Niels Bohr, the Danish physicist and philosopher-king of quantum theory, once said that great truth is a statement whose opposite is also a great truth. This pretty much captured the spirit of those elusive rules that govern the subatomic world, where light can be a wave -- no, a particle -- well, actually, whatever you need it to be for your particular experiment.
It also seems to me to sum up much of the history of science and philosophy, in which the learned consensus keeps swinging between the yin-and-yang theories of existence: free will and fate, change and eternity, atomicity and continuity.
These bipolar themes have been on my mind lately. This spring the theoretical physicist Lee Smolin published a new book, "Time Reborn," reopening a debate supposedly settled by Einstein and his acolytes a century ago: whether time is real or an illusion.
Meanwhile, other physicists have been arguing recently that the only way to understand the dark energy that is accelerating the expansion of the universe, and perhaps the mass of the newly discovered particle believed to be the Higgs boson as well, is to postulate that our universe is only one in an almost infinite ensemble of universes, each with different properties.
The reality of time and the plurality of worlds are only two of the eternal (so to speak) questions. Bob Dylan once wrote a song, "A Hard Rain's a-Gonna Fall," that consisted entirely, he said, of the first lines of songs he thought he would never have time to write. In that spirit I'd like to blurt out some of the Bohr-like questions about this vat of stars that I'll never be able to answer before my own time runs away.
Is nature discrete or continuous? Is the universe infinite or finite? Is life inevitable, or is it a lucky accident? Will we ever find company in the cosmos?
Is the truth of the world to be found in the ways things change, like the river that you cannot step into twice, or the ways they remain the same, like the law of gravity or, indeed, the name of that river?
I could go on all day. Feel free to write in with your own.
A final answer to any of these questions would be a landmark of human progress. But it might be in the nature of being human that we will never answer them but have to hug them both in a kind of Hegelian surrender. And so we live in the tension between opposites.
Take, for example, the history of cosmology. Only a century ago the universe was held to be eternal and unchanging. Then came the expanding universe and the Big Bang, an origin almost biblical in nature, like a girl bursting out of a cake, in the words of the cosmologist Fred Hoyle.
Hoyle and his colleagues cooked up a version of eternity, the Steady State universe, in which matter was created in the voids left as the galaxies sped away from one another, so over all the cosmos remained the same. The Steady State idea died in the 1960s and the Big Bang won.
Now a new version of the Big Bang, known as eternal inflation, is ascendant, in which there seem to be an endless number of universes bubbling violently forth from a background of primordial energy -- "false vacuum," in the jargon.
And so it goes, restless change manifested as immortality. Which part of the picture you think is most meaningful might depend on who you think you are -- a citizen of this planet and universe, or a creature of the endless possibilities of existence somewhere, at some time. Is there solace to be found in the vision of places and people we can never know or reach?
For we need solace. The latest cosmological wrinkle is dark energy, which is speeding up the flight of galaxies from one another. And the great question is whether this dark energy is going to suck the light and energy out of the universe so completely that some day billions of years from now nothing is left: no memory even of Homer, Jesus, Mozart, Elvis or Nelson Mandela, not to mention the rest of us.
Is this, then, the end of time, at least in our lonely corner of the multiverse, as it is known?
In the four-dimensional reality of Einstein's relativity, other times -- from the Big Bang to the Big Freeze -- are as real as other places. Nothing changes; we're just passing through. As Einstein once wrote, "People like us, who believe in physics, know the distinction between the past, present and future is only a stubbornly persistent illusion."
Dr. Smolin, like many philosophers, complains that this coldblooded mathematical formulation doesn't do justice to the experience we all have of being in time. Moreover, the only way to understand why the laws of physics are as they are, he says, is to imagine them changing -- like the Darwinian evolution of species -- in cosmic time. Real time. But neither he nor anyone else can say how it would work.
The party line among many theoretical physicists recently is that time (and space) are "approximations" that emerge out of a more primal entity, maybe information in some quantum process. You may wonder who cares what time is and whether it is worth your tax dollars. It's not a question that moves the markets, but as Bohr understood, it moves our hearts.
John Archibald Wheeler, the visionary Princeton physicist who was Bohr's disciple, once pointed out that the future and the past are theory. They exist only in records and the thoughts of the present, a fulcrum, in which all stories end and begin.
A single moment of insight or beauty or grace -- like hitting a perfect towering drive off the eighth tee -- can illuminate eternity.
It all depends on how you look at it.
This article originally appeared in The New York Times.