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MugaSofer comments on Closet survey #1 - Less Wrong
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I find it amusing that there are actual mechanisms that "our current understanding of the laws of physics" predict will allow both of these (zero-point energy and alcubierre drives, respectively.)
The Alcubierre drive is an highly speculative idea that would require exotic matter with negative mass, which is not considered possible according to mainstream theories of matter such as the Standard Model and common extensions and variations.
Zero-point energy is a property of quantum systems. According to mainstream quantum mechanics, Zero-point energy can't be withdrawn to perform physical work (without spending more energy to alter the underlaying physical system).
Among the perpetual motion/free energy crowd, Zero-point energy is a common buzzword, but these people are fringe scientists at the very best, and more commonly just crackpots or outright fraudsters.
Ah ... no.
Not exactly. ZPE has measurable and, in some cases, exploitable effects. I'm not saying it'll ever be practical to use it as a power source (except maybe for nanotech) but it can most definitely be used to perform work. For example, the Caismir effect. I note that Wikipedia (which I can't edit from this library computer) makes this claim, but the citation provided does not; I'm not sure if it's a simple mistake or someone backing up their citation-less claim with an impressive-sounding source.
Well yeah, anyone claiming to have an actual working free energy machine is lying or crazy. Just like anyone claiming to have flown to Venus or programmed a GAI. Likewise, anyone claiming to have almost achieved such technology is probably conning you. But that doesn't mean it's physically impossible or that it will never be achieved.
Uhm, I'm not a physicist, but that's a short paper (in letter to the editor format) regarding wormholes, which was published in 1988. The Alcubierre drive was proposed in 1994. Maybe somebody used an FLT drive to go back in time and write the paper :D
Anyway, while I don't have the expertise to properly evaluate it, the paper looks somewhat handwavy:
One can imagine the Moon being made of cheese, but that doesn't make it physically plausible.
AFAIK, there are multiple interpretations of the Caismir effect, but in most of them it is maintained that the phenomenon doesn't violate conservation of energy and can't be used to extract energy out of the quantum vacuum.
It can, in theory, be used to convert mass to energy directly. Bias quantum foam flux over an event horizon - and this need not be a gravitational event horizon, an optical one ought to work- and one side of the horizon will radiate hawking radiation, and the other will accumulate negative-mass particles. These should promptly annihilate with the first bit of matter they encounter, vanishing back into the foam and clearing the energy debit of the hawking radiation - effectively making the entire system a mass->energy conversion machine. Which does not violate CoE.
One second.. http://arxiv.org/pdf/1209.4993v1.pdf
AKA: A theoretical way to make a mass-annihilation powered laser amplifier. No way to tell if this is good physics without actually building the setup, but the theory all seems sound.
Eh... Only.. Do not point that lab bench at me, please? The amplification ought to stop when the diamond turns into a plasma cloud..
I'm not sure I understand what you mean. Sure, assuming that Hawking radiation exists, you could use a black hole to convert mass to electromagnetic radiation (although the emission power would be exceptioally low for any macroscopic black hole).
That paper seems to be discussing lasers with non-linear optical media.
Anyway, AFAIK, in physics, the term 'annihilation' is typically used in the context of matter-antimatter reactions. Both matter and antimatter have positive mass.
The point is that if hawking radiation is a physical phenomenon, then any event horizon should produce it, not just a gravitational one - and the non-linear optical medium forms two optical event horizons, which the laser pulse bounces between, picking up more input from hawking radiation each turn around. Very clever, limit should be the optical properties altering when the diamond sublimes into a fine carbon plasma.
Might be an energy source that makes fusion look like cave men burning dried dung, might be a way to disprove the physicality of hawking radiation, might be a lab demonstration that it exists that cannot be engineered to the point of net energy gain (you have to fire quite powerful lasers into the diamond to set things off. Even if it amplifies the laser pulse a lot, no guarantee you can get enough electricity back out to net positive..) Currently, it is simply an interesting computer simulation.
Strictly speaking, you're completely destroying the mass, but in the process gaining equivalent energy from nowhere. </nitpick> Of course, it balances out in the end.
I understand (I can't get past the paywall) that it describes how the Caismir effect creates an area that violates the positive energy condition, proving that it's not a law of physics. This is only part of their more general point (which is time machines, which are, of course, equivalent to FTL drives in any case. Harder to build though.)
