Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Anders on 08/07/2009 19:30:02
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Anders asked the Naked Scientists:
Hello,
thanks for an interesting show (http://www.thenakedscientists.com/HTML/podcasts/).
I read that scientists had managed to send information faster than the speed of light, coupled atoms/quantum states or something similar. Change one and the other changes at the same time.
As faster than speed of light is time travel does this means you can send information back in time, in theory?
As there is no mass involved, what kind of theoretical hurdles are there to send information faster than light?
Best regards,
anders
What do you think?
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You can send certain things faster than light, but to my knowledge, information isn't among those things. For example, taking measurements can effect two particles far apart at faster than the speed of light, but for you to get any information out of that, the observers looking at either particle have to contact each other and agree on how they're going to measure the particles, and that last step is limited to the speed of light.
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Sending information would be the simplest, as you can "attach" it to anything else, including light.
"Standard way" they consider is using wormholes due to GRT, but it would be rather practically impossible to realize.
Quantum mechanics is unitary and so time symmetric, what makes that it allows for retrocausality in delayed choice quantum erasure (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser) and Wheeler’s experiment (http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment). However they don't allow to send information this way.
But not only QM is time symmetric - Lagrangian mechanics we use in all scale from QFT to GRT is time or CPT symmetric, so I was thinking about constructing CPT analogue of laser (lasar: stimulating absorption instead of excitation) - conceptually simplest is free electron laser (FEL) (http://en.wikipedia.org/wiki/Free-electron_laser):
(https://dl.dropboxusercontent.com/u/12405967/freeelectron.jpg)
In standard FEL we use magnetic field to enforce electrons to travel in sinus-like pattern, what stimulates photon emission due to synchrotron radiation. These photons are later absorbed by the target, exciting it.
Let us now imagine CPT analogue of this situation: excited target deexcitates - producing photons finally absorbed by positron moving in opposite direction ... but the situation is stimulated by the sinus-like trajectory - we get stimulated absorption setting instead of standard stimulated emission.
So imagine we constantly excite the target (e.g. a sodium lamp), surround it with detectors - we should observe energy conservation.
Now if there is a small hole in detectors to the FEL in stimulated absorption setting (oppositely directed) and set to the same frequency as the excitement (it is rather not fulfilled in standard settings) - shouldn't turning it on change the lamp-detectors energy balance (before)?
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Quantum mechanics is unitary and so time symmetric,…
Please explain what it is that you mean by this assertions? I.e. what do you mean when you say Quantum mechanics is unitary. What do you mean by “and so” time symmetric,…?
what makes that it allows for retrocausality in delayed choice quantum erasure (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser) and Wheeler’s experiment (http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment). However they don't allow to send information this way.
This is all nice and all but it’s best to describe what it is that you’re talking about rather than assuming everyone here knows and is familiar with these things. Otherwise nobody will comment on your posts
But not only QM is time symmetric - Lagrangian mechanics we use in all scale from QFT to GRT is time or CPT symmetric, ..
Please rephrase. I am unable to understand what this means by the way it’s worded.
Most people in these forums don’t know what these terms mean. I myself am not certain what you mean by CPT. I can only guess by context, i.e. CPT – Charge, Parity and Time.
(https://dl.dropboxusercontent.com/u/12405967/freeelectron.jpg)
In standard FEL we use ..
[/quote]
Who are “we”?
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Hi, I apology for not explaining everything - I have attached links to wikipedia articles about these standard experiments.
Indeed CPT symmetry means charge, parity and time - it has to be conserved in quantum field theories ( http://en.wikipedia.org/wiki/CPT_theorem ) - says that fundamentally physics is time symmetric (when we additionally perform P and C).
Another view on Lagrangian mechanics like QFT is by action optimization - that physics finds the action optimizing history (e.g. trajectory in classical mechanics, shape of spacetime in GRT, history of field configuration in field theory). We can imagine that the present is action optimizing equilibrium between past and future - spacetime is kind of 4D jello.
By "quantum mechanics is unitary" I intended to say that its evolution is unitary. There are also problematic wavefunction collapses, but they are believed to be results of interaction with exterior - imagining complete QM: of the Universe, it has no longer exterior and so have purely unitary evolution.
