Death by Black Hole: And Other Cosmic Quandaries Read Online Free PDF

surly people. A few years ago I was having a hot-cocoa nightcap at a dessert shop in Pasadena, California. I had ordered it with whipped cream, of course. When it arrived at the table, I saw no trace of the stuff. After I told the waiter that my cocoa was plain, he asserted I couldn’t see the whipped cream because it sank to the bottom. Since whipped cream has a very low density and floats on all liquids that humans consume, I offered the waiter two possible explanations: either somebody forgot to add the whipped cream to my hot cocoa or the universal laws of physics were different in his restaurant. Unconvinced, he brought over a dollop of whipped cream to test for himself. After bobbing once or twice in my cup, the whipped cream sat up straight and afloat.
    What better proof do you need of the universality of physical law?

THREE
     

SEEING ISN’T BELIEVING
     
    S o much of the universe appears to be one way but is really another that I wonder, at times, whether there’s an ongoing conspiracy designed to embarrass astrophysicists. Examples of such cosmic tomfoolery abound.
    In modern times we take for granted that we live on a spherical planet. But the evidence for a flat Earth seemed clear enough for thousands of years of thinkers. Just look around. Without satellite imagery, it’s hard to convince yourself that the Earth is anything but flat, even when you look out of an airplane window. What’s true on Earth is true on all smooth surfaces in non-Euclidean geometry: a sufficiently small region of any curved surface is indistinguishable from a flat plane. Long ago, when people did not travel far from their birthplace, a flat Earth supported the ego-stroking view that your hometown occupied the exact center of Earth’s surface and that all points along the horizon (the edge of your world) were equally distant from you. As one might expect, nearly every map of a flat Earth depicts the map-drawing civilization at its center.
    Now look up. Without a telescope, you can’t tell how far away the stars are. They keep their places, rising and setting as if they were glued to the inside surface of a dark, upside-down cereal bowl. So why not assume all stars to be the same distance from Earth, whatever that distance might be?
    But they’re not all equally far away. And of course there is no bowl. Let’s grant that the stars are scattered through space, hither and yon. But how hither, and how yon? To the unaided eye the brightest stars are more than a hundred times brighter than the dimmest. So the dim ones are obviously a hundred times farther away from Earth, aren’t they?
    Nope.
    That simple argument boldly assumes that all stars are intrinsically equally luminous, automatically making the near ones brighter than the far ones. Stars, however, come in a staggering range of luminosities, spanning ten orders of magnitude—ten powers of 10. So the brightest stars are not necessarily the ones closest to Earth. In fact, most of the stars you see in the night sky are of the highly luminous variety, and they lie extraordinarily far away.
    If most of the stars we see are highly luminous, then surely those stars are common throughout the galaxy.
    Nope again.
    High-luminosity stars are the rarest of them all. In any given volume of space, they’re outnumbered by the low-luminosity stars a thousand to one. The prodigious energy output of high-luminosity stars is what enables you to see them across such large volumes of space.
    Suppose two stars emit light at the same rate (meaning that they have the same luminosity), but one is a hundred times farther from us than the other. We might expect it to be a hundredth as bright. No. That would be too easy. Fact is, the intensity of light dims in proportion to the square of the distance. So in this case, the faraway star looks ten thousand (100 2 ) times dimmer than the one nearby. The effect of this “inverse-square law” is purely geometric. When starlight spreads in all directions,