Scientists Are Starting to Take Warp Drives Seriously
sciencealert.com
sciencealert.com
No they aren't. This is just a fluff piece for a highly speculative but excitingly sci-fi sounding propulsion mechanism with not even the remotest prospect of practical implementation. See also EM Drive, etc.
https://en.wikipedia.org/wiki/Causality_(physics)
Anyone who wants to build a warp drive has to start by proving Einstein wrong. Anything else is a scam.
This creates nasty phenomena that we don't seem to observe in nature (i.e. if the third party observes effect before cause, they can interfere with cause. Everyone loves a good temporal paradox ;) ).
http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
I don't understand the problem here. The ship couldn't call Earth before Earth placed the call. It would see the call being received before Earth placing the call, but if it then called up Earth on their FTL phone and said "hey, don't make that call to Proxima Centauri we just saw you make," wouldn't Earth just reply "Uh, we already made the call, you seeing old light doesn't mean these events didn't already happen." Why does it matter what the third observer sees? Cause and effect aren't violated just because it can appear that way.
The source doesn't do the math on the final step, but you can arrange the third observer so they emit light that reaches Earth before the phone call is sent (because we are assuming FTL tech).
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Now, you might say "wait, light takes a finite amount of time to travel. You've just shown what times the spaceship will assign to various events, but they can't see it immediately. That'll save us!" Sadly no. Here's when the ship actually gets the light from the events. [complicated figure, but it shows there's enough time in the light-cone chart for the ship to receive the 'Proxima received the phone call' event and then travel to Earth slower than speed of light and tap Earth on the shoulder before the phone call was sent]
As you can see, the light from the phone call reception arrives well before the light from the placing of the phone call. Again: causality is violated.
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In fact, that message can be "Place the call," which means the call is placed because Earth was told to by the third party because the third party knew the call had to be placed because they observed the effect because the call was placed... FTL allows for closed-causal loops.
"As you can see, the light from the phone call reception arrives well before the light from the placing of the phone call. Again: causality is violated."
It's still only speaking about the perspective of the ship, and it seeing effect before cause.
Yeah, it's tough. I don't think I can explain it without bootstrapping a college semester of relativistic physics, and I'm afraid I'm not that good. :(
To start your search, "Objectively happens" is the intuition that doesn't hold water in relativity. There is no objective frame of reference (i.e. nothing in the universe is moving at 'speed zero', or more precisely, everything moves at 'speed zero' relative to itself). So everything is relative; there's no place anyone can stand and observe things objectively. Relativity changes the rules upon which reality operates so they hinge, loosely, on two fundamentals:
1) The speed of light, in a vacuum, must be observed to be the same by all observers
2) Observers do not agree on the times that they measure for when events occurred (for example, the "moving train" thought experiment shows that simultaneity is violated by relativity), but they can agree that the events align to each other subject to the Lorentz transformation when relative velocities are accounted for.
Under these rules, causality is maintained; I don't have the whole proof at my fingertips, but it can be shown that regardless of how you apply Lorentz transforms to sublight-velocity observers, they'll agree that events that caused one another have the same ordering (this is a subtlely different statement than "Two things happened at the same time," and it's partially a property of the events that are effects being within the 'light cones' of the effects that are causal). FTL travel allows one to exceed the "light-cone limit" and as a result, the causality constraint that 'effects are in the light cones of causes' is violated. The frame of reference where one event caused the other exists (i.e. there are velocities one could have where the light cones will line up that way), but there are also now velocities one can have where the light cones do not line up that way. It's only impossible for any observer to see effects happen before their causes if nothing can exceed light-speed.
For your specific question ("What mechanism allows for knowledge of the effect before the cause objectively happens?"), I think I can offer a short hypothetical thought experiment that might illuminate things. Imagine there were a door from Earth to Mars allowing instantaneous transit (so infinite velocity, in excess of speed of light). One day, the sun blinks out of existence. Earth will see this occur three light-minutes before Mars does. Someone steps through the door and yells "People of Mars! I come with a warning! In three minutes, the sun will go out! Evacuate now!"
From the Martian point of view, that person is a time traveler from the future, and the intuition relativity brings to us is that the Martian point of view is as "objective" as any other point of view. This (Lorentz-transform-violating) visitor has knowledge of an event that will definitely occur in three minutes before the cause of that knowledge has occurred.
There is a possibility antimatter could fit the bill for the "exotic matter" required in the model (we've never accumulated enough of the stuff and held it in a controlled environment long enough to do any tests on how gravity interacts with it), but I wouldn't bet money on that outcome yet.
Until a way is found to remove the need for exotic matter or to generate exotic matter it remains a purely theoretical exercise.
If you can access it, the doi link to the conference paper is: doi.org/10.2514/6.2019-4288
This is not a serious attempt, and in no way indicates that scientists are starting to "take warp drives seriously." It is pitching that same silly stuff White et al have been promising is "real close now" for around 15 years. I also cringed at the equations and figures that were obviously copy/pasted from White's PDFs to Word and back.
Having this kind of kooky thing hanging from a baby undergraduate's name--a name that I'm not going to propagate on the web for his own good--is not going to help his future career, and everyone involved should feel bad for wasting his time. Sorry this is boiling my grits so badly, am I over reacting or is this awful?
I'm sure it'll be informative to learn exactly why every theoretical avenue for faster-than-light travel doesn't work, but I feel like we can infer from the size of the universe that these effects don't exist; if they did, they'd dominate our observations of the universe and we'd never have thought the speed of light was a fundamental limit in the first place.
