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m74z00219
2009-Aug-11, 06:00 PM
If such a technology existed such as to launch a ship (by some special means) from one device (transmitter) to a another such device (receiver) almost instantaneously (only one planck time passes, but having nothing to do with light itself), how exactly would this cause causal paradoxes?

I've looked into it quite a bit using lots of Minkowski diagrams. There are of course frames where not only does a ship arrive before it left, but information about the arrival is received before the information about the departure.

While it seems strange that something that is causal could appear to happen in either order, it doesn't seem like there is an actual causal paradox. In the aforementioned set of frames, even once this information is possessed, there is nothing that can be done to cause a paradox.

More succinctly, it doesn't seem an FTL traveler in the special frame can do anything to affect the fact that he used FTL travel in the first place.

I don't normally get stuck on a topic like this and I know this is not science (no FTL anything): I just think such hypothetical problems illuminate the strength of a theory (special relativity).

Thanks everyone,
M74

korjik
2009-Aug-11, 06:20 PM
there would be certain frames where the ship would be in two places at once, and some frames where the ship would be nowhere.

Jeff Root
2009-Aug-11, 08:39 PM
Would that be a problem? The ship would never be in two places at once
or nowhere for those aboard the ship.

-- Jeff, in Minneapolis

undidly
2009-Aug-13, 08:58 AM
If such a technology existed such as to launch a ship (by some special means) from one device (transmitter) to a another such device (receiver) almost instantaneously (only one planck time passes, but having nothing to do with light itself), how exactly would this cause causal paradoxes?

I've looked into it quite a bit using lots of Minkowski diagrams. There are of course frames where not only does a ship arrive before it left, but information about the arrival is received before the information about the departure.

While it seems strange that something that is causal could appear to happen in either order, it doesn't seem like there is an actual causal paradox. In the aforementioned set of frames, even once this information is possessed, there is nothing that can be done to cause a paradox.

More succinctly, it doesn't seem an FTL traveler in the special frame can do anything to affect the fact that he used FTL travel in the first place.

I don't normally get stuck on a topic like this and I know this is not science (no FTL anything): I just think such hypothetical problems illuminate the strength of a theory (special relativity).

Thanks everyone,
M74

There is no causal paradox.
Time does not go backwards for FTL communications.
FTL of objects with mass is impossible for reasons of the required energy.

FTL coms just gets there quicker.

The paradox believers are too impressed by the math for objects with mass up to light speed.

Above light speed the math requires a division by the square root of a negative number.
And they believe the answer makes sense?.

cosmocrazy
2009-Aug-13, 12:35 PM
There is no need to go FTL for those on board, since no time would pass and no distance would be travelled as far as they are concerned, it would be instantaneous. FTL could be measured for the journey from another reference frame. Causality paradoxes only occur if you assume FTL means backward time travel within the same reference frame, or between different reference frames, which is what the math suggests in relativity.

Bynaus
2009-Aug-13, 12:43 PM
There are of course frames where not only does a ship arrive before it left, but information about the arrival is received before the information about the departure.

(emphasis mine)

So you do not let the FTL ship depart. What then?

korjik
2009-Aug-13, 04:41 PM
Would that be a problem? The ship would never be in two places at once
or nowhere for those aboard the ship.

-- Jeff, in Minneapolis

It isnt the aboard ship that is the problem. I think you could violate conservation of energy and conservation of momentum due to the ship being in two places at once to some and nowhere to others.

Not only that, but if c is still the speed of information, then if the ship does not teleport, but actually passes along the distance between origin and destination, you could end up with an information shockwave analogous to a sonic boom.

Jeff Root
2009-Aug-13, 05:35 PM
there would be certain frames where the ship would be in two places
at once, and some frames where the ship would be nowhere.



Would that be a problem? The ship would never be in two places at once
or nowhere for those aboard the ship.
It isnt the aboard ship that is the problem.
Thass what I said :p



I think you could violate conservation of energy and conservation of
momentum due to the ship being in two places at once to some and
nowhere to others.
Would that be a problem?

