Alcubierre drive research

[sojourner]

According to our current understanding of how the physics involved work, If an Alcubierre drive can be made to work, it will be limited to never exceeding the speed of light.

Wait, that's news to me. When did this change?

[K^2]

It did not. That statement is completely wrong. FTL and sublight Alcubierre drive requirements are identical.

[shynung]

What would a sublight Alcubierre drive good for? Interplanetary taxis?

[PB666]

What would a sublight Alcubierre drive good for? Interplanetary taxis?

Hmmm,not a bad idea, cut the time off that mars mission. Not certain but you preserve your starting momentum, so if you zipped to mars skirting the sun, when you came out of warp you and mars would be going opposite directions around the sun.

What would a unicorn or tooth fairy be good for?

[K^2]

What would a sublight Alcubierre drive good for? Interplanetary taxis?

Absolutely nothing. Like I said above, Alcubierre has same requirements for FTL ans sublight. If you can build Alcubierre drive, you should go FTL.

Alcubierre is a special case of warp drive, however. There are warp metrics that are inherently sublight. As such, they are not subject to CTC conjecture constraints. They can also have thicker bubbles, making them far more feasible.

In short, we might one day build a sublight warp drive for solar system exploration. Probably sooner than skeptics think, but still not soon. That drive will not be an Alcubierre drive. For starters, it will have Lorentz boosted interior metric.

As for uses, the obvious one is cycler ships. You still have to produce exhaust to change energy and angular momentum of your ship. Warp is not magic. That means you need to be in transfer orbit with correct semi-major axis, eccentricity, inclination, and argument of the ascending node. This leaves argument of periapsis and mean anomaly as free parameters. These the warp drive will have access to. Which means you can throw a ship into cycler orbit, ignoring phase sync, and warp yourself between the two planets in a tiny fraction of the time. Best part, if you can recover part of energy needed to create the bubble when you drop out, you can make the trips pretty cheap. And an Earth-Mars cycler can be making trips in days or weeks, depending on relative phases, with very reasonable energy investments.

[problemecium]

It could be convenient for going places without pancaking yourself against the back wall of your ship due to excessive G-force.

You could of course simply accelerate gradually, but that increases your total travel time. What if I want to travel to Saturn at close to the speed of light, but I don't want to spend weeks getting up to speed? Or what if I'm carrying a bunch of crates of nitroglycerin that I don't want to shake too much?

[K^2]

What if I want to travel to Saturn at close to the speed of light, but I don't want to spend weeks getting up to speed?

Run the numbers for a trip to Saturn at constant acceleration of 1G for half way, then deceleration at 1G. I'm sure you'll be surprised.

[PB666]

Run the numbers for a trip to Saturn at constant acceleration of 1G for half way, then deceleration at 1G. I'm sure you'll be surprised.

g*year = c so he wont be getting any were close to that, but if we consider earth is traveling 30,000 m/s then a g basically gives 36000 m/s*h saturn is 1.5 trillion meters, and so any average path is going to be slightly more around 1.6t. The closest path is not the fastest because you have to kill earths velocity for 1/2 hour and the accelerate toward saturn.

The suns gravity at earths orbit is rougly trivial compared to g so that the half path at 0.8Tm = 0.5 * at^2

160G = t^2 t = 400,000 or roughly 4.5 days to go half way, stop and 4.5 days to to stop for a total of 9 days. The differential between earths velocity and saturns velocity is roughly trivial since it takes half an hour of this period to correct the motion over the direct vector. When saturn is in front of earth in its orbit it saves a couple of hours and if th ships loses velocity in front of saturns orbit it can use saturns gravity to insert itself and retro at peri.

BTW, why limit acceleration to 1 g, humans can survive 3 g, you can accomplish the same trip in a little over 5 days. Put the humans in rollers and keep them turning to prevent RBC settling and go for 3. How many stages of a rocket would be required to obtain 3 g for 5 days. :^).

Edited October 26, 2015 by PB666

[K^2]

Surviving 3g is trivial. Enduring 3g for 5 days is not. If you give people an option between a 5 day trip at 3g or 9 day at 1g, everyone will chose the later. It will be cheaper too.

