Is the cat dead or alive: no one cares...

I recently saw this article posted on the physics subreddit, and it annoyed me. It didn’t annoy me because it had some misleading comments and some things that were just blatantly wrong. It annoyed me because people don’t try to learn things from proper sources, and they just let the mainstream media give them the information they think they need. I could go off on a side tangent about this, but I won’t go too far off topic. I just want to point out that if the mainstream media can get science topics so wrong, who’s to say they aren’t getting their other information wrong. I don’t watch the news anymore because I don’t trust news media (NBC/NPR/ABC/CNN/FOX) with my precious time and brain cells. Please don’t take everything you hear/see/read from media sources as credible until you fact check the fact checkers. Especially science Fridays on NPR, I usually get pretty upset with the way they explain a lot of things. Rant over.

So since a lot of people are very curious about the subject, I thought I would write a short explanation of quantum mechanics (I’ll refer to it as QM to save time typing) that hopefully even the lay person could understand. I know that this is not a good explanation for non-science people, but I just want to get the main point across that quantum mechanics just happens and no one knows why.

In order to properly understand the history and premise of QM, one must first understand what science is. Science according to google is: the intellectual and practical activity encompassing the systematic study of the structure and behavior of the physical and natural world through observation and experiment. That’s a really condensed definition. This basically means that science is a methodology. Scientists perform experiments and make observations. These are either experiments due to a proposed idea, or sometimes just observations out of curiosity. Based on the results of the experiment or data from observations, scientists develop a model for the behavior of the system. We find patterns, and attempt to explain those patterns with a hypothesis/theory/whatever you want to call it. Once we have the model for our system, other scientists will then perform experiments in an attempt to disprove and/or replicate the results of others. If the model cannot be disproved it is published, and then peer reviewed by other objective scientists. When the model is resilient enough to withstand the intense scrutiny of experiments and peer review it is then considered a law/theory/postulate/equation that describes nature. This is not to say however that we consider it to be truth, science does not look for truth like religion does. IN SCEINCE THERE IS NO ABSOLUTE TRUTH. There is only the best model we have come up with so far, and guaranteed we will find better models to describe nature in the future. This is a good lead in to how QM got started.

The history of quantum mechanics really starts in the mid 1600s with Isaac Newton. Newton was one of the first true modern scientists. He was one of the first thinkers to apply models to observed data. He was then able to show that his models were reproducible in the acceptable reference frame. Newton became the father of what is referred to as classical mechanics. This is usually the typical stuff you learn in an intro to physics class in high school or your freshman year of college. He came up with three ideas that were consistent and reproducible by anyone, which we now know as Newtons three laws of motion. I won’t go into the detail, but basically he came up with a model that described the world, and was able to predict outcomes of motion based on his mathematical models. Before Newton was a guy Johannes Kepler who had taken astronomical data and figured out a mathematical model to describe planetary motion. These are known as Kepler’s laws. When Newton came up with his laws of motion, Kepler’s laws fluidly were just a mathematical outcome of Newton’s laws for planets in orbit. Kepler wasn’t wrong; he just didn’t have the full model to describe things outside the scope of planetary motion. Newton was able to come up with a model that could be applied to any mass in motion.

There are many other scientists that come along the way after Newton that have a huge impact on science and its development. All of them had a significant impact on the field of science; I just don’t think most people want a history lesson on all of physics. I will skip now to the beginning of the 20^th century. By this time there had been experiments which showed that the speed of light is constant in all reference frames, which lead to the theory of special relativity. Special relativity just like QM is something that boggles the human mind. It’s something we don’t think is “intuitive”, well because it is not at all intuitive. We don’t travel at half the speed of light when driving to work so we can never just gain intuition on the subject. Instead scientists must remain completely objective when performing experiments and leave any personal bias out of their research. This is arguably why QM and relativity had such a slow start up, is because the older scientists of the day just had a hard time accepting theories that just “didn’t make sense”. Special relativity, however, has still yet to be disproven so it remains as the best model we have to describe such conditions as an electron traveling very fast through an accelerator.

