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QUANTUM PHYSICS : AN INTRODUCTION

by subhankar karmakar

WHAT IS QUANTUM PHYSICS?

To understand the quantum physics, one needs to understand the exact meaning of the word quantum. A quanta is the smallest possible amount of any substances, like the smallest possible amount of mass would a quanta of mass, similarly, for any electromagnetic wave a photon carries the smallest amount of energy hence called as a quanta of energy. Quantum is the plural of the word quanta.

Although, we experience matter as a continuous distribution over a certain amount of space, yet it's not true depiction of matter, somehow we feel and perceive the macroscopic definition of the size and shape of the matter, which is the gross behaviour of tiny microscopic discrete particles, which are named as sub atomic particles. So to understand nature, one needs to understand the behaviour of those tiny particles. The science of these tiny particles are named as Quantum science.

So we can define quantum physics as the science of things so small that the quantum nature of reality has an effect. Quantum means 'discrete amount' or 'portion'. Max Planck discovered in 1900 that you couldn't get smaller than a certain minimum amount of anything. This minimum amount is now called the Planck unit.

WHY IS IT WEIRD?

When I wrote an article on Quantum Mechanics, one of my closest friends, Utpal Brahma told me, that it is really a subject which goes over his head. It is not for Utpal only, a major population always insists that Quantum Mechanics is very weird and it is very hard to grasp the topic. But is it inherently so tough or it is our interpretations and basic assumptions are flawed, hence, naturally our theory would not produce any coherent conception. Like one of the major counter-intuitive quantum fact is "duality nature of matter". It is very hard to visualize a block of mass which has certain mass, shape and sizes as a "matter-wave" having a certain wavelength. But, according to quantum mechanics says that a matter exists neither as a particle nor as an wave, but it is certainly a such type of thing which behaves as an wave when there's no external disturbance imposed upon them, but it is not the wave itself, the fact is that it behaves like a wave. But when we try to watch it by impinging photons on it from external source, it starts to behave differently, it seems that all the spread energy as wave suddenly clumped together to give it a physical appearance of a particle. Here, also I want to stress the fact that it behaves like a particle although it's not a particle. As it is counter-intuitive phenomenon. We know from our daily life experiences that it is not possible, but this duality has been verified by several real life experiment. It is only possible when we do not experience all the events that occur, or all the phenomena associated with the matter. It may not be still clear to you, so let me explain the whole thing with some real life analogous examples.

We know that a particle mass may behave like a clumped together mass i.e. a particle-like appearance, or it may behave like a wave (more or less like a physical field). But, that is absolutely counter intuitive as we can't visualise an object which is particle as well as wave simultaneously. When does this kind of situation may occur? We shall try to use an analogy here to pinpoint the logic of the concept.

Suppose there are five people who have never seen an elephant neither heard about it. It means they have nil information regarding the object named "Elephant". Next, they are blind-folded and taken at a nearby place where an elephant is also kept. Now they are asked to feel through touches (human sensors) to describe the object they are touching, i.e. the elephant as they are placed at different places near the elephant. Then what will be their most probable replies? As their sensors are of limited capacity, none of them could give an exact description of the elephant. So, simultaneously it may be a pillar like as well as a rope like object to them.

Niels Bohr, the father of the orthodox 'Copenhagen Interpretation' of quantum physics once said, "Anyone who is not shocked by quantum theory has not understood it". Einstein once said, "God doesn't play dice" regarding the quantum wave collapse.

To understand the weirdness completely, you just need to know about three experiments: Light Bulb, Two Slits, Schroedinger's Cat. Other than these three experiments, there exists a lot of quantum phenomenon like "Quantum Entanglement" which is very hard to comprehend logicaly.

The Experiment One : TWO SLITS EXPERIMENT

Perhaps one of the most talked about physics experiment is Young's Double Slit experiment to demonstrate the interference phenomenon of waves.

TWO SLITS EXPERIMENT :

The simplest experiment to demonstrate quantum weirdness involves shining a light through two parallel slits and looking at the screen. It can be shown that a single photon (particle of light) can interfere with itself, as if it travelled through both slits at once.

A LIGHT BULB:

Imagine a light bulb filament gives out a photon, seemingly in a random direction. Erwin Schroedinger came up with a nine-letter-long equation that correctly predicts the chances of finding that photon at any given point. He envisaged a kind of wave, like a ripple from a pebble dropped into a pond, spreading out from the filament. Once you look at the photon, this 'wavefunction' collapses into the single point at which the photon really is.

