5 Light-Year long stick question

[Altabeh]

The whole idea of my last post was that even making a new theory that can describe something seeked out theoretically, which means has been proven to exist in reality, requires strict knowledge of all other related sciences to not be possibly violating them. In the Lisi's theory, there are many problems, the biggest of them being the fact that two generations of fermions do not have the correct quantum numbers in his model leaving much doubt about the capacity of the theory to be adequate in the long run to answer the fundamental questions. This kind of deficiency is structural and leaves no room for so many of physicists to believe it can do something. The other theory is suspected to have wrong predictions partially and that it is said to be a Higgs-less theory. These are from non-mainstream physics and it is clear that they could be largely subsided in comparison to a theory like standard model and this is normal unless one sees the theory from a unique angle where the outcome is not as important as the process or let's say the result can be anything in case you seem to be satisfied enough by the procedure itself to ignore all facts together!

In this case, since the outcome cannot be accepted by the ordinary kind of physics you and everybody else around here deals with on a regular basis even if the reasoning might make sense somehow, one the other hand taking for granted the fact that one can't call things such as a "timeless universe" or "no speed exists at all" pictures physics, there's no such thing as middle ground at least between me and the idea holder on the subject as long as I find myself in a situation attached to the observational aspects of the problem in question. I tend to not pass the red lines of "reality" and "imagination".

AB

 

[HallsofIvy]

It is precisely this thought experiment that shows that, even theoretically, there cannot be a "perfectly rigid" object.

perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

I said nothing about a five light year long stick. I said that this shows why, in relativity, objects cannot be perfectly rigid.

 

[Fredrik]

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical let's suppose..apparently its impossible to travel at the speed of light, but it didn't stop Einstein theorising.
I actually think the Original question is a bloody good one.
okay what if the stick is 5 metres long - and I poke you with it...you will feel the movement instantaneously - forgetting relativism, and internal factors for a second - so notionally, the resultant poke isn't governed by speed - time goes out of the equation...agreed?

You're overlooking something very important here. First of all, this is the relativity forum, so when someone asks about a perfectly rigid object, the natural interpretation of the question is "what does relativity say about these things?". Second, a perfectly rigid rod (of any length) in special relativity is a logical inconsistency. We don't mind questions that start with things that are impossible in practice ("If I eat a million hamburgers..."), but we understand that questions about things that are impossible in principle ("If I eat myself...") don't have any meaningful answers. (And yes, that example works better in a language where "eat" can't be interpreted as a sexual act, but it's still the best way I know to explain the difference between impossible in practice and impossible in principle).

where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation?

There is no such place. There's also no place where you'll get better answers than here.

it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

As I said, we don't mind unrealistic assumptions, but we can't answer questions that assume that the theory we're supposed to use to answer the question is logically inconsistent.

 

[Gatchaman]

I know where we can put this rod!

 

[Frame Dragger]

I know where we can put this rod!

...

...

Heh...

 

[Azeroffs]

So I came across this discussion via stumbleupon, and I'm astounded that no one spotted where q_interested was coming from. Everyone just decided to get all bent out of shape about creating the idea of a perfectly rigid stick.

q_interested states that when you poke someone 5 meters away from you the poke happens instantaneously so he's thinking that logically you could assume that the 5LY stick would also poke instantaneously.

The problem is that the 5 meter stick isn't poking instantaneously. The 5 meter stick actually does compress slightly as you push it forward. So the push-poke isn't happening simultaneously. There is a very tiny delay even in the 5 meter stick, but the delay is so negligible that it seems to be instantaneous. Multiply that delay by the proportion of 5 meters to 5 LY and you'll get a delay of >5 years.

Also, it's perfectly within the laws of physics to conceive of a perfectly rigid stick. Just as you can conceive of a frictionless surface and create thought experiments based on that using physics.

If there was a perfectly rigid substance, then it would be instantaneous no matter how long the stick, but as so many people so adamantly stated, such a thing does not exist and cannot exist in reality.

 

[Fredrik]

So I came across this discussion via stumbleupon,

Never heard of it, but now I have to check it out. Welcome to Physics Forums.

and I'm astounded that no one spotted where q_interested was coming from. Everyone just decided to get all bent out of shape about creating the idea of a perfectly rigid stick.

q_interested states that when you poke someone 5 meters away from you the poke happens instantaneously so he's thinking that logically you could assume that the 5LY stick would also poke instantaneously.

The problem is that the 5 meter stick isn't poking instantaneously. The 5 meter stick actually does compress slightly as you push it forward.

