Quantum Mechanics

[Philip Danziger]

This discussion is dedicated to the discussion of the field of Quantum Mechanics. All questions are welcome! The world of the Quantum has been introduced to Nerdfighters!

[Philip Danziger]

Just for anyone interested, quantum mechanics is the study of microscopic phenomena where the 'normal' laws of physics no longer appear to apply. In the quantum world, particles behave as both particles and waves, and you can never be sure of any particle's exact position or velocity, the famous Uncertainty Principle.

[Happy Smiley Robot]

This is a fantastic idea for a discussion, I've wanted to know more about this for a while. I know this is pop-science, but could you explain Schrodinger's cat properly? Everyone I ask gives a different answer and only half are related to quantumn theory!

[Philip Danziger]

This thought experiment was made by Erwin Schrodinger, in response to quantum theory, which he thought was just plain ridiculous. Now, one of the fundamental properties of quantum theory(or at least the popular Copenhagen interpretation) is that what we can't measure doesn't concern us or science, which is fundamental to all science, but quantum theory was the first time this fact had any relevance. So, back to our cat! Schrodinger made this new theory look ridiculous by showing how incompatible it was with the macroscopic world. In his thought experiment, he has a cat, a Geiger counter (radiation detector), and an unstable atom, along with some poison and a hammer. Since, as mentioned above, we don't care about what can't be measured, we can only predict probabilities, such as the chance of the unstable atom decaying. So our unstable atom has a 50% chance of decaying within an hour. When it decays, it will emit some radiation, which will trigger the Geiger counter, which triggers a hammer, which breaks the vile holding the poison, killing the cat (poor cat :( ). So if we came to the box after an hour, there is a 50/50 chance the cat is dead, and thus, according to Quantum mechanics, it is dead and alive at the same time!  Schrodinger found this ridiculous, and he was right. Even though in pop-culture today the cat is said to be dead and alive, the theory succeeds in finding the incompatibility between quantum physics and 'normal' physics, as we know the cat cannot be both dead and alive, while an atom can be in two states at once. This is all very confusing to many, so if you have any questions concerning the explanation, just ask!

[Happy Smiley Robot]

That was a fantabulous explanation, thankyou! :D

[Jack Copping]

How is it that quantum particles can behave as both a particle and a wave?

[Philip Danziger]

The 'Uncertainty Princple' I mentioned earlier stems from the fact that we can't observe the universe without affecting it. The reasoning goes that you see something simply with light, or on the small scale, bouncing a photon (particle of light) off of it. However, light with a longer wavelength doesn't accurately show the position, and shorter wavelengths have too much energy and affect the particle being observed too much to show proper momentum of the particle. Since we cannot ever know the exact position and velocity of a particle because of these factors, we have the uncertainty principle. And without knowing the exact position of a particle, physicists can only show an area of probability of where it might be, represented by a wave. And that, my friend, is where you get the wave associated with every particle! But remember, the wave isn't necessarily just a mathematical tool, the particles actually behave like waves!

[Shakir Ahmed]

I think you got it wrong. The Uncertainty Principle doesn't stem from the famous fact called "The Observer Effect" that you mentioned above.

It's true that particles change their properties (momentum, position) as soon as being observed. This is called The Observer Effect which states that we can't measure precisely the momentum and the position of a particle at the same time, whereas Uncertainty Principle states that a particle can't have a precise momentum and a precise position at the same time.

But the question is "Why can not a particle have simultaneously certain momentum and position?" Well, because of the wave-particle duality. And the interesting fact is that wave-particle duality is not yet explained. It is believed to be a fundamental property of universe. Scientists accept it without fully understanding it. No one has yet been able to answer the question “Why can a particle have wave-like properties?”  

[Philip Danziger]

Speaking of how we don't fully understand the wave particle duality, I recently read a book that included a theory where all particles are effectively little time machines, and being influenced by both the past, present, and future is what gives them their probabilistic and wave-like nature. If the theory is proven or not, it's still an interesting thing to think about.

[Philip Danziger]

Personally I usually think of particles' wave like nature in terms of the Copenhagen interpretation, where the wave associated with every particle is just a representation of its possible positions (I think I may have worded that poorly), but I'm sure there is something more fundamental we don't know yet that gives particles the wave-like behavior (or waves particle-like behavior).

[Shakir Ahmed]

I just wanted to say that the Observer Effect and the Uncertainty Principle are completely two different things. Particles can’t have an exact momentum and an exact position even though we don’t observe them. That’s why we measure the probability of corresponded variables instead of measuring precise value of them.

Yeah. There could be something more fundamental that could even explain wave-particle duality. We just don’t know it yet.

[Shakir Ahmed]

You know thermodynamics is some sort of statistical physics. Statistical physics uses probability and statistics and figures a mathematical model with large population to solve a physical problem. Thermodynamics expresses that entropy of our universe is increasing.  In other words, world is cooling down. Statistically it’s true for any system. But you can’t expect all the atoms or particles belonged to the system to comply simultaneously with the statistical result.

So, some individual particles can simply accomplish opposite thing. It necessarily doesn't hamper the second law of thermodynamics. But reducing entropy is as same as going back to the past. I think you've got the fact.

Many particles around us are actually traveling in time. They are virtual particles. They probably don't exist, but still they're important.

I’m pretty sure you are familiar with tachyons. As they travel faster than light, they probably travel back in time. 

[sina chiniforoosh]

I just love it, but what's more interesting I think, is the interpretations of quantum mechanics... I still don't know a lot... but I'm so interested...