Conservation of energy is already a limited conservation principle.
It is relative and only applies locally. It seems to me that it and
momentum would have to be modified to account for FTL in any
case, even if things didn't appear in two places at once.



Not only that, but if c is still the speed of information, then if the
ship does not teleport, but actually passes along the distance
between origin and destination, you could end up with an information
shockwave analogous to a sonic boom.
Would that be a problem?

-- Jeff, in Minneapolis

cosmocrazy
2009-Aug-13, 06:48 PM
(emphasis mine)

So you do not let the FTL ship depart. What then?

But this is just what the math predicts since its assuming that backward time travel occurs due to negative values in the equations. We all know that in reality negative values have no meaning when dealing with physical scenarios. 2 - 4 = -2 this makes sense mathematically but if i have 2 apples i cannot take 4 away only 2, so in reality the answer is always zero.. The information therefore would be instantaneous both ways no quicker, so it would be impossible to act on it accordingly. It would just happen or it wouldn't.
FTL has no meaning in space/time in our reality.

Jeff Root
2009-Aug-13, 07:03 PM
I take a Super-Duper-Ultra-Hyper-Laser with me in my teleporter, go to
Alpha Centuri, find the Sun in the sky and beam a message back to Earth,
then use my teleporter to return. I was gone three minutes in my time and
three minutes in your time. Four years or so later, astronomers detect
my message from Alpha Centuri.

What's wrong with that? :)

-- Jeff, in Minneapolis

korjik
2009-Aug-13, 07:50 PM
Thass what I said :p


Would that be a problem?

Conservation of energy is already a limited conservation principle.
It is relative and only applies locally. It seems to me that it and
momentum would have to be modified to account for FTL in any
case, even if things didn't appear in two places at once.

I was talking locally. The problem with this discussion is that it requires a rather fundamental error in our understanding of physics to even be possible. That throws the possibilities out of whack.



Would that be a problem?

-- Jeff, in Minneapolis

I would rather not get reduced to my component quarks when some idiot flies to close at 10^9 c. It would make a good Mythbusters, tho. 'Next up: Can flying at FTL too close to a white dwarf cause a supernova!'

:)

Problem is we are too far outside of my knowledge base to know for sure what would happen.

Jeff Root
2009-Aug-13, 08:26 PM
(I'm actually not convinced that conservation of energy only applies locally.
That is the way it appears, but I haven't analyzed it sufficiently yet.)

-- Jeff, in Minneapolis

pzkpfw
2009-Aug-13, 08:48 PM
This is a Q&A thread. A question is asked and gets mainstream answers.

Arguing those mainstream answers or making ATM assertions is not to be done here.

Undidly has been suspended already for 3 days for doing this. Jeff Root please start an ATM thread on this topic (if you haven't already had one in the past) if you wish to argue this point further. Some of your questions are skirting very close to being assertions.

cjameshuff
2009-Aug-13, 09:04 PM
But this is just what the math predicts since its assuming that backward time travel occurs due to negative values in the equations. We all know that in reality negative values have no meaning when dealing with physical scenarios. 2 - 4 = -2 this makes sense mathematically but if i have 2 apples i cannot take 4 away only 2, so in reality the answer is always zero.. The information therefore would be instantaneous both ways no quicker, so it would be impossible to act on it accordingly. It would just happen or it wouldn't.
FTL has no meaning in space/time in our reality.

Two ships simultaneously fly past earth in opposite directions, one at 0.5c and one at 0.75c, synchronizing clocks as they pass. Earth pings them an hour later by an instantaneous communications device. When do they get the ping? Say the faster ship gets it first on their subjective timeline...52 minutes for the 0.5c ship, 40 for the ones at 0.75c. So, if they each replied, sending a message at that time by their own clock, they would be heard by Earth at the same time, right?

