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Author Topic: Science Thread (and !!SCIENCE!! Thread!)  (Read 87725 times)

Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #195 on: December 01, 2017, 06:03:19 pm »

I'm not sure what you mean here. "A trajectory that permits photons from the end of its journey to reach it prior to launch" sounds suspiciously like the Bootstrap Problem.

Maybe this will make more sense: If you allow superluminal travel, you can send your FTL ship on a journey that ends within the past light cone of the ship at the time of departure -- so you could, if you want an easy-to-understand paradox, have the ship tell itself not to leave on the journey it just took. If you want, you can even put it in the same place in space but at an earlier point in time.

And no, those lines aren't going to bend. I explicitly drew the light cones at the ends of the time periods when the ship is motionless. Whatever distortion of spacetime you want to claim isn't going to happen when the drive isn't turned on in the first place, now is it? And yet C and D, among others, experience two ships at once. As you point out in bold text, this means there's now two of them, because
Put another way: you are your electromagnetic and gravitational interaction with the rest of the universe.

Incidentally, if you can't outrun your electromagnetic image you can't very well travel faster than light, now can you?
« Last Edit: December 01, 2017, 06:11:35 pm by Trekkin »
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Bumber

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #196 on: December 01, 2017, 08:49:13 pm »

Nicely done. Is there a way to color or otherwise label the photons according to the ship's position when they were emitted?
The numbers on the photon represent the ship's position, and the time of creation is relative to Y-coordinate. My only regret is not having them represent the ship's heading at the time.

Spoiler: The results (click to show/hide)

Incidentally, it might help you see explicit backwards time travel if you added a selector for reference frames that did the Lorentz transforms for you.
I don't know enough about Lorentz transformation to do anything like that.

And yes, the star will experience the ship when the photons arrive, because all the other force carriers (gravitons, etc) propagate at the speed of light as well. You can construct a trajectory in which an arbitrary number of the same ship are interacting gravitationally with the same observer, which kind of breaks the conservation of mass, does it not? This is ignoring the energy coming from those photons and its effect on the conservation of energy.
Is this not just a mere fact of the propagation of gravity through space-time as a wave? Gravity is not mass, as a ripple is not a stone.
The energy of gravity is being stored up like a capacitor in the surrounding space and released all at once. The observer is not a closed system.
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Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #197 on: December 01, 2017, 10:28:43 pm »

Incidentally, it might help you see explicit backwards time travel if you added a selector for reference frames that did the Lorentz transforms for you.
I don't know enough about Lorentz transformation to do anything like that.

Well, if you can't do the math then you can't meaningfully accept or reject any proof I might provide, so we'd just be stuck throwing metaphors at each other until one of us gets bored without actually generating any real insight, so I guess we're done here.
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McTraveller

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #198 on: December 01, 2017, 11:31:50 pm »

Yeah... generally speaking, putting FTL ships in a diagram that is designed with assumptions that you can't go FTL leads to paradoxical situations...  But there is an interesting interpretation I had a bit earlier that is surprisingly self consistent and provides an explanation for why you wouldn't end up continuously creating mass or having unbounded copies of FTL objects flying around.

Posting here will either cement or kill my chances to get in the history books of relativistic physics...

Consider three waypoints (starts, whatever) spaced 4 light years apart. I'll keep the trend going of calling them A,B,C.  An FTL ship starts at A, then goes to B at 2c, waits a year, goes to C at 2c, waits a year, then returns to A at 2c.  Check out the following...

Code: [Select]
        ref         obs A
  p  ----------   ----------
A 0  v0     ^10   v0     ^10!
  1
  2  v1     ^9    v3     ^11
  3
B 4  v2|v3  ^8    v6|v7  ^12
  5
  6  v4     ^7    v10    ^13
  7
C 8  v5     ^6    v13    ^14

So what we have there in the P column is "distance from station A".  The v and carat indicate direction of motion of an object at that position, and the number by it is the time the object is observed at that location by the given observer.  The reference observer shows that the FTL ship leaves A, stops at B, goes to C, then leaves and returns to A.

From the standpoint of A, we see something odd.  We see a ship heading from B to C at time 10, AND we see a ship materialize back at A at time 10!  But the observer at A never saw a ship approach - it had to have simply materialized.  Furthermore, we also see that a "ship" appears to travel backwards to C starting at time 10 - its position gets farther away from A as time increases.

The interpretation I have is this:  At time 10, an observer at A sees a ship - antiship pair spontaneously materialize. Think about it like pair production.  The "conventional" part of the pair remains at A - this is the ship that arrived back at A from the standpoint of the people on the ship.  The "antiship" travels backwards to C - where it happens to arrive just as an observer at C sees the "original" conventional ship ready to depart.  The ship and antiship appear to the observer at A to annihilate at time 14.  What's really notable - the time of apparent annihilation at time 14 for observer A is the same "time" that, if a ship were to have departed C at time 6, an observer at A would no longer be able to detect a ship at C. It is a wholly consistent interpretation, even if I can't say what mechanism would cause it.

