快猫短视频

Read more: 鈥�About time: Adventures in the fourth dimension鈥�

AMONG the concepts through which science tries to understand the world, time has an unmatched reputation for depth, mystery and paradox. Yet it is also one of the most familiar features not only of the sciences but of everyday life, and seems unproblematic for most practical purposes. 鈥淲hat then is time?鈥� asked St Augustine in the 4th century. 鈥淚f no one asks me, I know; if I wish to explain it to one who asks, I know not.鈥�

In many ways, there has been enormous progress in understanding time since Augustine鈥檚 day, much of it involving radical new concepts, as will become clear from this special issue of 快猫短视频. For example, Einstein鈥檚 theory of relativity has shown that time runs more slowly if one accelerates to high speeds, and that whether two distant events are simultaneous is not an objective fact but a matter of the observer鈥檚 perspective, and that space and time are inextricably combined in a four-dimensional 鈥渂lock universe鈥�.

鈥淭here has been enormous progress in understanding time, much of it involving radical new concepts鈥�

Cosmologists have discovered that time can have a beginning, in a big bang, but may have no end, according to mainstream theories. Neuropsychology has revealed that we re-sort our memories within fractions of a second so as to change the order in which we shall remember events happening, and that decisions we are going to make are detectable by brain scans before we are conscious of having decided anything.

Philosophers have drawn uneasy conclusions from this about the nature of consciousness (itself another ancient mystery) and free will. They have also realised that nothing objective distinguishes the present moment from any other, and that the notion of the flow of time is a nonsensical misconception despite being deeply embedded in common sense. Quantum theory 鈥� in its many-universes version 鈥� has revealed that moments of time, such as 鈥渢his time yesterday,鈥� are parallel universes too, distinguished from others merely by affecting us more strongly, and that time travel is not as paradoxical as it seems.

One might have expected all this progress to have resolved the mysteries that Augustine perceived. After all, he didn鈥檛 even know that days and years were not attributes of the sun and the stars as common sense had always assumed, but of his own motion along with the rest of the Earth, and that their durations are variable. Today we understand in exquisite detail how and why all that happens.

But the profound discoveries that science has made not only defy our intuitions, they often reveal new problems that are more perplexing. For example, quantum theory seems stubbornly to resist relativity鈥檚 unification of space and time. The fact that these theories 鈥� together containing our deepest knowledge of the physical world 鈥� contradict each other in this way is at the heart of the problem of constructing a quantum theory of gravity, the hard core of the 鈥渢heory of everything鈥� that particle physicists have been seeking for decades.

Another example is that known laws of motion treat the future and past symmetrically, while almost everything we observe in nature has an 鈥渁rrow of time鈥� that distinguishes between past and future directions. Causes precede effects; computer programs deliver their outputs after their inputs; the glove gets muddy and the washing machine that cleans it draws power from the electricity grid, never vice versa.

Other arrows of time, defined by the expansion of the universe, the spreading-out of ripples on a pond, and of electromagnetic radiation, our consciousness, our scientific measurements, and the creation of scientific knowledge itself, are all aligned with each other, not only over time but throughout space, for reasons that seem always to be just on the verge of being clarified.

The connections between many of these puzzles make it plausible that a conceptual breakthrough in one of them might solve them all. Perhaps it will. But if the future is going to be anything like the past in this respect, that would only be the key to a further, and even more delightful, Pandora鈥檚 box.