Flashlights on an FTL ship

[K^2]

FTL travel in one reference frame is also traveling backwards in time in another reference frame, so if you can travel FTL you can go back in time, then you wind up killing your grandfather or hitler or whatever...

A single FTL traveler does not break causality. The reason is that FTL travel is between space-like separated events by definition. So if in some reference frame event A causes event B, meaning they are time-like separated, a FTL traveler can observe event B occur before event A takes place. However, he cannot send a signal to location of A to reach it before A happens, so he still cannot cause any paradoxes.

It takes two FTL travelers going in different directions and communicating in transit. So the first traveler, having observed B take place, can send a message to second traveler saying, "I saw B happen because of A." The second traveler can now arrive at location of A before A takes place and avert it, causing a paradox. (Edit: I can draw some space-time diagrams, if anyone thinks it would help.)

The reason a single traveler cannot just turn around and head back to A is that acceleration will cause enough time dilation to prevent arrival in time. Take a look at twin paradox for a sub-light version of that.

The cool thing about the communication requirement? Two ships under Alcubierre Drive cannot communicate in such a manner without having their warp bubbles cross. Bubbles crossing would be very bad mojo, however, so we can exclude this possibility, meaning that we can have FTL Alcubierre-like warp ships without worrying about breaking causality.

There are other situations when FTL ship can act as a time machine, and there the Alcubierre Drive actually works, but it requires a space-time that's very far from flat.

Edited November 26, 2013 by K^2

[Savage117]

The faster an object goes the more mass it has. Closer to the speed of light the mass increases exponentially until approaching infinity. Since E=Mc², the amount of energy would also approach infinity.

Even if we had infinite energy, the mass of the object would also be infinite, so what happens when an unstoppable force meets an immovable object? Is it either impossible or is it just improbable that the object would move?

Einstein's theory of relativity does explain some observations better than preceding theories, and it made predictions which have subsequently been confirmed by new observations. So it has in fact been proven to a high standard. Denoting something a "theory" in science actually refers to a fairly complete idea with a large body of supporting evidence.

No, it's not a complete description of all phenomena in the universe, but it has been shown to be highly accurate. So if the most accurate theoretical framework we have says that light speed does present certain insurmountable practical barriers, then we'd be foolish to say otherwise.

I guess I did not make my point clear enough. I am aware of Einstein's theory, I am aware it is a nearly airtight theory, and therefor very likely to be correct. The operative words here are "Theory" and "Nearly", not "definitely" and "impossible". Einstein did not have all the variables, he predicted many of them, but he could not have had all of them. Without knowing those variables one can not say something is impossible, that's why it is still a theory and not a physical law, because we have not proven it beyond a doubt to be true. If people keep thinking this way nothing will ever get done, as soon as someone smart says something is impossible and gives supporting evidence, no one will try to overcome it, making it a self fulfilling prophesy.

Many decades ago man knew the world was flat, it was impossible to go beyond the edge of the world without destruction, and in reference these people who had this theory were correct, they even had compelling evidence, they were very smart people. They were working off known evidence and predictions, they missed a lot more than Einstein did, but they had more restrictive thinking practices due to religious intervention. They may seem dumb now but hindsight is 20/20. If we break the light barrier, those who thought it was impossible will seem foolish. And in contrast the people who don't think its impossible will seem foolish if its proven impossible.

I am just saying that nothing is impossible until proven so. Even then there might be a way around it or to bend it in some way. It may sound like I'm getting defensive here, but I'm not, I'm just debating my point and giving my opinion. I appreciate your replies.

[ZetaX]

You know that with your definition of "proof", which seems rather close to the mathematical one instead of the scientific one, you will very likely never get a proof of anything. You can't prove gravity, you can't even prove 2+2=4. To do anything, you will have to agree on some objective reality, and then on objective language, and after that you might at least be able to do mathematics, and observe things. You will still not be able to make any "provable" statement on any real event in the future.

