Quantum mech, choices, and time travel too!

Physicist: Recently I sent a series of emails back and forth with a reader that seem interesting enough to post. Conversations (near a chalkboard especially) are the best way to learn just about anything.


I wake up at 6am.
I brush my teeth get dressed and go downstairs.
I eat my breakfast at 7am and I’m just about to leave out the door.
when i notice there is an apple and an orange in a fruitbowl on a table next to me
at exactly 7:02am. I CHOOSE to take the apple.

I then make a choice whether I should choose to take my car or save gas and take the bus.
I CHOOSE to take the bus.

SUDDENLY. when i walk into the office, a strange event occurs and time starts moving backwards.
It goes back and back and back until finally it’s 6am in the morning and i wake up,
brush my teeth get dressed and go downstairs to eat my breakfast.

Here’s my tough quantum mechanics question for you,
at exactly 7:02am will I still choose to take the apple?

And if this process were repeated over and over and over again for about a million times. will the choice I make ALWAYS BE the apple?


I’m working on a (to long) post about Bell’s theorem. The thought experiment you propose, about going back in time, is one of the better ways to understand it.

To actually answer your question: if choosing the apple is based on some quantum mechanical process in your brain (and there’s a good chance that at least some part of it is), then every time that choice is made the result is random. Time travel or not.
Part of the weirdness comes from the fact that every possible thing that can happen does. So (even when you time travel) some versions of you take the apple and some versions don’t.


Ok, remember how I took the bus in the thought experiment?

My question is, does quantum mechanics also apply in a reversal of time?

For instance, lets say that time started to slowly reverse.
Will I always get onto the bus backwards and head home.


will my car magically appear (even though i didn’t take it)
and will I backwards drive home in that?

So the concluding question is,
do quantum principles apply in a reversal of time as it does when time moves forwards?


You’ll often hear “everything that can happen does” so if a particle can take two different paths it will actually take both.
If I understand your question correctly, the answer is yes. It turns out that “everything that could have happened did”. The “branching” goes both forward and backward in time. This is demonstrated by things like the “Franson experiment” that demonstrates the interference of a single photon with an earlier version of itself.
Driving a car, for example, will leave telltale signs that later make it impossible for you to have actually taken a bus. Chair fibers, leaving tire tracks, you’ll remember it, etc.
But if, in every way, you could have done either one, then you did both (no magically appearing cars).
This is actually the backbone of the Feynman path integral technique.


So are you telling me that just next to us, could exist a place where the Nazis won the second world war?
A place where there exists a flying spaghetti monster? (to quote richard dawkins)

Or even a place out there in a dimension somewhere where there exists an all knowing omnipresent, omnipotent, all encompassing being who “watches over us” etc..


Sure. BUT, it’s impossible to interact with things that are even a little bit different. For example, a stream of identical photons (lasers) will all interact with each other strongly. You can see evidence of this in effects like speckling. Non-coherent (regular) light is made up of all kinds of different photons, and the best way to figure out how they’ll behave is to assume that they’ll ignore each other. This is sort of a metaphor, and sort of a concrete example.
So while, yes, there are almost certainly universes where the Nazis won, it doesn’t matter. It’ll never have any impact on our universe whatsoever.
A good rule of thumb is: if there is any conceivable way, whatsoever, for anything to tell the difference between universes, then they can’t interact (from the perspective of that thing that can tell the difference).


Couldn’t that then solve the entire God dilemma? I mean if in only one of these infinite dimensions there existed an all encompassing all knowing all powerful entity, wouldn’t this entity then transcend all dimensions? (since he is all encompassing)


If you want to consider God, then it’s best not to do it in any kind of physics based context. That being said:
Remember that if two universes are even slightly dissimilar, they won’t interact at all. By “slightly dissimilar” I mean something like a single electron being conspicuously out of place.
So any existing Gods that follow the most basic laws of logic and quantum mechanics will be stuck in their native worlds.
If you’re not worried about Gods that follow physical laws, then, again, physics is literally the worst possible forum.
Also, you have to be careful with this kind of reasoning. You can make up just about anything and claim that it should exist in every version of the universe.
The rule “anything that can happen does” carries a bit more heft that it seems to at first. If something can’t happen, then it doesn’t happen in any version of the universe.
For example, spaghetti can neither fly nor think, so the FSM (pasta be upon him) can’t exist in any universe, no matter how much anyone dresses like a pirate.

