Thursday, June 14, 2012

Is time travel possible?

Maverick quotes John Locke (great English philosopher and father of the American constitution) on the impossibility of two beginnings of existence, as follows.
When therefore we demand whether anything be the same or no, it refers always to something that existed such a time in such a place, which it was certain, at that instant, was the same with itself, and no other. From whence it follows, that one thing cannot have two beginnings of existence, nor two things one beginning; it being impossible for two things of the same kind to be or exist in the same instant, in the very same place; or one and the same thing in different places.
He infers from this the impossibility of a soul not existing from the death of its body in 1890 (say) to its rebirth in a different body in 1990 (say). Does this also refute the possibility of time travel? Dr Who gets into his trusty police box in 1999 and travels to the year 2101. He lives out the rest of his life in the 22nd century and never travels to the 21st century. Therefore, Dr Who never existed in the 21st century. But he exists at the end of the 20th, and exists again at the beginning of the 22nd. Is this inconsistent with Locke's maxim about the impossibility of two beginnings? It's odd. The maxim seems correct, and it seems impossible that the same thing cannot have two beginnings. It seems almost a logical truth. Yet the impossibility of time travel does not seem a logical truth at all.

9 comments:

William Connolley said...

In theory, yes, in certain universes (http://en.wikipedia.org/wiki/Time_travel#General_relativity, etc). The answer comes from physics, not philosophy :-)

Anthony said...

The link you give talks about backward time travel, and it doesn't even mention whether or not the time travel is continuous or discontinuous.

Also, of course, Ed is talking about "logical possibility", whatever that means.

Forward time travel, is, of course, quite natural. We are always travelling forward in time, and theoretically it would certainly be possible for someone to enter a "time machine" in 1999 and travel at the speed of light to the year 2101. The question is really whether or not time travel can be discontinuous. Even backward time travel wouldn't be a problem for Locke's argument, so long as it was continuous (travel from 2012 to 1500 goes through, at least for a moment, all the times in between).

Anyway, my problem continues to be the lack of definition over "logical possibility". Modern physics has shown quite a few things which were once thought to be "logically impossible" (and, depending on your definition, actually were "logically impossible"), which are now known to be physically quite possible.

The big split between myself and Ed in this realm is that I hold that logic itself is empirical, whereas Ed holds (I believe) that knowledge of logic is innate. The important implication of my position is that, even if we do come up with a definition of "logical impossibility", it is essentially arbitrary what truths we call "logical" and which ones we call something else.

AC said...

In Carl Sagan's "Contact", scientists travel through a "wormhole" in spacetime, i.e., a shortcut through both space and time.

Kip Thorne, a theoretical physicist, was consulted by Carl on this aspect of his novel and Kip was able to show that such a wormhole was a solution to the equations of General Relativity. Such a wormhole is, in fact, a time machine. (Hawking later convinced Thorne that quantum fluctuations probably would probably destroy the wormhole at the moment time travel became possible).

The key, it seems to me, to this logical puzzle is that, while in the wormhole, the scientists exist but they exist "elsewhere and elsewhen".

Consider the case that a wormhole existed such that one could step into the wormhole somewhere in the US and walk out a second later in the UK.

During that second in the wormhole, one cannot be located anywhere "in between" the US and the UK. Questions: does one cease to exist for that second? Would this also be a potential violation of Locke's maxim?

Edward Ockham said...

>>The question is really whether or not time travel can be discontinuous.

Surely that was obvious.

Edward Ockham said...

>>During that second in the wormhole, one cannot be located anywhere "in between" the US and the UK. Questions: does one cease to exist for that second? Would this also be a potential violation of Locke's maxim?

Yes and that is precisely the problem. It is a logical problem (rather than a scientific one) in that Locke's maxim seems to be true. You can't 'begin to exist' twice. Yet discontinuous time travel certainly seems a logical possibility, perhaps even a scientific possibility.

Hal said...

>>The question is really whether or not time travel can be discontinuous.

Surely that was obvious.


It seems obvious to me that the question is actually whether world lines can be discontinuous.

Hal said...

nor two things one beginning

As logical as this sounds, Nature evidently (or should I say effectively?) allows exactly that in the phenomenon of pair production.

Indeed, the entire edifice of Quantum Field Theory depends on the simultaneous creation (and destruction) of particles at a point.

For example, in pair production, a photon is destroyed and an electron and positron created at the same time and in the same place.

David Brightly said...

This is reminiscent of the 'ship of Theseus' puzzle. Is the ship built out of the recycled planks just the original ship 'reincarnated'? In which case, against Locke, do we have to say it has had two beginnings? While the planks were in storage the world had one fewer ships. Likewise, while I'm asleep the world has one fewer consciousnesses. When I wake up is yesterday's consciousness reborn or is today's consciousness something new? Is there a 'right' answer?

Anthony said...

>> "It is a logical problem (rather than a scientific one) in that Locke's maxim seems to be true. You can't 'begin to exist' twice."

So, it "seems to be true", therefore it is logically necessary? Is that what "logically necessary" means?