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EDG
2007-Sep-02, 04:42 AM
Over on one of the rpg boards I read, there's a long convoluted discussion about how FTL travel wouldn't work, and I don't think I follow why.

The way I've always imagined it is that if you on a ship that jumped instantly to the nearest star, spent a week there, and jumped instantly back to Earth then everyone's clocks be going forward at the same rate - so a week (and two instants!) would have passed both on Earth and on the ship when it returned.

But apparently that's not how it works. It sounds like what they're saying is that if you were to instantly jump from Earth to a nearby star and then jump back again, you somehow can end up going back in time. The reason is something to do with reference frames (as in, my assumption would mean that you're creating a privileged reference frame where time is going at the same rate?) and going outside of lightcones, but I'm not sure I follow. I think someone there phrased it as "FTL, relativity, or causality - pick two".

Can anyone here explain why my assumption is incorrect?

Tim Thompson
2007-Sep-02, 04:50 AM
The problem is that, if special relativity (http://en.wikipedia.org/wiki/Special_relativity) is correct, then it is impossible to travel through space faster than the speed of light, as observed by anyone in an inertial reference frame (http://en.wikipedia.org/wiki/Inertial_frames). So your idea of "jumping instantly" is simply impossible.

publius
2007-Sep-02, 05:21 AM
Here is what I think they're driving at. It's not so much backward time, but causality. *Assuming the rules of SR apply*, that is we're in flat space-time, and you somehow jump instantly from point A to B, then there are some reference frames which will see event B occuring *before* A.

So from their POV, you get to B before you left A, and causality is out the window. (Or you much abandon the postulate that all reference frames are equally valid, and say the frames that see B occuring before A are not valid, but are seeing an illusion -- and this gets very deep, mind you.) That is where the "going backwards in time" thing comes from.

The speed limit of c insures that no frame will see effects occurring before causes.

Now, in curved space-time, things can work differently. Take the famed Alcubierre Warp Drive, which is a space-time that has a little "bubble" moving at a coordinate speed must greater than 'c'. You don't have the above causality problem because space-time is different from SR's background and when observers take the bubble into account, everything is okay.

Of course, that requires a few solar masses worth of negative energy, not to mention a lot of other crazy things such as how you would get it started. :) But it is a valid GR solution, so we do have "FTL, relativity, and causality". However, GR plays games with FTL there. That's a mere coordinate speed to an asympotic observer. Light itself goes faster than light in the bubble. :lol: Nothing can go faster than light itself goes in a region, but GR allows you to cheat a bit by speeding up light (according to some coordinates). So in that sense, we don't have FTL, and so have still just picked two, relativity and causality. :lol:

When one speculates about a "jump drive" one is talking about something that is impossible according to our current understanding. So, it's either impossible, or it's not and involves physics we don't know yet. If the latter, then, well, it would work somehow, and causality would not be violated because of the new physics that make it possible. :)

So basically, if you assume a "jump drive" is possible, you are saying SR no longer applies strictly, but is superceded by something else, in the same conceptual manner as the Alcubierre Warp Drive "works". So I guess I would agree with the "pick two" statement. New physics might just allow one to play more tricks with "FTL" or "relativity". But I'm fairly confident that causality is written in stone. :)

-Richard

astromark
2007-Sep-02, 05:48 AM
In order to accelerate any object to ( or near ) the speed of light It would require a very big push. Nothing we have come upon yet delivers such a energy output to do this. Cosmologists and other astronomy minded folk would like nothing more than to instantly star hop across this universe. Unfortunately we except that it may never be possible. Other more technically minded will leap to explain this more accurately than I can, or know how to.
Its as if the faster you try to go the greater amount of energy consumed in doing so. If you were to have no rest mass at all then this is not only possible its necessary. We note that those light photons if stationary are gone. Which is fascinating when you consider that it ( the photon ) may have been headed toward your eye for billions of years only to vanish completely as soon as you see it. You me and the machinery we would need does not have a zero rest mass. To accelerate a vessel of some tones would require Solar amounts of energy. We do not have a spare, so that idea is shelved...., and even so. Thats not quick enough. Just in this Galaxy to star hop around the local group of interesting object would take decades not years to accomplish.
In no time soon are we going out of this Solar system with anything other than our telescopes.
As Tim has said... Imposible...
The science fiction writers would have you believe that fiddling with antimatter or tachyon particles. Messing about with Dilithium cristals... sounds all very interesting but, In the real world none of it seems to bare fruit. We are stuck here on planet Earth for a few more malinia yet. Wishhhh<<<<< :)

EDG
2007-Sep-02, 07:46 AM
Whether FTL is actually possible or not isn't the issue, so we don't need to dwell on that.

What I'm asking is IF it was possible, then why would you get the weird "arrive before you started" effects? I guess I just don't get what these "frames of reference" actually are. Can we maybe go through it step by step, from the perspective of someone on Earth, someone on the ship, and someone outside both watching the whole thing - what would they each see? I still don't get how time isn't passing at the same rate (and in a forward direction) for everyone here - or at least why causality isn't being preserved.

And is this anything to do with the problem of being in "two places at once" - i.e. if you jump instantly to say Alpha Centauri and then used a super-powerful telescope to look back at Sol, then you'd still probably see your ship being built (because the light you're seeing is 4.3 years old)... even though you're actually sitting in your completed ship at Alpha Centauri?

cjl
2007-Sep-02, 08:40 AM
Exactly. Also, if you were going faster than light, the time on board the ship would go backwards (though it would not go backwards for everyone, just people on the ship - so you would arrive earlier than you started by the ship's clocks, but later than you started by other clocks).

astromark
2007-Sep-02, 10:48 AM
No. this is an exaggeration of the facts..as I understand them.
If it were possible to accelerate up to and maintain C as a velocity then time would not stop from your reference point on board this craft. Time would be advancing as you would normally perceive. The point of interest is only applicable if you return to your place of departure. You could arrive there shortly after leaving according to you. while the ground station you left would have recorded a much longer travel time. This subject has been thrashed out on these pages many times and, the outcome is always the same. Its about your point of reference. whose clock is right? All of them are.
Relative to your point of view. No you can not arrive be-for you left. Its against the law.

EDG
2007-Sep-02, 03:50 PM
I'm wondering if people are actually reading what I'm saying... If a ship instantly jumps from A to B then how can time go "backwards" on it? And if no acceleration up to (or beyond) c is involved then you don't need to worry about that either.

It's just a simple thought experiment: IF you had a ship that could instantly jump from A to B (say B is four lightyears away), spends a week at B, and then instantly jumps back to A, then how does time pass for those on the ship, for those on A, and for those outside?

eburacum45
2007-Sep-02, 04:36 PM
Someone with better knowledge of relativity will have to correct me when I go wrong, but this is the situation as I see it.

The problem with FLT has to do with simultaneity; two moving objects can never really be simultaneous with each other, because of relativity. This means you cant move instantaneously to Alpha Centauri (which is moving a many kilometers a second relative to you) in an 'instant', because there is difficulty defining a real simultaneous instant across such distances and differences in velocity.

To define a suitable instant to arrive at our stellar neighbour you would have to draw light cones (http://en.wikipedia.org/wiki/Light_cone) for you and Alpha Centauri- and the faster the relative speed of each location, the more skewed the light cones are. When two objects are moving close to light speed with respect to one another, spacetime becomes so skewed that an instantaneous jump of the kind you are describing (or a movement through space at a speed much faster than light) can often move you back into the past of the destination point. Jumping back would then send you back into your own past.
http://www.theculture.org/rich/sharpblue/archives/000089.html
Richard kindly let OA use a simplified version of his argument here

Now that doesn't mean that all movement at FTL speeds will cause timetravel paradoxes, but some of it will. Even the relatively small difference in velocity between Earth and Alpha Centauri might cause a tiny reversal of causality of a fraction of a second or more, although I am not sure about that. (But even a fraction of a second's travel back into the past could be used to seriously violate causaility, especially if you keep jumping back and forth).
Not all FTL travel leads to paradoxes- the trouble is, some of it does, and that leads to restrictions on how it could be used.

