Professional philosophy news

How do we know time-travelers aren’t constantly changing the past?

wonders movie website i09 here. They give seven reasons how we can know this. 1) Time travel is impossible. 2) Even if it were possible, you couldn’t change the past. 3) You might only be able to travel back as far as the point where the first time machine was invented. 4) Nobody’s showing off. 5) There would be physical traces. 6) Screwing with cause and effect would change the laws of probability. 7) History proves they’re not doing it.

The linchpin is offered by Gary Feldman (physics, Harvard):

“If time travelers are constantly changing the past, they are not very good at it. Why did they not avoid two disastrous and pointless world wars in the past century?”



One response to “How do we know time-travelers aren’t constantly changing the past?

  1. Richard Baron June 2, 2012 at 3:03 pm

    Here’s another argument. Suppose that people could travel back from the year 3012 to the year 2012, then back to their own time, with ease. Someone in 3012 would take cash from their bank, bring it back to 2012, and deposit it in an interest-bearing account. Then back in 3012, they would withdraw and spend the enormous amount that had accrued. This would be such a popular pastime in the thirty-first century that huge amounts would be put into interest-bearing accounts in our century, and the banks would never have to pay interest rates greater than the rate of inflation. OK, they don’t right now, but sometimes they do, so time-travelling investment tourists can’t exist.

    There is one counter to this argument. Banknote designs are changed every now and then. Notes from 3012 would not be recognized or accepted by banks in 2012. Indeed, the need to guard against time-travelling investment tourists is a reason to change the design regularly, alongside the need to keep one step ahead of counterfeiters.


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