Speeds greater than the speed of light

[Makep]

Are there speeds greater than the speed of light? If so, then can we discard c to embrace the new faster speed as the universal constant to explain relativity?

 

[Vanadium 50]

No.

.

 

[yuiop]

Log of superluminal radio conversation.

Bob: "Superluminal active"
Alice: "Switch to the prototype superluminal radio channel and confirm you have done so by saying 'superluminal active'".
Bob: "O.K"
Alice: "I am going to ask you a series of simple test questions to test the new superluminal communicator, OK?"
Bob: "Seven"
Alice: "What is six plus one?"
Bob: "Tuesday"
Alice: "What is the next day after Monday?"
Bob: "Yes, this is wierd"
Alice: This is weird isn't it?"
Bob: "Yes, it means we have no free will"
Alice: "Does this mean we have no free will?"
Bob: "You do not have any choice"
Alice: "What happens if I don't ask the question in the future that corresponds to the answer I received in the past?"
Bob: "OK, I've switched back to normal speed comms channel"
Alice: "I've had enough. Switch the dammed superluminal communicator back to the normal speed comms channel and tell me when you have done so"

I won't give a link but you can google using keywords like "superluminal implies time"

 

[russ_watters]

Are there speeds greater than the speed of light?

Sure, I can easily calculate speeds greater than the speed of light. None of them achievable by an object with mass, in reality, though...

If so, then can we discard c to embrace the new faster speed as the universal constant to explain relativity?

You can't just discard a universal constant because you feel like it. It isn't an arbitrary thing.

 

[Naty1]

There are many examples of phenomena faster than the speed of light. One book about just such phenomena is FASTER THAN LIGHT, Superluminal Loopholes in Physics,Nick Herbert.

The point is none can be used to transmit information... that is causality...none is faster than light.

 

[Naty1]

Try the thread here LIGHTHOUSE PARADOX REVISITED...I have not read it but the subject discussion may give you some insights ideas and perspectives.

 

[mgb_phys]

>Log of superluminal radio conversation.
Isn't really a satisfactory explanation. Imagine a species that was blind, it could develop a similair theory based on sound, you obviously can't have any speed faster than sound because then you would be able to get the answer before the question had reached you.

A better explantion might be that relativity says there is a maximum speed of information and Maxwell's equation coincidentally say that the speed is 'c'.

 

[RandallB]

Log of superluminal radio conversation. ...

Nonsense, superluminal communications implies no such conversation.
If the parties both rebroadcast the conversation over a luminal transmission both could pick up a echo (time delayed) of both sides of the same casually ordered conversation from the other at a later time on the luminal communications channel.

They should also be able to figure out if they are on a preferred ref frame by simply checking over the superluminal communications channel to see if their local clocks are actually synchronized as the luminal communications channel implies.

 

[RandallB]

A better explantion might be that relativity says there is a maximum speed of information and Maxwell's equation coincidentally say that the speed is 'c'.

I don’t think Maxwell or Einstein would consider that a coincidence. Neither should we.

 

[yuiop]

>Log of superluminal radio conversation.
Isn't really a satisfactory explanation. Imagine a species that was blind, it could develop a similair theory based on sound, you obviously can't have any speed faster than sound because then you would be able to get the answer before the question had reached you.

The difference is that a theory based on sound requires a medium which provides an absolute reference frame for a sound based theory. With relativity and light there is no medium and no absolute reference frame and the laws of physics are required to be the same from the point of view of any inertial observer. Any meaningful communications at superluminal sppeds topples that house of cards.

Log of superluminal radio conversation.

Nonsense, superluminal communications implies no such conversation.

Nonsense is what follows once the notion of the speed of light as an absolute limit on the speed of communication is rejected.

The attached time space diagrams clearly demonstrate that superluminal signaling predicts being able to signal backwards in time from the future to the past. (The space time diagram was generated by a nice program created by Mentz114 available https://www.physicsforums.com/blog.php?u=67670.)

The first diagram is the reference frame of Alice whose worldline is shown in red. She sends a superluminal signal (green worldline) at twice the speed of light to Bob. Bob,whoes worldline is shown in blue is moving at 0.9c relative to Alice. In Bob's reference frame the signal is traveling backwards in time. If we accept the principle that the laws of physics are the same in any inertial reference frame (and that superluminal signalling is possible) then a signal can be sent backwards in time in any reference frame. On receiving the signal, Bob sends a superluminal signal (magenta worldline) back to Alice at 3c. It is easy to see that Alice receives the reply before she sends the question in both reference frames and this is true for any observer at any speed relative to Alice. With superluiminal signalling, effect precedes cause.

We can take a more extreme example to show how the known laws of physics and rational logic all break down once superluminal signalling or superluminal transmission of particles is accepted.

Imagine Alice has a superluminal weapon that fires antiparticles at 2c. Alice does not like the look of Bob and makes a pre-emptive strike against him using her superluminal particle weapon. Bob's craft is damaged and he is injured but he is intact enough to return fire with his 3c superluminal particle weapon and completely destroys Alice's craft and kills Alice. Bob claims it was self defence. However, his case that he killed Alice because she attacked him in the future is going to have a hard time in court. The only available evidence according to witnesses is that Bob killed Alice before she fired a shot and it looks like Bob made a completely unprovoked attack on Alice. Does Bob's craft still get damaged in the future by Alice and if so, how does that happen if Alice and her craft were destroyed before that happens? This is the sort of nonsense that follows if superluminal transmission is proposed.

To accept superluminal communications requires that at least one of the following is rejected:

1) Cause precedes effect in any reference frame.
2) The laws of physics are the same in any inertial reference frame.
3) There is no absolute reference frame.

Which of these principles do you propose to reject?

They should also be able to figure out if they are on a preferred ref frame by simply checking over the superluminal communications channel to see if their local clocks are actually synchronized as the luminal communications channel implies.

It has nothing to do with clock synchronization. Bob does not even have to have a clock. The sequence of events can be entirely determined by Alice's single clock. Alice is aware that she receives the reply to her questions before she transmits the questions using her single stationary clock.


P.S. As I hinted at before, if we accept the notion that superluminal communication is possible, we probably have to reject the notion of free will and accept that the future is predetermined.

 

[atyy]

Log of superluminal radio conversation.

 

[atyy]

Are there speeds greater than the speed of light? If so, then can we discard c to embrace the new faster speed as the universal constant to explain relativity?

There are theories in which Lorentz invariance is violated. Experiments constrain the violation to be very tiny. This article reviews the theories and experiments - the comments on "doubly special relativity" may be relevant to your question: Mattingly, Modern Tests of Lorentz Invariance http://relativity.livingreviews.org/Articles/lrr-2005-5/ .

P.S. As I hinted at before, if we accept the notion that superluminal communication is possible, we probably have to reject the notion of free will and accept that the future is predetermined.

I'm not fond of free will, but some people claim it's possible: Conway and Kochen, The Free Will Theorem, http://arxiv.org/abs/quant-ph/0604079.

These guys claim we have free will and the future is determined: Aharonov and Tollaksen, New Insights on Time-Symmetry in Quantum Mechanics, http://arxiv.org/abs/0706.1232.

