# Relativity and time travel for all!!!

Last night at work, we somehow got around to a discussion of time and interstellar travel. I couldn’t do the subjects justice; can anyone here help me to explain these concepts?

1. No one believed me when I said that a person approaching the speed of light would appear to an outside observer to slow down. Wouldn’t this also mean that suspended animation would be unnecessary when traveling at the speed of… (geez, can I just call it C?) since the traveller’s motion would go to zero also? (and yes, I’m ignoring such things as acceleration and that travelling at C would probably require an infinite amount of energy.)

2. Last week on PBS, they did a thing on time travel. First off, several physicists claimed that it * could * be possible in the future to go back in time, but it would take an incredible amount of energy.

3. One physicist suggested that he had proven mathematically that it was impossible for something going back in time to affect the current state of affairs (I think he set up the equation for the case of a ball rolling off a table, back in time and then across its own path; it could not knock itself off its original path.) Did anyone see this, or can someone explain the ideas behind the claim?

Yes, it is true that if you could travel fast enough, hibernation or whatever would not be necessary. We could send a pilot to destination 100 light years away, and have him experience the trip as being a single day. Of course when gets there, all of his friends on Earth will be dead. Again that is ignoring the complications of the time it would take to accelerate to that speed.

The possibilities of time travel into the past are speculative to the extreme. They just haven’t been proved impossible. Yet.

In 3), the physicist is talking about causality. It can be argued logically in a proof by contradiction that we can’t change the past, at least not in a significant way. It sounds like he has a different angle on proving that, but it’s the same idea. This fact makes time travel to the past seem pretty unlikely, because we have no explanation for how someone would be restricted to protect causality.

Explaining time dilation for casual coffee talk ain’t real easy. When I try to explain it, I usually babble on for a couple pages, and even then it may be hard for the reader to accept. If your primary purpose is to convince them that it is true, you might be able to convince them with references.

Here’s one good one. http://math.ucr.edu/home/baez/physics/twin_paradox.html

You could also bring in a copy of A Brief History of Time, and find an apt exerpt. Who knows, they might have even heard of Hawking.

A clock carried by a person traveling near light speed will register time more slowly than a clock remaining at (relative) rest. This is a well-known principle of Einstein’s Theory of General Relativity. Ignoring the (current) impossibility of attaining such speeds, this would have the effect of propelling said traveler into a (relatively) motionless observer’s future, a feature already inherent in our ability to travel in time.

I have found nowhere in my readings on the subject any credible hypothesis as to how travel in the time dimension might be reversed. The current state of Superstring Theory makes no mention of the practice, even among its mathematical calculations requiring a eleven-dimensional (nine spatial and one time) universe, or the expressed possibility of tearing the very fabric of spacetime.

The closest you’re going to come is the curious condition of the photon, which always travels at light speed. Time is frozen for the photon, and they remain unchanged, always at the moment 15 billion years ago when they were created in the big bang.

Of course, I meant ten spatial dimensions.

I’ve always wondered about that Photon thing.

Ok. I’m in a dark room–no photons. Then I strike a match. Photons! They’re brand new photons, so they can’t be the same as the photons 15 billion years ago.

Photons only exist when moving, right? When they stop moving, they cease to exist, right?

Then where did my match photons come from?

Sign me confuzzled.

I believe the photons were formed through energy conversion… i.e. the heat energy from the match is converted into photons to produce light. You’re not creating matter here, just converting energy.

I explained time dilation to a friend thusly: Imagine you’re traveling away from a clock at the speed of light. The clock would appear to you to be motionless, stuck at the time of your departure, because you’re traveling at the same speed as the light reflected off the clock’s face. And the light from the clock after your departure never catches up to you. After you stop, the clock would appear to be moving normally again (Imagine you have a magical telescope that can see a clock from one light year away), but the time would be one year behind.

Those who do not learn from the past are condemned to relive it. Georges Santayana

But that’s not time dilation. That’s just travel lag. The time dilation effects are described after corrections have been made for the travel time of the light. Note that time dilation does not require you to be going away from the other clock. After accounting for travel time, you will see the other clock running slow even if you are going straight toward it.

Regarding the OP’s second question, the PBS special in which physicists speculated on the possibility of time travel into the past…

I hesitate to even get into this, as I too cannot do this justice… BUT it goes something like this:

It involves finding and harnessing a wormhole. one end of the wormhole would be sent on its way at the speed of light, while the other remains safely stationary on earth. the light-speed wormhole would be cruising around for a while at light speed, then returned to earth. Now, since time on the lightspeed wormhole would have effectively stopped, there would be a time difference between the stationary wormhole portal and the well-travelled wormhole.

So, you enter the wormhole which had been sent on its way, and exit the other end, and this would supposedly take you back in time, for, i guess the amount of time the light-speed wormhole had been travelling about.

I really shouldn’t have attempted this, but that’s how i remember it

The striking a match in the dark room thing now has me thinking (a potentially dangerous thing)… suppose you were in a room with perfect mirrored walls… and you lit a match, turned on a light, whatever.

when you extinguished the light source, would the photon continue to bounce about, providing light? I should go back to espn and ponder the braves’ fate

AHA! You really can go back home again!!! ;D You just can’t come back!

