8 ms·

This is my semi-educated guess as to what's going on here. Beware that semi-educated guesses are sometimes worse than uneducated ones, since they are more plaus

This is my semi-educated guess as to what's going on here. Beware that semi-educated guesses are sometimes worse than uneducated ones, since they are more plausible and not necessarily more correct. The Alcubierre warp drive requires both positive and negative curvatures. Positive curvatures are "easy" to make, since mass and energy bend space positively. Negative curvatures are typically said to require negative energy to produce, but the full story is thankfully more complex. The universe, despite being filled with mass and energy, is roughly flat. That means that the natural bending of spacetime, without any stuff inside it, has to be negative - otherwise all the mass would make it extra positive. If you believe the consequence of vacuum energy in QED, that the fields filling the universe are VERY heavy, even when no particles are around, then you must believe the natural curvature to be very negative indeed. So, it stands to reason, if you want to bend the universe backwards, take the weights off of it - really off of it, in this case using metal plates to forbid certain vacuum fluctuations, and let it unbend itself.

dotancohen 5y ago
> Beware that semi-educated guesses are sometimes worse than uneducated > ones, since they are more plausible and not necessarily more correct. This is one of the most insightful comments I've read on the internet ever. This is why we shouldn't take medical advice from people who have read a few Wikipedia articles. This is why years of experience in e.g. software are necessary to even begin to understand how deeply hidden security issues can be. This is why self-driving cars still elude us, but will be so much safer than human drivers when they finally do arrive.
- dekhn 5y ago
  The way I state the above point is: "The smartest people in the room are the most skilled at convincing people (including themselves) of a narrative that sounds logical but is wrong".
- sgt101 5y ago
  >This is why self-driving cars still elude us, but will be so much safer than human drivers when they finally do arrive. Isn't that just a by definition thing? My Tesla can already drive itself, I don't trust it and I don't think it's safe. When I get one that I trust and is safe then it will be safer than me, and by extension other humans. The problem is that the current generation of algorithms are brittle.
  - blamestross 5y ago
    The "just put a brick on the gas pedal" algorithm was particularly prone to crashing.
    - dotancohen 5y ago
      Yes, but it had the lowest CPU and memory footprint! There's also something to be said about the COTS hardware requirements and ease of installation.
  - deleted 5y ago
    [deleted]
- jl6 5y ago
  > This is one of the most insightful comments I've read on the internet ever. On the other hand, if I’m in the market for a guess, I’ll probably go for the educated guess rather than the uneducated one, because even though I recognize it’s not necessarily more correct, it probably is less wrong.
  - ALittleLight 5y ago
    Sometimes it's better to know you don't know than to think you know. If it's midnight and you're standing at the edge of a cliff you're better off thinking that you have no idea what's ahead of you than if you listen to a guy with "better than average night vision" telling you that there's a sidewalk ahead.
    - MarcoZavala 5y ago
  - Beltiras 5y ago
    Being less wrong is probably as good as it ever gets.
  - mnsc 5y ago
    If you are, as you say, "in the market for a guess" that would imply that you are actively looking at different guesses and comparing the credentials of the guessers. Then you would _probably_ be right to choose eg. the guess from the virologist over the proctologist or the mechanic when you need a guess on how the Covid situation will unfold. But the issue might be that an "educated guess" in itself could be worse because you wouldn't base any major decisions on a mechanic that present an "uneducated guess" that "the virus will die down in a month" but if you have a crackpot chiropractor presenting themselves as a "certified doctor with an expertise in everything concerning the body" that says the same thing, you might be worse off in saying that this "educated guess" is worth basing your travel plans on.
    - heavenlyblue 5y ago
- baq 5y ago
