vectored, it can't be negative... as far as taking the rubber sheet metaphor too far, i've never much cared for that metaphor, i've allways prefered to think of a 3d matrix with warped cells getting "smaller" as you get closer to the center of mass. that's what a spacetime warp would look like if we could see it in 3 dimensions, right?

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