 

[RandallB]

What you are doing here is accepting an unfounded assumption to support what amounts to a Science Fiction story line.

To accept superluminal communications requires that at least one of the following is rejected:

1) Cause precedes effect in any reference frame.
2) The laws of physics are the same in any inertial reference frame.
3) There is no absolute reference frame.

Which of these principles do you propose to reject?

First I don’t accept superluminal communications as possible at all – you’re the one making unfounded conclusions about reality based on assumptions on what would happen if you built such a system, like the idea we have no free will.

I do not need to reject a principle;
I reject the notion that “3) There is no absolute reference frame.” is a principle.
Certainly not based on Special Relativity and the Simultaneity Rule;
It does not say you can arbitrarily pick any frame as the absolute reference frame. (and here you even graph and use two different frames and selectivly use both as absolute)

It says although realism demands that separate events can happen simultaneously, you cannot determine if they are by any frame of synchronized clocks. Including any home frame you might prefer.
Simultaneity says the only time you can know two events are actually simultaneous is when they happen physically spatially next to each other.

It has nothing to do with clock synchronization. Bob does not even have to have a clock. The sequence of events can be entirely determined by Alice's single clock.

Nonsense, it has everything to do with clock synchronization
you cannot solve a problem by removing Bobs clock to reducing the amount of information available.
If Alice and Bob have synchronized their clocks but Alice can read Bob’s clock through an instant superluminal communications as being sometime in the future, then clearly their clocks cannot be used as a standard of absolute.
More importantly whatever the discrepancy is; Bob and Alice should be able to use that information to correctly establish what frame has synchronized clocks usable as coordinating when events are simultaneous.

… if we accept the notion that superluminal communication is possible, we probably have to reject the notion of free will and accept that the future is predetermined.

Only if we accept your notion that you can definitely establish that some future reading on Bob’s clock is absolutely simultaneous with the current time on Alice’s clock.
Assuming so is a complete violation of the SR Simultaneity Principle,
and until you legitimately update that principle you are dealing with Science Fiction here. [/QUOTE] Alice is aware that she receives the reply to her questions before she transmits the questions using her single stationary clock. [/QUOTE] Really when and under what conditions; prove it with an experiment designed convincing enough to update the Einstein Simultaneity Rule.

If the established SR principles are followed, there is no refutation of free will here.

 

[rbj]

i think that communication (or the "transmission of information") requires a physical action of some sort to bear that information upon (in Electrical Engineering, we call that "modulation"). the way we normally do that now is that the transmitter of information pushes some electrical charge around and that motion of charge perturbs some other charge that the recipient has and his/her receiver senses that perturbation and, from that detection of the movement of charge, information is conveyed. perhaps, if the transmitter of information was big enough, he/she could wave around a planet or a star or something, and the disturbance due to changes in gravitational attraction to it of a planet that the recipient has could be detected and information could be conveyed upon that.

trouble is, both physical actions (and the other fundamental actions, so i am told) propagate at the same speed of c. so, without having some physical action that is superluminal to carry your information, how can anything be communicated at a speed that is faster?
superluminal communication

 

[yuiop]

To accept superluminal communications requires that at least one of the following is rejected:

1) Cause precedes effect in any reference frame.
2) The laws of physics are the same in any inertial reference frame.
3) There is no absolute reference frame.

Which of these principles do you propose to reject?

What you are doing here is accepting an unfounded assumption to support what amounts to a Science Fiction story line.

First I don’t accept superluminal communications as possible at all – you’re the one making unfounded conclusions about reality based on assumptions on what would happen if you built such a system, like the idea we have no free will.

I do not need to reject a principle;
I reject the notion that “3) There is no absolute reference frame.” is a principle.

We seem to be at cross purposes here and possibly I am to blame for that due to the slightly confrontational tone of my last post. You do not seem to have noticed that I do NOT accept the notion that superluminal communication is possible either. I was trying to demonstrate that IF such a notion is postulated then the consequences are ridiculous (and therefore the notion of superluminal communications should be rejected).

You claim that I am "making unfounded conclusions about reality based on assumptions on what would happen if you built such a system". Clearly, in this quote below you are reaching conclusions about what would happen if such a system was built:

Log of superluminal radio conversation.

Nonsense, superluminal communications implies no such conversation.
If the parties both rebroadcast the conversation over a luminal transmission both could pick up a echo (time delayed) of both sides of the same casually ordered conversation from the other at a later time on the luminal communications channel.

They should also be able to figure out if they are on a preferred ref frame by simply checking over the superluminal communications channel to see if their local clocks are actually synchronized as the luminal communications channel implies

Your response suggests you have a clear idea of what superluminal communications implies, but you have not made it clear what exactly you think it does imply.

When I first read your response I thought you were suggesting that superluminal communications would not predict anything unusual and that it would shoehorn straight into the special theory of relativity without any modifications to the theory. I was trying to make the case that superluminal comms does not fit into an unmodified special relativity theory.

It is often stated that there is no law of physics that forbids particles that move at greater than the speed of light as long as they as never slow down to subluminal speeds. It is easy to calculate that such particles have imaginary length, imaginary proper time and imaginary mass. In other words superluminal particles are purely imaginary in every way. These are the hypothetical tachyons. Personally I believe the laws of physics does forbid the existence of such superluminal particles and it almost certainly forbids the interaction of any such particles with normal matter making them useless for communications as they can not be detected.

You reject the notion that “3) There is no absolute reference frame.” is a principle. Maybe I was wrong to call it a principle, but it is certainly an implication of "2) The laws of physics are the same in any inertial reference frame." In fact it just another way of stating principle 2 if we define an absolute reference frame as a special unique reference frame. The principle that the laws of physics are the same in all reference frames clearly implies there is no special, unique or absolute reference frame.

It does not say you can arbitrarily pick any frame as the absolute reference frame. (and here you even graph and use two different frames and selectivly use both as absolute)

I half expected I would be accused of selectively using reference frames because I was being lazy to keep the explantion brief and perhaps I could have been clearer. I will break down the explanation further here to try and make my reasoning clearer.

Alice sends a superluminal signal to Bob at 2c in her reference frame.

In Bob's reference frame, Alice is going away from him at 0.9c and from Bob's point of view, her signal appears to be going from the future to the past. This is a simple Lorentz transformation.

If we accept the Lorentz transformation as valid then we see that Alice sending a 2c superluminal signal in her own reference frame is the same as sending a signal into the past in Bob's reference frame. Now this is the crucial part of the argument:

IF Alice can send a superluminal signal to Bob in her own reference frame then Alice can send a signal backward in time in Bob's reference frame. IF Alice can send a signal backwards in time in Bob's reference frame then Bob can send a signal backwards in time in Alice's reference frame because the laws of physics are the same in all inertial reference frames.

We can now do it all just in Alice's reference frame.

Alice sends a superluminal signal to Bob (forward in time)
Bob sends a reply to Alice backwards in time. (We have established that IF superluminal signalling is allowed by the laws of physics, THEN sending a signal back in time is also allowed by the laws of physics.)
Bob's reply is received by Alice before she sends the initial query.