  you might be interested in Kahneman's body of work, especially 'Thinking, Fast and Slow'.
- belter 5y ago
  I am afraid many times when talking about educated guesses we mean schooled guesses. The problem is that, more than once, we learn per Mark Twain's famous quote ...that schooling has a way of interfering with education...:-) Specially for subtle but critical concepts: "Researchers misunderstand confidence intervals and standard error bars" https://pubmed.ncbi.nlm.nih.gov/16392994/ https://pubmed.ncbi.nlm.nih.gov/16392994/
- armchairhacker 5y ago
  I was thinking the opposite point: sometimes the experienced doctor or researcher made a mistake, and the guy who posted something on Reddit is correct. Usually not, but sometimes. Generally speaking, you should trust someone whose professional or experienced more than some random stranger. Especially if it's not one person but a large group of experienced professionals (most of the controversial "anti-science" issues today). But it's actually important to "do your own research", and not just listen to authority. Provided you actually know how, i.e. you can find accurate sources and distinguish fact from fiction. Because even the most qualified, unbiased, genius authority are occasionally wrong. The big issue today is that a lot of people don't know how to research and distinguish lies, but (although it's probably impossible for most) finding a way to teach someone how to find the truth, will always be more effective having some authority tell them the truth. Key example: doctors often miss symptoms and diagnoses that a patient can discover on their own. It's not that the doctor is stupid or unqualified, in fact they have way more medical knowledge than you. But the doctor only has a series of questions and maybe a few tests to make a diagnosis, whereas you have the full experience of symptoms and an internal understanding of your "normal". IMO people actually should "play Internet doctor" and research their symptoms, not to reach any 100% conclusions, but to suggest diagnoses and tests to their real doctor.
  - dotancohen 5y ago
    > IMO people actually should "play Internet doctor" and research their symptoms, not to reach any 100% conclusions, but to suggest diagnoses and tests to their real doctor. I agree, and do this. But I never tell the doctor "I suspect that I have X", rather I tell the doctor "I've observed J, K, and L under Foo conditions" and let the doctor decide where to go from there. In short time we can learn to give the doctors better information, but we cannot replace their breadth of knowledge.
    - armchairhacker 5y ago
      Yeah sometimes people say "doctor I have X, I'm trying to get diagnosed with X". You're biasing the doctor to give you a diagnosis so you don't drop them, and you're biasing the doctor to ignore your symptoms because you're just saying them to get the diagnosis and maybe you just want to get certain medication. Even if you don't outright say it, you should never convince yourself you have a diagnosis. The human body is extremely complex and some particularly weird issues need patient research, doctor's research, and experimentation to solve.
- codexcodexcodex 5y ago
  He's just saying n=1. Medical advice on the internet can be n=1, but read enough to get to n=100+, while still limited, can be quite valuable. I can't ask 100 doctors what’s wrong with me, but I can ask my question to hundreds in an online forum. They might lead me astray, but so can a single doctor’s misdiagnosis. A doctor can prescribe treatment that helps or harms, but I can ask a forum of people, with the same problem I have, if I should take his advice. I can't, however, ask 100 of his colleagues or peers if I should trust his advice, and that's why we end up with so many charlatans in medicine. But he's a doctor. Of course you can trust him! I don't know about the rest of you, but I've found a fair number of doctors to be mostly useless.
ToJans 5y ago