I don't know why you think no one understood that. The finite propagation speed was brought up in the very first reply the OP got. (See post #2 by JesseM).

Also, it's perfectly within the laws of physics to conceive of a perfectly rigid stick. Just as you can conceive of a frictionless surface and create thought experiments based on that using physics.

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

In principle, you could still get the different parts of the stick to start moving at the same time in a specific inertial frame by attaching a rocket to each component part and set a timer in every one of them, so that they all turn on their engines at the same time.

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

 

[Azeroffs]

Never heard of it, but now I have to check it out. Welcome to Physics Forums.

Thanks

I should probably add a disclaimer.. Stumbleupon is like procrastinator crack. You've been warned :P

I don't know why you think no one understood that. The finite propagation speed was brought up in the very first reply the OP got. (See post #2 by JesseM).

well its clear that q_interested wasn't aware.

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

In principle, you could still get the different parts of the stick to start moving at the same time in a specific inertial frame by attaching a rocket to each component part and set a timer in every one of them, so that they all turn on their engines at the same time.

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

You'll have to bear with me because I'm not entirely familiar with these concepts, but I think the idea of a completely rigid stick would be equivalent to one very large inertial frame.

Tbh, I don't understand a lot of that post, so I'll just take your word for it. Correct me if I'm wrong, but I'd think there are logical inconsistencies with frictionless surfaces and instantaneous acceleration as well.

 

[JesseM]

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

It depends what you mean by "rigid" I think. As you say:

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

This does show you'd get a logical inconsistency if you assumed that pushing a particular stick at a particular time would result in the other end moving "instantaneously" when the same set of events was analyzed using different inertial frames--according to the relativity of simultaneity, if two events are simultaneous in one frame (like the event of pushing one one end and the event of the other end accelerating) then they are non-simultaneous in other frames (in some frames the far end will accelerate after the near end was pushed but before there's been time for a light signal to get from the event of the near end being pushed to the far end, in other frames the far end will actually accelerate before the near end was pushed). However, this doesn't necessarily show a problem with the idea of a stick made out of some kind of tachyonic material which always accelerates instantaneously in its own rest frame, which you discuss below:

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

You correctly point out that FTL signalling (including 'instantaneous' signalling) would lead to causality violations (sending information backwards in time) in SR, but I don't think that's the same as a "logical inconsistency". As I'm sure you're aware, causality violations appear in solutions of general relativity which contain closed timelike curves but most physicists don't think that means we can rule out such solutions a priori on purely logical grounds, instead they usually invoke something like the Novikov self-consistency principle to show how such solutions need not lead to any logical paradoxes. This seems to be the type of resolution that Demystifier was talking about in post #137 on that other thread.

 

[sfan]

I've found a good site here that actually backs up the OP's theory and possibly prove some of the doubters wrong, it's backed up with technical drawings as well, so if you could get a pole long enough, it would be possible.

http://bit.ly/9SjS2u

 

[Dale]

Hi sfan, welcome to PF

That website is pretty clearly intended to be a joke making fun of the OP and other similarly wacky ideas. I especially enjoyed the 9-down-6-up generator idea. Very funny.

 

[sfan]

Yeah i know, i just came across this thread from stumbleupon and it instantly reminded me of those troll science cartoons

 

[JesseM]

I've found a good site here that actually backs up the OP's theory and possibly prove some of the doubters wrong, it's backed up with technical drawings as well, so if you could get a pole long enough, it would be possible.

http://bit.ly/9SjS2u

Hahaha, that page is hilarious, thanks for the link!

 

[Fredrik]

...something like the Novikov self-consistency principle to show how such solutions need not lead to any logical paradoxes.

It sounds like that principle is just dismissing the solutions that contain actual inconsistencies, while permitting the ones that don't. I agree that there are consistent solutions that contain a CTC. (I'm thinking specifically about the scenario described in the article that involves a billiard ball coming out of a wormhole in a way that knocks a younger version of itself into the other end of the wormhole). This idea makes sense in GR where spacetime containing the CTC and the motion of matter in it are different aspects of the same solution of Einstein's equation. But in SR, matter is added "manually" to a background spacetime, so the principle would at least have to be modified to deal with that. The only modification I can think of that avoids paradoxes is to simply say that we're not allowed to add matter to spacetime in a way that leads to paradoxes. So the principle doesn't really "resolve" the paradox I described in the other thread; it just says that I'm not allowed to construct it because it contains a contradiction. It doesn't say which of the ingredients are disallowed, it just says that I'm not allowed to include all of them.

