Hacker News new | past | comments | ask | show | jobs | submit login
Positive Energy Warp Drive from Hidden Geometric Structures (arxiv.org)
171 points by ctoth 18 days ago | hide | past | favorite | 188 comments

Everyone always talks about FTL travel for spacecraft, but there's also an interesting perspective from weapons side (and this a reason to be a bit careful and think about some ethics as we move along). A FTL kinetic weapon would be devastating. If it comes with a collapsing warp bubble then there's basically a gamma ray "gun" that travels right in front of your mass that is traveling at tremendous speeds. You look at impacts on the ISS and you can see how much damage a small object does[0]. This is also especially important when trying to develop such a device for travel as you'll probably be sending small objects before a spaceship, or rather a "bullet". Where that object ends up may be in serious danger.

Edit: And as others are pointing out, communication. There's a whole slew of technologies that could be improved/invented if you can move things/information FTL.

[0] https://www.esa.int/Safety_Security/Space_Debris/Hyperveloci...

If you can twist the mass of jupiter into a bubble and accelerate it to lightspeed, you probably already have the means to destroy an earth-like planet.

This is like a medieval person cautioning against the development of jet engines due to how fast they could make a catapult swing.

> the mass of jupiter ... means to destroy an earth-like planet.

What? That's a bit overkill, don't you think? I got a this pebble that'll do the job just fine.

If we're talking surprising applications of FTL, what about reducing latencies within CPUs?

Funnily enough, the writers of ST:TNG had this idea too. The main computer of the Enterprise-D was huge (9 decks tall) and generated a subspace field so that optical processing ran faster than light.

Star Trek's tech vision is astounding. How many things in it have actually been built? I count:

• Tablets (the padds)

• VR "holodecks"

• Computers that you can talk to and ask questions

• "Tricorders" (smartphones?)

• Turbolifts (https://www.dezeen.com/2017/07/12/thyssenkrupp-unveils-world...)

Maybe we'll get to add a warp drive to the list by the time I die.

What else?

Communicators would be something like Bluetooth earpieces.

You've got Tricorders wrong, they're a generalized non-contact diagnostic tool; someone was working on one but it was just a load of diagnostic devices stuck together. We've got body temperature, and pulse is doable, but not sure what else can really be added to a Tricorder, maybe a tiny mass-spectrometer gets you part way there.

> You've got Tricorders wrong, they're a generalized non-contact diagnostic tool; someone was working on one but it was just a load of diagnostic devices stuck together. We've got body temperature, and pulse is doable, but not sure what else can really be added to a Tricorder, maybe a tiny mass-spectrometer gets you part way there.

In fairness, a tricorder would probably need to be a device capable of gathering data about quantum fields within an extremely local region of space and then analyzing based on known frames of reference and data models, so we're probably a ways off on that one.

The harder part of that challenge is building a device capable of analyzing quantum fields in the most abstract sense. But once that's done, even if it's in the form of a massive machine that can only analyze spacetime in a small chamber, even if that machine is rooms-big, the race to the first tricorder begins.

(I could be completely off-base with my comment on analyzing quantum fields. I'm not at all an expert.)

I literally bought a Bluetooth enabled TNG-style comm badge for my partner for Christmas.

TOS-style communicators are basically flip-phones with absurd range.

Given that WiFi signals can be used as wall-penetrating radars capable of pose, breathing, and pulse determination, that could be added without too much difficulty.

Likewise broad range IR and UV cameras (which I assume will become standard when phone manufacturers can no longer differentiate by zoom, just as they added zoom when everyone had enough megapixels).

Geiger counter would probably just upset people when they realise even organic bananas contain radioactive potassium, but could be done.

Carbon monoxide and other air quality sensors are fairly plausible IMO. Not sure how many you could put in a phone (zero? Just CO? Whole mass spectrometer?), but I can see the desire.

Make an atomic clock good enough and small enough, and you could detect temporal anomalies caused by the nearby football stadium filling up (no, really: https://news.ycombinator.com/item?id=26582161)

My head-cannon is the small cylinder prop used in the medical tricorder is supposed to be a miniature MRI or CT scanner. I suspect either would be a mistake to add to a consumer product.

Is it so wrong? My phone can:

- Identify places and objects with the camera and give me data about them (Google Lens)

- Plot graphs of the local geomagnetic fields, my exact position on earth (the built in compass and GPS)

- Plot graphs of local radio traffic, signal strengths etc (wifi scanners)

- Control other devices

- Diagnose medical conditions

Smartphones can do a lot already just with the sensors they already have.

Interestingly, most of those ideas were far earlier than ST:TNG was. Roddenberry had written a lot of them down based on 1960s science fiction during the time he was working on the original series.

He wanted to do the holodeck already back then, but there was no way to do it reasonably convincingly given the technology of the time.

If you ever try to follow what a holodeck can and can't do, you easily see that few of our technology predictions can even hold a candle to it. There's some finger-waggling in-universe to explain away the discrepancy, but the ‘fact’ is that a holodeck would create matter with which you can interact, out of thin air, and expand this volume of matter seemingly for kilometers around, while purportedly being located in a room. VR is but a feeble emulation of holodeck's glory.

In fact, if you consider the in-universe explanation of ‘holodeck creates fake matter that can't actually exist and function outside of the holodeck’, then things may possibly be even more impressive. So, presumably people discovered some kind of a different matter, with which we could interact, but which exists only temporarily on the holodeck thing. Since it flips in and out of existence when the holodeck activates or deactivates, it sounds like some kind of energy that masquerades and works as matter. But how much regular energy would you need to expend to hold this matter-energy in place and make it work? All discussion of teleportation and ‘transporter’ devices comes to conclusions that disassembling and assembling human-scale objects would require huge amounts of energy, like on levels of Kardashev Type II. But a holodeck pretty much does about the same kind of thing continuously, all the time while it's in use. Imagine, at the vague minimum, running a present-day country's worth of energy supply to power your VR box.

Yes I know how the holodeck was hypothesised to work, but the actual goal of the deck was VR entertainment and we have that.

"The Game" episode of TNG predicted addictive viral mobile games

Walking around with the sum total of human knowledge. That featured heavily in a lot of episodes where the ship carried around a library of everything and and could communicate back to the network on Earth for everything squared. Often problems were solved by database search algorithm.

And the VR Holodeck is spot on actually. I was having moments where I was questioning how much better the experience really would be with a matter-energy converter. It already works pretty well using stupid human perceptual tricks.

We used to have the sum total of human knowledge, or at least a good facsimile of it. Unfortunately, with the Internet as we know it transformed into a handful of walled gardens, finding detailed knowledge is increasingly difficult.

