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If it is a wormhole (or by using a "warp" drive) then the scenario I described may not work for producing a paradox. However, if you have two separate wormholes, one connecting points A and B and another connecting points C and D and you position them such that they are far enough from each other, not to affect each other's regions of spacetime but close enough that you can travel between A and C and between B and D at slower-than-light speeds then you can still violate causality and create paradoxes. There is a short description of that here: http://www.physicsguy.com/ftl/html/FTL_part4.html#sec:stmanipulation and there is a more detailed discussion in Everett's book Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space (this covers both warp drives and wormholes, as well as other topics related to FTL and causality).

they all hinge on the principle that for something to exist it has to be observed. To me that's as stupid as a man claiming the sun doesn't exist when he can't see it in the sky.

No, it has nothing to do with that (which is indeed garbage). You actually do influence your past. For example you can send your FTL message to a satellite and have it send it back to you and depending on the speeds of the message and the satellite you can receive the message before you sent it. This can create a paradox because you can setup your experiment so that you only send the message if you haven't received it at time t. But after you bounce it off the satellite it can arrive at time t-1, which would cause you not to send it, but if you didn't send it, then how did you get it in the first place?

FTL communication resulting in being able to send a message to your past follows from special relativity. Here are a few resources that show the details of how it works:
http://www.theculture.org/rich/sharpblue/archives/000089.html
https://en.wikipedia.org/wiki/Tachyonic_antitelephone#Two-way_example
http://www.physicsguy.com/ftl/html/FTL_part4.html (unlike the others this also covers the case of non-flat spacetime, such as warp drives or wormholes)

This still allows causality violations: http://www.physicsguy.com/ftl/html/FTL_part4.html#sec:stmanipulation
Special Relativity is very unforgiving when it comes to FTL and causality and things like warp drives, wormholes, etc. do not help.

There *are* workarounds. A fairly comprehensive list is here:
http://www.physicsguy.com/ftl/html/FTL_part4.html#subsec:specialframe

Don't give people too much hope. Those are not really workarounds. A workaround is IMHO something you can do. But this is only a list how the universe had to be so that ftl travel is possible. If it is not... tough luck.

Here is a relatively understandable explanation of why beating a photon to its destination implies time travel, even if you don't locally travel faster than light: http://www.theculture.org/rich/sharpblue/archives/000089.html. Basically, if you can pass stuff along at FTL between people at sub-light speed, and those people are moving relative to one another, you can send stuff into the past.

There *are* workarounds. A fairly comprehensive list is here:

http://www.physicsguy.com/ftl/html/FTL_part4.html#subsec:specialframe

They're all kind of about relativity being wrong, and there's no evidence any of these are true. Mostly wishful thinking on the part of people that want to believe we can have an interstellar civilization but can't quite let go of causality. Briefly:

1. FTL takes you to a parallel universe. So if you try to kill your past self, there's no paradox and you keep living because it was actually your counterpart in a different universe.
2. There's some unknown physics that would prevent using FTL to violate causality. So even though there's technically time travel in some sense, it has no practical use and therefore you could say it isn't "really" time travel.
3. A specific case of the above: perhaps the act of travelling FTL prevents any other FTL travel within a certain spacetime "radius".
4. Violate relativity by having a "true" frame of reference with a "true" sequence of events. All FTL takes place in that context and is theoretically unlimited in speed. Within any other frame of reference, it looks like a speed limit, but still possibly faster than light speed.

http://www.physicsguy.com/ftl/html/FTL_part4.html# sec:ftlnotes

6.1 A Few Notes On The Meaning of FTL Travel

Before we begin the discussion, I wanted to go over the basic idea of what we mean by FTL travel. To do so, we should start by noting that most of space-time through which we would want to travel is fairly flat. For those who have not read Part III of this FAQ, that means that special relativity describes the space-time fairly well without having resorting to general relativity (which applies when a gravitational field is present). Sources of gravity are few and far between, and even if you travel "close" to one, it would have to be a significant source of gravity in order to destroy our flat space-time approximation. Now, some FTL travel concepts we consider will involve using certain areas of space-time which are not flat (and I will go over them when we get there); however, the important thing for us is that all around these non-flat areas, the space-time can be approximated fairly well as being flat.