Zuma: Yes, in a perfectly mirrored room, the light would bounce around forever. There are two problems, though. First, there’s no known way to make a perfect mirror. Even the best ones absorb some light (and turn it into heat). Secondly, you wouldn’t have any way to see the light bouncing around “forever”, because seeing means using up some of those photons.

Of course, you could always turn on the light (remotely) and turn it off the same way, then come back in a zillion years. If the light’s still bouncing, you are safe to assume that you’ve got some pretty darn good mirrors there.

Travelling at Light Speed: Let’s say there was some way to get up to light speed in a space ship. I think you’d want to get close to C, but not actually reach it. If you reached it, time would stop, and you’d have to “time” to turn off the darn space drive.

In other words, once you hit light speed, you’d keep going forever (or until you hit something, which would cause an interesting kaboom).

Ooops. In my last message I said:

you’d have “time”

I meant to type:

you wouldn’t have “time”.

Time dilation makes my brain go all funny.

zuma,

You wrote:

The only way that I have heard that worm holes might carry you back in time is based on the theory that time may be curved into a sphere, similar to the way space is curved by gravity. The worm hole would have to take you across the chasm of non-time to arrive at a different location on the curvature of time. This same theory would require that we would be having exactly this discussion a gazillion years from now (or however long it takes to wrap around).

If you could find an ideal (non lossy) reflective surface and you had your room properly designed so that the photons never escaped, then yes, they would bounce around forever. Of course, you would have to be careful not to put any other light absorbing obstacles in the room, like photon emittors, cameras, light sensors, or human observers.

The idea that zuma suggested is the only time travel idea that I have actually heard proposed by physicists (as opposed to SF writers), except that they don’t suggest that the wormhole travels at c, but merely very close to c. A massive object travelling at c is not a possibility.

It is a semi-legitimate idea. It has two major problems. The first problem is that the wormhole idea is highly speculative. We don’t know if they really could exist yet. The second, and more serious problem is that it requires a wormhole to be navigable. Now that’s a big stretch to believability, and even the physicists who throw out the idea know it. So even this possibility is extremely unlikely to be possible. It just hasn’t been clearly demonstrated to be impossible.

If wormholes do exist, and you could put matter in one end and have it come out the other, I would venture to guess that the results would come out in a form completely unrelated to the form that entered (in other words, you’d be smashed to bits). That would serve to maintain the idea that no information can be transported FTL.

Oh, and P.S.

zuma, that was a pretty decent explanation of the idea. Another minor comment is that the wormhole of course would not be on Earth, but merely in the neighborhood of Earth. Remember that wormholes resemble black holes in their properties, so we certainly wouldn’t want one on Earth.

AFAIK, this theory doesn’t require time to be circular in any fashion. Actually, I’ve never heard of any model that would predict that time would run in a circular manner. That in itself seems to be a causality violator.

Undead Dude, if one goes BACK to Earth, toward that clock, watching it with my magical telescope, you’d see it moving FASTER than normal. You’re running into the light reflected from its face. When you got back to Earth, at least two years would have passed (one lightyear out, one lightyear back, assuming no time spent on acceleration either way), but for you, NO time would have passed. Time would have dilated down to ZERO.

Let’s say you do need acceleration time. (And you do.) Think of this: your computer screen shows the Earth’s position during your journey. It would have to show the Earth standing still after you reached light-speed while on your flight out, but on the flight in, it would show the Earth making two (or more) revolutions about the Sun, and rotating very rapidly on its axis at the same time.

In both directions, time has slowed down for you, but it has passed normally on Earth. But to your eyes, it seems that time has stood still on Earth while on the flight out, but passed very rapidly on Earth on the flight back.

This is what Einstein meant by relativity: That the rate of the passage of time depends entirely on how fast you’re moving, that it slows down more the closer you get to the speed of light, and that it would stop entirely if you ever got all the way. But it would take an infinite amount of energy to accelerate any mass (even a single proton!) to the speed of light.

Those who do not learn from the past are condemned to relive it. Georges Santayana

jab1, re-read my post an when you do, in your mind, bold the part that says after accounting fot travel time.

If you still disagree with what I am saying, I have more to say, which I currently have saved locally. I figured I’d hold off on that because I think there may be a misunderstanding here.

But still, let me re-affirm what I said in that the effect of slowing down that is directly observed is irrelevant to relativity. The slowing down that is observed is far greater than the actual slowing of time, which is observed by accounting for the time it took light to reach you.

You’re right. My Boo-boo.

Those who do not learn from the past are condemned to relive it. Georges Santayana

One <additional> thing I’ve wondered about being accelerated to nearly C, and that is…

relative to what?

The Doppler effect can tell us that the most distant galaxies are traveling away from us at a significant percentage of C. That means I’m already traveling at the same percentage of C relative to them. So, in order for me to observe the effects of near-C time dilation, what do I have to be relative to to? Other stuff near me, or can I be traveling at near-C relative to something farther away? I don’t think direction has anything to do with it, just acceleration.

But acceleration relative to what where?