I don't think a "resolution" like that has any value. It's essentially just saying that "OK, you have a scenario where tachyons cause a contradiction, but that doesn't mean that tachyons don't exist. It could also mean that if you try to set up that experiment, a ninja turtle will appear out of nowhere and chop your head off". Demystifier argued that I would simply be unable to choose to complete the setup, but I don't find that any more plausible than ninja turtles, since it violates the illusion of free will. This was discussed in the other thread:

If the reply you get is "the message you sent hit your daughter in the head and killed her", and you're still unable to stop yourself from sending the message, you don't even have the illusion of free will.

There are ways to make sure that the reply can be trusted, at least to such a degree that you would feel that sending the message would be to gamble with your daughters life with nothing substantial to gain. You can e.g. use encryption and digital signatures, and put someone you trust at the other end. That someone doesn't even have to be a person. It could be a computer that you programmed yourself, and rigged to explode if tampered with.

 

[JesseM]

It sounds like that principle is just dismissing the solutions that contain actual inconsistencies, while permitting the ones that don't.

How could a "solution" contain an inconsistency? A solution is a spacetime manifold where the Einstein field equations hold at every point (and if you consider other laws of physics like electromagnetism, they should hold locally at every point too), I don't see how you can imagine a spacetime that contains an "inconsistency" but where this is still true. For example, if you try to imagine an inconsistent scenario where a billiard ball goes into a wormhole, then emerges in the past and knocks its younger self away so it doesn't go into the wormhole, then either you'd need to imagine two parallel versions of the region of spacetime where the billiard ball is headed for the wormhole (or at least parallel truths about whether the billiard ball's worldline passes through certain points in spacetime, like points near the wormhole mouth where the ball's trajectory would take it if it wasn't hit by its future self), or you'd have to imagine a discontinuity in the worldline of the billiard ball such that after being knocked out of the way, it suddenly jumps in position so it's back on course to go into the wormhole and go back in time and knock its younger self out of the way.

I agree that there are consistent solutions that contain a CTC. (I'm thinking specifically about the scenario described in the article that involves a billiard ball coming out of a wormhole in a way that knocks a younger version of itself into the other end of the wormhole). This idea makes sense in GR where spacetime containing the CTC and the motion of matter in it are different aspects of the same solution of Einstein's equation. But in SR, matter is added "manually" to a background spacetime, so the principle would at least have to be modified to deal with that. The only modification I can think of that avoids paradoxes is to simply say that we're not allowed to add matter to spacetime in a way that leads to paradoxes.

When you state it that way it sounds like a circular argument, but I would again state it in terms of the idea that all the laws of physics, including the ones governing the behavior of matter, work the same way in each local neighborhood of a point, and we're looking for a single 4D manifold where this is true in the neighborhood of every point on the manifold. The absence of "paradoxes" should follow directly from this, it isn't an additional requirement.

I don't think a "resolution" like that has any value. It's essentially just saying that "OK, you have a scenario where tachyons cause a contradiction, but that doesn't mean that tachyons don't exist. It could also mean that if you try to set up that experiment, a ninja turtle will appear out of nowhere and chop your head off".

But the sudden appearance of some weird phenomenon like a ninja turtle would actually be a violation of the principle because it would mean the same local laws don't apply in each local region, instead you need new laws that only get invoked when the danger of paradoxes looms. Pay particular attention to this section of the wikipedia article:

The Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes either that there is only one timeline, or that any alternative timelines (such as those postulated by the many-worlds interpretation of quantum mechanics) are not accessible.

Given these assumptions, the constraint that time travel must not lead to inconsistent outcomes could be seen merely as a tautology, a self-evident truth that cannot possibly be false, because if you make the assumption that it is false this would lead to a logical paradox. However, the Novikov self-consistency principle is intended to go beyond just the statement that history must be consistent, making the additional nontrivial assumption that the universe obeys the same local laws of physics in situations involving time travel that it does in regions of spacetime that lack closed timelike curves. This is made clear in the above-mentioned Cauchy problem in spacetimes with closed timelike curves, where the authors write:

"That the principle of self-consistency is not totally tautological becomes clear when one considers the following alternative: The laws of physics might permit CTC's; and when CTC's occur, they might trigger new kinds of local physics which we have not previously met. ... The principle of self-consistency is intended to rule out such behavior. It insists that local physics is governed by the same types of physical laws as we deal with in the absence of CTC's: the laws that entail self-consistent single valuedness for the fields. In essence, the principle of self-consistency is a principle of no new physics. If one is inclined from the outset to ignore or discount the possibility of new physics, then one will regard self-consistency as a trivial principle."

 

[Fredrik]

How could a "solution" contain an inconsistency?

