Taking Warp Drive Seriously:

[beecee]

I was prompted to start this subject by an article I came across, as follows.....

https://www.sciencealert.com/scient...-drives-seriously-especially-this-one-concept

Scientists Are Starting to Take Warp Drives Seriously, Especially This One Concept:

MATT WILLIAMS, UNIVERSE TODAY
1 MAR 2020
It's hard living in a relativistic Universe, where even the nearest stars are so far away and the speed of light is absolute. It is little wonder then why science fiction franchises routinely employ FTL (Faster-than-Light) as a plot device.

However, in recent years, the scientific community has become understandably excited and skeptical about claims that a particular concept – the Alcubierre Warp Drive – might actually be feasible.

This was the subject of a presentation made at this year's American Institute of Aeronautics and Astronautics Propulsion and Energy Forum, which took place from August 19th to 22nd in Indianapolis.

This presentation was conducted by Joseph Agnew – an undergraduate engineer and research assistant from the University of Alabama in Huntsville's Propulsion Research Center (PRC).

As part of a session titled "The Future of Nuclear and Breakthrough Propulsion", Agnew shared the results of a study he conducted titled "An Examination of Warp Theory and Technology to Determine the State of the Art and Feasibility".

As Agnew explained to a packed house, the theory behind a warp propulsion system is relatively simple.

more at link......

https://arc.aiaa.org/doi/10.2514/6.2019-4288

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OK, Of course this being achieved anytime soon is totally fanciful, and highly unlikely to be achieved at anytime, in my very humble opinion. And as we do know, sometimes pop science articles, can stretch a point for the sake of sensationalism.

My question is in regards to the following extract and particularly the highlighted parts....                                                                             

"In layman's terms, the Alcubierre Drive achieves FTL travel by stretching the fabric of space-time in a wave, causing the space ahead of it to contract while the space behind it expands.

In theory, a spacecraft inside this wave would be able to ride this "warp bubble" and achieve velocities beyond the speed of light. This is what is known as the "Alcubierre Metric".

Interpreted in the context of General Relativity, the interior of this warp bubble would constitute the inertial reference frame for anything inside it. By the same token, such bubbles can appear in a previously flat region of spacetime and exceed the speed of light.

Since the ship is not moving through space-time (but moving space-time itself), conventional relativistic effects (like time dilation) would not apply."

Any further comments are welcome as will be the answers to my question.

[taeto]

52 minutes ago, beecee said:

 

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The "Nomenclature" section alone makes it look like a bad joke. 

Edited March 3, 2020 by taeto

[Mordred]

You might find this Warp field mechanics paper supplies a lot details.

The fundamental problem is generating negative mass density. So whether that's feasible or not is another question.

[Markus Hanke]

1 hour ago, Mordred said:

You might find this Warp field mechanics paper supplies a lot details.

The fundamental problem is generating negative mass density. So whether that's feasible or not is another question.

The other fundamental issue I see is that, even if one was able to generate an Alcubierre bubble somehow, it would be entirely useless for all practical purposes. For starters, there is no practical way to control how this construct propagates - you couldn’t change its direction of propagation after it has been created, or even slow it down and bring it to a halt relative to some external reference point, by any means I can think of (excepting perhaps non-linear interactions with strong background curvature, which isn’t practical). The other awkward problem is that the “walls” of the bubble would constitute a region of extreme curvature, so anything entering or exiting the warp bubble would be ripped to shreds by tidal gravity. Lastly, once created, I don’t see any way of collapsing such an Alcubierre bubble again; effectively, any ship in the interior would end up being trapped forever. One must also wonder what the vacuum in the interior of the bubble, and especially around the walls, would look like from a QFT perspective - Unruh radiation?

All in all, it is an interesting concept from a purely academic point of view, but wholly impractical as a means for FTL travel.

[beecee]

OK thanks for those answers fellas. Would it be valid to say that research on warp drive potential continues because  warp drive hasn't been disproven  as yet? Yuk! I hate using that word proven [or disproven]. So, there may possibly be hope for a future with FTL? The problems listed may be in time overcome?

Found some more potential problems at Wiki....

https://en.wikipedia.org/wiki/Alcubierre_drive

extract:

Although the metric proposed by Alcubierre is consistent with the Einstein field equations, construction of such a drive is not necessarily possible. The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter. So if exotic matter with the correct properties cannot exist, then the drive could not be constructed. 

or is this a possibility?

"At the close of his original article,[4] however, Alcubierre argued (following an argument developed by physicists analyzing traversable wormholes[5][6]) that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive."

another probably difficulty.....

