The Universe in a Nutshell:
By Stephen Hawking

Nuclear Binding Energy
Nuclei are made up of protons and neutrons held together by a strong force.  But the mass of the nucleus is
always less than the sum of the individual masses of the protons and neutrons that make it up.  The difference is
a measure of the nuclear binding energy that holds the nucleus together.  This binding energy can be calculated
from the Einstein relationship: nuclear binding energy = m*c², where m is the difference between the mass of
the nucleus and the sum of the individual masses.  It is the release of this potential energy that creates the
devastating explosive force of a nuclear device.


The Shape of Time

The Universe is like a casino.  When it is big, as it is today, there are a very large number of rolls of the dice, and
the results average out to something one can predict.  That is why classical laws work for large systems.  But
when the universe is very small, as it was near the big bang, there are only a small number of roles of the dice,
and the uncertainty principle is very important.

What we think of as gravitational forces are just an expression of the fact that space-time is curved.  In the theory
of relativity each observer has his own measure of time.  General relativity incorporates the effect of gravity by
saying that the distribution of matter and energy in the universe warps and distorts space-time, so that it is not
flat.  Objects in this space-time try to move in straight lines, but because space-time is curved, their paths appear
bent.  They move as if affected by a gravitational field.

By curving space and time, general relativity changes them from being a passive background against which
events take place to being active, dynamic participants in what happens. In the mathematical model of general
relativity, time must have a beginning in what is called the big bang.  The big bang theory is supported by the fact
that the universe is currently expanding as galaxies move farther away from each other at increasing speeds.  
This means that earlier, the galaxies must have been closer together.  Similar arguments show that time would
have an end, when stars or galaxies collapse under their own gravity to form black holes.  

If there is more than a critical amount of matter, the gravitational attraction between the galaxies will slow them
down and will eventually stop them from flying apart.  They will start moving towards each other and will all come
together in a big crunch that will be the end of the history of the universe in real time.  If the density of the
universe is below the critical value, gravity is too weak to stop the galaxies from flying apart forever.  All the stars
will burn out, and the universe will get increasingly emptier and colder.  Either way we still have a few billion years.

If the mass of a star is concentrated in a small enough region, the gravitational field at the surface of the star
becomes so strong that even light can no longer escape.  We now understand that when any sufficiently heavy
non-rotating star, however complicated its shape and internal structure, runs out of nuclear fuel, it will necessarily
collapse to a perfectly spherical black hole, with its radius depending only on its mass.


Dark Matter and Dimensions

Various cosmological observations strongly suggest that there should be much more matter in our galaxy and
other galaxies than we see.  The most convincing of these observations is that stars on the outskirts of spiral
galaxies like our own Milky Way orbit far too fast to be held in their orbits only by the gravitational attraction of all
the stars that we observe.  This discrepancy indicates that there should be much more matter in the outer parts of
the spiral galaxies.

The brane/dimension on which we live, may be spherical in space-time.  A 5th dimension would exist on the
interior of the sphere.  As the brane expands, the volume of the higher-dimensional space inside would increase.

According to the idea of holography, information about what happens in a region of space-time can be encoded
on its boundary.  So maybe we think we live in a four-dimensional world because we are shadow cast on the
brane by what is happening in the interior of the bubble.

• Brane:  An object, which appears to be a fundamental ingredient of M-theory, which can have a variety of
  spatial   dimensions.  In general, a p-brane has length in p directions, a 1-brane is a string, a 2-brane is a
  surface or a membrane, etc.
  

• Dark Matter:  Matter in galaxies and clusters, and possibly between clusters, that cannot be observed
  directly but that can be detected by its gravitational field.  As much as 90% of the matter in the universe is
  dark matter.
  

• M-Theory:  A theory that unites all five string theories, as well as super gravity, within a single theoretical
  framework, but which is not yet fully understood.
  

The Future

We are at the beginning of a new era, in which we will be able to increase the complexity of our internal record,
the DNA, without having to wait for the slow process of biological evolution.  There has been no significant change
in human DNA in the last ten thousand years, but it is likely that we will be able to completely redesign it in the
next thousand.  Of course, many people will say that genetic engineering of humans should be banned, but it is
doubtful we will be able to prevent it.  Genetic engineering of plants and animals will be allowed for economic
reasons, and someone is bound to try it on humans.  Unless we have a totalitarian world order, someone
somewhere will design improved humans.

Within the next hundred years, Hawking expects we will be able to grow babies outside the human body, so the
limitation on brain growth will be removed.

The body’s chemical messengers responsible for our mental activity are relatively slow-moving.  This means that
further increases in the complexity of the brain will be at the expense of speed.  We can be quick-witted or very
intelligent, but not both.

One way in which electronic circuits can increase their complexity while maintaining speed is to copy the human
brain.  The brain doesn’t have a single CPU that processes each command in sequence.  Rather, it has millions of
processors working together at the same time.  Such massively parallel processing will be the future for electronic
intelligence as well.

Within two decades a thousand-dollar computer may be as complex as the human brain.  Parallel processors
could mimic the way our brain works and make computers act in intelligent and conscious ways.

Neural implants may allow a much faster interface between the brain and computers, dissolving the distance
between biological and electronic intelligence.

In the near future, most business transactions will probably be made between cyber personalities via the World
Wide Web.

Within a decade, many of us may even chose to live a virtual existence on the Net, forming cyber friends and
relationships.

Our understanding of the human genome will undoubtedly create great medical advances, but it will also enable
us to increase the complexity of the human DNA structure significantly.  In the next few hundred years, human
genetic engineering may replace biological evolution, redesigning the human race and posing entirely new ethical
questions.

Space travel beyond our solar system will probably require either genetically engineered humans or unmanned
computer-controlled probes.

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