Maybe I'm old-fashioned, but I think that there is plenty of room for a small number of forums where the people who are very doubtful about the way t… - Graham Farmelo

‘The greatest living theoretical physicist’ – many commentators in the past few decades have described Steven Weinberg in such terms. When I rather cheekily asked him what he thought of that statement, he shot back: ‘It is quite ridiculous to rank scientists like that’, adding with a twinkle in his eye, ‘but it would be impolite to dispute the conclusion’. That reply was classic Weinberg: self-aware, intimidatingly direct but always ready to lighten the moment with humour.

... One of the most vocal skeptics is the Standard-Model pioneer Martin Veltman: 'String theory is mumbo jumbo. It has nothing to do with experiment.'...
But is clear from the comments that Dirac repeatedly made in his lectures on the way theoretical physics should be done that he would have disagreed with those criticisms: he would have counselled string theorists to let the theory's beauty lead them by the hand, not to worry about the lack of experimental support and not be deterred if a few observations appear to refute it. But he would have cautioned string theorists to be modest, to keep an open mind and never to assume that they are within sight of the end of fundamental physics. If past experience is anything to go by, another revolution will eventually follow.
Such was the advice of this extraordinarily unemotional man offered to his colleagues: be guided, above all, by your emotions.

'Einstein is completely cuckoo'. That is how the cocky young Robert Oppenheimer described the world's most famous scientist in early 1935, after visiting him in Princeton. ... Einstein had been trying for a decade to develop an ambitious new theory in ways that demonstrated, in the view of Oppenheimer and others, that the sage of Princeton had lost the plot. Einstein was virtually ignoring matter on the smallest scale, using quantum theory. He was seeking an ambitious new theory, not in response to puzzling experimental discoveries, but as an intellectual exercise—using only his imagination, underpinned by mathematics. Although this approach was unpopular among his peers, he was pioneering a method similar to what some of his most distinguished successors are now using successfully at the frontiers of research.