Say the 0.75c ship relays it to the 0.5c ship immediately. The two ships are moving at 0.9c relative to each other, have been since the beginning...neither of the ships nor the Earth has accelerated by any significant amount since the clock synchronization, each of them has every right to have considered itself at rest the whole time, with only the others moving. The local time of arrival at that ship is 17 minutes. If the other ship relays that back to Earth, how does it not arrive before it was sent?

Even simpler, just one ship and one planet. Relative velocity such that time dilates by a factor of 0.5. Again, no acceleration after they pass and synchronize. Each considers itself at rest and sees the other dilated by that amount. So at what subjective time does each see a message transmitted by the other at 60 minutes by their clock?

It's not a matter of saying "negative numbers don't make sense, so stop at zero"...everything falls apart if FTL communication is allowed. It's much worse than getting negative or imaginary values that don't make physical sense. The length-contraction and time dilation relationships...which are observed to be very nearly correct, at worst...no longer hold together.

korjik
2009-Aug-13, 10:30 PM
But this is just what the math predicts since its assuming that backward time travel occurs due to negative values in the equations. We all know that in reality negative values have no meaning when dealing with physical scenarios. 2 - 4 = -2 this makes sense mathematically but if i have 2 apples i cannot take 4 away only 2, so in reality the answer is always zero.. The information therefore would be instantaneous both ways no quicker, so it would be impossible to act on it accordingly. It would just happen or it wouldn't.
FTL has no meaning in space/time in our reality.

It is worse than that. There is a square root in that equation. That changes the result to a complex number

Jeff Root
2009-Aug-13, 11:23 PM
This is a Q&A thread. A question is asked and gets mainstream answers.

Arguing those mainstream answers or making ATM assertions is not to
be done here.

Undidly has been suspended already for 3 days for doing this.
Jeff Root please start an ATM thread on this topic (if you haven't
already had one in the past) if you wish to argue this point further.
What point? That the conservation of energy law only applies locally?
That it doesn't? Or what?

What do you think is the mainstream answer to that question?



Some of your questions are skirting very close to being assertions.
My questions have all been restatements of the original question.

-- Jeff, in Minneapolis

Sam5
2009-Aug-13, 11:26 PM
Has anyone here ever read Einstein’s original comments about FTL in his 1907 paper? Since Einstein invented the no-FTL-signal concept, it would be interesting to read someone’s explanation of his opinion on this subject.

m74z00219
2009-Aug-13, 11:36 PM
Two ships simultaneously fly past earth in opposite directions, one at 0.5c and one at 0.75c, synchronizing clocks as they pass. Earth pings them an hour later by an instantaneous communications device. When do they get the ping? Say the faster ship gets it first on their subjective timeline...52 minutes for the 0.5c ship, 40 for the ones at 0.75c. So, if they each replied, sending a message at that time by their own clock, they would be heard by Earth at the same time, right?

Say the 0.75c ship relays it to the 0.5c ship immediately. The two ships are moving at 0.9c relative to each other, have been since the beginning...neither of the ships nor the Earth has accelerated by any significant amount since the clock synchronization, each of them has every right to have considered itself at rest the whole time, with only the others moving. The local time of arrival at that ship is 17 minutes. If the other ship relays that back to Earth, how does it not arrive before it was sent?

Even simpler, just one ship and one planet. Relative velocity such that time dilates by a factor of 0.5. Again, no acceleration after they pass and synchronize. Each considers itself at rest and sees the other dilated by that amount. So at what subjective time does each see a message transmitted by the other at 60 minutes by their clock?

It's not a matter of saying "negative numbers don't make sense, so stop at zero"...everything falls apart if FTL communication is allowed. It's much worse than getting negative or imaginary values that don't make physical sense. The length-contraction and time dilation relationships...which are observed to be very nearly correct, at worst...no longer hold together.

@cjameshuff
I agreed with pretty much everything you said. My hypothetical explicitly allows for only one frame in which FTL is allowed. This, it seems, would preclude causal paradoxes.