The quote is handwavy. Then again, I don't know much about quantum foam. OTOH, considering their paper concerns a mechanism for holding wormholes open, it's not an unreasonable proposition (and it's not the only way to get a wormhole, after all, merely a possible way.)
The Caismir effect isn't the only example. ZPE keeps liquid helium liquid and probably contributes (although it's not the only contributor) to the expansion of the universe. Conservation of energy simply doesn't apply on a quantum scale; it's an emergent property of quantum mechanics, like, say, chairs.
IIUC, while the Caismir effect has been observed, it is still debated whether it is actually evidence for the vacuum zero-point energy, since the calculations aren't completely developed and there are other proposed mechanisms.
Anyway, even in the vacuum zero-point energy explanation, the vacuum energy density in the geometrically constrained region is still positive, it is just smaller than the vacuum energy density in the unconstrained empty space. It's only negative if you arbitrarily consider the energy density of empty space equal to zero.
Without a theory of quantum gravity, the speculative connection between vacuum energy density and gravitational effects (the cosmological constant) is highly debatable: typical attempts at calculating the cosmological constant from vacuum energy yield absurdely high values, while astronomical observations are consistent with a very small strictly positive cosmological constant.
Even if the vacuum energy density generates gravitational effects by influencing the cosmological constant, the lower than average energy densiity of a "Casimir vacuum" is probably not the same thing as the absolutely negative gravitational effect of exotic matter with negative mass, which, IIUC, is required by the Alcubierre drive (I don't know about wormholes).
BTW: I've found this post on Physics Forums
EDIT:
And in any case, the Casimir effect can't be used to extract energy out of nothing: the Casimir forces are attractive or repulsive depending on the geometric configuration. If you use these forces to extract work, the system will eventually transition to a configuration where the attractive and repulsive effects are balanced. You have to pay back the same work you extracted to return the system to the original configuration. You can't complete a cycle with a net gain.
This is the same problem of most of the proposed perpetual motion contraptions: you can extract work in an one-shot transition, but you have to perform the same work on the system (actually more, once you account for the inevitable thermodynamic losses) to return to the initial configuration.
You know, you're right. ZPE is far less certain/accepted than Alcubierre drives.
I'm going to go on being amused just the same, though. Those really were unfortununate examples to pick :)
Thanks ;)
Just one last technical nitpick, if you don't mind: Zero-point energy is a property of all quantum systems, and this is essentially uncontroversial. The existence of a quantum vacuum with a positive zero-point energy is considered less certain, but relatively plausible in the mainstream models such as the Standard Model. The idea that is possible to extract work from the zero-point vacuum energy is generally considered wild fringe science speculation/crackpottery/fraud.
I was referring to using it as an energy source, as in the original comment.
That seems a little strong. Still, it's certainly impossible with current tech, and there's no method anyone's come up with to do it with a higher tech level.
It's not just matter of technology. Such a feat would most likely require a violation of the principle of conservation of energy. While there are still some unresolved issues with renormalization and general relativity, it is generally believed that conservation of energy applies to the universe. The discovery of a violation of conservation of energy (which would imply that the laws of physics are not invariant under time translation) would be a groundbreaking result.
Wrong link? The abstract (full text is paywalled) says:
I don't see any connection to Alcubierre drives. Classic Kip Thorne, though.
Without even pretending to be anything other than an amateur layman in such questions, I found this on arxiv, quote:
(Lastly, if you're wondering why I'm replying to you a lot, it's just because you are a prolific commenter with whom I occasionally disagree.)
looks embarrassed
I just grabbed a citation from someone talking about how the Caismir effect can be used to create negative energy (in the context of stabilizing wormholes.) I should probably have checked that, I would have found it wasn't actually in the abstract.
Nevertheless! My point was that negative energy is pretty obviously physically possible, since it's what predicts the Caismir effect working. (There has been some attempt to claim the CE is actually predicted by some other theories, but that's not widely accepted.)
From what I understand it may be closer to say "doesn't rule out" rather than "predict will allow". Even that much of a possibility is somewhat mind-blowing.
Um, the current definition of speed prohibits FTL motion.
Only locally. And 'local' is rather malleable (which is the principle alcubierre drives theoretically rely on).
It's distance and time which are more malleable; if light travels through a vacuum and arrives in x time, the arrival point is defined as being x distance away from the departure point of the light when it arrives. The Alcubierre drive would (given a couple of facts not in evidence) allow you to change the distance. Light emitted from you at the time of departure would still beat you to the destination.