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Hi, I apology for not explaining everything - I have attached links to wikipedia articles about these standard experiments.
Indeed CPT symmetry means charge, parity and time - it has to be conserved in quantum field theories ( http://en.wikipedia.org/wiki/CPT_theorem )
No problem. Thank you very much for understanding! :)
Why are you talking about quantum field theories? So far you’ve been talking exclusively about quantum mechanics. Per your link – the CPT theorem states that any canonical quantum field theory is invariant under the CPT operation, which is a combination of three discrete transformations: charge conjugation, parity and time symmetry. But that’s quantum field theory. In particle physics this is different, e.g. parity can be violated as can CP.
So far your entire post was about quantum mechanics and not field theory. So why bring it up?
- says that fundamentally physics is time symmetric (when we additionally perform P and C).
I want to make sure your readers understand what Jarek means by this, or at least what is meant by it – It refers to the fact that the equations of physics remain unchanged when you replace t by -t. However you are incorrect in this assumption. That only applies to certain areas of physics. E.g. the Shroedinger equation is symmetric in t as are Newton's laws and Maxwell's equations. What isn't symmetric in time are things such as thermodynamics. If you make a movie of a thermodynamic system then you can tell if the movie is running forwards or backwards. This is related to why people get old with age and not younger. It's why a gas will expand naturally but won't collect in a beaker naturally.
Another view on Lagrangian mechanics like QFT is by action optimization - that physics finds the action optimizing history (e.g. trajectory in classical mechanics, shape of spacetime in GRT, history of field configuration in field theory). We can imagine that the present is action optimizing equilibrium between past and future - spacetime is kind of 4D jello.
Nope. Sorry. This is all still too unclear for me to understand. Try again please. To be specific let’s break this apart. Please find a source (text, wiki etc) which defines and describes “action optimization.” I did an Internet search and basically came up with bubkiss. Did you make this term up yourself?
Please define and describe “action optimizing history”. What do you mean when you say that the action is “optimized”?
By "quantum mechanics is unitary" I intended to say that its evolution is unitary.
And what does that mean?
Let’s be precise for the rest of the folks here. The only way that I know of the term “unitary” being used in quantum mechanics is in a type of operator called a “unitary operator” which is defined as an operator U which has the property that UU* = I. There is a theorem which proves that the eigenvalues of a unitary operator are complex numbers of unit modulus.
Given that meaning of “unitary”what do you mean when you say “its evolution is unitary”?
There are also problematic wavefunction collapses, …
Such as?
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I won't go into the question of whether information CAN be sent faster than light, but I shall address the question of whether FTL communication implies the ability to send information back in time. The short answer is YES, but only in reference frames which are in motion relative to some absolutely motionless reference frame. (It's a good guess that the cosmic microwave background is motionless relative to such a preferred frame.)
Special relativity suggests that all inertial reference frames are equivalent, but that is only true if no phenomenon can ever exceed the speed of light. Once you prove the existence of FTL phenomena, you will have proven the existence of a unique preferred reference frame, in which there is no relativistic time shift relative to the FTL phenomenon. Let's call that frame Σ.
For simplicity, let's postulate the existence of a method to communicate instantaneously in Σ. Another reference frame ζ has motion in the +x direction relative to Σ. A message is sent in the +x direction from A to B. According to clocks synchronized in Σ, the message is received at B in the same instant that it is sent at A. Clocks synchronized in ζ would indicate that the message is received at B at an earlier time than when it is sent at A. If receipt of the message is instantaneously acknowledged by B, A will receive the acknowledgment at the same instant that he sent the original message. That is because the time shift in the -x direction is equal and opposite to that in +x direction.
A person cannot send a message from A and receive his own message at B before he sent it. Nor can he send a message to his grandfather to be received before his is born. Such events would require sending messages back in time in Σ. Effect preceding its cause in Σ would be paradoxical, which is good reason to assume it cannot happen. It is not paradoxical, however, for effect to precede cause at different locations in a reference with motion relative to Σ.
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Pmb, I am only saying that fundamental physics does not distinguish past and future.
