Alcubierre Drive: Constant Velocity or Acceleration?

[codeman_nz]

Hi everyone,

I have been reading the fascinating paper by Miguel Alcubierre about the warp drive and one thing is confusing me.

He says that the total co-ordinate time is

T= $\left[d/v + \sqrt{(D-2d)/a}\right]$

Would that mean that the warp bubble is constantly accelerating so its velocity is continuously increasing? Why can't it be a constant velocity?

 

[Simon Bridge]

Hmmm... well you use the drive to accelerate (was my reading). That's just how the math works out. Have you followed the problems with the idea as well? It's fun to consider exotic physics so long as you bear the limitations in mind.

 

[codeman_nz]

Hmmm... well you use the drive to accelerate (was my reading). That's just how the math works out. Have you followed the problems with the idea as well? It's fun to consider exotic physics so long as you bear the limitations in mind.

Yes I know you need to accelerate to faster than the speed of light but from what I read it is constantly accelerating for half the trip and then decelerating for the rest of it.

What I am confused about is why does it need to constantly accelerate? What can't it get up to speed and then continue on at that speed until it reaches the destination?

 

[Simon Bridge]

Well ... the way you travel at a constant speed in space is to switch off the drive.
What happens when you switch off this drive is you lose the special conditions that allows FTL.

 

[codeman_nz]

Well ... the way you travel at a constant speed in space is to switch off the drive.
What happens when you switch off this drive is you lose the special conditions that allows FTL.

Can you explain? I see nothing in there that says that constant acceleration is required.

Also I don't get how he derives the volume expansion equations.

 

[Simon Bridge]

I see nothing in there that says that constant acceleration is required.

... apart from the bit you pointed out in post #1.

Also I don't get how he derives the volume expansion equations.

Neither do I ... :)

It's all GR and topology ... what kind of grounding do you have in these subjects?

 

[HallsofIvy]

While there is a lot of deep relativity and topology involved in the Alcubierre drive, the basic question you are asking, "why does it need to constantly accelerate" is basic Newtonian physics (or perhaps Gallilean). There is little to no resistance to motion in space- you simply don't need a "drive" of any kind to go at constant speed. You don't need to constantly accelerate but we are talking about using the drive and acceleration is the only thing you need the drive for.

 

[codeman_nz]

While there is a lot of deep relativity and topology involved in the Alcubierre drive, the basic question you are asking, "why does it need to constantly accelerate" is basic Newtonian physics (or perhaps Gallilean). There is little to no resistance to motion in space- you simply don't need a "drive" of any kind to go at constant speed. You don't need to constantly accelerate but we are talking about using the drive and acceleration is the only thing you need the drive for.

Yes but why? The paper only introduces constant acceleration when he calculates total co-ordinate time. Until then he makes no mention of it so aside from getting up to speed I just do not see the need for it.

 

[Dickfore]

Hi everyone,

I have been reading the fascinating paper by Miguel Alcubierre about the warp drive and one thing is confusing me.

He says that the total co-ordinate time is

T= $\left[d/v + \sqrt{(D-2d)/a}\right]$

Would that mean that the warp bubble is constantly accelerating so its velocity is continuously increasing? Why can't it be a constant velocity?

No, it's accelerating until the midpoint between A and B, and then starts decelerating with an opposite by direction, but equal by magnitude acceleration until it reaches a distance d to the star B

 

[mnielsen1]

From what I unnderstood about it, the "bubble" that is created needs to be on while traveling at speed, this is what makes it possible to not have the infinite amount of fuel not to mention special relativity would take hold. The drive doesn't cancel or break relativity, it just works around it. Without it on, it might just turn out to be a bad day for those inside. I see what you are saying, why can't it just accelerate and stay at "warp 1, 2, 3, 4, etc." but I just don't think it is really that cut and dry. It all depends on where you want to go and how fast you want to get there. Leave the earth, stop, engage the drive to a certain point in space, disengage and you decelerate and the use your conventional drive. Don't think we are going to go to the galactic center any time soon. Radiation might suck...

