How accurate is the KSP interstellar Alcubierre Drive?

[Technical Ben]

The diagrams look like it's been fixed, but original example had time shifts backwards. If you are flying towards an event, it appears to have happened earlier. So both rockets need to be departing away from Earth. It still causes a "paradox", but honestly, time travel isn't an issue with real physics. The fact that you prevent event's cause using a CTC resolves itself trivially in field theory. Like many things in QM, it's easiest to understand from MWI perspective. All you did was create another branch in the many worlds. One that is, admittedly, a bit odd for anyone involved. But there are no fundamental contradictions in logic anywhere.

From what I understand (which may be off, but bear with me). Having consistent histories seems to be a fly in the MWI soup. As both parties could check history, and see it still matches up. Now if there tuely is many universes with many possible outcomes, there is no reason to prefer one history over another when checking. So they should theoretically have the same probability of finding a different history, as they do for branching into a new branch. (For example I can record an event, then check the same event again to see if I get a different result)

When I check for historic results, I don't get a new branch of the MW, I get the same one every time. Whenever I check, I am always born on the same day, I never find it "flipped" to a new one. However, in the example of travelling back in time, they branch into a new history, anything could change. This would seem to be a difference in the theory, which does not match observation.

Basically, we need to be very careful when drawing assumed realities from perfectly relevant and predictive calculations. Geo-centric calculations are perfect and fit the observations. They are elegant and predictive. They are not "correct" in the understanding of the construction of a solar system though. Similar with MWI and quantum effects so far calculated an observed. Especially until we find the transformation calculations to integrate special relativity (or replace it but in the QM world).

Thankfully, believing in Geo-centric theory would only cause problems if launching a rocket and risking your life on it. Similarly with MWI, it's not a big risk if we get it wrong (either for or against), however we need to admit to ourselves we do not know which way it will end up resulting in (pun intended! ).

On a side note, I don't see why each particle checking all future and past possibilities to see if there is a break in physics is not just as elegant and simple as MWI is.

Edited November 22, 2014 by Technical Ben

[K^2]

From what I understand (which may be off, but bear with me). Having consistent histories seems to be a fly in the MWI soup. As both parties could check history, and see it still matches up.

It's called entropy. The number of future states is nearly infinitely higher than the number of past states. That means that there is that much more branching into the future than into the past. This is basically the reason memory works in the first place, and why you can remember yesterday, but not tomorrow.

There should be some branching into the past, but it is by definition indistinguishable from random loss of information.

For the record, if you think you disproved MWI, you would have also disproved Quantum Mechanics in general, because all interpretations are mathematically identical. If there is something wrong with MWI, there is something wrong with Copenhagen Interpretation, and any other interpretation you can come up with. It would mean that QM is fundamentally wrong. Feeling lucky?

Now where did i say that time travel is impossible? I am only saying it's not possible in scenarios like doing some warp drive roundtrip ending up in your own past and place. Or scenarios where you would somehow magically warn of someone what is going to happen.

N_las has actually proven that it does work this way. I can confirm that with the exception of initial mistake with rocket's direction, his proof is correct. Your argument is, "No, I still don't think that's how it works." Since you don't seem to be capable of producing an actual proof, what exactly are your qualifications to say that it's wrong? Mine are graduate work in Relativistic Quantum Mechanics.

[Technical Ben]

"For the record, if you think you disproved MWI, you would have also disproved Quantum Mechanics in general, because all interpretations are mathematically identical."

Never tried to disprove MWI. But both Geo Centric theory and Aether theory fitted the mathematics and observations perfectly. Both described the systems beyond current observations incorrectly.

So QM describes the current observations perfectly. MWI goes on to suggest there are multiple universes we have yet to observe. Until we do (and have some conversations with them, how interesting THAT would be), I will read any interpretations with as much salt as those telling me "It's obvious the marks on Mars are trees and rivers". Our current observations are not adequate to say. So, I'm not a betting man, but I could be 100% certain I would draw winnings as QM is already not complete in it's theory. So it will be shown "fundamentally wrong" just as Newtonian mechanics are if we consider relativity instead (though it's not "wrong" within it's scope of observational predictions, jjust outside them).

As said, MWI branching to each possible possibility (so all exist simultaneously) or each particle/collapse checking every possible possibility (checking so no need to actually exist) are each as complex and eligant as each other, so I cannot favour one over the other until we make further observations.

As for time travel or FTL travel though, we have seen it is not possible. This is mainly because we are asking the wrong question. Asking to go "faster" when we do not understand what "fast" is. Those who have understood relativity, then realise these things converge. So just as it's impossible to ask "to get closer than being in the same position" we cannot go "faster than light". "Being in the same position" means the closest thing, and "the speed of light" means "the fastest speed". (Roughly speaking )

Edited November 23, 2014 by Technical Ben

[K^2]

Aether did not fit with many experiments. Michelson-Morley was specifically designed to disprove aether. The Geocentric model is a fair comparison, though.

