# Superluminal travel.

1. Jan 21, 2010

### NPSF3000

Hello,

I am attempting to write a script (for fun) which outlines a realistic scenario in which humanity travels to the stars within, say, 30 years. Given the short time frame, i'm thinking about using various forms of FTL travel. By my definition, FTL includes any means of getting from A to B faster than c without stuffing the universe.

While the special theory of relativity apparently does not rule out FTL, it does impose some interesting bounds that I would to see if there is a way out off.

I have a good understanding of Newtonian physics, and a basic grasp on some of the concepts behind quantum mechanics and relativity. I hope this in the right area to ask.

I have done some research on this topic, but the answers seem to be conflicting.

Questions:

1) Is it true that any form of FTL creates time travel?

2) Some claim a single bi-directional wormhole does not allow time travel. Is this true?

3) Are there any theory's out there, currently, that actually sound possible?

2. Jan 21, 2010

### Nuno Amiar

Speed travel faster than light is not permited by any established theories, with the exception of transfer of information in quantum entaglement.
In view of GR we are always moving trough spacetime and thus always traveling to a different point in time. The question is not how can we achieve FTL speeds, but rather, if it is possible that by moving in a closed line in spacetime, we arrive at a time in past. That configuration is the popular wormhole.
If you don't want to study (which would be the only way for you to understand the answers to your own questions), I suggest reading How to Build a Time Machine by Paul Davies.

Last edited: Jan 21, 2010
3. Jan 21, 2010

### S.Vasojevic

1) Depends what you mean by FTL. If you mean to outrun photon, then yes.

2) Wormholes are theoretical possibility, nothing more. Anyway any matter passing through it would be subjected to forces which are far from our technological or biological capabilities to withstand.

3) No.

4) You can't.

4. Jan 21, 2010

### bcrowell

Staff Emeritus
Below I've pasted some info from a FAQ I maintain. My opinions about your questions:

1) Yes.
2) I'm pretty sure this is not true. Novikov and others wrote a series of papers on whether closed timelike curves produce paradoxes, and the jumping off point for all that was "what if we had a wormhole?"
3) Maybe for godlike beings, provided they have access to exotic matter.
4) You can't.

I believe there's work showing that the situation is even worse than that: that it may simply be physically impossible, for fundamental reasons, for any matter or even information to traverse a wormhole without being destroyed.

==============

FAQ: Why can't anything go faster than the speed of light?

In flat spacetime, velocities greater than c lead to violations of causality: observer 1 says that event A caused event B, but observer 2, in a different state of motion, says that B caused A. Since violation of causality can produce paradoxes, we suspect that cause and effect can't be propagated at velocities greater than c in flat spacetime. Special relativity is one of the most precisely and extensively verified theories in physics, and in particular no violation of this speed limit for cause and effect has ever been detected -- not by radiation, material particles, or any other method of transmitting information, such as quantum entanglement. Particle accelerators routinely accelerate protons to energies of 1 TeV, where their velocity is 0.9999996c, and the results are exactly as predicted by general relativity: as the velocity approaches c, a given force produces less and less acceleration, so that the protons never exceed c.

The corresponding speed limit in curved spacetime is far from being established. The argument from causality is not watertight. General relativity has spacetimes, such as the Godel solution, that are valid solutions of the field equations, and that violate causality. Hawking's chronology protection conjecture says that this kind of causality violation can't arise from realistic conditions in our universe -- but that's all it is, a conjecture. Nobody has proved it. In fact, there is a major current research program that consists of nothing more than trying to *define* rigorously what the chronology protection conjecture means.

There are certain things we *can* say about FTL, based on the fundamental structure of general relativity. It would definitely be equivalent to time travel, so any science fiction that has routine FTL without routine time travel is just plain wrong. It would probably require the existence of exotic matter, which probably doesn't exist. If it were possible to produce FTL artificially, it would certainly require the manipulation of godlike amounts of matter and energy -- so great that it is unlikely that beings able to carry it out would have anything like ordinary human concerns.

There are many ways that velocities greater than c can appear in relativity without violating any of the above considerations. For example, one can point a laser at the moon and sweep it across, so that the spot moves at a speed greater than c, but that doesn't mean that cause and effect are being propagated at greater than c. Other examples of this kind include a pair of cosmic-sized scissors cutting through a gigantic piece of paper at greater than c; phase velocities greater than c; and distant, observable galaxies receding from us at greater than c, which is interpreted as an effect in which space itself is expanding in the space in between.