 

[mnielsen1]

From what I unnderstood about it, the "bubble" that is created needs to be on while traveling at speed, this is what makes it possible to not have the infinite amount of fuel not to mention special relativity would take hold. The drive doesn't cancel or break relativity, it just works around it. Without it on, it might just turn out to be a bad day for those inside. I see what you are saying, why can't it just accelerate and stay at "warp 1, 2, 3, 4, etc." but I just don't think it is really that cut and dry. It all depends on where you want to go and how fast you want to get there. Leave the earth, stop, engage the drive to a certain point in space, disengage and you decelerate and the use your conventional drive. Don't think we are going to go to the galactic center any time soon. Radiation might suck...

Not just the infinite amount of fuel, of course I mean all the other limitations that special relativity would place on say Discovery traveling at or attempting light speed.

 

[mnielsen1]

Anyone have an idea on the effects of Hawking radiation yet? Obviously we are not going to quit with this idea, but what are some possibilities for sheilding? Of course the ship can't be huge, the negative or exotic matter already has to be more than the universe?

 

[Simon Bridge]

Anyone have an idea on the effects of Hawking radiation yet?

What on?

Hawking radiation is black body radiation that is predicted to be emitted by black holes, due to quantum effects near the event horizon. It's effect would be just the same as any other black-body radiation - there's nothing special about the radiation itself. But what has this to do with the warp drive?

 

[mnielsen1]

What on?

Hawking radiation is black body radiation that is predicted to be emitted by black holes, due to quantum effects near the event horizon. It's effect would be just the same as any other black-body radiation - there's nothing special about the radiation itself. But what has this to do with the warp drive?

My bad, I hadn't read the paper yet. I saw the special on "how the universe works" I think it was and then stumbled upon this forum as I was looking up the warp drive. I understand the idea about it. My son Aspen (4) and I are very much into space and time. For instance, we were in the doctor's office and he quoted that another planet had crashed into Earth and the debris had created the moon. He explained it in much more detail and anunciated very clearly. no one, not my wife and he does not go to school yet, or his older brother, has every told him this stufff. I asked him how he knew this and he cocked an eye, looked up as if to the sky and said," I figured it out."
Anyway, I had heard some scuttle-butt and read around the net that Hawking radiation would eventually cook those inside the craft using this Alcubierre drive. I have understood since the idea was proposed that this was emitted from dark bodies i.e. black holes and that sort of thing. Before I read the paper I knew that it was going to need some sort of exotic matter and had read of worry of Hawking radiation. Basically...jumped the gun. I just assumed that the scuttle-butt I heard was somehow attaching Hawking radiation to exotic matter.
So, by what does Alcubierre mean by exotic matter...whatever works?

 

[Simon Bridge]

Oh I see, you mean Hawking radiation from the warp bubble?
See: https://www.physicsforums.com/showthread.php?t=305091

http://www.centauri-dreams.org/?p=10826
http://en.wikipedia.org/wiki/Talk:A...bout_energy_requirement_and_Hawking_radiation.

 

[mnielsen1]

First you start loving physics and fall in love with space, then join the military, have a family, raise some kids and like me get back into exploring the cosmos and realize that the physics world is a bunch of kids having a pissing contest. Ever notice that? lol

I am by no means Einstein, Kaku, or Tyson but whenever something seems impossible we always exceed it. if not, we would probably still be sleeping in caves.

 

[Nugatory]

realise that the physics world is a bunch of kids having a pissing contest. Ever notice that? lol

That's not a property of the physics world, it's a property of the internet
PF is better than a lot of it - at least there are some grown-ups around

I am by no means Einstein, Kaku, or Tyson but whenever something seems impossible we always exceed it. if not, we would probably still be sleeping in caves.