"Brendan McMonigal, Geraint F. Lewis, and Philip O'Byrne have argued that when an Alcubierre-driven ship decelerates from superluminal speed, the particles that its bubble has gathered in transit would be released in energetic outbursts akin to the infinitely blueshifted radiation hypothesized to occur at the inner event horizon of a Kerr black hole; in the case of forward-facing particles, energetic enough to destroy anything at the destination directly in front of the ship."

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The Geraint Lewis was a much respected participant on  another forum I was on that is now defunct. He is also Professor of Astrophysics at Sydney University.

[Markus Hanke]

7 hours ago, beecee said:

Would it be valid to say that research on warp drive potential continues

I think it is fair to say that FTL travel is a concept that will always be of interest to us as a species. Nonetheless, there are good reasons to conjecture (based on currently known physics) that FTL travel is not a valid concept, due to various fundamental issues with it.

7 hours ago, beecee said:

the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive.

Firstly, I am very sceptical of this, as the Casimir vacuum does not represent a “true” negative energy density in the physical sense; it is not exotic matter. Secondly, even if it does fulfill the requirements of the Alcubierre solution, it still wouldn’t be of practical use, since the Casimir effect cannot easily be scaled up to the size of an entire spaceship.

 

[MigL]

Wormholes are also consistent.
And they also require 'exotic' matter to keep from collapsing.
Even better, they eliminate the need for a spaceship; you just step through the 'hole', and you are there.

One method is not more realistic/feasible than the other  .

[Markus Hanke]

3 hours ago, MigL said:

Wormholes are also consistent.
And they also require 'exotic' matter to keep from collapsing.
Even better, they eliminate the need for a spaceship; you just step through the 'hole', and you are there.

One method is not more realistic/feasible than the other  .

That’s true, though technically speaking - and I know I’m nitpicking here - they are not a means of FTL travel, since everything happens at subluminal speeds.
On a purely practical level, my main issue with wormholes would be not so much their stability, but the fact that - at the time of their creation - there is no way whatsoever to control where/when the other end of the passage will form, not even in principle. This makes them rather useless for practical purposes.

Still, if for the sake of argument it could be made to work somehow, the possibilities would be fascinating - The sci-fi author Dan Simmons has explored this concept in his Hyperion Cantos, with his “farcaster” device. A very interesting read, so far as sci-fi goes.

I would like to briefly mention another - lesser known - topological construct, which could in principle be created in the real world, given enough energy: the Krasnikov Tube. It’s a permanent, stable distortion of spacetime that can be left behind in the wake of a correctly configured spacecraft moving at close to the speed of light. Any subsequent craft travelling that same route will find its trajectory shortened, much like in a wormhole, but without the need for exotic energy. This is theoretically feasible, and avoids all the issues of warp travel and wormholes, but does require a lot of energy to put into practice, at least for the first journey when the tube is created. 

[MigL]

Did a little reading ( as per your link ) on the Krasnikov tube.
Essentially a one-way wormhole, such that you need two to cause a closed timelike loop, and a causality violating ''time machine'.

Interesting.

[Markus Hanke]

12 hours ago, MigL said:

Essentially a one-way wormhole

It’s similar to a wormhole in the effect it has - i.e. providing a “shortcut” between widely separated regions -, but the geometry of spacetime is quite different. No exotic matter or any other special constructs are needed to create a stable, traversable Krasnikov tube; just lots of energy in the right configuration.

[md65536]

Does GR tell you how much time is measured outside the warp "bubble" or at the other end of a wormhole? Or "when" it is on the other side? With the Alcubierre drive, is there a horizon between inside and outside, so you can't interact with the outside?

Suppose there was a way to create the warp in one place, and something else to cancel it out in another place, so you could travel there. Then suppose there was a way to do that in the opposite direction, so you can come back. Does GR tell you when you'd come back? Eg. if you could spend a day traveling at 10c, then immediately spend a day returning at 10c, is the idea that you could return to Earth with 2 Earth days having passed? Or is it some other time or unknown or undefined?

 

When it comes to the wormholes, "when" is it on the other side of the wormhole? Is that arbitrary? Can you have a wormhole or a series of them connecting you to beyond our visible universe? I can't make sense of the idea of "now" on the other end of a set of wormholes, yet no universal now.

For example, if you had 3 extremely distant events A, B, and C, each connected to each other with a wormhole, either the relative timing of the events would be arbitrary, or it would establish a common 'now' between the 3 locations???

[Markus Hanke]

12 hours ago, md65536 said:

Does GR tell you how much time is measured outside the warp "bubble" or at the other end of a wormhole? Or "when" it is on the other side?

Yes, you could run the maths on it, though potentially this isn’t a straightforward calculation (especially in the case of the Alcubierre metric).