Wikipedia is excellent, as usual. But it is an encyclopaedia, not a definitive source of original knowledge, and it remains the case that I can browse my modest (physical) bookshelves and find a book on, say, Russian modern history that will give me far more in-depth information in the first ten pages than I can find online. Same applies for almost any genre, except perhaps fiction. Seven Years In Tibet taught me more about Tibetan geography and culture than any amount of Googling could do.

The idea of an infinite knowledge bank is appealing and I think the Internet could have made it - imagine a suite of applications that complement Wikipedia - but commercialism has ruined the utopia, as it is designed to do.

Technically, all the books are online now thanks to Google's book scanning programme and Kindle. You might have to buy it to see the pages but that's OK. Of course Star Trek didn't have money, so ...

The navigation was done by whales.

"Computers that you can talk to and ask questions"

Pity the answers are so often rubbish and the "side-effects" so obnoxious. ST (and most other SF) seems not to have predicted that.

This is a good one. The limits on the mass needed to construct the system are quite large, but perhaps you can get a weaker effect much more easily.

This would also be neat. And of course any information communication. While Ansibles[0] are typically used to verbally communicate with someone why not any information?

[0] https://en.wikipedia.org/wiki/Ansible

That was sort of a primary plot point of the Ender's Game series.

I wouldn't say primary plot of Ender's Game. I'd say it was a higher focus in Speaker for the Dead. Still, the device is used in a lot of Sci-Fis. Card wasn't even the first to use the term Ansible, though he did use that exact word.

It was most definitely one of if not the most central plot point of the series. Not sure how far you got, but the entire series including the prequels is worth a read imho.

Here's why I believe you are being downvoted:

1. The original topic was about Ender's Game only, not the entire series, so you basically shifted the goalposts. 2. More importantly, I think you are using the wrong literary terminology and you and the others are talking over each other. Where you say "plot point" I believe you meant "plot device."

The Ansible is definitely the primary plot device that makes so much of the story and how it flows possible, but the ansible is not the point of the plot itself.

Throughout Ender's Game the plot point is reinforced that the rules of civilization will be broken when a credible threat shows itself. That is shown with the various physical conflicts Ender has with the other children at the individual level, it is shown with the tolerance the military has in order to shape Ender at the organizational level, and it is shown with the vilification of Ender so society at large does not have to grapple with just how badly it broke its own rules - so it can be done again if necessary for survival.

FWIW I explicitly said "Ender's Game series". I think you're also redefining "plot point" to just be "plot". A plot point is simply an event that impacts what happens next. The reveal of FTL data happens late in the book but it is definitely both a plot point and a plot device.

For the greater series though, it's basically the core concept of the story arc's plot.

Appreciated, I wasn't meaning to be rude, just love talking about this series in particular.

Dropped by to see what was up, only a drive-by downvote so let me explain since you didn't.

Its becomes clear by the end that the ansibles being used were creating philotic threads throughout the universe. So the very thing they had been using for FTL communication was building a huge network. This allowed them to not only save the trees by uploading them, it akso allowed Ender to be brought to no-space where he was able to subconsciously create young Peter and Valentine.

So, they ended up being central to the entire story.

You're welcome.

You don't even need FTL for that, just move quicker than your cpu!

I assume that in theory this would also work the other way around ... find a way to make your CPU move (spatially) a lot slower than you and perform millenias worth of computation in your coffeebreak.

"I will slow myself down relativistically and declare that the universe has sped up." seems like the equivalent of the mathematical joke of "I build the fence around myself and declare that I am outside it."

I highly approve.

There's at least three sci-fi universes I know of that used ubiquitous nanoscale wormholes to speed up their processors.

If you are willing to assume CTC: https://www.scottaaronson.com/democritus/lec19.html

There's more than one way for the Fourth World War to be fought with stones.

FTL Sneakernet would be cool!


What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity that allow you to pop up anywhere and anywhen you want in spacetime.

[0] https://en.wikipedia.org/wiki/Light_cone

General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.

AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories.

What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric.

I wonder now how a violation of causality could either circumvent of contradict the second law.

This is the content I visit the orange site for. Thanks.

To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series

It's not creating new energy, right? It reads to me like causality violating configurations simply move energy around time as we'd move mass through space.

The second law, like all physical laws, does not violate conservation of energy. However, reversing causality can cause the reversal of entropy, which is what the second law deals with.

I don't believe it reverses entropy, if we consider the entire system to include all of time.

Moving energy around through time may serve to increase entropy overall.

It's not necessarily about reversing causality, it can simply be breaking it (ie, the effect happens before the cause without being able to influence the cause).

I would envision two chambers A and B connected by two vents X and Y, like Maxwell's demon but with two vents instead of one and both vents permanently open. Whereas X is an ordinary vent, Y has the property that any particle passing from A to B through it reaches B shortly before it leaves A. Any particle passing from B to A through Y disappears for a short time before reappearing in A. It seems to me that this setup would induce a current of air blowing from A to B through Y and from B to A through X, but I'm only an amateur and could be full of hot air myself.

Let's look at the two possible timelines here:

At some point a particle pops up into existence in B, increasing B's pressure.

Some time later, a particle from A disappears, decreasing A's pressure.

This pressure differential would cause flow from B to A through X.

Alternatively, a particle randomly disappears from B, decreasing B's pressure.

Some time later, a particle appears in A, increasing A's pressure.

This would cause flow from A to B through X.

The more particles that cross from A to B via Y, increasing B's pressure, the more likely it is for particles to pass from B to A via Y, keeping these two effects equal.

These two pressure differentials would cancel out for large numbers of particles.

You've essentially just hooked up the system to itself, but at a future point in time. Unless the overall pressure of the system is changing, there is nothing to induce the flow. If you are increasing the pressure, valve Y is essentially just a standard one-way valve.

I don't think we know that time's arrow is caused by the second law. Isn't that just a theory?

8. Bonus misuse: The term "science words" is not really used in academic literature

You can't be a scientific american if you keep using the science words bad.

It's like cargo culting science: Use these words correctly if you want to seem to know what you're talking about!

But anyone who actually knows what they are talking about would use these words correctly, or at least not be mistaken about their meaning.

So this article is teaching people that have no idea what they're talking about, how to sound like they do.

Maybe it's time we start using another language for science. Clearly it's confusing the Scientific Americans.

I've never managed to see how causality is a problem for FTL travel

If we perceived the world through sonar then causality would be violated by anything supersonic. If you were shot then you'd feel the bullet first and then "see" it moving back into the gun.

AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality

Comparing to sonar isn't valid since objects at subsonic and supersonic speeds still obey the rule that 1 + 1 = 2. For high velocities, this rule is no longer true and you need to resort to using a spacetime diagram to compute velocities.

If you draw a tachyon on a spacetime diagram going faster than the speed of light, you can create situations where the tachyon travels back in time for certain observers.

> AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality

If information can travel faster than c, then you can create situations where it's also violating causality.