OK, that's a good point. It can't, of course. But that has to mean that the principle doesn't actually say anything about general relativity. The text you're quoting (from the Wikipedia article) suggests that all it does is to rule out spontaneously appearing ninja turtles and similar stuff that aren't part of the theory anyway.

When you state it that way it sounds like a circular argument, but I would again state it in terms of the idea that all the laws of physics, including the ones governing the behavior of matter, work the same way in each local neighborhood of a point, and we're looking for a single 4D manifold where this is true in the neighborhood of every point on the manifold.

So how you would you apply this principle to SR, where matter is added manually to a fixed spacetime? (This is true both for classical SR and special relativistic QM). How does it invalidate the argument that instant messages makes SR inconsistent?

 

[JesseM]

So how you would you apply this principle to SR, where matter is added manually to a fixed spacetime? (This is true both for classical SR and special relativistic QM). How does it invalidate the argument that instant messages makes SR inconsistent?

Whatever particles/fields/etc. are added, it's assumed there are some known Lorentz-invariant laws governing their behavior, laws which can be stated in local form like the differential form of Maxwell's equations. And if tachyons exist and don't have a preferred frame, one should be able to write down some local Lorentz-invariant laws governing their behavior too. So, can't you just impose the condition that the global collection of local facts about particles/fields at each point in spacetime must satisfy these same local laws at every point? This global condition on allowable "histories" of particles/fields throughout the entire 4D spacetime should guarantee that the Novikov self-consistency principle would hold.

 

[PAllen]

It always struck me that tachyons observe the math of SR, but not the physics; more that the derivation of SR would break down, so you would need something different if tachyons existed.

Simultaneity could be objectively defined a frame independent way, for example. That would seem to require a major modification of SR.

 

[JesseM]

Simultaneity could be objectively defined a frame independent way, for example. That would seem to require a major modification of SR.

Only if you assume causality must hold would tachyons necessarily pick out a frame-invariant definition of simultaneity. If you allow tachyons to travel backwards in time in every inertial frame, which is the idea I and Fredrik have been discussing, then they can still behave in a frame-invariant way and satisfy the postulates of SR, at the cost of violating causality and allowing information about a given event E to be transmitted into E's own past light cone.

 

[PAllen]

Only if you assume causality must hold would tachyons necessarily pick out a frame-invariant definition of simultaneity. If you allow tachyons to travel backwards in time in every inertial frame, which is the idea I and Fredrik have been discussing, then they can still behave in a frame-invariant way and satisfy the postulates of SR, at the cost of violating causality and allowing information about a given event E to be transmitted into E's own past light cone.

I am not understanding this. Can you exlain more how causality fits in? I'm just thinking I can send signals as fast as I want, so I can set up absolute simultaneity between any two regions of space to any desired precision. Why would either party think they are getting messages from the future, each believing the other's messages were from the future? One would instead believe a common 'now' has been established to arbitrary precison over arbitrary distance.

 

[Fredrik]

Whatever particles/fields/etc. are added, it's assumed there are some known Lorentz-invariant laws governing their behavior, laws which can be stated in local form like the differential form of Maxwell's equations. And if tachyons exist and don't have a preferred frame, one should be able to write down some local Lorentz-invariant laws governing their behavior too. So, can't you just impose the condition that the global collection of local facts about particles/fields at each point in spacetime must satisfy these same local laws at every point? This global condition on allowable "histories" of particles/fields throughout the entire 4D spacetime should guarantee that the Novikov self-consistency principle would hold.

I don't see how that's different from just stating that scenarios that contain contradictions are not allowed.

By the way, a minor change to the computer program in my thought experiment would eliminate the contradiction. Is the scenario where the computers run "safe" programs allowed by the laws of physics? If yes, then what exactly is preventing me from setting up that scenario, and then edit the computer program? Will I get hit by a meteor that's been heading my way for a billion years the moment before I save the changes? Will I change my mind for no apparent reason, and choose not to save the changes?

Why would either party think they are getting messages from the future,

See the post I linked to in #37.

Note that for the reply message to go into the past, it's necessary that the messages are instantaneous in the transmitter's rest frame. If they're all simultaneous in some specific inertial frame, then no messages are being sent to the past, but the principle of relativity is violated instead.

 

[JesseM]

I don't see how that's different from just stating that scenarios that contain contradictions are not allowed.

