The Fabric of the Cosmos: Space, Time, and the Texture of Reality

by Brian Greene

the real question is whether all your ponderings and analyses will convince you that life

is worth living. That’s what it all comes down to. Everything else is detail.

Surely, reality is what we think it is; reality is revealed to us by our experiences.

By deepening our understanding of the true nature of physical reality, we profoundly reconfigure our sense of ourselves and our experience of the universe.

you know the positions

and velocities of all objects at a particular moment, Newton’s equations, together with their Maxwellian updating, can tell you their positions and velocities at any other moment, past or future.

the combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

the maximum speed through space is reached when all light-speed motion through time is fully diverted into light-speed motion through space—one

Time stops when traveling at the speed of light through space. A watch worn by a particle of light would not tick at all.

the size of a wave at a given point in space is proportional to the probability that the electron is located at that point in space. Places where the probability wave is large are locations where the electron is most likely to be found. Places where the probability wave is small are locations where the electron is unlikely to be found. And places where the probability wave is zero are locations where the electron will not be found.

the electron simply does not have a definite position before the measurement is taken.

Quantum theory thereby sets up its own duality: you can determine with precision certain physical features of the microscopic realm, but in so doing you eliminate the possibility of precisely determining certain other, complementary features.

You can therefore know precisely where the electron is, but you cannot also know precisely how fast, at that moment, it was moving. Conversely, you can measure precisely how fast an electron is moving, but in so doing you will contaminate your ability to determine with precision its position.

the uncertainty principle is completely general: it applies to everything.

A particle, according to quantum theory, cannot have a definite position and a definite velocity; a particle cannot have a definite spin (clockwise or counterclockwise) about more than one axis; a particle cannot simultaneously have definite attributes for things that lie on opposite sides of the uncertainty divide.

Particle properties, in this majority view, come into being when measurements force them to—an

When they are not being observed or interacting with the environment, particle properties have a nebulous, fuzzy existence characterized solely by a probability that one or another potentiality might be realized.

entropy is a measure of the amount of disorder in a physical system.

The tendency of physical systems to evolve toward states of higher entropy is known as the second law of thermodynamics.

the forward-in-time arrow points in the direction of increasing entropy.

Just as there are no signposts in the deep darkness of empty space that declare this direction up and that direction down, there is nothing in the laws of classical physics that says this direction is time future and that direction is time past.

not only is there an overwhelming probability that the entropy of a physical system will be higher in what we call the future, but there is the same overwhelming probability that it was higher in what we call the past.

Whereas classical physics describes the present as having a unique past, the probability waves of quantum mechanics enlarge the arena of history: in Feynman’s formulation, the observed present represents an amalgam—a particular kind of average— of all possible pasts compatible with what we now see.

observations reflect averages of all possible histories.

Quantum mechanics is starkly efficient: it explains what you see but prevents you from seeing the explanation.