When the universe is big, there are a very
large number of rolls of the dice, and the results average out to something one
can predict. But when the universe is very small, near the Big Bang, there are
only a small number of rolls of the dice, and the Uncertainty Principle is very
important. In order to understand the origin of the universe, one therefore has to
incorporate the Uncertainty Principle into Einstein's general theory of relativity.
This has been the great challenge in theoretical physics for at least the last thirty
years. We haven't solved it yet, but we have made a lot of progress.
Now suppose we try to predict the future. Because we only know some
combination of position and speed of a particle, we cannot make precise
predictions about the future positions and speeds of particles. We can only
assign a probability to particular combinations of positions and speeds. Thus
there is a certain probability to a particular future of the universe. But now
suppose we try to understand the past in the same way.
Given the nature of the observations we can make now, all we can do is assign
a probability to a particular history of the universe. Thus the universe must have
many possible histories, each with its own probability. There is a history of the
universe in which England win the World Cup again, though maybe the
probability is low. This idea that the universe has multiple histories may sound
like science fiction, but it is now accepted as science fact. It is due to Richard
Feynman, who worked at the eminently respectable California Institute of
Technology and played the bongo drums in a strip joint up the road. Feynman's
approach to understanding how things works is to assign to each possible history
a particular probability, and then use this idea to make predictions. It works
spectacularly well to predict the future. So we presume it works to retrodict the past too.
Scientists are now working to combine Einstein's general theory of relativity
and Feynman's idea of multiple histories into a complete unified theory that will
describe everything that happens in the universe. This unified theory will enable
us to calculate how the universe will evolve, if we know its state at one time. But
the unified theory will not in itself tell us how the universe began, or what its
initial state was. For that, we need something extra. We require what are known
as boundary conditions, things that tell us what happens at the frontiers of the
universe, the edges of space and time. But if the frontier of the universe was just
at a normal point of space and time we could go past it and claim the territory
beyond as part of the universe. On the other hand, if the boundary of the
universe was at a jagged edge where space or time were scrunched up, and the
density was infinite, it would be very difficult to define meaningful boundary
conditions. So it is not clear what boundary conditions are needed. It seems there
is no logical basis for picking one set of boundary conditions over another.
There is something special
about three space dimensions. In three dimensions, planets can have stable orbits
around stars. This is a consequence of gravitation obeying the inverse square
law, as discovered by Robert Hooke in 1665 and elaborated on by Isaac Newton.
Think about the gravitational attraction of two bodies at a particular distance. If
that distance is doubled, then the force between them is divided by four. If the
distance is tripled then the force is divided by nine, if quadrupled, then the force
is divided by sixteen and so on. This leads to stable planetary orbits. Now let's
think about four space dimensions. There gravitation would obey an inverse
cube law. If the distance between two bodies is doubled, then the gravitational
force would be divided by eight, tripled by twenty-seven and if quadrupled, by
sixty-four. This change to an inverse cube law prevents planets from having
stable orbits around their suns. They would either fall into their sun or escape to
the outer darkness and cold. Similarly, the orbits of electrons in atoms would not
be stable, so matter as we know it would not exist. Thus although the multiple-
histories idea would allow any number of nearly flat directions.
Well, are there possibilities of some changes in this 3 dimensional world ?
What! Why?
Due to the expansion,perhaps...
We're not sure though ...here my philosophy comes again: as we're in a computer 💻 and let's see now, as we're expecting the changes so how come & how it comes.
We'll be talking about some more, many more things.
For here, this is it.
I am awesome ...💯