A Brief History of Time

A Brief History of Time


Stephen Hawking

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A Brief History of Time Summary

Where did the universe and everything in it come from, and where is it all headed? New technology has allowed modern science to offer answers to such questions. First, Stephen Hawking details some of the major scientific breakthroughs throughout history that have brought our understanding to this point.

Though in Ancient Greece people worked out that the earth is round, thinkers such as Aristotle still thought the earth was at the center of everything else. This theory wasn’t really challenged until 1514, when Nicholas Copernicus showed that the planets, including the earth, orbit the sun. In 1687, Sir Isaac Newton devised his own laws related to gravity, theorized that all stars should exert this force on one another, and wondered whether the universe was infinite. In 1929, Edwin Hubble discovered that galaxies everywhere are rapidly moving away from each other. The idea that the universe is expanding, in turn, suggested there was a time when everything was in a single, tiny, infinitely dense place. God, Hawking notes, could fit into this theory.

Science’s ultimate goal is to find one theory that ties all the others together. For now, the two main partial theories scientists have include the general theory of relativity and quantum mechanics. The former focuses on gravity and massive celestial bodies, while the latter deals with the tiniest types of matter known to humanity. These theories are inconsistent with each other, however. Scientists are still search for a unifying theory to fulfill to the deepest longing of humankind: to understand where we come from.

Newton put forth the idea that objects are naturally in motion, and that forces cause them to speed up or slow down. This challenges the idea of absolute space; think of a ping pong ball bouncing on a train going 40 miles an hour. The distance the ball has moved according to someone on the moving train will be very different when compared to the observation of someone outside the train.

The idea of absolute time, however, took longer to overcome. In 1865, James Clerk Maxwell discovered that light has different wavelengths, such as radio waves or microwaves. Albert Einstein later pointed out that light always moves at the same speed and is faster than anything else in his theory of relativity. The general theory of relativity also put forth that gravity warps—bends and curves—space-time. For example, time moves more slowly near objects with larger masses. Thus, though space and time affect objects’ movements, objects’ movements also affect space and time; neither is absolute. Einstein also proposed a sort of antigravity force, or cosmological constant, that would repel objects and explain the (incorrectly) assumed static nature of the universe.

In the mid 18th century, astronomers identified the Milky Way and described it as a spiral galaxy. In the 20th century, Hubble showed that there are galaxies other than our own—and from the red shift seen in these far-off stars, it is clear that these galaxies are moving away from us. This explains why the universe is not collapsing in on itself, without the need for Einstein’s cosmological constant—which Einstein deemed the worst mistake of his career.

Russian physicist Alexander Friedmann suggested that the universe looks roughly the same in all directions. This was later proven by measuring the universe’s uniform microwave radiation. Friedmann also offered various models of the universe, both finite and infinite, with most featuring a big bang at the beginning. Wider acceptance of the big bang theory came with Roger Penrose’s work on black holes, which occur when matter collapses in on itself until it takes up zero size and is infinitely dense. Hawking, then a doctoral student, saw the relevance of the reverse version of this for the big bang theory, and later released a paper to that effect with Penrose.

The Marquis de Laplace suggested in the early 1800s that because modern science seemed to be doing such a good job predicting things, human beings would be able to predict everything if only they knew the exact state of the universe at one point in time. But the end of that idea came when Werner Heisenberg tried to exactly measure the position and velocity of a particle. The more accurately Heisenberg wanted to measure this, the more the light he had to use—which, in turn, would affect the particle’s position or velocity. Thus, it was impossible to say exactly where particles were, something scientists now call the uncertainty principle. This lead to the creation of quantum mechanics.

Theories about the atom built up slowly, but by the early 1900s scientists had identified electrons, neutrons, and protons. Murray Gell-Mann won the Nobel Prize in 1969 for discovering that these, in turn, were made up of quarks, of which there are different varieties. Every kind of particle also has certain types of spin, which relates to how many times a particle must be “turned” 360 degrees to look and behave the “same.” Spin can be used to determine various forces. Matter particles also obey the Pauli exclusion principle, which states that particles cannot exist in the exact same place: they inevitably repulse each other. Further work discovered anti-particles (against which particles collide, resulting in both being annihilated), as well as force-carrying particles that are undetectable apart from their effects on other particles. Force-carrying particles are categorized as either gravitational, electromagnetic, weak nuclear, or strong nuclear forces.

