A Briefer History of Time

 

A Briefer History of Time

By Stephen Hawking and Leonard Mlodinow

Preview

A briefer history of time is a short history of physics. It was written by world renowned scientist Stephen Hawking and popular science writer Leonard Mlodinow. This book documents in a few words, the long history of time and the physics which evolved with it. According to the authors, Science today has become vast and complex. This makes it impossible for an individual to specialize beyond one area at best. Thus, books like these are necessary to carry broad features of scientific discipline to the masses.

 

About the Authors

Stephen Hawking is a world renowned theoretical physicist and bestselling author, well known for his work in Cosmology. His pathbreaking scientific work includes the hypothesis known by the name of "Hawking Radiation". Leonard Mlodinow is a theoretical physicist turned popular science writer. He has written many books which include "The Grand design", Feynman's rainbow, and the Drunkyard's walk.

 

 

What to expect? Explore the fascinating world of Cosmology

The roots of astronomy are found as early as 340 B.C., when Aristotle wrote his book: "On the Heavens." He argued that the Earth was a sphere, based on the shape of lunar eclipses. However, he was mistaken about the structure of the Solar System. He assumed that the Earth lay at the center, while the Sun and other planets revolved around it.

 

Ptolemy extended this idea into a complete model where eight rotating spheres surrounded the Earth, and every successive sphere was larger than before. The stars occupied fixed position on the last rotating sphere. This last sphere was the final boundary of the observable universe. What lay beyond, were perhaps heaven and hell! Nonetheless, He too was wrong!

 

Years later modern astronomers realized the current model of the Solar System. Copernicus proposed that the Sun is fixed, and that everything else in the solar system, including the Earth, rotates around it in a circle. Later, Kepler arrived at the corrected trajectory of the motion of planets. He proposed that the orbital paths of the planets were elliptical, and this matched the observational data.   Finally, in what was to become the dawn of modern astronomy, a complete picture of the Solar System was given by Sir Isaac Newton, who, using his theory of gravitation, explained the motion of planets in one sweep, and that too with mathematical precision.

 

As it turned out, even the Newtonian picture was incomplete, as shown by Einstein's General Theory of Relativity, and later on, even more so, with the emergence of Quantum Mechanics. While we have made much progress in Science, the truth still remains out of bounds!

In this book, you will learn:

1)      What is a theory, and what are its traits?

2)      Why is the Universe expanding and what does it mean?

3)      What is an Event Horizon of a black hole?

4)      What would the unification of the theories of Physics imply?

 

 

In theory, theory and practice are the same. In practice, they are not!

 

In simplest words, "a theory is a model or a part of it, with a set of rules that relate the quantities in the model to the observations that we make." A good theory fulfills two requirements: -

(1) The theory should contain minimum arbitrary elements and explain a large number of observations.

(2) It must be able to make predictions which can in principle be falsified or disproved.

 

Thus, every theory is temporary. There is no certainty that any future observation will not prove it wrong. For example, if one has the theory that all swans are white, a simple observation of a black swan will falsify the theory. Hence the theory is falsifiable, meaning that it can be disproved by observations which are contrary to what the theory predicts.

 

In practice, most modern theories are an extension of previous 'correct' theories. They extend the previous models and explain observations not understood earlier. Einstein refined Newton's model of gravitation; he did not 'disprove' it. The reason we accept them both is because they meet our rules of a good theory. Today many partial collections of theories exist, but the goal of the scientists is to give a single unified theory which can explain everything.

 

Newton versus Einstein. Standing on the shoulder of Giants.

According to Newton, anything which has mass has an attractive force called gravity. The force of Gravity between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This means that if the mass of each body is doubled, the gravitational pull will increase four times, and if the distance between the two is doubled, the pull would reduce by three fourths. In his time, Newton faced a challenge of mathematically formalizing his theory, because the extant Mathematics was just not capable of describing such a model. Therefore, he devised 'new mathematics' in the form of calculus to accurately describe his model.