I don't see what it would even mean for a scenario to "contain a contradiction". Suppose we are given a complete set of facts about all local physical variables at every point in an infinite Minkowski spacetime--as long as we assume there is a single correct fact about the state of affairs at each point (i.e. we can't have two contradictory 'parallel' truths about what's happening at a single point in spacetime, like whether a given type of particle is present or absent at that point), then what would it mean for this list to contain a contradiction? Of course if we don't impose the rule that the same local laws of physics must apply at each point, then it might be possible for this complete set of facts to contain weird discontinuities, like a tachyon-receiving device that records having received a "1" at points on its worldline shortly after it received a tachyon message, but then at some point before sending a message it abruptly and for no lawlike reason changes to being in a state where it records having received a "0". But a discontinuity is not really a "contradiction", it's just a breakdown in the usual laws of physics at some point in the spacetime. And if we impose the rule that the only "complete set of facts" allowed are ones where the same local laws are obeyed at each point, then this sort of discontinuity won't happen, and the complete set of facts will necessarily be a globally self-consistent set of facts.

By the way, a minor change to the computer program in my thought experiment would eliminate the contradiction. Is the scenario where the computers run "safe" programs allowed by the laws of physics? If yes, then what exactly is preventing me from setting up that scenario, and then edit the computer program? Will I get hit by a meteor that's been heading my way for a billion years the moment before I save the changes? Will I change my mind for no apparent reason, and choose not to save the changes?

Yes, something like that would presumably have to happen in either an SR scenario with tachyons or a GR scenario with a traversable wormhole that allows you to travel into your own past--if at some point you decide to try to create a contradiction, events will "conspire" to stop you or at least change your mind.

This need not imply any "intelligence" on the part of the laws of physics, of course--with a sufficiently powerful computer we could use brute-force methods to generate a simulated universe which followed the same local laws at every point, which allowed "time travel", and which was guaranteed to be self-consistent in this way. To see this, consider a simpler analogy. Imagine you want to write a computer program to generate a possible chess game. One way is to start with the pieces in their starting configuration, then have the program generate each successive configuration on the next turn from the configuration on the previous turn, using only legal chess moves. But here's another, more elaborate way to do it. have the computer generate an entire series of configurations at once, completely randomly, so it just picks randomly which pieces to put in which positions on which turn. It is very unlikely that the resulting series will look like a legal chess game--a piece might randomly be on a particular square on one turn, but then the next turn randomly be on some totally different square that it shouldn't be able to get to in one move by the rules of chess. But suppose you have access to an idealized computer with nearly infinite speed and memory, and you have it generate a gigantic number of random series this way--if your number is large enough, chances are at least some of the series would just happen to satisfy the rules of a legal chess game. So you could specify that the computer should throw out all series which violate the rules of chess, and be left only with series that represent legal chess games. But since you are dealing with an entire series at once, you could also place other constraints on them, like "throw out all series where white wins", or "show me only series where the black rook checkmates the king in 25 moves", whatever you want. For sufficiently detailed conditions, it might be very hard to generate a chess game that matched them in the traditional way of starting from the beginning and basing each new configuration of pieces on the configuration of the previous turn, but using this brute-force method of generating a near-infinite number of entire histories, and throwing out all but the ones that satisfy your constraints, it's easy to get a game that satisfies any conditions you like without even having to think about it or plan the details of the game.

And suppose we want to come up with a game of "4D chess" which is similar to ordinary chess but with some extra rules that allow you to send pieces "back in time" to earlier time-increments in the game, but only in a self-consistent way where history is not changed. For example, suppose there are two squares labeled A and B on the middle of the board, such that if at any time-increment a piece is moved onto square A, then the rules say it is transported to square B four moves earlier (and say the player who controls the piece has to immediately move it when it appears on square B, and pieces can't move directly to square B by non-time-travel routes, to avoid the issue of multiple pieces occupying square B on a particular time-increment). It would be pretty hard to generate self-consistent games following these rules by the usual method of starting from some initial configuration and evolving it forward step-by-step, but if you just generate some astronomical number of random histories, the computer can algorithmically check any given randomly-generated history to see if it actually is a self-consistent 4D chess game that obeys the rules at every point, so with enough memory and computing power it should be able to find some valid games.

Similarly, suppose you were using this incredibly powerful computer to generate a simulation of an entire universe--instead of picking some initial conditions and then letting it evolve forward according to some set of laws of physics, you could again specify your "laws" in terms of constraints on entire histories, with the computer generating a huge number of random histories and then throwing out all the ones that don't satisfy the conditions. If the "laws of physics" you pick happen to allow time travel, then obviously any universe that respects the laws of physics locally at every point in spacetime must be globally self-consistent, and the computer will find some histories satisfying this condition. But the computer does not need to have any intelligence to do this, it's just randomly generating a huge number of possibilities until it finds one that satisfies the constraints. From the point of view of a simulated sentient being in this universe with access to a time machine, it might seem like the universe was cleverly finding ways to "outsmart" them and thwart their plans every time they tried to change history, but it would actually be the result of a fairly simple rule, just not a dynamical rule based on picking initial conditions and evolving them forward, as with normal computer simulations.