We use the word "impossible" in at least three different ways:
1) Impossible using any currently imaginable technology; In the 18th century flying to the moon was impossible, but by the 20th century it was different story.
2) Impossible unless our current theories are wrong. Perpetual motion machines would fall in this category.
3) Mathematically impossible, like finding integers I and J such that (I/J)²=2.

#1 has a pretty poor track record, #2 impossible has generally held up pretty well, and the #3 stuff you can take to the bank.

 

[mnielsen1]

That's not a property of the physics world, it's a property of the internet
PF is better than a lot of it - at least there are some grown-ups around



We use the word "impossible" in at least three different ways:
1) Impossible using any currently imaginable technology; In the 18th century flying to the moon was impossible, but by the 20th century it was different story.
2) Impossible unless our current theories are wrong. Perpetual motion machines would fall in this category.
3) Mathematically impossible, like finding integers I and J such that (I/J)²=2.

#1 has a pretty poor track record, #2 impossible has generally held up pretty well, and the #3 stuff you can take to the bank.

Amen

 

[mnielsen1]

Looking at PF i do see that, love the conversation

 

[George Jones]

See also the review

http://arxiv.org/abs/0710.4474.

 

[mnielsen1]

So, any idea what this "exotic matter" is or might be? Whatever works?

 

[PAllen]

So, any idea what this "exotic matter" is or might be? Whatever works?

There is no known form of it or place for it in current theories except in very limited quantities. For example. look up the Casimir effect for a limited case known from theory and experiment. So you would be looking to to somehow do this on a very large scale.

 

[djg1508]

Supposing the energy problem had been solved and your ship had all the energy it needed at its disposal. My question is; "What would you do with the energy?"
How would it be expended? How would it be radiated? - If this was just an engineering problem, what would be the solution?
Would it be channelled as electricity through metal coils to generate intense electo-magnetic fields? Or would it need to be used in some other way?

Sorry if its a basic question, but all the talk seems to be about generating the energy and not how you would use it!

 

[Simon Bridge]

Welcome to PF;
Once you have the energy - finding uses for it (guessing you mean: "when it isn't being used for the drive") usually takes care of itself. I imagine the military may want to release it all in one go at the destination for example.

A lot of the answer will depend on what form the energy takes and how much there is.
Without that information - there is no way to answer.

 

[PAllen]

Supposing the energy problem had been solved and your ship had all the energy it needed at its disposal. My question is; "What would you do with the energy?"
How would it be expended? How would it be radiated? - If this was just an engineering problem, what would be the solution?
Would it be channelled as electricity through metal coils to generate intense electo-magnetic fields? Or would it need to be used in some other way?

Sorry if its a basic question, but all the talk seems to be about generating the energy and not how you would use it!

I hope you realize that what is needed for all these devices is 'exotic matter' not normal mass/energy. This exists, so far, only in tiny quantities due to quantum effects. I don't think anyone knows how, if it existed in large quantities, how it could be maniputated or would behave.

 

[djg1508]

Welcome to PF;
Once you have the energy - finding uses for it (guessing you mean: "when it isn't being used for the drive") usually takes care of itself. I imagine the military may want to release it all in one go at the destination for example.

A lot of the answer will depend on what form the energy takes and how much there is.
Without that information - there is no way to answer.

Thankyou Simon.

No, I think you misunderstand me (probably because I didnt explain itvery well)
What I mean is; What do you do with the energy in order to make it create a warp-bubble?

If you had tonnes of anti-matter and tonnes of matter - all contained and ready to be borught into contact to generate energy the the energy generated would be in the form of photons - essentially heat & light - how would you then use this (electromagnetic) energy to create a warp bubble?

 

[PAllen]

Thankyou Simon.

No, I think you misunderstand me (probably because I didnt explain itvery well)
What I mean is; What do you do with the energy in order to make it create a warp-bubble?