Thus, for our purposes, we can use the following to describe FTL travel. Consider some observer traveling from point A to point B. At the same time this observer leaves A, a light beam is sent out towards the destination, B. This light travels in the area of fairly flat space-time outside of any effects that might be caused by the method our observer uses to travel from A to B. If the observer ends up at B in time to see the light beam arrive, then the observer is said to have traveled "faster than light".

Notice that with this definition we don't care where the observer is when he or she does the traveling. Also, if some space-time distortion is used to drive the ship, then even if the ship itself doesn't move faster than light within that distortion, the ship still travels faster than the light which is going through the normal, flat space-time that is not effected by the ship's FTL drive. Thus, this ship still fits our definition of FTL travel.

So, with this basic definition in mind, let's take a look at the problems involved with FTL Travel. /quote

cliff's notes: Wormholes -- non-localized FTL travel.

http://www.physicsguy.com/ftl/html/FTL_part4.html

http://www.physicsguy.com/ftl/html/FTL_part4.html# sec:ftlnotes

6.1 A Few Notes On The Meaning of FTL Travel

Before we begin the discussion, I wanted to go over the basic idea of what we mean by FTL travel. To do so, we should start by noting that most of space-time through which we would want to travel is fairly flat. For those who have not read Part III of this FAQ, that means that special relativity describes the space-time fairly well without having resorting to general relativity (which applies when a gravitational field is present). Sources of gravity are few and far between, and even if you travel "close" to one, it would have to be a significant source of gravity in order to destroy our flat space-time approximation. Now, some FTL travel concepts we consider will involve using certain areas of space-time which are not flat (and I will go over them when we get there); however, the important thing for us is that all around these non-flat areas, the space-time can be approximated fairly well as being flat.

Thus, for our purposes, we can use the following to describe FTL travel. Consider some observer traveling from point A to point B. At the same time this observer leaves A, a light beam is sent out towards the destination, B. This light travels in the area of fairly flat space-time outside of any effects that might be caused by the method our observer uses to travel from A to B. If the observer ends up at B in time to see the light beam arrive, then the observer is said to have traveled "faster than light".

Notice that with this definition we don't care where the observer is when he or she does the traveling. Also, if some space-time distortion is used to drive the ship, then even if the ship itself doesn't move faster than light within that distortion, the ship still travels faster than the light which is going through the normal, flat space-time that is not effected by the ship's FTL drive. Thus, this ship still fits our definition of FTL travel.

So, with this basic definition in mind, let's take a look at the problems involved with FTL Travel. /quote

cliff's notes: Wormholes -- non-localized FTL travel.

http://www.physicsguy.com/ftl/html/FTL_part4.html

And almost none of them acknowledges that `science thing'. For once, most space-based stories assume faster than light travel. Read Relativity and FTL Travel for a scientific treaty on that topic. And then check your so-called Hard Sci-Fi books and look how many handle the science right.

Frankly, I can think of very few honest Hard Sci-Fi efforts in books. In fact, there is only a few promille in the thousands of books that I own from that category.

So, my message to all posters here (not just you) who decry the missing science: Get a life. The question is not `how good is the science', but `how well is it written'. This is fiction, after all.

From this explanation. Twin A stays on Earth and Twin B sets off in a spaceship going 0.995 c (time and space will dilate to 1/10th). He reaches a point C that is 9.995 light-years away and heads back at the same speed. Let's assume accelleration is instantaneous. When Twin B leaves earth, both twins agree their clocks read zero. When Twin B reaches point C, Twin A sees that his clock reads 10 years and Twin B's clock reads 1 year. Twin B thinks his clock reads 1 year and Twin A's clock reads 0.1 year. As soon as he turns around, Twin A still thinks B's clock reads 1 year and his clock reads 10 years, but Twin B thinks his clock reads 1 year and Twin A's clock reads 19.9 years. It all depends on your frame of reference, and the accelleration changes that.

Personally, I don't think I will ever understand it. I think it's all philosophical because it is dependant on definitinitions. What does "observe" mean. What is "simultaneous"? Until you start studying special relativity, these terms are pretty easy to understand. I think physicists should come up with new words to describe these relativistic concepts and not use "observe" and "simultaneous" anymore in physics discussions. I have a special relativity textbook and the book contradicts itself on the meaning of those words in the first few chapters.