Now to implications for free will:
You will notice that I said "we probably have to reject the notion of free will and accept that the future is predetermined." The word "probably" means I am not sure. Certainly if superluminal signalling is allowed then are implications for the concept of free will that would have to be carefully evaluated. If we extend the idea of superluminal signalling to the idea of sending physical particles at superluminal velocities then there can be physical iinteractions in the past that are caused by events in the future. If those physical interactions (such as Bob killing Alice) prevent Alice from carrying out the actions in the futre that caused Bob to kill Alice in her past, then we have physical effects with no cause which is generally against the spirit of physics in general and not in agreement with actual observations.

At this point I do not want to delve to deeply into free will as it touches on the very meaning of our existence and I don't want to go there. I think it would be depressing to find out that there is a proof that there is no free will as it makes life seem pointless. I do not claim to have presented a proof or refutation of free will. I merely suggested that there are implications for free will if superluminal signalling is allowed by the laws of physics. I hope I have made it clear that that I do not believe superluminal signalling is is allowed by the laws of physics. I have presented my arguments for why I think superluminal signalling is not allowed. I would be interested to see your arguments for why super luminal signalling is not allowed beyond the "it just isn't" argument.

Simultaneity says the only time you can know two events are actually simultaneous is when they happen physically spatially next to each other.

I agree. That is why I suggested only using Alice's clock. The events "sending query" and "receiving reply" happen in the same location and the order of those events is determined by one single stationary clock (Alice's) so that no ambiguity of the simultaneity of spatially separated events is involved.

We both know how to synchronise clocks in different reference frames and how special relativity works and how to do a Lorentz boost so there is no need to establish all that again. All we have to do is accept the special theory of relativity and accept that the Lorentz transformation is a valid transformation and show that the Lorentz transformation of a superluminal signal or particle results in a signal or particle going backwards in time.

 

[JakeStan]

Well the problem with our superluminal communication argument negating free will is contextual. That is you are using our intuition to disregard a flaw in your reasoning. That is the time it takes for a message to get from sender to receiver. That time can never be less than zero because there will always be a distance covered. So if it takes 10 seconds for a luminal signal to traverse from Bob to Alice, then a superluminal message sent at 2c will take half the time, not negative time. All you are doing is further dividing the distance by the rate which will give you ever decreasing time it takes for the message to reach it's destination, never giving you a negative number (going back in time). It is a funny conversation but unfortunately not an accurate portrayal of our universe.

That being said, this isn't a endorsement of speeds greater than that of light, just that superluminal messaging isn't a solid argument against it.

 

[yuiop]

...

That is you are using our intuition to disregard a flaw in your reasoning. That is the time it takes for a message to get from sender to receiver. That time can never be less than zero because there will always be a distance covered. So if it takes 10 seconds for a luminal signal to traverse from Bob to Alice, then a superluminal message sent at 2c will take half the time, not negative time. All you are doing is further dividing the distance by the rate which will give you ever decreasing time it takes for the message to reach it's destination, never giving you a negative number (going back in time). It is a funny conversation but unfortunately not an accurate portrayal of our universe.

That being said, this isn't a endorsement of speeds greater than that of light, just that superluminal messaging isn't a solid argument against it.

Hi Jake ( and welcome to PF )

You are right that if it takes 10 seconds for a luminal signal to travel from Alice to Bob then a superluminal message sent at 2c will take half the time. You are wrong that "time it takes for the message to reach it's destination, never giving you a negative number (going back in time)." In a different reference frame a superluminal signal going forwards in time can become a signal going back in time. It is your intuition that is leading you to the wrong conclusion and intuition will nearly always let you down in relativity as it far removed from our everyday experience.

Intuition is the romantic glow of a candle lit dinner. Maths is the cold light of day.

Here is the maths:

I am using coordinate notation such that (x,t) is (distance,time)

Alice's frame (S)

Event A: Alice sends a superluminal signal at (0,0)
Event B: Bob receives the superluminal signal at (10,5)

The signal travels 10 lightseconds in 5 seconds (2c) in Alice's frame.

Bob's frame (S')

Event A' = (0,0)
Event B' = (x',t')

Using the Lorentz transformation

x^{\prime} = \gamma (x - vt)

t^{\prime} = \gamma \left( t - \frac{vx}{c^2} \right)

where \gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}

Assume Bob is moving at 0.8c relative to Alice and units such that c=1.

\gamma = \frac{1}{\sqrt{1 - \frac{0.8^2}{1^2}}} = 1.666

x^{\prime} = 1.666 ( 10 - 0.8*5) = 10

t^{\prime} = 1.666 \left( 5 - \frac{0.8*10}{1^2} \right)= -5

There is the negative time interval: minus 5 seconds.

I have shown that there is a reference frame where the superluminal signal travels backwards in time.
As I mentioned before a basic principle of relativity is that the laws of physics are the same in all inertial reference frames. Therefore I only have to show that a superluminal signal can go backwards in time in one inertial reference frame to show that a superluminal signal can go backwards in time in any inertial reference frame, (if superluminal signals are allowed by the laws of physics).

I have drawn the spacetime diagrams and done the maths. Everything else is handwaving.

As soon as you admit superluminal signalling as a law of physics you allow signalling backwards in time as law of physics. That is an inescapable conclusion. (That does not mean I am endorsing superluminal signalling as a law of physics either.) The implications for free will are debateable.

 

[robousy]

Hey Jake,

I contemplated the idea that c is a quantity that is derived from some higher dimensional ultimate speed [\tex]\tilde{c}[/tex] https://www.physicsforums.com/showthread.php?t=266958. Bentheman will argue otherwise. ->

 

[Jonathan Scott]

Well the problem with our superluminal communication argument negating free will is contextual. That is you are using our intuition to disregard a flaw in your reasoning. That is the time it takes for a message to get from sender to receiver. That time can never be less than zero because there will always be a distance covered. So if it takes 10 seconds for a luminal signal to traverse from Bob to Alice, then a superluminal message sent at 2c will take half the time, not negative time. All you are doing is further dividing the distance by the rate which will give you ever decreasing time it takes for the message to reach it's destination, never giving you a negative number (going back in time). It is a funny conversation but unfortunately not an accurate portrayal of our universe.

That being said, this isn't a endorsement of speeds greater than that of light, just that superluminal messaging isn't a solid argument against it.

If superluminal messaging is only relative to some absolute "ether" frame, then it doesn't result in causality problems. However, if it were possible to send messages even slightly faster than light relative to any general moving frames, and Bob and Alice are moving relative to one another, then this would mean that messages moving at superluminal speeds relative to Bob could be traveling back in time relative to Alice and vice versa.

 

[RandallB]

You claim that I am "making unfounded conclusions about reality based on assumptions on what would happen if you built such a system". ...

I do not see how you can conclude Backwards or Forwards motion in time without make assumptions outside the rules of SR Simultaneity. You will need to be much more detailed to show how you do that.

Your response suggests you have a clear idea of what superluminal communications implies, but you have not made it clear what exactly you think it does imply. ...

