This isn't the first time I've asked this question, I even asked it in a lecture years ago. I just haven't had a proper answer yet.

Time dialoation (which has been demostrated) shows that time inside a fast moving object is lsowert than time outside. So, as you approach LS, time slows to zero inside, then when you exceed it, time iside reverses resulting in the objects inside being younger when they finish their journey than when they started. At least that is how I understand it.

Forgive all those typos please. It's late in the workday on a Wednesday. The fingers stopped working an hour ago.

Yeah, Draug got it from my understanding for backwards time travel. What my question is is how we can go to the future? Do we use the same principle where we theoretically are still, defined as being perfectly having no velocity. It is then we start to progress where time starts to go by really fast (using the same logic). So, when we view that future, do we accelerate ourselves past the speed of light to return to where were before, effectively going into the future and travelling backwards?

So, is time linear? Does it follow one straight, direct path that cannot be undone or are there multiple ways to go around that path? It is sort of like the Grandfather paradox (I think it's called something like that).

I'll ask my physics teacher tomorrow to see what he thinks.

@acmac10,

"So, is time linear? Does it follow one straight, direct path that cannot be undone or are there multiple ways to go around that path? It is sort of like the Grandfather paradox (I think it's called something like that)."

That depends on the shape of spacetime. Kurt Godel showed that there exist universes (that is, solutions to Einstein's field equations) in which you could go backward in time by travelling very fast along certain paths.

Most people do not think our universe is like that, though (most known solutions to the GR equations don't have that property).

I was born in 2053, nuff said.

12 Monkeys was on yesterday, thats all I need to know

I should add, lest I give false hope -- it's not only that most solutions to GR don't allow time travel, it's also that the ones which do display characteristics not found in ours (such as uniform rotation everywhere).

"12 Monkeys was on yesterday, thats all I need to know"

Easily the best time travel movie of all time.

I have to disagree with you there.

Primer

Although, 12 Monkeys is very, very good.

First of all there is no evidence for time stopping when one moves at the speed of light. We just don't know for sure. This is scientific speculation based on the current understanding of special relativity. Light speed has been take as the universal speed limit precisely because nothing has ever been measured to move faster then light before recently and that light has shown, thus far, to exhibit zero mass. So it simply could mean that there is no cosmic speed limit. And that time just gets slower and slower and slower the faster one goes in relation to time experienced by an observing. It could mean that there is a cosmic speed limit based on the neutrino speed. If it is confirmed that neutrinos can move faster than light, it is certainly a game changer and the scientific community will have to rethink much of the basic assumptions about speed, light, neutrinos, time and such. This does not necessarily have any implications on time travel or more specifically backwards time travel for we are all time travelers but we can only travel in time in one direction, as far as we know. :)

Consider that time is consciousness based, it may then be possible for a consciousness to jump forward to backward in time. But this is a completely different line of thought and reasoning.

Although an interesting question, and the discovery of neutrinos possibly going faster than the speed of light is game changing if found to be true, I don't believe we'll see any major developments due to the fact that our mass is thousands upon thousands of times bigger than a neutrinos (or does a neutrino even have mass)

time as conscious experience seems like a largely separate definition.

that's like saving the state of a computer and waiting for it to go into the future, the loading it again... at least from a physics perspective.

@patizcool: if we can harness this ftl as a communication technology it will not require us moving ftl to have a huge effect.

I think Draug has done a decent job explaining why faster than light means backwards in time. (better way of explaining it i think than i could)

but he didn't address the causality question.

But basically, if i can "send a signal which goes back in time to earlier be picked up and used to determine if i should send the signal or not" then causality is at risk.

I would like to point out that our understanding of special relativity does some interesting things. First there is no special 0 velocity.

So for a moving object with time dilation (as mentioned this has been experimentally verified) in the objects frame of reference (the is measurements done by the object) it is stationary and everything else is moving relative to it at speeds high enough to experience time dilation.

This might seem to lead to what seems like a paradox. Where a pair of twins are moving relative to each other - each one seems to age faster than the other.

It seems obvious that if both go on a round trip and return to the earth then you can actually measure which has aged the most.

That said, special relativity only account for constant velocities, it does not state what happens in gravitational fields of during an acceleration (which are seen as equivalent in general relativity and cause their own time dilation effects)

http://en.wikipedia.org/wiki/Twin_paradox#Resolution_of_the_paradox_in_special_relativity

i can't seem to explain this any better... :(

Two prefaces:
1.) General and Special Relativity would be broken if someone travels faster than light. We'd have to adjust physics for it, which would be tough, when general/special relativity has been a guiding light for us up to now.
2.) I'm pretty sure they've determined the neutrino has, in fact, not traveled faster than light.

However, it is a fun topic.

I like to look at astronomy to explain faster than light travel.
Suppose you look at a star 100 light years away. In your telescope you see it supernova.
That supernova you see happened 100 years ago. It took the light of the supernova 100 years to get here.

