A Briefer History of Time

What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.

It's the gravity that shapes the large scale structure of the universe, even though it is the weakest of four categories of forces.

Intelligence is the ability to adapt to change. Stephen Hawking

The uncertainty principle signaled an end to Laplace's dream of a theory of science, a model of the universe that would be completely deterministic. We certainly cannot predict future events exactly if we cannot even measure the present state of the universe precisely! We could still imagine that there is a set of laws that determine events completely for some supernatural being who, unlike us, could observe the present state of the universe without disturbing it. However, such models of the universe are not of much interest to us ordinary mortals. It seems better to employ the principle of economy known as Occam's razor and cut out all the features of the theory that cannot be observed.

If a star were a grain of salt, you could fit all the stars visible to the naked eye on a teaspoon, but all the stars in the universe would fill a ball more than eight miles wide.

Yet if there really were a complete unified theory, it would also presumably determine our actions—so the theory itself would determine the outcome of our search for it! And why should it determine that we come to the right conclusions from the evidence? Might it not equally well determine that we draw the wrong conclusion? Or no conclusion at all?

El siglo XX también vio nacer otra gran teoría parcial de la naturaleza: la mecánica cuántica. Esta teoría trata los fenómenos que se producen a escalas muy pequeñas. Nuestra concepción del big bang nos indica que debió de haber un momento en que el universo muy primitivo era tan pequeño que, incluso al estudiar su estructura «a gran escala», no es posible ignorar los efectos de pequeña escala de la mecánica cuántica. Nuestra mayor esperanza de obtener una comprensión completa del universo desde su principio hasta su final implica combinar estas dos teorías parciales en una sola teoría cuántica de la gravedad. [...] Cuando se combina la relatividad general con el principio de incertidumbre de la mecánica cuántica surge la posibilidad de que tanto el espacio como el tiempo sean finitos, pero sin tener bordes ni fronteras. Y es posible que las leyes ordinarias de la ciencia se cumplan en todos los sitios, incluida la región inicial del tiempo, sin necesidad de que haya en ella singularidad alguna.

Por lo tanto, la mecánica cuántica introduce en la ciencia un elemento inevitable de impredecibilidad o aleatoriedad. Einstein se opuso rotundamente a ello, a pesar del importante papel que había desempeñando en el desarrollo de estas ideas. [...] La prueba de una teoría científica es su capacidad de predecir los resultados de un experimento. La teoría cuántica limita nuestras posibilidades. ¿Limita la teoría cuántica la ciencia? Para progresar, la manera en que hacemos ciencia debe ser dictada por la naturaleza. En este caso, la naturaleza exige que redefinamos lo que entendemos por predicción: quizá no podamos predecir exactamente el resultado de un experimento, pero podemos repetirlo muchas vces y confirmar que los diversos resultado posibles ocurren con las probabilidades predichas por la teoría cuántica. Así, a pesar del primer principio de incertidumbre, no es necesario abandonar la creencia en un mundo regido por leyes físicas.

If we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God.

Dios no solo juega a los dados, a veces los tira donde no se pueden ver.

(Alexander) Friedmann sólo dedujo un modelo de universo, pero, si sus suposiciones son correctas, hay en realidad tres posibles tipos de soluciones de las ecuaciones de Einstein, es decir, tres diferentes tipos de modelos de Friedmann, y tres diferentes comportamientos del universo. 1. En el primer tipo de solución (el que descubrió Friedmann) el universo se expande con suficiente lentitud como para que la atracción gravitatoria entre las galaxias vaya frenando la expansión hasta llegar a detenerla, tras lo cual las galaxias empiezan a aproximarse las unas a las otras y el universo se contrae. 2. En el segundo tipo de solución, el universo se expande tan rápidamente que la atracción gravitatoria no puede llegar a frenarlo nunca, aunque sí va reduciendo su ritmo de expansión. 3. Finalmente, en un tercer tipo de solución, el universo se expande con el ritmo justo para impedir que se vuelva a colapsar. La velocidad con que las galaxias se separan va disminuyendo progresivamente, pero nunca llega a alcanzar el valor cero. [...] Otras observaciones recientes indican que la expansión del universo en realidad no se está frenando, sino que se está acelerando.

Remember to look up at the stars and not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist. Be curious. And however difficult life may seem, there is always something you can do and succeed at. It matters that you don’t just give up.

Despite this, I have a bet with Kip Thorne of the California Institute of Technology that in fact Cygnus X-1 does not contain a black hole! This is a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would bring me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse. When we made the bet, in 1975, we were 80 per cent certain that Cygnus was a black hole. By now, I would say that we are about 95 per cent certain, but the bet has yet to be settled.

.... attempts to combine these two forces with the strong nuclear force into what is called a grand unified theory (GUT). This title is rather an exaggeration: the resultant theories are not all that grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories, because they contain a number of parameters whose values cannot be predicted from a theory but have to be chosen to fit in with experiment. Nevertheless, they may be a step toward a complete, fully unified theory.

It is just that modern physicists seem to have more imaginative ways of naming new particles and phenomena - they no longer restrict themselves to Greek!

contains on average some one hundred billion stars. If a star were a grain of salt, you could fit all the stars visible to the naked eye on a teaspoon, but all the stars in the universe would fill a ball more than eight miles wide.

There is a good chance that the study of the early universe and the requirements of mathematical consistency will lead us to a complete unified theory within the lifetime of some of us who are around today, always presuming we don't blow ourselves up first!

The twentieth century saw man's view of the universe transformed: we realized the insignificance of our planet in the vastness of the universe, and we discovered that time and space we curved and inseparable , that the universe was expanding, and that it had a beginning in time.

In other words, the theory of relativity requires us to put an end to the idea of absolute time! Instead, each observer must have his own measure of time, as recorded by a clock carried with him, and identical clocks carried by different observers need not agree.