[讨论] 给有志于穿越的童鞋一点点理论支持

马丁

Desktop Big Bang Shows Time Travel May Be Possible After All (时空旅行至少在这个简易版的bigbang模型中可行)

来源：http://www.wired.com/wiredscience/2011/06/model-big-bang/

The first desktop model of the Big Bang reportedly showed why time travel doesn’t work. But a new look suggests time travel may be possible after all, at least on the lab bench.

Electrical engineers Igor Smolyaninov and Yu-Ju Hung of the University of Maryland announced in April that their model Big Bang — made from metamaterials that move light just as particles move through mathematical representations of space and time — suggested time travel’s impossibility.

Light couldn’t be steered in a circle, they said. Therefore particles couldn’t loop back to the space-time point where they began. So much for time travel.

But physicist Ulf Leonhardt of the University of St Andrews in Scotland disagrees. “They considered the wrong polarization,” he said. “Making a loop in space is perfectly possible in their model. Therefore, for this model, time travel is possible.”

The discrepancy comes down to the direction in which the light waves wiggle. Light can be considered as two fields, electric and magnetic. These lie at right angles to each other and to the direction in which light is traveling. In the model Big Bang, the electric field has to point upwards from the plane of the metamaterial, or the analogy to space-time falls apart.

According to Leonhardt, the equations used by Smolyaninov are valid when the magnetic field — not the electric field — points upwards. Make the necessary correction, and the barrier to creating time-like loops in the metamaterial disappears.

“That doesn’t mean that real time travel is possible,” he said. “But for this analog model, it is so.”

Smolyaninov stands by his original conclusion, noting that only certain kinds of light can propagate through the model Big Bang in the first place.

“You could create a different physical system, maybe [time travel] would be possible in another model,” he said. “But not in this one.”

minchi

Let's say it is possible to go back in time, but it shouldn't be possible to change anything. If you do, according to the butterfly effect or grandfather paradox, you might be indirectly wiping out your existence. In essence, it's not the "traveling" that's not possible, it's the "change" you leave behind that's not possible. This way, the most one can do in the past is "observe", which is no better than watching a video or looking at old photographs (unless it's so far back that's there's no video or photograph, then I guess time traveling may be beneficial to archeologists and historians?). I believe the main reason one wants to go back in time is to change something, which is impossible, so, what is the point of time traveling?

Unless...you travel back to a parallel world...then you can do anything you want, like the Xiao Lou novel, but sadly nothing will take effect in your "original world"~

马丁

Let's say it is possible to go back in time, but it shouldn't be possible to change anything. If you ...
minchi 发表于 2011-6-11 05:38

    Your presence in the past world itself is a change after all. There is not an ultimate rule that we cannot make anychanges to ourselves. OK, following your logic~~~Have you ever played a Möbius Strip game? An ant can start from the other side and arrive at the same end. Although splitted, it just forms a bigger and more twisted ring... We might not change the result but we may at least change the reason, together with the whole process.

minchi

Yes, history always remains. It's a possibility that whatever you do in the past results in the same future because the "change" you made was actually what happened before you traveled. However, let's say someone tries to change a known future (ie. kill one's grandfather), the change can never be made, because a supreme force will stop the action from ever succeeding....
There's another possibility that changes that does not affect one's eventual birth is //possible//. It may then cause a corruption in the time and altered the future as one knows it (but still satisfy the conditions for one to build a time machine and travel back, although the conditions will be unbelievably strange..)...

Anyways, most of the theories support that the future can not be changed. Thus, time traveling with the intention of changing something, can be deemed futile.

马丁

We started from a physical experiment, and fell into logic trap o>_<o~

Do u mean that only if one finds a reasonable path leading to his future existence can time travel sound true? The matter does not extinct. Even if one kills his grands in the past, it is possible he comes out from another gene combination. We understand that 4+5=9, while 3 and 6 also makes 9~~~

^_^)Y

minchi

Lol, I guess I was off topic from the start.

There are a lot of theories that provide a solution to time paradox, yours is one of them. What I said before, supreme force that prevents change, is another one.
I think we both agree that it's impossible to change the result though?

What I think I'm trying to say is, time traveling seems too dangerous (//possible//risk of time corruption) and reckless to be worth the trouble.