Our current observations are not adequate to say.

Here you are making a mistake. It's not that we don't have information. We fundamentally cannot have that information. The interpretations are indistinguishable. Just like Geocentric view isn't wrong. There is absolutely no way you can say that "X is center of the universe." But having Earth go around the Sun rather than the other way around does make the math easier. Heliocentric model is simpler and more elegant. It is no more or less wrong than Geocentric model. They are equivalent.

The only way MWI will ever be proven wrong is if entire QM is wrong. If you trust predictions of Quantum Mechanics, you trust that MWI is an adequate description of the universe. And if you start arguing for distinction between "There are multiple worlds" and "Universe behaves in every way as if there are multiple worlds, but there aren't," you should be having that discussion with theologists and philosophers, not scientists.

The most important aspect of this is that Quantum Field Theory can look after time travel paradoxes in any interpretation. MWI just makes it easiest to visualize what is happening. It's easy to understand something showing up from alternative future and preventing that alternative future from happening than it is to work out how it even took place in other interpretations. If you'd like to get a feel for what it's like, try to work through the logic of Delayed Choice Quantum Eraser under Copenhagen. It works, but it's almost impossible to understand. In MWI, it is a trivial effect. So is Quantum Teleportation.

Whether you like the idea of many-worlds from philosophical perspective or not, MWI is a valuable tool, and it makes valid predictions. So if MWI is fine with time travel, real world is fine with time travel.

As for time travel or FTL travel though, we have seen it is not possible. This is mainly because we are asking the wrong question. Asking to go "faster" when we do not understand what "fast" is. Those who have understood relativity, then realise these things converge. So just as it's impossible to ask "to get closer than being in the same position" we cannot go "faster than light". "Being in the same position" means the closest thing, and "the speed of light" means "the fastest speed".

Total nonsense. Again, you are trying to look at it as a philosophical question. Next, you'll be telling me that Achilles cannot catch up with a turtle.

And we have observed FTL to be possible, because we have observed Universe expanding faster than light. Just like General Relativity predicts it should. Because GR states that light limit is purely local and does not apply globally. Which means you can travel faster than light with respect to a remote object, such as the place you are going to. And we see it actually happening, and it is what Alcubierre Drive does as well.

I'm sure it's fun to make blanket statements like that in other places, but until you can read and understand at least the Alcuberre's original paper, you really shouldn't be talking such nonsense on a science forum. It makes you look foolish.

[Aningaaq]

We have observed the universe expanding faster than light? How can such a thing be measured? Couldn't it be possible that the universe is expanding at the speed of light and the observable light just can't catch up because it itself cannot go faster?

[pellinor]

We have observed the universe expanding faster than light? How can such a thing be measured? Couldn't it be possible that the universe is expanding at the speed of light and the observable light just can't catch up because it itself cannot go faster?

In a curved space(-time), the whole concept of relative velocity only makes sense for objects which are at the same place. Say there are two houses 1km apart from each other, on a flat plane. Then, a mountain grows between them, so the shortest walkable path between them now is 2km long. Then the distance between the houses changed, but none of them did move. There is also no upper limit on how fast the distance can change with time. In general relativity, the 'line of sight'-distance behaves like such a mountain path, following the shortest way across a curvature landscape (that also changes with time).

The belief that there are regions of space whose light will never reach us is based on the assumption that space is homogenious. So we extrapolate that the 'relative velocity' of distant galaxies converges to the speed of light, but their distance does not converge to infinity. If space os homogenious, then behind that limit there must be more space similar to the observable universe.

Edited November 23, 2014 by pellinor

[Technical Ben]

Aether did not fit with many experiments. Michelson-Morley was specifically designed to disprove aether. The Geocentric model is a fair comparison, though.

Here you are making a mistake. It's not that we don't have information. We fundamentally cannot have that information.

In which case there is a fundamental problem with our theory.

Just as a theory allowing time travel within this universe would be fundamentally wrong, because we cannot allow causality to be broken (and many other things), a theory with no observable proofs is fundamentally not a scientific theory.

So we either have to propose a test (observe another universe, which should not be too difficult) or our theory is wrong. That is how I understand science. I'm not arguing from philosophy here, I'm arguing from science, saying to suggest anything else is philosophy.

We may be able to linguistically and theoretically describe "faster than light" or "backwards in time", but so far from observation, the real way the universe is constructed does not allow for such descriptions.

Total nonsense. Again, you are trying to look at it as a philosophical question. Next, you'll be telling me that Achilles cannot catch up with a turtle.

As far as I know, the agreed definition is that light takes the quickest and/or shortest distance between any two points. From the perspective of the photon the travel is instant. So we cannot get "faster than an instant". Are you saying my interpretation of the maths is wrong here, and instead philosophical?

