Alcubierre drive research

[cicatrix]

We don't, though. A simple warp transit in our frame of reference works exactly as you would expect - in this case, you see the ship arrive at mars in 20 minutes (light speed lag) plus one second (travel time). Distant observers may see it in a different order depending on their frame of reference, but their clocks aren't important to us.

But wouldn't the ship be travelling back in time too? I think that we should see it appearing near Mars in less than 20 minutes.

[Bill Phil]

But wouldn't the ship be travelling back in time too? I think that we should see it appearing near Mars in less than 20 minutes.

We have to aim it 20 minutes ahead of Mats anyhow

[Rune]

Here's a simpler illustration of FTL paradox:

Imagine we have a FTL-capable ship waiting in low Earth orbit (let's assume it can warp at the speed of 20 light minutes per second). We also have a very good telescope able to detect a baseball on the orbit of Mars.

Let's assume, the current distance to Mars is around 20 light minutes. We point our telescope to Mars, start looking through it and then issuing an order for the FTL ship to depart to Mars.

Now, will we see the ship appearing on the orbit of Mars BEFORE 20 minutes have passed?

No we won't. Because the light that allows us to see that ship will still take 20 minutes to get to us. What could feak us out is if the ship came back before those 20 minutes were up, we would be seeing both the past and the not-so-past images of the ship, in both Earth and Martian orbit "at the same time".

Rune. Doesn't take a lot of knowledge about relativity to figure that out.

[Findthepin1]

We have to aim it 20 minutes ahead of Mats anyhow

*Mars

But wouldn't the ship be travelling back in time too? I think that we should see it appearing near Mars in less than 20 minutes.

If the ship is traveling back in time, it would appear at Mars before it began its backwards-time-warp, not any time after, AFAIK.

[cicatrix]

*Mars

If the ship is traveling back in time, it would appear at Mars before it began its backwards-time-warp, not any time after, AFAIK.

Yes, but I'm taking into account the 20 min delay, so.. we would see it appearing there only after had departed.

Though, I'm wondering if it's possible to see the ship appearing somewhere BEFORE it had departed and then prevent the ship from flying off.

[K^2]

The ship arrived before Earth saw it leave the faraway point. It's an optical illusion.

Does not matter how many times you repeat the optical illusion counter, does not make it true. Please, learn some relativity, or stop arguing with people who have. Relativity of simultaneity is not an optical illusion and allows for actual violations of causality if you have FTL means.

[DestinyPlayer]

I still don't quite understand, how would FTL enable time travel? Wouldn't it just leave a whole bunch of afterimages behind the ship from all points of view? Or am I missing something, and that makes it time travel?

[rtxoff]

I don't think it makes much more sense to argue here, in the end it doesn't matter either. I highly doubt any of us will live long enough to see that they have been right or wrong.

[magnemoe]

The ship arrived before Earth saw it leave the faraway point. It's an optical illusion.

Its not an optical illusion, the ship arrives in LEO, you see that the drive activates 20 minutes later same as you see lighting before you hear the thunder.

In relativity all frames of reference is equal so someone on Mars will see the drive activate and the ship disappear then appear in earth orbit 20 minutes later and both are right.

However this will not create an paradox outside of this, where is no way to bring information back in the past and stop the ship from leaving after it activates warp.

Or is it possible if you have an message system faster than the ship, say ship moves at 2 * c while the message uses less than an second?

[ZetaX]

You didn't say anything that implied time travel. You simply show that the ship arrives back to Earth before it appears to have left the faraway point. That's not time-travel, just an after-image. Any message sent from the destination would not have arrived yet either.

I'm aware of time-dilation, and very familiar with relativity. I do not see any reason to confuse FTL travel with time travel though. Only the appearance of it.

K^2 has already fully explained that this is not what he describes and not what is happening. Please read his post where described the time traveling methods again instead of just wildly interpreting things. He definitely did not say what you attribute to him.

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I don't think it makes much more sense to argue here, in the end it doesn't matter either. I highly doubt any of us will live long enough to see that they have been right or wrong.