This has several interesting aspects. One important one : because the ship is an "antiship" there is no loss of conservation of mass or charge - when the observer at A sees the pair produced spontaneously, there is still a total of one net ship in the universe (in this case, two ships and one antiship).  Also, an observer at A cannot "retain" multiple copies of a ship - it can only have one, which is an interesting way to address the "passengers on the ship can't feel like they are in two places at once" paradox.

I actually worked out the same diagram for observers at B and C - and they are actually all consistent.  B and C are interesting though, because each of them see two ship-antiship productions and two annihilations.  It's super late here right now though, but if there is interest I can post those cases tomorrow.

Spoiler (click to show/hide)
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Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #199 on: December 02, 2017, 04:07:53 am »

Posting here will either cement or kill my chances to get in the history books of relativistic physics...

I'm really hoping this was meant as a joke. Moreso than is normal for this thread, I mean.

As to your diagram (which is just half of a really coarsely quantized Minkowski diagram displayed as a table, by the way): I thought we decided that handwavium spacetime distortion meant the ship couldn't outrun its own electromagnetic emissions because Alcubierre = Star Trek warp drive according to the Internet?

And yeah, not having any mechanism for that is...not a great sign, man. I can make up invisible chronology protection ninja pirates that hijack ships approaching lightspeed (and hey, that's why they appear to gain mass, right?), but that doesn't mean it happens, or that it explains anything. Working backwards from the outcomes you want (in this case, that FTL is possible in a causal universe with special relativity) in the absence of empirical proof rarely gets you anywhere useful in science, let alone working without any math.

And actually you're right about the ship going backwards. If you just plot an FTL course crossing a stationary observer, you see that they see a ship flash into existence and recede as two ships, one going forward to its destination and one going backwards to its origin. No such thing as an anti-ship, though; it's just nonsense that happens when you selectively ignore part of physics.
« Last Edit: December 02, 2017, 04:29:28 am by Trekkin »
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McTraveller

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #200 on: December 02, 2017, 08:19:40 am »

Yes - the table is indeed a quantized Minkowski diagram. I was attempting to understand where in the original diagram there is a ship that has "arrived before it left" in a single location, and never saw one - just some points where an observer would see a ship appear without an approach and the apparent motion of the "multiple ships" that would be present after that appearance.  And I offered an explanation that has physical implications of how an observer could interpret those events that is consistent with physics, even if it isn't "real" - so it would at least make for good storytelling :).  After all, we already accept pair production, we already accept that there is no difference between antimatter traveling forward in time and matter traveling backwards in time (this is effectively what the "antiship" is), and those things apply here.

(Incidentally, I don't think this invokes any kind of chronology ninjas - it just says "pair production, antimatter, and annihilation seems to match the observations".)
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Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #201 on: December 02, 2017, 11:40:45 am »

Okay. Apparently I wasn't clear enough when I said, multiple times now, that you need moving reference frames to get the ship to arrive in the same place before it leaves. Moving reference frames complicate the diagram in ways that you're going to say don't make sense to you, but if you're going to misinterpret CP symmetry and conjure "antiships" out of whole cloth, I guess there's nothing to be lost by it:

Spoiler (click to show/hide)

Ship starts off motionless in the green reference frame. It accelerates in an arbitrarily short nonzero time to the red, moving reference frame, skewing its space and time axes but keeping them reflected around the edge of the light cone as normal. It now coasts a bit, then moves at FTL speeds; this part is in the bright red, and since every point along this path is farther along the red frame's time axis, from the standpoint of an observer in this reference frame it's moving forward in time. Now it stops in arbitrarily short nonzero time (so it's in the green reference frame again), waits a bit, and jumps back to its original space coordinate. That part is in green -- again, a stationary observer sees it moving forward in time per the green frame's time axis.

And yet it arrives back where it started but before it left, so now it's free to shoot its grandfather or whatever paradox-inducing thing you would like it to do. It doesn't have to just sit motionless like I've diagrammed, before anyone says anything about how "well clearly it's a closed loop!"

