Alcubierre Warp Drive

clark · 2004-04-28 08:57:17

Ha! I'm hanging with the big boys in physics. Believe me, I'm treading water here, so it's comforting to know that I'm not to far off... well, not that far off... okay, near enough to sound semi-literate. I can settle for that. :laugh:

Algol, I like your explaination. When I approach stuff like this, I try to think of similar analogies that might guide my understanding of the overall idea.

Now, in a universe where we could not see, would we have such a hard time dealing with the issues of when it looked like we took off, or when we arrive?

Think about a plane travelling past the speed of sound. You see the plane, yet the sound waves travel much further behind it. Technically, it might be possible to circle the earth and arrive within your own sound wave, no? (assuming you could go fast enough)

Same thing here with light. It's just an effect- an observation registered by our senses. Now, perhaps there is a way to circumvent causality, however, FTL won't do it.

Shaun Barrett · 2004-04-28 19:13:56

Sorry to butt in here - I'll be brief.
Just wanted to tell Algol his A,C,E thought experiment is one of the most digestible and entertaining ones I've read about the whole light-barrier question.
Thanks, Algol!
:up:

Euler · 2004-04-28 19:37:23

The problem with the A,C,E explanation is that all 3 points are in the same inertial reference frame. If you try the same experiment with moving stars and time dialation, you should end up with different results.

The problem with using sound as an example is that sound has a prefered reference frame, while experimental exidence sugests that light does not.

ERRORIST · 2004-04-28 19:37:35

Light has mass. If at rest it must therefore also have mass.

Euler · 2004-04-28 19:48:55

Light has mass. If at rest it must therefore also have mass.

You have already discussed this a lot. Light is never at rest.

Trebuchet · 2004-04-28 22:47:58

Well, mathematically Alcubierre's drive works in relativity. So faster than life travel results in paradoxes... it's not like physics isn't screwy enough (quantum physics) so what's one more bit of insanity?

Mark Friedenbach · 2004-04-29 00:10:46

an alcubierre drive isn't a faster-than-light drive. the contraction and expansion of space effects everything, including electromagnatism, so you'd still be going less than light speed. people at your destination would still see you coming before you arrive. assuming you didn't distort spacetime too much, and you took a roundabout path to your destination, you could turn around and watch the preparations for your own launch, but you wouldn't have gone backwards in time. if you stayed two hours and then went back with your alcubierre drive, you'd find that two hours would have passed on earth. the ability to catch up with light that left you and travelled a different path is nothing new. this happens with strong gravitational lensing, for example. one lensing image can be delayed compared the other.

Josh Cryer · 2004-04-29 00:36:30

Screw reference frames. I see it this way. If I go to Sirius in ten minutes (for the sake of argument, obviously), I'd be arriving at a Sirius 8 years ten minutes in the observable future, but my trip would have only lasted ten minutes with regards to a universial, unobservable reference frame. If you were there when I arrived, it'd still take you some arbitrary amount of time to return to tell anyone I left. Even if both ends of the trip have difference levels of time passage, time is still passing.

Time paradoxes with wormholes, as I understand it, essentially works like this; you make a wormhole, take one end of it and move it an arbitrary speed and distance, and it winds up expericing less time than the time of the other end. Now, if you send something through it, it should arrive before it gets sent, and once you see it sent, you opt to not send it; how does it arrive (there's your time paradox)? Wormholes could easily cirumvent this paradox if they had a fixed length and vector (say, because wormhole physics required it), if you move one end, the other end moves precisely as much.

But taking into consideration moving time and reference frames; if I leave from a hub which is expericing 1/3rd time, and arrive at a hub experincing 1/4th time (to the observer between both paths), all you would do is see some thing blink into existance at the destination, and out of existance at the origin a momment later. But you would be completely unable to communicate to the thing leaving that it had already left, in an attempt to keep it from leaving (and causing a time paradox). Because this observation is of something which happened awhile ago with regards to a magical reference frame. You go to either destination with your faster than light ship, you arrive at least as long as it took to see the event.

And even then, that would be cirumventable for the same reasons that a wormhole could; your speed must be exact when you enter and exit existance- perhaps your speed would have to be exactly that of the expansion of space in whatever vector you're travelling or some such.

Those are basically the two time paradox situations I can think of, I don't understand time paradoxes with other forms of FTL travel.

Edited By Josh Cryer on 1083220774

clark · 2004-04-29 09:27:49

Time paradoxes with wormholes, as I understand it, essentially works like this; you make a wormhole, take one end of it and move it an arbitrary speed and distance, and it winds up expericing less time than the time of the other end. Now, if you send something through it, it should arrive before it gets sent, and once you see it sent, you opt to not send it; how does it arrive (there's your time paradox)? Wormholes could easily cirumvent this paradox if they had a fixed length and vector (say, because wormhole physics required it), if you move one end, the other end moves precisely as much.

I'll probably drown on this one.

Imagine the wormhole with two end points (start, finish). We open the wormhole (the start) and it experiences normal time. We open the other end (the finish) and it experiences half of normal time. So 1 year at the start, is only half a year at the finish. Do you travel back in time when you go through the wormhole?

No.

This is all about refrence frames from the way I look at it. This might be easier to deal with if we start thinking of "time" as a local phenomenon. The speed of time is dependant upon the locality and/or the observer.

If you go through the wormhole, to the finish, then go back through to the start, you will arrive at the start some positive time after when you first went through. Causality can't be cheated (at least how I see it). Sure, the math might say so, but the math is invalidated with a violation of causality.

