How to Build a Time Machine

frank249 writes "The September issue of Scientific American has an article discussing the possibility of time travel. They say that it wouldn't be easy, but it might be possible. It could be a while until we can expand worm holes and tow them to a neutron star but didn't someone say that if it is possible it will happen. If it is impossible it will just take a little longer."

Comments

Travelling through time can't be possible.

[Frobozz0]

I've given this some real thought and if it's possible to time travel at all, it would not be as how we see it in the movies. I'm a philosopher at heart and I think these points have been heard in many different forms:

• If I could travel back in time, then why would we not have seen people doing so already? Wouldn't travelers from OUR future visit us now?
• If it is at all possible, then it can't be possible to effect the future or the past-- you may only observe.
• There are very strong arguements that TIME DOES NOT EXIST. Everything is relative to the observer, and many arguments have been made that suggest that Quantum reality is true-- that all possibilities in the universe are played out and live in discrete "strings" of reality. Sometimes, those strings cross, and you get phenomina such as Deja Vu. Take a lok at the following book: The End of Time, by Julian Barbour. Also, anything by Stephen Hawking.
• The effects and observations we make concerning time travel may be directly influenced by our transendental means of observation. That is, how we perceive reality is completely based on how we observe it. Really take a second to think about that, because it's one of the most profound concepts I've come accross.

I just don't see it as a reality. I think what will actually happen is something altogether different-- but not a physical human being traveling into the past to hang out with Babe Ruth. Know whut I mean, vern?

Nothing Can Move in Time, Forward or Backward

[Louis Savain]

Forward time travel is of course possible right now

This is sad. Why does the physics community insist on putting out such unmitigated crackpottery? The truth is that nothing can move in time, forward or backward. The entire spacetime of relativity is changeless, from the infinite past to the infinite future. Karl Popper had a name for it: Einstein's block universe. More details can be found at this site:

Voodoo Physics

It's simple, really.

[TrumpetPower!]

Time travel isn't possible, except for the everyday kind that your wristwatch measures.

If time travel were possible, somebody (human, alien, whatever) from the future (perhaps billions of years into the future, or maybe just next week) would have traveled into the past already.

So, let's consider what can happen. Somebody will travel back in time to before the initial discovery in order to beat the ``original'' researcher to the punch. Now, we've got a cascade of ``inventions'' of the time machine racing backwards through time. Life and time-travel technology reach the earliest time after the Big Bang that the two are sustainable and both are prolifically spread throughout the infant universe. Clearly, that hasn't happened.

Don't think that some sort of morality would prevent this from happening, either. Time travel is an incredibly powerful weapon; consider what a knife to the throat of the infant Hitler would have done to history, and how many people would leap at the chance, consequences be damned. All it would take is one person to do so...at any time in the next many billion years.

The instant time travel becomes possible, the only possible method for self-preservation is to race to the beginning. After all, how do you know that some far-distant alien race with souls of pure evil won't do the same just out of spite?

There's a wonderful quote, and I wish I could remember who said it. ``Time is Nature's way of keeping everything from happening all at once.'' The obvious corollary is that, if you can break time, then everything will happen all at once.

Some people try to get around this in a few different ways. For one, there's the many-universes ilk: each act of time travel creates a whole new universe. In such a case, all of those universes would be on the same headlong rush to take time travel as early as possible. Besides, think of the incredible amount of energy and information needed to duplicate the universe--but I digress.

Others try to justify it by saying that it requires huge energy sources or otherwise make it hard. To this I say, ``so''? All you're talking about is a hard engineering project that'll take a lot of time. And--guess what? Even if it takes ten thousand years to build and the energy output of several stars, the payoff is worth it. Again, the alternative is to let somebody else do it...and invite certain disaster.

I take the mere fact that I'm typing this note as all the proof that I need that time travel is pure fantasy.

Cheers,

b&

Re:It's simple, really.

[Dr. Manhattan]

If time travel were possible, somebody... would have traveled into the past already.

As has been noted, GR time machines can't go back any further than when they were assembled. So you can't goi back any further than the first one.

...consider what a knife to the throat of the infant Hitler would have done to history...

It's impossible to know. History is chaotic. Consider a simple thing, like weather. That's chaotic, with a lambda on the order of a few days. You appear, kill baby Hitler, disappear. A few days later, it's raining instead of sunny.

All the weather, subsequently, is different. That affects when people make love; even a small difference in position and timing changes which sperm reaches the egg. The next generation consists of completely different individuals from the one in "our" history. Madonna and Nelson Mandela are never born.

If you can change the past, then you must, and you can't predict how you will change it.

I cover all this and more in my time travel page.

Re:The flaw:

[sgross]

An important aspect in relativity is that you have to take into account the observer. Relativity says a massive object cannot be travelling at the speed of light in any reference frame.

In the reference frame of an outside observer, your objects a and b are each travelling at half the speed of light. You are correct in saying their relative velocity is c, based on how relative velocity is defined (simply the addition of two velocities) but neither of the objects is travelling faster than the speed of light.

To find out the speed of one object in the reference frame of the other object, we must use the special relativity formula for the addition of two velocities u and v:

(u + v) / (1 + uv/c^2)

If obejcts a and b are spaceships, an astronaut in spaceship a would see spaceship b travelling away from him at 0.8c, obviously not in excess of the speed of light. For a good discussion of this, see this site.

As to your second question, obviously the speed of light is not infinite speed. However, a massive object travelling at the speed of light would have to have infinite momentum. The relativistic formula for momentum is:

p = mv / sqrt (1 - v^2/c^2)

as v approaches c, p approaches infinity. It would require an infinite amount of force to accelerate anything to a state of infinite momentum (or a finite amount of force applied over an infinite time). Since neither of these things are possible, everything with mass must travel at less than the speed of light.

Also note that by definition an object's momentum cannot be greater than infinity, so just by the limit of the momentum formula we can deducde that velocities greater than the speed of light are impossible.

just a thought

[steinerik]

If we ever was to be able to build a time machine, shouldnt we then at some point have had visitors from the future? As far as I know, we still havent so I believe it's very unlikely that a time machine ever will be built...