Here is a page about the restrictions on wormhole travel in OA (http://www.orionsarm.com/whitepapers/wormholes_and_causality.html), caused by similar effects- not all wormhole jumps are permitted, as it is fairly easy (as Kip Thorne has pointed out) to make an array of wormholes into a time machine.

EDG
2007-Sep-02, 07:24 PM
Ok, that's more useful :).

The problem with those explanations is that it assumes that there's a skewed lightcone involved, because two observers are moving. But what if there aren't any observers moving at sizeable enough fractions of the speed of light to skew the lightcones?

Figure three on Baker's page seems to be what I have in mind:

By itself, this single use of the ansible doesn't create a causality violation. If Bob transmits a signal back towards Alice using a conventional light-speed transmitter, she receives it a later time than when she signalled to Bob. Even if Bob re-transmits with his ansible, Alice receives the reply just a little after she sent out her signal.

That sounds like what I said earlier - the ship jumps, spends a week out there and then it jumps back. For all involved (on earth and on the ship), a week and two "instants" has passed, so nobody's arrived in anyone's past.

So as long as another inertial frame isn't involved, there shouldn't be a problem right?

astromark
2007-Sep-02, 07:35 PM
All a little irrelevant don't you think? This zipping across the space in an instant can never actually be possible. Nothing ever has or can or will ever do that. To sagest that time for an observer of something that can not happen is going to stop or move back is daft. Lets try and keep this forum for questions that are real. The what if... in this thread is fictional so the ATM might be a better place for this ? yes...

Disinfo Agent
2007-Sep-02, 08:06 PM
My understanding was that while relativity prohibits a spacecraft from gradually accelerating beyond (or until) light speed, theoretically there is no impediment to an instantaneous jump from point A to point B. The problem is that no one has come up with any plausible way of performing such instantaneous jumps. Wormholes are an intriguing and physically plausible idea, but they seem to create more difficulties than solutions.

grant hutchison
2007-Sep-02, 08:39 PM
The problem with those explanations is that it assumes that there's a skewed lightcone involved, because two observers are moving. But what if there aren't any observers moving at sizeable enough fractions of the speed of light to skew the lightcones?The problem arises just because it is possible for this to occur: you break causality right there, as soon as it is possible.

Grant Hutchison

EDG
2007-Sep-02, 09:15 PM
The problem arises just because it is possible for this to occur: you break causality right there, as soon as it is possible.

So basically the only way around this is to make it impossible to move so fast that the cones become skewed enough to send a message into the past?

What if it might be theoretically possible to do go back in time or send a message to the past, but in practice it just doesn't work? (e.g. you could try to send a message into someone's past, but if you do then your ship instead gets enclosed in an event horizon that pops up around it because the universe can't deal with that? That seems to be what happens when you get an object with a mass in an infinitely small space and infinitely high density after all, which is another thing that the universe can't deal with).

EDG
2007-Sep-02, 09:21 PM
All a little irrelevant don't you think? This zipping across the space in an instant can never actually be possible. Nothing ever has or can or will ever do that. To sagest that time for an observer of something that can not happen is going to stop or move back is daft. Lets try and keep this forum for questions that are real. The what if... in this thread is fictional so the ATM might be a better place for this ? yes...

Astromark - with all due respect I'd appreciate it if you just didn't post to threads that you don't find interesting, and if you did actually read what is said in the posts for once instead of rambling about some misinterpreted view of what you think the poster is talking about.

This is nothing to do with "ATM". I'm trying to get a handle on how relativity and reference frames prevent my thought experiment from working. I'm not saying that it should work or that relativity is wrong or anything like that. There's plenty of "what if" questions being asked on this board, because it's for Questions and Answers.

grant hutchison
2007-Sep-02, 09:44 PM
So basically the only way around this is to make it impossible to move so fast that the cones become skewed enough to send a message into the past?Yes. As soon as you can pop from here to there instantaneously, there will be a whole bunch of potential observers who would see you travelling rather less than instantaneously (not a big problem), and a whole bunch who would see you travelling in the opposite direction (big problem, since now the effect precedes the cause). If everything is restricted to less than lightspeed, causality never gets broken in this way, for any observer in any state of motion.

What if it might be theoretically possible to do go back in time or send a message to the past, but in practice it just doesn't work?Cosmic censorship. A take on the various general-relativity "time machines" that have been described is that perhaps they work, but can only ever produce internally consistent results: that you just can't rig up a system that will produce a paradoxical loop of causality.

Grant Hutchison

EDG
2007-Sep-02, 11:29 PM
Cosmic censorship. A take on the various general-relativity "time machines" that have been described is that perhaps they work, but can only ever produce internally consistent results: that you just can't rig up a system that will produce a paradoxical loop of causality.

So is that "censorship" basically the same as what I was saying about event horizons popping up around things that just aren't possible, except a bit less severe? Do we know if that censorship actually happens or not?

I just looked up CC on wikipedia and it does sound like what I was saying earlier - that the universe basically can't abide a singularity, so it wraps it in an event horizon so that it's not "naked" (this is basically what a lecturer told me on a relativity course I did about 12 years ago, most of which went over my head). So could trying to send a message back in time like this count as a singularity too?

grant hutchison
2007-Sep-02, 11:44 PM
I just looked up CC on wikipedia and it does sound like what I was saying earlier - that the universe basically can't abide a singularity, so it wraps it in an event horizon so that it's not "naked" (this is basically what a lecturer told me on a relativity course I did about 12 years ago, most of which went over my head). So could trying to send a message back in time like this count as a singularity too?Yes, "cosmic censorship" is the black hole thing. It seemed like you were invoking something similar for time travel.
I know nothing about that, apart from what I mentioned earlier. :)

Grant Hutchison

transreality
2007-Sep-03, 12:40 AM
A historian that could travel instanteously could trade distance, and therefore resolution, to look further into the past, but I can't see there is anyway he can actually affect it.

Ken G
2007-Sep-03, 12:56 AM
I don't think one needs to invoke causality to eliminate FTL in the context of relativity, the key postulates of relativity will do that already. Those postulates are that there is no fundamental difference in "what happens" as seen from two different frames (the differences are purely quantitative and can be transformed back and forth with no essential changes), and the speed of light is the same in all frames. If you apply these postulates to an observer zooming past another observer at a FTL speed, you get a violation. If each observer is emitting light, each will think the other's light is making a bow shock, like Cerenkov radiation or a sonic boom. But that's a fundamental difference-- light arranged in a shock formation versus light that is redshifted in various ways but the photons never overlap, never stack up. Different observers could not agree as to whether or not that was happening, in violation of the relativity requirement that physics work out the same in all frames.

astromark
2007-Sep-03, 09:42 AM
Sorry Ken but, I got told off by this poster for being real... No bow shock. This guy wants us to imagine instantaneous travel across the universe. So much faster than C as to be instant. I agree with every thing you just said...
A second point is he mentions worm holes. Now thats inviting me to leap back to print and respond.

From EDG., quote; -" Astromark - with all due respect I'd appreciate it if you just didn't post to threads that you don't find interesting, and if you did actually read what is said in the posts for once instead of rambling about some misinterpreted view of what you think the poster is talking about.;" end quote.

Good grief, don't hold back... I did not say I was not interested. I obviously am. I also think my ability to comprehend your post is not at issue here.
I was simply attempting to steer you away from this fiction stuff... You talk briefly of worm holes. We understand what you are saying but, You must except that I ( and others ) may not agree with you. The worm holes I am familiar with are on my lawn... I have no understanding of the cosmic type. Thats the sort of fiction I am talking of. So I try to avoid it. I do have an excellent imagination but, this is not the place for it. I note that in a thread I have been watching a moderator warned a contributor to except the views of others with more grace. I took that on board as good advice. I do understand what you are asking... I was just attempting to inform you of the facts. As I see them. That may not be as you do. That is fine . Is it not?.