If you had tonnes of anti-matter and tonnes of matter - all contained and ready to be borught into contact to generate energy the the energy generated would be in the form of photons - essentially heat & light - how would you then use this (electromagnetic) energy to create a warp bubble?

You continue to misunderstand that you can't make a warp bubble at all from normal matter or energy. You need 'exotic matter', which means matter with properties unlike any known matter or energy (except or tiny regions like the Casimir effect).

 

[djg1508]

You continue to misunderstand that you can't make a warp bubble at all from normal matter or energy. You need 'exotic matter', which means matter with properties unlike any known matter or energy (except or tiny regions like the Casimir effect).

So why is it that all of the posts and articles refer to "not having enough energy"

If it is 'exotic matter' you need and not energy then I re-phrase my question; "What would you do with the exotic matter to make a warp bubble"?

 

[PAllen]

So why is it that all of the posts and articles refer to "not having enough energy"

If it is 'exotic matter' you need and not energy then I re-phrase my question; "What would you do with the exotic matter to make a warp bubble"?

Exotic matter has an energy equivalent; the problem is worse, the more you need. The scholarly articles do mention it, but don't always highlight it because it is one of those obvious things the intended audience doesn't need to be reminded of. As for popular articles, they are simply quite commonly erroneous.

I am not aware of any theoretically well motivated approaches for building a warp bubble. The main effort in such theoretical investigations is simply to reduce the required amount of exotic matter.

Please see the abstract [also, the contents if you want more detail] of the review paper posted by George Jones above:

http://arxiv.org/abs/0710.4474

There are also analyses of the what would happen to the occupant of a warp bubble, and to the planet they got near. These show the occupants would be vaporized, and the destination would be vaporized. It is fun to speculate on these ideas, but it is wildly unlikely anything will come of it. In pop sci speculative mode, or sci fi stories, you ignore or minimize the problems.

 

[Simon Bridge]

No, I think you misunderstand me (probably because I didnt explain itvery well)
What I mean is; What do you do with the [negative] energy in order to make it create a warp-bubble?

I did, indeed, misunderstand you :) Let's see if I have you now: PAllen has already explained that the energy is a special kind - it's negative. This implies that the matter is "exotic" - i.e. it has a negative mass.
http://en.wikipedia.org/wiki/Exotic_matter#Negative_mass

To get a warp-envelope, you have to make your exotic matter into a particular shape. That's in the articles... some sort of torus is favorite.
Probably the simplest way to think of this is that you are trying to make a gravitational field that is a very unusual shape. In GR, gravity is described by curving space-time - and space-time curves in response to energy distributions... which is why there is all this talk about energy requirements: matter has mass which is very dense energy - so it is handy for making gravity.

Unfortunately the gravitationat field needed to go FTL without breaking any of the Rules is so weird that the matter/energy needed may not even exist (up until recently, imaginary matter would mean that the situation is certainly not possible!) but the investigation may tell us something about the relationship between GR and QM.

That help?

 

[djg1508]

I did, indeed, misunderstand you :) Let's see if I have you now: PAllen has already explained that the energy is a special kind - it's negative. This implies that the matter is "exotic" - i.e. it has a negative mass.
http://en.wikipedia.org/wiki/Exotic_matter#Negative_mass

To get a warp-envelope, you have to make your exotic matter into a particular shape. That's in the articles... some sort of torus is favorite.
Probably the simplest way to think of this is that you are trying to make a gravitational field that is a very unusual shape. In GR, gravity is described by curving space-time - and space-time curves in response to energy distributions... which is why there is all this talk about energy requirements: matter has mass which is very dense energy - so it is handy for making gravity.

Unfortunately the gravitationat field needed to go FTL without breaking any of the Rules is so weird that the matter/energy needed may not even exist (up until recently, imaginary matter would mean that the situation is certainly not possible!) but the investigation may tell us something about the relationship between GR and QM.