But yeah, the point was that the limiting factor here is propulsion. If you can build an engine that can pump out 1g worth of thrust for a passenger ship for over a week, Solar System isn't such a big place. Incidentally, this is right around the theoretical limits for nuclear-powered propulsion. So even without warp, we can expand throughout the Solar System. Beyond Sol, though... If we can't do FTL warp, we're not in a good place as far as building interstellar civilization goes.

[magnemoe]

Hmmm,not a bad idea, cut the time off that mars mission. Not certain but you preserve your starting momentum, so if you zipped to mars skirting the sun, when you came out of warp you and mars would be going opposite directions around the sun.

What would a unicorn or tooth fairy be good for?

Well, first it would make total joke of any travel inside the solar system, that is even with 0.1 c. You could travel to Pluto and back in a week, normal stuff including antimatter engines would not be able to touch this performance.

Say you manage 0.5 c or higher, well that is less than 8 years to Alpha Centauri. Shorter than New Horizon used to Pluto.

And it would not be an flyby, you should be able to use the drive to jump around locally too.

Interstellar the main benefit is that you don't have to face the rocket equation you just have to power the drive.

[PB666]

Well, first it would make total joke of any travel inside the solar system, that is even with 0.1 c. You could travel to Pluto and back in a week, normal stuff including antimatter engines would not be able to touch this performance.

Say you manage 0.5 c or higher, well that is less than 8 years to Alpha Centauri. Shorter than New Horizon used to Pluto.

And it would not be an flyby, you should be able to use the drive to jump around locally too.

Interstellar the main benefit is that you don't have to face the rocket equation you just have to power the drive.

g * year = c (alright 0.95c)

You need a year to accelerate, and a year to decelerate. d = VoT * 0.5 a * t^2 a = c/year therefore d = 0.5 ly and so you need 4.9 ly

Shielding is a major problem

0.5 g * year = 1/2 c

you need 2 years to accelerate and 2 years to decelerate. d = 1 therefore for 4 years you are accelerating gaining 2ly which means you have 1.9 years to travel under no acceleration = 5.9 ly (longer considering we aren't going C, but there is also time dilation for the travelers

Shielding is a major problem.

0.25 g * year = 1/4c you spend all your time accelerating and then decelerating and it takes 8 years.

Shielding is a major problem. Collisions of hydrogen with the ship and non-ionized molecules is creating gamma radiation and exotic matter.

Much of the ships mass is devoted to protecting contents from gamma radiation (lead and other metals) and cosmic radiation.

At 0.9c 300,000,000 m/s a grain of sand

A grain of sand 1.985E-9 kg x 9E16 roughly = .17 GJ of energy. it is equal in energy to a kilogram traveling at 19000 m/s

0.1 g for 2 ly then -0.1g for 2 ly. 2ly = 0.5 * 0.1 * T^2 40 = T^2,

6.32 year accel and 6.32 years decel = 12.6 years (peak speed 0.6c)

For a tiny fraction of the power used you take 2.57 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

grain of sand impacts are:(unsurvivable)

0.01 g for 2 ly then -0.01g for 2 ly. 2ly = 0.5 * 0.01 * T^2 400 = T^2,

20 year accel and 20 years decel = 40 years (peak speed 0.2c)

For a much smaller tiny fraction of the power used you take 9 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

the grain of sand now has the energy of a kilogram traveling at 760 m/s (unsurvivable)

0.001 g for 2 ly then -0.001g for 2 ly. 2ly = 0.5 * 0.01 * T^2 4000 = T^2,

63 year accel and 63 years decel = 126 years (peak speed 0.063c)

For a much smaller tiny fraction of the power used you take 25 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

the grain of sand now has the energy of a kilogram traveling at 76 m/s (survivable)

However I would think that 0.01c is probably a better margin of safety.

[magnemoe]

g * year = c (alright 0.95c)

You need a year to accelerate, and a year to decelerate. d = VoT * 0.5 a * t^2 a = c/year therefore d = 0.5 ly and so you need 4.9 ly

Shielding is a major problem

0.5 g * year = 1/2 c

you need 2 years to accelerate and 2 years to decelerate. d = 1 therefore for 4 years you are accelerating gaining 2ly which means you have 1.9 years to travel under no acceleration = 5.9 ly (longer considering we aren't going C, but there is also time dilation for the travelers

Shielding is a major problem.

0.25 g * year = 1/4c you spend all your time accelerating and then decelerating and it takes 8 years.