The biggest issue - based on your explanation - seems to be the need for negative energy... I might have a non-educated theory that could explain negative energy; could you tell me if this is potentially correct or am I completely wrong here? Disclaimer: I haven't read the article (it doesn't load), and I am (probably) lacking even the basics of the required knowledge. Let's assume the law of energy conservation applies to the universe: if you assume the total energy in the universe is constant, the negative energy is a relative imbalance instead of an absolute one, caused by the universe expanding at a different speed relative to the "things" it contains. As the universe is "expanding" constantly, the "energy amount per expanding area" gets "less dense", so the "things" that are not "expanding" at the same speed get "more dense" in comparison. This imbalance creates different types of energy perturbation. From the point of view of the things that do not expand as fast, the energy levels relative to yours seem to evolve in a "negative way". It's like looking at a stationary train from a train that is moving: from the point of view of the moving train, the train standing still seems to go backwards. Does this make any sense at all, or am I completely missing the mark here? Updated: reworded some things and provided extra context.
- MaxikCZ 5y ago
  As another complete layman just pooling my intuition: Relatively more or less dense "things" (to each other) will always generate positive curvature of spacetime. What you want is relatively less dense area than vacuum in another region of space, since vacuum is (for now) what we constitute as (somewhat) flat. That's what Casimir effect tries to accomplish: define a small enough region of spacetime where some larger quantum fluctuations don't have enough space to happen, thus lowering its energy below vacuum. Since the idea is to define small enough space-like interval in spacetime to curb some energies off, could defining small enough time-like interval help the same purpose, multiplying the former?
  - ToJans 5y ago
    Thanks, this makes a bit more sense to me now, but let's figure out if I understood it correctly: The Casimir effect is about creating a spacetime "corset" so to speak, that is so tight that even the regular quantum fluctuations don't have "enough room to pass", so they "almost freeze", and when they "come out on the other end", they could for example break or delay entanglement? On the other hand I think your assumption about the time aspect might be correct as we are talking about spacetime after all. The biggest issue might be that measuring time deltas might be harder than space deltas?
raducu 5y ago
> using metal plates to forbid certain vacuum fluctuations Just curious, why can't fluctuations happen inside matter? Isn't matter 99% vacuum anyway?
- XorNot 5y ago
  They can and do, and are an important part of intermolecular interactions (van der Waal's forces have a substantial Casimir effect component). Matter just ordinarily isn't structured to produce a strong version of an interesting effect - which when you get down to it is essentially what all human technology is based on. It's quite possible there are some weird molecular structures (if this paper is correct) which could also induce natural warp-field structures out of their crystal lattice.
  - l33tman 5y ago
    Going down this route, it strikes me as more feasible to find lattice effects or nano-scale effects facilitating cold fusion than warp fields. After all fusion itself is at least provably existing, just difficult to arrange.
    - zardo 5y ago
      https://www1.grc.nasa.gov/space/science/lattice-confinement-fusion/ https://www1.grc.nasa.gov/space/science/lattice-confinement-... They don't use the term 'cold fusion', and obviously the fusion sites would be hot, but the bulk material isn't.
wanderingmind 5y ago
While I understand your reservations, I think you are really underselling the power of Ansatz. Ansatz reduce the search space of possible solutions because it adds various constraints that makes it possible to get to solutions much faster. For example, without these constrained searches we will never have developed any analytical solutions to differential equations that are pervasive across science, engineering, finance among others.
Certhas 5y ago
As a trained physicist, I think you are getting this relatively right. Some of the issues at play are simply unknown according to the current state of theoretical physics. Our Quantum Field Theories naively predict an infinite energy density all throughout space time. A naive guess at regularizing this leads to absurd predictions 120 orders of magnitude off [1]. So there has to be something else happening here. One of the things that supersymmetry brings to the table is that it makes this energy density exactly zero (but then it get's complicated because you need to break super symmetry...). The thing is though, without gravity the absolute energy density of a quantum field is just some unobservable