We can instead ask to "go to before the event". But that is now a different question from asking to get there the fastest. There are other reasons we cannot go back in time, practicality is one. I could for instance break it down to a local event. Say I wish to go back 100 years and travel to earth 100 years from now. Lets take just one aspect of what is required. Theoretically physics is time invariant. I see no problem with that mathematically and physically. So let's try to send the earth back 100 years for me to travel to the past. I'll take just the first step. I need to send the earth backwards in it's orbit over 100 years.

Well, any KSP player knows, if I wish to just reverse the orbit of the earth, before even considering every elementary particle being pushed in reverse, I'm going to need a LOT of delta V. So yes, sending individual quantum systems backwards in time is trivial. Sending anything else requires more energy than can be applied to the system under our current understanding of physics.

Which is where my post is still on topic (honestly! ). To go back in time (thus travel faster than light) seems trivial and possible when observing things on the quantum level. As far as we can tell, things DO on the quantum level. However, while one particle or Quantum Superposition is travelling backwards, the rest of the universe is still travelling forwards. It's not practical to suggest making the entire solar system "flip" it's orbit, let alone the universe.

Likewise, MWI works with singular systems we force into a super position. To conclude all systems are in the same superposition, would be like me concluding all satellite orbit retrograde because I can make one orbit retrograde.

Edited November 23, 2014 by Technical Ben

[Guest]

Total nonsense. Again, you are trying to look at it as a philosophical question. Next, you'll be telling me that Achilles cannot catch up with a turtle.

Actually, Technical Ben is pretty close. There's no such thing as "faster than light", because light always moves "instantly" as far as geometry is concerned. We perceive this as it moving at a set speed because of geometry of the universe. c, if interpreted as a speed, can be exceeded in various ways by various things, but it's more than a speed. It's a fundamental geometric constant that defines how reference frames interact. The truth is, we've got serious difficulties when it comes to relativity. You can always go "faster", if you add more momentum, you'll reach your destination in shorter time. Period. However, beyond a certain point, you'll seem to be moving at near c for a stationary observer, and your time will start slowing down.

The problem indeed comes down to "what fast is". If you tried measuring speed as distance/travel time, you would seem to have traveled faster than light if the flight was done at near c. But you'd underestimate distance you traveled, due to Lorenz contraction. On the other hand, an outside observer doing that would measure your speed at being near c, but he'd overestimate your travel time due to time dilation. So which approach is correct? Turns out, there's no such thing as "correct time/distance". When everything is relative, everyday terms like "fast" and "far", "slow", etc. become useless. Even "moving", "simultaneous" and other such terms need clarification to work with relativity.

You mentioned universe expanding at FTL speeds, but that's merely because it isn't matter that is moving. What is expanding is the very fabric of space. Limits that apply to matter don't necessarily apply to space, because they're different entities. Alcubierre drive uses the same effect. That said, c is indeed a local limit, if you think of it as a speed, it can be "bypassed" with trickery such as Alcubierre drive.

That's what I like about it, in a nutshell. You're reshaping the universe so that the place you're in moves to the place you want to be in, all without moving an inch yourself. It isn't 100% accurate, but, IMO, gives the right idea.

[K^2]

We have observed the universe expanding faster than light? How can such a thing be measured? Couldn't it be possible that the universe is expanding at the speed of light and the observable light just can't catch up because it itself cannot go faster?

The universe is 14 billion years old and 90 billion light years across. Good place to remember intermediate value theorem.

In which case there is a fundamental problem with our theory.

Just as a theory allowing time travel within this universe would be fundamentally wrong, because we cannot allow causality to be broken (and many other things), a theory with no observable proofs is fundamentally not a scientific theory.

You are convoluting two different concepts, and you are completely wrong on both of them.

First of all, theory does not have to explain how things work. It has to make correct predictions. Quantum Mechanics is a Scientific Theory. It has multiple interpretations which imply different, but indistinguishable sets of fundamental reality. The goal of scientific theory is not to probe that reality, but merely predict what our measurements are going to be. And all interpretations make identical predictions.

Again, this is not uncommon in science. Quantum Mechanics is just the most blatant example.

Second issue is causality. Just because you can't imagine a world without causality, doesn't mean the real world has to follow your rules. Causality is routinely violated on micro-scale, and there is absolutely no reason to suppose that it has any fundamental properties on large scales. We just happen to live in a fairly flat part of space where things are pretty casual on scales we observe. That's all. You are simply projecting your expectations on the rest of the world, and the rest of the world doesn't work that way.

As far as I know, the agreed definition is that light takes the quickest and/or shortest distance between any two points. From the perspective of the photon the travel is instant. So we cannot get "faster than an instant". Are you saying my interpretation of the maths is wrong here, and instead philosophical?

Light does not follow the shortest path. You are stuck on high school explanation of refraction. It has very little to do with actual physics of light or relativity. Light takes path that exteimizes its action. So does anything else. For light, the action is proportional to the proper distance traveled. And that works out to be the local maximum in relativity, which happens to be zero for light, or anything else that is light-like.