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6 Responses to Quantum mech, choices, and time travel too!

  1. Scott says:

    You know, the problem I have with many worlds theory, and string theory, and the anthropic principle, is that they make the universe more complicated, rather than less, which has usually been a sign that we’re headed in the wrong direction.

  2. The Physicist Physicist says:

    Almost none of the infinite versions of you have talked smack about the many worlds hypothesis and the anthropic principle and lived to tell the tale.
    Happy anniversary by the way!

  3. Martin says:

    Now my issue with many worlds, and please stop me if I’m being an idiot, is similar to that of the “if a tree falls in the woods…” debate. If it is completely impossible to see, hear, interact with these other universes, do they really exist?

    Or am I needlessly crossing physics and philosophy?

  4. The Physicist The Physicist says:

    That’s a completely valid criticism!
    And, at least in a physical sense, it isn’t that different than a tree falling in the woods. If trees and woods follow all the relevant physical laws to the letter, regardless of who’s looking or not looking, then a falling tree will definitely make a sound (“sound” here defined as the vibration of the air, as opposed to the act of perceiving that vibration). In order for a falling tree not to make any sound, but only when no one is around, requires some pretty fancy physics. So while no one has ever heard a tree fall when they weren’t there to hear it, most people (physicists at least) assume that the noise still happens.
    Same idea with Many Worlds. You just keep using the physical laws we know about, and you end up with the MWI. Other interpretations of quantum mechanics tend to need a lot of very weird physical laws that violate other, well-established laws.
    The laws you need to get around MWI aren’t a lot more complicated, or less weird, than (taking form this example) “the moment that no one is within earshot, air suddenly loses the ability to vibrate”.

  5. Xerenarcy says:

    there is one interesting scenario i’ve been trying to understand in depth: what happens when two identical particles approach one another at a low angle (in the sense of marathon runners holding hands), such that it becomes impossible to determine if you’re dealing with one particle or two.

    suppose you have particle 1 and particle 2, they’re moving from left to right, 1 slightly downwards and 2 slightly upwards such that for a time they interfere with one another in close proximity…
    i’m speculating here but i expect there to be a double-entanglement of sorts (for lack of a proper term for it), where due to the positions becoming identical, the identity of p1 and p2 has a 50% to be switched around when they separate again. however when they do, i suspect the momentum too would become correlated due to the mixup of whether 1 or 2 had the more downward or more upward momenta. so assuming we are following their identities, p1 and p2 have a 50% chance to swap positions and a 50% chance to swap momentum, giving 4 potential results.

    however, due to them existing in the same position at the same time (with some probability of this being possible), the system of two particles potentially behaves as a superposition of a single particle, that has a 50% chance of slightly up or slightly down momentum. but this would suggest that one particle has vanished somewhere! so to conserve that vanishing particle, it can be accounted for by appearing in another universe.

    to recap: p1 and p2 are made to interfere with one another, such that their momenta are different but for a time their position could have been identical. under the circumstances the system is ambiguous as to the number of particles you are dealing with for that instant. if p1 and p2 cannot be told apart, the system should collapse to one particle with 50% probability to have p1’s momentum and 50% to have p2’s. because there are only two outcomes, the identity of the particle is lost and arguably you are left with either p1 or p2 not knowing which. and since position was the same, one or the other identity cancels out each time, meaning that in one universe you get p1 with p1 or p2’s momentum, and in another universe you get p2 with p1 or p2’s momentum.

    hoping that makes sense, and curious if this has been looked at before as i’m having a hard time making sense of this confused state of things.

  6. Devon S says:

    If you take Life is Strange’s answer, I think the universe keeps killing your friends and laughing about it.

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