It was Richard Feynman who proposed that particles like positrons -- the antimatter equivalent of the electron -- were actually normal particles traveling backward in time. Feynman would develop the idea with John Wheeler, and it continues to resonate with John Cramer (University of Washington), whose 'transactional' interpretation of quantum mechanics works with particle interactions that depend upon movement forward and backward in time. In December we looked at an experiment Cramer is developing to study this effect, which is best known as retrocausality. So it's nice to see that Patrick Barry's fine article on Cramer's work, and retrocausality in general, is available online from the San Francisco Chronicle. Originally written for New Scientist, the article is thus freed from that magazine's firewall and available for general access. A snippet: While Cramer last week prepared to start a series of experiments leading up to the big test of retrocausality, some researchers expect...

'Gravity's rainbow' calls to mind a novel by Thomas Pynchon, but in this case I'm thinking less in literary terms than scientific ones. Let's talk about the full spectrum of views on the subject of gravity itself. It's always a pertinent question because we can make sense out of the universe, up to a point, using Einstein's understanding of gravity. But when we get down to the quantum level, we have no insights into what happens at the atomic level and below. Thus the search for a 'quantum theory of gravity,' one we're likely to be a long time establishing. In that context, two quotes caught my eye over the weekend. The first is from Freeman Dyson, from a short piece that's now published in his new collection of essays called The Scientist as Rebel (an unfortunate title in this context, and one I suspect a marketer rather than Dyson chose). Dyson had been discussing "...those who build grand castles in the air and those who prefer to lay one brick at a time on solid ground," and he...

Because it's hard to argue with people once involved in Nobel Prize-winning work, I take Warren Nagourney (University of Washington) at his word. At one time Nagourney assisted Hans Dehmelt, the UW scientist who won the 1989 Nobel Prize in physics. Now he's working with John Cramer on a project so bizarre that, as this Seattle Post-Intelligencer story reports, he understands it only faintly. That makes Centauri Dreams' chances of understanding it all but infinitesimal. And because the work involves the paradoxical quantum behavior called 'entanglement' and implies communicating information backwards in time, it also conjures up memories of another man one hesitates to challenge. It was Einstein who called certain weird quantum behaviors 'spooky action at a distance' and cultivated a continuing distaste for the paradoxes of quantum mechanics. These are formidable scientists, but then so is Cramer, and in a way he seeks to confirm something Einstein said a long time ago. Einstein...

The evidence for dark matter keeps piling up, even if we still don't know what it is. Back in the 1930s, Fritz Zwicky noted that the galaxies he was observing weren't massive enough to account for the way they cohered into clusters. Vera Rubin later took the idea of missing mass further, seeing that stars in the outer parts of galaxies rotated too quickly for the presumed mass of the galaxy. So we've known for a long time that something mysterious is out there. Groups studying the phenomenon focus on odd, hypothetical particles called WIMPS -- Weakly Interacting Massive Particles. To make them fit a theoretical model of how dark matter works, scientists assume WIMPS are neutral in charge and about 100 times more massive than a proton. The problem is that they don't interact with most matter. If they're to be found, they'll have to be detected in a rare collision within extremely sensitive detectors. Image: The CDMS II detectors (hexagons) are stacked in an icebox with six insulating...

More on growth scenarios for interstellar flight soon. But I don't want to let the recent dark matter news get past us, so a quick nod to the University of California at Santa Cruz, where researchers have run a powerful computer simulation to probe the dark matter halo that evidently surrounds our Milky Way. It's a further step toward understanding the stuff that makes up 82 percent of the matter in the universe, and that in turns helps us see how the large scale structure of the cosmos has evolved. Image: Density map of dark matter in a halo the size of the Milky Way galaxy's dark matter halo. Credit: J. Diemand, UC-SC. I grew up in a time when it was thought that everything in the cosmos was explicable through gravitational forces produced by objects we could see. From solar systems on up to galaxies, it made sense -- and the textbooks did this quite neatly with colorful diagrams -- to show how matter found its way into configurations that would turn into celestial objects visible...

I see that Stephen Hawking has a new book under contract. The Grand Design is to be co-authored by Leonard Mlodinow, who also worked with Hawking on A Briefer History of Time. This one takes on an issue that is challenging even for Hawking, namely the question of why the laws of physics act as they do and, if a Bantam Dell publisher is to be believed, the question of why there is a universe in the first place. Meanwhile, Hawking spent the week of September 24 to October 1 visiting CERN in Geneva, meeting with physicists in the Theory Unit of the Physics Department there and touring the facilities of the Large Hadron Collider, due to be started up in 2007. Note that two Hawking lectures are now available over the Web, one of which, titled The Origin of the Universe, anticipates the new book. The other, The Semi-Classical Birth of the Universe, is aimed at a specialist audience. And one other note apropos of great physicists for an otherwise quiet weekend. The BBC offers a 1981...

We seem to be awash in exotic physics, an administrative category I created on this site only a couple of days ago to house the trillion-year crunch story and the 'light in reverse' work at the University of Rochester. It seems an appropriate time, then, to look at an investigation reported in the Physical Review Letters that takes us, like Alice through the looking glass, into the universe before the Big Bang. Penn State researchers are behind this study, combining quantum physics tools that Einstein didn't have with general relativity to punch through to a universe on the other side. So let's talk again about what might have been there before the Big Bang. This analysis says the previous universe had a spacetime geometry much like our own expanding universe, except that it was a contracting universe. Gravititational forces were pulling the previous universe together until the quantum properties of spacetime caused gravity itself to become repulsive. What follows is aptly described...

If you thought the trillion-year crunch was mind-boggling, how about light that moves backwards, and does so at speeds faster than c? From the University of Rochester comes word that Robert Boyd, a professor of optics there, has slowed light to negative speeds. To do this, the experimenter sent a pulse of laser light through an optical fiber laced with the element erbium. Leaving the laser, the light pulse was split, with one pulse going into the fiber and the other left undisturbed for reference. The remarkable result: The peak of the pulse emerged from the other end of the fiber before it entered the front of the fiber, and ahead of the reference pulse. "Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses," says Boyd, who acknowledges "I've had some of the world's experts scratching their heads over this one." Centauri Dreams hardly qualifies as an expert, but head-scratching does seem in order...

How to explain dark energy, which is pushing distant galaxies away at an accelerating rate? The cosmological constant that would account for the phenomenon -- originally conceived but then rejected by Einstein -- is far smaller than one would expect from conventional Big Bang scenarios. In fact, the observed vacuum energy (a possible explanation for the repulsive force) is smaller by a factor of 10120 than it would need to be to do the job. But if the universe were older than today's estimate of 13.7 billion years, and I mean a lot older, then this tiny value might make sense. So say Paul Steinhardt (Princeton) and Neil Turok (Cambridge, UK), who put forward a startling concept: there was indeed a time before the Big Bang. There is remarkable solace in this for all of us who grew up asking what happened before the Big Bang, only to be told that the question made no sense because it was unanswerable. So said a kindly astronomy professor in a long-ago college course, raising his shaggy...