Shielding is a major problem. Collisions of hydrogen with the ship and non-ionized molecules is creating gamma radiation and exotic matter.

Much of the ships mass is devoted to protecting contents from gamma radiation (lead and other metals) and cosmic radiation.

At 0.9c 300,000,000 m/s a grain of sand

A grain of sand 1.985E-9 kg x 9E16 roughly = .17 GJ of energy. it is equal in energy to a kilogram traveling at 19000 m/s

0.1 g for 2 ly then -0.1g for 2 ly. 2ly = 0.5 * 0.1 * T^2 40 = T^2,

6.32 year accel and 6.32 years decel = 12.6 years (peak speed 0.6c)

For a tiny fraction of the power used you take 2.57 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

grain of sand impacts are:(unsurvivable)

0.01 g for 2 ly then -0.01g for 2 ly. 2ly = 0.5 * 0.01 * T^2 400 = T^2,

20 year accel and 20 years decel = 40 years (peak speed 0.2c)

For a much smaller tiny fraction of the power used you take 9 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

the grain of sand now has the energy of a kilogram traveling at 760 m/s (unsurvivable)

0.001 g for 2 ly then -0.001g for 2 ly. 2ly = 0.5 * 0.01 * T^2 4000 = T^2,

63 year accel and 63 years decel = 126 years (peak speed 0.063c)

For a much smaller tiny fraction of the power used you take 25 times as long

Shielding is much less of a problem, but still a major problem. Collisions of particles with the ship is creating lower energy gamma and xrays.

the grain of sand now has the energy of a kilogram traveling at 76 m/s (survivable)

However I would think that 0.01c is probably a better margin of safety.

I thought we was talking about an warp drive who was slower than light.

it would escape all the issues with acceleration and probably lots of the shielding issues, if not an faster than light warp drive will face this problem too.

My point was that an slower than light warp drive would be very practical and open the road to interstellar probes even manned missions.

And anything slower than 0.05 c is pointless, it would put so large demands on self repair and AI / life support for manned that it would be simpler to make an faster ship.

Grains of sand will also kill most probes and satellites and its not a problem even in around earth with all the space junk.

And you will not accelerate until turnover, the last acceleration part does not add much to your travel time, exception is if trust is totally free like an solar sail.

[K^2]

g * year = c (alright 0.95c)

Magnemoe is talking about warp. Warp doesn't have the same limits on acceleration. It's not quite instant, but the limiting factor is how fast you can put necessary energy into bubble. In any case, ship's crew doesn't experience any acceleration during warp. So 1g has no significance at all.

Oh, and warp scheme can incorporate partial shielding. It will certainly atomize anything it encounters, making it much easier to deflect resulting radiation with mag fields or w/e.

[Yourself]

If we can't do FTL warp, we're not in a good place as far as building interstellar civilization goes.

That is, unless we replace ourselves with robots.

[K^2]

That is, unless we replace ourselves with robots.

If we "replace ourselves" with machines that can spend millenia in interstellar void and then reintegrate into society that's been busy all that time, it will not be us. Anything that makes us human will be foreign to these creatures. They will travel the stars. Not us. For us, it's FTL or bust.

[PB666]

Magnemoe is talking about warp. Warp doesn't have the same limits on acceleration. It's not quite instant, but the limiting factor is how fast you can put necessary energy into bubble. In any case, ship's crew doesn't experience any acceleration during warp. So 1g has no significance at all.

Oh, and warp scheme can incorporate partial shielding. It will certainly atomize anything it encounters, making it much easier to deflect resulting radiation with mag fields or w/e.

If you can actually move FTL, clearly FTL yes that would be shielded, because essentially you have moved matter into another dimension of space. But if its LS or sub-FTL you have essentially the same problem.

The inertial shift cause by the drive might disperse the matter, but if you are heading at C then the level of dispersion is minimal, a grain of sand would impact an area slightly larger than a grain of sand. The cohesion of the molecules in the grain of sand is immaterial at C, its the energy that is contained within its mass. Therefore deflecting the matter a tenth of a millimeter does not really fix the collision problem.

