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/sci/ - Science & Math

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10989456 No.10989456 [Reply] [Original]

>FTL travel violates causality

>> No.10989484

>>10989456
How so?

>> No.10989487

>>10989484
because according to special relativity, from one reference frame if you see something take a FTL trajectory, then from a reference frame moving with respect to that one, it would appear to be a backward-in-time trajectory

>> No.10989503

>>10989487
>would appear to be

Because I observe the events happening backwards doesn't mean they are.

If i watch a video in reverse am I violating causality?

>> No.10990548

>>10989456
Causality is not a law. It is a mistaken assumption.

>> No.10990582
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10990582

>>10989503
>If i watch a video in reverse am I violating causality?

>> No.10990636

>>10989456
>Implying 'causality' exists
>Actually believing we don't live in a many-dimensional block universe and falling for the illusion of time having a 'flow' independent of consciousness

>> No.10990637

>>10989456
Please someone explain to this here sped how there is no single 'absolute' time at which things are ocurring. Stuff is happening here, stuff is happening a meter from me, and stuff is happening 100 light years away. Just because the information does not hit me at the same instant does not mean those events did not occur at the same instant, right?

>> No.10990657

>>10990637
just read a fucking book. i recommend Ohanian. special relativity is not a meme you can condense into one post

>> No.10990789

>>10989456
Casualty is relatively

>> No.10990831

>>10990637
If you know the speed at which the light you observed traveled, you can take the time at which you observe an event and subtract d/c, where d is the distance and c the speed of light, to find out when it actually occurred. This distinguishes time of observation from time of occurrence.
It's easy to see why two differently located observers will record different observation times for the same events. However, if they each subtract d/c for those events, they should get the same time of occurrence, right? Or, if their clocks aren't synchronized, at least the same difference in time, or at bare minimum, the same order of occurrence of events? In special relativity, none of these are necessarily true if the observers have different velocities. Some pairs of events don't have an objective order of occurrence (specifically, those events between which light doesn't have enough time to traverse the distance).

>> No.10990966

>>10990831
You shouldn't talk about things you think you understand after watching a couple of popsci videos on YouTube.

>> No.10991461

>>10990657
But feynman said if you cannot 'splain you do not 'stand
Yeah yeah yeah I'll read the goddamn book

>> No.10991470

>>10989456
Causality violates causality.

WHAT NOW, HUH? HUH?!

>> No.10991493

>>10989456
My dick violates your mom

>> No.10991514

>>10990637
>Just because the information does not hit me at the same instant does not mean those events did not occur at the same instant, right?
it absolutely means that

>> No.10991526

http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Everett%20-%20Warp%20drive%20and%20causality%20-%20prd950914.pdf
you can probably preserve causality if you never perform round trips with your FTL device but the moment you have more than one FTL drive keeping track of where you can't go becomes... messy

it's also important to note that there's nothing that explicitly means FTL is impossible just because it can be used to break casuality. it's just that we have no idea what breaking causality would even mean practically

>> No.10991589

FTL wouldn't break causality if it were to happen through travel by wormholes or if we managed to entangle particles at a distance to transfer information.
Assumption of going at a faster velocity than light is just not possible, so not even worth discussing.

>> No.10991622

>>10991589
it's still possible to break causality with FTL communication

>> No.10991648

>>10991622
Why would it break?
It's not like your information is traveling through time, it's just being transferred in another location.
Whatever photons hit that location will still need to travel the long road, just like any other effect of such information arriving at destination.
It's not like I'm expecting to see a led turning on right off the bat from a planet several light years away, since I'm still receiving photons from years ago anyway.

>> No.10991706

>>10991648
http://www.theculture.org/rich/sharpblue/archives/000089.html

>> No.10991794

>>10990966
Are you retarded? What issue do you have with anything I said?

>> No.10991797

>>10989484
it's like programming, you get an overflow value and go back in time

>> No.10991818

>>10991797
That actually makes no sense

t. Programmer and Physicist

>> No.10991839

>>10991706
>http://www.theculture.org/rich/sharpblue/archives/000089.html
Those graphs make assumptions, for all you know the FTL trasmission could be able to just send the information to the closest absolute current frame time of all reference frames irregardless to the varying granurality caused by velocity/mass.
I bet a FTL radio would have silly issues like hearing people speaking at a slower rate in virtue of being subject to heavier velocity/masses, or faster in the opposite case.

