So a few months ago, in one of the science threads that sporadically pop here now and then, the discussion touched a little bit on quantum mechanics (QM). I mentioned that it would be great to have a separate thread just to discuss QM and someone PM'ed me telling me I should create one. At the time, I didn't know much about QM myself, so I didn't really consider it. But after a few lectures in physical chemistry combined with exposure to the official Space thread that gets bumped frequently, and the fairly successful official evolution thread, I've decided to take the plunge. While the chances are likely this will fall on deaf ears, I'd love for anybody interested to participate, because as seen from the quote above, there are no stupid questions.
Since people seem to be very interested in evolution and space, I thought it would be good to have a thread dedicated to another mind-bending aspect of the universe, especially one that is not understood very well. There is good reason for this. Quantum mechanics is complicated, and has several weird implications. But as it turns out, it's not as weird as one may think. Quantum mechanical theories have proven to be more successful than any other scientific theory in human history. They agree with experimental results to an absurd amount of significant figures. QM is largely responsible for the technological boom in the last 30 years and will be of increased importance going forward by introducing things like quantum computing.
So where do we start? Well, you might ask:
WHAT EXACTLY IS QUANTUM MECHANICS?
Quantum mechanics refers to the idea that things that we once thought were continuous are actually quantized. What does this mean? Quantization means that only specific values are applicable. For example, the grades of school are quantized. There is no such thing has 10.1th or 9.9th grade, it is either 9th grade or 10th grade. They each have their own discrete value.
That sounds easy enough. Why is it so complicated?
If you think about it, quantization is very hard to wrap our heads around since there's no real reason this should be the case. For example, when you run, your body does a certain amount of work. Your intuition suggests that the work you did is continuous and could take any value depending on the distance you ran. Quantum mechanics says this is not the case. Instead, the work you did can only take on certain values, which are integer multiples of a base value. For example, if the base value is 3 "units", you can only do multiples of 3 units of work. You cannot do 5, 7, 14, or 67.3 units of work, no matter what.
Ok, but how do we know this is the case? And how did anybody figure this out?
Quantization depends on h, the Planck constant. Since h is 6.626*10^-34 J*s (where J is joules, a unit of energy, and s is seconds) is an incredibly tiny number, it's hard to see the effects of QM in everyday life. That said, as technology got better, we were able to probe deeper and discovered a whole new universe, figuratively of course.
The History of Quantum Mechanics
At the end of the 19th century, when Newton's Laws had been the dominant laws of physics for ages and electromagnetism had been figured out by Maxwell, physics was considered a finished field. Children were encouraged not to go into physics because people believed everything had been solved, and only a few inconsequential problems remained. Unfortunately those problems turned out to destroy the entire fabric of the science. What were these problems? There were quite a few, some of which deal with relativity which I'll touch on later. The problems which lead to the discovery of quantum mechanics are blackbody radiation, the photoelectric effect, and atomic spectra. All of these are summarized here: http://en.wikipedia.org/wiki/Planck_constant#Origins
A rough summary is that blackbody radiation showed that energy is not continuous and takes on quantized values. The photoelectric effect shows us that light itself is quantized into individual photons. This is what Einstein won the Nobel Prize for, not relativity. At the time, light was thought to be a wave based on countless experiments. Quantum theory did not dispel this notion. Unfortunately, light is a wave and a particle. The same is true for every single thing in the universe.
Atomic spectra showed that atoms could only absorb and emit specific frequencies of light, meaning atomic structure was also quantized. These theories revolutionized the field and the quantum era was in full swing.
The Math
I'll try to make this as painless as possible while still explaining the main consequences. For reasons I won't go into, quantization implies two very bizarre things which are closely linked.
First, we have the Schroedinger Equation
I linked the time independent equation because it is the simplest case. What it implies is that every particle can be described by a wavefunction which through the Schroedinger Equation tells you its energy. Yeah, I know it looks and sounds complicated, but that's because it is. Psi, the pitchfork looking symbol, is the wavefunction, E is the energy, and H is the Hamiltonian operator which tells you what to do with the wavefunction (those of you that have taken linear algebra may recognize it). What exactly is a wavefunction? It's a mathematical function describes the "wave" nature of the particle and is dependent on at least one quantum number. Schroedinger himself, when applying it to the electron and its negative charge, thought the wavefunction was the representation of the "spread" of the charge, as if someone had used the smudge tool on photoshop and spread the charge all over the canvas. For other physical reasons, this idea does not work. Then came Max Born who theorized that the wavefunction* was actually describing a probability of where the electron was in space. Areas where the wavefunction is large means the electron has a high chance of being found there. This is the dominant view of physics today. But this means that we will never know where the electron actually is as well as the fact that the electron has a chance of being practically anywhere. This leads us to:
The Uncertainty Principle
There are many misconceptions about this principle so I'll be as straightforward as possible. All the principle states is that we cannot simultaneously know the exact values of momentum and position of any particle, but only a region of their values. I will provide a loose analogy. Imagine you're in a dark room and you can't see anything and aren't allowed to touch anything. You are given the task of finding six basketballs in the room. In order to do this, you are given an infinite amount of footballs to try and hit the other basketballs. You will know where the basketball is once you've hit it with your football. The problem is that once you hit the basketball, it moves and is no longer in the place it was. So, you can never know the exact position of any basketball at any present time. Since Planck's constant is so small, this has zero effect on everyday life. So referees in American football do not need to take it into account when measuring for a first down. An important aspect though, is that the uncertainty is intrinsic to the universe itself, not the measuring devices that we use. The universe says the room is always dark.
Implications and Consequences
While the math above has proven to be perfect on countless occasions, the ideas from the above section sound pretty terrifying and/or complicated. But as mentioned before, the tiny value of Planck's constant makes these bizarre things only observable at subatomic levels. Those of you who have taken introductory physics should know that Newton's 2nd Law F = ma holds true for pretty much every practical purpose. As a matter of fact, when the Schroedinger equation is applied to these practical purposes, it reproduces Newton's 2nd Law! That is actually quite astonishing since it makes F = ma no longer a postulate (of course, the Schroedinger equation takes its place as the postulate). In general terms, when you use high enough quantum numbers in the wavefunction, the behavior predicted by classical/Newtonian mechanics is also predicted by the Schroedinger equation.
That said, the whole wave-particle duality thing is kind of disheartening. How can a matter be both a wave and a particle? What does that even mean? That's a little too complicated for me to explain in simple terms so I'll just link to this video given by Feynman himself and leave the rest up to discussion: http://www.youtube.com/watch?v=_7OEzyEfzgg
That's as far as I'll go for now but I'll be sure to update the OP with any useful links and information. I'm by no means an authority in this field and I'm sure there are several of you who have a vast knowledge of it. Please feel free to correct/elaborate on anything I've said and add anything you find interesting.
I'm sure this thread will not be as popular as the evolution or space threads since it doesn't have the controversy or the pretty pictures. But there are definitely some fascinating things to be discussed. The most popular topic is perhaps the various theories that are trying to solve the inconsistency between relativity and quantum mechanics. The most popular of these is of course string theory, which I'm sure will lead to some of the best discussions in this thread. I'll be sure to put some more information about it in the OP, but that's for another day.
Until then, why we still got point particles?
