Tag Archives: quantum physics

The end of days

S104 is really picking up the pace now – I’ve just submitted iCMA 48, with 93%. So that’s good then. And I’m zig-zagging through TMA07, which is due in on September 1.

Actually, it’s going quite well. I still have trouble deciphering some of the question wording, and suspect that they are set by people for whom English is not their first language, but you can’t have everything.

Sometimes, things just snap into place. You need to worry about them for a day or so, fret that actually, you’re rather stupid and you’ll never get this, and then it happens. A golden moment, a small firework in your mind, and there it is: enlightenment and understanding.

Question 2 (c)(i), I have the measure of you. I challenge you to a duel; pick your pistol. I’m confident, knowledgeable, and I shall have my satisfaction, sir.

I’ve very much enjoyed Book 7 – Quarks to Quasars. I’ve struggled a little with the specifics, such as energy levels, and the subtle effects electrons have on one another, not to mention the strength of the various interactions. But the concepts, the wider questions that border on the philosophical as well as the scientific – those, I love.

The feeling of stretching your mind so wide open that you feel it’s entirely possible there may be a permanent split is a heady rush. Have you ever stood on the edge of a cliff, or a very tall building, and had that momentary – just a split second – urge to throw yourself into the void? It’s a little like that.

The Universe started as a very dense, very hot mass of energy, then exploded and expanded. But how? Where did the energy come from? Was it always there, or did it just pop into existence? Lawrence Krauss maintains that yes, it came from nothing. I’m afraid I can’t accept that – which is why I shall keep reading, and watching, and learning.

And what about the “edges” of the Universe? What is it expanding into? Well, nothing that we can comprehend. The Universe has no edges, so to speak. It is everything. Or, it is everything in our comprehension. But that is not to say that there isn’t some”thing” out there beyond that, far beyond our comprehension, made of stuff that we could never know…

The more I learn about our Universe, the more fascinating I find it. I worried that I would lose the meaning of life if I was truly convinced of how insignificant we are – but, if anything, I have experienced the opposite.

Perhaps everyone has (or wants, or needs) to believe in something. I’m not sure. I don’t believe in a god, I know that now. This worried me for a time, as I see some of those I care for deeply, and their faith gives them strength and purpose. What would I have? I think my drive comes from a deep-seated desire to understand our Universe, to find out as much about it as I can. I believe it is within our grasp as a species, if we can manage not to destroy ourselves first. And what we find out may turn out to be completely unexpected.

And, I have faith in people. They are extraordinary.

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Particle wanted: dead or alive

Book 7 – From Quarks to Quasars – is going exceedingly well. I was not quite sure what to expect from a crash course in quantum physics, as it’s a subject I’ve long been fascinated by. However, with great fascination comes great bafflement, so I was expecting to be totally befuddled.

As the great Niels Bohr said: “If quantum physics hasn’t profoundly shocked you, you haven’t understood it yet.”

It is a source of some pleasure that in fact, I’m not struggling with this book at all. It’s heavy going, and there are some difficult concepts – not to mention some downright weird concepts – but with really thorough reading it is all making sense.

Why do some concepts come so easily, while others seem so difficult? Some people struggle with ideas that others find simple; for me, that is engineering and abstract concepts. Quantum physics, though, makes perfect sense to me. I don’t know what that says about the way my brain and mind work…

According to the Book, most people find it difficult to come to terms with the idea of quantum indeterminacy, with Albert Einstein himself finding the idea abhorrent.

I can understand that: Newton’s world was ordered and completely predictable. I like order; I thrive on it, in fact. Some of my favourite things are lists. But quantum uncertainty makes perfect sense to me.

One cannot say for certain exactly where an electron is in an atom and simultaneously know its velocity, so instead of the “classic” atomic model with electrons orbiting the nucleus, Schrödinger (he of the cat – and more on him later) proposed a new and improved model. The nucleus is surrounded by a fuzzy cloud of electrons, denoting the range of possible positions the electrons may occupy.

"Classic" model of the atom

So, why can one not predict the position and velocity of an electron? Theoretically, an experiment could be devised to measure its position or velocity, but can they be measured simultaneously?

