After the rise in popularity of a certain Albert Einstein, thought experiments have become a big part of science. So it’s only natural that the field of physics, Einstein’s own expertise, has so many interesting ones. We all know Schrodinger’s cat, but what if I told you that there was more to the story? Read on to find the extensions to the famous thought experiment
#1: Schrodingers Cat:
Schrodinger’s Cat is by far the most famous quantum physics thought experiment. It imagines a cat trapped into a box, that becomes simultaneously alive and dead. For more detail, check out my other blog post on this topic.
Link to post: http://quantum17.info/schrodingers-cat/
#2: Schrodingers cat extension: Wigner’s Friend
The lesser known companion to Schrodinger’s Cat, is the extension. Here, it imagines a room containing the cat in limbo, with another person in the room. This procedure is the same as the original, just with you outside the room. In the original, the uncertainty is resolved when the person checks the box. However, now that you are an ouside observer, does the uncertainty resolve when the person inside the room opens the box? Or does the uncertainty persist until you check on them?
You can read more about it here: https://www.nature.com/articles/d41586-018-06749-8(you can skip to the subheading “What’s in the box?” if you only want the thought experiment)
#3: Schrodingers cat extension: Superposition contradiction
Now lets take it one step further. Now we have 2 labs, both with one scientist inside(friend a and b) and one scientist outside(scientist A and B). Friend a starts the experiment by flipping a coin. If heads, friend a makes a particle with down spin. If tails, friend a makes a particle of equal parts up and down spin(superpostion). Friend a then sends this particle to friend b. Using this particle, friend b should be able to know what side the coin landed on(if up spin, the coin had to be tails).
Now comes the main paradox. Scientist A and B both measure the entire lab, including their respective friends, classifying them each as one system. Keep in mind that this is only possible if quantum theory applies to all things. Now Scientist A makes a certain measurement that puts a quantum system into a superposition.
Using this measurement, Scientist A can deduce what Scientist B’s friend measured, and therefore what Scientist A’s friend’s coin landed on. Scientist B can do the same thing.
Now both scientists compare their results. Apparently, in 1/12 of the cases, the scientists will get contradictory answers. Something is wrong here.
The three assumptions we made were:
- Quantum systems are universally applicable
- Predictions made by different agents using quantum theory should not contradict each other
- If a coin is measured to be heads, then the coin cannot simultaneously have landed on tails
One possible resolution to this conundrum is the many worlds theory, that allows assumption 3 to be wrong. In the many worlds theory, every outcome simultaneously exists on different timelines. Therefore, on a universal scale, the coin is able to be on both heads and tails.
You can read more about it here: https://www.quantamagazine.org/frauchiger-renner-paradox-clarifies-where-our-views-of-reality-go-wrong-20181203/
The experiment referenced is here: https://www.nature.com/articles/s41467-018-05739-8
Bonus: Maxwell’s Demon
In this thought experiment, Maxwell imagines a demon that will seemingly generate energy out of nowhere through the movement of hot and cold air through a heat engine. This demon is in the middle of a room divided in half, filled with a gas at a certain temperature. Due to the properties of gases, we know that certain molecules will move faster, and certain molecules will be slower. Lucky for us, the demon can observe these molecules and can open a small door in the wall to allow faster molecules to pass through into the right side of the room. This would eventually seperate the gas into faster moving, or hotter gas, and slower moving, or colder gas. This can then be used to run a heat powered engine.
So why is the demon allowed to seemingly create energy from nothing? Well, this paradox is resolved by realizing the demon is actually using up energy itself, and will result in a net equal amount of energy in the system.
You can read more about it here: http://webhome.auburn.edu/~smith01/notes/maxdem.htm