12 hours ago, md65536 said:

With the Alcubierre drive, is there a horizon between inside and outside, so you can't interact with the outside?

My understanding (though I’m no expert on this particular solution) is that there is no horizon, but a narrow region of extreme tidal gravity. The two regions remain causally connected though.

12 hours ago, md65536 said:

Suppose there was a way to create the warp in one place, and something else to cancel it out in another place, so you could travel there. Then suppose there was a way to do that in the opposite direction, so you can come back. Does GR tell you when you'd come back? Eg. if you could spend a day traveling at 10c, then immediately spend a day returning at 10c, is the idea that you could return to Earth with 2 Earth days having passed? Or is it some other time or unknown or undefined?

Again, I don’t know the answers to this straight off the bat, but of course one could sit down and do the actual maths (definitely not a trivial calculation!). The relationship between clocks here would be well defined, and can be calculated. My intuition is that causality would remain preserved in all cases that are actually physically realisable (and there is a big question mark in that regard, so far as the Alcubierre solution is concerned).

12 hours ago, md65536 said:

When it comes to the wormholes, "when" is it on the other side of the wormhole? Is that arbitrary?

Wormholes connect potentially distant regions in spacetime, so in principle they span intervals of both space and time.

12 hours ago, md65536 said:

Can you have a wormhole or a series of them connecting you to beyond our visible universe?

In principle, yes.

12 hours ago, md65536 said:

I can't make sense of the idea of "now" on the other end of a set of wormholes, yet no universal now.

“Now” is always a local notion - it would be very difficult, perhaps meaningless, to try and define a notion of simultaneity in a spacetime with a topology that is multiply connected.

12 hours ago, md65536 said:

For example, if you had 3 extremely distant events A, B, and C, each connected to each other with a wormhole, either the relative timing of the events would be arbitrary, or it would establish a common 'now' between the 3 locations???

This is a very complex question, not just because this spacetime is multiply connected, but also because the existence of a wormhole does not necessarily imply causal connections (i.e. information may not be able to propagate through that wormhole, depending on its exact geometry).
Interesting question though!

[md65536]

8 hours ago, Markus Hanke said:

“Now” is always a local notion - it would be very difficult, perhaps meaningless, to try and define a notion of simultaneity in a spacetime with a topology that is multiply connected.

Yes, extra connections don't seem to add any meaning. The Krasnikov tube wiki you linked has an example with 2 one-way tubes that can bring you to Earth 6000 years before you departed. Not that that's assumed to be possible, but a connection between Earth 6000 years apart wouldn't make it a common 'now' away from that connection.

[MigL]

I can't do the math for wormholes either, but I'm sure Kip Thorne could have a good go at it.
The following is his example...

Bart and Lisa Simpson have two wormhole generators and they establish a 'portal' between the two, so that Bart can 'see' Lisa when he looks through his wormhole, and Lisa can 'see' Bart when she looks through hers.
Bart takes his wormhole generator in a spaceship, and travels to a distant star at relativistic speeds, all the while looking at Lisa through his wormhole.
Once he gets there, he turns around and comes back to Earth, all the while looking at Lisa through his wormhole.

When he gets back to earth he finds that 100 years have elapsed, as opposed to 2 years for himself, and while Lisa is long dead, he can still 'see' her through the wormhole. So he steps through the wormhole and is transported back in time 98 years via a closed timelike loop.

This would imply that K Thorne thinks there is a 'locality' between the two ends of a wormhole.

[Mordred]

Lol I tried doing the mathematics for a wormhole using partial derivatives years ago. After three months of losing track of where I was at I threw my hands up. 

 They are not trivial by any imagination.

 

[GaryV]

I am not well versed on the Alcubierre or Krasnikov work.  I just have a related question.  Why couldn't someone use a standing wave to create the warp?  If you used many different synchronized wave emitters to create a point of constant high energy in front of the ship, as per Relativity, the amount of energy at that point would have the equivalent effect as the corresponding amount of matter.  That means that you would warp space as much as that amount of matter would.  

If such a thing is possible, I could go further to postulate that someone could create a device that would turn such an effect on and off in a way to create gravitational waves.  The same as above could be used to create a standing gravitational wave.  Since all waves have both a crest and trough, it should be possible to create the negative point with no exotic matter.

While I realize that it would not generate faster than light travel any time soon, a proof of concept device (for the first paragraph) should be possible with current technology.  The enormous power demands could be mitigated somewhat by utilizing super conductors.  If using a craft in space is assumed, such a craft could have the electrical system shielded from electromagnetic radiation, but open to the vacuum of space.  In this way you could keep the temperature of the material low enough to have super conductivity with current level metals.

