Book Summary: A Brief History of Time (Part 2)

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When a star with a very high mass dies, it collapses into a singularity called a black hole.

During their lifetimes, stars need enormous amounts of energy to produce heat and light. Yet, this energy doesn’t last forever; eventually it runs out, leaving the star to die. What happens to a star when it dies depends on its size. When a very large star runs out of energy, something spectacular is created: a black hole.

A black hole occurs because the gravitational field of most massive stars is so strong. While the star is alive, it is able to use its energy to keep itself from collapsing. But when the star runs out of energy, it can no longer overcome the gravity and its decaying body collapses in on itself. Everything is pulled inwards toward an infinitely dense, spherical point called a singularity. This singularity is the black hole.

When a black hole forms, space-time is curved so steeply by its gravity that even light bends along it. Not only does a black hole pull in everything nearby, it also prevents anything that crosses a certain boundary around it from escaping again: this point of no return is called the event horizon, and not even light, which travels faster than anything else in the universe, can escape back over it.

This raises a question: if a black hole absorbs light and anything else that crosses its event horizon, how can we know they are there? Scientists search for black holes by looking for their gravitational effect on the universe and for the X-rays produced by their interaction with orbiting stars. For example, scientists look for stars orbiting dark and massive objects that could be black holes.

They also look for the X-rays and other waves that are commonly produced by matter when it is being sucked in and torn up by a black hole. There is even a source of radio and infrared waves at the center of our galaxy that could be a supermassive black hole.

 

Black holes emit radiation, which can lead to their demise through evaporation.

If the gravitational pull of a black hole is so strong that not even light can escape it, then you’d think nothing else could escape either. But you’d be wrong. In fact, black holes must release something; otherwise they’d break the second law of thermodynamics.

The universal second law of thermodynamics states that entropy, the tendency toward greater disorder, always increases. And as entropy increases, so must temperature. An example of this is the way a fire-poker, after being in a fire, glows red-hot and releases radiation as heat.

According to the second law, since black holes suck in disordered energy from the universe, the entropy of the black hole should also increase. And with this increase in entropy, black holes should have to let heat escape.

The escape of heat is possible because, although nothing that has passed a black hole’s event horizon can escape, virtual pairs of particles and antiparticles near the event horizon conserve the second law of thermodynamics. Virtual particles are particles that cannot be detected but whose effects can be measured. One of the partners in the pair has positive energy and the other has negative energy.

In a black hole, gravitation is so strong it can suck the negative particle into the black hole and in doing so give its particle partner enough energy to possibly escape into the universe and be emitted as heat. This allows the black hole to emit radiation, and thus follow the second law of thermodynamics.

The amount of positive radiation emitted is balanced by the negative particles being sucked into the black hole. This inward flow of negative particles can reduce the black hole’s mass until eventually it evaporates and dies. And if its mass becomes small enough, the black hole will most likely end in a massive final explosion, as large as millions of H-bombs.

 

Although we can’t be sure, there are strong indicators that suggest that time can only move forwards.

Imagine a scenario where the universe began to contract and time started running backward. What would that be like? Perhaps clocks would run backward and the course of history would reverse. Scientists haven’t completely ruled it out, but there are three strong indicators that suggest time only moves forward.

The first indicator showing that the passage of time goes from past to future is the thermodynamic arrow of time. According to the second law of thermodynamics, entropy – the disorder of a closed system – tends to increase with time. This means that time can be measured by the tendency of disorder to increase.

For example, if a cup rolls off a table and breaks, it has become less ordered, and its entropy has increased. Since a broken cup would never spontaneously reassemble and increase its order, we see that time is only going forward.

The broken cup and the thermodynamic arrow of time are also aspects of the second indicator of forward time: the psychological arrow of time, which is dictated by memory. After that cup has broken, you can remember it being on the table; but before this, when it was still on the table, you can’t “recall” it’s future position on the floor.

The third indicator, the cosmological arrow of time, refers to the expansion of the universe, and this also follows along our perception of the thermodynamic arrow of time. This is because as the universe expands, entropy increases.

If the disorder in the universe were to reach its maximum point then the universe could start contracting, reversing the cosmological arrow of time. However, we wouldn’t know about it because intelligent beings can only exist as disorder increases. This is because we rely on the process of entropy to break down our food into energy.

Therefore, as long as we’re around, we will observe the cosmological arrow of time as going forward.

 

In addition to gravity, there are three fundamental forces in the universe.

What kinds of forces are at work in the universe?