 

 

 

The second breakthrough was achieved when Einstein published his "General Theory of Relativity." In this theory Einstein delved deeper in to the nature of Gravity itself. He explained that Gravity is a curvature of space itself caused by the presence of massive objects like Stars and Planets. The higher the mass, the greater is the distortion and thus, higher is the curvature. This curvature is responsible for gravity being an attractive force. Using this Einstein was able to explain the phenomenon of bending of light when passing close to a heavy object, like a star. The Theory of relativity may have been unintuitive, but it has managed to pass through every experimental scrutiny it has been put to. Therefore, as a scientific theory, it still stands tall.

 

The universe has been getting bigger and bigger

In 1924, astronomer Edwin Hubble demonstrated that the Milkyway was one of the millions of Galaxies which exist in the universe. He used measurements of its brightness to determine the Earth's distance from the other galaxies indirectly.  Hubble also noticed that all the light from these galaxies were red-shifted, which meant that they were moving away from our galaxy! This was a simple extrapolation of the Doppler effect, in which the wavelength of light emitted from a source moving away, increases.

 

The redshift corresponded to the increase in wavelength since Red and Blue occupied opposite ends of the spectrum of light, with Red being the longest wavelength and Blue the shortest. This implied that the universe itself was expanding. The model developed from this observation is now called "The Big Bang Theory." It states that in the beginning there was a point size condensed universe. Then there was a cosmic explosion which threw out 'matter' with great speed and energy. This was the beginning of the universe, and of time itself.

 

we do not know what preceded this explosion, and what caused it, but it has determined how our universe looks today.

 

Blackholes are the giant sinkholes of the universe.

The Big bang theory also paved the way for the prediction of the existence of black holes, which are dead stars. 'Death' of a star essentially means their running out of nuclear fuel. For extremely massive stars, that is stars with masses thousands of magnitudes larger than our own Sun, Gravity becomes a killer! These stars may collapse under the force of their own gravity and get condensed to an extremely small size.

 

These cosmological entities have extremely high gravitational pull, and anything which crosses their boundary, which is also called the 'Event Horizon,' is pulled down into them for eternity. Nothing, including light, can escape from these 'Black Holes.'

 

At the center of the blackhole is a point with 'infinite' density. It is here that the equations of Physics break down, and thus, space and time as we know them, cease to exist. The strong gravitational pull of black holes is considered to be one of the forces holding galaxies together. It is believed that in the center of the milky way there is a super massive black hole, around which everything in the milky way, revolves. Humanity, then, is a very tiny piece of the grand puzzle of the cosmos.

 

 

Quantum mechanics and the General Theory of relativity are not a match made in heaven!

 

While the General Theory of Relativity explains the universe at a large scale, Quantum Mechanics, or QM, arose to explain the properties of sub-atomic particles. QM relies on the uncertainty principle. This principle states that all the physical quantities cannot be measured with exact precision. The precision of one measurement like momentum, or velocity, comes at the uncertainty of the measurement of position. This means that accurate predictions of the position, as well as the momentum of a particle, are impossible! This throws a spanner in the works as far as deterministic models go.

 

Combining relativity with quantum mechanics often ends up yielding results with infinite or indeterminate values which cannot be true in reality. For a long time, scientists used these two theories to explain many observations successfully, but to explain the properties of the universe and its constituents completely, it is important to combine the two theories.

 

While QM does a great job of explaining the nature of the subatomic world, its lack of applicability at a cosmological scale and incompatibility with Relativity, led to the search for a "Quantum model of Gravity". Eventually, a model called as the 'String Theory' came into the picture, and many physicists the world over, claim it to be the only worthy candidate for a "Theory of everything."

 

Interstellar, time travel, and wormholes!

 

Intergalactic space travel and Time travel has been an intriguing concept and inspiration to many works of science fiction like Star Trek, and Star Wars. However, it is unlikely that such travel would be possible in the near future. It may even never be possible! Even if time travel is possible, it can happen only in one direction, that is, into the future and never into the past.

 

Traveling back in time may imply the nonexistence of free will. The logic goes like this, we know history, and it is fixed. However, the future is unknown and will be determined by our actions, acted upon by our own will. What if someone travels back in time and then changes it in such a way that the present, or the 'future of the past,' change completely? For example, if the time traveler kills his grandparents in the past, he cannot exist!  A solution to this puzzle is that the time traveler from the future may have a limitation on how he or she interacts with the elements of the past so that the past and the future remain consistent. This implies that both past and future may well be preordained, and thus, free will does not exist!