I made a conclusion not an implication.
I concluded that by using SR rules (including causality) and adding new information from a fictitious FTL Communications Network we would be able to establish a Preferred frame.
At least accurately enough to know that other frame clock synchronizations are well out of sync with realty and not useful for determining real Simultaneous events.
A determination not possible within the realistic rules of SR.
Since FTL communications does not exist in reality, SR alone implies that a preferred frame cannot be defined. But saying a preferred frame cannot be defined does not conclude that one may not exist, only that it cannot be defined.

The problem I see is so many that think they know SR, somehow believe SR Simultaneity gives them permission to use any arbitrary frame as preferred.
The Einstein SR Simultaneity Rule is clear, no frame of synchronization is to be trusted to define Simultaneous.

I half expected I would be accused of selectively using reference frames because I was being lazy to keep the explantion brief and perhaps I could have been clearer. I will break down the explanation further here to try and make my reasoning clearer.

Alice sends a superluminal signal to Bob at 2c in her reference frame.

In Bob's reference frame, Alice is going away from him at 0.9c and from Bob's point of view, her signal appears to be going from the future to the past. This is a simple Lorentz transformation. ...

Why would you only half expect that?
Please explain how you established this using SR rules without arbitrarily picking a preferred frame and then using a frame that then gives the appearance of Backwards as also preferred and “real”.
It requires violating the Simultaneity Rule twice, by using two frames as correct in defining "real" time. Simultaneity does not let you even pick one to do that.

We can now do it all just in Alice's reference frame.

No, not without violating the Simultaneity Rule at least once maybe twice more.

Simultaneity says the only time you can know two events are actually simultaneous is when they happen physically spatially next to each other.

I agree. That is why I suggested only using Alice's clock. The events "sending query" and "receiving reply" happen in the same location and the order of those events is determined by one single stationary clock (Alice's) so that no ambiguity of the simultaneity of spatially separated events is involved.

But you didn’t use just Alice’s clock you used other clocks as preferred to show events as moving forward or backwards with respect to Alice’s clock then switched back to Alice’s clock as preferred.
Not within the rules of SR and completely unrealistic.

So NO
If you accept Lorentz Transformation and Simultaneity Rules as valid, you cannot imply anything about free will by violating Simultaneity three or four times.

 

[JakeStan]

Doesn't the Lorentz Transformation include c in it in the form of the Lorentz Factor? That would be using an argument that already accepts c as the limit of velocity in the universe to prove that nothing can go faster than c. Then it becomes an argument of terms, when you say "you can't have superluminal messaging because it eliminates free will and causality" and then cite the Lorentz Transformation you are really saying "you can't have superluminal messaging because c is the speed limit of the universe". I would agree to the latter point but the prior is an extra step in rationale that is unnecessary.

Edit: Also, thanks for the welcome, I'm really enjoying the site so far and please forgive any faults in my form or etiquette!

 

[yuiop]

Doesn't the Lorentz Transformation include c in it in the form of the Lorentz Factor? That would be using an argument that already accepts c as the limit of velocity in the universe to prove that nothing can go faster than c. Then it becomes an argument of terms, when you say "you can't have superluminal messaging because it eliminates free will and causality" and then cite the Lorentz Transformation you are really saying "you can't have superluminal messaging because c is the speed limit of the universe". I would agree to the latter point but the prior is an extra step in rationale that is unnecessary.

Hi Jake,
Let's start with two postulates that Special Relativity is built on. The postulates are the main assumumptions implicit in the theory.

1. First postulate (principle of relativity)

The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.

2. Second postulate (invariance of c)

Light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.

Note that no where in the postulates does it explicity state that c is the maximum speed limit of the universe, so the maximum speed is a conclusion rather than an assumption. We can use the Lorentz transformations for mass and length to conclude that any particle traveling faster than the speed of light has imaginary properties. That in itself does not exclude the possibility of particles that naturally travel faster than the speed of light and tachyons have been seriously proposed as a hypothetical particles with just such properties although as far as I know none have been detected. Now if tachyons could be emitted and detected by normal matter then they could be used to communicate faster than light (FLT).

Now,
IF (the special theory of relativity is a valid description of the universe
AND the Lorentz transformations are a valid description of the special theory)

THEN
IF (FLT communications are allowed by nature THEN causality is violated/reversed)

I have tried to show why this is the case in the diagrams and calculations of my previous posts, so now I will have to resort to an appeal to authority to make my case.

In this post https://www.physicsforums.com/showpost.php?p=719347&postcount=6 by Pervect, a former staff member of PhysicsForums and well respected contributer with over 5000 posts to his name, Pervect gives a link to FAQ site that eventually points to this webpage http://www.physicsguy.com/ftl/html/FTL_part4.html#chap:unsolvableparadoxes

That link by Pervect explains that

IF
FLT communications are allowed by nature (and SRT is a valid theory)
THEN
causality is violated/reversed
AND
this leads to unsolvable paradoxes.


So if we assume nature does not have unsolvable paradoxes (and SRT is a correct theory)it is assumed nature does not allow FLT transmission of information, energy or matter.

There is a way to avoid the causality violations/reversal (and unsolvable paradoxes) of FLT comms and that is to discard the first postulate of relativity which of course cripples special relativity as a theory and the Lorentz transformations will have to be reformulated.

To sum up what we have so far, there is nothing in relativity that says that nothing can go FLT. It just says we cannot use FLT particles (tachyons) to communicate or transmit normal matter FLT.

Now of course Relativity might be wrong and the Lorentz transformations might be wrong and physics FAQ page on FLT linked to by Pervect might be wrong and in that case Randall would be right in his assertion that FLT does not imply causality violations/reversal and unsolvable paradoxes.

So now to the question of free will which is really a question of whether the universe is deterministic or not.

Lets say for the sake of argument some physicist has really discovered FLT comms in a lab and not just got confused over group and phase velocities of light as has happened often in the past. This would mean (assuming relativity is a correct theory) that in certain circumstances effect will precede cause. In a non deterministic universe it is easy to show that actions could be taken that that prevent the cause in the future that caused the effect in the past. So in a non deterministic FLT universe we could have effects with no cause making a chaotic universe with random inexplicable events that seems to follow no logic. (hmmm.. maybe that IS the universe we live in from a quantum point of view ) If the FLT universe is rational and every effect has a cause then the universe would have to be deterministic with some "outside" influence ensuring that the future unfolds in such a way that the appropriate future causes occur to explain the events of the past. Take your pick.

Edit: Also, thanks for the welcome, I'm really enjoying the site so far and please forgive any faults in my form or etiquette!

 

[JakeStan]

I think we are agreeing here, I don't believe that FLT communications are possible but only because I believe that the speed of light is the speed limit of the universe. My point was more that the causality and free will argument is hollow because it uses the speed of light as a given to present the argument. A way to look at it is if you chance c in the Lorentz Factor to 1/2c then you use c for any communications in the examples, you can prove that luminal communication would give you negative time and imply free will and causality do not exist. It is using word in its own definition.