Most people will have heard about only one: gravity, the force that attracts objects to one another and which is experienced in the way that Earth’s gravity pulls us to its surface. However, most people are unaware that there are actually three additional forces that act on the smallest particles.

The first is electromagnetic force, which can be observed in everyday life when a magnet sticks to a refrigerator or when you recharge your cell phone. It acts on all particles with electric charges, such as electrons and quarks.

Electromagnetic force, like the north and south poles on a magnet, can be attractive or repulsive: positively charged particles attract negative particles and push away other positive particles, and vice versa. This force is much stronger than gravity and dominates at the small level of the atom. For example, electromagnetic force causes an electron to orbit around the atom’s nucleus.

The second is weak nuclear force, which acts on all the particles that make up matter and which causes radioactivity. This force is called “weak” because the particles that carry it can only exert force at short distances.

At higher energies, the strength of weak nuclear force increases until it matches that of electromagnetic force. The third is strong nuclear force, which binds protons and neutrons in the nucleus of an atom, and binds the smaller quarks within protons and neutrons. In contrast to electromagnetic force and weak nuclear force, strong nuclear force gets weaker at higher energies.

At a very high energy called grand unification energy, electromagnetic force and weak nuclear force get stronger and strong nuclear force gets weaker. At that point, all three forces reach equal strength and become different aspects of a single force: a force that might have played a role in the creation the universe.

 

Although scientists believe that the universe started with the big bang, they are unsure of exactly how this happened. 

Most scientists believe that time began with the big bang – the moment when the universe went from an infinitely dense state to a rapidly expanding entity which is still growing today. Scientists, however, don’t exactly know how this big bang occurred, although a number of theories have been proposed to explain how this huge expansion might have happened. The most widely accepted theory of the universe’s beginning is the hot big bang model.

In this model, the universe started with zero size and was infinitely hot and dense. During the big bang, it expanded, and as it grew its temperature cooled as its heat was spread. In the first few hours of this expansion, most of the elements in the universe today were created.

As the universe continued to expand, gravity caused denser regions of the expanding matter to start rotating, creating galaxies. Within these newly forming galaxies, clouds of hydrogen and helium gases collapsed. Their colliding atoms caused nuclear fusion reactions, which created stars.

When these stars later died and collapsed, they created huge stellar explosions that ejected more elements into the universe. This provided the material for the birth of new stars and planets. Although this is the generally accepted version of the big bang and the birth of time, its not the only model.

Another model is the inflationary model. This model proposes that the energy of the early universe was so enormously high that the strengths of the strong nuclear force, weak nuclear force and electromagnetic force were equal.

As the universe expanded, however, the three forces took on different strengths very quickly. As the forces split, an enormous amount of energy was released. This would have had an anti-gravitational effect, causing the universe to expand rapidly, and at an increasing rate.

 

Physicists haven’t been able to unify general relativity and quantum physics.

In their desire to understand and describe the universe, scientists have developed two major theories. The first is general relativity, which concentrates on a very large phenomenon in the universe: gravity. The second is quantum physics, which describes some of the smallest known objects in the universe: particles smaller than atoms.

While both theories provide great insights, there are big differences in what is predicted with the equations of quantum physics, and what is predicted and observed with general relativity. This means that currently there is no way of combining them together to make one complete unified theory of everything.

One issue that prevents the two theories being brought together is that many of the equations scientists use in quantum physics result in seemingly impossible infinite values. For example, according to the equations, the curve of space-time would be infinite, something observations have shown to be false.

To cancel out these infinities, scientists try to introduce other infinities into the equation. Unfortunately, this keeps scientists from being able to predict accurately. As a result, instead of using the equations from quantum physics to predict events, the events themselves have to be added and the equations tweaked to make them fit!

A second, similar problem is that quantum theory suggests that all the empty space in the universe is made up of virtual pairs of particles and antiparticles. However, the existence of these virtual pairs causes difficulties for general relativity.

Since there is an infinite amount of empty space in the universe, the energy of these pairs would have to have infinite energy. This is problematic because Einstein’s famous equation E=mc2 suggests that the mass of an object is equal to its energy. So the infinite energy of these virtual particles would mean that they would also have infinite mass. And if there were infinite mass, then the whole universe would collapse under the intense gravitational pull and become a single black hole.

 

Final Summary

Many people are put off physics because they see it as an impenetrable  world of lengthy equations and complex theories. And, to a certain extent, this is true. But the complexity of physics shouldn’t stop us non-experts from learning how and why the universe works.

There are a number of rules and laws that help us understand the mysteries of the universe around us. Rules and laws that most of us can comprehend. And once we understand them, we can begin to see the universe in a new light.

 

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