[2013-Jun-19] Wibbly Wobbly Timey Wimey.

[MysteryMan]

Doesn't "real" time travel basically already work like this?

I mean, in general, we have a hard limit of the speed of light, right? If you stay with me, I'll explain why that's relevant.

Let's say we've got a flat earth.

Let's say we are at a train station on the flat earth with one train heading west away from us at 10 mph and another train heading east away from us at 10mph.

From our perspective, the west train is heading away from us at 10mph.

From the east train's perspective, it'd be heading away from them at 20mph.

This breaks down when we approach/reach the speed of light.

Let's say instead of going away from us at 10mph, the east and west trains are powered by bad news and move at almost the speed of light.

From our point of view, the west train is heading away from us at almost the speed of light.

From our point of view, the east train is heading away from us at almost the speed of light.

That means from the east train's point of view, it seems the west train is heading away from it at almost twice the speed of light, right?

Well no. That would mean, from the perspective of the east train, the west train is traveling faster than the speed of light. So what happens?

Something's got to give to make this whole MPH thing to add up, right? Well, in our case it's time. The relative rate that time passes from the point of view of the various angels changes so that the numbers still add up correctly without breaking the rules of passing the speed of light.

edit: I'm talking about the relative part, not the going backwards part.

[YLM]

I'd like to say that the idea the comic presents kind of breaks down. One machine being "inside" the other doesn't matter, because that's a relative thing. One machine would go forward and be at point (x,y,z), and another machine would go back and be at point (x0,y0,z0). Now one isn't inside the other, but that's not some sort of physically-definable quantity.

[ChuckV]

I figure that in the first paradox, the premise that the second device has to stay inside the first doesn't hold. A time machine can hold regular things inside it. A second time machine could simply break out of the first and no longer be subject to time motion of the first.

Therefore, no paradox.

*edit - looks like YLM and I had the same idea.

[Skippy]

If I understand where you are going with this line of discussion, you wish to use time dilation as a method of time travel. This (practical constraints aside) is likely possible, but has many constraints. Most importantly, you cannot go backwards in time using solely time dilation, and time travel is far from instant (especially if you are travelling "far" in time). This gets really confusing when you try to use a time 'machine' on a vessel travelling near the speed of light.

[Primer]

This is only a paradox if you interpret a time machine as a device that encloses a volume and aren't instantaneous, which is silly.

In Back to the Future and many other media, time machines enclose a volume, but they instantaneously warp you to your target time.

In many other media, time machines are devices that open up wormholes (or magical portals that function like wormholes) that take you to other times. Wormholes do not enclose a volume.

In Primer, time machines ARE devices that enclose a volume and AREN'T instantaneous, but they only go backwards. Primer actually does explore the issue of what happens when you put one time machine inside of another.

There are lots of weird things about time travel, but this comic sets up a strawman version of a time machine that is represented in very few media.

[Karthas077]

I think part of the apparent 'confusion' that the female in the strip is creating is her inappropriate use of the word "while". One machine isn't going forward WHILE one is going backwards, it's misusing a word that refers to time as a constant linear progression while trying to reference a situation in which time functions like a dimension that can be traversed in multiple directions at multiple rates.

[Lethal Interjection]

I had a conversation about Time Travel popculture the other day. And how I like watching them because I'm always able to find problems/faults.

Two exceptions are Twelve Monkeys and Rant (by Chuck Palahniuk).

The former has nothing change.
The latter is the same, kind of, though it feels a little different. Essentially the time travel doesn't actually change anything prior to the point in which the first traveller goes back, and anything that changes after is slowly revealed to have been the case the entire time, but the way the story is told (oral biography) makes it a slow reveal.

Chuck Klosterman wrote that if you like 12 Monkeys you are probably a Calvinist. Made me laugh because both of these are true of me.

[ReasonablyLazy]

What happens when you walk backwards on an airplane?

I think this is just one of Zach's super tired/drunken comic rants that he thought was a good idea at the time.

[Metalknight1]

The comic only has a point if the time machines work by travelling through time themselves; if you're using time machines that remain when they are, but send their contents through time, then nothing interesting really happens here.

[nobody_special]

Doesn't "real" time travel basically already work like this?

I mean, in general, we have a hard limit of the speed of light, right? If you stay with me, I'll explain why that's relevant.

Let's say we've got a flat earth.

Let's say we are at a train station on the flat earth with one train heading west away from us at 10 mph and another train heading east away from us at 10mph.

From our perspective, the west train is heading away from us at 10mph.

From the east train's perspective, it'd be heading away from them at 20mph.

This breaks down when we approach/reach the speed of light.

Let's say instead of going away from us at 10mph, the east and west trains are powered by bad news and move at almost the speed of light.

From our point of view, the west train is heading away from us at almost the speed of light.

From our point of view, the east train is heading away from us at almost the speed of light.

That means from the east train's point of view, it seems the west train is heading away from it at almost twice the speed of light, right?

Well no. That would mean, from the perspective of the east train, the west train is traveling faster than the speed of light. So what happens?

Something's got to give to make this whole MPH thing to add up, right? Well, in our case it's time. The relative rate that time passes from the point of view of the various angels changes so that the numbers still add up correctly without breaking the rules of passing the speed of light.

edit: I'm talking about the relative part, not the going backwards part.