My view is; That the time would record the event perfectly from each point of observation. As it is. From the ship it would be as if when you pushed that button, then in a instant you are at the destination. No time has passed.
From the point of departure. As you have prohibited travel time, accelerations and decelerations etc., Only the time spent at your destination would be passing. So on your return the clocks would be the same. I can not see any other explanation for this while the instant travel time is in use. If your craft were to cover this distance at a set velocity, say C, then it would be ... From your ship clock would record the actual trip. whatever it actually was. From your point of departure the time away would be very different. much more than your clock would show.
Is this what you wanted?

eburacum45
2007-Sep-03, 11:52 AM
Yes. As soon as you can pop from here to there instantaneously, there will be a whole bunch of potential observers who would see you travelling rather less than instantaneously (not a big problem), and a whole bunch who would see you travelling in the opposite direction (big problem, since now the effect precedes the cause). If everything is restricted to less than lightspeed, causality never gets broken in this way, for any observer in any state of motion.
Grant Hutchison

I'd not looked at it that way before; instantaneous travel creates a 'whole bunch' of potential observers who would see effect precede cause. It doesn't really matter if those observers exist; they exist as a potential, so it is as if they did exist. Otherwise there would suddenly be a cosmic prohibition on certain types of slower-than-light movement which didn't apply beforehand.

Delvo
2007-Sep-03, 02:07 PM
But the existence of observational positions from which effect would be observed before cause doesn't violate causality. If it did, then supersonic planes, funky odors you have to search for the source of by sniffing around for a while, and my friend's becoming pregnant before I had met her husband, would all have been paradoxical and impossible.

For one thing, causality only pertains to the order in which things happen, not the order in which information about them becomes available to observers

But more importantly, the observer in the Centauri system watching your ship arrive at Centauri and depart from Earth isn't observing effect and cause; (s)he's observing two separate effects, both of which did indeed come after their separate causes even from his/her perspective. First there's the light your ship shed on arrival, which did indeed arrive at the observer sometime after the ship arrived, and then there's the light your ship shed on departure, which did indeed arrive at the observer sometime after the ship departed.

grant hutchison
2007-Sep-03, 02:20 PM
But the existence of observational positions from which effect would be observed before cause doesn't violate causality.In what we're discussing above, the problem is not that the visual appearance is of effect preceding cause: it's that for some observers the effect does precede the cause. That actually, in their frame of reference, the effect occurs at an earlier time than the cause does. (You can imagine that they figure out the light travel time and allow for it, so as to get a handle on the real order of events, or that they have previously seeded the Universe with clocks that are synchronized in their reference frame, and need only refer to the clock they see next to a given event as it occurs.)
This problem arises because observers in relative motion disagree about which events are simultaneous with other events in other places.
If EDG_ drops a glass just before he makes his instantaneous transfer, and it breaks on the floor after the transfer, then there are states of motion relative to EDG_ in which observers would find the broken glass preceding the intact glass. Not just as a visual illusion, but as a real state of affairs in the Universe.

Grant Hutchison

EDG
2007-Sep-03, 05:19 PM
Hang on, now you're losing me.

This all seems to me to be very much like the Observer Effect in quantum physics... if you don't have anyone observing from a different frame of reference then why should it matter? Isn't the only thing that matters that Causality isn't being violated for the observers within the "instajump" frame, so if nobody outside is watching it then why does how they'd see it become a problem?

grant hutchison
2007-Sep-03, 06:29 PM
Causality isn't being violated for the observers within the "instajump" frame, so if nobody outside is watching it then why does how they'd see it become a problem?Because the Universe is full of frames of reference that will see causality being violated, if instajumps can occur. Suddenly it's not just simultaneity that is influenced by relative motion, but causality. And if you jump far enough, a velocity of just a few metres per second (relative to your departure and arrival points) will do the trick to reverse causality in the case of the dropped glass I mentioned earlier.
Meanwhile, in other reference frames, the same events will procede in the usual cause-to-effect sequence. The Universe will contain events that bear no definite causal relationship to each other.

So either you invoke some sort of cosmic censorship which requires that the participants in the instajump are utterly unable to interact with any reference frame that might see reversed causality, or you rewrite physical law globally in order to accommodate it to an acausal Universe. Either option is rather a problem.

Grant Hutchison

ASEI
2007-Sep-03, 06:45 PM
So from their POV, you get to B before you left A, and causality is out the window.

What's special about causality though, that it must be one of the ones we pick when figuring out our FTL frame?

Disinfo Agent
2007-Sep-03, 07:28 PM
Because the Universe is full of frames of reference that will see causality being violated, if instajumps can occur. Suddenly it's not just simultaneity that is influenced by relative motion, but causality.How far away from the jumper will those reference frames be, if the question makes sense?

grant hutchison
2007-Sep-03, 07:54 PM
How far away from the jumper will those reference frames be, if the question makes sense?Well, the reference frame is everywhere, really: it just needs to be moving at some velocity relative to the jumper's transit points. So an observer could be stationed in that moving frame so as to be passing right next to the jumper's arrival or departure, for instance.

On another matter: I think some people on this thread are wondering why this causality issue is a problem for the jumper; but that's looking in the wrong place. The worry is that it's a problem for the whole Universe if it contains such jumpers, because the presence of such jumpers (or conditions that permit such jumpers) implies that cause and effect are no longer distinguishable.

Grant Hutchison

grant hutchison
2007-Sep-03, 08:24 PM
BTW:
My reply to Disinfo Agent above shows how easy it would be to use the "instajump" as a time machine.
1) Jump some large distance across the galaxy.
2) After having arrived, accelerate on a vector pointing directly away from your departure point, so that your line of simultaneity drops into the past of your departure point. The farther you have jumped, the lower the change in velocity necessary to produce any desired magnitude of "time jump".
3) While in motion, jump back to where you came from. You have now arrived in your own past, moving at some velocity relative to the departure point. Cancel that velocity, and you are ready to prevent your own birth in some inventive way.

Grant Hutchison

EDG
2007-Sep-03, 08:51 PM
So either you invoke some sort of cosmic censorship which requires that the participants in the instajump are utterly unable to interact with any reference frame that might see reversed causality [...]

Hm... OK then, so let's say that we have to invoke cosmic censorship. Let's assume that doing anything that could actually screw up causality for anybody means that an event horizon pops up around the ship and all intents and purposes destroys the ship.

So is there any way that the ship can still jump while being unable to interact with any reference frame that might see the reversed causality?

For starters, let's take an extreme example - if the ship jumped to a star four lightyears away, and actually took four years to do the trip (say it's in another universe in that time), and then arrived at its destination, that wouldn't violate anything would it? Because then it's just taking as long to get there as it would have if it traveled at the speed of light in our own universe, right?

Or what if we flip this round - if you DID actually enclose the ship in an event horizon in order to do the jump then isn't it by definition cut off from any other external reference frame while it's within it? So if it enclosed itself in the event horizon, and then the whole thing (ship + event horizon) jumped, and then the event horizon was dismantled at the other end, could that work?

grant hutchison
2007-Sep-03, 09:10 PM
For starters, let's take an extreme example - if the ship jumped to a star four lightyears away, and actually took four years to do the trip (say it's in another universe in that time), and then arrived at its destination, that wouldn't violate anything would it?Yeah, that just seems like you're invoke a way of sneaking around the infinite energy requirement to travel at the speed of light.

Or what if we flip this round - if you DID actually enclose the ship in an event horizon in order to do the jump then isn't it by definition cut off from any other external reference frame while it's within it? So if it enclosed itself in the event horizon, and then the whole thing (ship + event horizon) jumped, and then the event horizon was dismantled at the other end, could that work?But this is just the same instajump isn't it? We know nothing of what happens in the instant of transit, but we can inspect events at each end of the journey and find (if we're in the right state of motion) that you arrive before you depart.

A compromise that I think would work would be if the jump took no time for those aboard, but there was a lag corresponding to light-travel time for the journey as measured by those at rest in the departure/arrival inertial frame.
It's only when the journey takes less time than light-travel time that you can find reversed-causality observers.

Grant Hutchison

EDG
2007-Sep-03, 09:20 PM
A compromise that I think would work would be if the jump took no time for those aboard, but there was a lag corresponding to light-travel time for the journey as measured by those at rest in the departure/arrival inertial frame.