5. Jan 24, 2010

### NPSF3000

On the topic of me wanting to 'study', I want to, I'm actually thinking of doing a degree in a couple years majoring mathematics so that I can understand some of this stuff(and a lot of other high-end stuff). But, that doesn't help me in the hear and now - which is why I appreciate your time and effort. Thank you.

1)

A common example of special relativity is the near c spaceship travelling to far-flung star system.

From Earth, the time in the spaceship appears to slow down. While, the spaceship see the universe shorten. These can both be explained by using a metaphor of a 'light' clock, and the universe constant c.

What I don't understand, is why that from one frame a high velocity object's time slows down, while from the opposite frame the high velocity object's time appears to rapidly accelerate.

What am I missing?

2)

The only thing that appears to be certain, within modern physics, is that faster than light travel will result in going back in time. (remembering that i'm approaching this from a Sci-Fi angle, and considering the last couple hundred years of advancement) Would it be possible for you to explain, what this actually means, if one was to instantaneously travel to Alpha Centauri (and later back) for example.

Once again, thank you for your time.

NPSF3000

0010 1011 OR ! 0010 1011 = -1

6. Jan 24, 2010

### cylinder

Here, say that you travel instantaneously to Alpha Centauri. If you look at the earth, you'll see what it looks like 4.3 years in the past, because the photons are sill getting there. Now, if you were to wait for an hour, then travel back to earth instantaneously, would you arrive at the scene that you saw from Alpha Centauri, or would you end up an hour later on Earth?

7. Jan 24, 2010

### NPSF3000

As far as sci-fi goes, either the method of which you traveled (Wormhole/Alternate Universe) maintains a consistent time frame or some how 'magically' there is a yet undiscovered rule of causality that saves you.

Problem is that people who know more than me claim that any FTL causes timetravel...

Last edited: Jan 24, 2010
8. Jan 24, 2010

### cylinder

By conventional means, the faster a massive body moves through space, the faster it will move through time. So, when that body approaches the speed of light, time around it begins to stop. Then, if you found some way to move faster than the speed of light (by use of pushing off of something (rocket, wheels, maglev, ect)), you'd begin to move backwards through time.

9. Jan 24, 2010

### NPSF3000

Which is why I used the word instantaneous. AKAIK the only problem with instantaneous travel is that you may 'somehow' put yourself back in time - that is what I want to know about.

A common belief about Star Trek for example, is that they utilize an alternate universe that is either very dense, or has a different c as a method for going around our laws of "c". It's the time travel element that appears to disagree.

10. Jan 24, 2010

### cylinder

Say you build a stable wormhole from Earth to Alpha Centauri. You've got two portals, one on Earth, and one in Alpha Centauri. Now, say you drive your craft (that can go .99% c) into the portal, and you end up in Alpha Centauri. You drive back to Earth without the wormhole, and it takes 4.4 years. You wouldn't have moved back in time, you'd only have arrived at Earth right after you'd left (relative to the people on Earth, but it would have been 4.4 years for you).

11. Jan 24, 2010

### NPSF3000

Okay, assuming that instantaneous travel allows you to go to x number of light years, but at x number of years in the past you would have an interesting situation - those at home would be able to receive data from you immediately, but you would still be x years away from them when you decided to return home. Defiantly an interesting sci-fi plot.

12. Jan 24, 2010

### jacksonwalter

It'd be a lot easier to just use a physical principle that's not too complex like SR. If you consider time dilation you can travel to distant stars in only a few years proper time if you're moving close enough to the speed of light, and you can accelerate to those speeds in under a year at 10 $$\frac{m}{s^{2}}$$.

13. Jan 24, 2010

### NPSF3000

1st - cause thats boring. (Not that i'm not considering it for other stories). But this story only works if some form of FTL is accomplished .

2nd, if I where to be using time dilation I would use an acceleration much higher, at least 10-20 G's.

14. Jan 24, 2010

### Frame Dragger

Just a small correction: you're all talking about FTL and not "Superluminal" effects. One is breaking c, the other is merely the apparant breaking of c because of local vs. distant conditions. Otherwise Bcrowell has said it all, and he's being generous on the fiction side. The CPC is just a conjecture, but one that was created to describe an apparant truth at all times in THIS universe' history. That such exotic geometries might exist at some future time... who knows. That shouldn't effect your writing however. You're NEVER going to make a convincing FTL machine, but you can make one that isn't openly offensive. You're the author, so you can also turn a posit for principle like Consistant Histories, or The MWI plays a role in your FTL effect. That's why it's science FICTION. Don't get bogged down and hurt the story because of an attempt at scientific fidelity that few will appreciate and won't be perfect anyway. Good luck with the writing btw!

