--- Log opened Wed Mar 10 00:00:53 2021
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06:13 < lsneff> https://www.uni-goettingen.de/en/3240.html?id=6192
06:13 < lsneff> Lots of warp drive stuff recently
06:29 < docl> I still haven't twigged to a reason to take warp drives seriously. might just be above my current level but I've been told FTL implies time travel (still not quite sure I understand why) and stuff like quantum entanglement doesn't violate causality. on top of that, time travel is a super extraordinary claim with a worse kind of fermi paradox than aliens because we *know* there are future humans
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06:44 < docl> so it would be surprising to me (and, I get the impression, most physicists) if we live in a universe where time travel is possible and unsurprising if warp drive ideas are mirages. that being said, I don't have a *direct* argument against warp drive because all I understand about those is the layman descriptions, vaguely. I suspect exploring the concept might be useful to disambiguate the math terms i
06:45 < docl> nvolved in ways that will turn out to be important (just not for producing warp drives). if warp drives are possible, it will probably turn out the "ftl violates causality" claim is itself a misunderstanding.
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07:02 < lsneff> Keep in mind that "warp drive" doesn't always mean superluminal.
07:08 < lsneff> That particular press release (and the original paper) are about superluminal warp drives I believe, but I personally think subluminal warp drives are more interesting, and they’ve also been starting to show up in the scientific literature
07:57 < docl> that is interesting
07:58 < docl> how would you explain the concept / what makes it appear like a credible prospect? can such drives allow you to move around the solar system without expending reaction mass?
10:32 < maaku> docl: for that you could just use a magnetic solar sail
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10:33 < L29Ah> sounds slow
10:33 < maaku> docl: time travel is possible in general relativity as we know it. you can't travel back prior to the creation of the machine though
10:40 < maaku> docl: a physicist considers arriving before your prior light cone to be time travel
10:41 < maaku> any superluminal travel including the above warp drive achieves that, but like the GR time travel machine you can't use it to go back prior to the start of the journey.
10:42 < maaku> L29Ah: you can go fast or you can go cheap. pick one.
10:43 < maaku> if you're worried about reaction mass, then I presume you're wanting cheap travel. solar wind mag sails are great for that.
10:43 < maaku> you want to move quickly, then a fusion torch drive is best
11:07 < docl> I feel like defining it as time travel doesn't make it so, and on the surface it just seems like a needlessly confusing way of thinking about time. If I had insta-FTL and jumped to alpha centauri such that my view of earth is of 4 years ago and the stuff I see at ac is 4 years in the future of what I saw through a telescope from home, it's all symmetrical -- if I jumped back the process would reverse i
11:07 < docl> tself exactly and I'd get there moments after I left.
11:13 < docl> so presumably physicists can describe (or at least some physicists think they can describe) a situation where it actually breaks causality to have ftl. I'm just not able to describe it myself. something to do with accelerating objects and frames of reference? it's all very confusing.
11:17 < lsneff> It comes down to relativity making the sequence of events change if information is transmitted faster than the speed of light
11:18 < lsneff> http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel
11:19 < lsneff> There are some people who say that it works out if you use a wormhole rather than ftl travel, but I’m not sure how that works
11:19 < docl> hmm. the sequence of events changes if you move things faster vs slower subliminally as well, doesn't it? you get to the planet quicker and can intervene in its timeline sooner, etc.
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11:37 < lsneff> Not really, light will always arrive from events in the order they happened in
11:39 < lsneff> Rather, order that they happen in, in your frame of reference
11:42 < lsneff> Jeez this stuff is complicated
11:43 < lsneff> I took a class last semester on relativistic and quantum physics
11:45 < lsneff> This is a good example I found:
11:45 < lsneff> Bob and Alice make a bet about who will get to Alpha Centari first. On year 1, Alice builds a ship that can travel nearly the speed of light. So much near, that after she's done her instantaneous acceleration, one day to her dilates to 1 year to Bob. Therefore, it's only going to take 4 years to get to Alpha Centari (and to her, 4 days).
11:45 < lsneff> On year 2, Bob develops instantaneous teleportation and decides to jump over to Alice to brag. He does so.
11:45 < lsneff> Now, when is he?