Again, I'm not trying to use this as an argument for FTL communications, just that if you were ever to break down the speed of light (thus changing how the Lorentz Transformation works) you could have FTL communications but it would be moot because you've already gone past the speed of light anyways!

 

[RandallB]

IF FLT communications are allowed by nature THEN causality is violated/reversed

I have tried to show why this is the case in the diagrams and calculations of my previous posts, so now I will have to resort to an appeal to authority to make my case.

No authority can support that case without violating the Simultaneity Rule.

 

[yuiop]

I think we are agreeing here, I don't believe that FLT communications are possible but only because I believe that the speed of light is the speed limit of the universe. My point was more that the causality and free will argument is hollow because it uses the speed of light as a given to present the argument. A way to look at it is if you chance c in the Lorentz Factor to 1/2c then you use c for any communications in the examples, you can prove that luminal communication would give you negative time and imply free will and causality do not exist. It is using word in its own definition.

Again, I'm not trying to use this as an argument for FTL communications, just that if you were ever to break down the speed of light (thus changing how the Lorentz Transformation works) you could have FTL communications but it would be moot because you've already gone past the speed of light anyways!

The trouble is that if you change the the c in the Lorentz factor to to 1/2c then the Lorentz gamma factor becomes:

\gamma = \frac{1}{\sqrt{1 - 4\frac{v^2}{c^2}}}

rather than the usual

\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}


and time dilation and momentum measurements made in particle accelerators every day would not agree with the equations, so that new equation would not represent the physical laws of the universe we live in.

 

[Ich]

That is you are using our intuition to disregard a flaw in your reasoning.

There is no flaw in kev's reasoning, but a hidden assumption: if the principle of relativity still holds, superluminal speed implies time travel.

 

[RandallB]

There is no flaw in kev's reasoning, but a hidden assumption: if the principle of relativity still holds, superluminal speed implies time travel.

Galileo’s principle of relativity will still hold without implying time travel IF you hold to Special Relativity Principles like the Simultaneity Rule.
Violating the Simultaneity Rule is the flaw that allows the illusion of causality violations that imply “time travel”.

 

[DrGreg]

Galileo’s principle of relativity will still hold without implying time travel IF you hold to Special Relativity Principles like the Simultaneity Rule.
Violating the Simultaneity Rule is the flaw that allows the illusion of causality violations that imply “time travel”.

Can you please explain exactly what you mean by "the Simultaneity Rule"? This isn't standard terminology.

 

[RandallB]

Can you please explain exactly what you mean by "the Simultaneity Rule"? This isn't standard terminology.

See post #20

 

[Ich]

Galileo’s principle of relativity will still hold without implying time travel IF you hold to Special Relativity Principles like the Simultaneity Rule.

There is no Galileo's principle of relaitvity nor an Einsteinian one. There is a Principle of relativity which states that you can't find a preferred frame by whatever intrinsic measurement you do, and that means that speed be defined relative to the observer, not to a preferred frame. Especially talking about FTL, in the easiest case of infinite speed, this can only mean along the observer's line of simultaneity, as no other definition can be given. The "SR rules" concerning simultaneitiy are the only ones compliant with the principle of relativity.

 

[RandallB]

There is no Galileo's principle of relaitvity nor an Einsteinian one.

The “special principle of relativity” was established by Galileo 1639

Einstein used it to explain a new physics for Light, which is why Max Planck named the new theory “Special Relativity”.

Special Principle of Relativity: is a 17th century principle that still holds.
Special Relativity: is a theory from 1905 that offers rules or new principles.

The Special Principle of Relativity became an axiom of Special Relativity.
Without the ‘Special Principle of Relativity’ there is no theory of Special Relativity to establish SR – Rules.
Most importantly:
1) Uniform speed of Light
2) Simultaneity

Ref:Wiki

 

[yuiop]

The problem I see is so many that think they know SR, somehow believe SR Simultaneity gives them permission to use any arbitrary frame as preferred.

I am not sure where you think I selected a reference frame as preferred. In the diagram I uploaded in https://www.physicsforums.com/showpost.php?p=1945486&postcount=10" of this thread, it is clearly shown that Alice receives the reply from Bob before she sends the query in BOTH reference frames. The same would be true from the point of view of any other reference frame you care to choose. Here is the diagram again:

http://img520.imageshack.us/img520/3835/superluminalmf5.gif

The Einstein SR Simultaneity Rule is clear, no frame of synchronization is to be trusted to define Simultaneous.

It is true that in Alice's reference frame the superluminal query she sends to Bob appears to going forward in time while in Bob's reference frame Alice's query signal appear to be going backward in time. So we can say nothing in an absolute sense about whether Alice's original superluminal query signal to Bob goes forward or backward in time.

The two reference frames appear to contradict each other in that aspect and as you correctly say "no frame of synchronization is to be trusted to define Simultaneous" especially when two events like (Alice sending query) and (Bob receiving Alice's query) are spatially separated (by a space-like interval). Bob's point of view that Alice's superluminal query signal went backwards in time and Alice's point of view that her superluminal query signal went forwards in time are both equally valid even though they appear to contradict each other. That is a consequence of the Principle of relativity and we just have to live with the non intuitive ambiguity of the temporal order of spatially separated events!

When two events are not spatially separated (i.e they are separated by a time-like interval) then there is no ambiguity about the temporal order of the two events and ALL inertial observers, whatever their relative velocity, will agree on the temporal order of those two events.

The two events A= (Alice sending her superluminal query) and B = (Alice receiving the reply to her query from Bob) are NOT spatially separated in Alice's rest frame and ALL observers agree that B happened before A, so I am not preferring any reference frame to make that claim.


"[URL challenge of changing deeply held student beliefs about the relativity
of simultaneity[/B][/URL]

 

[Ich]

The “special principle of relativity” was established by Galileo 1639

Ok.
And how does this fit with a preferred frame? Not CMB matter, but different laws of physics, i.e. different tachyonic speeds depending on orientation and absolute state of motion?

 

[RandallB]

I am not sure where you think I selected a reference frame as preferred.

When you used both “Bob's point of view” and “Alice's point of view” as reliable to judge distant event synchronizations (causality) you did not just define a preferred frame, you used two of them!

As I said before; You need to be much more detailed:
Care to explain exactly when and where in the Alice's frame Bob was when the 2c FTL message was sent and recieved?
And exactly where and when was Alice in Bob’s frame when she sent it and when he received it to return a #C FTL reply?
How fast are Alice and Bob moving wrt each other and is that towards each other or apart?
Only “c” appears to be the same in all frames; how fast is the c2 signal measured in Bob's frame and how fast is the 3c signal when measured in Alice’s frame?

Exactly what established the foreword and backward time changes you are plotting if not some frames of reference?

What do you change that would have caused Bob’s replies to have come further in the future than of all Alice’s questions and responses to his replies.

 

[RandallB]

Ok.
And how does this fit with a preferred frame? Not CMB matter, but different laws of physics, i.e. different tachyonic speeds depending on orientation and absolute state of motion?

You claimed:
“There is no Galileo's principle of relaitvity nor an Einsteinian one.”
By "OK" Are you retracting that?