To imagine how nonintuitive can it be, GRT theoretically allows for non-orientable spacetime (e.g. with wormhole glued like in Klein bottle) - in which there exists a path switching past and future:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fdl.dropbox.com%2Fu%2F12405967%2Floop.jpg&hash=4fe0f285d5095a839582b2f4c344ddcb)
Here is discussion: http://www.thenakedscientists.com/forum/index.php?topic=46648
If fundamental physics does not distinguish past and future, why thermodynamics does? Because symmetry can and often is broken in concrete solutions, like when trowing a rock to flat water surface - it breaks the symmetry, starting propagation of events.
So what was "such rock" for our Universe? We call it Big Bang - when everything was well localized, and so had low entropy - the tendency of entropy to grow created our 2nd law of thermodynamics.
All low entropic objects we have come from reason-result relation chain starting in our Big Bang - from our past: through Earth creation, evolution, finally leading for example to an egg, which have tendency to increase entropy: scramble and can do it only toward our future.
However, if there would exist such time-reversing loops and we would send a rocket through it, e.g. an egg inside would come from our future and so would have tendency to scramble toward our past - from our time perspective it would unscramble.
About "action optimizing" I have meant the principle of lest action ( http://en.wikipedia.org/wiki/Principle_of_least_action ) - that in Lagrangian mechanics (like classical mechanics, GRT, QFT) we could fix values in past and future (for example in Big Bang in past and Big Collapse in future) and physics choses the action minimizing solution (like the history of the Universe).
Equivalently we could fix values and derivatives in the past (e.g. Big Bang) and Euler-Lagrange equations ( http://en.wikipedia.org/wiki/Euler-Lagrange ) defines their unique evolution.
Equivalently we could change time sign in Euler-Lagrange equations, what allows to find unique backward evolution for given values and derivatives in the future (e.g. Big Collapse).
About unitarity of quantum evolution, indeed I meant that the evolution operator is unitary and so can be easily changed to time-reversed.
Example of wavefunction collapse ( http://en.wikipedia.org/wiki/Wave_function_collapse ) is decoherence - loosing unitary evolution due to complex interaction with environment. If we would consider complete QM: of the whole Universe, there would be no longer exterior.
I would like to propose a good temporal exercise for this thread: what impact to our society would have appearance of technology to send information back in time?
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The following is a short extract from Richard Wolfson’s “Simply Einstein”.
“What are some events that are truly in the past, meaning they unambiguously occurred before your present event, that is the event of your reading these words? For one, your birth. There are no observers, in any state of motion, who would judge that event to occur after you’re here and now (although different observers will disagree about the amount of time between those events). We don’t have to restrict ourselves to events in relation to the here and now. We can also ask, for example, whether the event of the Titanic hitting the iceberg preceded the event of the great ship’s sinking. The answer is an unambiguous yes. Again, one event is clearly in the other’s past. Consider also that in 1987 astronomers observed a supernova – an exploding star – in a neighbor galaxy some 160,000 light-years away. Clearly the supernova event itself occurred before the astronomers observed it, since it took light from the supernova 160,000 years to reach the astronomers’ telescopes.
What do these pairs of events we’ve just considered have in common? They’re all causally related. Your birth is a necessary cause of your reading these words. Had the first event not occurred, the second could not have occurred either. Had the Titanic not hit the iceberg, it would not have sunk. Had the supernova explosion not occurred, the astronomers would not have observed it. In each case, the earlier event was capable of influencing the later one and, in fact, did influence it. That provides a more robust definition of the past: The past of a given event consists of all those events that are capable of influencing the given event. Similarly, the future of the given event consists of all those events that the given event can influence. Note that I’m talking about past and future in relation to a specific event; in a Universe in which simultaneity is relative, there’s simply no such thing as a universal past and a universal future. But when one event is in another’s past, that relationship is not ambiguous. All observers will agree about which event came first (although, again, they may disagree on the amount of time between the events).”
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Indeed our intuition is based on causal reason-result chains "anchored" in a very special event: the Big Bang.
This event breaks time/CPT symmetry of fundamental physics we use - is an asymmetric feature of concrete solution for symmetric equations, like rock dropped to flat water surface.