Sure, so long as the space being traversed remains consistent. Which it doesn't (always) given General Relativity. Hence Alcubierre drives.
No, it wouldn't. The drive in question is described thus:
Notice the link there to faster than light travel. That title is a literal description.
For emphasis: This is General and not Special Relativity.
Have you finished reading the paragraph the sentence you quoted comes from? And section "Alcubierre metric" from that article, in particular the fourth sentence?
Of course I have finished reading the paragraph. As soon as I encountered the notion. Because math that allows what is for most intents and purposes a warp drive within general relativity is freaking awesome. Even if it relies on pesky things like negative mass and ridiculous amounts of energy. Oh, and would utterly obliterate the destination. Still damn cool.
In answer to the presumed (and I hope I'm not misrepresenting you here) rhetorical intent of "The first paragraph demonstrates that your claim is wrong" I would (unsurprisingly) disagree.
The paragraph in question is:
And, assuming I can count periods correctly, the fourth sentence in the passage you refer to is:
Both of these are precisely correct. And the claim:
Is false. Light continues to be faster than you locally. That is, within the bubble. And the bubble goes faster than the speed of light. Light not inside such a bubble goes at the speed of light. You can get to a destination before that light does. Which is the entire point of a Faster Than Light drive.
I was assuming Decius wasn't assuming the light doesn't go through the bubble.
Rather than not assuming that it doesn't he would need to be actively assuming that it does, or he would have to make a different, more specific claim. And that more specific claim (that applies to light that travels in the the bubble) would not have supported the point Decius was using his claim to make in the context.
I understand recent formulations are better in this regard. </nitpick>
I assume you refer just to the ridiculous amount of energy required being a half dozen orders of magnitude less ridiculous than first calculated? Not that there are actually formulations that don't require negative mass? or don't obliterate the destination?
Oh, yeah. They're smaller, but they still need negative energy and they still obliterate anything directly in front of them - although that's hardly an impossible drawback.
What happens when the bubble overtakes light? Per my understanding, virtually all of the light emitted within the bubble in the direction of travel ends up in one front at the front of the bubble, along with all of the light overtaken. All of the matter overtaken accumulates along the edge of the bubble where space warps at their velocity, after experiencing some effects of misunderstood severity when space warps around them (does interstellar atomic H-1 fuse into He-2 when the distance between atoms falls inside the region where the weak force dominates? What happens when a solid chunk of mostly Pb-206 has the distance between multiple atoms is reduced to below the range of the weak force?). What happens when the bubble overtakes the gravity effect of the matter within the bubble?
'Obliterates the destination' might be a little bit of an understatement.
Do you mean 'poorly understood' rather than misunderstood? When talking about using negative mass and enormous energy to warp space itself to travel faster than freaking light. Most with even the most rudimentary grasp would see that the effects are inconceivably severe in relation to such a small object and bubble. Being unable to conceive of the scope or currently not knowing in detail is a very different state of knowledge to misunderstanding. At least, it is difference in epistemic states that seems rather important to me. (A wrong map leads you to walk into quicksand. An known to be incomplete map leads you to watch where you are walking or google up a better one.)
The same thing that happens when I overtake a car. I go around. What I definitely do not do is go around saying "Um, the current definition of speed prohibits FTL motion" because whenever I am racing light I must handicap myself and take the light I am racing along with me for a ride.
I meant "Everything that we currently understand about the phenomenon is almost certainly completely wrong." That's after accounting for what we know we don't know.
How, exactly, do you "go around" a wavefront which is propagating out from you in all directions? I'm still hazy on what the effects of autogravitation would be; once you overtake the light/gravational effect from you, are you accelerated towards your prior location proportionally to your mass and the inverse of the cube of distance from yourself?
Of course, if the travel is at some speed slower than that of light, no self-interaction effects are required. The warp drive doesn't beat the speed record, it beats the distance record.
Describing a method of travel as "faster than x" when x departs at the same time and arrives before you is the opposite of plain language. Distance and elapsed time between events is already agreed to be not constant even between colocated observers. That is an effect of postulating that light propagates at the same speed for all observers.
Travel, on the other hand, is a much looser term. Alcubierre drives, in theory, travel faster than their speed would suggest by distorting space. Until recently they were merely interesting mathematical curiosities, but recently new variations that allow them to be constructed by a non-godlike tech level have been discovered.