Primarily interstellar travel is an effort of nearly global cooperation, it would require building a massively survivable ship capable of sustaining flight for generations. This is all doable with nuclear technology, but the social aspects are the big problem. If a robotized ship sustains a collision and looses vacuum, the robots can survive in the vacuum of space, humans cannot, and robots sustain far less damage from ionizing radiation relative to humans, and if you concentrated the processors you could use local shielding. Humans you have to protect the whole body where-ever they might move. Basically it will be a hollowed out asteroid made into a ship, Might even be the shape of a Vogon construction ship, a long cylinder, but that spins to give artificial gravity. Robots could seed

The problem with the warp is that provision of the premises to the technology, which are often not considered, are more demanding (fantastic) than the technology itself.

How do you warp space in a fantastic manner on one side of a ship, and in the exact opposite manner on the other side. We are talking about warping of the level of approaching the event horizon of a black hole on one side, and a unfathomable opposing warp in space/time on the other. Warping space requires a huge energy density and negative-energy density, at least the first of which humans would not likely survive.

[K^2]

Nah, both FTL and hyper relativistic sublight warp ships will have the same kind of shielding. Warp bubble is going to atomize and likely ionize anything going in. From perspective of the ship, any obstacle just becomes a burst of particle radiation. If you can disperse it with a mag field, from there on you just need thick led lining to protect the crew. It's all quite manageable at high sublight or low FTL speeds. Unless you hit a sizable rock, of course, in which case, the radiation will just burn through whatever shielding you have.

The sublight we're talking about for in-System travel, circa 0.01c - 0.1c is a different matter. Yes, you can have stray microasteroid impacts, and grain of sand at .1c is not something you can just ignore. Fortunately, even here, warp can be of help. You can design the warp bubble geometry in such a way that by far most of the debris to miss the ship.

[Yourself]

If we "replace ourselves" with machines that can spend millenia in interstellar void and then reintegrate into society that's been busy all that time, it will not be us. Anything that makes us human will be foreign to these creatures. They will travel the stars. Not us. For us, it's FTL or bust.

I'm not sure I understand the importance of that distinction.

[K^2]

I'm not sure I understand the importance of that distinction.

Importance that it won't make a difference if it's machines we build or machines built by some other civilization on some other star. It's not a continuation of our civilization in any sense.

[PB666]

Nah, both FTL and hyper relativistic sublight warp ships will have the same kind of shielding. Warp bubble is going to atomize and likely ionize anything going in. From perspective of the ship, any obstacle just becomes a burst of particle radiation. If you can disperse it with a mag field, from there on you just need thick led lining to protect the crew. It's all quite manageable at high sublight or low FTL speeds. Unless you hit a sizable rock, of course, in which case, the radiation will just burn through whatever shielding you have.

The sublight we're talking about for in-System travel, circa 0.01c - 0.1c is a different matter. Yes, you can have stray microasteroid impacts, and grain of sand at .1c is not something you can just ignore. Fortunately, even here, warp can be of help. You can design the warp bubble geometry in such a way that by far most of the debris to miss the ship.

I don't know you can take that for granted. First, centauri proxima is not likely a best target, something on the order of 100 ly away is more likely to have habitable planets. over that distance ther is a pretty good chance that there is small objects in the path. Your warp drive places spacetime warping energy in front of the drive, that energy is going to draw particles at it, it will be a focal point of attraction. once the mass has reached that position it might be atomized but how much kinetic energy has it obtained relative to whats insidevthe bubble?

[Rakaydos]

I don't know you can take that for granted. First, centauri proxima is not likely a best target, something on the order of 100 ly away is more likely to have habitable planets. over that distance ther is a pretty good chance that there is small objects in the path. Your warp drive places spacetime warping energy in front of the drive, that energy is going to draw particles at it, it will be a focal point of attraction. once the mass has reached that position it might be atomized but how much kinetic energy has it obtained relative to whats insidevthe bubble?

Depends on the vessel's real velocity. If you gave the vessel just enough real velocity to escape intervening gravity wells, you could warp across them freely, and any debris would only have relative speed equal to yor real velocity. Your "Warp factor" has nothing to do with it.

[problemecium]

Okay so in this light, something's been bugging me for the last couple days:

According to reports on the Alcubierre Drive, its proposed ability to travel faster than light implies an ability to travel backward in time.

This didn't initially make sense to me, so I went back and refreshed myself on some things like Special Relativity. I get that for some observers, events might occur at the same time or in a particular sequence, while for others in a different state of motion, the same events might occur in a different sequence - time and the order of events is always relative to the observer(s). I also get that for all observers, objects moving near light speed relative to them will appear to experience time slower, regardless of whether you claim it's the object or the observer that is moving. And, as the cause of this, time slows down for anything approaching the speed of light.

