Alcubierre drive research

[PB666]

Causality need not exist, an oxymoron, but the problem is that our universe in its origin may not be causal, and the fact we cannot detect quantum gravity may suggest that causality is not neccesary. For example if dark energy were to reverse and the universe began to close back in on itself causality may cease to exist in the final moments. Local space time in the universe can be moving away at many times c....somewhere i read something like 58c....maybe higher. If this is reverse direction and return when they got close to each other then the comoving reference frames would be moving faster than c. In this way time is a function of the inflation of the universe, relativistic gravity may be the fix.

You are confusing what happens in our peaceful little backwater with all points. i think what hawkings and others are trying to say that if you have enough energy to travel back in time you also have energy comparable to other space time of the univers where this is possible, unfortunately, its too much energy for human, maybe matter, to exist. You could go back in time but it wouldn't matter because the local spacetime you are sitting would be complete and utter chaos characterized by exotic matter, inflation, etc. The outcome would be a honogenization of local space and future generations would not see specific outcomes but generalized outcomes, like CMBR. I think from a quantum perpective if you tried to tamper and persuade many anti-normal events, you would prolly loose all control of shaping those events, that is a refection of destructive chaos. Another way to look at this is that our time-line is like a pond and we are a fish in that pond, the pond has streams that flow in and out, and we can swim to those streams, but should we seek out to enter a birds nest or walk on the moon, we cannot expect our pond physics to work the way we expect.

Edited October 29, 2015 by PB666

[Specialist290]

The FTL page over on Atomic Rockets probably has the best way of breaking down all the mathematics and technical issues behind faster-than-light travel into layman's terms. The best summary, to quote the page itself, is: "Causality, Relativity, FTL travel: choose any two." Edited October 29, 2015 by Specialist290

[Rakaydos]

The FTL page over on Atomic Rockets probably has the best way of breaking down all the mathematics and technical issues behind faster-than-light travel into layman's terms. The best summary, to quote the page itself, is: "Causality, Relativity, FTL travel: choose any two."

Saw this on that page, thought it would be interesting to discuss...

"In some late-breaking news, physicists Daniel Greenberger and Karl Svozil have shown that the laws of quantum mechanics enforces Consistency Protection. You can read their paper here, but it makes my brain hurt. Translated into English, they maintain that time travellers going back into the past cannot alter the past (i.e., the past is deterministic). This is because quantum objects can act sometimes as a wave. When they go back in time, the various probabilities interfere destructively, thus preventing anything from happening differently from that which has already taken place."

So, if FTL was possible, and you could go back a few seconds of time with massive energy use... it still wouldnt do anything useful. Or am I reading that wrong?

[PB666]

Saw this on that page, thought it would be interesting to discuss...

"In some late-breaking news, physicists Daniel Greenberger and Karl Svozil have shown that the laws of quantum mechanics enforces Consistency Protection. You can read their paper here, but it makes my brain hurt. Translated into English, they maintain that time travellers going back into the past cannot alter the past (i.e., the past is deterministic). This is because quantum objects can act sometimes as a wave. When they go back in time, the various probabilities interfere destructively, thus preventing anything from happening differently from that which has already taken place."

So, if FTL was possible, and you could go back a few seconds of time with massive energy use... it still wouldnt do anything useful. Or am I reading that wrong?

This raises a whole bunch of new questions, according to relativity model, the existence of a point (or lets say an quantum object at that point, lets say a neutron) progresses through space time whose motion could be set forth. If i then took that neutron away at the speed of light, accelerated it into a different frame, then I created an electron and a proton and brought it back at FTL, I could then create a deuterium atom with that proton and the neutron.