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In response to the question you ask about sending information backwards I would like to say three things.
1) All things are possible. There is nothing that cannot be achieved in some way or another. It is merely a question of time and dedicated pursuit/study. If you don't believe me, then you are limiting your own mind and your experience in this realm. You may also look back into our history and you will see that much of what we achieved in our time on Earth was "proven" to be impossible in the past, using what they believed to be accurate Science, which we now scoff at the sheer simplicity and ignorance of. The same will be said about the Science and understanding you have right now, later in our timeline.
2) The concept that started people into saying that information s faster than the speed of light was not a measurement at all. It wasn't even the intent of the experiment. We were measuring other things when we noticed that the perceived information(I believe it was scaled to electrons in the act of their observation) was not able to be measured. You can not see something change from a multi-position wave into a single position particle. Which means it can not be measured(by current standards). However, the speed of light can. We have measured it's exact speed(and covered up the fact that it changes over time{https://www.youtube.com/watch?v=JKHUaNAxsTg}) and given it a number/value. Which means that it is slower than information as only one is slow enough for us to measure.
3) As for sending information backwards through time, even though I accept it to be possible, it is something we can not do, as we have no current means of sending anything backwards through time, at all. so it wouldn't matter what it was attached to, as it can only move forward by our current technological limitations. In the world of Theoretical Possibility, yes, it can.
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Nothing can go back in time. You cannot get to an event in the past since the photons illuminating that event have already been produced and the system has changed. Any photons you encounter along the way will be showing the history since that event in rapid succession. If you could travel instantaneously from your initial starting point to anywhere else in the observable universe you would get there in the 'now' and not in the past. Now is a relative term and involves a complex interaction of forces and vectors of vastly differing scales all combining in the present, instant, now. Let me know when anyone can reassemble a fractured pane of glass spontaneously then that will be the first step to time travel. The last step is undoing every action in the observable universe even at the scale of the Planck time.
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Hi Osirus, welcome.
Whilst I agree with you that many (perhaps most) of the things that have been made possible by science and technology in just the past decade would have been thought impossible a century or so ago; I also agree that people, a century or so hence, will probably laugh fatuously at our current efforts and beliefs. However, I think the statement “There is nothing that cannot be achieved in some way or another.” would have to be considered a bit rash. Recently, someone (I think it was Pete) resurrected the old question about God being able to create a weight “He” could not lift. To answer that we would have to establish the existence of God and the existence of gravity in Heaven; as well as answering a lot of questions about infinite space, mass etc. Personally, I’m not betting on that ever being established.
If you don't believe me, then you are limiting your own mind and your experience in this realm.
Could it not be said that you were also limiting yourself by denying out of hand the chance that something could be impossible?
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Nothing can go back in time.
I’m firmly with you on that point, but it has to be said that not all scientists are in the same camp. Have you read J. Richard Gott’s “Time Travel in Einstein’s Universe”?
Gott says: : “A time traveller who visits the past is just someone whose world line somehow loops back in time, where it could even intersect with itself.” To be fair to him, he doesn’t just leave it at that, but explains scenarios in which that could happen. He also points out that: “Time travel to the past would appear difficult at best, so don’t call your travel agent just yet.”
There are a few things in that book that would, I think, be worth discussing, but they might need threads of their own.
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Nothing can go back in time.
I’m firmly with you on that point, but it has to be said that not all scientists are in the same camp. Have you read J. Richard Gott’s “Time Travel in Einstein’s Universe”?
Gott says: : “A time traveller who visits the past is just someone whose world line somehow loops back in time, where it could even intersect with itself.” To be fair to him, he doesn’t just leave it at that, but explains scenarios in which that could happen. He also points out that: “Time travel to the past would appear difficult at best, so don’t call your travel agent just yet.”
There are a few things in that book that would, I think, be worth discussing, but they might need threads of their own.
