Now that March is here, Sirius, the Dog Star, shines due south after dinner. It sparkles icy white as high as it ever gets, a shiny dog tag on the chest of the constellation Canis Major, the Big Dog.
You’ll need a dark sky to see the dog’s faint triangular head and his belly stars, as drawn here. But his five brightest stars can be seen even from the city, forming a sort of meat cleaver shape.
Sirius is often called the brightest star in the sky. But that title belongs to the sun. The sun is only 8.3 light-
minutes from Earth, compared with Sirius’s distance of 8.6 light-years (50 trillion miles). Yet even at that enormous distance Sirius is one of the closest stars. Mirzim to its right is 490 light-years distant, and Aludra, the dog’s tail, is about 2,000.
As everyone learns in school, looking to such great distances means looking back in time. We see the sun as it was 8 minutes ago but Aludra as it was around the reign of Caesar Augustus. If all the stars vanished this instant, we would see the sun wink out in a few minutes, Sirius in 2021, and Aludra around AD 4000.
But that common idea is, in fact, wrong-headed, as Albert Einstein discovered more than a century ago with his special theory of relativity.
Time doesn’t exist?
Across great distances, there is no such thing as innately simultaneous events, such as turning off the stars at once, except as events communicate with each other slowly across space, at the speed of light. Moreover, different observers experience things happening in different time order, depending on the observer’s own velocity. Time itself proved to depend on your viewpoint.
It was a hard idea to swallow. A lot of people in the early 1900s thought that relativity would spell the death of certainty, science, morals, and everything else.
Worse was to come. In discovering the interrelationship of space with time, Einstein found no place left for the queer, indefinable thing called “the present.” He was stuck with a picture of the universe as a permanent, eternal, changeless block of four-dimensional “spacetime,” with past, present, and future all there at once, in a static, changeless whole.
Picture a book, in which the story is all there at once from start to finish. Or a DVD sitting on a shelf, containing the whole movie.
This is for real. Modern physics can locate no such thing as “the present.” The book of time is not open to a particular page that some unseen mechanism is turning. The DVD on the shelf is just a DVD; it has no laser readout gizmo playing a particular part of the movie right now. No readout device exists. So science is utterly stymied by a simple question: Why do we think it’s March 2013 right now, instead of, say, August 2005? What makes one of those two feel immediate to us, and the other not?
Some physicists have decided that they must accept the block-universe picture at face value; that the flowing present is some kind of illusion of the human brain, and that all cosmic history exists at once, eternal. Einstein saw no way around it. He urged friends to find comfort in this realization when their loved ones died.
Now this idea is coming under a serious challenge from today’s physics frontier. Theorist Lee Smolin at the Perimeter Institute for Theoretical Physics argues, in a book to be published next month titled Time Reborn, that the passage of time is the actual fundamental reality, and that physics’ picture of invariant space is the thing that needs to turn weird instead.
Smolin has already made his mark as an iconoclast. His “The Trouble with Physics” (2006) criticized the field’s preoccupation with string theory as a distraction that has diverted a generation’s best minds into a dead end.
Smolin points out that you can recast Einstein’s equations to make the passage of time into a real and fundamental thing, at the expense of making space, size and distance, into the part that is fungible and relative. You get to pick your weirdness. The equations of each version are equally correct, and any particular problem may be easier to tackle with the math of one than the math of the other.
Smolin assembles ingenious arguments that the time-is-real version is the fundamental one, bypassing string theory, and explaining built-in aspects of our universe that science cannot otherwise address. This version, he says, points the way to a grander, ever-evolving multiverse existing before and behind the current Big-Bang cosmos we inhabit. I suspect we’re getting a peek here at what the cutting edge of physics a few decades from now is going to look like.Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com).