It's only when the journey takes less time than light-travel time that you can find reversed-causality observers.

Hm. So the ship enters jump - according to those on board it then exits jump instantly four lightyears from its origin, but to an outside observer it arrives at its destination four years after it left Earth?

grant hutchison
2007-Sep-03, 09:29 PM
Hm. So the ship enters jump - according to those on board it then exits jump instantly four lightyears from its origin, but to an outside observer it arrives at its destination four years after it left Earth?Yes. At least, it would appear that way to an outside observer who was at rest in the inertial frame of the arrival and departure points. Other observers, in relative motion, might see the time lag differently, but no observer could find that the ship arrived before it departed. That gets rid of the time-travel and causality worries.

Grant Hutchison

EDG
2007-Sep-03, 11:17 PM
Yes. At least, it would appear that way to an outside observer who was at rest in the inertial frame of the arrival and departure points. Other observers, in relative motion, might see the time lag differently, but no observer could find that the ship arrived before it departed. That gets rid of the time-travel and causality worries.

Well that's something :).
But there's basically no other way this can work, right?

grant hutchison
2007-Sep-03, 11:28 PM
But there's basically no other way this can work, right?I dunno. I'm just rehearsing the usual objections and how I think they work.
I'm aware that serious thought by serious physicists has been applied to the prospect of causality violation, ever since Tipler's original paper suggesting it was possible to generate closed timelike curves under GR. I'm aware that some "benign" scenarios have been proposed. But I have no idea what they are or if they're applicable to your problem. :sad:

Grant Hutchison

grant hutchison
2007-Sep-03, 11:43 PM
If it were possible to use a GR time machine and somehow avoid causality problems, you could use it to shorten the elapsed time between departure and arrival on your interstellar journey. Leave home, zap back in time, continue to destination at a slower-than-light pace. You could then arrive at your destination shortly after you left home (in their reference frame), but would have aged hugely during the transit.

Roger MacBride Allen uses this approach in a couple of novels, with the crew frozen in transit, so that everyone feels like the journey takes just a few days.

Grant Hutchison

Ken G
2007-Sep-04, 02:08 PM
Yes, causality violations lead to paradoxes like preventing your own birth, and the concept of hypothetical observers has worked in all other aspects of physics and is commonly used. So Grant's right-- the logical flaw in FTL travel is the causality violation. Still, I point out that respecting causality is not a postulate of relativity, it's just something that the postulates of relativity have been set out to preserve (it seems like an even deeper principle than even Einstein's relativity, because Galilean relativity assumed that principle too). I think the key is that you can have all observers think one thing caused the other, or they can all think the other caused the one, but if you have some who think the causality went one way and the rest think the other, then the laws of physics are different in a qualitative way for the two groups. You could even have the same observer cross from one camp to the other, as in Grant's example. That's the violation of the spirit of relativity of any stripe.

John Mendenhall
2007-Sep-04, 05:08 PM
No dilithium crystals available yet.

DALeffler
2007-Sep-05, 02:17 AM
I always thought traveling back in time would be impossible because otherwise it would mean that there's a universe to travel back in time to...

If there's not a universe to go back to, then the only method of backwards time travel is to cause something to happen to the universe of the now - the entire universe of the now. We'd have to stop the expansion of the universe, reverse the expansion (as well as the arrow of time), stop that contraction, and then get the universe expanding again, all the while maintaining our own small section of spacetime outside of what we're doing to the rest of the universe.

Isn't that causality said a different way?

Why isn't causality a postulate?

Ken G
2007-Sep-05, 02:31 AM
I think what you are saying is that the principle of causality is deeply embedded in the concept of time, and how time progresses. I agree, but that doesn't necessarily by itself make it impossible to go backward in time. We can make the whole universe go in a big circle just by spinning ourselves around, so making it all go back in time could conceivably be possible. But it doesn't seem to be-- there seems to be a basic logic built into the universe that causality is part of. Note that individual particles may be thought of as going back in time (antimatter), but not observers-- the logic applies to observers, who cannot disagree on causality or reverse its meaning.

Why it isn't a postulate of relativity is simply because it isn't needed for that theory-- but the postulates of relativity do respect causality, as Grant has pointed out. I think that's because those postulates respect the qualitative similarity of the universe for all observers, which is a part of that big and mysterious logic of it all.

Exposed
2007-Sep-07, 10:48 PM
I think the "instajump" might be a little confusing. It kind of implies you have to get up to a certain speed and then slow down in an instant.

What about "instant teleportation"? Do the same rules of reference frames apply and possible time paradoxes still exist?

grant hutchison
2007-Sep-08, 12:00 AM
What about "instant teleportation"?As far as I'm aware, that's exactly what we've been discussing above, under EDG_'s name "instajump".

Grant Hutchison

EDG
2007-Sep-08, 01:19 AM
I think the "instajump" might be a little confusing. It kind of implies you have to get up to a certain speed and then slow down in an instant.

Well no, in my first post I was talking about "jumping instantly to a destination". If it's easier to think of it as "instant teleport" with no actual velocity change involved then yes, that's what I mean.

ASEI
2007-Sep-08, 04:20 AM
If each observer is emitting light, each will think the other's light is making a bow shock, like Cerenkov radiation or a sonic boom. But that's a fundamental difference-- light arranged in a shock formation versus light that is redshifted in various ways but the photons never overlap, never stack up. Different observers could not agree as to whether or not that was happening, in violation of the relativity requirement that physics work out the same in all frames.

Is that really a requirement being violated (something that invalidates the underlying math), or merely a description of the result of the scenario when relativity is applied?

I thought that simultaniety is already violated in relativity for STL speeds. Observers can't always agree on the order of events.

In any case, a violation of causality - is that such a big deal? Why the "need" for cosmic censorship? So your warp drive allows to to depart after you arrive in some frames - just factor it in to your scenario. Usually FTL also requires odd values for energy and entropy, such as the negative mass required in the Alcubierre drive, hinting at some sort of local relative inversion of thermodynamics.

Ken G
2007-Sep-08, 07:55 AM
Is that really a requirement being violated (something that invalidates the underlying math), or merely a description of the result of the scenario when relativity is applied?
Observations are always of paramount importance, so statements that seem built on theory or underlying math (like "the laws of physics are the same in all frames") are really extensions of something that is being said about the observations ("observed phenomena are equivalent in all frames to within translating the language of how the phenomena are described"). So relativity really comes down to answering, what is just a coordinate-language translation, and what is a fundamentally different observation. When you watch a movie backward, for example, you invert cause and effect in a way that is fundamentally different-- it looks undeniably "impossible" to you, and it is.

I thought that simultaniety is already violated in relativity for STL speeds. Observers can't always agree on the order of events.
Yes that's so, but simultaneity is not an observation, it is a construct that we form in our minds based on both observations and the coordinate system we have chosen to analyze those observations. The only way to really observe the order of two events is to be in the same place as those events, and if you are, they are causally connected in a definite way and no other observer will think otherwise. If you are not in the same place as the two events, their order becomes a construct of your coordinate system, within the constraint that their causal connection will still be agreed on by you and all other observers, including any that were at both places. In other words, you can only disagree on the order if you all agree they are not causally connected. That's the distinction between a "real difference in the observation" and simply a "different language translation based on coordinate system". I'm claiming the bow-shock effect is a real difference in observations, but a better example might be constructed.

In any case, a violation of causality - is that such a big deal? Why the "need" for cosmic censorship?
Perhaps not--those who argue on the existence of such censorship on the basis of "need" are doing natural philosophy-- those who simply observe that it is there are doing science, and if or when they observe it isn't there will be doing science again.

2007-Sep-12, 05:30 PM
publius

(Or you much abandon the postulate that all reference frames are equally valid, and say the frames that see B occuring before A are not valid, but are seeing an illusion -- and this gets very deep, mind you.) That is where the "going backwards in time" thing comes from.