I have not read the book but may at some point. Space is not only 3 dimensional but each axis is not unidirectional and covers all angles from any point in space. Time is 1 dimensional and is unidirectional. Time is a measure of how things change. Once they change causality is a severe obstacle to reversing that change. You would need to reverse causality in order to reverse time. They are connected directly to each other in their directionality. Think of the flow of energy that follows that causality. Considering the observable universe it is an unimaginable amount of energy. It cannot exist in two places at once otherwise conservation laws are violated. Therefore once it occurs the past is lost since every quanta of energy would have to have the potential to exist in an infinite number of places indefinitely in order to be re-accessed through time travel. This would require energy to be both static and active at the same time. Why would there be static active duality?
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... Could it not be said that you were also limiting yourself by denying out of hand the chance that something could be impossible?
Good point, Bill.
But ultimately, saying 'All things are possible' without qualification is self-contradictory; it means that it's possible that Osirus is wrong and all things aren't possible after all... [:P]
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... Could it not be said that you were also limiting yourself by denying out of hand the chance that something could be impossible?
Good point, Bill.
But ultimately saying 'All things are possible' without qualification is self-contradictory; it means that it's possible that Osirus is wrong and all things aren't possible after all... [:P]
That is an excellent use of logic.
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That is an excellent use of logic.
'Gödel started it Sir!'
'I did not - it was Russell!'
'Well, Russell?'
'It was Epimenides' idea, Sir'
'Epimenides? write out 100 times, "I refuse to write these lines", and see me after.'
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Here you are, Sir; I used my time machine and wrote them last night, Sir!
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I’m firmly with you on that point, but it has to be said that not all scientists are in the same camp. Have you read J. Richard Gott’s “Time Travel in Einstein’s Universe”?
Gott says: : “A time traveller who visits the past is just someone whose world line somehow loops back in time, where it could even intersect with itself.” To be fair to him, he doesn’t just leave it at that, but explains scenarios in which that could happen. He also points out that: “Time travel to the past would appear difficult at best, so don’t call your travel agent just yet.”
There are a few things in that book that would, I think, be worth discussing, but they might need threads of their own.
There's not a lot to discuss Bill. Gott is wrong. Palle Yourgrau talked about this in A World without Time: The Forgotten Legacy of Gödel and Einstein (http://www.amazon.co.uk/World-without-Time-Forgotten-Einstein/dp/0465092942). See this from page 142:
"Wheeler, unfortunately has conflated a temporal circle with a cycle, precisely missing the force of Gödel's conclusion that the possibility of closed, future-directed, timelike curves, ie time travel, proves that space-time is a space, not a time in the intuitive sense. Whereas a circle is a figure in space, a cycle is a journey undertaken along a circular path, one that can repeated, in Wheeler's words, "over and over again". Exactly how many times, one wants to ask Wheeler, is the journey supposed to be repeated? The question clearly cannot be answered, since the time traveller's journey is not over time, along the closed timelike curve: it is the curve itself. Just as one cannot ask of a circle how many times the points that constitute that figure have gone around, one cannot sensibly ask how often the time traveller in the Gödel universe has made his or her trip."
A worldline that loops back on itself isn't like Groundhog Day. You don't travel along a worldline. The length of your worldline relates to the length of your life. So if it's a loop that one day in circumference, you live for one day. It's Mayfly Day, not Groundhog Day. It's like you're born from the egg you laid before you died.
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In response to the question you ask about sending information backwards I would like to say three things.
1) All things are possible. There is nothing that cannot be achieved in some way or another. It is merely a question of time and dedicated pursuit/study. If you don't believe me, then you are limiting your own mind and your experience in this realm.
One what are you basing this assertion on? There's no reason to accept any assertion until a logical argument is made justifying it. What is the justification for this one?
You may also look back into our history and you will see that much of what we achieved in our time on Earth was "proven" to be impossible in the past, using what they believed to be accurate Science, which we now scoff at the sheer simplicity and ignorance of. The same will be said about the Science and understanding you have right now, later in our timeline.
That has never been true for anything. That is merely a common misunderstanding which is wide spread throughout non-scientists. There are several things wrong with that argument, the first of which is that it assumes that the process if science is to prove things to be true when in fact that isn't now nor has it been the case. The second thing is that science is about seeking models which describe nature and to do so as closely as possible as possible. That we have to modify physics along the way to make it better only means that we're accomplishing our goals.
