Monday, 1 September 2008

The Flaming Lips and the two-slit experiment

What is light? (an untested hypothesis suggesting that the chemical (in our brains) by which we are able to experience the sensation of being in love is the same chemical that caused the “Big Bang’ that was the birth of the accelerating universe) 4.05

> Track 5 from The Soft Bulletin by the Flaming Lips

It was in the early hours of Sunday morning - long after everyone else had departed for the land of dreams – when I rubbed my weary eyes and yawned. God I was tired.

But what was the exact time I wondered? So I looked up from the sofa I was slouched in toward the large aluminum-brushed Seiko clock at the far end of my long rectangular living room - only to become acutely aware that the glare on the face of the clock was in fact caused by the reflection of beams of light that were shooting toward my eyes at a velocity that was quite simply impossible for me to comprehend. 299,792,458 meters per second to be precise.


But what was it that my eyes were seeing?

Light you say.

But what exactly is light? I mean does light comprise of tiny particles that bounce off objects and things before they slam into our eyes? Or, alternatively, is light simply a wave which moves through the air in that familiar undulating motion that we know characterizes all waves?

Well, the answer, apparently, is both. Yep – light is indeed a wave but that it also comprises of particles.

This is shown quite clearly in what is largely regarded the most interesting lab experiment of all time: the classic two-slit experiment – an experiment that ultimately raises far more questions than it answers.

In the two-slit experiment, the light passes through both slits. There is a large peak intensity smack in line with the two holes – as you might expect – but there is also a very low intensity just either side of the peak, where the two sets of waves cancel each other out, and a pattern of highs and lows repeating alternately as we move along the screen. It looks like this:

Mathematically, the intensity of the light going through slit one can be said to be H².

And the light going through slit two can be said to be J².

But in this experiment, instead of finding that the intensity of the light is H²+J², it is actually (H+J)², which is the same as H²+J²+2HJ.

So what is the additional term 2HJ?

Well it is the contribution due to interference from the two waves, and making allowance for the fact that the H’s and J’s can be negative or positive, perfectly explains the peaks and troughs of the interference pattern.

Now that is all good and well but isn’t light also made up of particles?

Yep. And here is when it gets interesting. Because in the experiment above, any given particle must have passed through either the first slit or the second slit.

But if we somehow set up an experiment to actually see which of the two-slits the particles have passed through, then things get bizarre: the interference pattern (2HJ) disappears!!!

Just by merely “looking” at the spread out “wave” in terms of its individual particles, then the wave collapses!

Yet if we don’t look at the individual particles then the wave doesn’t collapse and we can see the interference pattern!!

Other questions remain of course: how does an individual particle know which slit to go through such that half of them go through the first slit and half go through the second slit?

This is highly freaky of course and totally baffled Einstein.

Welcome to the world of quantum physics.

And this phenomenon is also behind one of the greatest paradoxes we can imagine: Schrodinger’s cat.

Imagine a box that contains a radioactive source, a detector that records the presence of radioactive particles, a small glass bottle containing cyanide and a live cat.

The apparatus in the box is arranged so that the detector is switched on for just long enough so that there is a fifty-fifty chance that one of the atoms decays and that the detector records a particle.

Now if the detector records an event, the glass container is crushed, and the cat dies. If not then the cat lives.

But we have no way of knowing the outcome of the experiment until we look inside the box.

Thus, as in the two-slit light experiment (in which the particle could have gone through either slit, but we only know which one when we look), it should be logically the case that the equal probabilities for radioactive decay and no radioactive decay produce a superposition of states. Thus, the superposition is real until we open the box and look inside, at which instant, the wave function collapses and the atom either decays or doesn’t and the cat either lives or dies.

But before we look inside the box, the radioactive sample has both decayed and not decayed, the glass bottle is both broken and unbroken, and the cat is both dead and alive!

This ultimately leads to an infinite regression and that the whole universe only owes its existence to the fact that it is being observed by intelligent beings!!!

Well, I wonder what the Flaming Lips would make of that! Play on gentlemen!!!!