QUANTUM PHYSICS | QUANTUM MECHANICS

BY : SATYENDRA KUSHWAHA

"Quantum physics is the study of matter and energy at its most fundamental level. A central tenet of quantum physics is that energy comes in indivisible packets called quanta. Quanta behave very differently to macroscopic matter: particles can behave like waves, and waves behave as though they are particles."

What is Quantum Mechanics?

Quantum Physics  has a mystique of being complicated and hard to understand, in fact, Richard Feynmann who won the Nobel prize for his work on quantum electrodynamics said: “If you think you understand quantum physics, you don’t understand quantum physics”. Which is kind of disheartening for us because if he didn’t understand it, what chance do the rest of us have? Fortuitously this quote is a little misleading. We do in fact understand quantum physics really well, in fact, it is arguably the most successful scientific theory out there, and has let us invent technologies like computers, digital cameras, LED screens, lasers, and nuclear power plants. And you know, you don’t really want to build a nuclear power plant if you don’t really understand how it works.

(Photo Credit: Wikimedia Commons)

"So quantum physics is the part of physics that describes the smallest things in our Universe: molecules, atoms, subatomic particles things like that. Things down there don’t quite work the same way that we are used to up here. This is fascinating because you and everything around you is made from quantum physics, and so this is really how the whole universe is actually working."

NOW DOING SOME EXPERIMENTS

If we draw protons, neutrons, and electrons as particles we do as we do in our notebook, but in quantum mechanics, we really describe everything as waves. By the way, I'm using quantum physics and quantum mechanics interchangeably, they are the same thing. So instead of an electron looking like a particle, it should look something like some wave. This is called a wave-function.

But this wave-function isn’t a real physical wave-like wave on water or a sounds wave. A quantum wave is an abstract mathematical description. To get the real-world properties like position or momentum of an electron we have to do mathematical operations on this wave-function, so for the position we take the amplitude and square it, which for this wave would look something like this. This gives us a thing called a probability distribution which tells us that you are more likely to find the electron easily, and when we actually measure where the electron is, an electron particle pops up somewhere within that area. 

So with quantum physics, we don’t know anything with infinite detail, we can only predict probabilities that things will happen, and it looks like this is a fundamental feature of the Universe which was quite a departure from the clockwork, deterministic universe in classical physics, the kind of thing Newton derived.

This wave-function model predicts what subatomic particles will do incredibly well, but weirdly we've got no idea if this wave-function is literally real or not. No one has ever seen a quantum wave because whenever we measure an electron all we ever see is a point-like electron particle. 

So there is like a hidden quantum realm where the waves exist, and then the world we can see, which is where all the waves have turned into particles. And the barrier between these is a measurement. We say a measurement ‘collapses’ the wave function, but we don’t actually have any physics to describe how the wave collapses. This is a gap in our knowledge that we have dubbed the measurement problem, and this is one of the things that Feynmann was referring to with his quote.

Another confusing thing is how exactly to picture an electron. It seems to be a wave until you measure it, and then it is a particle, so what actually is it? This is known as particle-wave duality, and here is an example of it in action: the famous double-slit experiment.

Imagine spraying a paintball gun at a wall with two openings in it, you’d expect to see two columns of paint go through and hit the wall behind. But if you shrink this all down to the size of electrons you see something quite different. You can fire one electron at a time at the slits and they appear on the back wall, but as they build up over time you get a whole pattern of stripes, instead of just two bands, this pattern of stripes is called an interference pattern, something you only see with waves. 

The idea is that it is the electron-wave that goes through both slits at the same time, and then the waves from each slit overlap with each other, and where the waves add together you have a high probability of the electron popping up at the wall, but where the waves cancel out the probability is very low. So actually on the back wall, the highest probability of finding the electron is in the middle of the slits, and then it goes down and up again, and down and up again and this is the interference pattern. So when you fire one electron after another they follow this probability distribution and this interference pattern starts building up, and that's exactly what we see in experiments. So this shows that electrons behave like waves in this experiment.

Double Slit Experiment

A question is what actually happens to this spread-out electron-wave when you do a measurement? It seems like it goes from this spread out wave to this localized particle, but like I said, there's nothing in quantum mechanics that tells us how the wave-function collapses. And this is not only true for electrons, but for everything in the Universe, so this double-slit experiment has huge consequences for our model of the Universe, and it was very surprising the first time it was done. Physicists are still grappling with this question today and have come up with many interpretations of quantum mechanics to try and explain these results, and explain how reality actually works. {For Detail Article Of Double Slit Experiment Prefer This Link: Double-Slit Experiment }

Get back to the wave-function

 Now we can use this picture to explain other features of quantum physics that you may have heard about. So this is just one possible wave-function for an electron, but there are many others. Like this one for instance. This says that the electron has a probability of being over here, and a probability of being over here, and very little probability of being in the middle. This is perfectly allowable in quantum physics and this is where the phrase ‘things can be in two places at once' comes from. This is known as superposition, which comes from the fact that this wave can be made by adding, or superimposing these two waves. 