SCHROEDINGER'S CAT :

In this experiment, we take your pet cat and put it in a box with a bottle of cyanide. We rig it up so that a detector looks at an isolated electron and determines whether it is 'spin up' or 'spin down' (it can have either characteristic, seemingly at random). If it is 'spin up', then the bottle is opened and the cat gets it. Ten minutes later we open the box and see if the cat is alive or dead. The question is: what state is the cat in between the detector being activated and you opening the box. Nobody has actually done this experiment (to my knowledge) but it does show up a paradox that arises in certain interpretations.
If you dare to think about it (you're not really supposed to), you have to believe one of the following things:
Your consciousness affects the behaviour of subatomic particles

- or -

Particles move backwards as well as forwards in time and appear in all possible places at once

- or -

The universe is splitting, every Planck-time (10 E-43 seconds) into billions of parallel universes

- or -

The universe is interconnected with faster-than-light transfers of information
These are the results of the different interpretations of quantum physics. The interpretations all compete with each other. Otherwise respectable physicists can get quite heated about how sensible their pet interpretation is and how crazy all the others are. At the moment, there's about one new interpretation every three months, but most of them fit into these categories.
What does it mean?

The meaning of quantum physics is a bit of a taboo subject, but everyone thinks about it. To make it all a bit more respectable, it is better to say 'ontology' than 'meaning' -- it's the same thing. There are several competing interpretations and the one thing they all have in common is that each of them explains all the facts and predicts every experiment's outcome correctly.

Copenhagen Interpretation (CI)

This is the granddaddy of interpretations, championed by the formidable Niels Bohr of Copenhagen university. He browbeat all dissenters into submission (with the notable exception of Einstein) at a Brussels conference sponsored by a man called Solvay in 1927. Bohr thereby stifled the debate for a generation or two.

The CI has a bit of a cheek calling itself an interpretation, because it essentially says "thou shalt not ask what happens before ye look". He pointed out that the Schroedinger equation worked as a tool for calculating where the particle would be, except that it 'collapsed' as soon as you took a peek. If anyone asked why this was, he would say, "shut up and calculate" (or he might as well have done).

When you do try to take Copenhagen seriously you come to the conclusion that consciousness and particle physics are inter-related, and you rush off to write a book called The Dancing Wu-Li Masters.

More recently, Henry Stapp at the University of California has written papers such as On Quantum Theories of the Mind (1997). Stapp's central thesis is that the synapses in your brain are so small that quantum effects are significant. This means that there is quantum uncertainty about whether a neuron will fire or not - and this degree of freedom that nature has allows for the interaction of mind and matter.

What happens to the cat? You're not allowed to ask.

Many Worlds Interpretation (MWI)

The various paradoxes that the Copenhagen Interpretation gave rise to (famously Schroedinger's cat, and Einstein's dislike of "spooky action at a distance") led others to keep on trying to find a better interpretation.

The simplest was put forward by a student, Hugh Everett, in 1957. He simply said that the Schroedinger equation does not collapse. Of course, everyone laughed at him, because they could see that the photon, for example, was in just one place when they looked, not in all possible places. But after a couple of decades, this issue was resolved with the concept of decoherence - the idea that different universes can very quickly branch apart, so that there is very little relationship between them after a tiny fraction of a second.

This has led to what should strictly be called the 'post-Everett' Interpretation, but is still usually called MWI. It is now one of the most popular interpretations and has won some impromptu beauty contests at physics conferences. Unfortunately it means that billions of you are splitting off every fraction of a second into discrete universes and it implies that everything possible exists in one universe or another. This comes up with its own set of hard-to-digest concepts, such as the fact that a 500-year-old you exists in some universes, whereas in others you died at birth.

In 1997, Max Tegmark at Princeton University proposed an experiment to prove that MWI was correct. It involved pointing a loaded gun at your head and pulling the trigger. Of course, you will only survive in those universes where the gun, for whatever reason, fails to go off. If you get a misfire every time, you can satisfy yourself -- with an arbitrarily high level of confidence -- that MWI is true. Of course, in most universes your family will be weeping at your funeral (or possibly just shaking their heads and muttering).

What happens to the cat? It's dead in half the subsequent universes and alive in the other half.

Pilot Waves, Hidden Variables and the Implicate Order

David Bohm (1917-1992) was a very brilliant physicist and that's why ...
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