[MigL]

Not sure I understand you exactly, but...

Pick up a rock.
It takes the equivalent energy of the entire Earth's mass, to accelerate that rock at 9.8 m/s/s, at a distance of approx. 6300 km ( Earth radius ).
Yet you are resisting all that energy's gravity with your bare hand.
Similarly, you could impart that same acceleration to that rock, with a small amount of gunpowder.

Does that give you any idea of the magnitude of the energy needed to accelerate, even small masses, gravitationally ?

[Markus Hanke]

13 hours ago, GaryV said:

Why couldn't someone use a standing wave to create the warp?

You can’t “ride” a gravitational wave, pp-wave, or any other gravitational disturbance for that matter. The wave would pass right through the ship, and, if its amplitude is large enough, would tear it apart or at least cause structural damage of some kind. 

The other thing of course is that wave-like disturbances propagate at or below the speed of light, so you couldn’t achieve FTL travel this way.

[md65536]

21 hours ago, GaryV said:

If you used many different synchronized wave emitters to create a point of constant high energy in front of the ship, as per Relativity, the amount of energy at that point would have the equivalent effect as the corresponding amount of matter.  That means that you would warp space as much as that amount of matter would.

As MigL suggested, your idea is to warp space with a system of energy that has much more mass than the ship? And it pulls you along as you fall toward it? But to travel anywhere, wouldn't you have to accelerate it (using much more energy than accelerating the ship, since it's much more massive)?

Are you imagining something with a lot of gravitational mass but little inertial mass? I don't think anything known or predicted is like that.

Edited May 6, 2020 by md65536

[GaryV]

It is disturbing how much people read into comments and questions.  For example, I never claimed this would allow FTL travel.  My main question is purely related to the ability to warp space without the need for exotic matter.  The reason most people don't want to seriously take on this topic is the need for exotic matter.  If that was not needed, Warp Theory would become a serious discipline.  If warping space (I.E. causing a gravitational field) with only energy were possible, smarter people could use it for many things.  For example, such a point could be placed in the center of a ring shaped, or spherical, space station and create a small gravity on said station.  While never likely 1G, any field would be better than no gravity for the health of the inhabitants.

[Strange]

11 minutes ago, GaryV said:

My main question is purely related to the ability to warp space without the need for exotic matter.  

Normal matter warps space.

12 minutes ago, GaryV said:

 If warping space (I.E. causing a gravitational field) with only energy were possible

It is. In fact, mass doesn't appear in the equations of GR at all, only energy (and some other quantities like momentum flow and stress).

There seem to be a couple of problems with what you propose:

1) there is no such thing as "energy" that you can put at some point in space. Mass has energy, photons have energy, other particles have energy.

2) Mass is a far more effective way of concentrating energy than photons, for example. If you work out how much energy is required to produce the same effect as 1gram of matter, you will see it is enormous. And, if you use photons (because you can get an unlimited number of photons in a given space) then you have the problem that they travel at the speed of light so you need to constantly refresh them. Unless you can enclose them in a (near) perfect reflecting chamber or something.

3) Apart from the above, you would need to get the energy from your ship to the point in front of the ship. That will impart backwards momentum to your ship which, at best, could be cancelled by the gravitational attraction of that energy (but actually wouldn't be).

You effectively have a closed system (because the focussed point of energy is created by your craft) and therefore it is impossible to have net change of momentum. Your suggestion is therefore equivalent to firing rocks at the front wall of the ship and hoping that would make the ship move forwards.

[MigL]

1 hour ago, Strange said:

Your suggestion is therefore equivalent to firing rocks at the front wall of the ship and hoping that would make the ship move forwards

Didn't we have a thread, a few years ago, about making a sailboat move by mounting a fan on that sailboat, aimed at the sail ?

[Strange]

7 minutes ago, MigL said:

Didn't we have a thread, a few years ago, about making a sailboat move by mounting a fan on that sailboat, aimed at the sail ?

And that works, bizarrely enough. Related, I think, to the fact that a sailboat can go faster than the wind.

[StringJunky]

3 hours ago, Strange said:

And that works, bizarrely enough. Related, I think, to the fact that a sailboat can go faster than the wind.

I thought momentum from the fan pushing the boat backwards would be cancelled by the momentum imparted on the sail from the airflow going the other  way. leading to no net motion.

[Mordred]

Then you add the positive pressure region on the sail. The momentum is due to the pressure differential on one side of the sail compared to the other.

 

2 hours ago, StringJunky said:

I thought momentum from the fan pushing the boat backwards would be cancelled by the momentum imparted on the sail from the airflow going the other  way. leading to no net motion.

Your correct though on the above.