The "instant" you are talking about is proper time. In flat space-time, proper time of FTL trajectory would, indeed, be negative. But we are not dealing with a flat space-time. There is a metric involved, and that metric is the reason why ship's own world line remains entirely time-like. In other words, it keeps accumulating positive proper time.

Again, you are arguing out of ignorance alone, and it would be more constructive if you asked questions about things you don't understand, rather than assert things that aren't true and keep trying to prove them.

However, while one particle or Quantum Superposition is travelling backwards, the rest of the universe is still travelling forwards. It's not practical to suggest making the entire solar system "flip" it's orbit, let alone the universe.

Likewise, MWI works with singular systems we force into a super position. To conclude all systems are in the same superposition, would be like me concluding all satellite orbit retrograde because I can make one orbit retrograde.

False analogies. Traveling backwards in time is nothing like changing directions. The conjugate quantity to time is energy, not momentum.

There is a proof for MWI though. It would be detecting or communicating (as in effecting physically) other universes (not individual systems) or as you rightly say, travelling backwards in time (though only to a different universe).

Again, ignorance. The whole point of superposition principle is that things cannot self-interact. Detecting something from another world would prove both MWI and QM wrong.

[Technical Ben]

MWI is fine for a mathematical calculation, no problem there.

It has multiple interpretations which imply different, but indistinguishable sets of fundamental reality.

In that case, we cannot prefer one theory/interpretation over the other. We cannot say "MWI is better/more accurate/solves this/allows FTL so reality does" etc. We have to admit it is just a tool, and we cannot use it to make predictions outside of observation. We have not observed FTL travel (of matter/energy within space) or time travel (on a universal scale). So unless we observe these two things, we cannot say MWI is correct in predicting them to be possible.

Second issue is causality. Just because you can't imagine a world without causality, doesn't mean the real world has to follow your rules.

Again, you seem to be mixing my personal beliefs with the request on a scientific fact. I have no problem with anything reality proposes or does do. However, if MWI is a scientific theory, it has scientific requirements. If MWI allows for any breaks that other theories do not (FTL/Time travel or anything else), only scientific experiment will prove it scientifically.

(Not sure what you mean by "exteimizes", no dictionary response here. )

The "instant" you are talking about is proper time. In flat space-time, proper time of FTL trajectory would, indeed, be negative. But we are not dealing with a flat space-time. There is a metric involved, and that metric is the reason why ship's own world line remains entirely time-like. In other words, it keeps accumulating positive proper time.

Again, you are arguing out of ignorance alone, and it would be more constructive if you asked questions about things you don't understand, rather than assert things that aren't true and keep trying to prove them.

I still know of no space time that allows for negative travel through time. So a description of this would help. It is not an argument from "ignorance" but an argument from the current presented evidence. As said, Newtonian mechanics fits it's scope of observation. So if more is needed, say to progress to relativity, then such observations need to be presented. Without observations that prefer MWI or allow for FTL travel, how can I progress to them?

False analogies. Traveling backwards in time is nothing like changing directions. The conjugate quantity to time is energy, not momentum.

Changing direction requires energy. Sending a particle "backwards" in time requires energy. Is that correct? We can create a system that would "evolve", from our perspective, "backwards" through time. All that is required for us to create such a system is putting the energy in the right places. AFAIK, QM allows for this, while classical experiments do not: http://en.wikipedia.org/wiki/T-symmetry .

Checking up on the Wiki, it suggests that yes, we can make certain quantum states evolve "backwards", but it is still not the same as sending it backwards in time. The illustration was to show how some things are similar (we can send the earth back in it's orbit to send it back to where it was 100 years ago) and some things are different (it would still evolve forwards in time/entropy). However, both ideas require massive amounts of energy to complete, and are thus "impractical" (even if we are talking about negative energy or negative time in the literal time reversal earth).

Again, ignorance. The whole point of superposition principle is that things cannot self-interact. Detecting something from another world would prove both MWI and QM wrong.

But then any interpretation of MW (that is, interpretation that says those universes exist and evolve on their own separate from ours/detection) is not scientific. It's no different from the theory "Time travellers live in London, it's just they all know how to stay invisible while watching us". That's not a scientific theory... it's not testable, it's not preferable over others either.

I'm also confused as to why making a detection would prove them wrong. Would it not just be an additional complexity to their theories? As Relativity was a new observation added to Newtonian mechanics? Would a new universe detected not just be added to MWI? Why would there be no universes small/close enough to also detect? It does not require "self interaction", could we not detect an object/qm superposition not in this universe, but that is in another?