Time, and how we measure it, is an arbitrary thing. It's based off of observations to manage it into understandable bits and pieces. But the very premise of "time" is one of causality. "Time" is the forward progression of events based on a cause-effect relationship. No matter how we describe it or window dress it, that's what time is. Or am I completely off base here?

Cutting through the universe, squeezing it, folding it, or speeding through it, dosen't change cause and effect.

Josh Cryer · 2004-04-29 17:20:59

Imagine the wormhole with two end points (start, finish). We open the wormhole (the start) and it experiences normal time. We open the other end (the finish) and it experiences half of normal time. So 1 year at the start, is only half a year at the finish. Do you travel back in time when you go through the wormhole?

Well, you would be "traveling back in time" if both ends of the wormhole were close together enough to have a causality paradox. I go in the wormhole at midnight on Janurary 1st, and I arrive sometime in June the year before. So what you can then do is kill me before I go into it, and have yourself a cute time paradox.

What you seem to be saying is that it doesn't matter that the clocks on either side of the wormhole are different; one can enter a wormhole at any point but not exit until after the reference time point that they have entered. I really don't think so (unless, like I said, wormhole physics required it to be a fixed length and vector; that would result in the behavior you are talking about). Oh, of course if you go back through the wormhole you will return a positive amount of time later (ie, you'd travel forward in time from the time that you left).

I do believe the universe allows for paradoxial observations. I don't think, however, that the universe allows for paradoxes; it still takes time for one to act on an observation. The quantum tunnelling effect does result in observations before a signal is sent; the key is that one cannot stop themselves from sending the signal at that point because of how infinitesimally quick it happens.

Mark Friedenbach · 2004-04-29 18:15:24

hrm, correct me if i'm wrong, but wouldn't the distance through the wormhole expand to compensate for the time lag? so if the other end of the wormhole is "2 years behind" the first, there'd be at least 2 light-years from entrance to exit?

Josh Cryer · 2004-04-29 20:03:21

I'm not sure what you're asking. The internal structure of a wormhole theoretically doesn't have length. You go in, you pop out on the other side somewhere else in the universe. If we can move the wormhole "doors" about arbitrarily, both ends could be ten feet from each other, for all we care.

Here's a wormhole, each '-' is a light minute. The O on the left will be the entrance, and the one on the right will be the exit.

O---------O

distance: 10 light minutes.
entrance: 12:00
exit: 12:00

Now we take the exit off on a journey at nearly the speed of light; as it goes on its journey, it experiences almost zero time.

O------------------O

distance: 20 light minutes
entrance: 12:10
exit: 12:02

Now we bring the exit back to a close enough point to observe our magical paradox.

O-O

distance: 1 light minute
entrance: 12:20
exit: 12:04

If I go into the entrance, when will I exit? Relativity would posit that you would exit 16 minutes earlier, no? clark says that you would exit a few momments later.

Mark Friedenbach · 2004-04-30 01:31:05

I'm not sure what you're asking. The internal structure of a wormhole theoretically doesn't have length. You go in, you pop out on the other side somewhere else in the universe. If we can move the wormhole "doors" about arbitrarily, both ends could be ten feet from each other, for all we care.

Well, a wormhole is not an instantaneous connection between two points, just a shorter one. Moving one end of a wormhole for time dilation might stretch the connection to compensate. You time dilate two hours, you stretch it two light-hours, that's all I'm suggesting.

clark · 2004-05-03 09:17:21

If I am in the United States, and I call someone living in Japan, which is a day behind, am I calling into the past?

If Byron on the East Coast calls me on the West Coast, is he talking to me in his past? If I call him, am I talking to him in my future?

Relativity seems to posit that variations in the refrence frames of the inertial observors cause these weird instances between them. Common sense tells me it's fancy math.

My 1:00 PM is Byron's 4:00 PM. Japan's Tuesday is my Wednesday.

I don't see how going faster than light, or going through a wormhole, changes any of this.

Mark Friedenbach · 2004-05-03 11:41:55

If I am in the United States, and I call someone living in Japan, which is a day behind, am I calling into the past?
If Byron on the East Coast calls me on the West Coast, is he talking to me in his past? If I call him, am I talking to him in my future?
Relativity seems to posit that variations in the refrence frames of the inertial observors cause these weird instances between them. Common sense tells me it's fancy math.
My 1:00 PM is Byron's 4:00 PM. Japan's Tuesday is my Wednesday.
I don't see how going faster than light, or going through a wormhole, changes any of this.

??? not sure I follow what you're trying to say. Relativity gets rid of universal reference frames, so there's no way that you can say one place is truely an "hour ahead" or an "hour behind".

But using your example, if you call your friend on the east coast, what relativity posits is that there will be a delay in communication between you and him. with wormholes, this delay may be become arbitrarily close to zero, but never negative (meaning you will never hear his response prior to you saying anything). I think that's all you can read into your example.

clark · 2004-05-03 12:07:48

But using your example, if you call your friend on the east coast, what relativity posits is that there will be a delay in communication between you and him. with wormholes, this delay may be become arbitrarily close to zero, but never negative (meaning you will never hear his response prior to you saying anything). I think that's all you can read into your example.

My analogy aside, this is exactly my point.

You can't cheat causality, which relativity implies. Our own individual refrence frames determine the squence of events.

Mundaka · 2004-05-03 17:16:40

~Eternal~ · 2004-05-03 17:36:27

LOL!
Fashionable Faster Than Ligh Drives.

Mark Friedenbach · 2004-05-03 17:36:46

My analogy aside, this is exactly my point.
You can't cheat causality, which relativity implies. Our own individual refrence frames determine the squence of events.

Oh cool, we're on the same wavelength then. Forget I said anything

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