@korjik
I agree, the breaking of the principle of conservation is no good, but I never asserted this to be corrected. I merely asked if there would be causal issues wrt the initial hypothetical.

korjik
2009-Aug-13, 11:50 PM
@cjameshuff
I agreed with pretty much everything you said. My hypothetical explicitly allows for only one frame in which FTL is allowed. This, it seems, would preclude causal paradoxes.

@korjik
I agree, the breaking of the principle of conservation is no good, but I never asserted this to be corrected. I merely asked if there would be causal issues wrt the initial hypothetical.

there are all sorts of causal issues with the violation of conservation laws

m74z00219
2009-Aug-13, 11:58 PM
i don't see how. even if you pretended that the ship was cloned (that is, pretend that's part of the process), they would rapidly become distinguishable.

On a more physical note, the quantum foam allows for the law of conservation of energy to be "broken" because of uncertainty. Let's say it's absorbed into the quantum foam.

m74

pzkpfw
2009-Aug-14, 12:21 AM
My questions have all been restatements of the original question.

Firstly, I wrote (my underline):


Some of your questions are skirting very close to being assertions.

I was giving a simple warning, that you were getting close to the edge - not saying you'd gone over the edge.

Secondly, when you end your scenario-that-you-think-"proves"-your-point, with:


What's wrong with that? :)

...it is quite clear you are trying to make a point, not "merely" asking (or restating) a question.

It would be unfair to suspend Undidly, but let you continue on the path you were on.

Finally, as rule 17 of the forum states: "If you disagree with a moderator action, then PM or email the moderator, a different moderator, or an administrator".

Your post arguing my warning was off-topic for this thread and adds to the mess. My need to give this further warning, further messes up the thread. Please don't do it again.

Van Rijn
2009-Aug-14, 12:37 AM
@cjameshuff
I agreed with pretty much everything you said. My hypothetical explicitly allows for only one frame in which FTL is allowed. This, it seems, would preclude causal paradoxes.


In your idea, is it possible for something in this special frame to move to, communicate with, or be observed from a "normal" frame? Or is it completely, permanently, cut off from our universe?

korjik
2009-Aug-14, 12:45 AM
i don't see how. even if you pretended that the ship was cloned (that is, pretend that's part of the process), they would rapidly become distinguishable.

On a more physical note, the quantum foam allows for the law of conservation of energy to be "broken" because of uncertainty. Let's say it's absorbed into the quantum foam.

m74

The problem wouldnt be the ship, but its effect on the rest of the universe. Causing an asteroid to slam into a planet would be not good.

Uncertainty does not allow conservation laws to be broken. It allows you to get around them in very small, limited ways.

cjameshuff
2009-Aug-14, 02:47 AM
@cjameshuff
I agreed with pretty much everything you said. My hypothetical explicitly allows for only one frame in which FTL is allowed. This, it seems, would preclude causal paradoxes.

Only if the "FTL nodes" are forbidden from communicating with anything in other frames. Otherwise, two ships can use radio with nearby FTL nodes and achieve superluminal communications with each other.

m74z00219
2009-Aug-14, 04:05 AM
In your idea, is it possible for something in this special frame to move to, communicate with, or be observed from a "normal" frame? Or is it completely, permanently, cut off from our universe?

For my idea, I would say that this frame is not different from any other frame than it allows FTL. Sure, there would be no restrictions of normal (read: real) communication and motion from this "special" frame to any other frame.




The problem wouldnt be the ship, but its effect on the rest of the universe. Causing an asteroid to slam into a planet would be not good.

Uncertainty does not allow conservation laws to be broken. It allows you to get around them in very small, limited ways.


Hi korjik, I can't quite get a handle on what you mean. How would this situation cause an asteroid to slam into a planet? Although improbable, I think QM allows for the slight possibility of a whole ship's worth of matter appearing for a plank time.


