Physical Warp Drives
iopscience.iop.org
iopscience.iop.org
[] https://eriklentzphd.blogspot.com/2021/04/review-and-respons...
Would you mind explaining?
I agree, the positive energy, spherically symmetric warp bubble isn't particularly interesting and certainly not a new effect.
What do you mean by that? Not to be a negative Nancy but the titles of both the paper and the video are a bit click-baity. None of this will be possible in our lifetime, the amounts of energy required are way too large, even if it could be engineered (which I have doubts about). Imo as a society we need to focus on getting off earth and establishing colonies in our own solar system first, before we can even start dreaming about going to other stars.
How about people who are interested in warp physics study warp physics, and those interested in establishing off-world colonies spend their time on that persuit?
How can you accumulate enough mass to make this possible without destabilizing the entire solar system? Even getting to the Oort Cloud to collect mass outside of our solar system is 3 light years away.
The universe heat death theory makes me think of comics where people extrapolate the data they have while lacking other critical data. https://xkcd.com/605/
I think it's fair to assume that life will have practically zero impact on the universe. Even if the universe was teeming with it, it would be of such little significance at scale that it might as well be a rounding error in reality.
Think of a virus. A handful of specially shaped molecules has permanently changed human society.
Avoiding heat death requires a renewable source of negentropy, to bypass the second law. Some sort of perpetual motion machine that you can pump energy out of. E.g. if you could extract work out of the expansion of the universe fast enough to build expansion-work-extractors faster than they decay, and expansion continues indefinitely, then you can avoid heat death.
We can't predict what life will be doing in 100 years. What will it look like in 100,000,000 years of exponential growth?
I just prefer not to presume when I link... .
Is there some data on this, or are you just making it up? It's long been a feature on one of the largest internet sites. I've never not had it work on the desktop, at least on Firefox, Brave, and Chromium on both Mac and Linux.
Everyone papers over these very real problems like they’ll just get solved. But if you can keep your ship at .05c while moving at .4c relative to your starting point that’ll be a huge deal.
The writer has a blog [0], which is worth reading if you're interested in next steps for the theory.
And if the video @gmartire just posted is explaining the paper correctly, this warp drive also doesn't accelerate anything. You have to figure out how to speed up on your own.
I don't really see why anyone would bother doing any of these things. To dilate time better? I'd rather invest the same amount of mass towards a larger engine/fuel so that I can go faster, dilating time further and getting me there sooner.
Because Star Trek called their thing a warp drive and that's become the general term for this kind of speculative technology.
The rocket equation is every bit as unsympathetic to your desires as the laws of thermodynamics are. Fuel requirements go infinite very quickly.
It seems like this idea is not correct. The authors write in section 5.2: "Warp drives, being inertially moving shells of normal or exotic material, do not have any natural way of changing their velocities. They are just like any other types of inertially moving objects. Similarly, just like for any other massive objects, achieving a certain velocity for a warp drive requires an externally applied force or, more practically, some form of propulsion.", from https://arxiv.org/pdf/2102.06824.pdf#page=17
But also they're interesting as a theoretical construct for better understanding the whole idea of intentionally warping space.
The original Alcubierre drive required the energy equivalent of the mass of Jupiter to create the warp field, with an impossible shortcut in the form of exotic particles with negative mass. The sun burns loses about 6x10^9 kg of mass a second which is the upper limit for the amount of energy a Type I civilization can extract in our solar system. Jupiter's mass is roughly 2x10^27 kg, so assuming there are 4x10^11 stars in our galaxy and our star represents the average, a Type II civilization in our galaxy wouldn't have enough energy to produce a warp field. The drive doesn't just need a little bit of working mass to expel, it needs enough energy/mass to bend space time around it!
Chemical rockets are limited in how much energy they can put into the reaction mass (by the energy density of their fuel) and therefore the maximum exhaust velocity they can achieve - which is the input into the rocket equation
But while nuclear rockets get away from the constraint of chemical energy density they still have to operate by putting energy into reaction mass and shooting it out the back to create a momentum change.
And the rocket equation still applies to the reaction mass and its exhaust velocity.
In fact in theory you can build a chemical rocket engine that burns a fuel for heat, then uses that heat to energize some other reaction mass that you eject - it’s just that that is inherently less efficient than using the spent fuel as reaction mass since otherwise you’re needlessly pumping momentum into spent fuel. Far better to throw the exhaust products away as reaction mass.
So if you could 'warp' space around you rather than travel through it, you could go at an arbitrary speed. Unfortunately warping space-time is typically accomplished by large amounts of mass. Hence a large amount of energy. If I remember correctly it also requires we bend space in a way we have never seen nor have any reason to believe is possible to accomplish beyond it being allowed by general relativity. It's quite a pickle.
Without such a device humanity will still populate the galaxy but it'll take hundreds of millions of years. Assuming we don't wipe ourselves out.
To me that would indicate that an Alcubierre warp drive still shouldn't be possible despite the negative energy requirement being lifted.
The warp drive in the article is subliminal, though, so it doesn’t have that problem.
Does FTL really inevitably mean that, or is that a consequence of saying “no preferred frame of reference” and therefore having e.g. “100c” meaning different things to observers in relative motion?