The term black hole was coined in 1969 by John Wheeler, but the idea has been around for 200 years or so. John Michell said in 1783 that any star that was big and dense enough would have such strong gravity that even light could not escape it. While we might not be able to actually see these objects, we could measure the effect of their gravity on surrounding material. This would happen at the end of a star’s life, when its energy has been used up—meaning it can no longer fight against its own gravity and begins to collapse. Subrahmanyan Chandrasekhar measured how big a star would have to be to end up as a black hole, which in part earned him a Nobel Prize. Not all stars end up this way, however, especially if they are of a similar size to our own sun.

If black holes can stop light from escaping, nothing else can escape either. The boundary from which nothing can escape a black hole is called its event horizon. Hawking and Penrose showed that at the center of black holes are singularities, points of infinite density and gravity where the laws of physics cease to operate.

Hawking was the first to note that black holes can only grow bigger as more matter falls in. This is similar to the law of thermodynamics, which states that the entropy—essentially disorder or chaos—of an isolated system can only increase. Jacob Bekenstein suggested that the area of the event horizon is a measure of the black hole’s entropy. This meant that black holes must emit heat and particles, which has been proven to be true. An old, big black hole could therefore be harnessed to provide massive power output, but we do not have the technology to do so.

Hawking attended a conference at the Vatican in 1981, which reawakened his interest in the start and end of the universe. The Pope told the scientists present not to look at this aspect of science, as it was God’s work, but Hawking had in fact recently discussed the possibility there was no beginning to the universe at all, because the universe has no boundary. There are many different theories as to what the early universe looked like, however. In the hot big bang model, the early universe had infinite heat, meaning the particles were moving very, very fast. As the universe expanded, it cooled. Eventually, once the universe cooled off enough, matter clumped together; galaxies formed, then stars, then planets, and finally organisms.

Scientists need a quantum theory of gravity to really know what happened at the beginning, but this doesn’t exist yet. The anthropic principle, meanwhile, searches for an answer to the question of why the universe is compatible with the existence of intelligent life; the “weak” version of this principle essentially says that if the universe were not fit for intelligent life, intelligent life would not exist.

The second law of thermodynamics, entropy, states that things tend to get more disordered, so this is one arrow of time. The second arrow of time is psychological and refers roughly to the formation of memories. The third arrow refers to the cosmological arrow of time, in which the universe is expanding. The theory of the universe as having no boundaries and the weak anthropic principle make a case for why these three arrows of time all point forward, and why this is the only situation in which intelligent beings could exist. Even making memories increases disorder, as it takes energy to make a new memory, thus creating more disorder as that energy is emitted.

The question, then, is what happens when the universe eventually starts to contract? Time will not flow backwards, because disorder can still only increase—meaning the thermodynamic and psychological arrows of time would still point forward. The universe would only be suitable for life in the expanding phase, as all celestial bodies would have burned out before the collapsing phase begins.

In some models of the early universe, space-time might have been so disordered that time travel was possible, but observations of the uniform radiation across the universe suggest this was not the case. As time is not absolute, however, it would be possible for space travelers to return to earth in what seems like a short time to them, though thousands of years would have passed in earth’s perception of time. Traveling faster than light, however, would allow one to leave point A at the same time as an event and arrive at point B before that event had started. It might also be possible to warp space time to create a wormhole between two points. These “Einstein-Rosen bridges” could allow travel into the past if an advanced civilization could stabilize them.

The quest to unify physics—to bring together the general theory of relativity with quantum mechanics—is ongoing, as scientists only have partial theories so far. Any theory must include the uncertainty principle, and as such the first step must be to incorporate this into the theory of relativity. String theories offer possible answers. They visualize particles as waves on one-dimensional lines in two dimensions of space-time, instead of as dots. Particles are then visualized as waves passing down that “string.” The main problem with string theory is that it requires many more dimensions to work—either 10 or 26.

We can still be fairly certain that there is a unifying theory of physics, as the partial theories we have are getting closer and closer to explaining everything. But even if scientists were to find such a theory, they still could not predict everything exactly because of the uncertainty principle. The real aim, Hawking says, is to understand our own existence and, indeed, why anything exists. Once we know that, we will know the mind of God.