@cjameshuff
Well, it seems to me that this would not be necessary...as far as I can tell.
You're right about two ships moving relative to the FTL nodes being able to communicate. Though, I see no paradox. If there is a ship moving at relativistic speed relative to the "special" frame, it could send a signal out that would be received by one FTL node and transmitted to the other, but it would be in the same frame as the transmitting FTL node. Anyone on the other end could do nothing with the information to cause a paradox.

For god's sake, if I'm wrong, please show me. I'm really excited about this idea for sure!

mugaliens
2009-Aug-14, 04:06 AM
We all know that in reality negative values have no meaning when dealing with physical scenarios.

Exactly. Negative mass? Ain't gonna happen. Time is the same way.

korjik
2009-Aug-14, 05:04 AM
Exactly. Negative mass? Ain't gonna happen. Time is the same way.

I'll say it again. The value of gamma dosent go negative, it goes complex. It would lead to completely different behavoir

korjik
2009-Aug-14, 05:14 AM
For my idea, I would say that this frame is not different from any other frame than it allows FTL. Sure, there would be no restrictions of normal (read: real) communication and motion from this "special" frame to any other frame.



Hi korjik, I can't quite get a handle on what you mean. How would this situation cause an asteroid to slam into a planet? Although improbable, I think QM allows for the slight possibility of a whole ship's worth of matter appearing for a plank time.


@cjameshuff
Well, it seems to me that this would not be necessary...as far as I can tell.
You're right about two ships moving relative to the FTL nodes being able to communicate. Though, I see no paradox. If there is a ship moving at relativistic speed relative to the "special" frame, it could send a signal out that would be received by one FTL node and transmitted to the other, but it would be in the same frame as the transmitting FTL node. Anyone on the other end could do nothing with the information to cause a paradox.

For god's sake, if I'm wrong, please show me. I'm really excited about this idea for sure!

Maximum mass that can appear due to the uncertainty principle in 1 plank time is about a miligram.

E*t<hbar

E< hbar/t

m< hbar/(tc^2)

hbar~ 10^-34 Js
t ~ 10^-44 s
c^2 ~ 10^16

m< 10^(-34+44-16)=10^-6

since this is SI units m is in kg giving a mass on the order of a miligram.

a 1000 ton spacecraft would only last on the order of 10^-50s

WayneFrancis
2009-Aug-14, 05:36 AM
There is no need to go FTL for those on board, since no time would pass and no distance would be travelled as far as they are concerned, it would be instantaneous. FTL could be measured for the journey from another reference frame. Causality paradoxes only occur if you assume FTL means backward time travel within the same reference frame, or between different reference frames, which is what the math suggests in relativity.

Now that is what I used to think but I've seen a few explanations that say it isn't actually travelling back in time but travelling in imaginary time. IE the issue comes up where you are getting the √ of a negative number not that the √ of the number is negative. So it becomes more an issue of a dilation of i and not an actual negative dilation, travel back in time. What this means in the bigger picture I'm not so sure.

Can anyone point me to some more information along these lines? It seems to me that it is a problem with how the maths has been presented to the lay person and it became easier for them to say you travelled back in time because there is a negative value in the formula instead of the more accurate you travelled through imaginary time what ever that is.

WayneFrancis
2009-Aug-14, 05:43 AM
Two ships simultaneously fly past earth in opposite directions, one at 0.5c and one at 0.75c, synchronizing clocks as they pass. Earth pings them an hour later by an instantaneous communications device. When do they get the ping? Say the faster ship gets it first on their subjective timeline...52 minutes for the 0.5c ship, 40 for the ones at 0.75c. So, if they each replied, sending a message at that time by their own clock, they would be heard by Earth at the same time, right?

Say the 0.75c ship relays it to the 0.5c ship immediately. The two ships are moving at 0.9c relative to each other, have been since the beginning...neither of the ships nor the Earth has accelerated by any significant amount since the clock synchronization, each of them has every right to have considered itself at rest the whole time, with only the others moving. The local time of arrival at that ship is 17 minutes. If the other ship relays that back to Earth, how does it not arrive before it was sent?

