"The EmDrive Just Won't Die"

[Incarnation of Chaos]

12 hours ago, Nuke said:

its a known property of waves that when they interact with each other they can either cancel out, or stack up, scientists call it interference. this is likely true for gravity waves. considering that there are multiple sources in the universe at various frequencies and amplitudes, the universe is effectively a sea of gravity fields which are constantly in flux. sort of like the gravitational equivalent of radio static as a large number of radio sources interact in such a way as to create a chaotic mess. so its no aether, just a product of multiple wave interactions. 

enter mega drive, em drive's mechanical cousin. the real difference between the two is that the mega drive is built on a lot of peer reviewed science (from theory to device, while em drive attempted to go the other way). the wikipedia article is really out of date, but it does cover the theory of operation pretty well. the article describes the use of capacitors and inductors, but they have moved on to stacks of pzt disks, which are electrostrictive. the short explanation is that if you physically oscillate an energy storage device while simultaneously charging it up you should get some thrust. the claim is that it exchanges momentum with matter in the distant universe. now i have to admit that woodward and his team have been getting slammed hard in peer review in the past year or two. in part because em drive, and also because of the unfavorable outcome of tests of their thruster in dresden. and another guy from their team (rodal sp? i think) put out a paper that stated that mega would be impractical as a thruster. 

my idea was simply to find a relatively strong gravitational wave source, tune the frequency of the drive to match that of the gravity waves (it doesnt have to be 1:1, i figure you could hit a harmonic), point the drive at it, run the test and see if the results are any different. i dont think this would work for an em drive unless it was designed to hit a specific harmonic of the target gravity wave, which means making a custom cavity, but its possible that it could be tested in the same way with a one off test article. with the way that the mega drive is supposed to work, i was wondering if you could selectively use a single gravity source as the thing from which to borrow momentum, rather than 'the distant universe'. 

i should point out that i dont expect these drives to go anywhere, give me old boom boom any day, and i think that it may turn out to be quack science. but quacks doing quack science doesn't harm anyone (people arguing about it on youtube now, or worse using it to 'prove' some other wackjob ideology. thats more a symptom of general scientific illiteracy). wouldn't be the first time a scientist chased their own shadow. but you wanted me to play scientist, im out of my league there, so i dont have any equations for you. but there are woodward's equation from the wikipedia article. i need to better familiarize myself with the math behind gravity waves. since im making an assumption that gravity fluctuation + mass fluctuation = moar boosters. 

I know what destructive interference is....i even mentioned it. I wasn't questioning if Gravity Waves could suffer from interference, i was questioning how Microwaves could affect them in the slightest.

The piezoelectric one iv'e heard of, but the issue with it isn't the same as the EMdrive. They can demonstrate thrust, the results are repeatable. The explanation they're giving for why it's giving thrust is why they've been getting "Slammed", because it doesn't pass muster. As for the rest, i actually didn't have any issue with the idea of your test. I was mostly just saying there's absolutely no reason it couldn't be conducted on earth, even if the test needed a custom cavity.

And i shouldn't have said "You", because i wasn't actually thinking you should sit down and do their work for them. So that's my bad, and the burden of proof is on them (The people who keep testing EMdrive) to write such equations and then demonstrate they have predictive power.

 

[Nuke]

24 minutes ago, Incarnation of Chaos said:

I know what destructive interference is....i even mentioned it. I wasn't questioning if Gravity Waves could suffer from interference, i was questioning how Microwaves could affect them in the slightest.

The piezoelectric one iv'e heard of, but the issue with it isn't the same as the EMdrive. They can demonstrate thrust, the results are repeatable. The explanation they're giving for why it's giving thrust is why they've been getting "Slammed", because it doesn't pass muster. As for the rest, i actually didn't have any issue with the idea of your test. I was mostly just saying there's absolutely no reason it couldn't be conducted on earth, even if the test needed a custom cavity.

And i shouldn't have said "You", because i wasn't actually thinking you should sit down and do their work for them. So that's my bad, and the burden of proof is on them (The people who keep testing EMdrive) to write such equations and then demonstrate they have predictive power.

 

but i admit this kind of out of the box thinking, even if it doesn't lead to a drive, might lead to a better understanding of gravity or dark matter/energy. the whole thing fascinates me. 

[DDE]

10 hours ago, Dragon01 said:

Russia was in a hurry because of their (historically contentious) Chinese border, so they just went with missiles that had "growth potential", while US, being further away, decided to be more subtle.