The speed limit of c insures that no frame will see effects occurring before causes.

this is from a long way back in the thread but i thought i'd give an example.

imagine 2 points (ie: the 2 planets) on a line separated by 10 units distance.

we set an observer point at 2 units left of centre....so we have a distance of 3 units from the observer to the planet at left...and 7 units to the planet at right.

the pilot travels from the right planet to the left planet (commencing at T=0).

since the travel is instantaneous we can only see the pilot on one or the other planets.

and since the travel distance for light (to the observer) is shorter from one planet than the other, we will see a mismatch in the timing of events.

in the example given:

at T=3 our pilot appears on the planet at left (according to the observer)...but he also still appears to be on the planet at right.

finally when T=7 the pilot disappears from the righthand planet.

********************************************

if we reverse the example and have the pilot travel from the lefthand planet to the righthand planet?:

at T=3 the pilot will disappear from the planet at left (did he wink out of existance?)

finally at T=7 he appears on the righthand planet.

Ken G
2007-Sep-12, 09:09 PM
But seeing a pilot at two different places at once is not necessarily a violation of any causality-- it could be an illusion. For example, you could set a mirror 15 LY out in space, let the pilot fly between the planets at c/2 for 20 years, and then wait 3 more years for the pilot to appear to be on the planet for your observer. So after 23 years he appears on the new planet, but for 30 years he will continue to appear to be on the old planet, in the mirror. I think this must be what publius meant about it getting "very deep" when you try in a general way to distinguish an illusion from what is "really happening". The point is, the problem is not when you "see" the pilot in two places at once, it's when you infer, using your physics, that he actually is in two places at once, which requires you consider a moving observer. If you get that result, you either have a big philosophical problem of how someone can be in two places at once, or there's something wrong with the physics you used to infer that, or it just can't happen. We prefer option 3, so hope that's the right one.

2007-Sep-13, 01:11 AM
i agree...existance isn't an observer-based "double or nothing" affair.

Delvo
2007-Sep-13, 02:10 AM
...the problem is not when you "see" the pilot in two places at once, it's when you infer, using your physics, that he actually is in two places at once, which requires you consider a moving observer. If you get that result, you either have a big philosophical problem of how someone can be in two places at once, or there's something wrong with the physics you used to infer that, or it just can't happen. We prefer option 3, so hope that's the right one.We're starting with a problem that postulates that it's possible to disappear and immediately reappear somewhere else. In a universe where that is possible and the observer does not take it into account, the answer is simple: it's your second option as listed above. The observer expected that the traveller couldn't do something that the traveller actually could do, so the observer's idea of the laws of physics was wrong. If the observer used the correct laws of physics in a universe where the traveller actually did that, then the observer would understand what had really happened and not think the traveller was in two places at the same time.

tonyman1989
2007-Sep-14, 10:57 AM
Over on one of the rpg boards I read, there's a long convoluted discussion about how FTL travel wouldn't work, and I don't think I follow why.

The way I've always imagined it is that if you on a ship that jumped instantly to the nearest star, spent a week there, and jumped instantly back to Earth then everyone's clocks be going forward at the same rate - so a week (and two instants!) would have passed both on Earth and on the ship when it returned.

But apparently that's not how it works. It sounds like what they're saying is that if you were to instantly jump from Earth to a nearby star and then jump back again, you somehow can end up going back in time. The reason is something to do with reference frames (as in, my assumption would mean that you're creating a privileged reference frame where time is going at the same rate?) and going outside of lightcones, but I'm not sure I follow. I think someone there phrased it as "FTL, relativity, or causality - pick two".

Can anyone here explain why my assumption is incorrect?

Ftl travel is impossible because as you move towards the speed of light the enegry used to speed you up adds to your mass and then it takes even move mass to move you and then more enegry becomes mass.

We have tested this in particle accelerators, no matter how much enegry we put into the particle it can't go over the speed of light,

I believe the faster speed in a particle accelerator was 99.99999% the speed of light.

ASEI
2007-Sep-14, 01:07 PM
What if you jumped from moving slower than light to moving tachyon-like (negative proper time, losing energy to speed up, and all that jazz) then back to moving slower than light? Both are covered under relativity. We just haven't ever measured tachyons or anything like them in the real world. Like magnetic monopoles, unfortunately.

Delvo
2007-Sep-14, 01:42 PM
Your whole body/ship would have to act like a single particle; otherwise if your individual particles started acting like tachyons they'd fly apart immediately and you'd simply vanish. And there's no known way to make a whole object move like a single particle.

Ken G
2007-Sep-14, 02:46 PM
Yes, the difference between a particle and a macroscopic system is quite important. There really is no unique cause/effect relationship for any time reversible system, because the causes become the effects when you reverse time-- one light cone is mapped into the other. Individual particles obey CPT symmetry, so far as we know, so that's a little more complicated than time reversal but it's a similar idea. Thus there is no unique concept of cause/effect for individual particles. However, there is a more fundamantal principle, which is that there can be no cause/effect or effect/cause relationship between particles that are "spacelike separated", i.e., outside both light cones. That's the violation you'd encounter if you went FTL, including tachyons (which are pure science fiction because they violate this principle and nothing has ever been observed to do so). Thus we can easily interpret particles as going backward in time if we assert we got the parity of the universe wrong and the particles are also really antiparticles (CPT symmetry), but we should not be able to interpret particles as going FTL-- any observational evidence for tachyons would force the discarding of the principle of "locality", that spacelike separations are fundamental. (Also note that FTL is a misnomer, we really mean FTc, faster than c, because light can be slowed and things can go faster than light.)

For macrocopic systems, even the cause/effect effect/cause symmetry is broken, because the noisiness of the system means we have to start lumping classes of states together into essentially indistinguishable configurations, forcing us to interpret reality as indistinguishable actualizations of some probability distribution function rather than as some unique entity. When we do that, we lose time reversal symmetry because we find some classes of configurations are more likely than others, and less likely always evolve to more likely, not the other way around. That is what is meant by a definite "causality", and would be violated by FTL travel.

William
2007-Sep-14, 11:00 PM

Ftl travel is impossible because as you move towards the speed of light the energy used to speed you up adds to your mass and then it takes even move mass to move you and then more energy becomes mass.

What you say is correct. However, if matter could be changed (I am not say it could be, only if it could be) such that inertial mass does not change with velocity, conservation of energy holds. The object can move faster than the speed of light, the problem is not velocity but rather than the change in matter that occurs when matter moves faster. This affect in science fiction (I am not saying such a change is possible) might be called an inertial shielded drive.

The problem with attempting to rationally discuss these types of questions, (questions as to what is or isn't physically possible) is the weakness or limitation of the current physics models. As Galileo showed, a mathematical model (black box model, sort of curve fitting) can be used to make predictions without a physical explanation as to what is happening, or why something is happening. (Deep fundamental causes, at a subatomic level.)

It is my understanding that string theories, which are now called m-theories, are an attempt to define what space and matter are. M-theory is an attempt to develop a model that matches reality at a subatomic level (both space and matter). With a more advanced model, it might be possible to answer your question. At this point in the evolution of physics, it is my opinion, there is no answer to your question.

ASEI
2007-Sep-14, 11:52 PM
any observational evidence for tachyons would force the discarding of the principle of "locality", that spacelike separations are fundamental Right. But how do you express "fundamental" in math. That's what I don't get. Why are they fundamental, except for our current observations not including FTL objects? Why is the principle of locality fundamental, except as a description of a limitation of STL objects?

Ken G
2007-Sep-15, 12:27 AM
It is fundamental only in the sense that it is a powerful unifying principle that has been obeyed by all observations to date, sort of like a fundamental symmetry of nature. That doesn't mean it's true-- it just means it is a simplifying principle we'll keep in every context where it seems to apply, which so far, seems to be always.

Grand_Lunar
2007-Sep-15, 01:40 AM

What about warping space, so that you don't violate the limit of light speed travel?

This is what I commonly hear about FTL:
"Nothing can go faster than light in space. But there is no telling how fast space itself can go."

I also hear that new physics is mainly required for this.

I personally hope that however we manage it, it won't require antimatter. Antimatter just seems so dangerous and tough a stuff to use, not to mention construct.
My hope is a powerful thermonuclear reactor, fuled by hydrogen and deuterium. Just refuel at the nearest nebula or gas giant near you!