Edited November 23, 2014 by Technical Ben

[Z-Man]

Ben, you misunderstand what the difference between MW and the other interpretations is. Let me give you an equivalent example:

We label our electric charges so that electrons carry negative charge and protons carry positive charge. That's the analogy for the Copenhagen interpretation. But we could just as well label the electrons positive and protons negative (and switch the charges of other particles around). That would be MW. Both yield the same predictions for all experiments if you take into account that some axes will be labelled differently. Either both views are right or both are wrong. You will not find an experiment that distinguishes between the two. It's just that for some problems, causality loops being one of them, MW turns out to be the better tool for understanding what may happen.

CNR, doubly relevant:

[K^2]

Technical Ben, you are still arguing on things that you don't understand. This conversation can be way more constructive.

In that case, we cannot prefer one theory/interpretation over the other.

We cannot prefer one interpretation over the other. So long as they are interpretations of the same theory. We can, and should, prefer one theory over the other, or they are the same theory. With me so far?

We cannot say "MWI is better/more accurate/solves this/allows FTL so reality does" etc. We have to admit it is just a tool, and we cannot use it to make predictions outside of observation. We have not observed FTL travel (of matter/energy within space) or time travel (on a universal scale). So unless we observe these two things, we cannot say MWI is correct in predicting them to be possible.

You start out with the correct statement, and then BAM go on to make a completely unrelated conclusion which happens to be false.

If MWI makes a prediction, Quantum Mechanics makes the same prediction. In some interpretations, it's just more work to demonstrate it. We only use MWI to get to the result faster. The result is the same.

Quantum Mechanics and General Relativity are both the most precisely tested theories we have. Nothing comes even close to the amount and precision of testing that was done on these two. General Relativity predicts time travel as a possibility. Quantum Mechanics predicts how conflicts are resolved.

If you don't think we can make predictions based on that, you do not understand anything about how science works, or why it is useful.

Again, you seem to be mixing my personal beliefs with the request on a scientific fact. I have no problem with anything reality proposes or does do. However, if MWI is a scientific theory, it has scientific requirements. If MWI allows for any breaks that other theories do not (FTL/Time travel or anything else), only scientific experiment will prove it scientifically.

Quantum Mechanics and General Relativity predict time travel and FTL to be possible. There is no theory that predicts otherwise. The way science works is that you'd have to prove that it's impossible in order to overthrow the standing theories. Not the other way around.

(Not sure what you mean by "exteimizes", no dictionary response here. )

Makes action to be extreme. Extreme in the mathematical sense. Action does not vary with minor variations in path when it is extreme. Usually, it means that action is maximum or minimum, but that's not a strict requirement. I'd take a glance at Principle of Least Action. In classical Mechanics, least action is the extremum we are looking for. Article talks about that at least briefly.

I still know of no space time that allows for negative travel through time. So a description of this would help.

The solution to a rotating black hole is the Kerr Metric. The feature you're looking for is closed timelike curves (CTC). Same article talks about naked singularity case, where there is no event horizon, and you can actually use such a singularity as a time machine by following a CTC trajectory.

These solutions are known to be unstable, so they will not exist naturally. Whether stable configurations with CTC exist is still being researched. We do know that stable configurations exist if we assume negative energy densities are possible. Hence the Alcubierre Drive, traversable wormholes, etc. They all have that as a requirement for a stable configuration. Of course, stability of the solution isn't exactly a strict requirement either, if we are looking for something artificially generated.

Changing direction requires energy.

Really? So how much energy is required to keep Earth going in Circles around the Sun?

Sending a particle "backwards" in time requires energy. Is that correct?

No, for the same reason. You just need space-time that's sufficiently curved to send it the "wrong" way.

We can create a system that would "evolve", from our perspective, "backwards" through time. All that is required for us to create such a system is putting the energy in the right places. AFAIK, QM allows for this, while classical experiments do not: http://en.wikipedia.org/wiki/T-symmetry .

You got yourself confused somewhere along that way, but I'm not untangling that mess. You should probably avoid anything with the word "symmetry" in it until you learn more about the significance of symmetries in physics. Start with Noether's Theorem.

Checking up on the Wiki, it suggests that yes, we can make certain quantum states evolve "backwards", but it is still not the same as sending it backwards in time.

You are confusing GR time, which is a coordinate, with Statistical Mechanics time, which is essentially an evolution parameter. They can both be used for time travel, but for completely different reasons.

Seriously, this is messy in physics. We need more terms than just "time". There are three or four completely different, only slightly related concepts we call "time". In relativity, they got it sorted with "map time" and "proper time". We need that for other "times".

I'm also confused as to why making a detection would prove them wrong.

That's because you need to learn the difference between theory and interpretation, then learn some Quantum Mechanics, and then learn how the interpretations arise.

Edited November 24, 2014 by K^2

[NERVAfan]

If the different interpretations of QM are indistinguishable within QM, is it possible that a future 'theory of everything' might make them not indistinguishable in the broader context? Or is which one is "right" forever purely a philosophical question?