Well, let's be honest with our hawks, there was a clearly-voiced interest in deploying them to Europe to counter US BMD assets. That, and you had the "sawed off" ICBM the Rubezh, which waa reported to have a maximum range of exactly 6000 km, which could obviously be brought lower (at one point there was a program dubbed Rubezh-Avangard, implying they wanted to marry the much heavier HGV to it) and had the same design approach as the Pioneers that the INF treaty outlawed - it was rightly mentioned during the early anti-INF campaigns from the US side, but was weirdly missing during the final Iskander-K blitz.

[Incarnation of Chaos]

2 hours ago, Nuke said:

but i admit this kind of out of the box thinking, even if it doesn't lead to a drive, might lead to a better understanding of gravity or dark matter/energy. the whole thing fascinates me. 

Pretty sure the more conventional idea for how the Piezoelectric drive supposedly works has something to do with quantum spin. So indeed, and i try to not be too stuffy about things like this.

The only way we'll ever go beyond conventional physics, the standard model, etc. Is by pushing them to the absolute breaking points, and since there's no convenient way to access black holes, neutron stars or other similar objects near the solar system anything else that looks sufficiently novel should at least get a second look.

I think the only reason I'm so hostile towards the EMdrive specifically is because of just how thoroughly it's been busted, and the even more frustrating fact that there's real propulsion technologies that could be developed with technology we have now instead. 

Everyone wants a quick fix, a silver bullet or some miracle, and honestly that to me is just missing the point entirely. We want Space for it's resources, those resources will allow further exploration etc.

Brute-force isn't pretty, but at the end of the day getting to Mars and beyond with big dumb boosters is likely to catapult us further technologically than sitting around waiting for some "Magic technology" would. Assuming we stay, and don't lose sight of the ultimate objective in the end like what happened with Apollo. And if you make that step, then you likely won't have to wait as long in the first place is the most hilarious part. 

[K^2]

17 hours ago, Nuke said:

enter mega drive, em drive's mechanical cousin. the real difference between the two is that the mega drive is built on a lot of peer reviewed science (from theory to device, while em drive attempted to go the other way). the wikipedia article is really out of date, but it does cover the theory of operation pretty well.

Yeah, that's garbage. The only reason it "works" on paper is because authors don't account for the mass flow responsible for making capacitor heavier or lighter. Gravitational mass is part of a conserved current of a fundamental symmetry from which gravity arises in the first place. Conservation of momentum follows from the same conserved current. So all they're doing is constructing a classic proof by assuming falsehood. "If we assume a violation of Poincare symmetry, we can derive this thing that violates Poincare symmetry." Well, no crap. Except that this would also mean that General Relativity isn't correct, and we have pretty good evidence for it. To be specific, when considered along with other gauge theories, such as Quantum Electrodynamics, we have better evidence for symmetries in GR than any other scientific theory. And within confines of that theory, what they're proposing there is mathematically impossible by contradicting Noether's Theorem. To properly emphasize, time travel doesn't have nearly as many constraints on it, and would be far more believable.

To show that any such device is a violation, you don't need to understand anything about the physics of the abstract device. Take the device in inert state. Take a coordinate system where total momentum of the device and its surroundings is exactly zero. Place an imaginary bubble fully enclosing device and activate the device. If device never makes it out of the bubble, then everything is fine, but it's not much of a drive then either. If device makes it out of the bubble, it must pass certain amount of mass through the boundary. That would mean that energy density at the boundary increases from zero. Because energy is part of a conserved current, it means there is momentum flowing through the boundary. (In a fixed frame, rate of change of energy at a point is equal to divergence of momentum flow.) As the device passes outside, it carries some amount of momentum with it, so this quantity of net momentum has been taken out of the sphere, and inside the sphere we now must have something containing the exact opposite quantity of momentum. If the device produced no exhaust, and the sphere is empty, that is impossible. Q.E.D.

Exhaust can take any form. It can be matter, it can be light waves, and it can even be gravity waves. But in order for you to start from rest inside an imaginary sphere, and then leave that sphere, you have to have had produced exhaust of some kind that either stays inside the sphere or passes through it in the opposite direction. Absolutely anything else would violate local Poincare Symmetry.

The other thing to keep in mind is that if exhaust has no rest mass, such as light or gravity waves, then we have a very simple formula for minimal energy use of such a drive. For massless fields, E = pc, so you expect about 300MW of power for 1N of thrust. And that's the absolute best you can do for any drive if you don't bring propellant with you.