As to “preferred frame”; it is up to kev[/v] to show how he established the time sequences in his problem not me.

 

[yuiop]

When you used both “Bob's point of view” and “Alice's point of view” as reliable to judge distant event synchronizations (causality) you did not just define a preferred frame, you used two of them!

It is not a case of defining a preferred frame. That is sort of the whole point of relativity. there is no preferred frame. I used two frames to describe the situation from different view points and I could have used an infinite number of different frames. Being able to describe a situation from any number of arbitary reference frames is the same as having no preferred frame.

When
As I said before; You need to be much more detailed:
Care to explain exactly when and where in the Alice's frame Bob was when the 2c FTL message was sent and recieved?
And exactly where and when was Alice in Bob’s frame when she sent it and when he received it to return a #C FTL reply?
How fast are Alice and Bob moving wrt each other and is that towards each other or apart?

I mantioned in the first post with the diagram that Bob and Alice are moving away from each other at 0.9c. The rest of your questions are answered in this updated diagram.

http://img383.imageshack.us/img383/4903/superluminal2zn9.th.gif

Note that observers in Alice's frame do not agree with a lot of things that observers in Bob's frame measure and vice versa. That is the failure of simultaneity of space-like events.

Only “c” appears to be the same in all frames; how fast is the c2 signal measured in Bob's frame and how fast is the 3c signal when measured in Alice’s frame?

In Bob's frame Alice is moving at -0.9c (right to left) and by the velocity addition formula her query signal of +2c (left to right) as measured in her frame, is 1.235c (left to right) according to observers in Bob's frame.

Note that the sign of the velocity does not tell you whether the signal is going forward or backwards in time but only tells you whether it is going left or right.

In Alice's frame, Bob is moving at +0.9c (left to right) and by the velocity addition formula his reply signal of -3c (right to left) as measured in his frame, is -1.375c (right to left) according to observers in Alice's frame.

Note once again the negative signal velocity in itself does not imply backwards in time but only give direction (right to left).

To determine the order of events, the time intervals have to be calculated using the Lorentz transformations as demonstrated in https://www.physicsforums.com/showpost.php?p=1947893&postcount=17".

Exactly what established the foreword and backward time changes you are plotting if not some frames of reference?

The Lorentz transformations establish the forword and backward time changes.

What do you change that would have caused Bob’s replies to have come further in the future than of all Alice’s questions and responses to his replies.

If Bob is at rest with respect to Alice or if Bob is going towards rather than away from Alice, then Alice would receive the replies after the queries (normal time order) even with superluminal signal speeds.

However, I don't think that is exactly what you are asking and it hard to interpret what you mean. Could you rephrase it using normal syntax?

Also, "Bob's replies" is a vague. There are two events involved in a reply, namely, Bob sending a reply and Alice receiving Bob's reply. If you mean that Bob sending a reply is in the future of Alice receiving the reply then that is entirely subjective. There is no way to determine if one event is in the future or past of another event in any absolute sense, if those two events are space-like separated.

 

[RandallB]

It is not a case of defining a preferred frame. That is sort of the whole point of relativity. there is no preferred frame.

But you diagramed two frames with causality lines over a distance. And then selectively used both of them for different parts of a single solution to create time travel as you pleased – a double error as I said before.

I used two frames to describe the situation from different view points and I could have used an infinite number of different frames.

Fine use the velocity addition formula to create a third frame in the middle with A and B frames moving away from it at the same speed in different directions.

Being able to describe a situation from any number of arbitary reference frames is the same as having no preferred frame.

That does not mean you get to generate multiple frames, use one some of the time and another for another time interval to assume whatever you want about how time at a distance is behaving.
The requirement is to solve the entire problem IN COMPLETE DETAIL as in real numbers not just lines. From beginning to end in just one reference frame (not using two frames at the together as you are doing); including using Lorentz transformations to define Where and When each event happened in the location and time coordinates of the other two frames.
Then pick another frame and do the same again.
And since you claim you can do it with any number of arbitrary reference frames do it for a third as well.

Only if all three frame views each give the same results for all three frame coordinate times and locations for each event can you claim to have a solution that all frames agree on.

That means real numbers, not just a line that says “time goes backwards here”.

If you can show that a “reply” arrives at Alice before Alice sent it; Then you have to be able to show When and Where Bob was in each Ref Frame: 1) When Bob received it and replied [One event] 2) When Alice sent it. 3) And when Alice received the reply

You have not offered anything like that.

And the simultaneity rule makes it clear you cannot. If there is a preferred frame there can only be one, you cannot use two frames at the same time as if they were both preferred. You don’t need FTL signals to establish synchronized clocks in one frame and have SR show a second frame synchronized clocks out of sync indicating forward or backward time events. Any Twins problem will do that.
Simultaneity does not say you can pick the one you like; Simultaneity says you cannot trust anyone of the time lines as correct for defining causality.

Uses real numbers and do the math, you will get stuck soon enough.

 

[gdsandkes]

Is that not exactly what is postulated in inflationary theory in cosmology, in an attempt to explain the observable universe?

 

[yuiop]

And the simultaneity rule makes it clear you cannot. If there is a preferred frame there can only be one, you cannot use two frames at the same time as if they were both preferred.

Yes, IF there is a preferred frame... , but the first postulate of relativity clearly states there is NO preferred frame.

Uses real numbers and do the math...

I did, in post#17 and I note that you have not done any.


Look at it another way.

Let's assume, a priori for the sake of argument, that Alice sends a superluminal signal to Bob and Bob sends a reply traveling backwards it time to Alice. This is all in Alice's frame. Now switch to Bob's frame. We now see that what was a superluminal signal in Alice's frame has become a backwards in time signal in Bob's frame. The reply that was a signal going backwards in time in Alice's frame becomes a superluminal signal going forward in time in Bob's frame.

It should become clear to you that superluminal signalling and signalling backwards in time are two sides of one coin. They go hand in hand. If you have one then you have the other. If you don't have one then you don't have either. The most likely option is the latter because superluminal signalling and time travel have never been observed and if they did exist, the paradoxes are unresolvable.

 

[RandallB]

Is that not exactly what is postulated in inflationary theory in cosmology, in an attempt to explain the observable universe?

?
Explain what you mean by “that”
And what is “what is postulated in inflationary theory”
I have no clue what you are talking about,
This is a SR issue AFAIK not a Big Bang one.

 

[gdsandkes]

I am responding to the original question in this thread "speeds greater than the speed of light"

 

[DrGreg]

RandallB, there's nothing wrong with kev's argument. You cannot insist on considering a single frame only; we have to consider multiple frames in order to use the principle that all frames are equally valid. An argument that relied on a single frame could not be a relativistic argument.

You demanded some real numbers, so here goes (and let's hope I haven't made an arithmetical error!).

Let's consider Alice and Bob moving apart at speed v=4/5 (in units where c=1), so that \gamma = 5/3 and \gamma v = 4/3.

