What Really Exists?

If you’ve been to Cute Qubit before, you’ve probably noticed the 2 month summer vacation that this blog has taken. It wasn’t intentional, but my incredible laziness made it possible. Luckily, the majority of my thoughts have been uninteresting or simply not blog worthy, so you haven’t missed out on much. I’ve made lots of chocolate, watched plenty of movies, and spent a considerable amount of time in Waterloo.

On one such visit to Waterloo I found a book titled Quantum Physics and the Philosophical Tradition. Being in love with Quantum Physics, especially where it meets with philosophy, I couldn’t ignore this book. I started reading it and soon found an excerpt from a scientific journal that interested me. I went to the Davis Centre, rolled through hundreds of meters (which really isn’t that much) of microfilm, and found the full article that had been quoted in the book.

The article was called “Does the Neutrino Really Exist,” and it was about the way people perceive physical theories, like the existence of particles. I read and reread the article; I was in love! I thought the best thing to do was to type it up and let the world enjoy a truly beautiful piece of writing.

So with out much surprise, I’ve posted the article here on Cute Qubit. Before you read it, I’ll give you some background information. It was written in the 50’s when not too much experimental data existed for the Neutrino (though it is a widely accepted and loved particle today). Also, you may what to familiarize yourself with the uncertainty principle and the correspondence principle. They are (simply stated) as follows:

Uncertainty Principle: The position and velocity of a quantum particle cannot both be known with absolute accuracy at the same time.

Correspondence Principle: Quantum objects can be viewed as either particles or waves, and these two views complement each other.

Hey cool, now everyone knows some Quantum Physics! Enjoy the read!



Does the Neutrino Really Exist?
By S. M. Dancoff
From Bulletin of the Atomic Scientists 8, 139 (1952)

I must admit at the beginning that this isn’t a very good title for the talk I propose to give. In fact, the choice of a brief but accurate title presented somewhat of a problem. Perhaps the best short name for the talk would be something like the following: “Does the announced title for today’s lecture make any sense?”

My thesis will be: No, it doesn’t make any sense to argue about questions like the reality of the neutrino, or for that matter of the electron or proton. I would hold such discussion to be meaningless and based on a misunderstanding of the proper role of the electron, proton, neutrino, etc., in physics. I’ll do my best to justify for you my view on this matter.

Probably it’s not necessary to remind you that arguments of the sort “Is the neutrino a particle?” – “Is the neutrino a real particle.” And other variations—crop up around here pretty often, namely every time Professor Sherwin makes a report on his neutrino research. Now with Professor Allen joining our staff and adding his reports on his neutrino research, it would appear that the arguing is going to get pretty serious and it strikes me that it would be worthwhile to try to dispose of the matter once and for all, if possible.

First, I would like to review the situation with regard to the neutrino. You recall that in radioactive beta decay of a nucleus, an ordinary electron is observed to come off. The energy is not fixed, but in a large sample of decaying atoms the electrons coming off have an energy from 0 up to a certain maximum. On the other hand, the difference in energy between initial and final nucleus is known to be always precisely E(max). Then what happens to the extra energy in the cases when the electron takes off less than E(max)? Pauli postulated that this was removed by another particle called the neutrino. Since experimental tests to date have failed to give any evidence of the escape of such a particle from the source region, it has been necessary to assign the particle just exactly those properties that would enable it to get away without being detected. Namely, it has to have zero rest mass, no charge, no magnetic moment (or very small).

Fermi formulated Pauli’s suggestion into a theory which involved the use of the neutrino as a concept. With this theory, Fermi could predict the form of the electron distribution curve, and the best evidence seems to indicate that the prediction agrees with experiment. He could predict radioactive decay rates, and these too seem to be about right. Everywhere that the theory can be subjected to test, it appears to hold up well. In the most recent extremely delicate recoil tests of Professors Sherwin and Allen there seems every evidence that the theory checks on the momentum which the nucleus gets after beta decay. That is, this momentum is what you would get if an electron plus a neutrino were emitted in the process.

As to the neutrino itself, it is apparently gone and never heard from again. According to theory, the chance of stopping it somewhere is very tiny and much too small for present experimental techniques. In other words we don’t notice it while it’s around but miss it only after it’s gone away.

This being the case, it has been said by some that the neutrino has not yet become respectable, not acquired full stature as a particle – sort of a hypothetical particle when compared to others which are more real. This is what we want to talk about.

Problem of Terminology

Now, when we ask what is and what isn’t a particle, we have to keep in mind very clearly the point that Professor Hartree emphasized in one of his lectures here. When you set out in a new field and choose a terminology you have the choice of using old, familiar words in new meanings or else you can make up new words for the new meanings. If you use old words you make the theory look homey and inviting, but you run the risk of confusing the issue every time the old word is used. If you use new words you make the thing look excessively highbrow and frighten off any who might be interested.