How would the neutron know that the proton came from itself? Are you saying the proton produces a wave that the neutron suddenly recognizes. This sounds like the equivalent of the Sci-fi phase, the proton would have to be in another phase, its vibrations out of tune with the proton making them invisible to each other. For this to occur time would have to be discrete at the quantum scale, it would be like time-sharing of quantum time. We know the vibrations of the atoms E = hv, this is arguing that that this wave is carrying much smaller waveform, much like system clock on a computer. Even so if its a wave sometimes the harmonics will match and allow the interaction. So its not a full proof method.

For example, the second that ship heading back in time tries to take an observation, everything would disappear, the universe would become black, or it would enter another timeline, maybe a timeline that has all the anti-matter left from the nucleogenesis.

[ZetaX]

What does intuition has to do with causality? Even a farmer knows that the apple can't hit the ground until it falls off the tree.

You are claiming that your own intuition is an objectively obvious fact, while it clearly isn't. You are confusing your oppinions with universal truths here.

Pointing to farmers (which generally have no clue about these topics and thus are neither experts nor relevant) is not strengthening your claim even a bit; it actually weakens it, as it creates the impression that your best argument is to point to the oppinion of an uneducated guy on the fields.

[Bill Phil]

What exactly is causality, and why can it not be violated? Could it not be plausible, albeit unlikely, that causality could be somewhat incorrect? I'm not saying it's guaranteed, but...

Time me could be completely different than our thoughts. It's beyond observation. It is merely an illusion, a completely human thing to define the different instances of the universe. But it's based on something that does exist, a process of changing instances that the universe inherently has. But it's not time, it is a part of it.

[K^2]

Well this is something which still has to be observed, my standing point is we can't persist onto such things only by drawing a 2D diagram. Causality is basically the most important law of nature i really can't understand why an scientist would claim anything that will break it.

Causality is not the most important law of nature. It's not even a law of nature. It's a nice feature for some field theories. However, any field theory that supports gravity must also support causal loops, because thy are a feature of General Relativity. It's that simple. The fact that we rely on causality so much in our day-to-day lives means nothing to the rest of the universe.

[GeneralVeers]

Here's the real problem with the Alcubierre Drive, as far as we know: the traveler inside the warp bubble is causally disconnected from normal space. The traveling ship can't control the bubble, because any force or signal sent ahead of the bubble to control it would have to pass outside the bubble and travel faster than light. That means the warp bubble can only be controlled from the outside, and THAT means the infrastructure to control the bubble and convey traveling ships would have to be sent to the destination in advance.

At sublight speed.

[problemecium]

I still feel like we haven't established a concrete connection between warp and time travel.

We've successfully addressed that the structure of relativistic field equations is such that it does not preclude time travel for an FTL traveler, and that barring an unknown variable, this means that somewhere out there, a "solution" must exist to the equations. So has anyone actually found one? Has anyone proven mathematically that such a solution must not only exist but be at least as feasible as the warp drive was to begin with?

That's what I'd like to know. I'd like to see either the actual math that equates to backwards time or an example of a specific sequence of actions that results in time travel.

The closest I've seen so far is Silver_Swift's story about two locations in which events occur outside one another's light cones. So for third parties flying around the universe, Event A1 may occur before B1 or vice versa. Makes sense. But in order for information about which happened first to get to any party involved, wouldn't both events need to enter his or her light cone first?

So on Planet B a huge missile is fired toward Planet A. By sheer coincidence, Planet A's scientists launch a big satellite around the same time. Shortly thereafter, astronomers on Planet A see a huge missile coming, and there's nothing anyone can do about it. But when the missile arrives, by sheer coincidence the satellite blocks it and the world is saved. Physicists on Planet A surmise that based on the missile's speed, it was launched before the satellite was ever launched, and that they lucked out when they decided to launch it.

Meanwhile Jeb is in a rocket flying past and sees the satellite launch first and then the missile. Aha! So the physicists were wrong. He goes to tell them about it, but in so doing he must travel slower than light, so by the time he gets there, the missile has already hit the satellite. More importantly, the missile has entered Planet A's light cone, so the fact that it had been launched had been visible to astronomers already. His perspective was perfectly valid way out in space, but in this reference frame he's wrong. Right?