In Alice's frame consider the three events E, F, G given by

(t_E, x_E) = (0, 0)
(t_F, x_F) = (12, 48)
(t_G, x_G) = (-21, 0)​

Note that events E and G both occur at distance zero from Alice; Alice experiences both events directly and the elapsed time of 21 seconds between them is Alice's proper time, measured by a single clock. Therefore all observers unambiguously agree that event G occurs before event E.

Bob moves at speed 4/5 in the positive x direction, synchronising his clocks and distances to zero at event E. So, applying the Lorentz transform

t' = \gamma(t - vx) = \frac{5t - 4x}{3}
x' = \gamma(x - vt) = \frac{5x - 4t}{3}​

we measure the 3 events in Bob's frame as

(t'_E, x'_E) = (0, 0)
(t'_F, x'_F) = (-44, 64)
(t'_G, x'_G) = (-35, 28)​

So far so good. No mention of signalling or velocities, just a slightly painful*[/color] exercise in the Lorentz transform for 3 events.

Now, in Alice's frame, if a signal were to travel from E to F, it would travel forwards in time (according to Alice) at a speed of 48/12 = 4c, faster than light. And in Bob's frame, if a signal were to travel from F to G, it would travel forwards in time (according to Bob) at a speed of (28 - 64) / (-35 + 44) = -4c, faster than light. (The minus indicates the direction of travel.) But now we have sent a message from E to F to G, and G occurs unambiguously (absolutely) before E.

What did we assume above?
(a) that a signal could travel at 4c relative to Alice in the + direction
(b) that a signal could travel at 4c relative to Bob in the - direction
(c) that the Lorentz transform is valid.

If you accept all the above assumptions are true, then I have shown how to send a message backwards in time. If you think that's impossible then at least one of the assumptions above must be wrong. (And I could have picked different numbers for any speed greater than c instead of 4c.)

If you think (a) could be true but (b) could be false, that would be to deny the principle of relativity: the laws of physics should be the same for both Alice and Bob. If you believe in relativity and also believe in causality, then both (a) and (b) must be false.

_______
*[/color]More painful for me than for you, because I had to work backwards from my objective of getting 4c for the answer!

 

[RandallB]

RandallB, there's nothing wrong with kev's argument. You cannot insist on considering a single frame only; we have to consider multiple frames in order to use the principle that all frames are equally valid. An argument that relied on a single frame could not be a relativistic argument.

What is “the principle that all frames are equally valid”.
I’ve never seen, or used it. Who declared it? how did they justify it? and
when did the science community acknowledge it in peer reviewed publication?

I do not “considering a single frame only” at all.
I reject relying on even one frame as correct in SR.
Because SR simultaneity is clear that you can not declare synchronized clocks as simultaneous. In SR synchronized and simultaneous are two different things and no one (or Two) reference frames can resolve that in SR.

" ... I had to work backwards from my objective of getting 4c for the answer! "
And yes I can see you worked backwards from only the detailed answers that support you objective without giving any of the details I asked for.
IF “all frames are equally valid” then pick any two additional frames and see if you can them to give same amount of time causality error. Since kev was already indentifying how to pick frames that would show a forward rather than backward time causality error, how could they be “equally valid”?

 

[yuiop]

What is “the principle that all frames are equally valid”.

It is usually know as the 1st postulate of relativity that is usually paraphrased in casual terms as "the laws of physics are the same in all inertial reference frames".

I’ve never seen, or used it. Who declared it?

Einstein.

how did they justify it?

By showing that the implications (predictions) of making that assumption (postulate) agreed with actual experimental results better than Newtonian predictions.

and when did the science community acknowledge it in peer reviewed publication?

.. I can not give it an exact date, but the world at large accepted Einstein's Relativity Theories in 1919 when Eddington's observations of an eclipse confirmed Einstein's predictions over Newton's. The scientific community accepted his theries much earlier when it explained the null MMX result and the "anomalous" precession of the perihelion of Mercury.

 

[gdsandkes]

Then according to E=Mc2, if you increase the speed of light then you violate the equation, (unless you alter the total mass and/or energy)

Thus my question about inflation in Cosmology.

 

[RandallB]

What is “the principle that all frames are equally valid”.

It is usually know as the 1st postulate of relativity that is usually paraphrased in casual terms as "the laws of physics are the same in all inertial reference frames".

Einstein.

kev this is old ground. Just repeating old errors is not helpful.
We already covered the “special principle of relativity” by Galileo from 1639 and that it became part of SR.
That has nothing to do with how you are applying “all frames are equally valid” in defining events based on their synchronized clocks – regardless of what other frames say.

Fact is Einstein showed that this is not true with SR! That is the whole point here.

Sure that could mean SR is “incompatible with the first postulate” (oh no what to do?)
Except the first postulate does not mean all inertial frame have the same “classical laws” it says they all use the same laws of physics; And SR comes with an addition Law, Rule, whatever you want call it.

And that is the Simultaneity Rule where NO FRAME CAN BE TRUSTED to define any from of absolute measure of simultaneous only synchronized clocks that can give the appearance of distant simultaneous events but only in the view of that frame no others.

Is that the same in all frames,
YES because no frame is allowed to define simultaneous events as real!

And you cannot define Backwards time without picking using a frame or two or three or four all defining their own version simultaneous events in synchronized clocks giving multiple versions of different absolute times.
Which you keep doing demanding to do in violation of SR Simultaneity, so whatever your doing it is not based on relativity.

Down side of Simultaneity;
since the rule says no frame can be trusted to absolutely define separate events as simultaneous, then a the Newton idea of absolute time, cannot be supported.

Physics is currently based on not using Newton absolute time. Other than appling Simultaneity as I've discribed it, just how did science come to abandon Newton absolute time?

gdsandkes
This issue here has nothing to do with GR, BigBang, inflation e=mc2 or Cosmology. It is just a matter of correctly using the SR Simultaneity Rule.

 

[rbj]

The trouble is that if you change the the c in the Lorentz factor to to 1/2c then the Lorentz gamma factor becomes:

\gamma = \frac{1}{\sqrt{1 - 4\frac{v^2}{c^2}}}

rather than the usual

\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}and time dilation and momentum measurements made in particle accelerators every day would not agree with the equations, so that new equation would not represent the physical laws of the universe we live in.

kev, keep in mind that any physical measuring instruments in any particle accelerators or anything anywhere/anytime, that in all of these measurements, what is essentially being measured are dimensionless quantities. you never measure a length or a time by itself apart from some other reference (tick marks on your meter stick or ticks on your clock). no measurement of c is made without reference to some other velocity. it is only the dimensionless ratios of these like-dimensioned physical quantities that matter, in the final analysis. this, plus the fact that the speed of light (or some other EM radiation) can be so handily used in a lab along with a good (cesium) clock to define the meter as a unit length such that c is defined to be 299792458 m/s.

so what you're saying about changing c to c/2 is just non-sensical. even if you were a some kinda god-like being with access to the control knobs of nature and changed it (in some manner so that you would know the difference), it would still be c to us mortals that are governed by physical reality and we would not know the difference (unless some dimensionless "constant" like \alpha changed, and then the net essential issue is that \alpha = \frac{e^2}{\hbar c (4 \pi \epsilon_0)} changed and which of the dimensionful factors inside of it that changed (e \ \hbar \ c \ \epsilon_0) is only a matter of how your units of measure are defined. and nature doesn't care which units we use. so the it doesn't matter and a changing c is meaningless. could just as well be a changing \hbar.

may i suggest looking at:

Comment on time-variation of fundamental constants

Trialogue on the number of fundamental constants

 

[DrGreg]

What is “the principle that all frames are equally valid”.
I’ve never seen, or used it. Who declared it? how did they justify it? and
when did the science community acknowledge it in peer reviewed publication?

kev has already answered that.

