Various new states of matter have been created. Someone already mentioned Bose Einstein Condensates. Related to that, there’s also Fermionic Condensates. Also, I think we may have created quark-gluon plasma.
High temperature superconductors (discovered in 1986) strike me as something else that could prove to be very important, and it’s certainly a very active area of study right now.
If the LHC discovers the Higgs boson or superpartners, that would be quite major. (Although perhaps even more significant if it fails to discover them).
Of course, all of these are experimental achievements. Perhaps 25 years isn’t really long enough for a major new theory to prove its value. (At least, theories toward the more recent end of that 25 year period may still be largely unproven.) Obviously, if string theory/M-theory turns out to be correct, it’s huge. But who knows when we’ll know that. If you extend the window back another decade, you’d include things like asymptotic freedom.
Just because those questions are obscure and highly technical doesn’t mean they won’t yield real-world advances. The problem, I think, is that it’s generally impossible to predict what those advances will be a head of time. Who would have thought in the early days of quantum mechanics that it would lead to something like the transistor? Or who could have forseen that we’d someday use lasers for everything from eye surgery to playing music?
To predict this sort of thing in advance, I think physicists would have to know the answers to their questions before they ask them. It’s kind of a rough situation for people who may be applying for funding from a government agency that wants to know what practical applications their work will have.
Don’t confuse theory with applied science. People were using chemistry for centuries before anyone knew what an atom was. One of the stages I’ve already mentioned was the one where a theory is regarded as a useful model. There’s a subtle but important conceptual leap between accepting that compounds behave as if they are made of atoms and accepting that compounds actually are made out of atoms. And the same thing applied to quantum theory; it was accepted as a mathematical model long before it was accepted as a description of reality.
The transistor was not made by people randomly poking wires into things. The idea for a transistor came from an understanding of how semiconductors work and then building it.
Saying QM wasn’t fully accepted until the '70s seems pretty late to me too. Let’s look at when some of the major contributors got their Nobel prizes:
Planck - 1918
Einstein - 1921
Heisenberg - 1932
Schrodinger, Dirac - 1933
Pauli - 1945.
Planck’s and Einstein’s Nobel-winning contributions weren’t exactly QM as we think of it today, but the others sure were. Heck, Tomonaga, Schwinger, and Feynman won the 1965 Nobel for Quantum Electrodynamics. Can you really contend that basic quantum mechanics wasn’t accepted by that time?
In contrast, how many people have won Nobels for string theory. None, because we still don’t know if it’s right.
I should add, I didn’t mean to suggest they will necessarily be proven correct. Just that it will take significant time before they become the generally accepted theory, and only after that can they be considered a major breakthrough in physics.
Tim, you wrote Einstein won a Nobel Prize in 1921 for his contributions to quantum theory. That would no doubt be a surprise to him because he never really accepted quantum theory. As late as 1954 he wrote, “Probably never before has a theory been evolved which has given a key to the interpretation and calculation of such a heterogeneous group of phenomena of experience as has quantum theory. In spite of this, however, I believe that the theory is apt to beguile us into error in our search for a uniform basis for physics, because, in my belief, it is an incomplete representation of real things, although it is the only one which can be built out of the fundamental concepts of force and material points (quantum corrections to classical mechanics). The incompleteness of the representation leads necessarily to the statistical nature (incompleteness) of the laws.”
I’d say Einstein is an example of exactly the kind of thing I’ve been talking about.
The fact that he never really accepted quantum theory doesn’t change the fact that he was a leading contributor to its early development. This is an often observed irony, that Einstein was both a key pioneer of quantum mechanics and one of its most famous doubters.
Einstein’s Nobel Prize was officially given “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”. Einstein’s explanation of the photoelectric effect relied on the notion of light quanta – this was primordial quantum mechanics in the same vein as Planck’s blackbody radiation law. In fact, I’ve seen it claimed that at the time Planck viewed quantization as basically a mathematical trick, whereas it was Einstein who first genuinely believed that light was composed of such quanta.
Incidentally, Einstein died in 1955, and his most productive years were decades earlier than that, so it doesn’t exactly make sense to cite him as evidence that quantum mechanics wasn’t fully accepted until the '70s.
The fact that somebody of Einstein’s reputation could openly express doubts about quantum theory as late as the 1950s clearly shows it was hardly a universally accepted part of the physics mainstream.
It doesn’t show it was hardly universally accepted in 1970. Or are you adjusting your claim to say that quantum mechanics wasn’t accepted in the '50s? It’s fine if you are – nothing wrong with changing your mind.
Still, the thing to remember was Einstein was about 75 years old when he made the statement you referred to. I don’t think it’s uncommon for an older scientist (even a brilliant one) to cling to outdated ideas even when most of the scientific community has moved past them. I submit that if Einstein had been born in 1900 instead of 1879, then by 1954 he’d probably have been convinced of quantum mechanics correctness. But he’d had the idea of a fully deterministic universe reinforced in his mind for decades, and by the time the evidence really started to come down the other way, he was just to set in his views. (Although if he’d lived another decade, I have to think he would have come around.)