[K^2]

If the different interpretations of QM are indistinguishable within QM, is it possible that a future 'theory of everything' might make them not indistinguishable in the broader context? Or is which one is "right" forever purely a philosophical question?

They are mathematically identical and are a part of a theory. Another theory can have its own set of interpretations, but it will not have any bearing on these, even if some of the interpetations sound like they are the same thing.

If I had to wager on future developments in Field Theory, I'd say that we are going to find that principle of superposition is bogus. It's too specific, and looks too much like linearization. So I'm guessing, and that is a conjecture, that we simply don't see the non-linear behavior at energies we have access to. In that case, the corrected, general field theory will have a distinction between a single history or multiple histories. But that won't prove or disprove MWI. Even if we find that due to decoherence or something else the worlds collapse to single history, it would still allow use of MWI to explain some local phenomena.

[Technical Ben]

"If MWI makes a prediction, Quantum Mechanics makes the same prediction. In some interpretations, it's just more work to demonstrate it. "

I don't buy that until it's demonstrated to me, sorry. How is it not the same as Newtonian being different than relativity? How is MWI the same as any other interpretation?

An observation might show all (or some of) QM to be wrong, but I still don't understand how it would effect all interpretations equally. Unless they are just different ways to calculate the same results. Just as we can find the angles of a triangle via different methods. However, as said, it would be wrong to consider reality reflecting our calculations. So if MWI suggests FTL and time travel are possible, I'd still consider it a great way to calculate Quantum Mechanics, but a poor way to describe current observations.

"Quantum Mechanics and General Relativity predict time travel and FTL to be possible."

I thought you said the requirement of negative energy, entropy and other things make these practically impossible?

Basically, it's possible for the universe to have produced a moon made of cheese (very unlikely, but still possible). However, observations are that it is not. As you said, time travel via spinning black holes is theoretical, but not achievable in nature.

"Really? So how much energy is required to keep Earth going in Circles around the Sun?"

Oh, come on. I was referencing asking for a moving object to be sent "back" using a practical means we have available, say a rocket. That would require energy. Sending the earth back with a gravitational body still requires us to move that body towards the earth.

Edited November 24, 2014 by Technical Ben

[K^2]

Unless they are just different ways to calculate the same results.

They are. They all give exactly the same result. Always. Like I said, it might be more work in one than the other. But all interpretations will give you the same answer. They make the same predictions, and they will either all pass or all fail any particular experimental test.

I thought you said the requirement of negative energy, entropy and other things make these practically impossible?

There is no known way that is practical. We don't know if there is a different, practical way to achieve these.

But more importantly, it's theoretically possible. That means, there are no fundamental contradictions with these things happening.

If we find a moon made out of cheese, we will be stunned with such a find. But it would not invalidate any of our science.

Oh, come on. I was referencing asking for a moving object to be sent "back" using a practical means we have available, say a rocket. That would require energy. Sending the earth back with a gravitational body still requires us to move that body towards the earth.

In this thread, we are fundamentally talking about curvature of space-time. FTL only works in curved space-time. Time travel is only knwon to work in curved space-time. Gravity is just a side effect of such curvature. My analogy is entirely fair. You are just trying to restrict the problem to simple flat space-time picture you have some understanding off. Time travel doesn't work in that simple picture. No sh*t. That's why we have to discuss more complicated situations.

[Technical Ben]

That's great then.

The interpretations are calculations on how to reach a required result. So just like calculating the area of a triangle? ( http://en.wikipedia.org/wiki/Triangle )

There are many different ways to reach the same results. However, those calculations don't allow us to make predictions outside of that calculation. We cannot conclude that "all triangles come in pairs" just because we use pairs of triangles to speed up our calculation of area. Can we? Is that a fair comparison?

Likewise, if MWI allows us to do great calculations, we still cannot conclude that it means reality also has these additional "worlds", can we? It only allows us to calculate some part of our observation. Is that fair? Or do we also conclude all triangles have a second mirrored triangle connected to them, which is just "unobserved" but provides the full triangles area and angles? Would that not be a fanciful idea?

We don't know if there is a different, practical way to achieve these.

Unknown unknowns are no more accurate than fiction. So how can we say KSP Warp drives are close to reality or not then?

But more importantly, it's theoretically possible. That means, there are no fundamental contradictions with these things happening.

As said, there is no theoretical impossibility to many things, however we only have one observed reality. I can conclude that it's theoretically possible to have a binary star system, however sol only has 1 star. It is theoretically possible for a pot of flowers to appear in mid air through quantum fluctuations, but we don't observe such happenings. It's also theoretically possible a teapot orbits a distant star, it's rather hard to push such a theory as indicative of further facts.

It's a "whole 'nother level" of theoretical possibility with time travel/FTL travel. As not only does it require improbable instances, but possibly "impossible" things such as infinite energy or negative energy. I'm not sure how much better a theory mixed with possibilities and unknowns is, compared to a theory only having possibilities that are known. Observations of CTCs or negative energy etc would all help such theories. Until then, their very much wishful.