[wumpus]

On 9/12/2020 at 5:55 PM, Dragon01 said:

Yeah, that sort of thing is why I'm worried about KSP2 featuring metallic hydrogen doing the same thing to that theory. Myths, especially too good to be true hypothetical technologies, die hard. Cold fusion still has its proponents, too (Stellaris fell into that trap, though luckily it doesn't sell itself as hard SF). This is a waste of taxpayer money, and a good example of bad science doing real harm.

Anchoring bias.  The first time people hear about it is the excited "breakthrough" reported from the researcher's PR agency.  Anything failure to reproduce the effect is dismissed as "git gud".  Not to mention that anyone seriously pushing any such nonsense has a significant chance of absolutely refusing to admit their (ok, typically "he's") wrong.

My guess is metallic hydrogen will be in KSP if it fills a needed place in the tech tree and won't be included if not.  There's always the chance than one of the devs really likes the idea of metallic hydrogen (hopefully knowing full well it won't work that way) and we're stuck with it.  We can always push for the term "zip fuel" (what they called the hypothetical high-Isp fuel in Ignition!), but I suspect the devs would prefer a more well known (and googleable) term.  If you want realism in KSP, you need RSS/RO.  Otherwise you have to live with little green men.

[kerbiloid]

If attach two similar inertialess EM-drives with backs and switch on, they stop violating the conservation laws, lol.

[Guest]

3 hours ago, wumpus said:

Anchoring bias.  The first time people hear about it is the excited "breakthrough" reported from the researcher's PR agency.  Anything failure to reproduce the effect is dismissed as "git gud".  Not to mention that anyone seriously pushing any such nonsense has a significant chance of absolutely refusing to admit their (ok, typically "he's") wrong.

That depends. Sometimes, even something overtly optimistic (but plausible at the time) is picked up, and even if it's later disproven, it lingers on. There's a good chance we'll be eventually be laughing at "realistic" Alcubierre drive depictions the same way we do at metallic hydrogen. At least that is somewhat understood as far-fetched and nearly (but not definitely) physics-breaking. 

3 hours ago, wumpus said:

My guess is metallic hydrogen will be in KSP if it fills a needed place in the tech tree and won't be included if not.  There's always the chance than one of the devs really likes the idea of metallic hydrogen (hopefully knowing full well it won't work that way) and we're stuck with it.  We can always push for the term "zip fuel" (what they called the hypothetical high-Isp fuel in Ignition!), but I suspect the devs would prefer a more well known (and googleable) term.  If you want realism in KSP, you need RSS/RO.  Otherwise you have to live with little green men.

You can't get the same performance out of a zip fuel, this tech is in liquid core NTR range of Isp. Liquid core NTR, with its own mechanics, would actually be an awesome addition. Also, from the last video on that I suspect they don't know if it doesn't work. I don't think they read this particular forum (nevermind material science journals), and nobody had a chance to talk to them since the discovery was made. We need something like Snark's interview or an AMA, and someone knowledgeable in place to bring this issue up.

[magnemoe]

8 hours ago, kerbiloid said:

If attach two similar inertialess EM-drives with backs and switch on, they stop violating the conservation laws, lol.

More fun, add one on an long arm in vacuum, as its power use and acceleration is constant while energy in device is square of velocity* mass once you spin it up fast enough you can start bleed of extra energy with an generator at the pinon.  
Yes this has some challenges like physic. On the other hand its legal to break physical laws  

[Clamp-o-Tron]

On 9/13/2020 at 5:18 AM, kerbiloid said:

To make the EM-drive work, you just need to use metallic hydrogen superconductors.

  Reveal hidden contents

 

At room temperature. While sprinkled with memory water.

[Incarnation of Chaos]

On 9/14/2020 at 4:44 AM, K^2 said:

 For massless fields, E = pc, so you expect about 300MW of power for 1N of thrust. And that's the absolute best you can do for any drive if you don't bring propellant with you.

300MW PER NEWTON!?

At that point you're better off just using the colossal nuclear/fusion reactor you have to boil water and send it out the back!

[Bill Phil]

19 minutes ago, Incarnation of Chaos said:

300MW PER NEWTON!?

At that point you're better off just using the colossal nuclear/fusion reactor you have to boil water and send it out the back!

From an energy used per unit thrust perspective, yes. But a photon rocket should have the highest possible specific impulse.