In the case of electron, proton, neutron, neutrino, etc., physicists used an old word – “Particle” – but they gave it a new meaning, a very very new meaning. I believe that here is a place we could have afforded to use a new word, because an enormous amount of confusion results when people try to apply to these “particles” ideas they’ve always had about what particles should be like.

Example of “Electron”

Let’s consider a “particle” that has achieved considerable respectability, the electron. Let’s ask: What is an electron?—is it a particle?—does it really exist?

It’s an obvious fact that no one has ever seen an electron, no one has ever weighed an electron, felt an electron, or in fact made any observation whatever on an electron. What we have seen are certain experimental phenomena—scintillations of a screen, water droplets in a cloud chamber, deflections of a dial, black spots on a photographic plate, etc. By themselves, they represent just a lot of observations having no particular connection with each other. But when we use the Schrodinger wave equation, or perhaps the Dirac wave equation, we find it’s possible to calculate the results of the various experiments mentioned and get agreement between theory and experiment.

Notice that all that’s entered here are the experimental data on the one side and some equations on the other, along with rules for calculating from the equations. At no time has the idea of a particle entered; in fact it would be possible to dispense entirely with the word electron and yet not affect the procedure of comparing experiment with theory. Strictly speaking, an electron is merely that thing—that state of affaires—which is defined by Schrodinger-Dirac theory.

Role of Theory

Now I must say a word about the role of theory in physics. In physics we start with experiments—always with experiments. We write down the results of many experiments on a sheer of paper. If all the experiments are entered, it has to be a very big sheet of paper indeed. In fact, we have decided this method of storing the data is rather clumsy; wouldn’t it be nicer if we could find some neater, more abbreviated way to do the job? So we set to work some characters with badly-kept hair, called “theorists,” whose job it is to find some trick way of correlating the data in as big lumps as possible. The theorist works a while then tells you: “All the data in this region over here—plus a little of the data over here, plus some over there—can be all correlated together and simultaneously by means of this equation, using these rules for calculating experimental results from the equation.”

Then along comes another theorist with the announcement: “That’s all very well what you say, but I’ve got a better equation because I can correlate even more of the data with my equation.” And so we consider the second theory to be an advance over the first one.

Generally speaking, these theoretical formulations are of such a nature that they can be extrapolated. That is, they can be used to predict the results of experiments that have not yet been performed. Now if this extrapolation is not too great, it usually works out that the theory holds up more or less well. But if the new experimental conditions are greatly different from the ones for which the theory was derived, the extrapolation generally fails, and a new theory, or new correlation has to be worked out.

Place of “Electron” in Theory

The Schrodinger-Dirac theory is just such a correlation. It gives very accurately the results of a large number of experiments. Now where does the electron come in? When I use the word electron, I mean the Schrodinger-Dirac theory, neither more nor less. I don’t think there’s anything else you can mean and still make sense. The particle name is simply a title to differentiate for convenience one group of data from another. In other words I would say: “Electron” = Theoretical Formulation = Group of data.”

When on asks: “Does the electron exist”—that is the same as asking: “Do the data exist? Are the experiments real?”

I realize that in the course of this talk, I am being more dogmatic than is customary in a physics lecture, and I apologize for this. My excuse is that I have found in discussing this subject that it is very hard to make oneself understood. In any case, there will be plenty of opportunity for rebuttal from those who view the matter differently.

Existence of Electron

There are many who want to think of the electron as something more than a conveinient grouping of data. They like to think that in some extra-physical sense there “really is” an electron. I don’t know what that means, but I’m describing as well as I can the position of others. The physicists of whom I’m speaking feel that an electron exists independently of our experiments and theory. It is really, absolutely there.

I don’t say that such a point of view can be shown to be logically false. I wouldn’t try to do that. But I do think it is a very dangerous and unproductive point of view. It actually hampers the work of physics.

Now take this question of the electron. If it “really, truly exists,” then how dare we, for example, try to describe nature in terms of any theory that doesn’t have electrons in it? To a believer in the reality of the electron such a theory would be unimaginable. But theorists don’t want to be bound in developing new theories by somebody’s prejudices about what it is that really exists. If I can find a new theory which is superior to quantum mechanics and which doesn’t mention electrons anywhere, then away with electrons—let’s hear no more about them! But the man for whom electrons have become real and unquestionable is forever limited in his conception of possible new theories.

We have all heard about how the electron has properties like those of a particle and also others like those of a wave. That is to say, the solution of Schrodinger’s equation exhibits similarity in some cases to classical equations describing the motion of a point particle. Under other conditions the solutions of Schrodinger’s equation are more like those of wave motion as in optics. The believers in the absolute reality of the electron will often say, for example, that the electron really is a particle but it is the perversity of nature which prevents us from making exact measurements on this particle. We see this particle only through a ground glass, darkly. There is a principle, unfortunately called by some the uncertainty principle, which is said to express the limitations on measuring the position and velocity of the electron. I prefer to say there is no uncertainty involved anywhere. It is not necessary to assume that there really is a point electron; consequently, there is no position and velocity of anything to be uncertain about. The electron may behave only approximately like a particle, or approximately like a wave. But it always behaves precisely and without any uncertainty, like a solution of Dirac’s equation, i.e., precisely like an electron.

