Monday, January 20, 2020

An argument against time travel or for temporal parts or for internal time

Start with this plausible claim:

  1. If two objects are composed of the very same particles at the same time, then they have the same shape.

But now consider a statue of a horse that is reshaped into a statue of a tree and then time-travels back to sit besides the statue of the horse. Then the statue of the horse and the statue of the tree are composed of the very same particles at the same time, and yet they do not have the same shape.

I see three ways out of this paradox.

  1. Deny the possibility of time travel.

  2. Subscribe to temporal parts theory and modify (1) to speak of temporal parts of particles instead of particles.

  3. Distinguish external and internal time, and qualify (1) to refer to internal time.

My preference is (4).


Martin Cooke said...

So, to make sense of time travel you need there to be internal time?
That seems like an argument against time travel, to me.

Your plausible claim is a bit like saying that if two objects are composed of exactly the same stuff at the same time, then there are not actually two objects, they are the same object; and hence those two objects have the same shape (and every other property in common too). That is plausible, until you introduce the concept of time travel, and then it becomes implausible. But you want to say that it is still plausible, and use its plausibility to justify discovering an equivocation in "time". I wonder why it is still plausible.

Alexander R Pruss said...

A lot of people think two objects can be composed of the same stuff at the same time. E.g., the lump and the statue. But the two objects will have the same shape, and have some other properties in common--but not ALL other properties (e.g., the lump may be older than the statue; the statue may have aesthetic properties not shared by the lump).

Martin Cooke said...

Still, it is a bit odd to think of the statue and the lump as two objects, rather than as the very same object being considered in two ways, It is a bit like thinking of a person as a parent and as an employee. And surely that reason for the truth of your plausible claim would not be able to withstand the intuition that if time travel was possible then it would be false that the two "objects" would have to have the same shape. I find it hard to believe that many of those who think that two objects can be composed of the same stuff at the same time would retain shape, as one of the common properties of those two objects, were time travel possible.

Alexander R Pruss said...

Suppose I have a statue of Hermes made of a lump of clay, and then I squash it into a ball. Then we would say "I destroyed the statue." (But I still have the lump.) The analogue for the employee case would be firing. But if Alice is working for Bob, and Bob fires her, we would not say "Bob killed Alice", unless this led to her suicide or a drug overdose or something like that. So we don't think of the employee as an entity that comes in and out of existence on its own in the way we do of the statue.

You may be right in the last sentence.