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Thought Experiment that's bugging me

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backpack Since: Jan, 2011
#1: Aug 29th 2015 at 7:47:00 PM

Ok, this is a thought experiment that's been bothering me for some time. I came up with it on my own, but if it has a formal name let me know. A ran it by a friend of mine, who basically asserted that it can't be true, but didn't give a good reason.

Now, let me start with my premises:

-Firstly, I accept that Quantum Mechanics is true, meaning that quantum phenomena do not exist in a single state until observed, and what state they collapse into is chance-based.

-Secondly, while I don't have the expertise to refute it, the Many World's interpretation always seems too good to be true to me, so I assume that there's only one world, and any possibility that isn't realized ceases to exist.

Now, let me present the scenario I came up with to explain my issue to my friend:

You perform the Schrodinger's Cat experiment. Then, I render you insubstantial, so you can observe the universe, but cannot affect it in any way (I know this is impossible, but it's a thought experiment).

Then, I send you back in time, and you observe yourself performing the same experiment. You can't change any of the things leading up to the experiment, but since the probability is 50% there's still a 50/50 chance you might observe a different outcome than what you saw before.

Here's the thing that bugs me: Since your presence had no effect, is the time-traveler even necessary for this to happen?

And if the time-traveler is unnecessary, then why can't the past just randomly chance? Couldn't some quantum phenomena from a billion years ago just suddenly happen differently, so that we all cease to exist?

Fighteer Lost in Space from The Time Vortex (Time Abyss) Relationship Status: TV Tropes ruined my love life
Lost in Space
#2: Aug 29th 2015 at 8:16:57 PM

We can have this conversation if you want, but I have to tell you, sadly, that your rules for the experiment contradict its premise.

  1. An observer that does not affect the thing observed is impossible. That you have stipulated this means the experiment does not conform to the physical laws of our universe.
  2. Time travel backwards is impossible in our understanding of physics.

Also, Schrodinger's Cat is a thought experiment, not an actual thing you can do in a laboratory. The cat is not literally simultaneously alive and dead. It is one or the other, but we don't know until we observe it. That is what the experiment is intended to demonstrate. It doesn't matter whether the hypothetical observer has a tangible effect on the thing being observed.

Quantum mechanics does not mean that stuff can "un-happen". Causality still exists. If you insist on backwards time travel being possible, then you have removed one of the foundations of our understanding of physical reality and are now working in the realm of fiction.

edited 30th Aug '15 6:30:36 PM by Fighteer

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
DeMarquis (4 Score & 7 Years Ago)
#3: Aug 30th 2015 at 5:51:22 AM

But even granting all the premises of your thought experiment, the answer is "no". According to the latest models, the universe exists simultaneously throughout time. What happened happened, and in fact there is no such thing as "change". Change is a kind of illusion born from the fact that we only occupy one point in time at a time.

I'm done trying to sound smart. "Clear" is the new smart.
storyyeller More like giant cherries from Appleloosa Since: Jan, 2001 Relationship Status: RelationshipOutOfBoundsException: 1
More like giant cherries
#4: Aug 30th 2015 at 1:35:45 PM

I suspect that this is a question which could be answered just by learning more physics. Speculating about quantum physics without understanding it isn't very useful.

Also, I'd argue that Schrodinger's cat isn't just a thought experiment. The fact that we can't do it in a lab is a limitation of current technology, but I think that in principle, it could be done.

edited 30th Aug '15 1:36:48 PM by storyyeller

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backpack Since: Jan, 2011
#5: Aug 30th 2015 at 3:17:46 PM

@Fighteer: That's why I specified that it's a thought-experiment. The whole point was that the time traveler is actually irrelevant, but I had to explain it in terms of a time-traveler, because otherwise my friend couldn't wrap his head around what I was asking.

@De Marquis: How is that reconciled with probability in Quantum Mechanics?

@storyyeller: And yeah, but I seriously doubt I'll ever achieve the level of understanding needed.

DeMarquis (4 Score & 7 Years Ago)
#6: Aug 30th 2015 at 3:39:10 PM

One way to reconcile the two is to consider the design of the universe and our experience of the universe to be two different things. The sequence of events since the Big Bang is like a computer program- at any point in the sequence when probability determines what happens next, the event is determined randomly. Then, at a "later" point (so to speak) the program is run or activated by allowing a "present moment" to begin at the beginning and end at the end. It's this moving "present moment" that we experience, but its the timeless design of the universe that we model with physics equations. This is all speculation on my part, of course.

I'm done trying to sound smart. "Clear" is the new smart.
backpack Since: Jan, 2011
#7: Aug 31st 2015 at 5:06:32 AM

Wow, that actually makes sense, thanks!

BlueNinja0 The Mod with the Migraine from Taking a left at Albuquerque Since: Dec, 2010 Relationship Status: Showing feelings of an almost human nature
The Mod with the Migraine
#8: Aug 31st 2015 at 6:00:52 AM

[up] Actually, the original scenario has me thinking of a simpler adaptation of it.

Take a "Groundhog Day" Loop. Someone not-the-person-looping sets up the Schrodinger's Cat experiment.

If the looper is the observer, will the outcome change? Will it change even if they ensure they do the same thing every day? If the looper is not the observer, will the outcome change?

That’s the epitome of privilege right there, not considering armed nazis a threat to your life. - Silasw
DeMarquis (4 Score & 7 Years Ago)
#9: Aug 31st 2015 at 7:23:32 AM

It shouldnt change. In fact the looper should simply go back to being the exact same person with the exact same mental state as they had the first time through the loop. Which means they wont remember the future.

I'm done trying to sound smart. "Clear" is the new smart.
BlueNinja0 The Mod with the Migraine from Taking a left at Albuquerque Since: Dec, 2010 Relationship Status: Showing feelings of an almost human nature
The Mod with the Migraine
#10: Aug 31st 2015 at 7:47:20 AM

[up] But that's not what a "Groundhog Day" Loop is. The looper**  is the only thing that doesn't reset. They would remember the previous test(s).

That’s the epitome of privilege right there, not considering armed nazis a threat to your life. - Silasw
Fighteer Lost in Space from The Time Vortex (Time Abyss) Relationship Status: TV Tropes ruined my love life
Lost in Space
#11: Aug 31st 2015 at 11:47:25 AM

It's irrelevant. The outcome is not determined by the observer; the outcome is resolved by the observer. If nothing changes in any way about the quantum state of the experimental environment, then the outcome will be the same each time.

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
DeMarquis (4 Score & 7 Years Ago)
#12: Aug 31st 2015 at 4:47:57 PM

@Blue Ninja 0: I'm working off of Backpack's scenario. Are you proposing a different one?

edited 31st Aug '15 4:48:53 PM by DeMarquis

I'm done trying to sound smart. "Clear" is the new smart.
TotemicHero No longer a forum herald from the next level Since: Dec, 2009
No longer a forum herald
#13: Aug 31st 2015 at 5:39:58 PM

Keep in mind that Schrodinger's cat takes on new significance now that we've realized that "macro" biological structures can exist that exploit quantum mechanics, such as chloroplasts utilizing quantum-level vibrations to make photosynthesis more efficient.

In particular, the one thing that article doesn't mention is that the plant in question would have to be considered an observer, since it "observes" the extra energy it receives as a result of this, and automatically factors in it in regards to its own internal processes and growth. It may not be a conscious process, but it's happening. And if a plant is technically an observer, why wouldn't Schrodinger's cat be?

Expergiscēre cras, medior quam hodie. (Awaken tomorrow, better than today.)
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