 

Space travel is not impossible, man has been on the moon after all, but forging deeper seems rather improbable, because of the huge distances involved. While even exploring the Solar System remains a distant dream, traveling to another Star is close to impossible with current technology. The nearest star to the sun is four light years away, and even a one-way journey with existing resources is impossible. The obvious solution is to surpass the speed of light and make it within reach of a human's lifetime. However, the theory of relativity puts the upper limit on the maximum attainable speed at the speed of light.

 

What if you could traverse from point A to point B by placing them closer together? This is the theoretical concept of a "wormhole."  Imagine a flat 2-dimensional space-time much like a sheet of paper. The distance between two points A, and B, on this sheet, can be reduced to almost zero by curving, or folding the piece of paper. The "hyperspace" within this curvature which connects the two points on the sheet is what is called a wormhole. It is like going through a mountain in a tunnel, instead of hiking up and down to cross it. However, according to relativistic equations, the worm holes do not last long enough for them to serve any practical purpose, and nobody knows how to prolong its collapse.

 

 

The arrow of time moves "forward" towards a higher state of disorder

Philosophers have debated the nature of time, with many stating that it is an illusion. What it really is, is a matter of debate, but there is no mistaking the 'perception' of the direction of its flow. Time seems to flow only forwards.

 

No one has ever seen a broken cup on the floor, assembling together on its own, and flying up on to the table from where it fell from! No, this sequence only happens in the reverse. This is the thermodynamic arrow of time, which flows in the direction of increased disorder, or entropy. Quite simply, the arrow of time moves towards higher entropy.

 

The Universe itself is moving towards higher disorder as it expands. If there is a theoretical limit to the achievable entropy, then at some point the Universe would achieve it. and then either remain stagnant, or reverse its direction towards orderliness! Reversal towards lower entropy may imply the reversal in the flow of time!

 

 

The pursuit of truth has led us to String Theory, which may just unify all of Physics!

A theory which can combine all four fundamental forces: electromagnetic, gravitational, strong, and weak force into one model is the greatest dream and challenge of physicists. This theory should reconcile the differences between quantum mechanics and theory of relativity.

 

Around 1984, a theory called string theory became famous for explaining many observable properties of the Universe. String theory assumes that all point- like fundamental particles are not actually zero dimensional but are vibrating strings.

 

The dimension of these strings and their vibrations are so small that they appear point like for all practical purposes. Each fundamental particle is a result of differences in the frequency of vibrations of these strings. The theory by way of its mathematical formulation necessitates the presence of extra spatial dimensions. While the Theory is still under development with various versions doing the rounds, many scientists already believe a version of String theory to be the much-touted Theory of Everything.

 

Final summary

The history of our understanding of the universe and laws of physics is quite rich itself. Starting from the 16th century, thinkers like Copernicus, Kepler, and Galileo, introduced heliocentric models and made other revelations about the Universe. After that, in the 17th century, Sir Isaac Newton with his theory of gravitation, blazed a trail never before taken. He wrapped up all doubts with his accurate predictions based on formal mathematics. 

For the next two centuries, there were many advances in Science, and in the early 1900s, Albert Einstein with his Special and the General Theory of Relativity brought another paradigm shift in our understanding of the world. This was followed by the emergence of Quantum Mechanics, which revealed the probabilistic nature of the world. The incompatibility of Relativity and QM, necessitated the research for a theory of everything, which aims to explain all the observed properties of the Universe at macroscopic as well as microscopic scales.

 

 

Stand-out sections

The best sections of the book are:

 

Chapter 3: The Nature of scientific theory

 

Chapter 5 & 6: Relativity & Curved space

 

Chapter 8: The Big Bang and the evolution of the universe

 

Chapter 10: Wormholes and the time travel

 

The common underlying theme in all these chapters which makes them outstanding is that they all discuss unintuitive and complex scientific concepts and explain them in very simple words. Sir Arthur Eddington once said there are only two people who understood the theory of relativity. He would have been baffled with this concise, simple, and yet an accurate description of relativity in motion, and its implication of gravity being the result of a curvature of space-time.