Schrodinger's model of the atom

The Heisenberg uncertainty principle says “no”. If you know the exact position of a particle, you cannot know anything about its velocity at the same time. The reason why is actually rather simple, and makes perfect sense (once you understand how energy levels work in atoms…)

To measure something, you have to “see” it. And to “see” it, you have to shine a light on it. But by shining a light on an atom (which is where the electron we’re looking for is) one of two things will happen

  1. A photon from the light may be absorbed, causing the atom to change energy levels. The quantum state of the atom will have changed, and the electron will no longer have the same position or velocity.
  2. The photon may not interfere with the atom at all. In which case nothing has been measured.

This is the basis from which comes the adage that the act of observation changes the observed. This is certainly true on a quantum level, and true of a subject that knows it’s being watched.

(It’s a good example of how armchair psychologists and philosophers grab an idea from quantum physics and give it a little tickle, without fully digesting the concept!)

The above aside brings me neatly back to Schrödinger and his beknighted cat. It’s a wee niggle of mine that much of the populace seize on Schrödinger’s thought experiment and bend it to a vague philosophical notion along the same line as a falling tree making a sound when there’s nobody there to hear it. Or not, as the case may be.

Quantum cat: observation fail

It’s not as simple as not knowing for sure if the cat in the box is dead or alive before you open the lid. (And if they’re my cats, there’s no uncertainty at all, because they’d be screeching threats through the lid.)

The cat in the box is awaiting its fate – a possible death by poison. A vial of poison is positioned in the box, along with a radioactive isotope. If the isotope decays, the poison will be released by a trigger system. However, here is where quantum uncertainty comes in: the isotope may decay immediately. It may not decay at all. And all the possibilities in between. But the point is that until the box is opened, nobody knows whether or not the isotope has decayed, and so whether or not the cat is dead or alive.

The isotope now has a wavefunction describing two states: decayed and undecayed (because we can predict the probability of its state at any time, but not its actual state).

Is this all making sense?

(Schrödinger had also had enough of quantum weirdness, which inspired him to anger animal rights activists the world over in the first place.)

He then carried the theory of quantum uncertainty a little further: the cat is also made of atoms, and is a quantum system (a huge and complicated one, but a system nonetheless). So, stretching this, the cat must then also have a wavefunction describing a live cat and a dead cat, using probabilities based upon the probability of the isotope’s decay – because the cat’s fate was now bound inextricably with that of the isotope.

Clearly, this is silly. And Schrödinger knew it, which is why he challenged the upstarts Bohr and Heisenberg and their quantum theories of oddness.

It is resolved though: the cat is shut in a box. We can have no knowledge at all of the real state of the cat: probability is just a string of numbers. Without measuring reality, we cannot describe it – so we do not try. When it comes time to open the box, we can use the probability of the isotope’s decay to predict the probability of the cat’s state, but beyond that – nothing.

There’s an excellent book: “Quantum – A Guide for the Perplexed” by Jim Al-Khalili. It explains a lot of difficult concepts really well.

I hope I’ve made a start. It makes perfect sense to me, anyway!

Stay tuned for nuclear decay…

A billion billion billion billion billion times bigger…

Book 7 of S104: Exploring Science is entitled, rather niftily, “From Quarks to Quasars”.

Quarks are the smallest things of all, the fundamental constituents of the Universe, measuring 10-19 m across; quasars are the most distant objects we can observe, and are around 1026 m away.

There’s really no way to get your head around these extremes of sizes; suffice it to say that quasars are a billion billion billion billion billion times larger than quarks. Even analogies are impossible. Imagine a marble and a… no. There’s nothing big enough. Or far away enough. Imagine a marble and something MUCH further than a quasar?

“Common sense is the deposit of prejudice laid down in the mind before the age of eighteen.” Albert Einstein

Well, leaving aside ludicrous quantities of billion, cosmology is the study of the very, very large and particle physics is the study of the very, very small. This aspect of the module combines both of these studies into one neat package, and that package helps to answer the fundamental questions:

  • How does the Universe behave?
  • What rules does it follow? Or is it an anarchist, breaking glasses, listening to the Sex Pistols, and throwing sofas out of hotel windows?
  • How does the Universe change with time?

I’ll get back to you on those when I’ve worked out the answers. Quantum physics will help.

In the meantime, here’s a philosophical take on the very, very small by those reknowned poets, They Might Be Giants:

Looking at the nature of the Universe takes you outside of the everyday into the realm of the fascinating, the baffling, and the just-plain-weird. Particles that are in two places at once; antimatter; eleven-dimensional space-time.

“If quantum physics hasn’t profoundly shocked you, you haven’t understood it yet.” Niels Bohr

Hang on to your hats, because Kansas is about to disappear…