Do you know of any good layman-level explanations?
Like, if I FTL from point a to point b, 10 light years apart, in my super duper warp vessel. It takes me, for the sake of argument, 10 minutes to make that journey. Now say I set off a big comms laser at point a, sending a message to point b, before I left. I don't see that laser until 10 years later.
What am I missing? I know I'm missing something, but that seems straightforward to me. It's weird to butt up against that seemingly incomprehensible.
I'm eager to check out more of their channel!
However, this is only true from the perspective of someone on Earth. From the point of view of someone on the spaceship, the opposite is true. From their perspective, the spaceship is stationary, and Earth is travelling away from it at 0.8c. Therefore, for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth.
Suppose there was a way of instantaneously communicating between the two. On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective. The spaceship then sends a message back, and due to the same time dilation effect, the message arrives on Earth 3 hours and 36 minutes into the mission (60% of 6 hours). In other words, the reply from the spaceship will arrive 6 hours and 24 minutes before mission control sends the original message.
In the case of your explanation, what sticks out to me is the "Suppose their was a way of instantaneously communicating" part - it seems more intuitive to me that the warp bubble would not allow any communication across the threshold, effectively becoming a pocket universe.
If you have a ship with warp speed, then you have superluminal communication, because you can just carry a message on board. Even if you can't communicate inside the warp bubble, as long as you can exit the bubble at some point, then you can travel via warp, pop out, and transmit your message conventionally.
In my earlier example, if the ship and mission control had messenger drones capable of travelling many times faster than light, then the ship's response drone could arrive on Earth before mission control's messenger drone was launched.
And from their perspective - "for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth." - the message was sent at 3:36 of the Earth time. They immediately send a response message which immediately arrives at 3:36 of the Earth time from their perspective. No paradox so far.
from which POV? From the ship's POV - the message is sent at 3:36 Earth time , and the immediately sent reply is received at 3:36 Earth time.
From the perspective of someone on Earth, when it's 10:00 on Earth, it's 6:00 on the spaceship.
But from the perspective of someone on the spaceship, when it's 6:00 on the spaceship, it's 3:36 on Earth.
From the ship's reference frame, Earth is in the past; from Earth's reference frame, the ship is in the past. If you have superluminal communication, the Earth can send a message to the ship's past, which can then send a response to Earth's past.
no. The "when it's 6:00 on the spaceship, it's 3:36 on Earth" doesn't mean "From the ship's reference frame, Earth is in the past". It is just different speed of time. The 3:36 of "slow time" isn't "less" than ("past" of) the 6:00 of "fast time". Say you have 2 clocks, and took one on a spaceship ride. When brought back it will be showing say 6 why the twin clock which didn't left the Earth would be showing 10 - the ship clock wouldn't be in the "past" as both clocks are on the table in front of you in the "present".
It amounts to the same thing. From the perspective of Earth, time travels slower on the ship than on Earth. From the perspective of the ship, time travels faster on the ship than on Earth. Superluminal communication allows you to abuse the difference in reference frames to break causality.
Put it another way: when it's 6 on the ship, what time is it on Earth? It depends who you ask. Mission control would say it's 10. The people on the ship would say 3:36. Both are correct.
> Say you have 2 clocks, and took one on a spaceship ride. When brought back it will be showing say 6 why the twin clock which didn't left the Earth would be showing 10 - the ship clock wouldn't be in the "past" as both clocks are on the table in front of you in the "present".
In your example, the ship needs to change velocity in order to get back to Earth and compare the clocks. The ship is not always at rest in any reference frame; it either needs to stop and turn around, or accelerate to catch a retreating Earth.
It just so happens that in this scenario, whatever reference frame you choose, the ratio between the time on the ship and the time on Earth works out to be the same.
But this changes if the ship doesn't need to alter it's velocity. If the ship is travelling at 0.8c away from Earth, then it's equally valid to say that the Earth is travelling 0.8c away from the ship. It is therefore equally true to say:
* For every 10 hours on Earth, 6 hours pass on the ship
* For every 10 hours on the ship, 6 hours pass on Earth
In some reference frames Earth's time progresses faster than the ship; in other reference frames it progresses slower. If you're able to pass around information between reference frames instantaneously, then you can pass information backward through time.
The point you're completely missing here is that there's no objective yardstick to measure "past" and "present" against.
http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
The Alcubierre solution avoids this by being symmetrical, so that observers don’t have to agree on which direction is the destination and which is the origin
We see that matter and energy and all their fields and stuff move through time and space continuously. They don't spontaneously relocate themselves a million light-years away, and they don't generally relocate themselves a million years into the past. They trace a continuous path through time and space. (Oh, sure, for individual particles there's some quantum weirdness about exactly where it is along the way, but I think we're dealing with larger systems here?)
Now, physics have told us that space and time are glued together into a four-dimensional spacetime. So pick any two given points in spacetime — in whatever reference frame you like, doesn't matter if you're on Earth or in a space or next to a black hole — and take measurements of the distance between them, and the time between them. Ask yourself, "can some matter or energy travel along a path from the one point in spacetime to the other?" This is what it means to say, "are these points casually connected?" Because causes and effects have physical carriers, and they must move through time and space.