On 9/14/2020 at 4:44 AM, K^2 said:

The other thing to keep in mind is that if exhaust has no rest mass, such as light or gravity waves, then we have a very simple formula for minimal energy use of such a drive. For massless fields, E = pc, so you expect about 300MW of power for 1N of thrust. And that's the absolute best you can do for any drive if you don't bring propellant with you.

Only in the case of rockets. A photon rocket may not use physical propellant in the same way a conventional rocket does but it does use mass to generate thrust. 

There are other propulsion schemes, however. For example, photon sails get 150MW per N, because of the reflection. Some concepts create a resonance cavity between the sail and the origin and can get hundreds of kW per N. And in theory a vehicle could reflect an incoming mass stream and get good performance as well. 

Such drives are not rockets, and can achieve better thrust per unit power than photon rockets. This is because photons are extremely inefficient at transferring momentum. This can be improved by reflecting photons as many times as possible before they become impossible to reflect or by using particles with mass.

Within the context of interplanetary travel a network of beam stations (likely firing a mass stream but laser beams are possible too) could provide reasonably fast travel. And in theory such technology could be scaled up to provide interstellar launch capability as well, though that would be a far future endeavor.

[wumpus]

1 hour ago, Incarnation of Chaos said:

300MW PER NEWTON!?

At that point you're better off just using the colossal nuclear/fusion reactor you have to boil water and send it out the back!

Dawn used 25kW per Newton, and holds the record for the most delta-V of any spacecraft ~10km/s.  It is quite possible that a deepspace probe would burn through a first stage of Argon, then a much smaller second stage of Xenon, and finally use photons until the nuclear power ran out.  To be honest, when using photonic drives, make sure your heatsinks are design with the black sides facing in the direction you want to thrust.  They will be a significant part of your thrust (i.e. you aren't going to have much at all).

[kerbiloid]

1 hour ago, Incarnation of Chaos said:

300MW PER NEWTON!?

Think positive. 1% of heat waste will melt the rocket way before.

[K^2]

3 hours ago, Bill Phil said:

There are other propulsion schemes, however. For example, photon sails get 150MW per N, because of the reflection.

Right. Inherent assumption in all of the derivations above is that your vessel starts out as a closed system in vacuum. If you have external medium or external power source, that changes everything. Extreme case being a solar powered airplane, which requires neither its own power source nor propellant to generate significant thrust.

For a more interesting example, modern field theory predicts huge zero-point energy of vacuum (in fact, seemingly impossibly large, but that's another story,) but it's fundamentally impossible to push from it. Explanation for why you can't push off from vacuum despite it having mass until you excite it is a bit mathy, but in a nutshell, excitations are what carries momentum, and these excitations are also presented as particles, so you need to either have or create particles to push from. However, if we are missing something in Standard Model, there might be something in zero-point vacuum that we can push from. That would allow not truly reactionless, but certainly propellantless drives. It seems extremely unlikely that such a feature exists, because we'd be almost guaranteed to see some side effects, but it's hypothetically possible within framework of fundamental principles we understand to be true. It would kind of put it on the same level as an idea of hyperspace. Hypothetically possible, but entirely speculative, with absolutely no evidence for it found.

At this point, we have good enough coverage on experiments that I expect any novel propulsion method, if ever discovered, to come from sound mathematical model first. It's not going to be something we stumble on by accident. And with things like EM Drive, whose "theory" is entirely bogus, if it were to work, it would be entirely by accident. We don't have theoretical framework that predicts that something like this should work.

[Incarnation of Chaos]

5 hours ago, wumpus said:

Dawn used 25kW per Newton, and holds the record for the most delta-V of any spacecraft ~10km/s.  It is quite possible that a deepspace probe would burn through a first stage of Argon, then a much smaller second stage of Xenon, and finally use photons until the nuclear power ran out.  To be honest, when using photonic drives, make sure your heatsinks are design with the black sides facing in the direction you want to thrust.  They will be a significant part of your thrust (i.e. you aren't going to have much at all).

1MW = 1000kw, so one MW could power 40 dawns...300MW would be 12,000!

I was mostly commenting on the absolutely abysmal energy/thrust ratio more than anything; ion drives don't produce much thrust (Isn't it in fractions of Kiloneutons?) but they use far less energy to achieve it. So i was mostly saying that if 1MW=1N for a potential "Reactionless" drive, and that's the limit not a likely achievable goal. Then you're better off just doing literally anything else with such mind boggling quantities of energy. Especially since current ion drives would be improved by being able to pump that much energy in them.