That help?

Thanks for sticking with me on this!

So to bring it back to a 'hypothetical' question of engineering;

Assuming you had access to the right kinds and amounts of exotic matter, you would simply put one kind of matter at the front of the ship and the opposite kind of matter at the rear of the ship to make it go?

Or, in terms of energy;

Emit the right kind of particles at the front and the right kind of particles at the rear?

 

[Simon Bridge]

you would simply put one kind of matter at the front of the ship and the opposite kind of matter at the rear of the ship to make it go?

No - you need it to be the right shape as well.
But that's the basic concept... very very basic.

You also need to revise your ideas about the relationship between particles and energy.

 

[djg1508]

No - you need it to be the right shape as well.
But that's the basic concept... very very basic.

You also need to revise your ideas about the relationship between particles and energy.

In what way "revise"?
I'm just interested in what set of physical\mechanical attributes and their distribution in relation the vessel would need to exist in order for the vessel to begin to create the space-time distribution around itself that would propel it in a given direction.

The emphasis has so far been on the impracticalities of;
Producing the energy required
Obtaining right kinds of 'exotic' matter
Handling the amounts of 'exotic maater'
The radiation problems
The problems of destroying your destination etc...

I'm aware of all those issues and I am not really interested in those issues, as I realize this ship isn't going to get built any time soon.

I was just wondering, theoretically, what you would have construct (schematicallY0 from an engineering point of view in order for the vessell to to go! Or rather space-time go around it.

So far the diagrams show the shape of the warp bubble and how space-time would need to be warped around with a dip at one side and a peak at the other etc... but I can't see any indication of what (mechanically) the ship would need to be doing to create this shape. (theoretically of course)

 

[1977ub]

If you could go FTL, you could go backwards in time. That in itself may be the most damning bit of evidence against the possibility.

 

[Simon Bridge]

In what way "revise"?

You know: "revise". Review, check, reassess - go back to the materials where you formed the ideas with a critical eye to your current understanding. You made a distinction between energy and mass for example. I will not go further here as I don't want to stray too far off topic. If you are still puzzled, after your review, feel free to make another thread.

I'm just interested in what set of physical\mechanical attributes and their distribution in relation the vessel would need to exist in order for the vessel to begin to create the space-time distribution around itself that would propel it in a given direction.

I get that and have answered to this point as clearly as possible.

The emphasis has so far been on the impracticalities of;
Producing the energy required
Obtaining right kinds of 'exotic' matter
Handling the amounts of 'exotic maater'
The radiation problems
The problems of destroying your destination etc...

etc etc... yeah I know.
To be fair - these are part of the set of physical\mechanical attributes and their distribution in relation the vessel that would need to exist in order for the vessel to begin to create the space-time distribution around itself that would propel it in a given direction.

Your question amounts to "if we have solved all that [on your list above] what else is needed?"

I was just wondering, theoretically, what you would have construct (schematically) from an engineering point of view in order for the vessell to to go! Or rather space-time go around it.

The pop articles usually include an artists impression of the kind of thing you'd get if the exotic matter took a form similar to normal matter.

Bottom line - the engineering of how to get the distributions needed, never mind how to turn it on and off, depend on the exact form that the negative energy takes. Therefore - nobody knows enough to answer the question you have posed in the way you seem to want - i.e. in an "engineering/schematic type of way.

So far the diagrams show the shape of the warp bubble and how space-time would need to be warped around with a dip at one side and a peak at the other etc... but I can't see any indication of what (mechanically) the ship would need to be doing to create this shape. (theoretically of course)

The space-warp diagrams can be misleading. The actual bubble is four dimensional and the articles can only print in 2D.

That is partly why I'm not going into great detail about the exact shape - it will be a 3D structure that looks like a pair of toruses with flattened crossections. No doubt the distribution will get tweaked further as more people work on the problem.

If you want the details - go to the original papers and learn the math.