Edited November 24, 2014 by Technical Ben

[Rakaydos]

The Earth centric model in kepler's day was becoming increasingly complicated to keep up with observations, as it had to model the known planets backtracking across the sky every so often. But the full complexity was a perfectly valid explanation- it's just that the kepler model gave all the same predictions significantly easier. Iccasionally you'll still see crackpots espousing the earth centric model, because if you throw enough complication into it, it can still be a valid interpretation- but the heliocentric model, Newton, and Relativity are simpler and provide precice results without fiddling with the models.

If I understand correctly, Many Worlds is a simpler interpretation, just as heliocentricity was, but not to the same degree. Is that correct?

[N_las]

The Earth centric model in kepler's day was becoming increasingly complicated to keep up with observations, as it had to model the known planets backtracking across the sky every so often. But the full complexity was a perfectly valid explanation- it's just that the kepler model gave all the same predictions significantly easier. Iccasionally you'll still see crackpots espousing the earth centric model, because if you throw enough complication into it, it can still be a valid interpretation- but the heliocentric model, Newton, and Relativity are simpler and provide precice results without fiddling with the models.

If I understand correctly, Many Worlds is a simpler interpretation, just as heliocentricity was, but not to the same degree. Is that correct?

The epicycles in the ptolemaic system only give approximatly the same results as keplers orbits. They give arguably different results. But the math behind MWI and copenhagen interpretation is identical. They can never give different results.

[Technical Ben]

Seems we have a similar argument as the "wave/particle" one.

We have wave-particle duality, that the reality is "neither" but "something else that is both". It would seem we have neither MWI or Copenhagen, but reality is something ahead of both of these. Possibly, as we found GR added to Newtonian, we will find what adds to QM. (Possibly that oh so desired Quantum Gravity?)

I still don't see why we cannot have MPI (Multiple Particle Interpretation) or MQSI (Multi Quantum State Interpretation) as a kind of mix of the two, where any group of Particles or Quantum system less than the total sum of the entire universe is under consideration.

PS, both Copenhagen and MWI have gaps in their theories/interpretations. So we definitely need some addition to them to complete the picture.

PPS, I still see it mentioned as a given the maths is the same between both. Is this a given?

Edited November 24, 2014 by Technical Ben

[Z-Man]

The triangle analogy is good for the kind of multiple worlds laymen usually think about: The world where you won the lottery last week, or the one where we all ride to work on dinosaurs. You will never interact or observe these worlds from where we are now. Therefore, you are free to consider them not real.

The multiple worlds physicists are concerned with usually do not differ so drastically. It's the world where the electron takes the right slot vs. the one where it takes the left slot. Those worlds all do influence the outcome of the experiment, the experience of the one doing the experiment. For pragmatic definitions of "real" (those where you ignore the possibility that you're a simulation of a brain in a jar dreamed up by some guy), they are as real is it gets.

PPS, I still see it mentioned as a given the maths is the same between both. Is this a given?

Yes. Well, depends on what you mean by 'maths'. MW is a² + 2 ab + b², Copenhagen is (a+². They look different, but the result is the same.

[Technical Ben]

" The world where you won the lottery last week, or the one where we all ride to work on dinosaurs. You will never interact or observe these worlds from where we are now. Therefore, you are free to consider them not real."

One of these things is a scientific consideration. One is not, which one follows?

1) I cannot even detect something via inference. I cannot even theoretically detect it ever. Thus it does not even exist, even theoretically.

2) I cannot even detect something via inference. I cannot even theoretically detect it ever. But I will consider it real anyway.

Giving the option "to sometimes consider it real" is still not scientific, is it? So I need a definite answer, is it "real" or not? If I say "we do not yet know" or "we cannot ever know", both answers are an admission that science considers those universes "not real". So is it just a part of the construction of our calculation?

If we are not talking about "worlds", but only "systems in a super position", this is more understandable (as it fits observations). However MWI still considers the branches continuous existence after an interaction/split/collapse (choose depending on interpretation).

"Yes. Well, depends on what you mean by 'maths'. MW is a2 + 2 ab + b2, Copenhagen is (a+b)2. They look different, but the result is the same."

Ok, thanks, so it is similar to the "area of a triangle" example.

Then both are fundamentally incomplete when talking about reality. That conversation is best left for me in another discussion as it's maths and stuff are off topic to FTL travel. But thanks for the heads up.

PS, I'll not need to apologise for the universe where an alternate me rides said dinosaur into your computer, trampling it into dust, as I'm certain the maths says said dinosaur riding me is not coherent or "real".

Edited November 25, 2014 by Technical Ben

[Z-Man]

One of these things is a scientific consideration. One is not, which one follows?

1) I cannot even detect something via inference. I cannot even theoretically detect it ever. Thus it does not even exist, even theoretically.