Ken G
2007-Sep-15, 02:39 AM
I think you can do pretty much anything if you are allowed to change physics. So far there's no evidence it's any easier to warp spacetime over a large enough scale to be interesting than to go FTL.

Grand_Lunar
2007-Sep-15, 02:47 AM
I think you can do pretty much anything if you are allowed to change physics.

Works for sci-fi. Not so well for real life, unfortunately.

So far there's no evidence it's any easier to warp spacetime over a large enough scale to be interesting than to go FTL.

This, then, would probably be the breakthrough we need. I'm not holding my breath for it, though.

Disinfo Agent
2007-Sep-15, 03:02 AM

What about warping space, so that you don't violate the limit of light speed travel?I thought that was what the OP was asking about...

Grand_Lunar
2007-Sep-15, 03:20 AM
I probably didn't look hard enough. It seemed only like stuff about standard propulsion being used. Maybe the OP can explain what he/she means.

eburacum45
2007-Sep-15, 07:27 AM
One way of jumping from one location to another instantaneously is via a wormhole. In an eariler post I suggested that using a wormhole need not cause problems with causality; many people remember that Kip Thorne has suggested that wormholes can be used for time travel, but it should be remembered that there are a whole set of cases where a wormhole does not lead to closed timelike curves (http://en.wikipedia.org/wiki/Closed_timelike_curve) and reversals of causality.

I have had it explained to me like this (bear in mind I don't really understand this stuff, so I'm only parroting here); space which doesn't have wormholes in it can be conveniently described as Minkowski spacetime (http://en.wikipedia.org/wiki/Minkowski_space)

If we introduce wormholes into this space, it is no longer Minkowski spacetime but it is instead multiply-connected. However it is possible for multiply-connected space to have no reversals of causality under certain sets of circumstances; so someone in this multiply-connected space would not be able to tell the difference if they did not actually encounter a wormhole.

It may be that there are wormholes (multiply-connected spaces) in our universe, left over from the extraordinary conditions of the Big Bang; if so, it is possible that no observers (even hypothetical ones) would observe a reversal of causality as a result of the existence of such 'holes. But then our universe would not strictly be describable by using the concept of Minkowski space

tonyman1989
2007-Sep-15, 10:41 AM
William

What you say is correct. However, if matter could be changed (I am not say it could be, only if it could be) such that inertial mass does not change with velocity, conservation of energy holds. The object can move faster than the speed of light, the problem is not velocity but rather than the change in matter that occurs when matter moves faster. This affect in science fiction (I am not saying such a change is possible) might be called an inertial shielded drive.

I'm sorry I don't see your point here are you saying that as you come close to the speed of light, you don't become more massive?

It is my understanding that string theories, which are now called m-theories, are an attempt to define what space and matter are. M-theory is an attempt to develop a model that matches reality at a subatomic level (both space and matter).

Yes, but there is no proof that string theory or M-theory is part of reality.

astromark
2007-Sep-15, 10:56 AM
Yes Tony there is no proof that this is any other than science fiction. As the Wormholes conversation suffers the same shortage of facts issue.

Delvo
2007-Sep-15, 11:21 AM
One way of jumping from one location to another instantaneously is via a wormhole.For a wormhole to work, the points you're coming from and going to must be near each other in some "higher" dimension even if they're far apart in the three we're familiar with. So even if you could create or find a macroscopic wormhole and survive going through it (remember that both ends of it are rather similar to black holes), either it would only get you from where you are to a few certain other places based on the universe's inherent shape in those other dimensions, or you'd have to be able to change the shape of the universe in order to get anywhere else.

Quantum mechanics does give us a few FTL/teleportational effects in nature. One is entanglement. Uncertainty itself pretty much requires it, which tunnelling is a manifestation of. We've even built some gadgets that take advantage of tunnelling, so you could say we're using FTL technology already. But nobody knows what fundamentally makes quantum FTL effects possible or how to manipulate them much.

eburacum45
2007-Sep-15, 11:42 AM
For a wormhole to work, the points you're coming from and going to must be near each other in some "higher" dimension even if they're far apart in the three we're familiar with.
It is difficult to imagine the geometry of wormholes, but I don't think that is really true, or if it is, I don't think it is really relevant. Two connected wormhole mouths can move with respect to one another in space but their internal dimensions do not change. Moving the wormhole causes no change in the shape of the metric of our universe, except in the respect that the wormholes probably resemble black holes in some respects, and are just as difficult, or as easy, to move.

it would only get you from where you are to a few certain other places I think that is the correct case; you can only go to the other end of the wormhole; but it is possible to move either, or both, ends of that wormhole in the same way it is possible (but difficult) to move a black hole.

tonyman1989
2007-Sep-15, 12:25 PM
Yes Tony there is no proof that this is any other than science fiction.

astromark

I know that what I was telling William I'm sorry if I didn't make that clear.

Ken G
2007-Sep-15, 01:38 PM
Quantum mechanics does give us a few FTL/teleportational effects in nature. Actually, it does not. No demonstrated physics allows information, let alone an observer, to travel FTL.
One is entanglement. Uncertainty itself pretty much requires it, which tunnelling is a manifestation of.That is a common misconception about both entanglement and uncertainty. Entanglement causes no surprises until the measurements on the entangled particles come into each other's light cones. There's no propagation FTL. Uncertainty only allows information to travel FTL if you use nonrelativistic quantum mechanics, so it would be wrong.

We've even built some gadgets that take advantage of tunnelling, so you could say we're using FTL technology already. Tunneling has to do with crossing a classically disallowed energy barrier, it does not permit FTL transport of information.
But nobody knows what fundamentally makes quantum FTL effects possible or how to manipulate them much.They cannot be manipulated at all, they are illusions. No real particle has ever been observed to travel FTL, and relativistic quantum mechanics never predicts that they can.

Disinfo Agent
2007-Sep-15, 01:58 PM

Ken G
2007-Sep-15, 03:13 PM
The interesting issue in that other thread is whether or not reality contains "nonlocal influences" or just doesn't obey "realism", but FTL propagation of information is not at issue-- that doesn't happen even in quantum entanglement, it's uncontroversial.

EDG
2007-Sep-16, 02:22 AM
I really, really wish that people would read the thread before they post. I've made it astoundingly clear from the very start that I know my question is "science fiction", and yet people keep coming along and "helpfully" explaining how what I suggest "can't happen" or implying that it's not worth answering because it's fictional. These are not useful contributions!

I've stated the parameters of the situation that I'm asking about, and that's what I wanted people to think about. Comments on how "possible" it is are irrelevant and a waste of time. By all means, discuss the hows and whys and wherefores of wormholes and/or how this could actually be made to work, but please don't waste posting space by telling us all how possible it is. I know that at the very least what I'm asking is incredibly difficult to actually do in practise (I hesitate to say "impossible" because for all we know there may be workarounds to relativity etc that we haven't discovered yet).

Ken G
2007-Sep-16, 02:42 AM
I rather wish you would read your own thread. Your question centered on why relativity and FTL travel ran afoul of causality, and was answered completely by publius in post #3, but for good measure, Grant Hutchison elaborated on the causality issue by underscoring how a moving reference frame is required to reach a problem. I felt the OP was all completely covered in posts #3 and #16, you might care to review those. After that, the thread rather evolved into the issue of why we should care if causality is violated, and what ramifications that might have. If you're not interested in that part, fine, but posts #3 and #16 still answer the OP. If there's still some issue you aren't clear on after that first page, perhaps it has to do with the erroneous information you received that a problem would appear even if you don't introduce any moving frames. That would not happen, you would not have gone back in time in the original frame, you were simply misled, or misheard, on that other forum. But I thought Grant covered all that pretty clearly in this thread, before it turned into all that unpleasant stuff you're not interested in.

EDG
2007-Sep-16, 03:31 AM

I did. I'm referring to posts #51 and #62. I know my question was largely answered by Grant (and a bit by Publius), but people are still chipping in saying that it's impossible or that they don't get what I'm asking, and I really can't see why they're doing that given all the previous explanations and clarifications.

