Hi everybody. It has nearly been 2 months since my last post. A friend even messaged me to ask if my blog is still alive :)) Yes, it is. It was just hibernating. Sorry for disappearing for too long.
Many of you might have heard of Schrodinger's cat. Those who haven't - don't worry, because I'm going to tell you now. It is one of my most favourite stories and I hope you'll like it, too:
"Imagine you have a safe with some kind of poison (or radioactive substance if you prefer) in it. This poison sprays out completely randomly, which means you cannot predict when it will do so. Now put a cat in the safe and close the door tightly. After 30 minutes, do not look inside the safe, can you tell if the cat is dead or alive?"
This thought experiment was devised in 1935 by an Austrian physicist Erwin Schrodinger. Classically and intuitively, the cat must (and can only) take either of the 2 states: dead or alive. But according to Quantum Mechanics, as long as you don't look inside the safe, the cat's state is a "superposition" of both states. Roughly speaking, it is BOTH DEAD AND ALIVE!
When I first told a friend this story, he was a very clever man and he asked me: "Well, doesn't make sense. What if I shake the safe and listen to the cat's sound? By that I can definitely tell if the cat is dead or alive without looking into the safe, can't I?" Good question. So I should have clarified from the beginning that by "look" I mean "any actions that allowed you to interact with the cat, including looking, touching etc...". (You might argue: "Touching, OK, that counts. But I don't really interact with the cat just by looking at it, do I?". But yes, you do! If you remember your first lesson about light, you'll also remember that to "see" something, you must first illuminate it with a beam of light and let that beam to be reflected back to your eyes. There must be some kind of interaction which reflected the light back. So by "looking" at something, you really do interact with it.)
Back to the case of Schrodinger's cat. Does this result make any sense? Well, though it seems counter-intuitive, it raises no paradox. Let's think about it this way: If you're very careful not to interact with the cat, you'll never know what it really is doing inside the safe. There is no evidence you can give to invalidate the conclusion "The cat is suspended in a state of both dead and alive". Otherwise, your action will alter its state and force it to choose either of the 2 states: When you open the safe and look inside, you can say immediately: "Ah hah, it is alive / dead. so you're wrong!" But remember I said the above conclusion only held as long as you didn't "touch" the cat. Therefore your evidence is invalid because you broke the rule.
"It is just in theory" - you might say. But in fact similar results have been found in practice. They prove the existence of a "superposition of states".
Does the above image ring a bell? Those who did Quantum Mechanics will find it familiar. This is in fact the double slit experiment with electrons. In this experiment, an electron gun accelerates electrons towards a screen which has 2 holes on it. The electrons then incident on another screen, allowing us to see their positions. When only one slit is open, most electrons arrive at the position directly opposite the slit, as expected, and as you get further away from this position, you find less and less dots, indicating less and less electrons arriving. So one might guess if we open both silt, there will be nothing difference: very bright at central region but gets fainter as we move sideways. But what was actually found is electrons form "fringes" on the screen, as shown in the image above, where there are alternative regions of brightness and darkness. HOW COME? Well, this pattern can only be explained if the following statements are true:
1, One electron goes through BOTH slits AT ONCE.
2, Electrons are waves. The pattern observed on the screen results from the interference of an electron with itself.
Does the first statement look familiar? You've met it in the story about Schrodinger's cat! One electron is at both slits at the same time - just like the cat is both dead and alive!
These properties of electrons cannot be explained by classical physics. But to a quantum-mechanical physicist, they are of no surprise. I will not go any deeper into this matter - just an example so that you see the idea of "superposition of states" does apply to our everyday world - it's not just a fictitious idea.
Double slit experiment with electrons is one of the most fundamental and important experiments in Modern Physics. I will discuss this cool subject, together with some basic ideas in Quantum Mechanics, in another post.
So, I will end this post here. Any questions? Fire away! I'll answer you by comments.
See you in post #3 !
--- Meo Fisica ---
PS: I don't own the images in this post. I found them on the internet.
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