Even simpler, just one ship and one planet. Relative velocity such that time dilates by a factor of 0.5. Again, no acceleration after they pass and synchronize. Each considers itself at rest and sees the other dilated by that amount. So at what subjective time does each see a message transmitted by the other at 60 minutes by their clock?

It's not a matter of saying "negative numbers don't make sense, so stop at zero"...everything falls apart if FTL communication is allowed. It's much worse than getting negative or imaginary values that don't make physical sense. The length-contraction and time dilation relationships...which are observed to be very nearly correct, at worst...no longer hold together.

I need to process this a bit.

WayneFrancis
2009-Aug-14, 05:51 AM
Exactly. Negative mass? Ain't gonna happen. Time is the same way.

I know we've never seen it but until we get a handle on gravity I'm not going to say that Negative mass does not exist personally. That said I'm not going to let "Negative mass" effect anything I do for the foreseeable future :)

Van Rijn
2009-Aug-14, 06:07 AM
For my idea, I would say that this frame is not different from any other frame than it allows FTL.

Sure, there would be no restrictions of normal (read: real) communication and motion from this "special" frame to any other frame.


Then you have the potential for causality violations, per relativity. Then again, if you have a "special" frame that allows FTL, you've broken relativity, so you can pretty much pick any rules you want for your fictional universe. :)

cjameshuff
2009-Aug-14, 12:58 PM
Well, it seems to me that this would not be necessary...as far as I can tell.
You're right about two ships moving relative to the FTL nodes being able to communicate. Though, I see no paradox. If there is a ship moving at relativistic speed relative to the "special" frame, it could send a signal out that would be received by one FTL node and transmitted to the other, but it would be in the same frame as the transmitting FTL node. Anyone on the other end could do nothing with the information to cause a paradox.

There's nothing special about that frame, though...you've only stated that they have to be in the same frame to have FTL communications. You can have multiple networks, each occupying a different inertial frame. If there is something special about that frame, then time dilation effects need to be due to velocity relative to that frame, etc...what you have is no longer any form of relativity. It conflicts with experimental observations, and I doubt it can be made to work even in theory.

cjameshuff
2009-Aug-14, 01:05 PM
I'll say it again. The value of gamma dosent go negative, it goes complex. It would lead to completely different behavoir

And again, it doesn't matter what the value of gamma is for superluminal objects. The positive, real values it has for subluminal objects are sufficient to cause problems when those objects can exchange information via superluminal channels. The problems have nothing to do with the amount of time experienced by the message.

chornedsnorkack
2009-Aug-14, 05:24 PM
If there is something special about that frame, then time dilation effects need to be due to velocity relative to that frame, etc...what you have is no longer any form of relativity. It conflicts with experimental observations, and I doubt it can be made to work even in theory.

NO. It does NOT contradict ANY experimental observation at all. And it can be made to work in theory.

This is, in fact, the Lorentz theory, that includes Lorentz contraction et cetera. In Lorentz theory, a preferred ether frame does exist, and ether interacts with objects moving in it in such a way that all experiments (STL) give exact same results no matter what the true speed is relative to ether.

Lorentz theory and relativity theory both work, and both give exact same predictions, consistent with all STL experiments. It is just that the Lorenz theory is more complex and incorporates a feature (ether frame) that is not needed in relativity.

If FTL exists then we are back at Lorentz theory. STL behaviour still fits all predictions of theory of relativity, and FTL depends on, and allows experimental observation of, ether wind. Proving Lorentz theory is like discovering that the Invisible Pink Unicorn actually is visible under certain limited condition.

m74z00219
2009-Aug-14, 06:03 PM
Well, Lorentz ether theory sounds ATM to me and that's not what I was getting at. I asserted from the beginning that my hypothetical has nothing to do with reality (as far as I know). Though the hypothetical happens to function the same LET, I was assuming some technological route that does the same thing (provides a special frame).

@cjameshuff

I think you're right about the multinetwork conflict, but I meant a single network operating in a single frame.

