Notes for 4/8/2026

 4/8/2026
[Philosophy Club every Monday, 4-5 pm, in the Buchtel College of Arts and Sciences room 436 ("The Cave")]
[Bioethics Club: Mondays from 5:30pm-6:30pm in Leigh Hall 408]

If you COULD travel into either the past or the future but the trip had to be ONE WAY:
a)    WOULD you opt to time-travel at all?
b)    Which “direction” would you travel?

 
If presentism is true, then neither the past nor future exist as possible destinations for time travel.


Modus ponens
If A then B
A
Therefore B

Modus Tolens
If A then B
Not B
Therefore Not A

Here is a related concern:

How is motion through time (even at the normal rate) possible if the future doesn’t exist?

How is any change at all of any sort possible under presentism?
(How is it possible for something that exists to become something that doesn’t exist or for something that doesn’t exist to become something that does exist? )

Empirical arguments against presentism:
-    Special relativity shows that there is no universal “now” and no universal rate of time passing.
-    Quantum physics also suggests that quantum events don’t have a fixed temporal order (and that “before” states can be altered on the basis of “after events).




The falsity of presentism seems like possible good news to proponents of time travel.
But is it?

Causal limitations on time travel

A brings about B

If time travel is possible into the past, then it would be possible to avoid B by making A not happen.

But B is what brings about your elimination of A.

Causal paradox (best known example = grandfather paradox)




In the Grandfather Paradox, how, exactly, does “chronology protection” work? What, exactly, prevents you from killing your grandfather?



The “parallel worlds” attempt to solve the grandfather paradox:
When you go back in time, you arrive in a parallel world where you can kill your grandfather in THAT world.

But this isn’t really a solution. It doesn’t solve the problem that led you to go back in time in the first place.

Going into the past to eliminate A results in your only eliminating A’


 
Another variation: Suppose you don’t have accepted a 1-way trip into the past. You track down the person who invented the time machine. If you steal a particular book from that person’s library, they will never invent the time machine and you will never have gone back in time. Problem solved?



Time travel into the past might be possible so long as causality is not violated.


 
So: You go back in time and meet the person who invented the time machine. They haven’t figured out yet how to build it. But there’s a book you know of that could help, so you obtain that book and give it to them making it possible for them to invent the time machine. (No paradox?)



Bootstrap “paradox” (is it really a paradox?)
You want to become rich, so you go back in time bringing with you the plans for the time machine. Now YOU “invent” (copy) the time machine and get rich from it. (No paradox?)

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