Assuming I thought that through, let's run it again.

Planet B launches missile while Planet A launches satellite around the same time. Val, in space, sees the missile launch and realizes that by the time the Planet A-liens see it, it will be too late to do anything. Incidentally, she notices that their satellite has not been launched from her point of view. No matter though - she has a world to save. She warps to Planet A at ludicrous speed to warn them. They are still debating whether to launch the satellite, and she convinces them that it's of world-shattering importance. Thus they launch it. A while later, astronomers spot the missile coming and are glad they launched the satellite. When the missile arrives, the satellite blocks it and saves the world. Everything's cool.

But wait! The physicists calculate the missile's incoming speed and surmise that it launched before their satellite did. How did Val manage to see it coming... wait no. They surmised that last time around too. Val has warp, so of course she can warn them before the missile comes into view.

Once more, with feeling!

Planets A and B launch their respective thingies around the same time. Secret Agent Kirrim is in space flying around (in the opposite direction as Val last time) and sees the satellite go up first. Cool, a satellite. Then he sees the missile launch shortly thereafter. Oh no! Better warn the A-liens. He warps to Planet A at ludicrous speed and tells them there's a big deadly missile coming. Unfortunately, there's no sign of any missile to be seen in the sky. People ridicule him and his claims to have outrun light. But then, a while later, a huge deadly missile appears on scope and the astronomers realize to their dismay that it's too late to do anything. Fortunately, the satellite blocks the missile and the world is saved. Kirrim is honored for his pure-hearted attempt at heroism.

But oh no - physicists have calculated that the missile was launched before the satellite, which was already in orbit by the time Kirrim landed. Yet Kirrim claims to have seen the satellite go up first. They argue at length over whether Kirrim has traveled in time or is simply insane.

As far as I understand it, the question is of trivial importance. Of course people from different reference frames will disagree on the sequence of (unrelated) events. Special Relativity warned us about that. By warning the A-liens, Kirrim had to enter their light cone and thus their frame of reference. His recollection of history ended up out of order, but as far as I can tell he's still in the present.

Edited October 29, 2015 by parameciumkid
blarg

[ZetaX]

Here's the real problem with the Alcubierre Drive, as far as we know: the traveler inside the warp bubble is causally disconnected from normal space. The traveling ship can't control the bubble, because any force or signal sent ahead of the bubble to control it would have to pass outside the bubble and travel faster than light. That means the warp bubble can only be controlled from the outside, and THAT means the infrastructure to control the bubble and convey traveling ships would have to be sent to the destination in advance.

At sublight speed.

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

[PB666]

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

Wrong, that kills the inhabitants. The warp bubble traps matter in front of the vessel just like a black hole, when the warp space needs to relax and move away from the vessel other wise when the feild collapse that material basically blows up right on the ship releasing large amounts of radiation.

We are not really getting the issue here, humans are peddly little biologicals that do not stand up well to the types of forces that pervade traveling between the stars, we are planetbund. The forces we are talking about to create the field are beyond our survival scope, and maybe beyond the reach of our technology, if not that which is available in our accessible universe. At every point of behavior of the system the forces humans would confront would be beyond the scope of any human experience, outside of an atomic bomb blast.

In answer to the kid, to state the issue the paradox of time travel is only a paradox if FTL is possible. SoL limit is not based on light, light is a sensor. The limit is a property of the way space evolves. Massless things do not cling to the inertial fabric of space and are free to move as fast as space allows. So that the big problem is not time reversal, but how space allows this phenomena, if my notion here is correct any attempt to create FTL will result in a disruption of space to a degree that the device itself is disrupted. It would be like making a nitrobenzene compund that was so explosive that any heat applied to it would make it explode, except it requires heat to make the compound, so as it is realizing its form it decays.