Here's a good explanation of why FTL causes causality issues.


The sonar example works only if everything is traveling in roughly the same frame of reference. You're not accounting for relativity and its effects on space and time in your analogy.

I still don't get it. That analogy assumes that light is the only way causality information can be transferred - i.e. if I see the phone call being placed before its sent then it seems like causality is violated. But it I look at what's happening via the FTL phone then I can see that it's not

It doesn't guarantee that causality is protected, but it certainly seems pretty suggestive that GR creates a big infeasibility region for positive-mass objects (outside of the future-past light cone). There are ways you could still violate causality despite this restriction, but it makes it a lot harder. There's no particular reason we couldn't have a ++++ spacetime, which wouldn't have this form of causal censorship, so I count this as one more piece of evidence towards "maybe causality violation is impossible".

That conjecture is one of the two foundational principles of relativity - that the laws of physics are the same in all valid frames of reference. "Violating causality" means that in some valid frame of reference A, event X caused event Y, and in some other valid frame of reference B, event Y caused event X. The laws of physics cannot be the same in A and B for both of those to be true.

This is not true. Relativity doesn't forbid and is not in contradiction with supraluminal phenomenona.

You need to impose additional constraints on the theory (e.g., real mass conditions) to get rid of them.

I'm watching two friends go through a nasty breakup that "violates causality" if you take both points of view into account, and am realizing that it's impossible to do any work with pop-sci physics because I'm relying on an extended metaphor to grasp the topic.

It's clear that your point of view matters in physics, but it's also clear that I have no idea what a "point of view" means in this context.

Point of view means coordinate system, or reference frame.

If I am stationary and measuring you flying past me in a relativistic train, I would measure you in a coordinate grid say, centered on myself. You may be watching me from your train and doing the same, but from a reference grid stationary with respect to you but moving with respect to me. Even if we both agreed on identical measures ahead of time, your measurement of me from your reference frame and mine of you from my reference will be off from a measurement we do ahead of time in the same reference frame as a calibration. However, that difference comes out in the wash through the magic of the Lorentz Transformation. Using the Lorentz Transformation, I should be able to negotiate transforming a measurement I do of you in my own reference frame to what it would look like from yours and you should be able to transform yours to mine, through which we'd both come to the same numbersassuming we're in spacetime that shares the same general degree of curvature (same gravity). This can even be generalized to arbitrary degrees of spacetime curvature. I.e. if your relativistic train is racing past a black hole with a near tangent intersect, and I'm stationary with reference to the black hole centric reference frame but further out in the system, as you blast through the high curvature part of your trip your clock may seem to de-sync from mine. This too, however, can be mitigated through copious amounts of math I don't get for more than half an hour after looking away from a text book. I just know if I ever have a problem to solve that involves hitting that book again, I have either made some terrible or awesome decisions in my life.

So we're traveling away from one another at 0.9c. I send a paper airplane in your direction at 500x the speed of light. You receive it and send it back (at 500c). Does it not arrive before I sent it, from my perspective?

I’m still studying this, but IIUC that’s only a problem when you and the other party both launch the paper plane at 500c relative to your own reference frames, and (separate thing I might have misunderstood) “no preferred frame” is because Occam’s razor says GR doesn’t need one rather than because such a frame would be incompatible with observations.

(Again I may have misunderstood, but the frame of the CMB sometimes gets talked about as if it might be a preferred frame).

Even at 500x the speed of light it took some time for me to receive it, and then you receive it some time after, so no it does not arrive before you send it. You'll just have to wait ~500x that time to see the corresponding visual. For me it's not weirder than sending and getting back the paper airplane at 500x the speed of sound and waiting for the sound. But I surely miss something.

See, the formula for velocity is v=s/t. We know the distance the object has traveled, so it's the time it took to get there that gets adjusted so as the resulting velocity never exceeds the speed of light. At least that's my understanding of it. I might be entirely wrong.

Does this reverse time though? 0.9c is 90% of lightspeed, and for the sake of your example, 500c is just instantaneous.

If light is just information, wouldn’t it just be a matter of seemingly getting something very fast?

If you're traveling at 0.9c then there will be significant time dilation. Since velocity is relative, both parties will see each other as severely time dilated which can lead to somebody receiving a reply to a message before they send it.

But how do you know that you need to send a message or what if you don’t send a message? How can you get a reply?

Everytime I think I get it, I don’t!

Well for one thing, a speed-of-light limited warp drive would still be effectively a reaction-less drive we could use for travel in the solar system.

But a decent way to beat Einstein would be find and implement an actual FTL engine: after all, if you can do it then obviously the theory isn't correct (alternately: the universe possible does not care about causality as much as we do in our little bubble of perception).

The whole point of warp drive FTL, and the reason it's the only kind of FTL idea taken seriously, is that it wouldn't violate GR. We already have widely accepted physical theories that contain exactly this kind of FTL 'travel': the Big Bang theory posits that space in the early universe expanded faster than the speed of light.

> would still be effectively a reaction-less drive

True, but if the energy requirements for warp drives continue to be measured in Jupiters, reaction-mass is the least of your worries.

I think with that much energy the light pressure alone would get you wherever you wanted to go.

I never understood the problem regarding causality and the light cone. Why a latency between photons and the FTL event break causality? Isn’t just a visual delay and the events aren’t still kind of “stacked”? Even if we alter photons we just break the visual ordering, the actions still happened in order.

The quantum world constantly needs to agree on what the state of the macroscopic world is, there is a limit to the speed of that agreement happening, hence the speed limit on the spread of information (speed of light). There are contradictions happening constantly in the quantum world, but they are resolved either way.

Any contradiction would be resolved on quantum level even if it can be interpreted at macroscopic scale as breaking of causality.

The important thing for physics would be to better clarify what time is. Why is the speed of light as much as it is, why is there time dilation related to mass and why is the time slowing down for objects travelling faster.

My guess is that the quantum world needs more of its own time to agree when there are more objects (more mass) close to one another, or that a fast travelling object needs to "agree" with its surrounding more frequently, and that overhead is manifesting as slowing of its time.

"What do the authors of these warp drive papers think will happen causality-wise"

According to §3.3, they aren't sure. If I parsed it correctly, they haven't mathematically resolved whether closed timelike curves are necessary, or precluded, or neither.

The last part of that section suggests (with a citation) that "horizon formations [may] prevent the configurations from transporting an inertial observer from the subluminal regime to the superluminal regime." So it's a distinct possibility that warp drives can't actually go FTL.

We're all traveling through spacetime at the speed of light, most of our velocity is in the time dimension, the faster you translate in the x,y,z dimensions, the less is left over for the time dimension, and thus time seems to slow down. (That's how this non-physicist things about it, anyway)

In a warp field, you're just somehow shrinking X,Y,Z while not doing the same to time. I see no reason to think you'd travel backwards in time.

