/sci/ - Science & Math

As far as time travel goes in anime and any other entertainment media, their basis on time travel and the speed of light are based entirely off of basic misconception that is common knowledge to most people.

Fun fact: Any science related technology in entertainment media can be explained with "Future Magic".
>How do they travel through time?
Future magic!
>How the crap can they go faster than light?
Future magic!
>How did a robot gain consciousness, go mad, destroy Tokyo (again) and the shady government organization has to recruit angsty teenage high schoolers to fight it with more big robots?
Future magic!

But, if you want a serious answer to your question, based in actual science, keep in mind it will destroy any hope of achieving anything you see in any show.

About the possibility of going faster than the speed of light. Stay with me, the idea will seem stupid, but the concept if what I'm trying to communicate:

What if, WHAT IF, we build a railroad in space. A GIANT, LONG RAILROAD, and put a train on it that travels (1/2)c+x, where x>/=1. Then we put another railroad inside that train, and put a train on that railroad that also travels (1/2)c+x. 2((1/2)c+x) > c, right?

>>3484054
Might want to use something other than a railroad or some other vehicle that requires constant gravity to function.

>>3484080
That's absolutely irrelevant to the question. It could be super fast monkey running on the back of super fast and long space whale.
Seriously, if you don't understand what he says you are a bit retarded.

>Bridging the gap between /a/ and /sci/

And you're not posting Franken Fran.

For shame.

>I've been wondering why going faster than the speed of light would cause time travel.

It doesn't. Instead of things taking a positive amount of time to happen, they take an imaginary amount of time to happen. Try plugging in some numbers into the time-dilation equation:

<div class="math">t = t_0 / \sqrt{1 - v^2 / c^2} </div>

Where:

<span class="math">t[/spoiler] = time elapsed as seen from someone stationary to space ship
<span class="math">t_0[/spoiler] = time elapsed as seen by someone in the space ship
<span class="math">v[/spoiler] = relative velocity between the spaceship and the "stationary" observer
<span class="math">c[/spoiler] = speed of light


>Also, some people think that it's impossible to go faster than the speed of light.

It is. The has been experimentally and mathematically proven up the ass.

>Are there any other methods you think would work other than traveling faster than the speed of light?

Wormholes, though they have been proven (mathematically) to be to unstable to work.


Time travel to the past is impossible for logical reasons as well: look up some of the paradoxes presented by retro-causality - the most famous of them is probably the grandfather paradox.

>>3484111
I don't have any pictures of her.

>>3484080
I don't suppose there are any, are there.

>>3484102
Exactly. If, putting sense aside, we could get monkeys like that, would the concept work?

Guys...

guys

Guys.

Guys listen.

I have the best ide

guys listen

I have the best idea ever

guys

Let's make a railroad


guys

a railroad

Let's make a railroad
guys listen here

Let's make a railroad... IN SPACE!

Basically, op, as we get closer to the speed of light, time slows down. Theoretically if we reached the speed of light time would stop, and if we went past it it would go backwards. This is all just what is implied by the mathematical formulas that relativity gives us. However, those same formulas tell us that it would take an infinite amount of energy to reach the speed of light, and thus it is impossible to reach it or go beyond it.

>>3484121
The equation was a bit complicated for someone like me with no actual science background, but I think I understood the basics of it.
Why exactly wouldn't a wormhole work?
I've read those paradoxes, but what about world lines?

>>3484132
Problem with that?

No. I don't want to bridge the gap. I want to fucking throw over 9,000 tons of tri-nitro-toluene at the gap.

>>3484054
No. That is not the way velocity addition works in relativity.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html

The velocity of the second train with respect to the railroad would be:

<div class="math"> v=\frac{c+2x}{1+(c/2+x)^2/c^2}</div>

Which (try plugging in numbers) is always < c.

>>3484054
No, because you can't go faster than light, period. Relativity.

>>3484157
>

>>3484158
So reality is screwing with us? That would also be why the speed of light in space is the same as the speed of light on Earth, even though (since the Earth revolves around Sol at about 100,000k/h) light on Earth should be traveling at c+100,000k/h?

>>3484242
>So reality is screwing with us?
.

>>3484267
I understand the very basics of the uncertainty principle already, but what does it have to do with nothing being able to go faster than c?

>>3484291
I only understand the basics too lol. I just found this infograph on /b/.