[K^2]

I guess I did not make my point clear enough. I am aware of Einstein's theory, I am aware it is a nearly airtight theory, and therefor very likely to be correct. The operative words here are "Theory" and "Nearly", not "definitely" and "impossible". Einstein did not have all the variables, he predicted many of them, but he could not have had all of them. Without knowing those variables one can not say something is impossible, that's why it is still a theory and not a physical law, because we have not proven it beyond a doubt to be true. If people keep thinking this way nothing will ever get done, as soon as someone smart says something is impossible and gives supporting evidence, no one will try to overcome it, making it a self fulfilling prophesy.

You do realize it's been nearly a century since General Relativity got published and more than a century since Special Relativity, right? You do realize we've been probing the hell out of both in the mean time. Not only have we not found any problems with GR, but it's now the most precisely tested theory. Furthermore, it packs neatly into our understanding of field theories in general. Yes, we now have many more, stemming from both relativity and quantum mechanics. We now understand the relationship between geometrical properties of space-time and the fundamental symmetry groups. And while precise mathematical formulation of Grand Unified theory still remains problematic, we do know how General Relativity fits into it and that speed of light is built in as one of its fundamental properties.

As for your misconception of what constitutes theories and laws, it could not be further from the truth. Laws are just observational rules of thumb. Like Newton's Laws. They are not derived from any deeper principles, but rather stem from empirical observation directly. A proper scientific theory is an entire construct which based on some postulates derives a model with mathematical precision. The fact that we call General Relativity a scientific theory puts it on the highest pedestal of reliability.

Many decades ago man knew the world was flat, it was impossible to go beyond the edge of the world without destruction, and in reference these people who had this theory were correct, they even had compelling evidence, they were very smart people. They were working off known evidence and predictions, they missed a lot more than Einstein did, but they had more restrictive thinking practices due to religious intervention. They may seem dumb now but hindsight is 20/20. If we break the light barrier, those who thought it was impossible will seem foolish. And in contrast the people who don't think its impossible will seem foolish if its proven impossible.

We already know how to break the global light barrier. There is warp drive and there are wormholes. You are not satisfied with that, for some reason, even though that's the practical way of "bending" the laws. Instead, you want to break the local light barrier which serves absolutely no purpose and only leads to a complete and total breakdown of all known physics. Even if it's possible beyond all measurable bounds we've established, why? What in the world would be the point of that?

[cr055H41rz]

Yep, it's science

Looks like I'm in the wrong neigborhood

[Starwaster]

The cool thing about the communication requirement? Two ships under Alcubierre Drive cannot communicate in such a manner without having their warp bubbles cross. Bubbles crossing would be very bad mojo, however, so we can exclude this possibility, meaning that we can have FTL Alcubierre-like warp ships without worrying about breaking causality.

There are other situations when FTL ship can act as a time machine, and there the Alcubierre Drive actually works, but it requires a space-time that's very far from flat.

Or how about having to travel through a space-time that's totally flat, and warships that enter your bubble can attack you but you destroy them and hey guess what? The mass of all the wreckage is still hanging out in your bubble slowing you down.... (Crest / Banner of the Stars)

[Ascension Islander]

The craft in question, even if it was only going over the speed of light by 1%, would be going ridiculously fast, there is no computer on this planet that could keep up with that kind of speed, you would blink and then be outside the solar system. You would have to figure everything before entering FTL or you would slam into a star or an asteroid before you even registered you had let your finger off the button. It would be like sailors plotting courses through unknown waters all over again, but this time if you mess up you don't sink, you get atomized.

You do realize that it takes light eight minutes to reach the Earth from the Sun, right? The speed of light is ridiculously, mind-bendingly fast, but space is HUGE. It takes well over four hours to travel from the sun to Neptune at the speed of light.

