Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Some Emerging Possibilities For Space Propulsion

nasa.gov

Post reply on HN
Print view View original

41–50 of 61 posts

Re: Some Emerging Possibilities For Space Propulsion

#41

Post by logfromblammo »

fooqux wrote: post #9
eudox wrote: post #4

I honestly can't imagine why NASA even acknowledges the idea of FTL travel - Is there some kind of propeller beanie PAC forcing their hand? They should just have a page that says: "FTL is impossible, but that's fine, once you get used to the way things are."

>They should just have a page that says: "FTL is impossible, but that's fine, once you get used to the way things are." No offense intended to you, but I feel this is a disastrously negative attitude to have. Except where morals dictate otherwise (which are outside the scope of science anyway) nothing is banned from exploration. Suggesting otherwise implies that we know all there is to know about a subject; to me, th…

I would amend that to "FTL is economically impossible", rather than an unqualified impossible. It may be allowed by physics, but if we have to effectively bankrupt a whole galaxy just to move quickly from one star to the next, humans simply won't ever do it.

As it is now, with our current level of understanding, the best way to move around in space is by throwing tiny things very fast in one direction, to make a big thing move in the opposite direction, then wait a few thousand years for that big thing to get somewhere.

Given that we sometimes have difficulty being alive after waiting a few thousand years, it is understandable that some people have a problem with this, and want to explore other options.

Re: Some Emerging Possibilities For Space Propulsion

#42

Post by trhway »

mrfusion wrote: post #5
Steuard wrote: post #3

Speaking as a professional physicist, this page feels awfully light on actual information. And as far as I can tell, that's merited: there's not a lot of solid science discussed on this page, or even especially tantalizing ideas for what might come next. They spend a lot of time discussing quantum vacuum energy, but the one short note about its "propulsion implications" includes the comment that "Those particular gra…

How do these "legitimate" ideas for FTL deal with causality? I've always wondered that. People always use causality to argue against FTL, but then readily acknowledge that wormholes or Albicurie might allow it. I'm not getting that disconnect?

>People always use causality to argue against FTL,

for some definitions of causality. Anyway, far-far galaxies are running away from us FTL using part of the same machinery as Albicurie drive - ie. space expansion and no causality violation so far.

Anyway before FTL, we need to get basic aneutronic fusion drive working - emitting 0.05c alphas - so that we can comfortably travel inside Solar system and send first near-by star systems probes. Unfortunately continuing work on non-starter schemas like Tokamak or laser-driven (better than Tokamak, yet laser driver is the worst one) neutronic fusion wastes so many resources and creates public distrust to the fusion as a technology.

Re: Some Emerging Possibilities For Space Propulsion

#43

Post by ceejayoz »

jp555 wrote: post #38
ceejayoz wrote: post #8

Earlier quoted context omitted.

Scientists, rightly so, are a little leery of stating "we know everything". There's no guarantees our current theories are perfect - in fact, it's extraordinarily unlikely that they are. Also of note: https://en.wikipedia.org/wiki/Alcubierre_drive

I consider how we understand that at the speed of light, all distances are zero. So even if you can somehow travel at C, there would be nowhere to go, because you're already everywhere. Given this, travelling faster than C can be seen as travelling a distance of less than zero... which makes no sense.

Humans have always had a big gap between "how we understand" and knowing everything. Plenty of stuff in physics doesn't make intuitive sense, and there are things like https://en.wikipedia.org/wiki/Alcubierre_drive theorized that wouldn't require brute force exceeding C.

Re: Some Emerging Possibilities For Space Propulsion

#45

Post by Steuard »

mrfusion wrote: post #26
Steuard wrote: post #19

Earlier quoted context omitted.

Here's a link to a good discussion of some related issues by cosmologist Sean Carroll: http://www.preposterousuniverse.com/blog/2011/09/24/can-neut... (It's in the context of that mistaken "neutrinos move faster than light" claim from a few years back, but the arguments are general. Bob Geroch's article mentioned near the end is pretty much the gold standard for dealing with these issues as far as I know.) I really n…

Thanks! Could you explain a bit more about how Alcubierre can't be used to send information? Wouldn't a spaceship traveling FTL be able to carry information?

You know what? I take that back. At some point, I'd had the impression that Alcubierre's solution required a fixed velocity (so you couldn't start or stop the spaceship), but evidently that's not the case. It gets to be problematic after all! :)

Re: Some Emerging Possibilities For Space Propulsion

#46

Post by Steuard »

pdonis wrote: post #34
Steuard wrote: post #19

Earlier quoted context omitted.

Here's a link to a good discussion of some related issues by cosmologist Sean Carroll: http://www.preposterousuniverse.com/blog/2011/09/24/can-neut... (It's in the context of that mistaken "neutrinos move faster than light" claim from a few years back, but the arguments are general. Bob Geroch's article mentioned near the end is pretty much the gold standard for dealing with these issues as far as I know.) I really n…

The causality question was addressed years ago in this paper: http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev... TL/DR: Yes, the Alcubierre drive does violate causality.

Thanks! I knew someone must have analyzed this before.