I mean, why would a FTL transmission need adhere to relativity? We're linking two points in absolute terms, points which are likely to have different flows of time.
Relativity is great to calculate effects that happen in relative terms, which is the only thing we can measure. But in absolute terms... who the fuck knows what's happening there, is there a fucking universal clock tick causing time as a side effect? How the fuck does it work? We can't even test it since we're inside the system and can only experience the relative side.
Also, if the FTL mean of trasmission involved entanglement, how the fuck would the delayed quantum eraser effect play into the equation. Wouldn't it just prevent any violation as it already does?

>> No.10991876

>>10991839
>for all you know the FTL trasmission could be able to just send the information to the closest absolute current frame time of all reference frames irregardless to the varying granurality caused by velocity/mass.
and for all you know that's not what happens at all. we're all talking right out our ass here

>> No.10991902

>>10991818
'twas a joke mein freund

>> No.10991945

>>10989456
btw are those guys that justin has his arms around fake sandnogs or real? they look pretty real tbqh

>> No.10991948

>>10989456
Scientifically speaking why is this picture so funny?

>> No.10991961

>>10991948
because justin trudeau in blackface is fucking hilarious. i like him better now

>> No.10991990

>>10990637
The reason is that classically it was assumed that there was only one time (frame invariant) and that distances between things were also frame invariant. It turns out that this is actually not true. Rather, the frame invariant quantities are the speed of light and the spacetime interval. When you use these as invariants instead, you get that there must be different time coordinates in different reference frames. This has nothing to do with the time it takes information to travel. If two things happen simultaneously in your frame of reference, someone in a frame of reference that is moving with respect to yours will not measure those events to be simultaneous.

>> No.10992003

the 'violation of causality' is just a retarded retard idea. it's not very different from the idea of seeing the back of your head while falling into a black hole, because the light reflecting off the back of your head is being bent around and back to your face. It doesn't mean that you're actually in front of yourself.

>> No.10992405

>>10990966
>Tells him he doesn't understand what he is talking about.
>Doesn't say what was wrong or didn't make sense.

>> No.10992531

>>10990637
Exactly

Think about conways game of life. In that, c is one pixel per time step. Things are happening simultaneously all over the space, but things can only have effects spreading out at c.

>> No.10992575

Are there historical examples of attempts at reasoning away lightspeed communication as it eventually climbed to its superior position over sub-lightspeed communication? Did the transatlantic morse-code cable bring naysayers concerned about it violating the natural order of things?

>> No.10992579

>>10989487
>use blackhole to fold space like oragami
>move from one fold to the other
>unfold
>you're instantly (from both points of view) lightyears away
it's that easy (well it's not easy at all since you have to control singularities uhhh)

>> No.10992606

>>10992531
yes, though each cell can be processed in parallel if you have the cores; which I guess is a more explicit version of the 'absolute time' question: is the universe not processed massively in parallel? Every little quark and gluon and electron stepping through states at some constant rate?
All this talk of inertial frames and different interpretations of time - to game-of-lifeify it, there'd just be groups of cells that happen to get processed at a different frequency (twice as often, half as often?)

>> No.10992843

>>10989487
>>10989487
True, I guess. But maybe it could be possible if we were to somehow compress space in front of us and decompress it behind us, however that doesn't really count as being 'faster' than light

>> No.10993180

>>10992606
Could it be possible that particles have maybe an "execution speed" vector/dimension?
This would allow the the existence of an "absolute time" while still keeping relativity intact.

>> No.10993463

>>10991648
The tachyon anti-telephone.
People would return your phone call before you left them your message.

>> No.10993489

There is a way to have both FTL and causality. But you need to assume a special frame of reference for FTL travel.

http://www.physicsguy.com/ftl/html/FTL_part4.html#subsec:specialframe

>> No.10993574

>>10992843
It makes your worldline spacelike instead of timelike, which is no bueno.
To an outside observer you could arrive at a place before you left. Diagram it.

>> No.10993710

>>10990636
Block universe still has causality.

>> No.10993742

>>10993574
Warping spacetime changes the metric itself, meaning what would previously have been a spacelike interval becomes timelike between the origin and destination. In GR, it's fine if you make it from A to B faster than light that follows a different path, as long as you locally travel slower than light everywhere along the path you take. You'll make it faster than light that travels outside the warped region, but everywhere inside the warped region, nothing locally moves faster than light.
For instance, if you're orbiting a black hole, you can fire a laser all the way around the black hole to come back to you. That makes it possible to travel between two points in your orbit before the laser pulse makes it between those points, since the laser followed a different geodesic through curved spacetime. However, you will never beat out any light in your immediate vicinity.