Edited November 27, 2013 by Ascension Islander
Fixed quote brackets

[tetryds]

It's size would contract to 0, time would stop and it's mass would become infinite.

actually, what is infinite at light speed is the kinectic energy, it doesn't change the rest mass

if you lit up a flashlight in any situation, even faster than light, you would measure the light traveling at c

its akward to comprehend, but if you theorically traveling faster than light relative to another physical body, you are warping back in time relatively to that object

as well as that object is warping back in time relative to you

the object would "see" your flashlight light beam traveling at c and you would see it traveling at c as well

Edited November 27, 2013 by tetryds

[K^2]

actually, what is infinite at light speed is the kinectic energy, it doesn't change the rest mass

Inertial/relativistic mass diverges as v->c. In old convention, which you'll find in early relativity books, the symbol m and the term "mass" was reserved for such, which is why many old articles and books on relativity will say that the mass increases. Modern convention is for the term to mean invariant/rest mass, which is always the same. In the old books, you would frequently see it denoted as m₀, but now we just use m. The conversion factor should look familiar. m_rel = γm. And since Lorentz factor, γ, goes to infinity as v does, so does relativistic mass.

[Seret]

If people keep thinking this way nothing will ever get done, as soon as someone smart says something is impossible and gives supporting evidence, no one will try to overcome it, making it a self fulfilling prophesy.

Holding to the current understanding of things doesn't preclude reality from occasionally upsetting your conceptual apple cart. Because if your theory isn't a good match for reality then sooner or later someone will make an observation that can't be explained by the current theory, and then people will be led to examine other possibilities. Until such time as someone makes such an observation then you stick to what the evidence you have suggests. This is the cornerstone of an evidence-based view of the natural world (ie: science), which can hardly be said to have resulted in a lack of progress.

[Jim DiGriz]

The reason a single traveler cannot just turn around and head back to A is that acceleration will cause enough time dilation to prevent arrival in time. Take a look at twin paradox for a sub-light version of that.

We've already gone to FTL travel so that means that clocks are ticking with imaginary time, mass and length are imaginary and we've used more than an infinite amount of energy to get going this fast. There's nothing special in this case about the plane of simultaneity that we start out with (for example in Kerbin's reference frame), and somehow we got from being at rest on Kerbin to travelling FTL. That means we certainly can turn around and head back. We can also pick a different reference frame, like one at our location going away from Kerbin at 0.9c and look at its planes of simultaneity, which certainly may intersect with Kerbin back in time, and decide we're going to travel FTL relative to that frame and go backwards in time relative to Kerbin. If you've already done 6 impossible things this morning, then there's no law that prevents that...

[K^2]

We've already gone to FTL travel so that means that clocks are ticking with imaginary time, mass and length are imaginary and we've used more than an infinite amount of energy to get going this fast. There's nothing special in this case about the plane of simultaneity that we start out with (for example in Kerbin's reference frame), and somehow we got from being at rest on Kerbin to travelling FTL. That means we certainly can turn around and head back. We can also pick a different reference frame, like one at our location going away from Kerbin at 0.9c and look at its planes of simultaneity, which certainly may intersect with Kerbin back in time, and decide we're going to travel FTL relative to that frame and go backwards in time relative to Kerbin. If you've already done 6 impossible things this morning, then there's no law that prevents that...

You are applying equations from Special Relativity to an accelerated frame of reference. That's a big no-no. If you turn around in transit, you have to use GR equations, or you have to treat everything from an external inertial frame. In either case, you'll end up showing that your FTL ship could not deliver information about consequences of event before it happens. You really do need at least two FTL ships on different trajectories communicating in transit. Like I said, I can prepare some diagrams if you think this will help. I just don't want to waste time on that if nobody is going to care.

As for imaginary time, etc. Yes, at face value, using SR equations, you end up with ship heading in an imaginary direction, which should be a warning sign. Fortunately, warp drive resolves this issue. The ship itself is actually at rest with respect to the space in the bubble, so there are no issues with time doing anything weird. However, the really cool thing about the warp drive is that so long as there are no other sources of significant curvature^*, we can forget all that. From perspective of exterior observer, the ship behave as an object traveling faster than light. The only difference is that time aboard the ship isn't governed by SR time dilation equations. In other words, all of the concepts about two FTL ships above can be applied to a pair of ships under Alcubierre warp metric.