 

[JesseM]

I am not understanding this. Can you exlain more how causality fits in? I'm just thinking I can send signals as fast as I want, so I can set up absolute simultaneity between any two regions of space to any desired precision. Why would either party think they are getting messages from the future, each believing the other's messages were from the future? One would instead believe a common 'now' has been established to arbitrary precison over arbitrary distance.

Well, it's just a matter of the relativity of simultaneity--if there is some frame where the sending of the tachyon message happens at the same time as the receiving of the same message at a different location (i.e. the tachyon signal traveled instantaneously, with infinite speed, in that frame), then it's easy to show using the Lorentz transformation that there must be some other frame where the signal was actually received at an earlier time then it was sent, i.e. the tachyon signal went back in time in that frame! And if the first postulate of relativity is obeyed, then if tachyon signals are allowed to travel back in time in one inertial frame, they must be allowed to travel back in time in any other frame as well. You can only use tachyons to establish a preferred definition of simultaneity if there is some preferred frame such that tachyons can travel with arbitrarily large speed in that frame but they cannot travel backward in time in that frame (and if that were the case, tachyons could never be used to create true causality violations where information about some event E could be transmitted into E's own past light cone as in these diagrams, with a preferred frame it would still be true that tachyons moved back in time in other frames but the information they carried would always remain outside the past light cone of the event of the tachyons being sent)

 

[PAllen]

Well, it's just a matter of the relativity of simultaneity--if there is some frame where the sending of the tachyon message happens at the same time as the receiving of the same message at a different location (i.e. the tachyon signal traveled instantaneously, with infinite speed, in that frame), then it's easy to show using the Lorentz transformation that there must be some other frame where the signal was actually received at an earlier time then it was sent, i.e. the tachyon signal went back in time in that frame! And if the first postulate of relativity is obeyed, then if tachyon signals are allowed to travel back in time in one inertial frame, they must be allowed to travel back in time in any other frame as well. You can only use tachyons to establish a preferred definition of simultaneity if there is some preferred frame such that tachyons can travel with arbitrarily large speed in that frame but they cannot travel backward in time in that frame (and if that were the case, tachyons could never be used to create true causality violations where information about some event E could be transmitted into E's own past light cone as in these diagrams, with a preferred frame it would still be true that tachyons moved back in time in other frames but the information they carried would always remain outside the past light cone of the event of the tachyons being sent)

What I'm claiming is that if tachyon messages were really possible, one would conclude distant simultaneity is well defined and would never derive or believe the Lorentz transform. The whole basis for deriving or believing it would gone. Thus, one would be forced to develop some radical alternative to SR.

 

[JesseM]

What I'm claiming is that if tachyon messages were really possible, one would conclude distant simultaneity is well defined and would never derive or believe the Lorentz transform. The whole basis for deriving or believing it would gone. Thus, one would be forced to develop some radical alternative to SR.

Why do you say that? If tachyons could really travel backwards in time in any frame as well as FTL or instantaneously, then I don't see how they could be used to establish any absolute notion of simultaneity, and the laws governing tachyons could still be Lorentz-symmetric (i.e.e the same equations would accurately predict their behavior in the coordinates different frames related by the Lorentz transformation) just like the laws governing other particles. Fundamentally it is the Lorentz-symmetry of the laws of physics that makes people "believe" in SR, the addition of tachyons wouldn't change this any more than the subtraction of photons and other particles capable of moving at c (even if absolutely no particle in the universe could move at c, the laws of physics governing slower-than-c particles could still be Lorentz-symmetric)

 

[PAllen]

Why do you say that? If tachyons could really travel backwards in time in any frame as well as FTL or instantaneously, then I don't see how they could be used to establish any absolute notion of simultaneity,

Ah, I finally get it, this is the key. If I observed a tachyon message sent from my future, I could not independently know how to interpret the 'time sent' for any tachyon message I receive. Then the rest of this approach follows. Thanks for your patience explaining this.

 

[Fredrik]

I don't see what it would even mean for a scenario to "contain a contradiction".

I guess I could have phrased it better, but you know what I mean. The technique of proving an assumption wrong by deriving a contradiction is clearly just as valid here as in any other area of mathematics.