You might quibble about using two reference frames, but per special relativity that's valid to do -- and if you use a subluminal ship, you can watch any part of the trip from any number of reference frames and it's clear you can't get back to where you left. At most you can go arbitrarily close to not moving in time at all, which doesn't actually get you into the past. Superluminal paths, though, let you thread observations together in relativistically valid ways to get to causally invalid results, suggesting either relativity doesn't work (no) causality is optional (no) or there are no superluminal paths.
« Last Edit: December 02, 2017, 01:57:44 pm by Trekkin »
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McTraveller

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #202 on: December 02, 2017, 03:40:07 pm »


It now coasts a bit, then moves at FTL speeds; this part is in the bright red, and since every point along this path is farther along the red frame's time axis, from the standpoint of an observer in this reference frame it's moving forward in time

I follow what you are saying - but I must be missing something in that drawing. To me it looks like the FTL trajectory (bright red line) is moving in the negative-time direction in the red (skewed) reference frame.  That is, taking "increasing time" to be in the direction of the t(relativistic) line moving up and to the right - because the portion that looks like it is 'coasting' in that frame is moving in that direction. The bright red trajectory does "always increase" if the relativistic time axis is in the direction of the blue line heading up and to the left, but I thought that was the light cone.  Can you clarify the diagram there? I feel like I'm missing something obvious... and it's been a long time since I've had general relativity and I'd rather continue a conversation than just one-directional reading.

Incidentally I do agree that general and special relativity definitely seem to hold, and I agree that causality holds, so it does sure look like (sadly) there are no real-world superluminal paths.
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Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #203 on: December 02, 2017, 04:06:41 pm »

I follow what you are saying - but I must be missing something in that drawing. To me it looks like the FTL trajectory (bright red line) is moving in the negative-time direction in the red (skewed) reference frame.  That is, taking "increasing time" to be in the direction of the t(relativistic) line moving up and to the right - because the portion that looks like it is 'coasting' in that frame is moving in that direction. The bright red trajectory does "always increase" if the relativistic time axis is in the direction of the blue line heading up and to the left, but I thought that was the light cone.  Can you clarify the diagram there? I feel like I'm missing something obvious... and it's been a long time since I've had general relativity and I'd rather continue a conversation than just one-directional reading.

That's kind of why I've been hoping to display this mathematically; it's obvious numerically but the diagram is nonintuitively skewed, so it looks confusing. Perhaps more so since I left the grids out. That said, I've added lines of constant time for both the arrival and the departure trips and labeled their intersections with the relevant time axes.

Spoiler (click to show/hide)
« Last Edit: December 02, 2017, 04:09:16 pm by Trekkin »
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Max™

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #204 on: December 02, 2017, 09:08:20 pm »

Lorentz transforms shouldn't be foreign if you're discussing relativity.
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Trekkin

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #205 on: December 02, 2017, 09:22:06 pm »

Lorentz transforms shouldn't be foreign if you're discussing relativity.
I've been saying that for days, but nobody's listened yet.
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McTraveller

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #206 on: December 02, 2017, 09:46:07 pm »

Ahhh.... yes ok, that helps - that wasn't the direction I expected for the lines of constant time.

That whole "the order of events occurring at different locations is actually relative" thing... fun.

Also - don't be afraid to throw equations in here. I, at least, tend to understand those easier than images.

And yeah, I've got to really dust the cobwebs off my relativistic physics.  It's only been 21 years since I had to know that stuff...
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martinuzz

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #207 on: December 04, 2017, 11:14:14 am »

Dutch Nobel Prize winner Ben Feringa made another breakthrough in nano-mechanics. Last year, he won the Nobel Prize for creating nano-carts that can move when stimulated by light.

Now, he succeeded in making artificial nano muscle fibers, powered by light.
In their experiments, they show how their nano-fibres are able to lift small pieces of paper.

The fibres are made by arranging the nano-engines used in the previous experiments with nano-carts into long braids. From the short molecules, microscopic fibres are formed using chemical tricks.
In water, the fibres form noodle-like strings.
The whole process can be classified as 'self-assembly'. Under the right conditions, the fibres automatically assume the correct position and fall into place.
The resulting complex consists of 95% water molecules, making it exceptionally applicable in biological systems.

A possible application could be using the fibres as support structures for growing organs from stem cells. The University of Groningen has already started a joint research with the team to try and force stem cells to grow into the right shape.
https://www.volkskrant.nl/wetenschap/nobelprijswinnaar-feringa-komt-met-nieuw-onderzoek-nanospiertjes-tillen-snippertje-papier-op~a4543216/
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Egan_BW

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #208 on: December 12, 2017, 05:19:03 pm »

Not that it's likely, but what would we do if we did find that it's probably artificial? Would be quite hard to actually go catch it at this point.
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smjjames

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Re: Science Thread (and !!SCIENCE!! Thread!)
« Reply #209 on: December 12, 2017, 05:48:53 pm »

Probably double, triple, and quadruple check your measurements plus confer with a bunch of other experts, after all, it'd be huge news if it was without a doubt (or as little doubt as possible) that it's artificial.

One possible explaination for the elongated shape is that it resulted from an enlongated string of hot rock that was spinning really fast and it cooled that way. Could be wrong. Another possibility is that the ends didn't get smashed off for whatever reason. There are enlongated asteroids and comets, but none that would envoke a spindle or needle shape description.

In what way is it spinning though?
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