When we talk about the way the FTL ship under warp drive perceives the rest of the world, on the other hand, warp bubble cannot be ignored. Crew aboard such a ship cannot see a portion of the universe immediately behind the ship, and they cannot send messages to anyone directly ahead of the ship. Since two FTL ships required to set up time travel have to be moving in roughly opposite directions, they actually cannot communicate with each other as required to set up time travel. So Alcubierre Drive preserves global causal structure of asymptotically flat space-time, which is a really neat feature.

^* In this case, "significant curvature" means in comparison to warp bubble's own curvature. So if you are flying a warp ship through an event horizon of a "small" black hole, you'd have to do some very hairy math to figure out what's going to happen. However, even with a black hole, if it's big enough to have gentle enough curvature at the event horizon, you can basically predict how the ship will behave just by assuming it can go FTL.

Edited November 28, 2013 by K^2

[Jim DiGriz]

I guess I did not make my point clear enough. I am aware of Einstein's theory, I am aware it is a nearly airtight theory, and therefor very likely to be correct. The operative words here are "Theory" and "Nearly", not "definitely" and "impossible". Einstein did not have all the variables, he predicted many of them, but he could not have had all of them. Without knowing those variables one can not say something is impossible, that's why it is still a theory and not a physical law, because we have not proven it beyond a doubt to be true. If people keep thinking this way nothing will ever get done, as soon as someone smart says something is impossible and gives supporting evidence, no one will try to overcome it, making it a self fulfilling prophesy.

Many decades ago man knew the world was flat, it was impossible to go beyond the edge of the world without destruction, and in reference these people who had this theory were correct, they even had compelling evidence, they were very smart people. They were working off known evidence and predictions, they missed a lot more than Einstein did, but they had more restrictive thinking practices due to religious intervention. They may seem dumb now but hindsight is 20/20. If we break the light barrier, those who thought it was impossible will seem foolish. And in contrast the people who don't think its impossible will seem foolish if its proven impossible.

I am just saying that nothing is impossible until proven so. Even then there might be a way around it or to bend it in some way. It may sound like I'm getting defensive here, but I'm not, I'm just debating my point and giving my opinion. I appreciate your replies.

Yeah, nothing is impossible. If you climb up to the top of a 100 story building and jump off of it, you /might/ fly. Gravity is just a theory. It would be presumptuous of me to assert that it was impossible that you might fly. I don't really have all the variables. You should try that and let us know how it turns out...

Or not...

The thing is that there's very little reason to doubt that you'll wind up a sidewalk pancake if you try that, and lots of evidence in favor of it. When you look at the notion that the Earth is round, that was believed as early as the ancient greeks, and you can point to glaring holes in the idea that the earth is flat. There's changes in how the stars rotate across the sky, when you climb up in altitude you can see further past the horizon, ships masts appear before they do as the come across the horizon, the Earth casts a round shadow on the moon, travel to the south and the sun will throw a shadow to the south, etc.

In contrast to the flat earth idea, there is basically no phenomenon in the entire observable universe that you can point to which violates SR right now. You have to go digging around in the center of black holes to find areas where SR might be broken (and that can't be 'observed' in the strict sense which is why I say nothing in the observable universe violates SR -- there's a very precise meaning of observable there).

At this point we do have a massive body of collected evidence across all of the history of science, which means that anyone who wants to claim something otherwise needs to come up with a theory which explains all the current observables, but in certain cases violates SR. The burden of proof is on the person who has the new idea. It must be able to explain everything that SR does and then more. That makes it hard.