I do not “considering a single frame only” at all.
I reject relying on even one frame as correct in SR.
Because SR simultaneity is clear that you can not declare synchronized clocks as simultaneous. In SR synchronized and simultaneous are two different things and no one (or Two) reference frames can resolve that in SR.

" ... I had to work backwards from my objective of getting 4c for the answer! "
And yes I can see you worked backwards from only the detailed answers that support you objective without giving any of the details I asked for.
IF “all frames are equally valid” then pick any two additional frames and see if you can them to give same amount of time causality error. Since kev was already indentifying how to pick frames that would show a forward rather than backward time causality error, how could they be “equally valid”?

To clarify what your position is:

Do you accept the logic that if (a) (b) and (c) were all true in post #42, then a message could be sent, unambiguously and absolutely, backwards in time?

- If no, which part of the logic do you not understand?

(i) Do you accept that all observers agree that G occurs absolutely before E? If not, what criterion would you accept to prove one event occurs absolutely before another?
(ii) Do you accept that if a signal can travel at 4c relative to Alice it can travel from E to F?
(iii) Do you accept that if a signal can travel at 4c relative to Bob it can travel from F to G?​

- If yes, is the sticking point that you think (a) could be true while (b) is false?

What are the "the details you asked for" that I haven't provided?

 

[Kopachris]

I don't know if this has already been said or not:

Nothing with mass can travel faster than the speed of light. Einstein postulated that as the velocity of an object increases, so does its mass. As said object approaches the speed of light, the mass approaches infinity. At the speed of light, any object with any positive non-zero rest mass would have infinite mass and so it would require infinite energy to accelerate it. There are many phenomena that appear to "break" this rule, but no particles are actually traveling faster than local c.

There is a loophole, however. Objects cannot travel faster than light, but spacetime can. Theoretically, if one could create a pocket of spacetime around a spaceship and get it moving faster than light, it would carry the spaceship with it without breaking any laws. There wouldn't even be any problems with time-dilation or the crushing effects of accelerational g-forces. Lookup "Alcubierre Drive" on Wikipedia for more info.

 

[RandallB]

Oh Good Grief!

kev has already answered that.

Not with a valid or logical answer.

Do you accept the logic that if (a) (b) and (c) were all true in post #42, then a message could be sent, unambiguously and absolutely, backwards in time?

Of course not – to borrow a coined phrase post #42 is just silly, much more so than von Neumann's silly mistake (ref: J Bell)

Do you accept that all observers agree that G occurs absolutely before E?
If not, what criterion would you accept to prove one event occurs absolutely before another?

With the rather obvious flawed logic in #42, of course not.

Since SR simultaneity makes it clear that no ref frame of synchronized clocks can be trusted to define its “simultaneous” clock times as an accurate reference of simultaneity with which to define causality, within the limits of SR I am willing to accept the “apparent simultaneity” (and resulting causality standard) defined by A REFERANCE FRAME.

But unlike you and kev, because I accept the simultaneity rule, I will only accept an analysis based on a standard built only within a single reference frame.

What are the "the details you asked for" that I haven't provided?

I asked details like when and where Bob claimed remote locations were at the event times for “E” and “F”.

You set the time standard as the Alice frame where Alice0 and Bob0 both have the time as t=t’=0 (Alice0 kind of likes Bob0).
- And far ahead you have Alice48 waiting for Bob0 to be coming her way at the “simultaneous” time of t=0 and reports that Bob80a (‘a’head of Bob0) is with her NOW.
- Also, both Alice48 & Bob80a can confirm that t’=-64 is on Bob80a’s clock.
Alice48 is waiting on Bob48a for a date, but he seems to be late, he must have a slow clock just as Bob80 does!
But Bob0 backs up his guy and tells Alice0 that Alice48 just cannot read her own clock and doesn’t know what real time is. (Alice0 frowns at his comment about one of her girls)

Still on your standard:
At t=12 for Alice0 and Alice48 with the 4c FTL signal reaching Alice48 (and returned at some new FTL speed) she is being visited by Bob64a who shows a time of t’= -44 claiming it’s the correct ‘real’ time.
- - Simultaneously Alice0 is visited by BOB16b (‘b’hind Bob0) claiming the correct time is t’=20 and saying her view of Time being defined by t not t’ is flawed.
Alice0 protests and puts a call out for “Where is Bob0”
– Alice9.6 (also at t=12) reports that Bob0 is with her and he claims that the real time is t’=7.2 and "we girls are all nuts" for thinking our time t is a standard for causality and simultaneity!
“Laired” declares Alice0, “kick them all to the curb girls - tell them to move on.”
(no wonder all the Bob’s are ‘moving on’ at 0.8c, bad news for Bob48 & that date)

But after you set the standard of using Alice0 and her girls t time as correct for "real" time;
you and kev decide to “ditch the witch” and switch to side with the boys!
- But only after establishing as “real” the time 12 for event F, and also including to use as "real" the boys claim of a "real" time of -44!
That is using two times as "real" for one event!

You have yet to rationally justify doing that!
- This is not just some twin moving from one frame to another frame; but continue to analyze both twins from the view of just one ref frame.
You are changing the frame by which you claim to analyze reality; from t & x to t’ & x’!

Nowhere in SR is there a rule that allows you to make such a change of view, and use two frames of reference to set two standards for what is "real"!

Depending on how the return speed of the FTL signal is defined – I am willing to continue with you in this problem:
but only in the Alice frame you started with.
There you could find the return signal reaching Alice0 at t=21 with her being visited by Bob28b at t’=35.

So unless you can provide a justification for arbitrarily changing the “real” time standard from t to t’ you are not following the SR rules.
To be sure, I am not implying that Alice is using the correct frame to define “real”; only that SR simultaneity will permit and tolerate the use of a single frame as defining the standard for “real” to work though a problem; but never can two frames as you use them by applied as if both could define a standard for real. The conflict between the boy and the girls here is why and how the SR simultaneity rule was established by Einstein in the first place!

You should redo the problem in only the t’ frame with Bob0 chasing after the signal.
If you do all the math with the extra detail you should be able to demonstrate for us how the when and where the FTL signal will return passing Bob0 to reach Bob28b.
By showing the simultaneous clocks in the Bob frame setting the standard for “real” the various Alice reading will become unrealistic and Bob and his guys will show a forward in time causality.
– that is if you willing to do the math with the extra details without making a faulty assumption just to reach a preplanned result.

[Take your time, so you can check your work; I’ll be out of touch for awhile until likely after Thanksgiving]