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As we shall see, the concept of time has no meaning before the beginning of the universe.
If there were events earlier than this time, then they could not affect what happens at the present time.
An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job!
A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations.
On the moon, where there is no air to slow things down, the astronaut David R. Scott performed the feather and lead weight experiment and found that indeed they did hit the ground at the same time.
In the theory of relativity there is no unique absolute time, but instead each individual has his own personal measure of time that depends on where he is and how he is moving.
As far as we are concerned, events before the big bang can have no consequences, so they should not form part of a scientific model of the universe. We should therefore cut them out of the model and say that time had a beginning at the big bang.
Einstein’s general theory of relativity seems to govern the large-scale structure of the universe.
Thus, in a sense, classical general relativity, by predicting points of infinite density, predicts its own downfall, just as classical (that is, nonquantum) mechanics predicted its downfall by suggesting that atoms should collapse to infinite density.
Like light, gravitational waves carry energy away from the objects that emit them.
second law of thermodynamics. It states that the entropy of an isolated system always increases, and that when two systems are joined together, the entropy of the combined system is greater than the sum of the entropies of the individual systems.
They all confirm that a black hole ought to emit particles and radiation as if it were a hot body with a temperature that depends only on the black hole’s mass:
the anthropic principle, which can be paraphrased as “We see the universe the way it is because we exist.”
It would be very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended to create beings like us.
Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together.
Einstein’s idea that the gravitational field is represented by curved space-time: particles try to follow the nearest thing to a straight path in a curved space,
either it has existed for an infinite time, or else it had a beginning at a singularity at some finite time in the past.
the quantum theory of gravity has opened up a new possibility, in which there would be no boundary to space-time and so there would be no need to specify the behavior at the boundary.
The increase of disorder or entropy with time is one example of what is called an arrow of time,
Disorder increases with time because we measure time in the direction in which disorder increases.
One has to use a quantum theory of gravity to understand how the universe began.
Regions in which the density was slightly higher than average would have had their expansion slowed down by the gravitational attraction of the extra mass. Eventually, such regions would stop expanding and collapse to form galaxies, stars, and beings like us.
The theory of relativity says that the laws of physics appear the same to observers moving at different speeds.
our aim is to formulate a set of laws that enables us to predict events only up to the limit set by the uncertainty principle.
we now believe inflation likely gives rise to a vast number of universes, known collectively as a multiverse.