Ken G
2007-Sep-16, 05:18 AM
I see, and indeed it's even a little strange for a post to refer directly to the OP after 50 odd posts have already been registered in a thread, though some were just seeking clarification. I thought you were talking about the direction the thread wandered, I misunderstood. And it's also true that I don't know if anyone directly dealt with your question around the misinformation you received that someone carrying out what is in the OP would show up farther back in time in the same frame that they left even without ever entering another moving frame, though you may have inferred that from Grant's explanation.

Disinfo Agent
2007-Sep-17, 07:13 PM
I know my question was largely answered by Grant (and a bit by Publius), but people are still chipping in saying that it's impossible or that they don't get what I'm asking, and I really can't see why they're doing that given all the previous explanations and clarifications.Lack of imagination.

Michael Noonan
2007-Oct-22, 08:22 AM
Lack of imagination.

You know the more I learn about physics the less I understand it and the more fun it becomes. Take this for instance Wiki here (http://en.wikipedia.org/wiki/Cherenkov_radiation) Čerenkov radiation it says is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. It must be good stuff because Russian scientist Pavel Alekseyevich Čerenkov won the 1958 Nobel Prize for it and you don't get them things in your standard packets of rice krispies.

It results when a charged particle, most commonly an electron, exceeds the speed at which light is propagating. So if the electron is now travelling as a particle at greater than the speed of light in that medium then by drawing energy from that particle wont actually slow it down but should in fact make it go faster still. Draw enough energy from it and transfer it to the vacuum of space and draw more energy from it and hyper accelerate the particle, normally an electron out the exhaust stack and voila, FTL drive.

Now lets look at a decent pulsed phase array and load some really good Čerenkov cannons aboard our Star Trek Enterprise.

Kirk ..... "Mr Čhekhov what sort of armament does that enemy vessal carry"
Čhekhov "They have standard sub light phazors Captain"
Kirk ..... "Mr Čhekhov lock Čerenkov cannons on that ship
........... and don't fire until they do"
Čhekhov "Čerenkov cannons locked on"
Kirk ..... "Very good, I want the primary array to show that they fired first
........... before we shot them out of existence before they could get
........... a shot off."
Čhekhov "Enemy wessel has fired and is now destroyed before getting
........... a shot off Sir"
Kirk ..... "Very good Mr Čhekhov, stand down blue alert.
........... Set course for the nearest Romulan star system".

OK I am wrong, I know I am wrong I just couldn't resist it :)

eburacum45
2007-Oct-22, 08:53 AM
If the Enterprise really did travel faster than light in our universe, and assuming that relativity still applies, the situation you describe would have been an everyday occurrence.
For example Uhura would have been routinely communicating with people who were in her effective past or future.

Michael Noonan
2007-Oct-22, 09:12 AM
If the Enterprise really did travel faster than light in our universe, and assuming that relativity still applies, the situation you describe would have been an everyday occurrence.
For example Uhura would have been routinely communicating with people who were in her effective past or future.

Good thing we didn't go for warped factor 4.1 then :lol:

Disinfo Agent
2007-Oct-22, 11:33 AM
So if the electron is now travelling as a particle at greater than the speed of light in that medium then by drawing energy from that particle wont actually slow it down but should in fact make it go faster still. Draw enough energy from it and transfer it to the vacuum of space and draw more energy from it and hyper accelerate the particle, normally an electron out the exhaust stack and voila, FTL drive.I think that would only be true if you could exceed the speed of light in a vacuum. You know, c.

grant hutchison
2007-Oct-22, 11:52 AM
ČhekhovHeh. :)
That hachek accent on Čerenkov has obviously caught your eye. If you're going to use it on "Chekhov", though, you need to spell it Čehov [sic], which is perhaps why Anton Chekhov generally manages to dodge the UN-approved transliteration of his name.

Grant Hutchison

Noclevername
2007-Oct-22, 04:36 PM
The interesting issue in that other thread is whether or not reality contains "nonlocal influences" or just doesn't obey "realism", but FTL propagation of information is not at issue-- that doesn't happen even in quantum entanglement, it's uncontroversial.

It hasn't been observed to happen. Always emphasize the importance of the limits of our observation! :)

John Mendenhall
2007-Oct-22, 04:44 PM
FTL makes for wonderful science fiction. Try Harry Turtledove's book "Going Home".

In the real world, see if you can figure out a way to keep the air from leaking out of a generation starship over the course of a few centuries.

Noclevername
2007-Oct-22, 08:33 PM
FTL makes for wonderful science fiction. Try Harry Turtledove's book "Going Home".

In the real world, see if you can figure out a way to keep the air from leaking out of a generation starship over the course of a few centuries.

Don't know what that has to do with FTL.

(Thought of an answer; You can't, so bring plenty extra.)

(Another one: Coat it with a 50-meter layer of molten metal and let harden. Test for leaks using 10x internal pressure. Repeat as many times as needed. Not a very practical solution, but it is technically possible.)

John Mendenhall
2007-Oct-23, 07:46 PM
Don't know what that has to do with FTL.

(Thought of an answer; You can't, so bring plenty extra.)

(Another one: Coat it with a 50-meter layer of molten metal and let harden. Test for leaks using 10x internal pressure. Repeat as many times as needed. Not a very practical solution, but it is technically possible.)

Notice that I didn't say impossible. But, if the hidden agenda in FTL is interstellar travel, what can we do with non-FTL starships? Retaining breathable air is one of many problems. All real world systems leak, including the tanks carrying the make-up air. Certainly your suggestion would greatly reduce the leaks, and if you made the layer of lead, it also cuts the radiation. Point is, there is endless imaginative and creative work to be done for interstellar travel in the real non-FTL world.

Michael Noonan
2007-Oct-24, 03:35 AM
Heh. :)
That hachek accent on Čerenkov has obviously caught your eye. If you're going to use it on "Chekhov", though, you need to spell it Čehov [sic], which is perhaps why Anton Chekhov generally manages to dodge the UN-approved transliteration of his name.

Grant Hutchison

Thank you Grant Hutchison,
I did find the letter hachek accent most interesting. So if the electron is already going faster than the speed that light can travel through the insulator, would removing energy from it get it to go faster still?

Just an idea but if it has already gone FTL in an insulator can it go faster, maybe even to where feeding it into a vacuum would have it still travelling faster than the speed of light for a vacuum :)

grant hutchison
2007-Oct-24, 07:38 AM
So if the electron is already going faster than the speed that light can travel through the insulator, would removing energy from it get it to go faster still?No. It's going slower than c.

Grant Hutchison

DyerWolf
2007-Oct-24, 01:57 PM
BTW:
My reply to Disinfo Agent above shows how easy it would be to use the "instajump" as a time machine.
1) Jump some large distance across the galaxy.
2) After having arrived, accelerate on a vector pointing directly away from your departure point, so that your line of simultaneity drops into the past of your departure point. The farther you have jumped, the lower the change in velocity necessary to produce any desired magnitude of "time jump".
3) While in motion, jump back to where you came from. You have now arrived in your own past, moving at some velocity relative to the departure point. Cancel that velocity, and you are ready to prevent your own birth in some inventive way.

Grant Hutchison

How does this allow you to move backward in your own time?*

If later this afternoon my wife and I were to InstaJump using my homemade spaceship/timemachine to HR 8210, (@150 ly away) to check out whether we needed to panic (http://www.newscientist.com/article/dn2311.html) - wouldn't it still be 2007 when I arrived? If I decided there was something to fear and in my panic fled using the vector you describe, and my wife - being smarter than me - pressed the InstaJump button to bring us back to here - wouldn't it still be 2007?

grant hutchison
2007-Oct-24, 02:41 PM
If later this afternoon my wife and I were to InstaJump using my homemade spaceship/timemachine to HR 8210, (@150 ly away) to check out whether we needed to panic (http://www.newscientist.com/article/dn2311.html) - wouldn't it still be 2007 when I arrived? If I decided there was something to fear and in my panic fled using the vector you describe, and my wife - being smarter than me - pressed the InstaJump button to bring us back to here - wouldn't it still be 2007?We can all agree on a date and time because we can all agree about what is simultaneous: I can synchronize my watch with yours if you give me a countdown and say "now" at some particular time. We can only do this because we are in the same place and moving at the same speed.
But if we are far apart, and in relative motion, we don't agree about when "now" is. Even once we've allowed for the inevitable delays as light travels between us, we can't reconstruct a common reference frame of simultaneous events.
Let's say, while we're separated and in relative motion, that you observe (after allowing for delay in light travel) that when it reads eight o'clock on your clock it also reads eight o'clock on mine. You find that our clocks are synchronized. But I find (and with equal validity) that when it reads eight o'clock on my clock it reads seven o'clock on yours. That disagreement is a fundamental implication of special relativity.
So if you could jump instantly to my location, you'd arrive at eight o'clock by my clock. (Because that clock reading is simultaneous with yours.) But if you then accelerate up to my speed and make the jump back, you find yourself at seven o'clock by your home clock (because that clock reading is simultaneous with mine.) Time travel!