@Van Rijn
Could you please explain to me how a single frame allowing FTL would cause causal paradoxes. My whole thing is that it seems this would not be the case.

M74

Sam5
2009-Aug-14, 08:41 PM
@Van Rijn
Could you please explain to me how a single frame allowing FTL would cause causal paradoxes. My whole thing is that it seems this would not be the case.

M74


I’m curious about the same issue. Maybe someone can explain how a hypothetical FTL signal of some kind could “violate causality” if both of two emitters/observers are stationary relative to each other?

cosmocrazy
2009-Aug-14, 08:57 PM
I’m curious about the same issue. Maybe someone can explain how a hypothetical FTL signal of some kind could “violate causality” if both of two emitters/observers are stationary relative to each other?

Because the signal received in theory could be acted on before it was originally sent. The moving signal itself creates a different frame between the 2. If we said for example that the emitter's original signal was a deadly virus that was sent to destroy the receiver but some how she managed to avoid destruction she could send back a retaliation before the original threat was sent out, destroying the sender before he had sent his virus..... and so it goes on... my brain hurts now.
Anyway i don't believe in reality that this would happen, since i don't believe FTL is possible because its not necessary, I don't see how imaginary time fits into reality. I will stick to my view until i'm suitably convinced otherwise.

pzkpfw
2009-Aug-14, 09:05 PM
I’m curious about the same issue. Maybe someone can explain how a hypothetical FTL signal of some kind could “violate causality” if both of two emitters/observers are stationary relative to each other?

Van Rijn pointed out the issue in post #22

cjameshuff in post #14 gave some related examples

Essentially, if the two emitters/observers were the entire Universe then there'd maybe be no issue. But since there can be third parties observing them - that's where causality issues arise.

This is not about when these third parties receive the light that tells them an event occured, but is about when they calculate/know the event "actually" occured according to them.

Due to time dilation and the Relativity of simultaneity, these third observers can observe events occuring in the "wrong" order.

Note the title of this thread. The OP (who generally accepts that FTL won't work (see prev. thread (http://www.bautforum.com/space-astronomy-questions-answers/91864-superluminal-travel-i-want-believe-p-would-work.html))) is looking for a "special" frame where FTL would work; the problem is that it only works if the "special" frame is the only thing there is.

publius
2009-Aug-14, 09:22 PM
We've hashed this over here in several threads. Basically, of the set (Relativity, Causality, FTL), pick any two. You can't have all three together.

There is what we can can call "strong causality" which declares that no observer will see anything run backwards, that is, effects occur before their causes. The ordering of events in the foward light cone is absolute to all observers.

Now, we can, as I think the OP was wondering about, loosen causality a bit and declare they can be no closed information loops in space-time. Some observers would see things run backwards, but no paradoxes would be possible.

The problem with that, and this is key, *key* to grasp, is it establishes a preferred frame of reference. And that frame is simply the one where all events run in their proper temporal order. Knowing this any observer could calculate his state of motion relative to this special frame by carefully conducting FTL experiments.

We have thus thrown out Relativity. We'd still have the Lorentz transform mind you, but we've thrown out the postulate that there is no special frame and the notion of absolute motion being meaningless. It seems odd the universe would base the Lorentz transform around 'c', but allow some special frame with some maximum information velocity greater than that. Why not "pivot" around this higher speed?

-Richard

Sam5
2009-Aug-14, 09:22 PM
cosmocrazy and pzkpfw,

Ok, that will give me something to think about for a while. :)

Thanks.

m74z00219
2009-Aug-15, 02:41 AM
This is not about when these third parties receive the light that tells them an event occured, but is about when they calculate/know the event "actually" occured according to them.

Due to time dilation and the Relativity of simultaneity, these third observers can observe events occuring in the "wrong" order.

Note the title of this thread. The OP (who generally accepts that FTL won't work (see prev. thread)) is looking for a "special" frame where FTL would work; the problem is that it only works if the "special" frame is the only thing there is.

