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Decius comments on Closet survey #1 - Less Wrong
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Comments (653)
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.
Definitely. Especially when it comes to one of the first uses people would consider putting this (or most other) technology toward. No need for a payload!
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.
If you leave Earth in a spaceship using an alcubierre drive and simultanously have someone emit a radio signal from Mars, reach alpha centauri in half an hour, then observe that same radio signal arrive at alpha centauri after approximately six years of lounging around on an alien planet, then you have beaten that light to your destination. You have traveled faster than light.
Your other questions, about your own gravitational force, I assume need to be answered by an actual expert on general relativity.
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.