Hello there. My first post here, so go easy and try to help me out with this if you can.

I'm a computer science student (first year, still a beginner), and part of the studies involve physics. A subject I'm quite interested in. I also like writing stories, especially science-fiction. So of course I'm interested in superluminal travel.

There is one part of the whole FTL thing which confuzzles me. Every time I read something about FTL on the web, people mention travelling backward in time. One explanation of this I have seen several times is as follows:

- Space ship and signal leave planet one at the same time.
- Space ship travels toward planet two way FTL.
- Signal travels at C/refraction index in space or whatever.
- Spaceship reaches planet two before signal.

And somehow that means the ship travelled backward in time. I don't see how. As far as I can see, travelling FTL involves simply outpacing the signal going in the same direction. And no, I haven't yet read through Einstein's relativity work.

I've read some on Lijun Wang's experiment, and as far as I know nobody has found a hole in it yet.

So my question is: How does travelling backward in time come into it? Now, before you answer, keep in mind I'm a beginner at all this stuff.

Adam, welcome to sciforums.

I think what you're talking about is time dialation.

Look at the FTL vessel. If you are on board, and the vessel is travelling near c then subjective time dialates. In other words you could age at a different rate to that which you would if in objective time.
If this is in real space, I suppose logically, if you procede to travel FTL then the time dialation continues to the point where time passes backwards.
Maybe.
:)

First, I don't quite know FTL, can you explain?
However, I know relativity.
First, in the point of view of an outside observer, you cannot travel faster then the signal.
The observer in the rest frame(planet 1) recorded time t has passed. You at the spaceship recorded time t' has passed. Since you're moving at high speed t'<t, but still you cannot traavel faster than the signal and reach planet2.
Whether FTL=timetravel? If I know FTL, then I can try to say things clearier, as you see my message is not complete!

FTL = faster than light (generic)

Welcome to Sciforums, Adam & Dreamsa.

Ftl=Faster than light travel

Ok, The light we see from distance stars arrived after a long travel. That light arriving is from what used to be. If we could somehow magnify that light so that we could see say the surface of some planet within a stellar system them we would be seeing it as it was and not as it is now. The light leaving now will take quite some time to arrive having only started it's journey. As you approach that system at faster than light you are passing light that left eariler than the light you originally saw. In that since you are traveling in time as you have seen light coming from your target sooner. Actually, you have shortened the distance between you and the star target so that the light arrives sooner.

Experiments have proven that there is indeed a time dialation effect between highly accurate clocks, one onboard a satellite and the other ground based. Because the speed is not as high as that of an FTL flight speed the difference is noticably smaller but there none the less. This tends to support the aspect that as you approach light speed time becomes faster to the outside observer and slower in comparision by the traveler. Notice that it is a comparision. It is postulated that the traveler would notice no difference in the passage of time or its speed. Though everything would be longer. Now I have read where this theory was "played with" by authors. That visual sight would be Blue shifted and narrowed down to the point that you would not see anything from the sides of the craft, that you would in essence have a tunnel of light comeing at you from the front and leaving from the rear. Hope this helps some.

That was one thing I was also wondering about, the clocks. I've heard of this experiment many times. As I understand it, these clocks work on the deterioration of radioactive elements or compounds. That radiation being electromagnetic in nature, or at least involving that force greatly as electrons or other are departing the atom. It seems to me that since EM events/waves can interact, we could expect to see different activity in such clocks based on their proximity to different external radiations. Say, for example, if one clock was in space and the other on the ground, or whatever. Could that be a possible explanation for the difference registered in those clocks? And is so, has there ever been any other evidence supporting time dilation?

Hi Adam,

The most famous evidence for time dilatation is the detection of muons on earth. Muons have a lifetime of about 2 microseconds. Since we know that muons that we detect on earth have been created in the upper atmosphere, there is no way they could survive their trip to the ground (even if these muons would travel at lightspeed, they would not be able to make it all the way down - roughly 30km.).