We found experimentally that there is generally a special speed limit — the "speed of light", though other things also are affected by this limit. We also found that this speed is constant in all reference frames. Some reference frames will disagree on how much space and time there is (your usual twin paradoxes and the like) but it all works out so that all observers agree on how much spacetime is between the points. (The mathematical objects for representing spacetime are quaternions, and the space and time vectors are just particular projections of the underlying quaternions, onto a set of measurements for a reference frame. But I don't know how to operate quaternions, and the specifics don't matter here.)
Now here's the thing:
If you are able to go faster than the speed of light (as measured in any reference frame of your choosing) it is geometrically equivalent to the power to go back in time. It doesn't really matter how you go faster, how you connected those two points — teleport there, open wormholes, warp space — there is some reference frame where you have travelled back in time. If you perform your maneuvers right, and take the right path, that reference frame could be Earth.
There's no other barrier: the speed of light itself was the barrier, and you just broke it. And if you have done so successfully, that means that the way cause-and-effect actually happens is ... something weird we only understand poorly.
Not necessarily, compare https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
That does sound like traveling faster than light, but I think there's a reasonable view that makes it not.
Suppose you traveled at some very fast speed around a black hole. It bends space time in such a way that you might, at some point, be able to collect photons that could be reconstructed into an image of you earlier in time.
In short, when space time is flexible and bending, there are paths in which you can see your previous self. An alcubierre drive is a means to bend space time.
You seem to imply that you think differently. Care to elaborate?
And imagining world with all those spooky quantum effects is already almost impossible, at least for me :)
...regular FTL travel...
The original Alcubierre metric does not, but that's because it only describes a single "warp bubble" that travels in one direction, never starting or stopping and never turning around.
As soon as you have more than one "warp bubble", or you let a single "warp bubble" turn around, then you will have closed timelike curves, which violate causality.
Relativity proves that FTL travel is identical to Time travel.
Time travel makes Causality impossible, since it can be used to create paradoxes. Note that to a physicist, it is not enough that time travel never happens to be used to make a paradox. The mere fact it is possible is enough to utterly destroy Causality.
1. So if you have Relativity and FTL, Causality is impossible
2. If you do not have Relativity, then FTL is not Time travel, so you can have Causality.
3. Or more mundanely you can have Relativity and Causality, but no FTL/Time travel
∴ Causality, Relativity, FTL travel: chose any two.
[0]: http://www.projectrho.com/public_html/rocket/fasterlight.php...
But the logic doesn't then mean "can't FTL" - it's still "can't have paradoxes". An FTL system, theoretically incapable of creating paradoxes, has no such problems.
An FTL system which prevents you from going faster then light in directions which allow violating causality would be totally fine, provided this was a theoretical restriction: i.e. your FTL engine just plain can't thrust in causality violating vectors, because it encounters some temporal restriction field or similar.
So the problem isn't "FTL is a causality violation", it's FTL without a mechanism to prevent causality violation is probably impossible.
The sci-fi conceit of it being difficult to plot efficient paths through hyperspace or whatever might well be quite a real thing, and simply related to charting causality-allowed courses through FTL-space.
I don't find their description very clear. In particular, they don't even define
- what "faster-than-light" is supposed to mean in a non-relativistic setting (Newtonian mechanics) where there's no such thing as a universal velocity and you're dealing with Galilean velocity transformations.
- what causality is supposed to mean exactly outside the relativistic setting where you have a universal velocity which induces the causal structure.
All in all, their definitions are very inter-dependent, so the "trichotomy" they present as truth doesn't make much sense to begin with. From my POV, they're not even wrong[0].
Moreover, they seem to only consider Special Relativity and completely ignore General Relativity where the speed-of-light bound is only a local bound. Basically, even if what they say were true, it would still not apply to the General Relativistic setting we're looking at here.
> Time travel makes Causality impossible, since it can be used to create paradoxes
This is not so clear, either, compare e.g. https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
No, it makes the idea that causal sequence and temporal sequence are identical and that the former mapped against the latter can’t have cycles impossible.
Making causality more complex than we'd like to think it is isn't making it impossible.
1) this paper exhibits, for the first time, a continuous family of warp drives which contains Minkowski space (see the end of S.4.3). This means that it is, in principle, possible to turn on/off a warp drive, which is a basic requirement if you ever want to build one.
The next basic feature we need to understand is how/whether warp shells can accelerate/decelerate. No paper in the literature does this yet, afaict. Would be very exciting if Bobrick and Martire can exhibit this in a sequel!
Where could we source so much mass? That is far more than the mass of the earth, and surely altering the mass of the sun by 10% would have catastrophic effects.
> physically possible
Does an element exist that is dense enough to compress 10% of the mass of the sun into a 620m sphere? If not, how can the claim that this is physically possible be supported?
So I assume 610m in diameter would be 8 earth masses?
That's still 4000 times less than 10% of the mass of the sun
I'll try to find the article.
looks like CQG spell-checker added a subliminal typo in the abstract :)