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added some comments on history to neutrino.
Please excuse my ignorance, but it might be nice to add some mathematics to the article, e.g., saying something about mass and spin which would characterize the neutrino in terms of an irreducible representation of the Pincaré group. (I would do this myself if I already felt confident doing so.)
Why did Pauli think neutrinos were so far out of reach?
I’ve done a terrible thing today, something which no theoretical physicist should ever do. I have suggested something that can never be verified experimentally.
He presumably knew we were bathed in them, so it was just a question of detection. Why underestimate technological advance?
Also, what is being implied here:
Notice that back then, predicting unobserved and possibly practically unobservable fundamental particles was not taken as lightly as in some circles it is these days (e.g. in supersymmetry and/or string theory)?
If there’s uncertainty as to the difficulty of detecting something, better to err on the cautious side?
Re #2, I seem to remember you Todd summing up very well the irreps of the Poincare group. Ah yes here. That should be copied over to unitary representation of the Poincaré group.
If there’s uncertainty as to the difficulty of detecting something, better to err on the cautious side?
Yes, that’s certainly the general attitude, that physics should not lightly postulate entities which are not in principle observable. Physics pre-history is regarded as a an example how things go wrong if one did. And of course rightly so. Nevertheless the issue is more subtle than some people these days, who cry “falsifiablility!”, seem to acknowledge.
Modern analogs of the neutrino in the 1930s are the inflaton field, dark matter and dark energy (until recently also the Higgs particle). None of these have have been (or had been, for the Higgs) directly detected, yet just as with the neutrino presumeably “we are bathed in them” (or the universe was, for the inflaton).
As with the neutrino, postulating these unobserved (directly unobserved) entities makes theory come to good agreement with experiment (but dramatically so, a good review was given a while back here).
These days, most physicists are happy with assuming all these unobserved entities, regarding the elegant form of the maths describing the universe, which would break without these, as sufficient support. But not all. Some proponents of MOND maintain that it is preposterous to postulate that more than 95 % of the observable Universe is in fact unobservable, and that instead it is the laws of nature that must be different from what we think they are. These MOND proponents are the Bohr-s of these days, ready to give up a nice general theory for a piecemeal description of phenonema, if only this reduces the amount of unobservables.
Some people these days find this debate a sign of crisis of physics even. But it is good to remember that the word “horizon” in cosmology has exactly the same epsitomological meaning as in everyday life: back some millenia mankind’s cosmic horizon was a few miles off shore in the oceans and already back then we had both schools: some proclaiming that clearly the world must end where our perception of it ends, others trying to infer from general principles (such as lengths of shadows in Alexandria) that there must be something beyond the current horizon, even if unobservable by direct means.
I don't think that any civilization ever had intellectuals who actually believed that the world ended at the horizon. A single long-distance journey disproves that. But if you're talking about the horizon as seen from the top of a tall mountain, then that's another matter; there is a lot of ancient literature suggesting that one could see the entire world from a sufficiently high mountaintop (with Matthew 4:8 as probably the most famous).
We're approaching Columbus Day here in the U.S., so I'm sensitive to claims of pre-modern ignorance about the shape of Earth. Of course, you said ‘shadows in Alexandria’, so you’re not talking about Columbus!
I am not into the history, but I gather that a) back in the times without global communication exchange “human knowledge” was a very inhomogeneous quantity, with some people somewhere knowing something of which other people elsewhere were completely ignorant, and that b) even the knowledge of the best informed may have decayed over time at some places. Medieval maps of “the world” tend to be much less informed than their ancient predecessors and tend to clearly suggest the proverbial disk. On the other hand, during the dark European ages the ancient Greek knowledge was kept in Arabia.
What is maybe interesting is to make surveys among scientifically lay members of your family and friends on the cosmic knowledge of our times. My experience is that almost no lay person has any clear idea of what is known about whether or how “the universe is infinite” and what our bounds on the knowledge about the extension of the universe are. Children seem to routinely ask this, but rarely get good answers it seems (from my limited experience, and of course I try to counteract ;-) But on the other hand I noticed that laymen tend to be very relaxed about the question in the big cosmic debate in the public domain of our times: when asked whether they can imagine that there are other parts of the universe where maybe different laws of nature hold, I often get an “of course” also from people who were not subjected to modern “multiverse” discussion.
On the other hand, I remember after the movie “The Fifth Element” came out that my scientifically lay friends were surprised to hear me make fun of the statement in the movie that the movie characters found an unknow new chemical element on some star or somewhere. I pointed out that it is one of the big insights of modern physics and astronomy that we know of all possible chemical elements and that these are the same everywhere in the observable universe, and that this is proved by spectroscopy of stars and is the reason for the amazing fact that by just analyzing light here on earth we can know about the chemical composition of stars billions of light years away. My friends on the other hand seemed happy to assume that the laws of nature may spatially change already within our cosmic horizon.
@Toby: Of course, Columbus knew where he was going, more-or-less, as it was known in Scandanavia and the British Isles that there was a land mass out there. There is a story in Galway that Columbus visited there and was told stories of voyages to the west, and in Bristol they claim that Bristol merchants were trading with the east coast of the North American continent before Columbus set sail. Most of these claims are probably unverifiable leading to a situation somewhat like unobservables in theoretical physics!
On Urs’ point:”during the dark European ages the ancient Greek knowledge was kept in Arabia” and on a point of detail, possibly Mesopatamia would be more accurate, but your point is valid. (There is however an interesting old map in Hereford cathedral which may show more than one would expect!)
What should one’s attitude be to particles that would be practically unobservable (requiring too much energy for their observation, say), but which would have an effect that might be explained in various other ways than from the usual model, and that effect was observable? The theory would be consistent with, but not dependent on the results of the experiments.
What should one’s attitude be to particles that would be practically unobservable (requiring too much energy for their observation, say), but which would have an effect that might be explained in various other ways than from the usual model, and that effect was observable?
i think the best general attitude is simply: to keep an open mind.
What is noteworthy are the time spans in between prediction and detection of the particles that we discussed: for the neutrino over 26 years, for the Higgs about 50 years. This is short on some scales, but is on the boundary of what single humans can usefully handle.
This was, by the way, Edward Witten’s reply in the question session after his talk at StringMath2012 last year, when a member of the audience used his question to openly accuse him by claiming that he has “mislead the entire field for 30 years”: Witten replied that with the current energy scale of particle physics, even a single simple idea like the Higgs mechanism takes half a century to experimentally verify, so that 30 years for something much more ambitious is not much.
Of course that does’t prove anything and won’t settle any debate. But it may be good to keep in mind when developing a feeling for prediction and experimental verification in fundamental physics.
added a Weinberg-quote on neutrino masses and the standard model as an effective QFT to neutriono (also to effective field theory). Mostly a reminder to come back to later…
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(under “References – history”)
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