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## The most misunderstood moggie in history

Let's start with an assumption. There's a pile of cookies. Maybe two thousand of them. Chocolate chip with a chewy centre. I take a cookie. This would leave one thousand, nine hundred and ninety nine cookies. Still a big pile of cookies.
I invite lots of people around and each take a cookie and leave with the sort of smile that only gooey cookie goodness can provide.

You know, after a while, there will be only one cookie left. Are we looking at a pile of cookies...er... cookie? What if I load on the calories by eating the last one? Can I look at an empty table and still call it a pile of cookies? If not, when did the pile cease to become a pile? You'll find you can't really quantify it - for 180 cookies might be a pile but 179 not - although you'd probably be hard pressed to find somebody that can tell the difference without actually counting. This is, of course, assuming you can find people that actually agree on when a pile ceases to be a pile.

This introduces us to the concept of a thought exercise. Which is exactly what Schrödinger's Cat is.

A long time ago - 1935 to be exact, but anything before Unixtime qualifies as "long long ago" - an Austrian physicist by the name of Erwin Schrödinger postulated a "reductio ad absurdum". This pseudoLatin phrase means to take something and keep on going until you reach its most ridiculous conclusion. He proposed the idea of the following experiment:

Let's assume you have a cat, sealed inside an otherwise impenetrable steel box. There is a mechanism that is inaccessible to said cat. This mechanism is a tiny piece of a radioactive substance, a Geiger counter, and a vial of hydrocyanic acid. It is arranged such that, maybe within an hour, you might get a single atom decaying. But on the other hand, you might not.
If an atom decays it will be detected by the Geiger counter which will trigger a relay which will release a hammer which will break the flask which will release the acid which will kill the cat.
So if we leave this setup running for an hour, we have the condition that if no atom decayed, the cat will be alive in the box, and probably rather annoyed. But if the atom decayed, dead moggy.
So it's been an hour. There's a box in front of us. With a cat inside. The question is - is this cat alive or dead?

Now, because it is an impenetrable box, we can't cheat and use a stethoscope (maybe the cat is sleeping? maybe the cat understands Quantum Physics and was so frightened it crapped itself and fainted?) or other such means. We must open the box and look.

Until then, what is the state of the cat? Well, given that this thought experiment is an extrapolation of Quantum Physics, the only viable response is that after the hour the cat exists in both states. It is both alive and dead, and it is only when we come to open the box and look at it that it assumes a definite state of being alive, or being dead.

Freaky, right? Well, Quantum Physics is always good for a trip into CloudCuckooLand where it seems like logic bends over backwards to defy itself. In this case, Schrödinger is asking:

• When does a quantum system cease being two simultaneous states and become one or the other?

• Sadly, I should point out that this is an analogy - there was no actual cat in an actual box.

All of this can be traced back to a school of thought on Quantum Physics known as the Copenhagen Interpretation, which states - in a layman's nutshell - that Quantum Physics does not exist in an objective reality but rather everything is in all states at the same time. It is only when something is observed that it assumes one of the potential states.

This is not an entirely unknown principle, for the act of measuring can affect that being measured. I'll give you two real world examples:

1. A multimeter. Necessarily high resistance, as you don't want to affect the readings you are trying to take.
2. A debugger. By necessity a debugger is a sanitised environment where the debugger will replicate a subset of the operating system to permit you to examine the operation of your program. This can lead to its own problems - one that caught me was a bit of code that worked just fine within the debugger, but gave spurious random errors in reality. The problem, as it turns out, was that I had called malloc() instead of calloc(). The difference is that the latter call not only allocates a wodge of memory, but it zeroes it too. The code then worked on the assumption that memory would be clear, when in fact it wasn't. Except under the debugger, which cleared all of the program's memory itself...

The point here, the point to the potentially unfortunate furball is twofold.

Firstly the cloudcuckooland angle - that things may be in an indeterminate state until they are observed to be in one state or another. This, I feel, is swaying into the realm of "if a tree falls in a forest but nobody is around to hear it fall, does it make any noise?".

Secondly, the real world practical angle - the action of observing something may well have an outcome upon the entity being observed. The cat can be both alive and dead. At the same time. But when we look at it, it needs to be either alive or dead.

But, to carry on, what constitutes "observing"? Is it something special and unique to us as humans? If we had a chamber in the box containing a bullfrog and we had this automatically open after an hour, would the frog see a live cat, a dead cat, or a cat in two states of being? How about if we put a video camera in the box? In order to affect the situation of the cat, does the video camera need to be turned on? Connected to something? Watched by somebody? What happens if we automatically record the cat and then later play it back to an observer? At which point would it assume a definite state?

Maybe, just maybe, we humans are special and have the ability to alter the state of objects in the universe merely by looking. Not because we are closer to God, but rather the opposite - that things must assume a state because we are far too limited to conceive of anything else.

Okay, so I've mentioned "Quantum Physics" several times now, but what the heck am I on about, really?