The argument around the culture of science fits better at the fringes today. Supersymmety, GUTs and String Theory have taken a huge beating from the LHC results, and its entirely possible that the last 20 years of theoretical high energy physics was all a study of a non-existent aether. There have been people who spoke out about the failure of String Theory to explain anything useful and they were mostly marginalized, while clever people who were very adept at advanced mathematics turned theoretical physics into a club of very smart mathematicians one-upping each other and looking down on anyone outside of their club. The few who claimed the emperor had no clothes and were looking for alternative approaches are starting to get a bit more attention now. It may turn out that its the String Theorists who look incredibly foolish in the future and their critics look like the wise sages in the future (or the 13 TeV LHC might barf up a stop particle and the String Theorists may turn out to be victorious after all). Right now String Theory has started to be backed into such a corner that leading physicists are starting to claim that its simply impossible to know the ultimate laws of nature, that we live in a multiverse and that the anthropic principle just accidentally gives rise to the universe that we live in out of a ridiculous bajillion different possible laws of physics, and that knowledge about fundamental physics is starting to come to an end -- now *that* sounds like old farts falling back on telling people its impossible to learn anything more, which really sounds *wrong*.

Just plain old violating SR, tough, sounds like jumping off a building just to see if you might happen to fly today, since it might not be totally impossible... Not a very good bet...

[sal_vager]

Please don't suggest people jump off buildings Jim DiGriz, it's not funny :/

[zekes]

The problem with light is it is IMPOSSIBLE to get to the speed of light, or faster. That's just the way it is. we just can't. Why? because if, for example, you were going .9999 times the speed of light (just slower than it), light would still go past you at the speed of light. Weird huh? Physicists have found that since light always goes faster than you, At the same rate, Time must be changing for you. so, if you go almost the speed of light, time is going much slower for you than in real life, so if the light would just slowly pass you, instead the increase in time relative to the outside world makes it zip by at normal speed.

So, going by that logic, if you went FASTER than light, you should be going back in time, which is impossible for several reasons. The speed of light is acually the asymtote for time, it cannot be zero or less than zero.

[sal_vager]

I may have linked to this before, but it's a pretty good explanation of why faster than light makes no sense.

[ZetaX]

Supersymmety, GUTs and String Theory have taken a huge beating from the LHC results,

Sources please.

[K^2]

Sources please.

Supersymmetry predicts that elementary fermions and bosons exist in pairs. None of the discovered particles are such a pair to any other known particle. LHC has effectively raised the energy scale at which superpartners might exist, and that's a problem for the theory. Wikipedia article on the topic goes into a bit more detail.

I don't know enough about string theory to say anything for sure, but there seem to be similar problems there. Some of the effects predicted at high energies are yet to be observed. Again, you can get the general gist of it from the Wikipedia article.

[ZetaX]

The last time I spoke to a string theorist on it and supersymmetry was before CERN was active and already then he told me that CERN is probably to weak for this anyway, so would not think that anything was unexpected here. So I was specifically asking for things that are were not discovered yet expected.

[Seret]

The last time I spoke to a string theorist on it and supersymmetry was before CERN was active

Do you mean the LHC? CERN has been around for decades.

[K^2]

Absolutely none of the predictions of supersymmetry or string theory, these that are distinct from standard model, have been observed. As such, we have no means of gauging at what energy levels these effects are meant to be found. Pure supersymmetry would require identical masses for superpartners. That's obviously not the case, so if such a symmetry exists, it is a broken symmetry. How badly broken? There is no way of telling. But the more broken it needs to be to fit data, the less plausible it becomes. And regardless of what somebody might have expected, LHC experiments are pushing these boundaries further back.

Like I said, I don't know much about string theory. What I can tell you is that the last few conferences I have been to on particle and nuclear physics, there have not been any talks focused purely on string theory, and very few on supersymmetry. People are interested in string theory and things like AdS/QCD as tools of studying QCD, but not as independent theories.

As for GUT, I have not seen any attempts at building a completely stand-alone theory. Which makes sense. We have Yang-Mills, and we know how to build a unified action. The key problem is that this does not, directly, lead to a renormalizable QFT. So most of the progress there stems from either trying to come up with renormalizable approximation or in learning to work with non-renormalizable QFT. There is progress in both areas, but it's outside of my expertise.