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

by Brian Greene

What is reality? We humans only have access to the internal experiences of perception and thought, so how can we be sure they truly reflect an external world?

physicists such as myself are acutely aware that the reality we observe—matter evolving on the stage of space and time—may have little to do with the reality, if any, that’s out there.

The overarching lesson that has emerged from scientific inquiry over the last century is that human experience is often a misleading guide to the true nature of reality.

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

The universe, according to quantum mechanics, is not etched into the present; the universe, according to quantum mechanics, participates in a game of chance.

Quantum mechanics challenges this view by revealing, at least in certain circumstances, a capacity to transcend space; long-range quantum connections can bypass spatial separation. Two objects can be far apart in space, but as far as quantum mechanics is concerned, it’s as if they’re a single entity.

Entanglement

Despite these many impressive insights, there remains one very basic feature of time—that it seems to have a direction pointing from past to future—for which neither relativity nor quantum mechanics has provided an explanation.

puzzle.Nothing in the equations of fundamental physics shows any sign of treating one direction in time differently from the other, and that is totally at odds with everything we experience.5

And since the stars provide a visual reference that allows you to distinguish spinning from not spinning, you would expect to be able to feel it, too.

What about a blind person?

“Make everything as simple as possible, but no simpler.”

Special relativity declares a similar law for all motion: the combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

While you can shield yourself from electromagnetic and nuclear forces, there is no way to shield yourself from gravity.

Since gravity and acceleration are equivalent, if you feel gravity’s influence, you must be accelerating. Einstein argued that only those observers who feel no force at all—including the force of gravity—are justified in declaring that they are not accelerating.

But as an important point of principle, the link Einstein found between gravity and acceleration means, once again, that we are justified only in considering stationary those observers who feel no forces whatsoever.

An accelerated observer carves spatial slices that are warped.

He found that warps and ripples—gravity, that is—do not travel from place to place instantaneously, as they do in Newtonian calculations of gravity. Instead, they travel at exactly the speed of light.

General relativity provides the choreography for an entwined cosmic dance of space, time, matter, and energy.

The weirdness of relativity arises because our personal experience of space and time differs from the experience of others. It is a weirdness born of comparison. We are forced to concede that our view of reality is but one among many—an infinite number, in fact—which all fit together within the seamless whole of spacetime. Quantum mechanics is different. Its weirdness is evident without comparison. It is harder to train your mind to have quantum mechanical intuition, because quantum mechanics shatters our own personal, individual conception of reality.

Quantum mechanics shows that the best we can ever do is predict the probability that an experiment will turn out this way or that.

Space, whatever it is fundamentally, provides the medium that separates and distinguishes one object from another.

According to quantum theory and the many experiments that bear out its predictions, the quantum connection between two particles can persist even if they are on opposite sides of the universe.

That is, there are points on the screen where it is very likely an electron will land, points where it is far less likely that it will land, and places where there is no chance at all that an electron will land.

Instead, according to Bohr and the Copenhagen interpretation of quantum mechanics he forcefully championed, before one measures the electron’s position there is no sense in even asking where it is.

when we measure the electron’s position we are not measuring an objective, preexisting feature of reality. Rather, the act of measurement is deeply enmeshed in creating the very reality it is measuring.

Nature has a built-in limit on the precision with which such complementary features can be determined. And although we are focusing on electrons, the uncertainty principle is completely general: it applies to everything.

Heisenberg’s principle does not just declare uncertainty, it also specifies—with complete certainty—the minimum amount of uncertainty in any situation.

Uncertainty is always present, but it becomes significant only on microscopic scales.

Uncertainty is built into the wave structure of quantum mechanics and exists whether or not we carry out some clumsy measurement.

Physics addresses only things we can measure. From the standpoint of physics, that is reality. Trying to use physics to analyze a “deeper” reality, one beyond what we can know through measurement, is like asking physics to analyze the sound of one hand clapping.

Physics in general, and quantum mechanics in particular, can deal only with the measurable properties of the universe. Anything else is simply not in the domain of physics.

quantum uncertainty applied to spin shows that just as you can’t simultaneously determine the position and the velocity of a particle, so also you can’t simultaneously determine the spin of a particle about more than one axis.

No.

Maybe?

Two observers in relative motion have nows—single moments in time, from each one’s perspective—that are different: their nows slice through spacetime at different angles. And different nows mean different now-lists. Observers moving relative to each other have different conceptions of what exists at a given moment, and hence they have different conceptions of reality.

even at low velocities relativistic effects can be greatly amplified when considered over large distances in space.

But, as Einstein once said, “For we convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”5 The only thing that’s real is the whole of spacetime.

By definition, moments don’t include the passing of time—at least, not the time we’re aware of— because moments just are, they are the raw material of time, they don’t change.

Human language is far better at capturing human experience than at expressing deep physical laws.

Entropy

The second law of thermodynamics seems to have given us an arrow of time, one that emerges when physical systems have a large number of constituents.

Statistical and probabilistic reasoning has given us the second law of thermodynamics. In turn, the second law has provided us with an intuitive distinction between what we call past and what we call future.

The second law actually says that if at any given moment of interest, a physical system happens not to possess the maximum possible entropy, it is extraordinarily likely that the physical system will subsequently have and previously had more entropy.

From any specified moment, the arrow of entropy increase points toward the future and toward the past. And that makes it decidedly awkward to propose entropy as the explanation of the one-way arrow of experiential time.

Physicists have come to realize that mathematics, when used with sufficient care, is a proven pathway to truth.

The main lesson of the second law of thermodynamics is that physical systems have an overwhelming tendency to be in high-entropy configurations because there are so many ways such states can be realized.

it is far more likely—breathtakingly more likely—that the whole universe we now see arose as a statistically rare fluctuation from a normal, unsurprising, high-entropy, completely disordered configuration.

So we can trace the low-entropy, nonanimal sources of energy to the sun.

Our most refined theories of the origin of the universe—our most refined cosmological theories—tell us that by the time the universe was a couple of minutes old, it was filled with a nearly uniform hot gas composed of roughly 75 percent hydrogen, 23 percent helium, and small amounts of deuterium and lithium.

for the initially diffuse gas cloud, you find that the entropy decrease through the formation of orderly clumps is more than compensated by the heat generated as the gas compresses, and, ultimately, by the enormous amount of heat and light released when nuclear processes begin to take place.

What the second law of thermodynamics entails is that in the formation of order there is generally a more-than-compensating generation of disorder. The entropy balance sheet is still in the black even though certain constituents have become more ordered.

The ultimate source of order, of low entropy, must be the big bang itself.