More experiments were conducted in the late 1800s to early 1900s. These experiments gave rise to results not previously explained earlier models of classical mechanics. One such of these examples is the photoelectric effect. Hertz (a scientist in the late 19^th century) observed that light of a certain frequency (energy) is shined on a metal it will start to emit electrons. There is however a minimum amount of energy required in order to “free” this electron from the metal. This is behavior totally unexplained by any theory at the time, and is part of the reason why we call this new model quantum mechanics. It was observed that these electrons had a discrete energy that bound them to the metal, thus the quanta (meaning discrete energy) in quantum mechanics. Other experiments showed that electrons behave as light waves, and that light waves exhibited particle behavior (if you’re interested look up electron diffraction and Compton scattering on Wikipedia). That was crazy talk for the time, and if these were indeed reproducible experiments (which they were), then a new model would have to arise to describe them. Thus quantum mechanics was born.

Quantum mechanics (just like Newton’s laws of motion) are predicated on some fundamental postulates. These postulates CANNOT BE DERIVED as they are first principles of QM. In other words, we observe these things happening, so we model the system based on this set of rules. Some people say that there are four postulates (postulate means thesis, theory, or idea), others say six, and some say anything in between. I will give what’s referred to as the Copenhagen interpretation as it is the most widely accepted point of view among physicists. If you wish to know more look up the Copenhagen interpretation on Wikipedia. I will attempt to restate the 6 principles that Wikipedia refers to in words that are easier to understand.

1.       A system can be described by a wave function which describes the state of the system. This wave function develops in time just like a wave does in acoustics, in the ocean, or any other type of wave. This wave isn’t a physical wave however. It merely describes the system (which could be a particle or several particles). When we take a measurement, however, it gives some kind of information about the system. Since our way of measurement says for example “is the particle here?” we then know where the particle is. In other words, this wave function which gives information about the state of the particle “collapses”. This is to say we took a measurement, and now we know the state of the particle, this happens because of the inherent way in which we measure a system. IE: A system is described by a wave function, until we measure the system. This gives us information about the system, so it is no longer described as a wave.

2.       We describe nature through probabilities. This probability is given by the square modulus of the wave function. I won’t go into the mathy details, but this essentially says we don’t describe things happening in absolutes, but we describe nature in terms of probability of something occurring. A good example of this is nuclear decay; an unstable isotope has some probability of decaying each second. This is why some isotopes are more unstable then others, it is because their probability of decay is higher.

3.       Heisenberg’s uncertainty principle is familiar to most people. It is not inherently a postulate per-say, but really it is a bi-product of the math that comes from the other postulates of QM. It says that systems information cannot be known perfect. Most people understand this to mean that a particles position and momentum cannot be known at the same time. This is a bit of an oversimplification. The certainty of our knowledge of some systems momentum is sacrificed with the certainty of knowledge about the systems position. In other words if I kind of know where I am, I can kind of know which way and how fast I’m going. If I know exactly where I am, I can’t know at all where I’m going. Like I say, this comes out of the math from the other postulates, but it also applies to things like energy and time. This is why a vacuum is not actually empty, it has an infinite amount of particles popping in with some energy, and they just have to disappear within a certain amount of time (this is what's referred to as the "quantum foam"). Weird stuff I know.

4.       The wave-particle duality basically just states that anything can be viewed as either a wave or a particle. This is why photons scatter off of electrons elastically, which one wouldn’t predict in classical mechanics. Don’t over think, just accept it and be in awe of the mystery. This is just how the model describes nature, and there is really no underlying derivation to any of these postulates.