Your thinking assumes a preferred reference frame. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

Why could it not somehow hold in all reference frames?

In relativity, the speed of light is the only constant speed; therefore, in which reference frame are you going at e.g. 10c?

Perhaps the CMB is the preferred frame of reference for all motion and this problem has a trivial solution (I am not a physicist), but everyone I’ve seen explain relativity starts with “there is no preferred frame”, and the rest follows.

"effectively traveling into your own past"

Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"

Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past. [0] If you can send messages to yourself in the past all manner of causality goes out the window.

First of all, it gives you the ability to solve NP-complete problems trivially.

Step 1: you receive a message from yourself from the future. It explains the problem, gives you a potential solution. Let's say you ask yourself to solve boolean satisfiability, gives you the problem statement, and then a potential solution. You check the solution: If the possible solution is correct, you send yourself an identical message. If the solution is not correct, you send the next possible permutation of the answer. So if you sent yourself the possible solution TTFTFTTTFTFFTTTFTF, you send yourself TTFTFTTTFTFFTTTFTT. If there are no further possible permutations, then you send yourself a statement saying that the problem is not solvable. You've created a time paradox; a future in which you send yourself an incorrectly solution is logically inconsistent. The only consistent universes are the ones where you send yourself the correct solution or not-solvable, and all other universes (starting with the one where you sent yourself the message FFFFFFFFFFFFFFFFFF) collapse into the valid one.


> Being able to send messages faster than the speed of light grants the ability to send messages to yourself in the past.

No, it doesn't. Even if we will be able to communicate instantly (0 delay = infinite faster than speed of light), we will not be able to send a message to the past (negative delay).

It does.

Read the numerical example on https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone

1) It's not applicable for this kind of drive. Isn't?

2) It's an unsolved problem, which waits for a correct solution.

3) Next time, please read what you are posting, please.

1) It's applicable for any kind of ... thing ... which lets you send a message faster than the speed of light. It doesn't matter if you build this kind of drive into a USPS truck, or if it's a special closet in an unused bedroom, or a carrier pigeon that flies really fast, or a garden gnome that you hand an envelope to and he's just not there when you go to look for him until he then pops up and says "speedy delivery!" and hands you an envelope from yourself from the future.

2) It's an unsolved problem because it's built on a foundation which is also an unsolved problem -- namely faster than light travel/communication. If -- if -- we solve the problem that it's built on, all indications are that sending a message to your past will become possible.

That being said, I expect that it's impossible because I expect that FTL travel/communication is impossible.

1) It's formula for time dilution. It's not applicable for a warp drive, because Alice will be shielded from relativistic effects, so no time dilution will occur. Alice's space will be dragged at speed > c, but Alice will live a normal life at normal speed, like a passenger in a supersonic airplane is protected from supersonic effects.

2) It's an unsolved problem because formula is defined for [0..c] only and for EM communication only. For a•v = c² it predicts dT=0, so time will stop for unshielded Alice, like it stops for photon, so she will not be able to communicate at all. For a•v > c², the formula is not defined at all (it is the unsolved problem). In reality, we expect to see Cherenkov radiation. In theory, according to this formula, EM processes should go in reverse, which is not possible at all, because it requires all photon emitted by Alice to reverse their direction and hit Alice at her new location.


We're sitting here talking about how 1+2=3 and you're telling us that we're wrong, and that the sky is actually blue.

I have no basis to object to your argument, other than to tell you the thing you're arguing against clearly doesn't have anything to do with what the rest of us are talking about.

These responses are correct.

I'll add: 3) yes I read it. It's an example Einstein created, so possibly worth responding to the points made there rather than attacking me for "not reading it".

If you read it and understood it, can you show us how to apply this formula to the warp drive, please? I cannot ask Einstein, obviously.

It exactly applies to a warp drive. Just saying "warp drive" doesn't have magical qualities - it's exactly what Einstein was talking about.

So you put in the numbers in the same way as that numerical example on Wikipedia, and it shows the same thing.

Edit: the only exception I can think of is if causality can be reversed.

No, not "observing your own past". Instead, your past self could observe your future self. And then your future self could turn their FTL ship around and run you over, killing their past self and creating a paradox.


> No, not "observing your own past".

Being able to observe your own past is a given after FTL travel, and the most important thing: it's a given that relies on almost no assumptions about the deeper mechanics of the universe. If you instantly teleport to Alpha Centauri, you will be able to see your old Earth photons for years.

> Instead, your past self could observe your future self.

Now that statement (like the Tachyonic antitelephone) relies on some assumptions that we can't test yet. My hunch would be that either an as-of-yet unknown non-local temporal dimension (or synchronization mechanism) exists at the foundations of the universe, or that FTL travel is indeed impossible. One excludes the other would be my intuition.

Sorry if I don't fully understand, I've read through link a bit but it just leaves me with one question in particular: Wouldn't your relativity have to be both different and the same simultaneously for this to work?

In the tachyonic antitelephone example, Bob and Alice send messages that travel at 2.4c, but neither observers are at that speed. Wouldn't the limits of their observation and time dilation cause the messages to appear in order?

If you were to theoretically travel into a black hole, and then somehow escape out of it very quickly, you would see yourself still in the black hole, but you wouldn't see yourself coming out of it, because that light was further from the black hole and thus moving at a different speed?

> turn their FTL ship around

I feel like this is the part that would probabally be hard (ie not possible) when going faster than light, assuming such a thing is possible.

Also whatever magical space warp it is that lets you go that fast could presumably also obscure your ability to see yourself in the distance.

"Turning around" is just stopping at some point (ie. some inertial reference frame) and then accelerating in the opposite direction with your FTL drive. You have all the time in the world to do this maneuver because you're already effectively in the past: https://physics.stackexchange.com/a/54242

There's no absolute coordinate system in space, so there's no way to "stop". You can only change your velocity relative to some other object, e.g. your past self in the example. What counts is acceleration / deceleration.

Slowing down to reach the speed of light might be as hard as speeding up to reach it; I remember it played a role in the theory of tachyons, hypothetical FTL elementary particles which could not be slowed down.

> acceleration / deceleration

Another question for the thread: Anyone here feel like they can feel the Coriolis effect if they focus enough? I haven't travelled across the equator enough to know if the north/south jetlag is different from east/west.

How would you feel the effect? If you do it's more likely to be placebo as you can't avoid knowing where in the world you are!

I imagine if there is any amount of angular momentum imparted to the water in our cells, the reversal of the force propelling it would create a turbulent episode of transition.

The differences in force are way too miniscule on that scale. If you're looking for a measurable force differential across the extent of your own body, look at gravity instead. When standing up, the gravity at your feet is about 0.5 ppm smaller than at your head (for a body height of 1.75 meters). You don't feel that either; there is just no evolutionary advantage to being able to measure such tiny differences in local gravity, especially since we have acceleration sensors in our ears that are way more efficient at telling up from down.

