Is an observer needed?

kw: science, quantum theory

After writing yesterday's review of The Age of Entanglement, I continued thinking about the issues raised by entanglement and the "observer problem" that is at the root of the disagreements over the philosophy of quantum theory.

One of the thought experiments that probes the issue is Schrodinger's Cat. A live cat is placed in a box that contains a tiny bit of radioactive material, a detector, and some means of the detector triggering the release of poison gas when the detector detects a radioactive decay. The amount of radioactive material and the placement of the detector are designed such that there is a 50% chance the device will trigger within one hour.

When the box is opened after one hour, is the cat alive or dead? Just before you open the box, in what state is the cat? According to some interpretations of quantum mechanics, the cat is in a superposition of states, both dead and alive. Opening the box to observe the cat "collapses the wavefunction" (is there meaning in those three words?) and produces either a dead cat or a living cat, out of an ambiguous state.

Then there is an extension of the matter, the "Wigner's friend" version: Wigner steps out of the lab, and while he is gone, his friend performs the experiment. Well after the hour has passed, Wigner returns, to be told the result by his friend. Now we are one step removed. Before Wigner returns, is his friend in an ambiguous state, a mixture of happy over a live cat and sad over a dead one? Does some wavefunction collapse when Wigner returns?

I always ask, "How about the cat? Is it an observer?" I guess the answer is, "Not to a physicist." But to me, the detector is an observer! It observes the radioactive decay (or not) and takes action accordingly. You could replace it with a piece of sensitive film, and develop the film. There will be a spot, or there will not be a spot. Does the spot exist before the film is developed? When does the wavefunction collapse? Whatever that might mean, I say it collapses when the particle's motion is diverted or stopped.

Consider interference, whether carried out with light (photons) or electrons, or even with viruses (this has been done!). Let's use electrons. A beam of electrons passes trough one very small hole, which leaves a coherent, spreading beam. This then strikes a plate that contains two holes or slits. A photographic film placed a suitable distance beyond the two slits is exposed for a while, enough time for many thousands of electrons to strike it. Then it is removed and developed. The banded pattern that shows up on the developed film indicates interference, that the electrons behaved as waves when passing through the slits.

It is easy to figure out how to lower the current in the electron beam such that the electrons arrive one at a time; indeed, such that there is never more than one electron anywhere in the whole beam, from emitter to film, at any particular time. It takes a long time to gather thousands of "hits" on the film, but what do we see when the film is developed? We see the same banded pattern. This has been done. It seems to prove that each electron somehow goes through both slits and interferes with itself! Yet the banded pattern is made up of many tiny dots, each indicating where an electron struck the film. They just didn't strike the film in the "low" bands, and they did strike in great numbers in the "high" bands.

Where is the observer in this? Is it the person who develops the film? Is it the film itself? Crucially for this setup, if no film is put into the path of the beam, do the electrons still pass through space as a fan of narrow beams that would produce a banded pattern if film were ever put into place? I say they do, but some interpreters of quantum theory say that there is no pattern unless something is there to record it. And that shows just how crazy this all can get.

Actually, there are a few things that are already "observing" the electrons! The first hole, that spreads the beam and makes it coherent, has "observed" the positions of electrons and selected only certain ones in a narrow range of positions. This, by Heisenberg's Uncertainty Principle, deflects them, though they have not, it seems, "touched" the edges of the hole. In actuality, they must have done so, their wave nature "feeling" the size of the hole so they'll "know" just how much to spread out! Sorry for the anthropomorphism here, but it is by far the briefest way to describe what is going on.

Then there are the two holes or two slits that turn a portion of the beam into two beams that can interfere with one another. They also "observe" electrons, and select certain ones which are in the "right" range of positions to pass onward.

This leads to my conclusion: anything that disturbs a particle's motion can be considered an observer. The universe was ticking along quite nicely for a long, long time before there were brainy macroanimals about who could think they were somehow privileged to be "the observers".

And one more consequence of the Uncertainty Principle: no particle is ever at rest. When a barrier "stops" a particle, that particle is either destroyed or it takes up a different kind of motion within the material of the barrier. Typically, bosons such as photons, if they are not reflected or refracted, are destroyed and turn into phonons (heat), while fermions such as electrons or protons "join" the material of the barrier, where they either travel through the bulk thereof or become bound in some way and take up vibratory motion that accords with the temperature of the material (they also release some of their kinetic energy as phonons). Even at "absolute zero" (0K), a little jitter remains, having the value of Planck's Constant divided by two pi. So I conclude that a particle has no existence unless it is in motion. Perhaps a "particle" is simply kinetic energy objectified.