Some have claimed that simply extrapolating the trend of time dilation implies that if you go past the speed of light, your perception of time will go into negative territory, but as I understand things, time dilation follows an asymptotic curve. Thus if you hurl yourself at light speed or faster, the only thing you can expect to happen to your time is for it to move infinitely slowly or stop.

But how is any of this even relevant to a device that doesn't move within space, but simply translates itself within the universe by surfing about on its own little wave of space?

So I ran a thought experiment. About 1400 light-years away is the star KIC 8462852, which has been in the news lately due to what could be an alien megastructure. So to find out, I accelerate to near light speed and jump over there. For me, I travel for a few hours, but when I get there, about 1400 years have passed and I find all the aliens are dead. I turn around, and using my super-duper-mega-ultra telescope, I can gaze upon Earth as it was 1400 years prior (i.e. the present). All well and good.

But in the meantime, humans have invented warp drive and a plucky young Kerballer from the year 3415 warps to my location over the course of a few minutes. Since he was in warp, he gets no time dilation, and because he was traveling at ludicrous speed, he can tell that, indeed, five minutes have passed. When we meet, he tells me all about the glorious exploits of future human civilization. When I look back at Earth, of course, none of that is evident, but I know that light is slow and that 1400 years have passed, so I believe him. He turns around and goes home to future Earth to hang out with his future friends at the future mall.

Meanwhile, I reverse-engineer his warp drive and try to troll the universe by time-traveling. I surmise that since it is now the year 3415, if I warp back home I can tell everyone all about the future before it even happens. So I build a warp drive and go home... to the year 3415 (plus five minutes). I run into my acquaintance from earlier and he laughs at my idiocy, embarrassing me in front of his friends.

Desperate to troll spacetime, I hop back in the time machine to try and tell all my alien ghost buddies about Earth's future. I warp myself over there, and it's still the year 3415, and all the aliens are still dead. I regale their ghosts on Earth's future, but it really doesn't matter to anyone because it's already the year 3415 anyway. By the time anything like a big evil death ray can make it from Earth to there, or vice versa, yet another 1400 years will have passed. Big whoop.

Any way I try to rearrange the characters, locations, or events, I can't seem to produce any time travel. What am I missing?

How, specifically, would an Alcubierre Drive act as a time machine?

Edited October 28, 2015 by parameciumkid

[Scotius]

I wonder about it too. Can you really complete a circular journey in a warp capable ship, and arrive at your starting point before you started? No matter how fast your warpship will be - flight somewhere will take a bit of real time. Even if you can fly to Alpha Centauri and back in four seconds flat - you will still return to Earth four seconds later. Where is the time travel supposed to come from?

[PB666]

You carry your space time with you in the drive, when you return to the sun it will be slightly shifted because of its rotation in the galaxy, but from your spacetime perspective you will have to stop and apply energy to match its motion and regain orbit which will take time. You did not travel in spacetime to get to a centauri, had you stopped you would have had to apply large amounts of energy to insert orbit and that period of thrust application you would have adjusted your space-time to that of the system.

I have to say, its one thing to imagine what it would be like to travel at SoL or close to it, imagining how this fictious entity behaves is silly. The energy requirements for the drive AFAIK or anyone knows do not exist, the energy requirements are fictitious. So predicting how things will behave, well, one behavior is that the energy system converts all the encapsulated energy into quantum pixie dust and uniformly transports it to everypoint in the universe. In that vien of thought it can go backwards, forward . . . .

Edited October 28, 2015 by PB666

[andrewas]

I wonder about it too. Can you really complete a circular journey in a warp capable ship, and arrive at your starting point before you started? No matter how fast your warpship will be - flight somewhere will take a bit of real time. Even if you can fly to Alpha Centauri and back in four seconds flat - you will still return to Earth four seconds later. Where is the time travel supposed to come from?

Look up 'relativity of simultaneity' - by changing reference frames, you can change the order of distant events. Accelerate away from Earth using a conventional drive, then warp back, and you can arrive before you left.

However, while its true that any method of FTL travel must also allow time travel in principle, it does not mean that time travel must be easy or even practical. Getting to the past with an Alcubierre drive requires the ability to accelerate to very high velocities without it.