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Hey Pete, see http://en.wikipedia.org/w/index.php?title=Shapiro_delay&action=history where you "Included quote from Shapiro's paper where he states the purpose of the experiment", and some new guy has "Removed fabricated quotation and added relevant details" .
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Hey Pete, see http://en.wikipedia.org/w/index.php?title=Shapiro_delay&action=history where you "Included quote from Shapiro's paper where he states the purpose of the experiment", and some new guy has "Removed fabricated quotation and added relevant details" .
I don't see your point since that page Shapiro time delay still says
http://en.wikipedia.org/wiki/Shapiro_delay
The Shapiro time delay effect, or gravitational time delay effect, is one of the four classic solar system tests of general relativity. Radar signals passing near a massive object take slightly longer to travel to a target and longer to return than they would if the mass of the object were not present. The time delay is caused by the slowing passage of light as it moves over a finite distance through a change in gravitational potential. In an article entitled Fourth Test of General Relativity, Shapiro wrote
Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased by almost 2x10-4 sec when the radar pulses pass near the sun. Such a change, equivalent to 60 km in distance, could now be measured over the required path length to within about 5 to 10% with presently obtainable equipment.
which is exactly what both and us has said.
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No problem. I was just alerting you to some guy removing what looked like a verbatim quote by Irwin Shapiro.
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Don't know Jarek. It's nice reading you though, and Pete, redefining and clearing up the nomenclature into more understandable language. I don't think it's possible myself, not as described 'inside' a universe. too many logical fallacies, and when solving those you will fall into a 'multi verse' not only defined by a local arrow, or 'universal time' 'splitting' with each outcome, but also a universe in where you now introduce reaction before action, splitting it anew.
that doesn't mean that you can't describe such a universe, just that I think you will need some sort of 'outside', making sense of this 'inside' we would find. My own notion is one in where this 'arrow' is what defines the universe, as contrasted to the idea of 'unfulfilled potentials' outside of it, to us inside it. One is the arrow, the other is not a arrow.
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to simplify it Anders. If you want a universe in where we can send information faster than light. 'c' split is a clock, that clock has only one direction, into a 'future' to all of us existing inside. Introduce 'ftl' and it will make no difference to that statement. It must then 'split' a universe, into new configurations, or else this universe is meaningless, and all ideas of progress, thermodynamics, action and reaction, time, loses any meaning. And there is no experimental proofs of anything else than just this local arrow, existing for all of us. So, even though it might be possible theoretically, I would expect some sort of 'cosmological censorship' to step in, unless, naturally, someone has a proof? In which case we now will need to introduce this 'outside', to make sense of our new type of 'inside', losing its former temporal direction.
(The hitchhikers guide to the galaxy thought about it, if you remember, finding that all history ultimately became meaningless finding us able to move backward in time. And as I define 'c' as a clock then anything moving 'ftl' definitely becomes time travels:)
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....when solving those you will fall into a 'multi verse'.....
The multiverse is often cited as a means of achieving past-directed time travel, but there are a lot of snags that seem to be overlooked. Going back to gill my grandmother, for example, is by no means straightforward.
Even in the multiple universe situations, this grandmatricide makes no sense, because, unless there is a single universe in which I did and did not kill my grandmother, the situation is such that each universe follows its own course, and no amount of frisking backwards through time can change what has happened in any one universe. It can, of course, be argued that in the multiverse I go back and kill my grandmother in a universe in which I never existed, and never will exist as anything other than a murderous time traveller, but this works only if the universe in which I start and that in which I kill my grandmother – who, quite obviously, is not my grandmother in that universe – are out of synchronisation by exactly the right amount
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True Bill, I get a headache fairly fast, trying to imagine what such a universe would look like. And if you also introduce reaction before action I lose all footing, if that now is a word? The way I see it, and that is purely naive, is the one between a arrow, and what is not. We know the arrow, that's our history, also what builds statistics. We do not 'know' what happens with those unfulfilled probabilities we define. It's possible that all of them exist, but I doubt us proving it. So, what do you have? Between dreams and what you see? And what do I want it to be?
A place of fair play?
And a place of compassion
It's not like we all have the same chances..