I have no affiliation with Elite: Dangerous, that's just how I learned about Alcubierre Drive
https://news.ycombinator.com/newsguidelines.html
Just a reminder.
But another valid way of looking at it is that scientists have moved it from "impossible", to "impossible", to "impossible", then, with some more work, upgraded it to "impossible". Or, in other words, no progress at all.
The negative mass is still only one problem with the Alcubierre drive concept. Last I knew, it remains unclear how to enter and/or create a bubble without being totally destroyed, or exit and/or destroy a bubble without being totally destroyed. Reuse of any of the components is probably also a problem; in the Einstein equations, playing fancy games with spacetime tends to want to rather explosively go back to normal with the entire mass-energy of the distortions in question. It's also somewhat unclear what would happen to an Alcubierre warp drive in the real universe, where the space between stars is not a perfect vacuum.
It's not a drive being taken "increasingly seriously", unless you mean it's gone from epsilon to twice epsilon. It's a particular solution of the Einstein equations that involves impossible quantities of things and a particularly complicated setup basically already existing. If we didn't have science fiction making FTL drives cognitively available to people's imaginations and perhaps even subconsiously bleeding over into people's impressions of what real is (i.e., the bizarre but clearly pervasive subconscious assumption that seeing something in a sci-fi show means the probability of that occuring in real life is higher), nobody would be taking this seriously right now. Between the actively impossible elements (sustained, enormous quantities of negative mass) and the things that may not be mathematically "impossible" but are probably engineering-impossible, this is just a thought experiment right now.
Now, for all that, it's a worthy thought experiment. I am a firm believer in putting down a bit of money on the very long-shot payoff research. I'm not asking anyone to stop working on it. I'm just asking for realistic assessments of the current state of the art, which is that the probability that this drive will ever work is basically indistinguishable from zero at this point.
I think that description used to apply to a lot of technology, which then progressed to "almost impossible", "slightly less impossible", and finally to "built a prototype". From my laymans' perspective, it's the trajectory that matters more than the current state-of-the-art.
We have every reason to believe that stable negative mass is impossible. Not just "we don't know how to do it yet", but impossible. Impossible is like "perfect"; technically, it doesn't admit of "degrees" of impossible. So moving from "impossible" to "impossible" is not progress.
People like to cite a lot of cases of various supposed boundaries being broken over time as evidence that maybe this one will be broken too, but there's a qualitative difference between some "elderly distinguished scientist" opining something is impossible, and the mathematics of physics saying something is impossible. It is not a sophisticated, open-minded position about the technological possibilities of the future to say that someday, man will break the barriers of the laws of thermodynamics and someday produce the perpetual motion machine; it is ignorance and scientific illiteracy. FTL is not quite that certain yet, but at the moment, the smart money is on it being the exact same sort of thing, not humanity someday overcoming it. At the moment I'd say that if you properly understand the science of the matter and just how thoroughly reality seems to stymie us in our every attempt to worm around the speed of light restriction, you are completely unjustified in giving even a .1% chance of FTL being possible, let alone that humanity will ever achieve it. It looks to be a lot closer to perpetual motion than breaking the sound "barrier", which barely even deserves the same English word as the speed of light barrier given their massive qualitative differences.
Now, as I say, the long shots sometimes pay off, so I don't advocate that nobody thinks about this. Even the process of discovering why the <.1% is better thought of as a flat 0% can be valuable scientific progress, plus there is always the chance I'm wrong. However, at the moment, it looks like FTL is a problem that is far harder than just waving a couple of hoary old quotes about scientists at it is going to solve. You're not fighting "scientists", but the math.
Personally, I tend to think Hawking probably got it right with his chronology protection conjecture, and that even if you do manage to build something that goes faster than light or travels back in time, the entire system will literally explode. FTL may not be just impossible because we don't know how to build it, but because it really is fundamentally impossible; spacetime will literally explode in your face even if you do manage it.
https://en.wikipedia.org/wiki/Chronology_protection_conjectu...
1) Negative mass, well, if you take one impossible thing you can make other impossible things happen. So you've just shuffled your impossibles around.
2) None of this still tells me what happens when you create a closed timelike curve and attempt hijinx.
> People like to cite a lot of cases of various supposed boundaries being broken over time as evidence that maybe this one will be broken too, but there's a qualitative difference between some "elderly distinguished scientist" opining something is impossible, and the mathematics of physics saying something is impossible.
I appreciate your distinction in the use of the word 'impossible'. I didn't actually see what you meant by it before. From what I remember from learning about the development of airplanes, it was considered physically impossible until proven otherwise. It wasn't an elderly scientist saying planes can't fly, it was the best science of the day declaring that something heavier than air was meant to stay on the ground (IIRC).
I'm simply skeptical of claims of impossibility, across the board. I do appreciate and understand the science of the reasoning behind the claim; but to a layman who knows a bit of the history of science, 21st century scientists declaring things scientifically impossible (because science has progressed so far since 100 years prior) sound eerily similiar to 19th and 18th century scientists saying the same thing, for the same reason.
But we don't even know if it's theoretically possible to create a baseball amount of negative energy. As long as that question hasn't been answered, any discussion about practical applications seems rather pointless.
"We may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense."
-- Albert Einstein
Quantum gravity may give us a way out but so far we've achieved little progress, mainly due to the very same stress-energy tensor being a different type than those found in other quantum fields. Viktor Roth provides a simple explanation of the problem: https://www.forbes.com/sites/quora/2017/05/17/the-marriage-o.... If you're into physics Viktor is a fun guy to follow on Quora.
Applying the term "aether" to other theories is uncommon, especially among non-physicists.