6 hours ago, Bill Phil said:

 

Only in the case of rockets. A photon rocket may not use physical propellant in the same way a conventional rocket does but it does use mass to generate thrust. 

There are other propulsion schemes, however. For example, photon sails get 150MW per N, because of the reflection. Some concepts create a resonance cavity between the sail and the origin and can get hundreds of kW per N. And in theory a vehicle could reflect an incoming mass stream and get good performance as well. 

Such drives are not rockets, and can achieve better thrust per unit power than photon rockets. This is because photons are extremely inefficient at transferring momentum. This can be improved by reflecting photons as many times as possible before they become impossible to reflect or by using particles with mass.

Within the context of interplanetary travel a network of beam stations (likely firing a mass stream but laser beams are possible too) could provide reasonably fast travel. And in theory such technology could be scaled up to provide interstellar launch capability as well, though that would be a far future endeavor.

Are we talking Solar Sails? Star Drives? Or using actual Laser beams as the propulsion?

2 hours ago, K^2 said:

At this point, we have good enough coverage on experiments that I expect any novel propulsion method, if ever discovered, to come from sound mathematical model first.

This is what i was trying to say, but rather crudely at the beginning. There's just no real justification for any potential propulsion method to not propose a model to explain itself that lies within conventional physics.

[magnemoe]

7 hours ago, K^2 said:

Right. Inherent assumption in all of the derivations above is that your vessel starts out as a closed system in vacuum. If you have external medium or external power source, that changes everything. Extreme case being a solar powered airplane, which requires neither its own power source nor propellant to generate significant thrust.

For a more interesting example, modern field theory predicts huge zero-point energy of vacuum (in fact, seemingly impossibly large, but that's another story,) but it's fundamentally impossible to push from it. Explanation for why you can't push off from vacuum despite it having mass until you excite it is a bit mathy, but in a nutshell, excitations are what carries momentum, and these excitations are also presented as particles, so you need to either have or create particles to push from. However, if we are missing something in Standard Model, there might be something in zero-point vacuum that we can push from. That would allow not truly reactionless, but certainly propellantless drives. It seems extremely unlikely that such a feature exists, because we'd be almost guaranteed to see some side effects, but it's hypothetically possible within framework of fundamental principles we understand to be true. It would kind of put it on the same level as an idea of hyperspace. Hypothetically possible, but entirely speculative, with absolutely no evidence for it found.

At this point, we have good enough coverage on experiments that I expect any novel propulsion method, if ever discovered, to come from sound mathematical model first. It's not going to be something we stumble on by accident. And with things like EM Drive, whose "theory" is entirely bogus, if it were to work, it would be entirely by accident. We don't have theoretical framework that predicts that something like this should work.

Yes the real problem is not propellant free drives. Its having something to push against. 
And you can not get and constant force and acceleration over time with say 100 kw power. 

Accelerate an car, as it go faster the the g force goes down as the kinetic energy increases with square of velocity. 
Yes you also have air resistance and gearing who ultimately limit your top speed, but this works the same way braking, the braking force is constant so braking length is four time longer for an twice as high speed. 
The constant acceleration with fixed power use made the em-drive an perpetuum mobile 

Rockets has constant acceleration because of they uses reaction mass. 
 

[Nuke]

im also somewhat curious if reactionless drives (not these ones, but something based on a better understanding of physics) might be an explanation for the fermi paradox. we don't see the aliens because they aren't constantly firing nuclear bombs out the tail pipe. they would leave no external signatures beyond their waste heat. need not be strictly reactionless drives either, perhaps drives that operate on the interstellar medium like bussard ramjets or space propellers (sort of like the turbomolecular pumps used in vacuum chambers, or operating on the medium electromagnetically). 

Edited September 16, 2020 by Nuke

[kerbiloid]

I strongly believe that no progress in propulsion will come from astronomy and mathematics until we start performing physical experiments at astronomical distances after sending a lab to the Oort and beyond, and get new astronomy and new math theories from it.

And from the brain vivisection to understand the relation between the mind, the "physical observer" presence, the memory, the physical (in)determinism, and so on.
(Not necessary right on humans, they can start from mice, see Pinky and Brain)

Until that the astronomy will keep producing self-predicting results, and pure mathematics will keep replenishing the cemetery of nice mathematical theories, each one so truthful and promising, like the Sun producing light from the gravity on shrinking.

Edited September 16, 2020 by kerbiloid

[K^2]

2 hours ago, magnemoe said:

Rockets has constant acceleration because of they uses reaction mass. 

