Eliezer S. Yudkowsky wrote:
> I happen to believe that time travel is explicitly
> permitted by General Relativity, which forbids a
> single correct direction of time just as Special
> Relativity forbids an "ether" or single correct
> reference frame.
Time is just movement of matter. The presupposition that dimensions exist is Platonism at its worst.
We "describe" wave-particles of matter and energy as having three dimensions. But in the end, they simply exist. And they move, so we say they move through the fourth dimension, which is fine as a description -- but they're just moving. Part of the problem is not seeing that there is a fabric to space (a unique fabric with special unintuitive properties no doubt -- like a speed limit). Matter isn't moving through anything -- it is a modulation of "something."
It is a leap to say that we are moving through "time" -- and a weird leap at that. Time is our description of movement of matter and energy, which are descriptions themselves of reality. So time is a meta-description.
Let's substitute the meta-definition back into the definition: matter is moving through movement of matter. What!? You see it's clear: a "real" thing can't interact with an abstract idea.
Now, it's true that general relativity says that there is no preferred arrow of time. But there's this thing called symmetry breaking, and the GUTs force and the E-weak force lost that symmetry. So there is an arrow of time. The 2nd law of thermo does apply, etc. These laws all describe the way processes happen -- the movement of matter and energy.
So we go back to the problem of time travel. Clearly, it's possible for time to pass more slowly or quickly for one individual compared to another (subject to the "movement" of clocks in their respective frames of reference and likewise their biochemical clocks). So, as long as it's understood what exactly is meant by the expression, it's fine to say that "traveling" into the future is possible (subject to some odd properties of movement of waves along the fabric of space). No confusion between abstract idea and reality exists here.
The problem is when someone talks about "traveling" into the past. You see, if you didn't properly understand what was meant by "traveling" in the previous paragraph, you are apt to make this mistake. Time doesn't exist as anything but a description. You can't physically travel through a description.
Our mathematics, by treating time as though it were real, does a wonderful job of describing movement along the fabric of space. But it is wrong to reify descriptions as if they were real. Math is not real. It's just a language -- a tool. And paradoxes are the inherent flaw of any language (as per Epimenides). As Gödel proved, all languages are essentially incomplete. Thus, we can expect to have to use many different languages to describe reality. The hope of unifying everything under one language stems from a complete misunderstanding of the inherent limits of "isomorphic description."
Eric (mailto:bijaz@mindspring.com)