That's not how it works. Already well-established facts and pure mathematics/logic imply that FTL can happen only if time travel can happen, too. If you think time travel is impossible, fine, but then FTL is impossible as well.

[GeneralVeers]

No, you simple create a bubble geometry that collapses by itself after a fixed distance.

From what I've read, that would only be possible with tachyonic matter. If that doesn't exist, then the above isn't possible. As I understand it (everything I've read is very unclear on exactly how the Alcubierre warp bubble is created) the Alcubierre bubble is something that collapses back into normal nearly-flat space the instant the force creating it is turned off.

Just a thought - what *IF* causality gets violated regularly, but the universe simply exludes these paradoxes as they are somehow deleted from objective reality and thus we don't perceive them. So if someone creates a paradox he simply disappears (along with all the memories and traces he had left in this reality) so we act as if that person never existed. I know that this theory is unfalcifiable and unscientific, but it can explain the apparent absense of time travellers.

Actually, the apparent absence of time travelers, and the attendant paradoxes, from our universe can be explained much more simply:

Suppose you, citcatrix, were to go back in time and prevent Justin Bieber from ever being born. After you changed the timeline, you would be the only person in the entire Universe who knew of the change. Nobody else would know that the timeline had been rewritten, because everybody else in the Universe would have had their own timeline--and their memories--rewritten along with it.

As to the absence of paradoxes--how do you know we're not ALREADY IN A PARADOX right now? Suppose you were to go back in time and kill your grandfather. Then you were never born, you never went back in time to kill your grandfather, the original timeline snaps back into existence, and the cycle repeats endlessly (at least, until the Uncertainty Principle kicks in and the Universe ends up in a temporally consistent state--shoutout to Rakaydos for mentioning this earlier on, by the way!).

How do you know we're not in the middle of one of those loops at this very moment......? This life you're living right now could simply be one iteration in billions, being played out normally from your viewpoint. And when time loops into the alternate history where your life never happened, you won't remember that it never happened because you never happened. Then, when time returns to THIS loop, you won't remember the PREVIOUS loop. That's what's really insidious about causality loops. There's no way to test whether we're in one.....

Long story short: there's no way for anybody existing outside the above scenarios to test them. Only the traveler himself would ever know that time travel had happened. And if the traveler's meddling created a causality loop, the traveler himself would never know it.....

Edited October 31, 2015 by WedgeAntilles

[cicatrix]

I don't think it makes much more sense to argue here, in the end it doesn't matter either. I highly doubt any of us will live long enough to see that they have been right or wrong.

Depending of how old you are you might live (or 'exist' to be precise since the term 'live' is open to many interpretations) much longer than you expect. Of course it's offtopic here.

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Long story short: there's no way for anybody existing outside the above scenarios to test them. Only the traveler himself would ever know that time travel had happened. And if the traveler's meddling created a causality loop, the traveler himself would never know it.....

The latter disturbes me. My memory is 'stored' within organic matter (my brain). If I create a causality loop then somehow the state of my brain should be changed - which means the matter will be re-arranged somehow. This requires work (energy). Where does it come from? I think I'm on the verge of inventing a machine that creates energy out of nothing.

Edited October 30, 2015 by cicatrix

[ZetaX]

Suppose you, ZetaX, were to go back in time

I think you wanted to address cicatrix there. At least you quoted him and I did not say anything that you seem to respond to in that part of your post.

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The latter disturbes me. My memory is 'stored' within organic matter (my brain). If I create a causality loop then somehow the state of my brain should be changed - which means the matter will be re-arrange somehow. This requires work (energy). Where does it come from? I think I'm on the verge of inventing a machine that creates energy out of nothing.

No, time travel does not rearrange matter at a distance or in any other way (sans the travelers themselves). It simply goes to an earlier point where your state of mind was different, it then continues to rearrange itself in a different way due to different circumstances. No energy involved. Philosophically, this is quite complex, but from the energetic point of view it is simple.