number. But how to reconcile gravity and quantum field theory is an unsolved problem to begin with, so it's completely unclear what that naive infinite number actually means. We are working without a firm theoretical (or empirical for that matter) foundation here. However, the Casimir effect is remarkable exactly because it appears that the local energy density between two conducting plates is lower than in the ambient space. This is an energy difference that can and has been measured [2]. It is entirely plausible that it interacts with gravity in the right way to cause an effective negative energy density. This has been speculated by serious physicists for a long time. However: The Casimir effect is exceptionally weak. Space time is exceptionally hard to bend. So the idea of an actual warp bubble from Casimir effect geometry seems extremely far fetched[3] right now. How stiff is space time? LIGO the gravitational wave observatory, measures changes to a 4000m distance that are smaller than an atomic nucleus, at its full design it will detect changes that are many time smaller than a proton. This is a feat of experiment that is the more astonishing the more you know. If you had told me you want to build something like this I would have assumed that it's likely impossible. Now, the first signal that could be detected by LIGO was a black hole merger that radiated away the energy equivalent of three solar masses. The peak power output of this event was larger than the combined energy output of all visible stars in the observable universe. How small is the Casimir effect? Micro Newtons/m force gradients for micro meter distances [4]. Again we are talking about an effect so subtle that it's only been measured reliably in recent decades. So these are the orders of magnitudes involved: White et.al. claim they can use an effect so small as to be barely perceptible, to manipulate space time in such a way to create interesting effects, when it takes the energy output of the entire universe to generate a ripple in space time strong enough to be detectable (a few galaxies over) by the most sensitive instrument ever devised. So I am sceptical. [1] https://en.wikipedia.org/wiki/Cosmological_constant_problem#Quantum_description https://en.wikipedia.org/wiki/Cosmological_constant_problem#... [2] https://en.wikipedia.org/wiki/Casimir_effect https://en.wikipedia.org/wiki/Casimir_effect [3] My physicist intuition says: Far fetched but not impossible. One of the heuristics for looking at this result though is this: The lead author is Harold White, one of the people who really pushed the EMDrive. The EMDrive is not far fetched but simply impossible (to a physicist the EMDrive is in exactly the same category as perpetuum mobiles). Worse, the original EMDrive paper by Harold White, despite supposedly showing evidence overthrowing three centuries of established physics, did not attempt to get published in a physics journal (which would have required extraordinary proof for such an extraordinary claim). Finally it involved some theoretical speculation that were just abject nonsense. These things mean I will not spend serious time on understanding and analyzing the details of this latest work. [4] https://www.nature.com/articles/ncomms2842 https://www.nature.com/articles/ncomms2842
Wildgoose 5y ago
I remember reading the suggestion that one possible interpretation of Dirac's equations was that "empty" space is actually a sea of negative energy.
pdonis 5y ago
> The universe, despite being filled with mass and energy, is roughly flat. No, it isn't. It is spatially flat (if we choose appropriate coordinates), but the spacetime of the universe is not flat, because it is expanding. The rest of your reasoning is wrong because it starts from this false premise.
- whatshisface 5y ago
  Which kind negative curvature does the drive need?
  - pdonis 5y ago
    "Negative curvature" isn't quite the right term. (For one thing, curvature in 4-dimensional spacetime cannot be described by a single number.) The Alcubierre drive requires "exotic matter", i.e., matter that violates the weak energy condition. One way of phrasing that condition is that the matter will have negative average energy density as measured by certain observers.
  - pdonis 5y ago
    > Which kind negative curvature does the drive need? The issue with your reasoning is not what kind of curvature the drive needs, but the idea that there is a "natural" bending of spacetime at all. There is no such thing.