The coriolis force differential, besides being rather small to begin with, has the additional problem that the coriolis force is proportional to the speed at which you're moving over the earth. If you were to accelerate to a speed where the coriolis force differential might be appreciable (e.g. by sitting in a bullet train or in a plane), it will be completely drowned out by any other momentary forces imparted upon you by the vehicle (trains or planes can move mostly straight, but not in a mathematical-grade straight line).

I'd guess there would be frames of reference (with relative high-speed motion) in which you'd appear at the destination before you've left the origin. The laws of physics are supposed to hold regardless of which frame of reference you use.

SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. It's that second rule that must be violated for FTL to work - FTL implies a reference frame in which light can, at the very least, switch directions! That is, light from behind you looks like it's coming from the front (but still going c).

> SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames.

Technically that's only one assertion (that is, the second is a strict subset of the first).

No, it isn't. Light is unique in this way.

Correct. In a bit more detail:

Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers.

The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity.

Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way.

(Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)

Moreover, gravitational waves are affecting speed of light, while EM waves are not affecting gravitation, so speed of gravitational waves is fundamental, while speed of light is not.

> (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)

Right - this is a subtle but important distinction! Photons don't have primacy, space-time does.

No it isn't. The speed of light in SR is not really about light. It's just a good way to explain things.

You can calculate the speed of light from other physical, measurable constants without using light (the permittivity and permeability of free space). Those you can measure with inductors and capacitors. So because those measurable physical behaviours are invariant in all reference frames under SR, it follows that the speed of light is also invariant.

c is defined. https://physics.nist.gov/cgi-bin/cuu/Value?c

You don't have to measure c, or calculate it. The causal chain goes the other way - we assume c is invariant and define capacitance and inductance in terms of it (c, that is).

No, the meter is defined (as c times a certain number of seconds).

No, the meter is defined in terms of c and the hyperfine transition frequency of Cs-133 is defined https://physics.nist.gov/cgi-bin/cuu/Value?nucs. If that link isn't good enough, you can check out the SI Mises en pratique on the subject: https://www.bipm.org/en/publications/mises-en-pratique

Edit: I'm totally this guy right now https://xkcd.com/386/

> the meter is defined in terms of c

That is what I just said: the meter is defined in terms of c, not vice versa.

I think you are confused. I suggest you read up on this stuff. It's really fascinating, but you're not using the words right. Cheers, and good night.

Try this: https://www.youtube.com/watch?v=au0QJYISe4c

c isn't defined as the speed of light - light travels at c, but so does everything else, when you factor in movement through time.

Good video! Thanks. c is still defined as "the speed of light" by NIST, even if it's not measured, and even if it's more universal than light. But I get your point (via the video) - photons are particles that move only in space, and not at all in time. Kind of makes your mind bend when you consider the cosmic background radiation hasn't aged a day since the beginning of the universe...and since it comes from everywhere equally, we really are the center of the universe. And so is everwhere else! Fun.

You could break the problem of closed timelike loops by having a preferred reference frame for the Universe where nothing could travel backwards in time relative to that reference frame. And the CMBR rest frame might be an artifact of that preferred reference frame existing in the early Universe.

Why would causality need to be one way?

It's not unusual for (anticipation of) future events to affect the present. For example if I think it will rain, I grab an umbrella. While typically my belief that it will rain is based on something in the past (previously weather patterns for example), does it really change anything if a time traveler claiming to be me from the day after tomorrow told me it's going to rain tomorrow?

We only think of causality as going from past to future because that is the way we live. We also find the idea that particles can have an indeterminate location or pop randomly into or out of existence is weird, because our minds which have evolved to experience and understand the macroscopic world have no method of really processing that, but our intuition is not reality. If we can accept that our concept of position of objects in space is merely an approximation of a more complicated reality, the same for position of events in time should not be much harder.

People get bent up about paradoxes, but we have things like the Novikov self consistency principle [0] which allow us to avoid paradoxes even without one way causality.

[0] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

Only (well-written) f77 is faster than c

I believe the answer to this is [0], and the answer to that is [1]. I'm rather over my head here, and would love some analysis from people who know what the heck they're talking about.

[0]: https://arxiv.org/abs/2105.03079

[1]: https://eriklentzphd.blogspot.com/2021/05/response-to-new-pa...

These drives don't violate causation, they warp space. Similar to the way expansion of the early universe could "move faster than the speed of light"

What happens if you're between the drive and the destination? Do you experience spatial contraction too?

If not then the spacecraft is traveling faster than 'c' with regard to everything not in its reference frame (right?). If this kind of thing is possible then the universe is considerably weirder than we thought even after a hundred years or so of QM.

If FTL is possible (and engineerable) then the Fermi paradox becomes a serious problem. There "should" be aliens all over the place unless...

(1) We are wrong about the nature of the universe and strong theism / creationism or the simulation hypothesis (which is the same thing more or less) are right.

(2) Life or complex life are so phenomenally unlikely that we are literally the only example in the local galactic supercluster at least, if not the whole universe. Speaking of creationists they often argue that the probability of life forming from non-life is insanely low. Maybe they're right about those probabilities and it only happened once.

(3) We live in a cosmic cemetery. There is a big nasty great filter ahead of us, and nobody has passed so far. Maybe the great filter is when you try to actually build FTL and cause a space-time catastrophe.

(4) We live in a "dark forest" and should be very, very quiet. Shhhhh....

(5) We live in a nature preserve. See #6.

(6) Last but not least, at least some UFOs really are ETs in which case there is no Fermi paradox. They're here and we have seen them. They're just laying low and not making overt contact.

My money would be on 6. If we demonstrated some kind of engineerable FTL I'd take the possibility that at least a few UFOs are ETs very seriously.

Or 7) FTL is possible, but the amount of energy required is so insanely large it doesn't matter. The paper says it takes energy equivalent to "four orders of magnitude smaller than the solar mass", which is an absurd amount.

This reminds of an interview I read with Donald Knuth about the P vs. NP problem. IIRC, Knuth said he believes there probably is a polynomial time algorithm to solve NP problems, but the constant factor will be so absurdly huge (e.g. Graham's number) that it could never be run on an actual computer. I've heard layman claim that if P=NP were proven, then it'd suddenly spell the end of encryption relying on NP problems, but it's entirely possible that it'd have no practical impact. The constant factor is analogous to energy requirements for FTL.

That’s why I said engineerable. If it takes the mass-energy of Jupiter it’s likely something that could never be done since no material could contain or control such energy in an organized way.

This paper does not offer an engineerable solution but it does argue that negative mass, which may not exist, may not be required. That brings us closer. Are there solutions that require less energy? Are they physical or just math artifacts? Get the energy down to LHC levels and it becomes testable if even at submicroscopic scale. Show it at any scale and it’s a physics revolution and front page news and loads of money starts being spent.