5.       The tools we have to measure things are classical instruments. We are limited in what we can measure, because we live in the classical limits of really big stuff. There are properties of matter like spin for example. We don’t really know what spin is, it’s just a property of particles. We can’t directly measure the spin of an electron, since we don’t have quantum mechanical devices. Instead we have to use a property of spin, which is a magnetic moment. This magnetic moment aligns with the direction of the spin of the particle, and so if we shoot an electron through a magnet we can measure the spin by the direction the electron bends towards (for more info on this example look up the Stern-Gerlach experiment).

6.       Classical mechanics is an approximation of QM on a really big scale. If you take average quantum probabilities of Trillions of particles in a system (say a block of aluminum) the math gives you classical mechanics. This is known as the Ehrenfest theorem. This means that Newton was right, because we was observing exactly what happens on a large scale. QM doesn’t necessarily TRUMP classical mechanics; classical mechanics is merely QM on a large scale approximation. Don’t think we’ve forgotten about Newton’s laws of motion because we found the Higg’s boson, if we did then humans would have never reached the moon.

Hopefully this gives a little bit of an overview of the principles of QM. These basic principles give rise to some phenomena that seems impossible (and indeed are) in classical mechanics, but that happen frequently in QM. In fact they are used in our daily lives. I will give some examples, and how we take advantage of the science contained in these phenomena.

·         Tunneling

Tunneling is the phenomena of a particle going through what we could view as in classical mechanics as a brick wall. In classical mechanics we don’t deal in probabilities, we deal in absolutes. In QM the probability of being outside a boundary is a finite possibility. An example of this is alpha decay. One can view an unstable (for example Uranium-235) nucleus as having a free alpha particle trapped inside a well. It is trapped because there is a force called the nuclear strong force that is attractive between nucleons. Although this alpha is trapped inside this nucleus, it has a probability of existing outside this nuclear potential barrier. If at some point the particle finds itself outside the nucleus its positive charge repels the positively charged nucleus, and it flies away. Quantum tunneling is used in all modern electronic devices. It is the reason that diodes and transistors work. Without quantum mechanics, we would not have computers today.

·         Quantization of energy

Quantum mechanics explains the discrete property of nature. Energy in nature is quantized; each state a particle is in refers to a certain discrete energy value. This is how we can know things in chemistry so well. Remember the periodic table? Well the entire thing is predicated on principles of quantum mechanics. The ionization energy of a certain atom has a discrete value associated with the energy state of an electron bound to the nucleus. Quantization of energy is the reason why our different molecules in our eyes are able to see different colors associated with different light wave energies. It is why colorblind people like me can’t see certain colors, because we are missing molecules that can view those energies of light. This is why purple lasers are purple, green lasers are green, why CO2 is a greenhouse gas, and why a microwave or a cell phone cannot physically cause cancer. These are all topics that are much more complicated than me just stating them, but the underlying principles of chemistry, climate change, and the like are all rooted in our quantum mechanical model.

·         Pauli exclusion principle

The Pauli Exclusion Principle states that no two fermions can occupy the same quantum state. Fermions are just a class of particles with ½ integer spin. Spin has nothing to do with spinning or rotation, so just view spin as a strange number that has some weird properties to it. Examples of fermions are electrons, protons, and neutrons. Because no two fermions can occupy the same quantum state, this gives rise to the way that orbitals are filled up in an atom. If electrons weren’t fermions, then all electrons bound to a nucleus would be in a 1s state. This does not apply to particles known as bosons. Photons (electromagnetic waves) are bosons, which means that they can essentially stack up on top of each other. That’s what a laser essentially is: a whole bunch of photons of the same energy all stacked up on top of each other.

There is a whole list of fantastically fascinating phenomena that occur, which we never understood until we came up with our model of QM. Who knows, maybe in fifty years we’ll come up with an even better model to understand how general relativity and QM can relate to each other. QM is not an absolute truth, just as Newton’s laws are not absolute truth. They are models that help us to understand the universe around us. We as scientists are not trying to find what people in religion refer to as truth. All we want to know is how the universe works, and we get that through experiments and observations. Arguably science will never find truth, just the best model of nature humans can find.