The Day We Found the Universe - Marcia Bartusiak [68]
With the globulars acting in a way like surveyor posts, marking the boundaries of our galactic borders, the Milky Way was growing by leaps and bounds. As a result, the globular clusters could no longer be thought of as similar in size to the Milky Way, as Shapley once thought. The clusters were now far smaller by comparison. “This is a peculiar universe” was Shapley's reaction to this new cosmic landscape.
So what did this mean for the spiral nebulae, which Heber Curtis and V. M. Slipher were now enthusiastically hawking as separate galaxies? Around this time Shapley's Mount Wilson buddy van Maanen was claiming to see some spirals rotate, an impossible feat if they were lying at a great distance. To perceive a rotation from so far away over a short period of time would mean the spirals had to be spinning at close to the speed of light!
To understand why this would be so, imagine a kitchen clock sitting right by you on the wall. The second hand is sweeping around the dial at a speed of about 1 centimeter per second. But then imagine the face of that clock covering the entire surface of the Moon, its apparent size looking just like the clock on your wall. Yet the clock in reality is now much bigger, so the second hand has to travel at a faster clip, about 110 miles per second, to make a full circuit over a minute's time. Now if that clock were as big as the Milky Way, the second hand would be moving at a demonic pace. If van Maanen's spiral nebula was truly a distant galaxy and he was able to detect its arms shift over a matter of years, then he was seeing it rotate at light-defying speeds.
Not willing to tolerate such bizarre behavior, Shapley at first was doubtful of van Maanen's finding. In fact, he published an article in 1917 saying that “the minimum distance of the Andromeda Nebula must be of the order of a million light-years,” based on some dim novae detected within the nebula and the faintness of its brightest stars. “The difficulty is obvious,” he continued, “in reconciling van Maanen's measures of internal proper motion with the hypothesis of external galactic systems. We are not prepared to accept velocities of rotation of the order of the velocity of light.” The issue in question was not whether spiral nebulae truly rotate. In the 1910s Vesto Slipher had already detected evidence that they spin around. The proof was found within the spectra of the spirals that he was examining. Like a Frisbee thrown outward and spinning in flight, the rotation on one side is directed forward, adding to the measured velocity, while on the other side the spin is aimed back, subtracting from the overall speed. This difference manifests itself as a slight inclination in the spiral's spectral lines. But this motion was certainly not rapid enough to notice by eye alone when comparing photos taken just a few years apart. Moreover, the spectral signatures indicated that a spiral was closing up, wrapping its arms tighter around the nebula's center, “like a winding spring,” reported Slipher. But this clashed directly with van Maanen's claim that a spiral was opening up. Slipher, so modest and reticent, didn't make an issue of this contradiction. If he had made a clamor, loudly and persistently publicizing his proof, van Maanen's assertions might have been dismissed far earlier. But, as it turned out, Slipher's conflicting result was essentially neglected, occasionally discussed among astronomers privately but rarely singled out in print.
Shapley soon asked his Princeton adviser, Russell, whether he too questioned van Maanen's rotations. “V. M. does a little, Hale a little more, and I much,” wrote Shapley. In time Russell replied that he was “inclined to believe in the reality of the [spirals'] internal proper motions, and hence to doubt the island universe theory. But if [the spirals] are not star clouds, what the Dickens are they?”
Considering what happened soon after