The reason why we do measure those muons is exactly because of time dilation: because of the great speed those muons have, time will pass slower for them. This enables them to travel large distances (eg. the distance upper-atmosphere to earth) in a "small timespan" (eg. their lifetime). When you work out the maths, it works out quite nice.

Bye!

Crisp

Yes, I had intended to ask about the muon thing as well. It seems they are detected high, their expected lifespan in our atmosphere is worked out to be one thing, but then they are detected lower than they should be. However, this seems like jumping to conclusions to me. Why not simply assume the most likely explanation: that they are travelling very fast? I haven't done any of the equations myself, I have to take the word of people who have done it, but an equation does not prove a reality; it merely explains an observation. The equation may not be the correct explanation, but it may still fit what is known, and of course it helps further research. I would only accept the time dilation of muons if specific muons could be tracked through our atmosphere. Simply detecting them at low altitude and assuming they time travel seems to be jumoing the gun in a huge way. However, if anyone has some really good links specifically about the muon thing, I'd love to see them please.

By the way, anyone hear about Super-K in Japan dying? I think it lost about a thousand photo-receptors or something.

Hi!
We can measure the speed of the muons, I think. Also we already assume its speed = speed of light to do the calculation and they still cannot reach the surface of Earth.
The other thing I am sure is that when we look at the muons, we see that they are moving at high speed and their time is dilated. However from the point of view of the muons, they do not think their time is dilated, instead they see the distance they need to travel is contracted. The distance between the upper atmosphere is shorter for the muons than the distance we measured.

Dreamsa

I am aware of other tests that have been done in particle accelerators. In these tests certain particles, I can't remember which maybe Crisp or Wet1 know, have been accelerated close to the speed of light and their half-lives have been measured. The measured value of their half-lives after being sped up close to the speed of light was longer than their measured half-life when at rest.

Hi all,

"Why not simply assume the most likely explanation: that they are travelling very fast? I haven't done any of the equations myself, I have to take the word of people who have done it, but an equation does not prove a reality; it merely explains an observation. The equation may not be the correct explanation, but it may still fit what is known, and of course it helps further research."

That is exactly what scientific research is about. Proposing a mechanism of how nature might work, performing the calculations and determine whether they match reality or not. If they do match reality, then you can take your proposed mechanism one step further and make predictions about untested situations. If your theory (e.g. special relativity in this case) still holds when your predictions are verified in an experiment, you can conclude that the theory describes the mechanisms of nature in a very good way. You can never prove that nature identically behaves like your theory (you cannot perform all possible experiments since there are an infinite number of them), but only say something about the lickeliness of your theory being correct.

"I would only accept the time dilation of muons if specific muons could be tracked through our atmosphere. Simply detecting them at low altitude and assuming they time travel seems to be jumoing the gun in a huge way."

We're not saying they time travel. We're saying that time passes slower for us than it does for them. According to the theory of special relativity, a proton going at 0.9999999c requires about 3 seconds to travel through our entire galaxy (a distance of about 150 million lightyears) - in the frame of reference of the proton. An observer on earth would say it takes the proton about 150 million years. The effect that time intervals change according to the observer is known as "time dilatation" and is well-explained by special relativity.

This is exactly what Dreamsa and SeekerOfTruth pointed out: for any particle going at high speeds (be it a muon going to the earth's surface or an exotic particle being accelerated in a particle accelerator), time goes slower for us than it goes for the particle. Hence we see longer lifetimes in our frame of reference.

Actually the whole reasoning behind time dilatation and Lorentz contraction (when distance shortens when you travel faster) is quite easy: by simply assuming that the speed of light is constant for all observers, you can deduce the formula's for dilatation and contraction in about 4 or 5 lines. You should really check <A HREF="http://www.pa.msu.edu/courses/1997spring/PHY232/lectures/relativity/dilation.html">this site</A> for a simple derivation.

Bye!

Crisp

Well, thanks for the replies. This has prompted me to go and actually read the material behind these theories, Lorentz and such. I'll be back when I've read more on the subject. Thanks.

Adam,
I think you might be interested in this link:

http://www.physics.purdue.edu/~hinson/ftl/html/FTL_part1.html

Damn groovy link, thanks heaps for that.