The concept of the drive is to creat a blackhole like warping of space confined to E-32 meters, space may not tolerate this, in which case it would decay before the metric forms, then there is no paradox.

[K^2]

I still feel like we haven't established a concrete connection between warp and time travel.

Forget how warp works. You have ability to go from A to B faster than light. From perspective of moving observer, you arrive at B before you left A. You understand that, right? Ok, now before engaging warp, lets have warp ship accelerate to a high speed, so point of origin, A, is such a moving observer. You can still warp to B. And now you arrive at B before you left A from perspective of people left behind at A. Repeat the SAME thing at B to get back to A. Presto. You've arrived before you departed.

[ZetaX]

Wrong, that kills the inhabitants.

I would prefer if you would answer in regard to the actual point, instead of being nitpicky about my choice of word "collapse". Feel free to read it as "break apart", "stops working" or similiar things instead.

[PB666]

Because like other aspects of the drive its not clear that the metric can be turned off, safely. As i stated in the same response the fields that humans would be exposed to while assembling, operating and ceasing may be beyond the survival capacity. Theoretically if you reduced the metric to subluminal speed, broadened the metric as you reduced the feild sthrength that eventually the metric would be so far away that humans might survive, but there is no guarantee that this could be done. Those grains of sand that would have struct the craft end up being exotic matter at the field minimum. As soon as these are not in an event horizon it would spew radiation in all directions.

[K^2]

Observer doesn't observe you arriving until your light come reaches him.

From observers perspective, you disappeared and reappeared soon after. Even if you get back before the light come of the destination reaches Observer, it's an optical illusion. Nothing more. Causality is not affected.

Draw yourself a diagram of these events. It's seriously infuriating when people with zero formal education on relativity argue about these things. You don't understand my explanations? That's fine. But I'm also not going to waste my time reading a full lecture on special relativity that it will take for you to understand this. If you want to understand it, find a textbook. Read it. Understand it. I'll be happy to answer questions.

FTL Capability = Time Travel. Again, it's a theorem in Special Relativity that carries over to General Relativity. I'm very sorry that you don't understand it, but your understanding does not change the physics.

[cicatrix]

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.

Edited October 30, 2015 by cicatrix

[Silver_Swift]

I still feel like we haven't established a concrete connection between warp and time travel.

I'm not 100% sure I got the scenarios you are proposing correctly so I'm going to formulate one of my own. I'm also going to assume instant FTL communications. The example generalizes to arbitrarily slow FTL communication or travel, but it is clearer with instant communications. (Note that this example is likely still way more complicated than needed)

A and B are 5 light minutes apart

A1 and B1 are events happening within 5 minutes of each other on A and B.

D and C are spaceships travelling at speeds such that:

- From C's perspective A1 happens before B1

- From D's perspective B1 happens before A1

Let's keep the events simple:

A1 = A sends FTL message to C and D with "A1 happens now"

B1 = B sends FTL message to C and D with "B1 happens now"

Now assume that the two spaceships are unmanned drones (so no one gets hurt) that can send FTL messages to each other. On top of that C has a self destruct system.

C has the following programming:

- If it receives the message "Please self destruct" it will self destruct.

- If C receives the message "B1 happens now" it sends the message "I have seen B1 happen" to D.

D has just one programmed action:

- If it receives the message "I have seen B1 happen" from C it will wait until it receives "A1 happens now" and send the message "Please self destruct" to C.

So from C's perspective A1 happens before B1 and it receives the self destruct message (and self destructs) before it can send "I have seen B1 happen" to D. But D won't send the self destruct message unless it receives the "I have seen B1 happen" message, so does C get destroyed or not?

Edited October 30, 2015 by Silver_Swift

[andrewas]

Observer doesn't observe you arriving until your light come reaches him.

From observers perspective, you disappeared and reappeared soon after. Even if you get back before the light come of the destination reaches Observer, it's an optical illusion. Nothing more. Causality is not affected.