And if we impose the rule that the only "complete set of facts" allowed are ones where the same local laws are obeyed at each point, then this sort of discontinuity won't happen, and the complete set of facts will necessarily be a globally self-consistent set of facts.

I agree. We're really just talking about a bunch of curves in Minkowski spacetime (curves representing the motion of particles), so there can't be any inconsistency. The "local laws" must be statements about what the curves are doing.

Yes, something like that would presumably have to happen in either an SR scenario with tachyons or a GR scenario with a traversable wormhole that allows you to travel into your own past--if at some point you decide to try to create a contradiction, events will "conspire" to stop you or at least change your mind.

This is what I can't accept. The properties of tachyons and their interactions are such that all scientists would choose to murder their children rather than change a 0 to a 1 in a computer program? If I use a telescope to look for meteors before I make the change, I will die in a terrorist attack instead? If I lock myself away where I'm safe from terrorists, I have a heart attack instead? If I make sure that I'm in perfect health...I guess we're back to ninja turtles.

What I just described doesn't sound anything at all like the universe we live in, and I also don't think it can be justified by a rule that says that the same "local laws" are obeyed in each region of spacetime. If these "local laws" say that I can build a device that sends instantaneous messages (in the device's rest frame), and the same thing can't be done in another region of spacetime, then I would say that the laws in that region are different from the laws in the first region, or alternatively, that the laws are the same everywhere, but different from what we previously thought that they were. I know the principle explicitly said the there won't be any new physics, but now I think it contradicts itself on that point.

Even if we assume that there are such global laws (for both tachyons and normal matter), they don't seem to describe something that resembles the universe we live in, where people have an illusion of free will.

 

[swapnanil_s]

My friends and I are having an argument over this question:

"Someone 5 LY away on a planet is getting "poked" by a 5 LY long stick from here on earth"

Does it take five years (or more) from the time one end of the stick is pushed until the person right next to the other end is poked by it? Or does it happen in next to no time at all?
- I personally believe that it will take alteast five years.. since if the person being poked had a telescope and watched the person push the stick, it would take the light ("information") 5 years to reach him.. and he can't get poked by the stick if it hasn't been pushed?

Is that right to say?

*Ignoring the fact of obvious problems with the situation, like requiring a massive force to move the stick.

My friend, light is not instantaneous. Suppose your friend lives next door, around 10 metres away from yours.If you hold a 10 m long stick in your hand, and poke him, that surely will be instantaneous. The same happens with electrons in a conductor... Their speed is in picometres. Then how is it that immediately you put the switch on, your pc starts? This has got to do nothing with the speed of light... But u poke him today, he'll fell it just now, but u can see him feel that 5 years later!

 

[JesseM]

I guess I could have phrased it better, but you know what I mean. The technique of proving an assumption wrong by deriving a contradiction is clearly just as valid here as in any other area of mathematics.

But like I said before, I'm assuming that we have some complete set of facts about the conditions at every local point in the spacetime, as would be true in the computer simulation example where the computer generates entire histories. There can't be a contradiction in such a complete set of facts. But there could be a discontinuity, like one region of spacetime including me sending a 0 while a later region of spacetime has you receiving a 1...even if the local laws work correctly in each region there'd have to be a violation on the boundary between them somewhere, an unexplained flip from 0 to 1.

This is what I can't accept. The properties of tachyons and their interactions are such that all scientists would choose to murder their children rather than change a 0 to a 1 in a computer program? If I use a telescope to look for meteors before I make the change, I will die in a terrorist attack instead? If I lock myself away where I'm safe from terrorists, I have a heart attack instead? If I make sure that I'm in perfect health...I guess we're back to ninja turtles.

What I just described doesn't sound anything at all like the universe we live in, and I also don't think it can be justified by a rule that says that the same "local laws" are obeyed at each point. I don't think it's even consistent with such a rule. For example, if these "local laws" say that I can build a device that sends instantaneous messages (in the device's rest frame), and the same thing can't be done in another region of spacetime, then I would say that the local laws are different in the two regions of spacetime.