The Case of the Neutrino

Now let’s get back to the case of the neutrino. As I’ve said, Fermi developed a theory which gave a good account of the experimental data known at the time. The wider implications, or extrapolations of his theory have been tested and one by one they have seemed to stand the test, at least so far.

It pleased Fermi to designate a certain set of symbols by the word “neutrino.” These symbols were an integral part of his theory. Their use is just as important as anything else in his theory for any experimental result that is calculated from it. Now I think Fermi had just as much right to name these symbols and their associated properties “neutrino” as he had to name his own daughter. There is no question as to whether these symbols describe something which “really exists”—the only things that “really exists” as far as a physicist is concerned are the data, and Fermi’s theory is in agreement with them.

I will now address myself to those of you who are not convinced by my previous arguments and continue to feel that the neutrino is not as real as some other particle or is not as respectable. Undoubtedly, being fair-minded people you have a set of crucial tests which you feel the neutrino should pass—prelims, so to speak. Maybe you want neutrinos to be produced artificially; perhaps you insist on the development of a neutrino detector. Some will be satisfied with a little less, others will want more, but each of you has in his mind some crucial experiment, after which you will break down and take neutrinos to your bosom.

Being talented physicists, you will perhaps undertake one or more of these crucial experiments yourself. With the advances to be expected in techniques it may be that you will actually before long be able to detect a neutrino. You may learn how to produce neutrinos artificially. You may learn how to observe the scattering of neutrinos, how to measure radiation from neutrinos.

As knowledge about the properties of neutrinos increases, new technical achievements will become available. You will be able to make neutrinos follow helical paths—to bounce up and down like rubber balls. You may even be able to fashion a club out of neutrinos with which to hit theoretical physicists on the head.

Finally, practical uses for neutrinos will appear and multiply. They will be used to power space ships, new plastics will be made out of them, they will become the essential ingredient in a nourishing soup.

You will become ashamed that you ever doubted that neutrinos “really existed.” You will come to be very fond of the neutrino. You will love is as a son. You will say: “there is a fine particle, a real particle.”

Out-Moding the Neutrino

At that time, however, some theorist will get up and say: “Too late—the neutrino is out! We have a new theory now. Everything gets described in terms of a series of inversions of the space-time metric. We can include in this way all atomic, nuclear, gravitational, electromagnetic phenomena, and lots more. And it can all be written down on a half sheet of paper. We’re all through with protons, neutrons, electrons, mesons, neutrinos, and all that nonsense. We use just a single concept now—we call it the metricon. The theory is completely worked out—in some unpublished notes of J. Schwinger.”

And so, after all the trouble of getting yourself convinced that the neutrino really exists you suddenly find yourself left holding the sack—of neutrinos. And if I’m not mistaken, your first reaction will be to attack the new theory on the grounds that the metricon is not a real particle—it doesn’t really exist.

You all know about the terrible shock to the nervous system the world of physics got at the beginning of this century when it was forced to swallow the double dose of relativity and quantum theory. Probably the great majority of physicists found that they were asked to surrender hallowed and cherished concepts that had become set in granite for them. Many of them balked and refused to have anything to do with the new theories.

It has been asked what we can do to guard against having ourselves terribly shocked in a similar way by the next radical new development that comes along. I would say that one of the things we must be most careful about is to refrain from attaching any absolute reality to concepts like the electron, energy, mass, etc. They are useful, they are expedient, but they are also expendable.

Thus, when someone says he doubts that the neutrino is a real particle, I am concerned not so much with his ideas about the neutrino, but rather with the implication that some other particles are real—real in the sense that when you describe the universe by using the particle you’re speaking truth, but if you don’t use this particle in your description, then you’re getting involved in error. This is actually an article of faith, and like any other article of faith it cannot be proved or disproved on physical or logical grounds. But, as I’ve attempted to show, the holding of a belief in the absolute reality of any given concept is a dangerous thing and inimical to the progress of research.

Summary

To summarize—the concept of a particle, such as an electron neutrino, etc., may be considered simply a chapter heading in some book of theoretical calculations. Each chapter covers all the calculations on a certain class of experiments. If the results of the calculations check the experiments, then the theory is good, and the concept may be said to be suitable, or appropriate.

The neutrino appears to be an appropriate concept on the basis of all experiments performed to date relating to Fermi’s theory. As to whether it is something more, something real in some sense or other, it doesn’t appear possible to define that question in a physically meaningful way.

Coming back finally to the question of the title of this lecture, I think we can now see how to modify it so as to give a better description of the talk. We simply add two short words: Who cares?

With this I retire and await the deluge.