[cicatrix]

No, time travel does not rearrange matter at a distance or in any other way (sans the travelers themselves). It simply goes to an earlier point where your state of mind was different, it then continues to rearrange itself in a different way due to different circumstances. No energy involved. Philosophically, this is quite complex, but from the energetic point of view it is simple.

The spacetime is a poor term, perhaps, because it combines 'space' and 'time' - two different concepts in our understanding of things right from the earliest childhood. I wish Einstein had come up with a different term. It took me quite a lot of time to understand that it is a one thing, not two different things bound together with some weird theory. To illustrate - let's strip one dimention, as we always do, from 'space' and substitute time instead. We now have a 3d object (say, a loaf of bread) where one end is the Big Bang and another one - the heat death of the Universe (or whatever end-of-the-world is in store for us). If we cut it we'll get a single moment of 'time' somewhere in between.

If I stick a toothpick inside it - it would represent my worldline within the spacetime - every single moment in my life is already there. The trajectory of each particle my body consists of has been 'fixed' at the moment of creation of our Universe. It's deterministic. At every single slice there will be only one copy of me. Normally. What I do now, what I will be doing in 10 minutes, 10 years from now - all of it is already there. The amount of matter and energy in each slice is constant.

Imagine now I somehow manage to overcome the entropy of that system and bend the toothpick (my worldline) in such a way it will appear twice on some slices. If we take space and time separately then I just added energy to the space '10 minutes ago' by introducing my own self into it twice, but at the same time I subtracted energy from my former 'now' by removing myself from there. Only if we take the whole spacetime as a whole the laws of conservation can be preserved.

This is the problem with time travel - I will need to reverse the entropy of the Universe. Will my toothpick (worldline) be bent to create a loop or will it break and create two separate worldines? Normally (without time mahchine) each particle has a worldline pointed in the direction the entropy increases so, if I re-arrange matter in the past, its particles will have no means to affect me also being in the past since without special means for time travel I will be the 'cause'. So I don't think that if we assume time travelling is possible my memory would be in any danger of changing. Even if I would really kill my granddad.

Edited October 30, 2015 by cicatrix

[K^2]

You didn't say anything that implied time travel. You simply show that the ship arrives back to Earth before it appears to have left the faraway point.

No. I have provided a scheme in which ship arrives on Earth before it left Earth. And as I have indicated, there is a general theorem that states that it is always possible if you can go FTL. There shouldn't be a discussion about it. If you don't understand Special Relativity, please, feel free to find a textbook to learn it. Otherwise, don't argue about things you don't understand. It's never a good idea.

[rtxoff]

Even in Wikipedia you can read that a wormhole can be used as a time machine only in a specific case and that one is when one of the ends is moving. In the other case you will only be traversing space. Still people here claim that it does not matter which method someone uses it will end always in time travel. https://en.wikipedia.org/wiki/Wormhole

[K^2]

Even in Wikipedia you can read that a wormhole can be used as a time machine only in a specific case and that one is when one of the ends is moving. In the other case you will only be traversing space. Still people here claim that it does not matter which method someone uses it will end always in time travel. https://en.wikipedia.org/wiki/Wormhole

A lot of mistakes start with, "Even in Wikipedia you can read..."

But yes, it's all about moving. Thing is, if you can create a wormhole between two points in a static frame, you can create it in a moving frame, because there is no difference between inertial frames. And then you get a wormhole with both ends moving, which is sufficient for time travel.

Relativity is a very important concept in relativity. Who would have thought?

[rtxoff]

So traveling through a wormhole between two points in a static frame won't cause CTC's?

[Silver_Swift]

Still people here claim that it does not matter which method someone uses it will end always in time travel.

What people are trying to refute is the idea that because a ship with an Alcubierre drive is somehow safe from causing CTC's because it doesn't locally go faster than C.

[rtxoff]

What people are trying to refute is the idea that because a ship with an Alcubierre drive is somehow safe from causing CTC's because it doesn't locally go faster than C.

Naturally, the whole point about this drive is not to go faster then C because it's simply not possible but still arrive where you want to in a very short time.

[Pixel of Life]

"Time travel" happens as long as you're either A) moving relative to the observer, or experiencing a different gravitational force than the observer. The warp drive itself has nothing to do with it - it also happens with normal spacecraft.