I never said anything about not being able to send instantaneous messages in every region. But to modify your argument a bit, it could be true that by self-consistency, the fact that I received a 1 in the past means that the sender at a later time is constrained so he can't send a 0 back in time, even if he knows I got a 1 and wants to create a contradiction, and even though another experimenter in a different part of spacetime could send a 0 with the same device because in her case doing so wouldn't create a contradiction. But then, even in a deterministic universe without time travel the conditions of one region of spacetime A may constrain the possibilities that lie in some other region B that lies in its future light cone, so certain events cannot happen in B that might happen in some other region of spacetime C with a different set of conditions in its past...that doesn't mean the local laws of physics are different in region B and C, it's just a matter of conditional probabilities! If you consider an ensemble of different spacetimes that all satisfy the local laws everywhere (like in the computer simulation argument where we imagine generating a huge number of random histories and then throwing out all the ones where the local laws aren't obeyed in each neighborhood, leaving us with an ensemble of possible histories that do obey these local laws everywhere) then any conditions that hold in region C in some members of the ensemble could be found in region B in other members of the ensemble, and vice versa. Same would be true for an ensemble of self-consistent histories, there'd be nothing that's consistently impossible in one region throughout the ensemble even though it's consistently possible in a different region.

Did you consider my computer simulation argument carefully? Suppose the underlying laws of physics in the simulation were something like lattice QCD or even a simple cellular automata type rule, where the law tells you what states are allowed in one "cell" given the states in surrounding cells, much the same way that in "4D chess" the piece that might be found on a square at one time-increment would depend on the positions of the pieces at other times (both past and future in 4D chess, just past in normal chess). Would you agree that for any such local laws governing a physical simulation, if we imagine a vast computer of the gods that has googleplexes and googleplexes of memory and can do googleplexes and googleplexes of calculations, then the computer could simply use the method of generating "random histories" where the state of each cell is decided in a completely random way and then the history looks at each and every cell at each time increment in the history to see if its relation with surrounding cells follows the correct local laws, throwing out the vast majority of random histories and only keeping the tiny tiny fraction that follow the correct local laws at each point? Would you agree that if the local laws allow for causality violation, the tiny fraction the computer didn't throw out would have self-consistent histories that obeyed the same local laws at each point? If so, suppose each self-consistent simulated history was so fine-grained and vast that over billions of simulated years you could observe the formation of stars, planets, and in some cases the evolution of life eventually leading to simulated sentient beings with brains as complex as ours...if some of those sentient beings tried to exploit the laws of physics in their simulated universe to send messages back in time and create paradoxes, wouldn't the mere fact that the computer had kept only the histories that were self-consistent and followed the same local laws everywhere, while throwing out all the histories that didn't satisfy this, mean that in any such history including sentient beings the beings would fail in their attempts to create paradoxes?

So we would at the very least have to talk about "global" laws about what the curves look like, and require that what we otherwise would have thought of as "local laws" are actually false, even though they work in all situations where no tachyons are involved. I know the principle explicitly said the there won't be any new physics, but now I think it contradicts itself on that point.

I disagree. Again, the mere fact that there is a single unique truth about what happens at each point in spacetime in any global "solution", and that the relation between what's happening at each point is constrained to relate in the same lawlike way to what's happening at nearby points, is enough to guarantee that everything is globally consistent. If you disagree, please think about my computer simulation analogy and tell me if you at least agree that this would be true in that case.

Even if we assume that there are such global laws (for both tachyons and normal matter), they don't seem to describe something that resembles the universe we live in, where people have an illusion of free will.

Well, we only have the illusion of free will because we haven't found any way to learn what choices we will make before we make them. I don't personally think that's ever likely to change but the simple fact that it hasn't happened yet doesn't prove it could never happen. And if we ever succeed in creating some type of artificial intelligence (for example, by mapping a human brain and simulating it in sufficient detail that the simulation behaved just like the original brain, an idea known as mind uploading) then the artificial intelligence could have experimental evidence it doesn't have free will, in the form of multiple parallel simulations of the mind in closed-off virtual environments running on deterministic computers with the same initial state, in which case each simulation should run in perfect lockstep. Of course this wouldn't be the same as knowing what you were going to do before you did it, but it would still be a demonstration that free will is an illusion, and it's the sort of thing I do expect to be possible in the real universe eventually if our civilization lasts long enough.

 

[dx]

Well, we only have the illusion of free will because we haven't found any way to learn what choices we will make before we make them...

Not sure what you guys are talking about since I didn't read the thread but..

Recognition of complementary relationship is not least required in psychology, where the conditions for the analysis and synthesis of experience exhibit striking analogy with the situation in atomic physics. In fact, the use of words like 'thoughts' and 'sentiments', equally indispensable to illustrate the diversity of psychical experience, pertain to mutually exclusive situations characterized by a different drawing of the line of separation between the observer and the observed. In particular, the place left for the feeling of volition is afforded by the very circumstance that situations where we experience freedom of will are incompatible with psychological situations where causal analysis is reasonably attempted. In other words, when we use the phrase 'I will', we renounce explanatory argumentation.