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Tuesday, April 27, 2010

Tuesday 4/27

Time Travel

The first part of a series of documentaries from the mind of Stephen Hawking aired last weekend.  I missed it, but my brother told me about it.  Here’s what I took from it.

My brother told me that he had watched the show last Sunday. 
“Cool,” I said, “how was it?”  (At this point, I didn’t yet realize that it was to be a multi-part series.)
“Pretty interesting,” he said.
“Isn’t he supposed to know how the world ends, or I’m guessing the probability of it, at least?”
“No,” he replied, “that’s next week.”

Wait a second.  Next week?  Bummer.  I guess I can cancel my summer vacation.  This is when I realized there was at least one more show.

“Oh,” I quickly pondered the end of the world, “well, what was it about?”
“Aliens and time travel.  Aliens are likely to invade our planet for resources.”
“And time travel,” I asked, “what’s that all about?
“Well, theoretically, it’s possible.”

I thought for second how cool that would be and that someone beyond our lifetimes might figure out how to do it.  It would be nice to float through time and fix some things here and there.  I wouldn’t have eaten those bad leftovers.  Maybe go back and get some wining power ball numbers… then my thought was cut short.

“But you can only go forward.”
“What do you mean?”  I asked him.
“Well you could travel through time, in theory, but you can only go forward.”

So let me get this straight.  The arguably smartest man in the world is on TV explaining that time travel is theoretically possible, but we can only go forward?  Maybe I’m missing something here, but… DON’T WE DO THAT EVERY SECOND OF EVERY DAY?  I’m no rocket scientist, but if that’s all it takes, then I’m fairly certain I’m traveling through time right now, at the rate of about one second per second (normal speed).  

I’m not bashing Stephen Hawking or the network, but can’t the people that are way smarter than all of us come up with cars that run on garbage or ways to build cities on water or something?  (That’s a lot of real estate, by the way!)  Just a thought.

By: S. Cole Garrett


Boxxy said...

I think Japan has aleady started building cities on water, and i think somewhere in the carribean they building islands. of course they will be selling for millions of dollars.

Chris said...

Ha! Yes, technically we are traveling through time just by existing, but that's not what is meant by "time travel" in this instance. Time for a crash course in special relativity.

Thought experiment: Imagine you are driving in your car at 10mph and you throw a ball out the window (in the forward direction) at 5mph. At what speed would a stationary observer view the ball to be moving? Easy, right? It would just be 10mph + 5mph = 15mph. ~Simple classical mechanics.

Now here comes the fun part. What happens if instead of a ball, you shine a laser beam forward? You're driving at 10mph and the laser light propagates forward at the speed of light, c, from your point of view.

But what about our stationary observer? At what speed would they observe the light beam? If we follow the logic of the classical example, then the speed of the light beam as viewed by the observer would be the speed your car is moving (your frame of reference) plus the speed of the light beam in your reference frame (c) = c + 10mph.

Congratulations, you've just broken physics! Since nothing can move faster than the speed of light, c, saying something appears to have a speed of c + 10mph is nonsensical. And this is where Einstein comes to the rescue.


Chris said...


In order to fix this minor detail, special relativity asserts that the speed of light must remain constant in all frames of reference. So both you and the stationary observer would "observe" the laser beam propagating at speed c.

No doubt some little voice is going off in your head saying that just isn't possible given the logic of the classical example, and you're right. That's because the classical example has an unmentioned assumption: that space and time are fixed and do not vary. Special relativity throws that inconvenient assumption out the window.

This is also a good time to bring up that "rate of about one second per second" business you mentioned - that of course is a meaningless tautology that reduces to one (unit-less). It's much more useful to think of time in a relative sense: your second is equivalent to the observer's second.

So what does all of this have to do with time travel? Don't worry, I'm getting there. In order for the speed of light to be equal in all frames of reference, spacetime must be made malleable. Because of this, many strange things happen to your frame of reference as you move relative to another. We don't notice any of this since the effects can only be detected at significant fractions of the speed of light (like if you were in a spaceship that was moving a 1/2 the speed of light). The only effect pertinent to this discussion is that of time dilation.

It goes like this: when your frame of reference (i.e. you in your car) is in motion relative to another (the observer) spacetime in your reference frame ceases to be identical to spacetime in the other. Speed causes time to dilate, so that the faster you're moving relative to an observer, the slower time appears to be moving in your reference frame to that observer. (Lost you yet?) To the observer, your second appears to be longer then theirs. Time has dilated so that you are now moving through time at a slower rate than the observer.

Here is where the time travel becomes theoretically possible. Instead of a car, let's put you into a very fast spaceship and send you barreling off into space at a relatavistically significant speed, something close to the speed of light so that the effects of special relativity are apparent. Since you're moving so fast compared to earth, time moves more slowly for you than it does on earth. At these high speeds, your second is no longer equal to their second, it is longer. Because of this, events will pass by on the earth more quickly than they do for you - 24 hours for them could be equivalent to only 12 for you.

Let's say you continue flying around in your rocket ship for about 10 years and then return to the earth. Because time has dilated for you, 20 years have passed on the earth while you were on your journey. You've traveled through time!

Yay physics!

Cole Garrett said...

Interesting. Malleable space time. So in order to theorize about time travel, we have to throw all the other rules out the window? That's pretty convenient. Just kidding. Theorizing about things plausible beyond our own lifetimes is the only way we arrive at them isn't it? Chew on that.

Chris said...

Not quite. Theorizing isn't mere abstract thought, it requires a lot of evidence first. None of these conclusions were arrived at from just thought experiments. VERY heavy mathematics led Einstein to the conclusion that spacetime had to be malleable. The thought experiment raised the question and mathematics/physics provided the answer.

Of course, this was terribly inconvenient for a time when humanity knew the world through Newtonian physics (absolute spacetime) alone. I mean, can you imagine? It was absolutely ground-shattering. Abandoning your entire concept of how the universe functions based on evidence that is completely counter-intuitive.

blondie said...

Wow. I had not put much thought into time travel past DeLoreans and flux capacitors. This is fascinating information.
And is everybody in your family a nerd? Also fascinating.

Thomas Wayne said...

Supposedly space-time is not smooth and connected, but made of foamy space-time bubbles. And it can be bent, to create wormholes and such. (So it's not connected, but it can all bend together. Doesn't seem to fit within common sense.)

I would explain it all to you, but there's a lot of math involved... :)

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