dotancohen 5y ago

> Beware that semi-educated guesses are sometimes worse than uneducated > ones, since they are more plausible and not necessarily more correct. This is one of the most insightful comments I've read on the internet ever. This is why we shouldn't take medical advice from people who have read a few Wikipedia articles. This is why years of experience in e.g. software are necessary to even begin to understand how deeply hidden security issues can be. This is why self-driving cars still elude us, but will be so much safer than human drivers when they finally do arrive.

dekhn 5y ago
The way I state the above point is: "The smartest people in the room are the most skilled at convincing people (including themselves) of a narrative that sounds logical but is wrong".
sgt101 5y ago
>This is why self-driving cars still elude us, but will be so much safer than human drivers when they finally do arrive. Isn't that just a by definition thing? My Tesla can already drive itself, I don't trust it and I don't think it's safe. When I get one that I trust and is safe then it will be safer than me, and by extension other humans. The problem is that the current generation of algorithms are brittle.
- blamestross 5y ago
  The "just put a brick on the gas pedal" algorithm was particularly prone to crashing.
  - dotancohen 5y ago
    Yes, but it had the lowest CPU and memory footprint! There's also something to be said about the COTS hardware requirements and ease of installation.
- deleted 5y ago
  [deleted]
jl6 5y ago
> This is one of the most insightful comments I've read on the internet ever. On the other hand, if I’m in the market for a guess, I’ll probably go for the educated guess rather than the uneducated one, because even though I recognize it’s not necessarily more correct, it probably is less wrong.
- ALittleLight 5y ago
  Sometimes it's better to know you don't know than to think you know. If it's midnight and you're standing at the edge of a cliff you're better off thinking that you have no idea what's ahead of you than if you listen to a guy with "better than average night vision" telling you that there's a sidewalk ahead.
  - MarcoZavala 5y ago
- Beltiras 5y ago
  Being less wrong is probably as good as it ever gets.
- mnsc 5y ago
  If you are, as you say, "in the market for a guess" that would imply that you are actively looking at different guesses and comparing the credentials of the guessers. Then you would _probably_ be right to choose eg. the guess from the virologist over the proctologist or the mechanic when you need a guess on how the Covid situation will unfold. But the issue might be that an "educated guess" in itself could be worse because you wouldn't base any major decisions on a mechanic that present an "uneducated guess" that "the virus will die down in a month" but if you have a crackpot chiropractor presenting themselves as a "certified doctor with an expertise in everything concerning the body" that says the same thing, you might be worse off in saying that this "educated guess" is worth basing your travel plans on.
  - heavenlyblue 5y ago
baq 5y ago
you might be interested in Kahneman's body of work, especially 'Thinking, Fast and Slow'.
belter 5y ago
I am afraid many times when talking about educated guesses we mean schooled guesses. The problem is that, more than once, we learn per Mark Twain's famous quote ...that schooling has a way of interfering with education...:-) Specially for subtle but critical concepts: "Researchers misunderstand confidence intervals and standard error bars" https://pubmed.ncbi.nlm.nih.gov/16392994/ https://pubmed.ncbi.nlm.nih.gov/16392994/
armchairhacker 5y ago
I was thinking the opposite point: sometimes the experienced doctor or researcher made a mistake, and the guy who posted something on Reddit is correct. Usually not, but sometimes. Generally speaking, you should trust someone whose professional or experienced more than some random stranger. Especially if it's not one person but a large group of experienced professionals (most of the controversial "anti-science" issues today). But it's actually important to "do your own research", and not just listen to authority. Provided you actually know how, i.e. you can find accurate sources and distinguish fact from fiction. Because even the most qualified, unbiased, genius authority are occasionally wrong. The big issue today is that a lot of people don't know how to research and distinguish lies, but (although it's probably impossible for most) finding a way to teach someone how to find the truth, will always be more effective having some authority tell them the truth. Key example: doctors often miss symptoms and diagnoses that a patient can discover on their own. It's not that the doctor is stupid or unqualified, in fact they have way more medical knowledge than you. But the doctor only has a series of questions and maybe a few tests to make a diagnosis, whereas you have the full experience of symptoms and an internal understanding of your "normal". IMO people actually should "play Internet doctor" and research their symptoms, not to reach any 100% conclusions, but to suggest diagnoses and tests to their real doctor.
- dotancohen 5y ago
  > IMO people actually should "play Internet doctor" and research their symptoms, not to reach any 100% conclusions, but to suggest diagnoses and tests to their real doctor. I agree, and do this. But I never tell the doctor "I suspect that I have X", rather I tell the doctor "I've observed J, K, and L under Foo conditions" and let the doctor decide where to go from there. In short time we can learn to give the doctors better information, but we cannot replace their breadth of knowledge.
  - armchairhacker 5y ago
    Yeah sometimes people say "doctor I have X, I'm trying to get diagnosed with X". You're biasing the doctor to give you a diagnosis so you don't drop them, and you're biasing the doctor to ignore your symptoms because you're just saying them to get the diagnosis and maybe you just want to get certain medication. Even if you don't outright say it, you should never convince yourself you have a diagnosis. The human body is extremely complex and some particularly weird issues need patient research, doctor's research, and experimentation to solve.
codexcodexcodex 5y ago
He's just saying n=1. Medical advice on the internet can be n=1, but read enough to get to n=100+, while still limited, can be quite valuable. I can't ask 100 doctors what’s wrong with me, but I can ask my question to hundreds in an online forum. They might lead me astray, but so can a single doctor’s misdiagnosis. A doctor can prescribe treatment that helps or harms, but I can ask a forum of people, with the same problem I have, if I should take his advice. I can't, however, ask 100 of his colleagues or peers if I should trust his advice, and that's why we end up with so many charlatans in medicine. But he's a doctor. Of course you can trust him! I don't know about the rest of you, but I've found a fair number of doctors to be mostly useless.