OK. Lets do this again. Also, go to wikipedia, look up 'Simultaneity of Relativity' and see what happens to the plane of simultaneity in different frames of references. Yes, I know that wikipedia isn't the best source, but its a good diagram and you can google/hit the library for better sources if you want confirmation.

Step one. We Warp from A to B. This happens in Earth's frame of reference so theres no time travel, we just need some distance to work with. B should be more than one light year away for every year we want to travel back in time, possibly much more depending how how much we can accelerate.

Step two - we accelerate away from Earth.

Step three - we warp back to Earth in our new reference frame, which results in travel back in time.

Now, from Earth's perspective, lets look at the events.

Step 1 - we see the ship vanish. If we have a big telescope and wait long enough, we also see the ship arrive at a distant point, and if we account for speed of light delays, we get the expected travel times.

Step 2- the ship accelerates, Earth watches a few years later.

Step 3 - this is the tricky one. Remember that the ship entering warp and emerging from warp are spacelike separated events and so can occur in either order depending on the frame of reference, and we have chosen a frame of reference that puts exit before the entry, in Earth's time. So Earth, a few years after the ship leaves, sees it entering warp for a second time. It saw it arriving a long time before that, though, and after you account for speed of light delays, you do not get the expected travel time as we did on the trip out. The ship, in fact, arrived before it set out.

Im not sure why people have a problem with this. Sure, time travel is absurd and almost certainly impossible - but so is FTL travel.

[Silver_Swift]

As an addition to my above post (to tie it back to actual time travel): if you have instant ftl travel you could groundhog-day-loop yourself by teleporting from C to D when B1 happens and teleporting from D to C when A1 happens.

[PB666]

Traveling into the past and creating a grandfather paradox is absurb in its own right.

FTL travel is not absurb until you create a speed limit for light and make it universal from all local inertial reference frames. This absurbity makes FTL travel absurb but it also facilitates the paradox. There in lies the problem.

Our observations are basically newtonian, we have to rely on machines to tell us that light has a certain speed and behaves in a certain non-newtonian manner. Visually that is not apparent until you compare input energies and velocity at c scalar velocities. That is were the problem lies.

[cicatrix]

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?

[Bill Phil]

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... Light prevents that. We'd see the ship 20 minutes later.

But how does this relate?

[cicatrix]

No... Light prevents that. We'd see the ship 20 minutes later.

But how does this relate?

If the FTL ship appears on the orbit of Mars at the instant we have issued the order, it would have appeared 'in the past of Mars' so we should see it appearing there instantly (or taking the 20 minute lag into account) 20 minutes before we told it to fly to Mars. In other words, we should see the ship there 20 minutes before we issue that order. What if we seeing it already there then decided not to tell it to depart?

Edited October 30, 2015 by cicatrix

[Bill Phil]

If the FTL ship appears on the orbit of Mars at the instant we have issued the order, it would have appeared 'in the past of Mars' so we should see it appearing there instantly (or taking the 20 minute lag into account) 20 minutes before we told it to fly to Mars. In other words, we should see the ship there 20 minutes before we issue that order. What if we seeing it already there then decided not to tell it to depart?

I don't see how that works... If it arrives instantly it's still the present, but... It's not even instant. It took a second to get there. And we would see it more than 20 minutes later, by 1 second. It's in the present of all the universe plus one second.

This is not my forte...

And even if it's in the the past it would take another 20 minutes to send the order, bringing it back to the present.

Edited October 30, 2015 by Bill Phil

[andrewas]

If the FTL ship appears on the orbit of Mars at the instant we have issued the order, it would have appeared 'in the past of Mars' so we should see it appearing there instantly (or taking the 20 minute lag into account) 20 minutes before we told it to fly to Mars. In other words, we should see the ship there 20 minutes before we issue that order. What if we seeing it already there then decided not to tell it to depart?

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.