Friday, May 3, 2013

Impossibility of time travel

The recent Rupert Sheldrake on "Science Set Free" podcast interviews a scientist with crackpot ideas. His excuse is that Thomas Kuhn discovered that science was about groupthink, and not truth. A comment defends pursuing untestable ideas because string theory is not testable either.

Here is a poll of philopher beliefs, but it does not directly ask about Kuhn's paradigm shift theory.

Speaking of crackpot ideas, Scott Aaronson's new book defends time travel:
Yes, the Grandfather Paradox has often been put forward as a “proof” that time travel into the past is logically impossible. But there are several loopholes in that “proof.” One of them is the possibility of resolving the paradox probabilistically or quantumly (as Deutsch proposed). Another loophole is that maybe Nature simply always finds a consistent deterministic evolution, no matter how unlikely it seemed a priori. (E.g., if you went back in time and tried to kill your grandfather, you’d always discover that the gun jammed, or you forgot to load it, or he recovered from the gunshot wound and went on to sire your parent, etc. etc.) So really the Grandfather Paradox should be seen as a central, obvious difficulty that any account of closed timelike curves needs to overcome.

Your resolution of the paradox, in your first comment, is actually a good way to describe or visualize what happens in Deutsch’s resolution. (Indeed, since Deutsch avidly believes in the Many-Worlds Interpretation, he would regard it not just as a convenient way to visualize, but as a literal description of what happens in his proposal.)

However, one can also invent more complicated time-travel scenarios: for example, what happens if you flip a fair coin, and go back in time and kill your grandfather if and only if the coin lands heads? The beauty of Deutsch’s proposal is that it gives you an automatic way to compute a consistent story for any possible such scenario.

(Spoiler alert: in the above example, the solution is that you’re born with probability 2/3 and not born with probability 1/3. Or if you prefer, you’re born in 2 of 3 parallel universes, and not born in 1 of them.)
This is pretty wacky. I say that the Grandfather Paradox disproves time travel.

Here is a completely separate proof that we will never see time machines. If some future advanced civilization ever got time machines, then surely someone would decide that they are a really bad idea, and go back in time to kill the first inventor before he can create a time machine.

If you believe in Many-Worlds, then I suppose a time machine could take you to a parallel universe. But Many-Worlds is another crackpot idea with no scientific merit.

Frank Wilczek is promoting time crystals. These are not as crazy as time machines, as you cannot use them to violate logic and physics laws.


  1. >> Thomas Kuhn discovered that science was about groupthink, and not truth.

    Not to defend cracked-pottery but I wonder what Kuhn might say in defense. I imagine he might say that among a group of accomplished, esteemed scientists who've cozily agreed upon a truth, such group being approached by Dazzling Young Apprentice Scientist With New Theory, in a case where History eventually proves Young Scientist correct, History will relegate the cozy group into one of three camps. 0. They were for whatever reasons unable to apprehend and/or comprehend the new theory. 1. Their groupthink prevented them from adopting the new theory. 2. Their open-mindedness paved the way for their eventual adoption of the new theory. Setting aside Case 0, all I feel I can safely surmise is that History will judge these hypothetical eggheads as either "wed to their own pet theory which spouse eventually proved unfaithful" (groupthunked) or "ready to jump into some other wifelike's bed" (nimble and successful). Critics abound who decry the "all-too-often ready to jump" types, but some make the leap to a more successful theory more quickly than others do. Those are the same type as I believe you are here -- the claim is that a group of esteemed scientists whose pet theory "loses bigtime sway" (for lack of more fetching term) can be safely said in many cases to have been "merely wrong" rather than "wrong, and because of groupthink". I take a strong version of this position to be "groupthink never enters into voting for any theories whatsoever" (because science by its nature is nonpolitical i.e. a voter for a theory "because my mentor did" is tossed out as an outlier because he/she isn't voting as an individual solely on "science" grounds) and a weak version is "sometimes scientists back wrong theories due to groupthink, but sometimes due to other causes".

  2. It's not supposed to be political, see ... [this just in]

    "The Royal Society has defended its election of the Duke of York as a fellow despite some of its members branding him 'over-colourful' and lacking a scientific background. [ ] Some fellows were in revolt over the Duke's appointment [...]"