ToJans 5y ago

The biggest issue - based on your explanation - seems to be the need for negative energy... I might have a non-educated theory that could explain negative energy; could you tell me if this is potentially correct or am I completely wrong here? Disclaimer: I haven't read the article (it doesn't load), and I am (probably) lacking even the basics of the required knowledge. Let's assume the law of energy conservation applies to the universe: if you assume the total energy in the universe is constant, the negative energy is a relative imbalance instead of an absolute one, caused by the universe expanding at a different speed relative to the "things" it contains. As the universe is "expanding" constantly, the "energy amount per expanding area" gets "less dense", so the "things" that are not "expanding" at the same speed get "more dense" in comparison. This imbalance creates different types of energy perturbation. From the point of view of the things that do not expand as fast, the energy levels relative to yours seem to evolve in a "negative way". It's like looking at a stationary train from a train that is moving: from the point of view of the moving train, the train standing still seems to go backwards. Does this make any sense at all, or am I completely missing the mark here? Updated: reworded some things and provided extra context.

MaxikCZ 5y ago
As another complete layman just pooling my intuition: Relatively more or less dense "things" (to each other) will always generate positive curvature of spacetime. What you want is relatively less dense area than vacuum in another region of space, since vacuum is (for now) what we constitute as (somewhat) flat. That's what Casimir effect tries to accomplish: define a small enough region of spacetime where some larger quantum fluctuations don't have enough space to happen, thus lowering its energy below vacuum. Since the idea is to define small enough space-like interval in spacetime to curb some energies off, could defining small enough time-like interval help the same purpose, multiplying the former?
- ToJans 5y ago
  Thanks, this makes a bit more sense to me now, but let's figure out if I understood it correctly: The Casimir effect is about creating a spacetime "corset" so to speak, that is so tight that even the regular quantum fluctuations don't have "enough room to pass", so they "almost freeze", and when they "come out on the other end", they could for example break or delay entanglement? On the other hand I think your assumption about the time aspect might be correct as we are talking about spacetime after all. The biggest issue might be that measuring time deltas might be harder than space deltas?

raducu 5y ago

> using metal plates to forbid certain vacuum fluctuations Just curious, why can't fluctuations happen inside matter? Isn't matter 99% vacuum anyway?

XorNot 5y ago
They can and do, and are an important part of intermolecular interactions (van der Waal's forces have a substantial Casimir effect component). Matter just ordinarily isn't structured to produce a strong version of an interesting effect - which when you get down to it is essentially what all human technology is based on. It's quite possible there are some weird molecular structures (if this paper is correct) which could also induce natural warp-field structures out of their crystal lattice.
- l33tman 5y ago
  Going down this route, it strikes me as more feasible to find lattice effects or nano-scale effects facilitating cold fusion than warp fields. After all fusion itself is at least provably existing, just difficult to arrange.
  - zardo 5y ago
    https://www1.grc.nasa.gov/space/science/lattice-confinement-fusion/ https://www1.grc.nasa.gov/space/science/lattice-confinement-... They don't use the term 'cold fusion', and obviously the fusion sites would be hot, but the bulk material isn't.

wanderingmind 5y ago

While I understand your reservations, I think you are really underselling the power of Ansatz. Ansatz reduce the search space of possible solutions because it adds various constraints that makes it possible to get to solutions much faster. For example, without these constrained searches we will never have developed any analytical solutions to differential equations that are pervasive across science, engineering, finance among others.