Going to the moon was once dismissed by many scientists as requiring silly amounts of energy until we figured out a bunch of things like modern rocket engines and staged rockets.

Fermi's paradox does not depend on FTL travel.

No, but the difficulties of conventional interstellar travel provide some cushion in any model. If you can build a USS Enterprise that cushion goes away.

There is the threshold where these theories rule in human plausible warp drives, and then there is the weaker threshold where these theories rule in that they should be taken as serious explanations for the alleged “advanced technology” demonstrated by UAPs. I wonder what we ought to expect to see from such a device as an observer if the geometric formulation here is correct, even if we can’t explain the energy management problems. It sure would be nice if this research adds tools to the scientific community to start forming decent falsifiable hypotheses within the scope of an assumption of a given incident being such a tech demo.

> then there is the weaker threshold where these theories rule in that they should be taken as serious explanations for the alleged “advanced technology” demonstrated by UAPs

No, it doesn't really do anything to weaken the threshold.

If humans discover a method of warp travel which is then validated by peer review, this does not do anything to increase the reliability of claims made by the US Air Force concerning the existence and performance characteristics of UAPs.

The reliability of the later claims are limited by the fact that there is no independent observation and verification from non-military sources, and due to the fact that former USAF employees such as Richard Doty have claimed they were hired to fabricate and provide false information to independent UFO researchers as part of a domestic counter-intelligence operation.

You're not understanding what I am saying. Also the US Air Force is explicitly not involved in the latest disclosure regarding UAPs, so I suspect you're not fully up to date on everything.

My point is that if a theoretical model for a warp drive emerges that provides falsifiable hypotheses in terms of what an external observer ought to see, this could be a useful tool for scientists to use if/when they are given the underlying UAP data I expect to see the government hand over the next 12-18 months.

The threshold I am referring to is when the theory becomes cohesive enough to provide such a tool for being able to generate such explanations, as opposed to the threshold where it could lead to actual applied physics.

"Warp drives in Einstein's general theory of relativity provide a unique mechanism for manned interstellar travel. It is well-known that the classical superluminal soliton[0] spacetimes require negative energy densities[1], likely sourced by quantum processes of the uncertainty principle. It has even been claimed by few that negative energy densities are a requirement of superluminal motion. However, recent studies suggest this may not be the case. A general decomposition of the defining variables and the corresponding decomposition of the Eulerian energy are studied. A geometrical interpretation of the Eulerian energy is found, shedding new light[2] on superluminal solitons generated by realistic energy distributions. With this new interpretation, it becomes a relatively simple matter to generate solitonic configurations, within a certain subclass, that respect the positive energy constraint. Using this newfound interpretation, a superluminal solitonic spacetime is presented that possesses positive semi-definite energy[3]. A modest numerical analysis is carried out on a set of example configurations, finding total energy requirements four orders of magnitude smaller than the solar mass[4]. Extraordinarily, the example configurations are generated by purely positive energy densities, a tremendous improvement on the classical configurations. The geometrical interpretation of the Eulerian energy thus opens new doors to generating realistic warp fields for laboratory study and potential future manned interstellar travel."

0: Soliton - A "single bump" wave that propagates. See youtube for examples in water. These are (well-studied) solutions to the wave equation. In spacetime, we want some local "warp bubble" bump that is capable of moving/propagating around.

1: Negative energy density: Colloquially, this is like requiring negative mass. Nobody knows if this exists; most aren't hopeful.

2: Nice pun.

3: Positive semi-definite energy: I.e. non-negative (>=0)

4: So we're still talking mass-energies of Jupiter-(ish)-sized planets

This is very cool stuff. Definitely going to read the whole paper after work. Current questions:

- Is this really superluminal? Famously, the Alcubierre "warp bubble" solution doesn't actually propagate you superluminally unless you start it off that way.

- Is there a Minkowskian interior? The warped part of space is like your engine. We also need to have it carry around some flat space if we want it to be capable of carrying passengers.

- Can we construe this into PoC tests? How much energy do we need to detect warping (e.g. laser deflection) with current tech? Could some large-scale LIGO-like setup achieve detection?

As a non physicist, the first question sounded like yes there is an ability to adjust speed if you can adjust the energy density distribution. I didn’t see an answer on the second beyond that the energy density required at the center is zero which leaves hope for a place to put a vehicle there at least, your question of flat space aside.

Curious if I read this right tho.

From p. 12:

This could serve as a method of generating acceleration without modifying the total energy on the hypersurface. One could start with a finite energy distribution highly concentrated and uniformly distributed in a spherical shell around a spacecraft. The energy density is then increased behind the spacecraft and decreased in front, without destroying or creating any additional energy. This increases the shift vector magnitude in the region where the spacecraft is located, transporting it to non-zero speeds relative to faraway observers. If the energy density is sufficiently concentrated behind the central observer, the relative speed becomes superluminal. This then operates as a tuneable solitonic configuration capable of superluminal speeds.

I honestly thought this article was going to lead to something written by Charlse Stross.

I mean with lines like "A geometrical interpretation of the Eulerian energy is found, shedding new light on superluminal solitons generated by realistic energy distributions" I'm still not sure it wasn't...

It might sound like gibberish to non-physicists, but it actually isn't. Eulerian energy is just the energy of something as observed by an eulerian observer, i.e. a normal observer traveling through spacetime (below the speed of light ofc). Solitons are just wave packets that keep their shape as they travel. None of the ideas presented here is new, not even the fact that these warp drives might work without negative energy. It's just a study of some more contrived geometries.

This sounds pretty amazing, but I also have no real idea what I'm reading.

> A modest numerical analysis is carried out on a set of example configurations, finding total energy requirements four orders of magnitude smaller than the solar mass.

Can someone smarter than me say whether this is within the (reasonable-ish?) realm of matter-antimatter reactions? Like Star Trek!? 4 orders of magnitude less than the sun's mass does sound like a positive development - still a lot but less than before, right?

The first paper detailing the Alcubierre drive required exotic particles with negative mass to achieve a spacetime bubble (soliton) which is an issue since particles with negative mass have never been observed. The next big paper figured out how to do it with regular matter and it brought the requirements down to the energy equivalent of four solar masses. That would put it squarely into the realm of a type II or III civilization (1 solar mass worth of energy is in the same ballpark as the amount a star puts out over its entire lifetime).

If this paper is correct, it brings the requirements down to a tenth of Jupiter's mass, which is probably somewhat achievable for a type I civilization. The Enterprise would be the size of a small gas giant to hold an equivalent amount of matter and antimatter. It'd be able to enter warp once.