I've been wrestling with this thought for a very long time, and it seems like a lot of smart guys are in this forum... So could anyone please answer how this doesn't violate the max speed of light? It would be most appreciated

[just a passing science guy]

Essentially what you think of as the "intuitive" answer to the addition of two velocities breaks down at the relativistic extreme. The following links regarding Lorentz transformations might help you out.

http://hyperphysics.phy-astr.gsu.edu/hb ... ltran.html
http://hyperphysics.phy-astr.gsu.edu/hb ... l2.html#c2

As for exactly why this transformation works the way it does, sometimes when it comes to relativity and quantum mechanics, well...the model is consistent with the observed data. Sometimes it's best not to wonder too hard about what was going through Einstein's head when he came up with all this stuff.

[graphitepen]

I find it exceedingly strange that, while the point is being argued, the thing that comes to my mind most immediately hasn't been pointed out (at least not specifically... or that I noticed anyway...) and that is this:

As I see it, the comic makes 4 relevant assumptions :
1. Time travel is possible
2. Time travel both forward and backward is possible
3. The outer time machine "creates its own internal time".
4. The machine "travels" through time - I take that to mean that the transportation of the passenger/payload is not instantaneous, but rather, moves through space/time only with a direction and speed as configured into the time machine.

Assuming these points to be true, there is absolutely nothing wrong with having another time machine inside the first/outer machine.
Lets assume the given scenario where there are two time machines, one inside of the other, each containing a person. Now, the outer time machine goes back in time, while the inner goes forward in time. Forgetting for now the speeds that they are each moving through time, the conclusion given in the comic is that once both have stopped and the person in the inner machine steps out, both people would then be "sharing space, time and age"...
This conclusion is incorrect.
As the outer time machine creates its own inner time, the inner time machine, traveling through the outer time machine's inner time ("outer-inner-time"), would be travelling forward in time as compared to the person in the outer time machine (and not in the inner). Having traveled forward in outer-inner-time an x amount of time, the person in the inner time machine exits into what would be x time in the future from when the person in the outer time machine stopped moving back in time.
The while point of the "outer-inner-time", from the perspective of the person in the outer time machine cancels out any noticeable time travel event other than the "normal flow" of time inside the machine. To reach the same inner time to which the person in the inner time machine has traveled to - the person in the outer time machine would have to allow x time to pass to reach the exiting event.

Man, that whole thing looks so much more complicated when put into words... but the thought is really simple.
Basically, the whole "inner time" point makes the "two time machines" moot. When the person in the inner time machine steps out of it and into "inner time" - the only relevant time!! - the other person would have gone through the same amount of time - but slower from his perspective.

[Kaharz]

I've been wrestling with this thought for a very long time, and it seems like a lot of smart guys are in this forum... So could anyone please answer how this doesn't violate the max speed of light? It would be most appreciated

I'm stepping outside my actual knowledge on this since I never studied relativistic speeds. But I think it comes down to the issue the frame of reference. The trains are non-inertial reference frames in relation to each other. An observer in an inertial reference frame will see both trains moving at nearly the speed of light. An observer on one train would see the other train moving away at nearly double the speed of light*, but the other train wouldn't actually be going that fast. Just as when the trains were going 10mph, an observer would see the other train as moving away at 20mph, but it wouldn't actually be going that fast.

*if you ignore all the messy time dilation stuff and the problems that come with approaching infinite mass

[Oldrac the Chitinous]

It's weirder than that.
One of the key ideas of relativity is that there is no such thing as an "actual speed." The laws of physics are the same whether you're standing on the ground or on a moving train. And one of those laws is that you'll never observe anything traveling faster than the speed of light.

If you put a lantern on a train, the light will appear to travel at the speed of light in all directions. So, based on regular Galileian relativity, where velocities add like numbers, if you sent the train off at 99% of the speed of light, then you'd expect the lantern light to go at 1.99 times light speed in front of the train, and 0.01 times the speed of light in in the opposite direction. But it doesn't. To an observer on the ground, the light travels at the speed of light in front of and behind the train. (That's why they call it the speed of light.) So in the train's reference frame, the photons are getting away from the front of the train at the speed of light, but to someone on the ground they're only going .01c faster than the train, and you just have to live with that.

[Durandal_1707]

I had a conversation about Time Travel popculture the other day. And how I like watching them because I'm always able to find problems/faults.

Two exceptions are Twelve Monkeys and Rant (by Chuck Palahniuk).

The former has nothing change.

Personally, I found that to be a fault, because there's no causality. The catastrophe always happened because Bruce Willis travelled back in time, but the only reason Bruce Willis travelled in time was to investigate what caused the catastrophe. There was nothing that independently should have caused either event to occur.

It's kind of like an apocalyptic version of the Shakespeare time travel paradox, where a time traveller goes back in time and gives Shakespeare a copy of his collected works, and then Shakespeare just copies them down instead of actually having to write them. Shakespeare has the plays because of the copy he was given, and the time traveller has the plays because they've been preserved through the generations from the copy he gave Shakespeare, so who actually wrote the plays? No one, apparently — they just winked into existence in the space-time continuum. Twelve Monkeys does the same thing with the apocalypse; it just happens for no reason.