Grant Hutchison

DyerWolf
2007-Oct-24, 03:41 PM
Hm. -- Working through this: Initially, that seemed more like an optical illusion (Edit: which I didn't recognize that you'd addressed...). My off the cuff response was:

It appears you've described a situation where you are one light hour away from me. If you told me to synchronize my clock to yours, and I look through my telescope at your clock and synchronize my 8:00 with the 8:00 I see on your clock - I think we're synchronized. However, when you look through your telescope at my clock and see it reads 8:00, your clock would actually read 10:00 (to which you might think "that twit can't synchronize anything...).

If to resolve the argument I InstaJump to you, I can see I'll need to wipe the egg from my face to synchronize my clock to yours - and that when I InstaJump back, my wife will wonder why my watch is now two hours fast.

Except you said relative motion. (Which I interpret to mean not moving parallel to one another)

So if I were to InstaJump to your position, I would retain the relative motion of my original vector and have to change vectors and acceleration to match yours (head starting to hurt here:doh::wall:). How then does the timeshift happen and it not be an optical illusion?

I guess one conceptual problem I have understanding Relativity is this: Even New Horizons and Voyager and our other deep space probes are operating within the reference frame of our solar system - and we can match clocks and account for the light-travel delay with those probes.

How different then must the relative motion be for the effect you describe to occur?

I presume that if we were to InstaJump between stars within the reference frame of our Galaxy we could agree on a "now." There is a "now" that coincides with a "now" at HR 8210 - isn't there?

grant hutchison
2007-Oct-24, 04:01 PM
Except you said relative motion. (Which I interpret to mean not moving parallel to one another)Yes, any situation in which the distance between us is changing with time: you could be moving away, towards, or on some diagonal path.

So if I were to InstaJump to your position, I would retain the relative motion of my original vector and have to change vectors and acceleration to match yours).That's right. We assume you stay in the same state of motion after your instajump. So you arrive next to me, but have to accelerate to keep up with me. As soon as you do that, your measurement of simultaneity matches mine: you arrived simultaneous with eight o'clock at home, but now you're simultaneous with seven o'clock at home. This is not an optical illusion; it's a real and fundamental aspect of the way the Universe is observed to work. If you now instajump home, you will arrive at seven o'clock. Then you need to decelerate to match the velocity of home again. But because this deceleration happens very close to home, the simultaneity shift is very much less. After deceleration, you end up still at seven o'clock, an hour before you departed.

I presume that if we were to InstaJump between stars within the reference frame of our Galaxy we could agree on a "now." There is a "now" that coincides with a "now" at HR 8210 - isn't there?There isn't. There is no "now" that can be agreed by any observers who are a) separated and b) in relative motion. Since HR 8210 is in motion relative to the Sun, there would be a disagreement about "now" between observers there and here. The faster the relative motion and the wider the separation, the worse the disagreement becomes.
We can agree a "now" as we shoot past each other in relative motion (assuming we occupy the same point at the same moment); and we can agree a "now" if we are far apart but not in relative motion.

Grant Hutchison

2009-Jul-23, 02:56 PM
After deceleration, you end up still at seven o'clock, an hour before you departed.Right in time to be arrested by time police.

Ken G
2009-Jul-23, 03:35 PM
Incidentally, I feel it is interesting to point out that the way grant hutchison is using the concept of "simultaneity" is a device for making a valid calculation. It is not, or should not be, interpreted as a physically real quality that can be tested with experiment. It is a way of coordinatizing spacetime, sort of like using x-y coordinates to talk about the motion of a cannon ball. There is actually "no such thing" as simultaneity, it is an arbitrary invention of the mathematics we use to do these kinds of calculations. What is real is the invariant outcome of the calculation-- all roads lead to Rome. Nevertheless, it is a useful construct, and that is why grant hutchison is relying on it. (My point is, I feel it is actually a conceptual flaw in special relativity that its language tends to elevate the concept of simultaneity to an ontological status of a "real thing" that actually has some demonstrable meaning for a given observer, but it has no such demonstrable meaning, it is completely arbitrary to within the general causality constraints that things that are simultaneous belong in the class of events that cannot affect each other.)

DrRocket
2009-Jul-23, 07:40 PM
Incidentally, I feel it is interesting to point out that the way grant hutchison is using the concept of "simultaneity" is a device for making a valid calculation. It is not, or should not be, interpreted as a physically real quality that can be tested with experiment. It is a way of coordinatizing spacetime, sort of like using x-y coordinates to talk about the motion of a cannon ball. There is actually "no such thing" as simultaneity, it is an arbitrary invention of the mathematics we use to do these kinds of calculations. What is real is the invariant outcome of the calculation-- all roads lead to Rome. Nevertheless, it is a useful construct, and that is why grant hutchison is relying on it. (My point is, I feel it is actually a conceptual flaw in special relativity that its language tends to elevate the concept of simultaneity to an ontological status of a "real thing" that actually has some demonstrable meaning for a given observer, but it has no such demonstrable meaning, it is completely arbitrary to within the general causality constraints that things that are simultaneous belong in the class of events that cannot affect each other.)

I think some care is necessary in such discussions.

There are two distinct 'relativities" involved. In special relativity you have the advantage of a global coordinate system in which to discuss "time" and hence "simultaneity". What changes with observers are their notions of "tiime" and "simulataneity" which are not invariant.

Then there is general relativity, in which you don't have a gobal coordinate system and in which special relativity appears as simply a local theory, an approximation that is valid only infitesimally. In that context it is rather difficult to talk about simultaneity, as there is no clear definition of either "tiime" or "space" applicable to distinct points in space-time.

It seems to me that we need to be clear of the context in which simultaneity is being discussed. In short, in order to speak precisely we need to be very clear as to the definition of the terms being used.

Ken G
2009-Jul-23, 08:09 PM
There are two distinct 'relativities" involved. In special relativity you have the advantage of a global coordinate system in which to discuss "time" and hence "simultaneity". What changes with observers are their notions of "tiime" and "simulataneity" which are not invariant.Yes, but all the aspects of "special" relativity of which you speak are done away with in general relativity. There is no question that general relativity replaces special relativity as the more fundamental and more demonstrable theory (by "demonstrable" I mean "built around an ontology that differentiates what is demonstrable from what is not"). Thus you are speaking about structural flaws in special relativity that are corrected in general relativity (even though they make no false predictions in the absence of gravity, they are awkward and untrue to the basic tenets of a good scientific theory). To wit, the artificial reliance of special relativity on one arbitrary coordinate system is replaced in general relativity (which is more or less the origin of the "general" moniker).

It seems to me that we need to be clear of the context in which simultaneity is being discussed. In short, in order to speak precisely we need to be very clear as to the definition of the terms being used.Simultaneity certainly has a definition in special relativity, that's not my issue. My claim is that the definition is not defining anything physically real, any more than my address says something physically real about my house. The nice thing about general relativity, independent of its ability to handle gravity, is that it is much more clear on what is an invariant aspect of physical reality that we should pay attention to, and what is an artifact of some arbitrary coordinate choice. Special relativity tends to use language that exposes a kind of stultification in regard to a single coordinatization, a dogma which is studiously avoided in most other areas of physics.