Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. — Stephen Hawking

“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” — Ernest Rutherford

“The ultimate goal of mathematics is to eliminate any need for intelligent thought.” — frequently but falsely attributed to Alfred N. Whitehead

“I do not like [quantum mechanics], and I am sorry I ever had anything to do with it.” — Erwin Schrödinger

“If we want to describe what happens in an atomic event, we have to realize that the word ‘happens’ can apply only to the observation, not to the state of affairs between two observations.” — Werner Heisenberg

“While I am describing to you how nature works, you won’t understand why nature works that way. But you see, nobody understands that.” — Richard Feynman

“The paradox is only a conflict between reality and your feeling what reality ought to be.” — Richard Feynman

7 Classical mechanics in a nutshell

“The most exciting phrase to hear in science, the one that heralds the most discoveries, is not ‘Eureka!’ (I found it!) but ‘That’s funny…'” — Isaac Asimov

“The appearance of a definite position x during an observation … is then regarded as a creation existing outside the laws of nature.” — Wolfgang Pauli

“Of all the theories proposed in the 20th century, the silliest is quantum theory… The only thing quantum theory has going for it is that it is unquestionably correct.” — Michio Kaku

“Whoever endows the wave function with more meaning than is needed for computing observable phenomena is responsible for the consequences.” — Nico van Kampen

13 Whence those “rules of the game”?

“Quantum mechanics makes absolutely no sense.” — Roger Penrose

“Electrons seem to have modes of being, or modes of moving, available to them which are quite unlike what we know how to think about.” — David Z. Albert

15 Why special relativity? (I)

16 Why special relativity? (II)

“The essence of science: ask an impertinent question, and you are on the way to a pertinent answer.” — Jacob Bronowski

17 The principle of least action

18 From quantum to classical (effects)

19 From quantum to classical (causes)

“Matter is not made of matter.” — Hans-Peter Dűrr

20 Spin, Zeno, and the stability of matter

“Einstein said that if quantum mechanics is right, then the world is crazy. Well, Einstein was right. The world is crazy.” — Daniel Greenberger

21 Entanglement, Bell revisited

“Unperformed experiments have no results.” — Asher Peres

“Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.” — Bertrand Russell

“I think science has enjoyed an extraordinary success because it has such a limited and narrow realm in which to focus its efforts. Namely, the physical universe.” — Ken Jenkins