Strange things are said about time: that it's illusory, that it has no direction. But what about space, or the space-time continuum? What exactly is space-time?
Common sense sees a clear division of labor between space and time. I am about 11 years older than Ken. I was born in Lincoln, Nebraska, while he was born in Sandusky, Ohio. Lincoln is roughly 800 miles west of Sandusky, and about 500 feet higher. So the events of my birth and his are distant in time and in space. These facts are not causally independent; some event might have led to our being closer in both space and time. But they seem logically independent. This spatial relation between two events is one thing, their temporal relation is another. For one thing, we can travel in space, but not in time, unless continuing to exist counts as time-travel.
We live in an age in which politicians and scientists tell us absurd things. Trump says he has more respect for women than anyone else in the world, and that he and Paul Ryan are going to give us a wonderful health-care system, with lower costs, better medicine, lower taxes and more choice, and that global warming is a lie invented by the Chinese. Scientists tell us that space and time are really aspects of a single thing, the space-time continuum, that time has no intrinsic direction, and that there is really no such thing as objective, observer independent simultaneity, and maybe time travel is possible after all.
It’s easy and wise to disbelieve what Trump says. It’s difficult but wise to believe what scientists say. People like me read popular explanations of what physicists and astronomers have to say, and accept that they must be right. But I can never remember exactly why they believe what they do.
I remember that it has a lot to do with the speed of light, and Einstein sitting walking up and down in a moving train, and then sitting on the cowcatcher as it approaches the speed of light. People smarter than I understand and are convinced. I am convinced, but never quite understand. When I imagine sitting on the cowcatcher, I don’t experience illumination but fear. What if there is a cow on the tracks?
In this week's program I’ll get another chance to understand; Tim Maudlin and Ken will do their best. Maybe I’ll finally get it.
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The Big Bang theory is the prevailing theory about the “birth” of the universe.
At the foundation of modern theoretical physics lie the equations that define our universe, telling us of its beginnings, evolution, and future.
All the matter we have ever observed accounts for less than 5% of the universe. The rest?
From airplanes flying overhead to the cellular activity inside us, all events that take place in the world obey the laws of physics.