I found is in Relativity and FTL(faster than ligth) FAQ http://www.physicsguy.com/ftl/

This explanation explain relativity in context with the faster than light travel. Needs elementary math and explains lucidly why the time dilation occurs. Highly recommened.

First of all, I understood you loud and clear. Your explanation was lucid but I have to respectfully disagree with your explanation.

I also cheated here, because what I explained was not my explanation. You can read it from
http://www.physicsguy.com/ftl/html/FTL_part1.html/ . The article explains that how the time slows down.

I am not a physicist so all my understanding comes about the relativity comes from this article.

My orginal question was how come the universe can expand at the rate which is faster than light and still be consistent with the relativity. I think I understand now why this is possible and this is all because of your and other posters.

Your explanation also explained for the first time to me what is background radiation and why its microwave. So thanks.

The nacelles that are in pairs contain the warp coils which are also in pairs. They allow the ship to 'submerge' into subspace. They inject plasma into the warp coils to cause a reaction which the subspace field then counters by creating a warpfield.

Thats a very brief explanination, you can read a hell of alot more about it if you have the time. I sadly, had that time...

Hm. That's assuming that what the law of causality describes as an 'observer' always uses his/her/its physical vision, which uses a speed-limited medium (light), and thus Causality's chain of events is defined by vision...

No. See my other reply, which is the one that should have been moderated up, not the one you are replying to.

I don't think that Causality cares much about who's there watching, when things happens. I always figured an 'observer' as something close enough, or able, to be in the same space _and_ time referential as the event, and above all _remain in the same referential_.

But you can't make that assumption; if a ship coming from Earth at high (but still sublight) speed crosses the Centauri system as you break the news, they will get the message although in their frame of reference the event really has not occurred yet. And if they also have FTL capability, they can send a message to Earth in time, effectively preventing an impact from happening thanks to the impact actually happening. Thus creating a time-travel-like paradox, which must be prevented, possibly by such means as you suggested.

About FTL itself, the whole point is making the travel's duration approach zero. An ideal FTL travel would make us go from one side of the universe to the other in no time flat, whatever size the universe is. No time travel involved. You wouldn't be 15+ billions years in the past, or future, or whatever, but exactly at the same time than anyone who didn't travel.

Anyone where? At what speed? In which gravity field?

Since, ideally, the events "departure" and "arrival" are in different places, they cannot be simultaneous in all frames of references; otherwise, you are implicitly supposing the existence of an "absolute" time, shared by everyone at every place in the Universe; this is exactly the hypothesis Relativity drops in favor of lightspeed being constant.

'Stretching it further' wouldn't change anything: the 'observer' (the FTL traveller) still doesn't stay in the same referential as the event during the whole incident (his time is 'compressed', while the event's is not).

Without knowing what kind of mechanism would produce FTL travel, one can't really argue about this, but the end result is the same. I really suggest looking at the FTL causality problem FAQ I pointed to in my other post.

If he actually managed to find an intact Earth _after_ it blew up, then A) it's the same Earth, when warning authorities hadn't worked, or B) it's a parallel earth, so the original event wouldn't have any impact (so to speak) to that universe's Causality.

Now that's another matter, and one which is, rightfully, just like time travel. And with such special provisions, then you can have FTL travel or communication, albeit restricted insofar as either some journeys or messages can be prevented in some circumstances (possibly including events which have not happened yet in your timeframe), or you may drop out into another Universe altogether. And then FTL travel isn't really fun, any more than time-travel in which you can't return to your particular time after killing your own grandfather...

I have been told that theory forbids any travel faster than light, no matter what the means ("warp drive", "hyperspace", "teleporter", whatever). My understanding is that if you could, some observers would see you traveling back in time, and this is forbidden.

Much better than my previous comment, see this relativity and FTL travel FAQ, which has a chapter about just that question.

I have been told that theory forbids any travel faster than light, no matter what the means ("warp drive", "hyperspace", "teleporter", whatever). My understanding is that if you could, some observers would see you traveling back in time, and this is forbidden.

Much better than my previous comment, see this relativity and FTL travel FAQ, which has a chapter about just that question.