Thanks pzkpfw for your response.
This conjures up some interesting philosophical notions. I certainly don't object that reality shows that strong relativity is the way of it (as Richard stated). What's interesting is that a single FTL frame would cause other frames to see a series of causal events happen in reverse: like rewind a video cassette, but this still would not allow an initiator of an event (person or otherwise) to undo the initiation of said event.

It is interesting that single-frame FTL travel causes no unresolvable paradoxes (like A causes B causes C cause A not to happen). Aside for there being no evidence of such phenomena (such as observing causal events happening in reverse), there must be other physical laws that are broken...






We've hashed this over here in several threads. Basically, of the set (Relativity, Causality, FTL), pick any two. You can't have all three together.

There is what we can can call "strong causality" which declares that no observer will see anything run backwards, that is, effects occur before their causes. The ordering of events in the foward light cone is absolute to all observers.

Now, we can, as I think the OP was wondering about, loosen causality a bit and declare they can be no closed information loops in space-time. Some observers would see things run backwards, but no paradoxes would be possible.

The problem with that, and this is key, *key* to grasp, is it establishes a preferred frame of reference. And that frame is simply the one where all events run in their proper temporal order. Knowing this any observer could calculate his state of motion relative to this special frame by carefully conducting FTL experiments.

We have thus thrown out Relativity. We'd still have the Lorentz transform mind you, but we've thrown out the postulate that there is no special frame and the notion of absolute motion being meaningless. It seems odd the universe would base the Lorentz transform around 'c', but allow some special frame with some maximum information velocity greater than that. Why not "pivot" around this higher speed?

-Richard

Richard, your comment on temporal order reminds me the second law of thermodynamics. If there's no issue with irresolvable causal paradox, then there's the breaking of the second law of thermodynamics.
Thanks for your comments.

M74

a quick unrelated question: what does OP stand for? :confused:

Sam5
2009-Aug-15, 02:44 AM
a quick unrelated question: what does OP stand for? :confused:


Original Post and/or Original Poster. I.E. the first post in a new thread.

Jeff Root
2009-Aug-15, 03:50 AM
Two ships simultaneously fly past earth in opposite directions, one at
0.5c and one at 0.75c, synchronizing clocks as they pass. Earth pings
them an hour later by an instantaneous communications device. When
do they get the ping? Say the faster ship gets it first on their subjective
timeline...52 minutes for the 0.5c ship, 40 for the ones at 0.75c.
Just to be sure we're talking about the same thing... the clocks
are identical and tick at the same proper rate. Gravitational
effects are ignored. This is special relativity only.

If the two ships turn around and return to Earth, it will be found
that the clocks on the ships lag behind the Earth clock in proportion
to their speeds. But it isn't clear to me what time their clocks
must read when the "ping" reaches them.

At the moment the ships pass each other and pass Earth:

- The observer on Earth sees the clock on the 0.5c ship ticking
more slowly than his own (0.866 of his), and the clock on the
0.75c ship ticking even more slowly (0.661 of his).

- The observer on the 0.5c ship sees the clock on Earth ticking
more slowly than his own (0.866 of his), and the clock on the
0.75c ship ticking even more slowly (0.433 of his).

- The observer on the 0.75c ship sees the clock on Earth ticking
more slowly than his own (0.661 of his), and the clock on the
0.5c ship ticking even more slowly (0.433 of his).

So the Earth observer calculates that one hour after the ships
pass, by his clock (at 1:00), the clock on the 0.5c ship must
read 0:52, and the clock on the 0.75c ship must read 0:40.

The observer on the 0.5c ship calculates that the clock on Earth
must read 1:00 when his own clock reads 1:09, and the clock on
the 0.75c ship must read 0:30.

The observer on the 0.75c ship calculates that the clock on Earth
must read 1:00 when his own clock reads 1:30, and the clock on
the 0.5c ship must read 0:39.

Have I analyzed that correctly?

What if the "ping" reaches each ship when its clock reads 1:00 ?

-- Jeff, in Minneapolis