I don't think a lot of people appreciate that about rocket propulsion and rocket equation. The power output of a rocket increases with velocity, thanks to the kinetic energy of propellant being used as additional energy source. It's all kind of counter-intuitive, and the idea that rockets must have fixed power output, and therefore can't go much faster than the exhaust, was pretty popular.

These days, we're used to having rocket equation as a tool and the fact that rockets, well, work, so we tend not to even think about this bit that would have made some otherwise competent engineers in the olden days argue that reaching orbital speeds with a rocket is impossible.

[Incarnation of Chaos]

15 hours ago, Nuke said:

im also somewhat curious if reactionless drives (not these ones, but something based on a better understanding of physics) might be an explanation for the fermi paradox. we don't see the aliens because they aren't constantly firing nuclear bombs out the tail pipe. they would leave no external signatures beyond their waste heat. need not be strictly reactionless drives either, perhaps drives that operate on the interstellar medium like bussard ramjets or space propellers (sort of like the turbomolecular pumps used in vacuum chambers, or operating on the medium electromagnetically). 

My favorite is that they're all in different universes laughing at heat death. Even if there aren't parallel universes extent when they become a Type III civilization (Yes i know the K-scale isn't perfect, but i like it as a rough measure); the math still works (As far as we know; hehe). So all they do is create one, and let it cool while they chill in massive jupiter brains for billions of years. Then once the conditions are right, they bail out and repeat the process. An endless loop of recursive universes, and if we assume this civilization would be there from relatively early then anything that could evolve billions of years later wouldn't even have the privilege of seeing them. They'd already skip out to their next home, and any sentient life billions of years later would be none the wiser.

 

[damerell]

On 9/14/2020 at 7:58 PM, Dragon01 said:

That depends. Sometimes, even something overtly optimistic (but plausible at the time) is picked up, and even if it's later disproven, it lingers on. There's a good chance we'll be eventually be laughing at "realistic" Alcubierre drive depictions the same way we do at metallic hydrogen. At least that is somewhat understood as far-fetched and nearly (but not definitely) physics-breaking.

Er - relativity, causality, FTL, pick two; doesn't matter how the FTL supposedly works.. The Alcubierre "Drive" is very definitely "physics-breaking"; it is an interesting mathematical model of something impossible.

[K^2]

3 hours ago, damerell said:

Er - relativity, causality, FTL, pick two; doesn't matter how the FTL supposedly works.. The Alcubierre "Drive" is very definitely "physics-breaking"; it is an interesting mathematical model of something impossible.

Global causality is overrated and not strictly necessary in field theory. Ability to build FTL drive absolutely implies ability to build a time machine, but there is nothing fundamentally wrong with the later.

Stating anything with certainty would require better understanding of quantum gravity than we possess, but taken individually, General Relativity allows time travel and Quantum Mechanics has mechanisms for resolving time travel paradoxes. I can't guarantee that something doesn't break spectacularly when you put the two together, but given that existing models don't have a problem with time travel, insisting that it's "physics-breaking" is at very least completely unfounded.

Alcubierre metric is not it, however; that much I can agree on. But it was only ever meant as a simple toy model, and expecting it to be a practical drive is no more reasonable than trying to build a gasoline engine from schematic representation of one in thermodynamics textbook.

[Guest]

4 hours ago, damerell said:

Er - relativity, causality, FTL, pick two; doesn't matter how the FTL supposedly works.

I'll take relatively and FTL, then, causality we can do without, as demonstrated by most modern news media.  Seriously, though, the entire reason this sort of thing was even brought up was that there's nothing in current physics that'd explicitly forbid it from working. It does mess with causality, but it's less of a problem than commonly thought, at least as far as field theory goes.

Edited September 27, 2020 by Guest

[damerell]

2 hours ago, K^2 said:

Global causality is overrated and not strictly necessary in field theory. Ability to build FTL drive absolutely implies ability to build a time machine, but there is nothing fundamentally wrong with the later.

That seems completely extraordinary. We have no evidence whatsoever of causality violation - and, while this is a slightly pat argument, if _is_ possible, it's not just that it hasn't been done _yet_. To suppose it is possible for the sake of not recognising FTL is impossible is not sensible.

If any leg of that triangle is going to give, I'd go for relativity, which does at least have a few problems around the edges like all this spooky dark matter whizzing about. The oddball theories that actually galaxies stick together because things are slightly different at very long distances are plausibility personified compared to causality violation.