Why black holes could be antimatter factories

Here’s an interesting chain of thought…

Imagine a black hole sucking in protons and electrons. With their higher mass, protons are likely to be preferentially sucked, giving the black hole a positive charge. (That’s not so unusual in space: a similar mechanism can give planets a charge because electrons escape their gravity more easily.)

But black holes also create such strong electrostatic fields at the horizon that positrons and electrons simply appear out of the vacuum.

In those circumstances, it’ll look as if the protons being sucked into the black hole are being converted into positrons.

So these kinds of black holes will look and behave like antimatter factories, say Cosimo Bambi from Wayne State University in Detroit and pals.

How might we we spot these exotic objects? Bambi and friends say a sure signature would be an excess of positrons in cosmic rays with an energy between 1 and 100 MeV coming from a black hole.

Anybody seen any of these?

Ref: arxiv.org/abs/0806.3440: Black Holes as Antimatter Factories

Musical relativity

Here’s a neat idea for a concert that’s going to blow a few minds if it ever takes to the stage.

A combination of three or more notes played together is called a chord. We know that certain musical chords sound happy while others sound sad (although nobody knows why). The mood of a piece of music then depends on the combination of chords being played. More than a few weighty tomes have been written about the way one chord can be transformed into another and the effect this has on the mood of the music.

But Kaca Bradonjic, a physicist at Boston University, says that musicians appear to have ignored one of the fundamental ways of changing the pitch of a note: the Doppler shift. He points out that it ought to be possible for an observer moving at a specific velocity to hear a sad sounding note as a happy one and vice versa.

Which means that the mood of a piece of music depends on the relative velocities of the audience and performers.

He calculates for example that to hear a C major chord as a C minor, the listener would need to be travelling at about 43 miles per hour, directly away from the source. That’s a fair speed. And the accelerations necessary to vary this effect from one note to another during a concert would make this one helluva roller coaster ride.

Talking of which, a (very quiet) roller coaster might be the perfect venue for the first concert of this type.

Ref: arxiv.org/abs/0807.2493: Relativity of musical mood

The magnetic magic of liquid mirrors

Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.

The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.

It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.

But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis. Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.

Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required. Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.

However, it may also be possible to use this technique to tilt liquid mirrors further than ever before. Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.

Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.

But here on Earth they could be made much more useful. It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.

Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments

Source: arxivblog.com