11:45 < lsneff> From Alice's standpoint, only a few minutes have passed on the Earth since she left (after all, time dilation works both ways). So, now Bon offers to take Alice back to Earth using his device. But, who's reference frame do we use?
11:45 < lsneff> If we use Bob's original reference frame, Alice gets to jump 2 years into the future. If we use Alice's frame, Bob gets to jump back in time 2 years.
11:45 < lsneff> There are lots of good pictures, but to really understand everything, the math is what describes it.
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12:38 < docl> nice example. but alice looking at earth will only see very slow changes through her on-board telescope, so you have to have length contraction otherwise she will assume it's 4 years passing on earth from simple observation that the picture doesn't go backward in time
12:41 < docl> length contraction is part of how time dilation works, i.e. as I understand it the angle wrt time vs space is changed so one substitutes for the other in a certain sense
12:41 < docl> (space in the direction of travel, that is)
12:52 < docl> so I think while she's moving, the passing of time on earth seems like it's moving ultra fast because the universe is squashed into a pancake like structure where the light signals don't have to cover much distance. not sure that holds up to scrutiny though because length contraction should only affect signals moving parallel to the vector she's moving
12:55 < docl> like, if bob sends a signal to Pluto during earth day 1 and this is orthogonal to the direction of alpha centauri, alice seeing that apparently reach its destination in a few seconds might not make sense
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15:23 < fenn> a big ball of wibbly wobbly timey wimey stuff
15:30 < fenn> i have a hard time taking seriously any explanation where the entire universe turns into a pancake for every observer
15:31 < fenn> it's just not logically self-consistent
15:33 < fenn> the light from earth just hasn't caught up to alice yet
15:34 < fenn> i don't think objections of the form "but then alice will have lost 2 years!" are valid, because she will have lost 4 years anyway, or 8 years if returning to earth
15:35 < fenn> there is the matter of conservation of momentum though
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16:59 < kanzure> .title https://news.ycombinator.com/item?id=26418307
16:59 < saxo> Xilinx Opens Up Vitis HLS Tool for FPGAs | Hacker News
17:01 < fenn> "The Vitis HLS is a high-level synthesis tool that allows C, C++, and OpenCL functions to become hardwired onto the device logic fabric and RAM/DSP blocks."
17:02 < fenn> https://www.xilinx.com/html_docs/xilinx2020_2/vitis_doc/introductionvitishls.html
17:09 < kanzure> essentially magic
17:10 < kanzure> https://github.com/Xilinx/Vitis_Accel_Examples
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17:27 < lsneff> I'm impressed. How long does that take to compile?
17:29 < lsneff> I ask because I've heard stories about opencl kernels taking literally days to compile into an fpga bitstream
17:32 < L29Ah> their bitstream generator is still closed it seems
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19:30 < lsneff> of course
21:54 < lsneff> i read incandescence and diaspora by greg egan today
21:55 < lsneff> incredible books
21:57 < superkuh> Greg Egan is the best scifi author alive.
22:26 < apotheon> superkuh: I'm having difficulty coming up with a counterargument.
22:27 < apotheon> The Subjective Cosmology series is mind-blowing.
22:28 < apotheon> Learning To Be Me needs to be better known.  Other science fiction authors could be less ignorant of what they're writing if they'd just (re)visit that story.
22:29 < apotheon> I need to finish reading his novels.  I haven't read all of them.
22:29 < lsneff> apotheon: https://www.gregegan.net/BIBLIOGRAPHY/Online.html
22:29 < apotheon> wow, nice
22:29 < apotheon> Thanks.
22:30 < apotheon> I forgot he had all that stuff online.
22:30 < apotheon> heh
22:30 < apotheon> He denies there's a series.
22:30 < apotheon> . . . but they still feel like they fit together like one.
22:31 < apotheon> I guess it's the Subjective Cosmology accidental series, then.
22:31 < apotheon> . . . or the Blue Period.  Whatever.
22:31 < apotheon> I need sleep.  G'night.
22:31 < lsneff> I've read permutation city—dust theory threw me for a loop for a couple of days
22:31 < lsneff> But I haven't read the other two books in the "series"
22:31 < lsneff> goodnight
--- Log closed Thu Mar 11 00:00:52 2021