Re: Some Emerging Possibilities For Space Propulsion

#47

Post by ctdonath »

eudox wrote: post #4

I honestly can't imagine why NASA even acknowledges the idea of FTL travel - Is there some kind of propeller beanie PAC forcing their hand? They should just have a page that says: "FTL is impossible, but that's fine, once you get used to the way things are."

I've long observed there's a small but profitable industry in proposing & researching megaprojects: suggest doing something so intriguing and outlandish and expensive that somebody will figure it's worth coughing up a minuscule percentage of the ginormous price tag to fund at least an amusing cursory overview, at a price sufficient to employ a small team for several years. Space elevators, floating (ocean) cities, bridging the Strait Of Gibraltar, world-spanning freeway, FTL interstellar travel, mile-high buildings, etc all are engaging concepts that people with deep pockets are willing to toss some "pocket change" at, giving a group a tidy sum to live on for a while doing something amusing.

Re: Some Emerging Possibilities For Space Propulsion

#48

Post by Steuard »

pdonis wrote: post #36
Steuard wrote: post #3

Speaking as a professional physicist, this page feels awfully light on actual information. And as far as I can tell, that's merited: there's not a lot of solid science discussed on this page, or even especially tantalizing ideas for what might come next. They spend a lot of time discussing quantum vacuum energy, but the one short note about its "propulsion implications" includes the comment that "Those particular gra…

> quantum gravity isn't far from my field, and I've never heard of this take on the origin of gravity and inertia That's probably because nobody in the field pays any attention to Haisch, Puthoff, and Rueda, who have been publishing papers on "zero point energy" and how they claim it causes gravity and inertia for a couple of decades now.

I know nothing about their theories, clearly. But if I had a nickel for every suggestion I've heard for somehow extracting energy or momentum or something useful from quantum vacuum energy, I'd have, well, a couple bucks.

The thing is, we've already got some awfully good theories about gravity and inertia, and a lot of their predictions are very well tested. Just as one example, if these folks think that what they've been working on is "a possible alternative to the Higgs" (a quote by Haisch himself in 2006 on the Talk page for his own Wikipedia entry), doesn't that mean that the actual recent discovery of the Higgs (and its good agreement with mainstream theory) strongly disfavors their alternative explanation for inertia?

There are too many fringe theories like this for any working physicist to have time to refute them all in detail. (I did my time years ago, trying to refute the nonsense of "Heim Theory" that had a crowd of Wikipedia proponents and a few overly credible mentions in the popular press.) At some point, when people start talking about replacing the foundations of modern physics you have to be skeptical, and raise the bar quite high before they merit serious attention.

Re: Some Emerging Possibilities For Space Propulsion

#49

Post by jerf »

Someone wrote: post #40
Strilanc wrote: post #15

Earlier quoted context omitted.

Whether or not you drift there or instantaneously teleport there, there are still causality issues. You can make it into your own past by jumping once, changing your speed to tilt the frame of reference you're moving "instantaneously" in, then jumping again. http://casa.colorado.edu/~ajsh/sr/simultaneous.html

That argument assumes that our understanding of physics is correct and complete. If FTL is possible, that is an incorrect assumption. But yes, any theory including FTL travel must explain how nature behaves if this happens.

No, it's much weaker than that... that assumes that our understanding of relativity is even remotely correct. You can obtain the FTL/causality problem from even a brutally simplified version of general relativity, such that one can literally draw the problem on a piece of paper. For FTL not to break causality essentially requires that A: you can only FTL jump once and only once in the entire history of the universe as long as you don't leap into your own past light cone, B: FTL is impossible or C: Causality is not absolute.

I've often thought that the announcement that we've developed FTL would actually be a very bad thing, as fun as it sounds at first. Sort of like this story, where it turns out a proof of P = NP is just about the worst possible thing: http://www.antipope.org/charlie/blog-static/fiction/toast/to...

Re: Some Emerging Possibilities For Space Propulsion

#50

Post by jerf »

logfromblammo wrote: post #29
mrfusion wrote: post #5

Earlier quoted context omitted.

How do these "legitimate" ideas for FTL deal with causality? I've always wondered that. People always use causality to argue against FTL, but then readily acknowledge that wormholes or Albicurie might allow it. I'm not getting that disconnect?

I have a hypothesis, unsupported of course, that everything has a fixed spacetime velocity. As beings with mass, we perceive the universe as having three spatial dimensions and one temporal dimension. We move relatively slowly in the spatial dimensions, and we find it easy to change direction. As a result, we are hurtling incredibly fast in the temporal dimension. A photon, on the other hand, moves incredibly fast in…

"We move relatively slowly in the spatial dimensions, and we find it easy to change direction. As a result, we are hurtling incredibly fast in the temporal dimension."

There exists an invariant such that you can look at anything and sum its spatial and temporal velocities in a certain manner to obtain c, but it's not a very useful observation because you end up with only two categories: Things moving at c, and things not moving at c. You don't have "slow" or "fast" things because that all depends on your frame of reference. For instance, from an object's frame of reference, it is always at rest.

(This is also why old sci-fi about a drive that can push you to FTL if you get "close" to the speed of light is gibberish. Your velocity is always zero relative to yourself. You can never get close to c because you can't even get closer to c. It is always and forever c beyond you.)

Post reply on HN