Certhas 5y ago

As a trained physicist, I think you are getting this relatively right. Some of the issues at play are simply unknown according to the current state of theoretical physics. Our Quantum Field Theories naively predict an infinite energy density all throughout space time. A naive guess at regularizing this leads to absurd predictions 120 orders of magnitude off [1]. So there has to be something else happening here. One of the things that supersymmetry brings to the table is that it makes this energy density exactly zero (but then it get's complicated because you need to break super symmetry...). The thing is though, without gravity the absolute energy density of a quantum field is just some unobservable number. But how to reconcile gravity and quantum field theory is an unsolved problem to begin with, so it's completely unclear what that naive infinite number actually means. We are working without a firm theoretical (or empirical for that matter) foundation here. However, the Casimir effect is remarkable exactly because it appears that the local energy density between two conducting plates is lower than in the ambient space. This is an energy difference that can and has been measured [2]. It is entirely plausible that it interacts with gravity in the right way to cause an effective negative energy density. This has been speculated by serious physicists for a long time. However: The Casimir effect is exceptionally weak. Space time is exceptionally hard to bend. So the idea of an actual warp bubble from Casimir effect geometry seems extremely far fetched[3] right now. How stiff is space time? LIGO the gravitational wave observatory, measures changes to a 4000m distance that are smaller than an atomic nucleus, at its full design it will detect changes that are many time smaller than a proton. This is a feat of experiment that is the more astonishing the more you know. If you had told me you want to build something like this I would have assumed that it's likely impossible. Now, the first signal that could be detected by LIGO was a black hole merger that radiated away the energy equivalent of three solar masses. The peak power output of this event was larger than the combined energy output of all visible stars in the observable universe. How small is the Casimir effect? Micro Newtons/m force gradients for micro meter distances [4]. Again we are talking about an effect so subtle that it's only been measured reliably in recent decades. So these are the orders of magnitudes involved: White et.al. claim they can use an effect so small as to be barely perceptible, to manipulate space time in such a way to create interesting effects, when it takes the energy output of the entire universe to generate a ripple in space time strong enough to be detectable (a few galaxies over) by the most sensitive instrument ever devised. So I am sceptical. [1] https://en.wikipedia.org/wiki/Cosmological_constant_problem#Quantum_description https://en.wikipedia.org/wiki/Cosmological_constant_problem#... [2] https://en.wikipedia.org/wiki/Casimir_effect https://en.wikipedia.org/wiki/Casimir_effect [3] My physicist intuition says: Far fetched but not impossible. One of the heuristics for looking at this result though is this: The lead author is Harold White, one of the people who really pushed the EMDrive. The EMDrive is not far fetched but simply impossible (to a physicist the EMDrive is in exactly the same category as perpetuum mobiles). Worse, the original EMDrive paper by Harold White, despite supposedly showing evidence overthrowing three centuries of established physics, did not attempt to get published in a physics journal (which would have required extraordinary proof for such an extraordinary claim). Finally it involved some theoretical speculation that were just abject nonsense. These things mean I will not spend serious time on understanding and analyzing the details of this latest work. [4] https://www.nature.com/articles/ncomms2842 https://www.nature.com/articles/ncomms2842

Wildgoose 5y ago

I remember reading the suggestion that one possible interpretation of Dirac's equations was that "empty" space is actually a sea of negative energy.

pdonis 5y ago

> The universe, despite being filled with mass and energy, is roughly flat. No, it isn't. It is spatially flat (if we choose appropriate coordinates), but the spacetime of the universe is not flat, because it is expanding. The rest of your reasoning is wrong because it starts from this false premise.

whatshisface 5y ago
Which kind negative curvature does the drive need?
- pdonis 5y ago
  "Negative curvature" isn't quite the right term. (For one thing, curvature in 4-dimensional spacetime cannot be described by a single number.) The Alcubierre drive requires "exotic matter", i.e., matter that violates the weak energy condition. One way of phrasing that condition is that the matter will have negative average energy density as measured by certain observers.
- pdonis 5y ago
  > Which kind negative curvature does the drive need? The issue with your reasoning is not what kind of curvature the drive needs, but the idea that there is a "natural" bending of spacetime at all. There is no such thing.