The word superposition just means the adding together of waves and we already saw this in the double-slit experiment, and is not really a very special phenomenon. You can even see superposition by dropping two pebbles into a pond where the ripples overlap. Now for entanglement. Let’s say two electron-waves meet. Their waves interfere with each other and become mixed up. This means that mathematically we now have one wave-function that describes everything about both electrons and they are inextricably linked, even if they move far away from each other. A measurement on one of the particles, like measuring if it is spin up or down is now correlated with a measurement on the other, even if they move billions of miles away. 

Einstein was very uncomfortable with this idea because if you measure one of the particles here you instantaneously know what the other will be even if it is billions of miles away, and that's got a sort of whiff of faster than light communication, which is not allowed by the theory of relativity. But it turns out you can’t actually use this to communicate information, because the measurements give you random results, but the fact that they are correlated means that somehow there is a link that stretches over that distance. This is called non-locality.

Quantum tunneling :

Quantum tunneling is where particles have a probability of moving through barriers, essentially allowing things like electrons to pass through walls. When a wave-function meets a barrier it decays exponentially in the barrier, but if the barrier is narrow enough the wave-function will exist on the other side meaning there is a probability of the particle being found there when a measurement is made. In fact, the only reason you are alive is because of quantum tunneling in the Sun which makes the Sunshine. Protons normally repel each other, but they have a small probability of quantum tunneling into each other which is what turns hydrogen into helium and releases fusion energy. All life on Earth exists because of energy from the Sun, except for life around hydrothermal vents.

Heisenberg Uncertainty Principle

Now on to the Heisenberg Uncertainty principle.  It said that that this wave function contains all of the information like position and momentum of the electron, we just have to do some maths on it. The position is given by the amplitude, or height of the wave, and the momentum is given by the wavelength of the wave. But for this specific wave, the position gives us a probability distribution, so we don't know exactly where the electron is. Also, there is an uncertainty in the momentum because this wave is made of many different wavelengths. But we can reduce that uncertainty, let’s have a wave that only has one wavelength, so a sine wave. Now we know the momentum exactly because the wavelength has a single value, but look at the position. There is an equal probability of the electron being found anywhere in the universe. 

Okay, let's do the opposite let’s make a wave that has only got one position. Now we know exactly where the electron is, but what is the wavelength of the wave? Now the wavelength is very uncertain. Basically, only a sine wave gives you a precise momentum, and any wave that isn't a perfect sine wave, you have to build out of multiple different sine waves, and each of those multiple different sine waves has got a different wavelength, and hence you have a range of possible different values of momentum for the particle. This is Heisenberg’s Uncertainty principle, you can only know certain things precisely, but not everything. Either you have got a definite value of momentum, and don't know anything about the position, or you know the position very well, but don't know anything about the momentum, or you are in some intermediate state. And this isn't a limit of our measuring apparatus, this is a fundamental property of the Universe! 

And finally, where does the name ‘quantum’ come from. Well, a quantum is a packet of something like a chunk of something, and one of the first quantum effects people saw was atomic spectra which is where atoms give off light with specific discrete energies. Imagine a string that is tied at both ends, like a guitar string. If you pluck it, only certain waves can exist because the ends are tied down, in this situation we say that the wavelengths are quantized to certain values. The same thing happens if you tie the ends of the string together because the waves have to match up, they can only vibrate in certain restricted ways. And this is what is happening to an electron in an atom. The electron-wave is constrained by the atom and quantized to certain wavelengths, short-wavelength have high energy and long wavelengths have a lower energy. This is why the light emitted by an atom looks like a barcode because each bar of light corresponds to an electron jumping from a wave with a high energy to one with a lower energy, and at the same time emitting a quantized photon of light when it does this. So the light from an atom is quantized to discrete packets of energy.

ROUNDING UP

So to round up. In quantum physics, objects are described with wave-functions, but when we measure them, what we see are particles, so this leads to particle-wave duality, and also the measurement problem. And the consequence of these wave-functions are the quantum phenomena of superposition, entanglement, quantum tunneling, the Heisenberg uncertainty principle, and energy quantization. So if you understand these things you have got a good basic understanding of quantum physics. Despite its reputation, I think quantum mechanics isn’t too difficult for most people to get the basics of what is going on. 

For me, the weird thing about quantum physics is that on the one hand, it is incredibly accurate and predictive but also it has got giant holes in it like the measurement problem which we just don’t understand. So we can wonder, will we ever actually understand quantum physics, or is it just too abstract for our human brains to comprehend? Well, I hope this article has helped you understand a little more about how quantum physics works.

"IF QUANTUM MECHANICS HASN'T

PROFOUNDLY SHOCK YOU, YOU

HAVEN'T UNDERSTOOD IT YET"