>>3484242
Well, the postulate that the speed of light is the same for all observers actually comes from maxwell's equations. The equation I posted:

<div class="math">u = \frac{v+u'}{1+vu'/c^2}</div>

Is based on that postulate. To answer your question though, if you want to calculate the speed of a photon (light) as seen by both people (the moving one and the stationary one), you just use u' = c, which by simplifying gives u = c. So both people see light traveling at the same speed.

This equation reduces to the more familiar (Newtonian) equation when the speeds involved are much less than that of light:

<div class="math">u \approx v+u'</div>

I wouldn't say that reality is screwing with us so much as our brains evolved in a world where we never see things move at a significant fraction of the speed of light, so the equations in special relativity seem to defy common sense.

>>3484291
They are unrelated concepts. He was posting that in regard to your comment that nature is screwing with us.

>>3484327
>So both people see light traveling at the same speed.
You could also justify how people can't see the difference with the Just-noticeable difference (psychology), right? It's too small of a change in relationship to c to be noticeable.
>I wouldn't say that reality is screwing with us so much as our brains evolved in a world where we never see things move at a significant fraction of the speed of light, so the equations in special relativity seem to defy common sense.
That last sentence threw me off a bit. Is this equation for the actual movement, or just how we observe it?

>I've been wondering why going faster than the speed of light would cause time travel.
everything is traveling trough time, depending on your relative speed some things travel faster trough time
i'm really not very good at explaining you'd be better of reading about Special Relativity on wikipedia

>Also, some people think that it's impossible to go faster than the speed of light.
all the experiments so far show that this is just how the universe is. also this is the basis of the theory of relativity, and this is the best theory of spacetime so far.
also if it were possible to travel back in time causality would be broken and all the laws of physics consequently
for example you'll be bringing energy back in time which would violate the thermodynamics conservation of energy law or quantum mechanics conservation of charge. basically it'll fuck up all the physics.

>Are there any other methods you think would work other than traveling faster than the speed of light?
well if we knew everything about the starting conditions of the universe and had a super fast computer we could simulate the universe and then join the simulation.
but then again we'll most likely never find out the initial conditions.

>>3484399
>You could also justify how people can't see the difference with the Just-noticeable difference (psychology), right? It's too small of a change in relationship to c to be noticeable.
I suppose that makes sense, though I think it's actually a deeper than that. Even our most sensitive equipment would have a tough time detecting relativistic effects at low speeds.

>That last sentence threw me off a bit. Is this equation for the actual movement, or just how we observe it?
How do you define what "actually" happens? Things that are true in one reference frame may be false in another. There is no "correct" reference frame. We can only judge events by what we observe.

>>3484158
>>3484327
I'm liking your posts dude.
I actually posted a proof of the relative velocity equation in a graphic some time ago.

Here it is fro anyone who is interested.

Are there 'equations of motion' in GR or since time is not the same for all frames it is not possible to calculate a trajectory?

>>3484457
That pic is awesome. Mostly because I like the concept of rapidity, and nobody seems to use it much anymore.

>>3484465
When you talk about GR you're usually implying that you're going to be dealing with accelerations and gravity. Of course there are equations which describe these, but they involve tensors so almost nobody will understand them.

To explain OP: Nothing can go faster than light in local space ,but general relativity does not forbid objects from going faster than the speed of light globally. To explain what I mean. In this picture a ship inside the bubble could not go faster than light C within the bubble. However outside the bubble the ship would be going faster than the speed of light. This is called an alcubierre drive and is mathematically sound,but not feasible.For one it would require exotic matter with negative energy density and two: it is now believed anything inside the bubble would be incinerated due to Hawking radiation.

>>3484489
Thanks!
I should really get to writing one with the proof of the Lorentz Transforms though.

>>3484527
Which derivation will you use. Most people seem to find this one the easiest to understand:

http://arxiv.org/abs/physics/0606103

>>3484512
nope...
spacetime's speed of curvature = c

>>3484547
Yeah, they way I learned it was pretty much just a vector addition too.
Except I would use wayyy more diagrams than this guy. Looks good though, thanks for the link!

>>3484054
>>3484102

In this situation, the first train/space whale would be going at 0.5c + x relative to a "stationary" object like a planet, and the second train/monkey would be going at 0.5c + x relative to the first train/whale.

But the second train/monkey would NOT be going at c + 2x relative to the planet. At high speeds, velocities don't actually add together in an intuitive way. If you do the calculations, the monkey would only be going at about 0.8 c relative to the planet (as long as x is small compared to c)