If you traveled there in a ship that goes faster than light and it takes, say, 1 year to get there (assuming you have shielding from the neutron star that has been left after the supernova), you look back at earth. What do you see? Earth 100 years ago.

You see how warped spacetime is? BTW - it is spacetime. They are combined.

Another fun example is if you travel in a circle (orbit the sun)... if you go faster than light, you will literally go around fast enough that you'll catch up to the time when you took off. So if you were in the space ship, just before you start, you can look out the window and watch yourself pass by. Funky.

@fk, this gets the essence of relative distance, but does not address the lack of absolute simultaneity which special rel implies.

maybe a stupid question
how is ligth mesured?
does ligth mantain the same speed all the time also after reflecting on something?

This is the best explanation I've seen: http://sheol.org/throopw/tachyon-pistols.html

"2.) I'm pretty sure they've determined the neutrino has, in fact, not traveled faster than light." [citation needed]

Are you sure?

It's only Wikipedia but this article says the jury is out.
http://en.wikipedia.org/wiki/OPERA_neutrino_anomaly

But this source say that some say that it has been disproved. I don't think you can that that there is yet a consensus on the matter tough.
http://www.physorg.com/news/2011-10-theories-emerge-opera-faster-than-light-neutrinos.html

*though

@taos, no, light does not maintain speed outside of vacuum conditions. The permitivity and permeability of a material will affect the speed of light in that material (light refracting in glass is a result of this effect)

@lenpw: great link!

Check out D. Overbye article in New York TImes Scinece section Oct. 25
Neutrinos & Opera "error"

Bartender: "We don't serve faster-than-light neutrinos here." A neutrino walks into a bar.
Besides the joke & CERN comments, Overbye notes Relativity theory allows Tachyons particles to go faster than light ... "they would have imaginary masses, [sic!] whatever that means".

FTL isn't possible:
http://www.engadget.com/2011/10/17/remember-those-faster-than-light-neutrinos-great-now-forget-e/

that is what the CERN paper was asking, if anyone else could come up with an explanation...

Ahhh yes, we Dutch people shattered the world's dream to be able to travel in time. We will however keep the steady export of marijuana and Heineken going to facilitate all other kinds of dreams.

@Fortknox...if you travel around the earth to watch yourself take off in a space ship, are you watching yourself, or just the photonic image of yourself? (does that make sense?) Rephrase...if you watch a star blowup today that is a hundred light years away, it is still a 100 year old explosion. If I traveled FTL AWAY from the star, I could eventually travel "past" the explosion and watch it occue again....but it still a 100+ year old explosion. You are watching the movie of it, so to speak...but the star is alreay gone. How is that traveling through time?

traveling round the world fast enough so that you can see yourself take off is traveling through time.

Say I travel to the extent that I am 1 light second away from where I took off when I took off. The image I am seeing is what happened 1 second ago since that is where light was 1 second ago,

That means that although say I traveled to Alpha Centuri and back in 2.2 years (distance being ~4.37 light years in total) for me time would have advance 2.2 years, but 1 light second away time only advanced 1 second, since the image that you see 1 light second away is the events that happened 1 second ago. Therefore I have traveled through time thanks to FTL.

@redhouse - Sending the pot is fine, but you can keep the Heiney. after all, it tastes like it's name (heiney=ass).

@Fasces -- so you are saying that....because time is relative....that as I approach light speed I travel through time slower....and therefore if I travel around the world as watch myself take off in a space ship, I am watching the ACTUAL event....not a photonic movie?

And...I do get this all....I'm just trying to see it explained in other ways to help those that do not grasp the theories to get it, too.


Of course...in the SCI FI world, you escape the constraints of time compresion, etc., by putting yourself in a "warp bubble"....such that, within the warp bubble, you are moving a relativistic speeds, even thought the region of space you are in is moving FTL.

THOUGHTS???

@taylor4 All imaginary mass means is that when a tachyons mass is being mathematically modeled, the mass is multiplied by i (the square root of -1). They are actually just theoretical particles that can only move faster then the speed of light.

@krellin - the theory is that as one exceeds time travel, their time bubble starts to flow backwards inside so that they are younger when they left than when they started. the issue becomes that insdie the bubble time has moved backwards but outside it hasn't. So they will return and relive the time but from a different perspective having never experienced the journey in their minds to begin with.

The Trek idea of outside time moving backwards to inside the time bubble is false. It's the other way around. to work the way it did in ST IV, time inside the bubble would have to speed up so the outside world slowed until it stopped relatively speaking then flowed in the reverse direction.

So, in short, you experience the same sequence of your life three times: once when it happened before you started the hjoureny, once *backwards* during the journey, and once again after the journey completes, but all this occurs at a continually moving outside time.

So, you experience a day of your life, hop into an FTL ship for a day (but a whole day runs in reverse to you) then come back out, you experience that you are physically two days younger than you should be of the universe. It almost, in effect, bcomes a fountain of youth.