Would be interesting if paired with the harvesting of energy from a black hole. It is theorized that you can do this 'remotely' by slingshotting a laser around a black hole. This functions the same way a normal gravity assist does, except in this case it's photons, and not spacecraft. Shoot a laser around one side of a black hole, it returns to you around the other side, but at a higher energy wavelength. In this way, perhaps you would not need to store the energy needed 'in' your spacecraft, but instead in a black hole that is some safe distance away.

If your laser beam doesn't spread so far that you can't harvest enough energy to be net positive, then the black hole is not going to be a safe distance away.

I supposed the term 'safe' is pretty relative in this case.

The scale is all wrong. A type 1 civilisation is, rougly, civilisation that covered earth with solar panels.

This drive needs a planet's mass in the form of pure energy, that's a totally different league. Also you dint have a planet to live on after that.

This is a major stretch for even a type 2

>It'd be able to enter warp once.

...why once? Are you assuming all matter+antimatter is destroyed in the single trip? If you have x amount of critical material, could you not warp for half the distance of the "fuel" in your "tank" and then come back?

IANAPhyicist. I'm assuming that if we have the ability to contain and harvest energy from a matter/antimatter reaction then we also have the ability to stop it & start it at will.

The way I said it is misleading for the sake of brevity. That's a small gas giant worth of matter and antimatter to power a warp drive for a ship so small that it's not even worth calculating its mass relative to the fuel. The entire mass of Mount Everest is something like 0.00000000001% of the mass you'd need to create the warp bubble. If the ship was in the center of the fuel, it'd be crushed into something so dense it'd resemble the earth's core.

Best case scenario we're talking a mass effect style planetary megastructure that forms a warp bubble around a ship and shoots it with just enough "momentum" for the warp bubble to fail right at the destination.

IANAPhyicist but if I understand what’s being said correctly the amount of energy necessary to create the warp bubble in the first place, would require converting something roughly the mass of Jupiter. So you’d twice that to “come back”. It sounds much more like surfing a wave that something that get’s turned on or off.

The paper also gives recommendation for a starting point for some finite element modeling and optimization for field configurations requiring less energy than that, improving other properties and such. (FEM modeling to search the irrotational sector of the Hamiltonian constraint...) They mentioned this with a fair amount of enthusiasm - sounding like there may be room to optimize.

I wonder what the gradients look like.

The Sun has a mass of about 2x10^30 kg. 4 orders of magnitude smaller would be 2x10^26 kg which is somewhere between the mass of Saturn and Neptune.

So turn one of those into energy and you're good to go.

> 4 orders of magnitude less than the sun's mass does sound like a positive development - still a lot but less than before, right?

That’s something like 10^43 joules. Still not achievable anytime soon.

Progress in warp field research though has resulted in significant energy reductions though - everytime a more efficient formulation is found its a step on the way to a possible practical system.

After all, if it was easy then everyone would be doing it.

We've definitely achieved progress on this sort of scale with transistor density. Not that it's at all similar, but it's interesting to observe that we do seem to be able to occasionally bring things within our grasp that would have sounded absolutely silly a lifetime ago.

Did we though? From milli- to nanometers is "only" 9 orders of magnitude. 10^43 J is about 30 order of magnitude from practical application than the first transistor was. Also, warp bubble generation might be more like electric motors, which have gone from ~2% to ~95% efficiency and will never ever surpass 100%.

"if it was easy then everyone would be doing it": And, I suppose, we'd see them. Little green men, I mean.

Well that supposes that they currently exist, that they're close enough and that they want to come here.

Maybe it is feasible to build an FTL engine, but maybe it can only feasibly go at like 4c. That doesn't make it very easy to drop by our solar system and have a look at what all that radio noise means, even if anybody's heard it.

Well, we went from theoretically impossible to merely practically impossible. (Only slightly tongue in cheek)

I think that's like 1/10th of Jupiter. Wasn't there a space drive that needed negative mass of about one Jupiter?

The lesson we should really take here is the power of wishful thinking. People dearly want to believe in FTL travel. Without it, the Universe just seems so vast, empty and distant. The truth is it is and it isn't. It's only really that on our current time scales.

Not to be a broken record (there's some new variation on some old idea like every week it seems), but I'm firmly in the camp that believes FTL isn't possible. There are two prongs to that.

1. Theoretical: a lot of these theories rely on people not understanding the domain of a function (eg plugging in negative mass or energy into an equation; just because a function can operate over real numbers doesn't mean real numbers are the domain of that function); and

2. The macro evidence in that if FTL were possible it makes it that much more unlikely that we haven't detected starfaring life. Specifically, the potential volume of spacetime goes from a line cone of several million light years to the entire Universe (not just the observable Universe). We don't know how big that is of course but it's may orders of magnitude larger.

So this paper tries to "solve" the negative energy/mass problem. I honestly can't speak to the soundness of this idea. I am however skeptical.

> The macro evidence in that if FTL were possible it makes it that much more unlikely that we haven't detected starfaring life.

I don't think this is a great argument. Even with FTL the galaxy (not universe) is huge. Let's just say you can go 2x (warp factor 1.25) it would still take more than 2 years to reach Alpha Centauri (our closest star, 4.37 lyr). Even at 8c (Warp 2) it would still take >6 months. At 100c it is 2 weeks. The energy to travel these different speeds presumably does not scale well and so the difference between 2c and 1000c is going to be different level of civilizations[0]. The Milky Way is 2 million light years in diameter. There's about 1000 stars within 50 light years. Here's a list of stars <5 parsecs (16 lyr)[1] or within 5 days at 1000c or 0.0008% of the Milky Way. There could be extensive networks of trade and travel with FTL civilizations and we'd probably never detect them. These routes would still be fairly local and if they aren't around us then we would have no chance of seeing them. This is basically peoples in the Western Hemisphere trying to observe people in the Eastern Hemisphere when neither civilizations have the ability to cross the ocean nor see across it to a reasonable resolution.

I think a lot of people miss how huge the galaxy is (let alone Universe). Nothing is accessible without FTL. FTL is _necessary_. That, or immortality, or generational ships. The problem is that these numbers are so incomprehensibly large that we think they are magnitudes smaller than they actually are.

However big you think the galaxy is, it is actually bigger.

[0] https://en.wikipedia.org/wiki/Kardashev_scale

[1] https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...

I don't think it's fair to say "or immortality, or generational ships." You can make a one way trip, sometimes, but you can't meaningfully communicate. FTL is necessary or the universe is kind of boring on a grand scale.

What I'm saying is that you're not going to travel a meaningful distance in the Milky Way (as in percentage) within several lifetimes. Longer life or generational ships are absolutely necessary, even at high speeds and time dilation.

It's not boring, it's just too big and therefore too slow for our tastes. If we lived 5,000 years, taking a 100 year trip to Alpha Centauri might not seem so bad. I think extending lifespan like this is more likely than building an FTL drive.