(But in fairness to Einstein, I don’t think he really thought that quantum mechanics was incorrect per se . . . he just thought it wasn’t the whole picture. He felt there had to be some purely deterministic theory behind it all, that somehow merely gave the appearance of being probabilistic.)
At any rate, if we’re trying to judge how well accepted quantum mechanics was in the '50s, I think it makes more sense to look at the attitudes of Dirac and Heisenberg’s generation (i.e., physicists born around the turn of the century), not a septuagenarian with one foot in the grave.
No, I was specifically refuting your previous claim that quantum theory was widely accepted back in the 1920’s. You used Einstein as one of your cites, you I used him also. I was unable to use any Einstein quotes from after 1955 for obvious reasons. But I stand by my original statement. Quantum theory was still held as tentative by many physicists in the 1960’s.
Now hold on, I never said QM was accepted in the '20s. I just listed several Nobel prizes from 1918 to 1945 that could reasonably be labeled as prizes for quantum physics. And those were just the first five I spotted from the list – I made some MAJOR omissions. Here’s a much better list.
Planck - 1918
Einstein - 1921
Bohr - 1922
de Broglie - 1929
Heisenberg - 1932
Schrodinger, Dirac - 1933
Pauli - 1945.
My point was not that quantum mechanics was widely accepted in the '20s. The theory was still in its infancy then, and those first prizes were for what could really be called primordial quantum mechanics. (And as you point out, Einstein never really reconciled himself to the full version of the theory.) But my point is that by 1945, when eight people had received prizes for their work on quantum physics, it was clearly pretty widely accepted. Einstein was probably one of the last few holdouts in the early '50s, and if he had been anyone else I doubt that much attention would have been paid to his objections.
You still haven’t provided any cites for your claim that the theory was viewed as tentative until the '70s.
Not necessarily. And the problem is that quantum mechanics is an almost perfect counterexample to the naive generalisation above.
The first paper - Heisenberg’s - was dated 18th September 1925. There was then an explosive growth, with a doubling time of about two months, so that about 80 different authors together published over two hundred papers on the subject in the next year and a half. By 1929, one estimate has fully 16% of the papers in Physics Review being specifically on the new mechanics. At roughly the same time, it became necessary for any institution with pretentions to being a research university to be teaching at least an advanced course on the subject, with the result that they were falling over themselves to hire new professors with experience enough that they could teach these courses.
In his Quantum Generations (Princeton, 1999, p169), the historian Helge Kragh summarises the situation thus:
His main point here corresponds with my impressions from what I’ve read in the technical literature of the period. By about 1930, at the latest, it became superfluous for those writing on quantum mechanics to have to defend or justify their assumption of it. In writing a paper on the subject you could just cite the specific papers relating to the narrow quantum mechanical topic you wanted to address and then get on with it. Familiarity with - and acceptance of - the broad ideas of QM was something you could expect of your readers.
This isn’t quite to suggest that Planck’s maxim about dissenter’s dying off, rather than being convinced, doesn’t apply. The period 1925-30 does see a generational split along attitudes towards the new mechanics. There is an old guard, educated in earlier times, who never publish using the new methods and language. But, for the most part, their reservations aren’t expressed as active opposition. The typical complaint from them is that it’s all too mathematical and complicated. That attitude not only explains why they don’t contribute to its development, it also explains why their stance becomes rather passive. Since, by their own admission, they couldn’t really engage with the details, they couldn’t really argue with those who did understand the newfangled stuff. However, most of them did recognise that this was, perforce, the way the subject was going and so it’s that generation who hire and promote the quantum mechanical young guns in the years immediately after 1925. Thus they were hardly speaking out against the new stuff, they weren’t debating it and they weren’t even much standing in its way.
Einstein is one of the very, very few exceptions - as he himself admitted at the time.
In summary, the people who were active in research in 1925 and their immediate pupils were all nearly instant converts to QM. Voices opposing the new physics were few and far between. By any reasonable measure, quantum mechanics had become accepted as mainstream physics in even the 1930s.
The next question is why this was the case? I’m not going to hazard a full answer, but there are several obvious factors.
One is that Old Quantum Theory provided a long gestation period that prepared the way. Many of the new ideas had been partially developed or foreshadowed in that period. Personal and institutional relationships were already in place that would help spread the ideas.
Another is that the data explained by QM didn’t come along later. Most importantly, atomic spectroscopy had become enormously sophisticated by 1925 and Old Quantum Theory had elaborated a whole forest of ad hoc explanations for all this data. Virtually at a stroke, QM either explained this data or provided tools that allowed theorists to address issues within it, all from first principles. People realised this very, very quickly and were both duly impressed and eager to contribute to applying the new tools to these problems.
No, they won’t. Arrayed against intellectual inertia in these matters is a delight scientists can take in being surprised, whether by an unanticipated experimental result, an unexpected insight or whatever.
To personalise the point, while I’ve long argued that string theory is a legitimate and fruitful part of quantum field theory, have occasionally had reason to cite string theory papers in my own QFT papers and have good friends who’re string theorist, I’m a classic example of a physicist who doesn’t expect string theory ever to make significant contact with experimental realities. But my pleasure in being proved wrong about that would outweigh any narrow consolation I’d get from being right.