15. Jan 24, 2010

### bcrowell

Staff Emeritus
For a good discussion of how a wormhole is equivalent to time travel, see Thorne's Black Holes and Time Warps. One argument is that you accelerate one mouth of the wormhole back and forth using gravitational forces, and this causes a kind of twin paradox. So if the two mouths are initially synchronized in their common rest frame, by the time you're done they aren't, and they constitute a time machine.

As an SF writer myself (I've had a few stories published), I would say that we're talking about a marketplace, and it's a marketplace for entertainment. Some people find hard SF entertaining. Some find soft SF entertaining. The only thing that grates on me as a physicist when I read SF is when authors offer obviously nonsensical, elaborate justifications for their FTL. To me, FTL is like the tooth fairy. If you want to write a story with the tooth fairy in it, just go ahead and do it. I don't believe in the tooth fairy, but I can suspend my disbelief, provided that you don't try to give me absurd pseudo-justifications for why a tooth fairy really *is* possible.

Some possible SF models for competent treatment of FTL:

In Sagan's Contact, the wormhole is both a spatial travel device and a time machine, which makes sense.

Hal Clement is generally known as the paladin of scientifically accurate SF, but the exception is that he throws in FTL in his stories, because he just needs it for the stories he wants to tell. No apologies. It's wrong. He knows it's wrong. I know it's wrong. It's okay. It's the tooth fairy.

Charles Stross has done some stories that explicitly acknowledge the equivalence of FTL and time travel.

One scientifically accurate feature of the Stross and Sagan examples is that the FTL is accomplished by godlike beings. Anything else isn't right scientifically, since anybody who can accomplish FTL is going to have to have mastery over godlike amounts of energy and matter. This just has to be so, based on the gross features of GR.

16. Jan 24, 2010

### Frame Dragger

Well said, not to mention that the author can just posit a universe with different constants, or geometry so that many planets and regions exist with a short path for a traveler. As the writer, you have huge freedom to make sure that you never run into FTL contradictions, or run right over them with a wink and a nod. Just don't pull that Catherine Asaro **** where somehow a mathematical construct like Imaginary Time becomes a metaphysical "hyperspace". Star Trek shamelessly warps around their galaxy, but they (usually) seem to be subject to some form of chronology protection, and it's as much obvious fantasy as sci-fi.

Writing isn't about being the best physicist, although there is some good "hard" scifi that DOES limit or do away with FTL travel. People often want to make a star-spanning adventure, when a local system within a ly or 2 is PLENTY. That is a loooooooot of space!

17. Jan 24, 2010

### hamster143

The key to understanding this conundrum is that, in special relativity, there's no absolute time and no absolute notion of simultaneity. Two events, one here and one on Alpha Centauri, may be simultaneous in one reference frame and non-simultaneous in another. A good analogy is a coordinate grid on a 2-dimensional plane. If we choose a grid, we can operate with concepts "moving parallel to y-axis", "getting from x=a to x=b in such a way that does not involve any changes in y coordinate", but none of them have absolute physical meaning, because any rotation of the grid redefines axes.

Once you forget about absolute time, you can start understanding why FTL is equivalent to time travel.

Suppose you start your trip on January 1, 2011 by Earth clock in the direction of Alpha Centauri. From SR point of view, the destination "(Alpha Centauri, 1/1/2011 by Earth clock)" is perfectly unremarkable. If there is a way for you to emerge on the other side on January 1, 2011 (such as a wormhole), there's no reason why the same mechanism shouldn't allow you to emerge in 2014 or 2008. You could emerge in 2008, turn around, and do the same in reverse, ending up on Earth in 2005.

In future, we could probably learn to create pairs of connected wormhole openings and manipulate them in such a way that there's always a bidirectional wormhole that goes from Earth, year X to Alpha Centauri, year X+3, and another wormhole that goes from Earth, year X to Alpha Centauri, year X-3, creating two "ladders" that let you travel arbitrarily far into the future or back into the past up to the moment of their creation.