杏吧原创

IN THE 19th century, astronomers were convinced that Venus sported its own version of our snow-capped Himalayas, that the moon was circled by its own set of mini-moons, and that the newly discovered Neptune boasted a ring system much like Saturn鈥檚. There were even claims by competent observers that when the moon sailed in front of a star in the night sky, the star would sometimes jump in front of the moon, hover there for a while, and then jump back behind it. Read Samuel Taylor Coleridge鈥檚 The Rime of the Ancient Mariner and you will see this peculiar observation immortalised in verse.

All of these observations proved to be mirages. Although Venus does have lofty volcanoes and Neptune flimsy ring arcs, the 19th-century claims were no more than lucky guesses 鈥� the telescopic technology of the day could not possibly have revealed such features.

These and other astronomical curiosities are highlighted by Richard Baum in The Haunted Observatory. Baum鈥檚 impressively researched and fascinating book prompts an obvious question: are there modern astronomical observations that will likewise turn out to be mirages?

Several spring to mind. First, there is the Pioneer anomaly. NASA鈥檚 Pioneer 10 and 11 probes have been racing out of the solar system since their launch in the early 1970s, but a mysterious force has been putting the brakes on their motion. Dozens of explanations have been proposed, from the drag of unseen matter to altogether new physics. There is talk of sending a follow-up probe to look for gravitational anomalies in the outer solar system. The most likely explanation of the anomaly, however, is a force generated on board the probes themselves, perhaps by a leakage of fuel or heat.

Then there鈥檚 the so-called 鈥渁xis of evil鈥�, a curious feature in the cosmic background radiation 鈥� the dim afterglow from the heat of the big bang. Standard cosmology predicts that the hotspots in this radiation should be randomly distributed across the sky, but they are not. Data collected by NASA鈥檚 Wilkinson Microwave Anisotropy Probe (WMAP) reveals a curious alignment of hotspots along a particular direction, now known as the axis of evil. If the apparent alignment is real, it could undermine one of the key assumptions of the big bang model: that the large-scale universe is, on average, equally dense in all places and in all directions. In fact, the alignment could indicate that the universe is shaped like a doughnut or a disc. The best bet, however, is that the axis of evil is nothing more than an instrumental mirage.

Could even bigger astronomical mysteries turn out to be mirages? Dark matter 鈥� the theoretical stuff that does not interact with light but does exert a gravitational pull and is believed to comprise 90 per cent of the matter in the universe 鈥� is a prime candidate. Astronomers need dark matter鈥檚 extra gravity to explain why fast-moving stars in the outer regions of spiral galaxies don鈥檛 cut loose and fly off into intergalactic space. They also need it to account for the apparent speed at which galaxies coalesced after the big bang.

But dark matter is not the only way of explaining all these observations. A simple modification to the law of gravity at low accelerations appears to fit the data far more economically 鈥� particularly in spiral galaxies 鈥� than dark matter does. This theory is called modified Newtonian dynamics, or MOND. If MOND is right, dark matter could go down in history as one of the biggest mirages of our time.

鈥淒ark matter could go down as one of our biggest mirages鈥�

Of course, dark matter, the axis of evil and the Pioneer anomaly may all turn out to be real. If so, things are about to get very interesting. At the end of the 19th century, there was just one peculiar observation in physics that did not fit with everything else: the anomalous spectrum produced by heat radiating from an ordinary furnace. That innocuous observation triggered the quantum revolution, which gave us computers, lasers, iPods and a new understanding of reality.