My concern is the mismatch between the timescale we communicate with people and the timescale of the trips (minutes or hours vs centuries). It would be such a massive transition during your trip that it is hard to adequately describe it.

In the past information had to travel by people or pigeon (some sort of physical movement). Realistically this is communication happening faster than travel does. Currently we have the inverse, where information is faster than travel. This should similarly happen in this FTL world because of Ansbiles and that you can not just send ships full of people FTL but information (and presumably more easily). So I think the only difference is that the travel time is scale. Right now there's nowhere it takes months to travel and we can communicate instantly. In this scenario we actually have a mixture of past and present. We can maintain near instant communication but maximal traveling distance is years instead of days.

The Milky Way is only about 100K to 200K light years in diameter; the 2 million diameter includes hypothetical dark matter.[1] If it takes a thousand years on average to spread civilization out by another light year, that's as little as 100M years to spread from one side of the normal-matter galaxy to the other. That's less than 1% of the age of the Milky Way.

Assuming exponential growth, an alien civilization could easily have populated the galaxy. Or heck, octopuses are pretty smart and have been around for 150 million years, if they were land animals and figured out fire they'd probably rule the galaxy already.

[1] https://en.wikipedia.org/wiki/Milky_Way

What if we're the first species to explore FTL? What if the Universe is vast enough that FTL-capable specifies are busy in other parts of the Galaxy or the Universe?

> Theoretical: a lot of these theories rely on people not understanding the domain of a function

We thought it was an aberration of mathematics when Einstein's equations blew up into singularities. A singularity couldn't exist in the real world, they said. Must just be an artifact of us using "real numbers", and we needed to be careful to respect the domains of those functions.

Well, turns out singularities do exist and the math led us right to them.

Event horizons exist, I don't think we have any evidence singularities really exist inside them, or any way gather such evidence.

Isn't there evidence starting to surface that we have actually detected alien contact?

You mean that UAP stuff? Occam's razor says this is probably some electronic countermeasure stuff by the US or something.

Maybe, or what if it’s exactly what the DoD is saying? That there’s verified advanced UAPs beyond our current technological understanding.

I don't they said "there are uaps beyond..." , they said they couldn't explain all of them, that's all.

“UAP clearly pose a safety of flight issue and may pose a challenge to U.S. national security. Safety concerns primarily center on aviators contending with an increasingly cluttered air domain. UAP would also represent a national security challenge if they are foreign adversary collection platforms or provide evidence a potential adversary has developed either a breakthrough or disruptive technology.”


I think breakthrough or disruptive technology are the key words here. They may not have said “beyond” but pretty much said everything short of that.

Occam's razor actually says that?

Yes. This is actually about as clear of an example as you can find of what it says.

Turing test failed.

Wouldn't space-faring civilizations be more likely to exist along the galactic plane, which we have a much harder time examining?

Previous work seemed to conclude pretty strongly that superluminal warp travel required negative energy. What's the insight here that gets around that?

The goal of warp drive is to produce a modified region of spacetime that makes it feasible to travel faster than light. There are an infinitude of different approaches. The authors in this paper find that if you impose specific constraints on the way the modified region of spacetime is shaped and evolves over time, you don't need the negative energy anymore.

However, you do need a very high energy density, strong enough to create a black hole. And there is, of course, no guarantee that such an energy source is feasible or even possible. But you no longer need exotic matter — you just need enough regular matter in one spot.

The sociology of this is fascinating. For my whole life I've heard that travelling faster than light is not only impossible, but so obviously absurdly impossible that only loons and sci-fi writers bother to think about it at all. Now I'm reading that "There are an infinitude of different approaches" to the problem!

What exactly is the breakthrough that has led to this recent spurt of warp drive papers? As far as I can tell the results are all purely theoretical and just re-arranging equations. None of them appear to rely on any recently lab discovered physics. Why couldn't this have been done in 1950, I wonder?

> What exactly is the breakthrough that has led to this recent spurt of warp drive papers? As far as I can tell the results are all purely theoretical and just re-arranging equations. None of them appear to rely on any recently lab discovered physics. Why couldn't this have been done in 1950, I wonder?

By "recent", you have to go back almost 30 years to the Aclubierre Drive (https://en.wikipedia.org/wiki/Alcubierre_drive) described in 1994. It forms part of the "we can come up with some interesting solutions to general relativity" literature.

That literature is not just a recent phenomenon. Research into theoretical wormholes (https://en.wikipedia.org/wiki/Wormhole#Development) goes back decades further.

> The sociology of this is fascinating.

It really is.

It's not like these are entirely new questions, but they used to be relegated to much smaller circles.

Hell, even recall any theoretical physics notions in other threads on HN and watch it burn.

It definitely seems like a bit of a philosophical change is sweeping throughout society, and it's probably mainly do to the increased speed and spread of information. It's easy to ignore, but those who don't are sort of forced to revaluate their world view and even their understanding of reality.

We have mainstream philosophers and neuroscientists giving talks on the limits of our perceptions in language "lay" people can understand. It's an interesting time to follow these developments.

It's almost like a new-age open-mindedness without the woo-woo. It was probably never really true, but growing up it felt like the answers were all written, and the universe was sticks and stones and the suggestion of anything else was nonsense. It seems more open now, and more intellectually encouraging, isn't it?

Bear in mind that the "infinitude of different approaches" means an infinitude of purely mathematical solutions, all of which seem to involve physically impossible concepts like negative energy.

As far as I can tell, there is no such insight here - in the middle of page 13 they admit that in their solution, the weak energy condition is violated, and even that:

"No amount of modification to the configuration could get rid of these WEC-violating regions."

If I'm reading this correctly, all they have done is found a solution that has positive Eulerian energy density according to one particular reference frame, but not according to other reference frames. (I'm not a physicist, so please correct me if I'm wrong.)

am i expected to know this in interviews now

By the time it works well enough to be useful, we'll all know about it, perhaps as soon as 50 years ago.

If aliens decide to visit us using the warp drive today, they will still arrive tomorrow, not 50 years ago.

Depends on how good their aim is. :-)

PBS Spacetime has a video discussing the recent theoretical advances in warp drives. It does not cover this paper but does discus a paper that also claims to solve the negative energy problem.


It's my understanding that warp drives can't travel faster than light (superluminal) into new destinations. You must first create the soliton in subliminal speed, then you can move faster than light.

In other words, if you wan to travel 100 light years, you must first spend more than 100 years warping the spacetime towards the destination, then you can move FTL.

It never ceases to amaze me how smart some people are.

Yeah... using Euler geometries to generate warp fields always seems like the obvious solution at first, until you wind up stranded in hyperspace with gimbal lock. That's why you need to use quaternions.

Don't worry, your R2 unit can fix it.

Applications are open for YC Winter 2022

Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact