Although he agreed it worked perfectly, Einstein was never happy with quantum theory because it denied a reality of things when they were not being observed.
[I can't accept quantum mechanics because] "I like to think the moon is there even if I am not looking at it."
Albert Einstein
"[T]he atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts."
Werner Heisenberg
But Heisenberg went on to insist that these philosophical issues raised by quantum mechanics applied to the big as well as the small.
Werner Heisenberg
"Anyone not shocked by quantum mechanics has not yet understood it."
Niels Bohr
"Observations not only disturb what is to be measured, they produce it."
Pascual Jordan
"When the province of physical theory was extended to encompass microscopic phenomena through the creation of quantum mechanics, the concept of consciousness came to the fore again. It was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness."
Eugene Wigner
"The doctrine that the world is made up of objects whose existence is independent of human consciousness turns out to be in conflict with quantum mechanics and with facts established by experiment."
Bernard d'Espagnat
"Nobody understands quantum mechanics."
Richard Feynman
"Is it not good to know what follows from what, even if it is not necessary FAPP? [FAPP is Bell's disparaging abbreviation of "for all practical purposes."] Suppose for example that quantum mechanics were found to resist precise formulation. Suppose that when formulation beyond FAPP is attempted, we find an unmovable finger obstinately pointing outside the subject, to the mind of the observer, to the Hindu scriptures, to God, or even only Gravitation? Would that not be very, very interesting?"
John Bell
"In the beginning there were only probabilities. The universe could only come into existence if someone observed it. It does not matter that the observers turned up several billion years later. The universe exists because we are aware of it."
Martin Rees