Here's a fun thought: The Voyager 1 probe is moving at tremendous velocities from our perspective so its clock runs slightly slower than ours, but it's also experiencing a lot less gravity (about 1 / 133^2 = 1 / 17689 of the gravity experienced by the Earth as of "autumn 2015") due to the huge distance between it and the sun, and this makes its clocks run faster. Since the force of gravity decreases to 1/4 every time the distance is doubled, but velocity doesn't, does this mean that at some point, the net effect of relativistic and gravitational time dilation might actually make time on the probe move more slowly than it does on Earth, thus making the probe "travel to the past" from our perspective?

Edited October 31, 2015 by CaptainKorhonen

[GeneralVeers]

I think you wanted to address cicatrix there. At least you quoted him and I did not say anything that you seem to respond to in that part of your post.

Oopsie. Yeah, I meant to name-drop him, not you. Previous post corrected.

The latter disturbes me. My memory is 'stored' within organic matter (my brain). If I create a causality loop then somehow the state of my brain should be changed

From your viewpoint in four dimensions, yes. It does appear as if the state of your brain is being changed.

But we need to look at it in five dimensions. Three dimensions of space, and two dimensions of time. The first dimension of time is our normal perceived progression from "past" to "future". When a time traveler changes time, and time takes a new path (i.e. "alternate timeline") that's where the second dimension of time comes in--effectively, alternate possible futures can be thought of as "sideways" in time.

In four dimensions, the time traveler's shenanigans appear to be changing things. In five dimensions, however, the Universe's path through time is fixed. History simply follows a curved path through various alternate futures (each curve in the fifth dimension being a change made by a time traveler). All changes made by time travel, and all the alternate futures resulting, are already included in the Universe's five-dimensional path.

- which means the matter will be re-arranged somehow. This requires work (energy). Where does it come from? I think I'm on the verge of inventing a machine that creates energy out of nothing.

That would come from the time machine and the time traveler, i.e. from whatever work the time traveler has to do to kill somebody, prevent somebody from dying, make Justin Bieber's music halfway decent, or whatever else. In five dimensions, those changes have already happened; the time traveler's actions are already included in the worldline.

Now running for cover in case somebody's brain explodes........

[PB666]

Does anyone else here get the feeling people are arguing in this thread just to be arguing. If i could place the pro "time" travel folks in front of this thread and the anti time travel folks in back of the thread could that blabber energy take the thread back before it was born and have a forum server power failure so it would not ever exist?

[problemecium]

I think I get it now, if I understand Silver_Swift's story about the drones correctly.

That said, I guess all of us on the doubting team were right in our hunch that it wouldn't be easy. Also, if I understand everything correctly, C will not self-destruct, because it is programmed to do so only after receiving the order from D, which cannot occur until C has itself informed D, regardless of whether A1 occurred. And since C is moving forward in time, in its own reference frame it cannot send a message and receive a reply to that same message before sending it.

We can only hope for the sake of future space colonization efforts that the universe does pardon CTCs and that the Chronology Protection Conjecture is wrong, or that there is an unknown law of physics that will come to the rescue.

[K^2]

So traveling through a wormhole between two points in a static frame won't cause CTC's?

First of all, CTC is frame-independent. It's either a CTC in all frames, or it's not. There are precisely two properties of that curve. 1) It is closed. 2) It is time-like. Neither property changes with frame of reference.

Second, it's not about "won't cause". It's about "can cause". Simplest way is to use a second wormhole. But you can also use that same wormhole twice after accelerating both mouths.

So if that's where you're stuck, no, you don't have to time travel to go FTL. But if you can go FTL you inherently can time travel. If FTL is permitted, then so is time travel, because you can always come up with a trajectory that uses FTL to bring you back to where you started, exactly the same time and place. Which also means you can arrive at your departure point before you depart. Can. Don't have to. But you always can.

And again, means of travel are irrelevant. You can use wormholes, warp drive, or pixie dust. It's a matter of space-time differential geometry only.