Einstein's Fridge: How the Difference Between Hot and Cold Explains the Universe

by Paul Sen

It led to his now-famous discovery that “Hawking radiation” leaks out of all black holes. But how did Hawking realize that although nothing—not even light—can escape the event horizon, a black hole can radiate heat in defiance of this principle? The answer lies in Hawking’s having decided to investigate the event horizon of a black hole from the perspective of quantum theory.

Hawking had a hunch, in part inspired by his conversations in Moscow, that if he investigated the empty space at and near the black hole’s event horizon from the quantum perspective, something interesting would turn up. To follow Hawking’s exact reasoning is tricky. For a rough intuition for what he did, we must consider one of the most outlandish consequences of the famous “uncertainty principle” of quantum physics—something known as “vacuum energy.” As the name implies, far from being inert, the vacuum is seething with activity. At any instant, bursts of energy appear from nowhere by borrowing equivalent bursts of energy from a tiny instant in the future. Mostly, we are unaware of these fluctuations because the positive burst of energy that appears at one instant is cancelled out by the negative burst that immediately follows it. Negative energy is a strange concept, but it does exist! These energy bursts can appear in many forms. They can appear as particles, such as electrons and positrons, and as photons of electromagnetic energy.

In the decades since Hawking’s and Bekenstein’s work, a consensus has formed that the surface area of a black hole’s event horizon is its entropy.

Using Bekenstein’s and Hawking’s formulas, physicists can say how much space a single digit from the binary number describing the gas’s entropy takes up on the event horizon’s surface. It’s a small area—around 4 × 10-66 square centimeters. This means you can think of the surface of a black hole’s event horizon as covered in tiny tiles, each carrying one “bit” of the information describing the entropy of everything that’s fallen inside.

Similarly, to observers outside a black hole, objects don’t appear to fall into it. Rather, they are smeared in a thin layer onto the event horizon. This has led physicists to compare the event horizon of a black hole to a hologram. These are two-dimensional surfaces that contain all the information needed to generate a complete three-dimensional image. These aren’t like the 3D images one sees in the cinema, which create an illusion of three dimensions. One can walk in a circle around a hologram of an object and it will appear as if one is walking in a circle around the actual three-dimensional object. Yet all the information needed to create the image is stored on a flat piece of film.

the two-dimensional information on the black hole’s event horizon is in a sense more “real” than the three-dimensional stuff that has fallen into it because the event horizon is still accessible to our part of the universe while whatever has fallen in is lost forever.

And the conclusion this brings us to is even more extraordinary: it could be that a two-dimensional surface shell surrounding our universe, in some sense, s...

The speed at which distant galaxies recede from us is growing, becoming ever faster. Physicists do not yet know what is behind this expansion—a mysterious “dark energy” has been suggested—but the effect is that space is flowing away from us, outward in all directions, at greater and greater speeds, the farther away we look. At a distance of around 16 billion light-years from earth, space is flowing away from us at a speed faster than that of light.

Assuming that the universe’s acceleration continues indefinitely, more and more galaxies will be swept away from us by space flowing faster than the speed of light, and they will disappear from view. Eventually, our own galaxy and a few of its closest neighbors will be all that’s visible.

A mysterious energy is making the balloon expand at a faster and faster rate, so all the dots are moving farther and farther apart. Think of one dot as our Milky Way, and imagine that a circular boundary is drawn around it and its nearby galaxies. Inside this boundary, the balloon is expanding slower than the speed of light, whereas outside it is expanding faster. Only objects inside this boundary are visible to us; everything outside the boundary is invisible.

This process should sound familiar. It’s as if our universe is “an inside-out black hole.” Instead of stuff flowing inward across a one-way boundary never to be seen again, stuff is flowing outward across a one-way boundary never to be seen again. That means that, like the event horizon surrounding a black hole, there is an event horizon around our universe.

Mathematical analysis suggests that all the physics on the outer shell of this universe can be completely described by quantum theory while in its interior, a gravitational force exists. But remember, the information on the outer shell fully describes everything inside of it. This suggests gravity is an illusion or an artifact, a way of interpreting data encoded on the boundary of the universe. And if gravity is not “real,” it could mean that a form of quantum theory fully describes all physics, with a gravity as a force that emerges from it.

As Stephen Hawking wrote, “We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.”

Tyndall had discovered what we now call the greenhouse effect. Water vapor and carbon dioxide in our atmosphere trap some of the sun’s energy. Effectively, they provide a warming blanket around our planet. Without these gases in our atmosphere, the earth’s temperature would plummet and would be well below zero even at the equator.

Should we commit to nuclear energy? Should we drive electric cars? How much tax should we pay on petrol, and how much should we subsidize wind farms? We will be in no position to answer these vitally important questions unless we have a basic understanding of the laws of thermodynamics. Informed debate, I am convinced, will provide these answers. The science of heat can and should improve the human condition without destroying the planet. It’s up to us.

This book focuses on the discovery and consequences of the first and second laws of thermodynamics. Over the twentieth century, scientists added two more laws for the sake of completeness. The first of these, now known as the zeroth law, was assumed to be true in the nineteenth century but didn’t then have the status of a law. The second of the additional laws, now known as the third law, pertains to materials at ultracold temperatures very near to absolute zero.

The Zeroth Law If two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other.

The First Law The energy of the universe is constant. The Second Law The entropy of the universe tends to increase. The Third Law The entropy of a system approaches a constant value as its temperature approaches absolute zero. (This law allows entropy to be expressed on an absolute scale, rather than only as a change in its value.)