Put simply, it is a branch of physics (the science that attempts to explain practical matters of what makes stuff work) that deals with very small things. By small, I don't mean like atoms with electrons and stuff whizzing around. That's the realm of the traditional sciences. But rather, what goes on at a deeper level than that. As it happens, electrons and such are appearing and disappearing all the time and all over the place. This, obviously, gives the middle finger to The First Law of Thermodynamics which states: Energy can be neither created nor destroyed. It can only change forms. Is this wrong? Maybe, maybe not. So where do these subatomic particles come from, and go to? One theory is a parallel universe. I know, I know, it sounds like some hokey sci-fi rubbish to handwave a crap theory. But it is a theory that carries some merit, surprisingly enough.

Now to understand where Quantum Physics begins - and you may well have performed this experiment in school physics classes but not entirely realised the implications - we must first make a distinction between a particle and a wave.

A particle is some kind of "matter". An actual real object. Take for example your old cathode ray television. It works by having a small heater which emits electrons (you can see a faint orange glow down the back of the telly). These electrons are particles. They are actual things. A high tension source in the front of the tube attracts the particles. Actually, the front of the TV can be running at a constant and powerful high voltage of 20kV or more, and as such this rips the electrons from the heater and sucks them, super-accelerating them, towards the front of the screen. Powerful magnetic fields (all those coils around the tube) warp reality around the particle in order to change its trajectory. A particle in motion goes in a straight line, so the magnetic fields bend this straight line in order that the spray of electrons can trace out a pattern on the tube face. Just before the highly charged tube face is a mosaic of tiny dots of phosphor. This is a compound that glows when electrons hit it. It ought to be obvious to you now how the picture appears - just join up all the dots. Yes, terrible pun intended.

A wave, on the other hand, is a disturbance in something. If you lob a rock into a pond, you can see the ripple spread outwards from the rock. The rock hasn't split into a million pieces to push the water around, in the end it is water pushing on water. So it is a disturbance rather than an actual object. Likewise, if you hear thunder, the associated lightning won't have hit literally beside your ear... well, not if you want to keep your pants clean and your heart beating. It happened, somewhere, the air around superheated and exploded. This created a shock wave where air pushed on air and so on until our ears perceive thunder.

This leads us to Young's Experiment, also known as the double slit experiment. If we take a source of "coherent light" (for example, a laser and not a tungsten bulb) and shine its output through a card with two slits in it, we would expect to see the light of the two slits shining on a card placed behind the laser. In effect, this is what we would expect:

That is, however, not what we would observe (if it is, you probably aren't using a coherent light source; a desk lamp won't cut it). I define coherent light below.

What you would observe is... well... this...

What is going on here is that the light is shining through, but interference patterns are creating zones of light and non-light. There are scary-looking equations to determine all of this, but one thing is clear - this is not possible if light is a particle (it would look like the example above) and it is not possible either if light is a wave. It is only possible if...

...if light is a particle and a wave. At the same time.

Wrap your noggin around that, and the exciting and freaky quantum world awaits you!

• Coherent light is a light that is both polarised and of a specific wavelength. Thus light from a laser or bright LED (as used in the example below), while light from a regular bulb, or daylight, will be a scattering of different wavelengths.
The reason this is important is because the effects are noticable due to interference patterns from the light. The waves reflect and merge with each other, in much the same way as a ripple from a stone in a pond when it bounces off the side of the pond. This is what creates the effect seen in the second diagram. If, however, we used normal (non-coherent) light, the effect would still be there, but as it would be happening at all sorts of wavelengths, all we would see is a sort of blur with no distinct visible pattern.

You can do a cheap'n'cheerful version yourself, in the comfort of your own home!

You'll need:

• A Smartphone with an LED flash that has a video recording mode where the LED can be used as a lamp.
(it needs to be a bright single point of light, an LED torch probably won't work)
• Some thick card. I used the pack of a Duracell USB stick, but a cereal box will probably do just as well.
• Scissors. (if you are an adult, please seek the supervision of a competent child...)
• A piece of blank white paper to see the results upon.

We cannot see the above effect as the tolerances of the double-slit are not something we can easily reproduce in a Blue Petery way, so we're doing something slightly different.

Take your card and cut a single slit in it. It must be as thin as possible, but wide enough that you can actually see through it if you hold it up against your eye.

Switch your smartphone to video recorder mode, and turn on the lamp. On Android and iPhone, you might have a "torch" app that does pretty much the same thing.

Hold the phone's light up to the slit in the card, and shine it onto the piece of paper.
You'll see something like this:

I will leave you with a lovely photo. This is a laser pointer/spirit level with the "straight line level" attachment fitted. This means I can point it at a wall and see a long red line. It works by using a special lens and a bit of Quantum Physics. If I point the laser into the camera (from a distance, and with the shutter partially closed, for it is a powerful lightsource!) you can see these amazing interference patterns.
The image has been digitally sharpened to enhance the effect (which was mostly messed up in scaling down from the 7mpix original).

Updated 2011/11/21:

• Added note that the cat-in-the-box is an analogy.
• Added a definition of coherent light (thanks mom for pointing that out! ☺)
• Changed "Quantum Mechanics" to "Quantum Physics", it's a more inclusive description.

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