2) I cannot even detect something via inference. I cannot even theoretically detect it ever. But I will consider it real anyway.

None are scientific. The question of what is real is a philosophic question. IIRC, you never gave your criteria, so I assumed "parts of the model that influence the outcome in a significant way". Which I personally consider incomplete and too strict, here is why. The way the universe is expanding according to our current best understanding implies that there is a cosmological horizon: A "sphere" around us where everything inside the sphere can potentially still influence us and everything outside can not (assuming FTL is impossible) and also cannot influence anything inside the sphere. And locally, that horizon is sweeping inwards towards us with the speed of light. As I write this, maybe another star has moved from the inside to the outside. Ten minutes ago (in its proper time), it could still send light that could reach us, now it can't. There is no reason to assume anything bad has happened to it, yet if you only grant reality to things that can possibly be observed, it stopped existing. That does not sound right to me. I'd allow extrapolation of reality across space and time within reasonable bounds. If you don't, fine by me.

So is it just a part of the construction of our calculation?

Everything in theoretical physics is just that. We have models with no ambition to match reality exactly, just to reproduce experimental results and make predictions. Any science talk that implies otherwise is just shorthand because constantly saying "Our current best model, the one reproducing experimental results most accurately, for this system is..." is so much more tedious that just saying "This system is...". And quantum mechanics isn't even a model, it's a framework for models.

If we are not talking about "worlds", but only "systems in a super position", this is more understandable (as it fits observations). However MWI still considers the branches continuous existence after an interaction/split/collapse (choose depending on interpretation).

Well, when do the branches stop existing, then, and why? Same situation as with the star vanishing behind the horizon, really.

Then both are fundamentally incomplete when talking about reality.

Yeah, taking a purist scientific view, they don't aim for that. But here's a rule of thumb IF you want to attribute reality to the objects they operate on:

If you think reality is continuous, that nothing can go from existing to not existing without a good reason, then MWI is the "correct" interpretation. It keeps superpositions "alive" for as long as you care about them.

If you think you are real and by extension everything that can interact with you or interact with things that can interact with you and so on, Copenhagen is the one for you. The collapse culls off branches that do not matter anymore.

If you have to do actual calculations, you use whatever fits the problem better.

[Technical Ben]

An observation is real. Anything not observable (though indirect observation is allowed) is not "real". If I suppose little fairies make Cosmic Rays, and someone else supposes it's elves, then we don't have a scientific theory or real observation if we both suggest "they are invisible, we can never tell if it's fairies or elves".

AFAIK things outside the sphere of the observable universe do not exist. Just as things outside of my observable light cone do not (IIRC). Something being detectable in the past, but not in the future does not seem to break this requirement, so the future and past are ok, as they are observable at some point. Something never can be observed by definition (MWI etc) is not a scientific consideration or request.

For example, the second triangle in the calculation of the area of a triangle does not exist. We need it (in some interpretations of area) for calculating the triangles area. But it can never be observed (that is detected by an instrument). It is not part of reality, and it is not scientific to claim "the universe is made of invisible triangle providing the correct area distribution of the visible ones.

Someone suggesting MWI provides the correct probability distributions and correct observations for the universe is likewise not a scientific claim. Suggesting MWI provides us the calculations for the probability distributtions (it does not, but that's another discussion ), is a scientific and testable claim.

"Well, when do the branches stop existing, then, and why? Same situation as with the star vanishing behind the horizon, really."

Less than the sum of the universe. If this is not testable, then why is that a problem, MWI is not either, but we happily follow it? Joking aside, we have observations, and can claim "as far as we have observed" or claim "to n" or "to infinity". Copenhagen claims "as far as we have observed" and guide/field/particle theories claim "as far as n". Those are currently incomplete, but are testable (the size of a superposition system).

"If you think reality is continuous, that nothing can go from existing to not existing without a good reason, then MWI is the "correct" interpretation. It keeps superpositions "alive" for as long as you care about them."

Copenhagen looses the superposition at every point, and fails to keep it alive, but allows for an interaction. MWI keeps the superposition alive at evey point, but does not allow for an interaction.

We observe interactions and superposition, we have neither theory keeping to the required observations. MWI is no better, but at least equally useful. (or as in other theories, particles and waves)

[K^2]

*Sigh*. Nobody is saying that worlds of the MWI interpretation are real. Nobody cares if they are, either, because they are impossible to interact with by definition.

Again, the only use of a theory is its predictions. Quantum Mechanics has the most precise predictions out there. Quantum Mechanics predicts that time paradoxes get resolved in a certain way. MWI is merely the simplest way to understand these.

But if you are an armchair philosopher who has no grasp of what a theory is or what is the value of interpretations, let alone any actual scientific understanding, then it won't make any difference. You will not understand how any of it works, because all you do is use your own ignorance as a shield. Which looks really silly to everyone else, I can assure you.