The Pipe Nebula (Barnard 59) is a dark nebula. It is conspicuous for the
absence of light it causes in this region of the sky, as dense swirls of other-
wise invisible dust block the light of the bright Milky Way behind it. ESO
32 ASTRONOMY • JUNE 2015
of waves, which require some medium through which to travel.
Sound waves need air to carry the thumping bass from a teen-
ager’s car radio to a pedestrian’s ear. Similarly, illumination
waves from the Sun or the stars must require a medium to carry
light’s pulsations from there to here. The anti-nothing lobby thus
included members of the scientific, religious, and philosophical
communities. They ruled. If you were pro-vacuum, you were a
nut job — though there were always a few dissenters, such as the
noted mathematician and physicist Blaise Pascal. The universal
stuff assumed to fill all space was first called a plenum, then an
aether, or ether. Its existence was a given for centuries.Evidence of absence
The ether-belief took a serious blow following one of the most
famous demonstrations in history: the Michelson-Morley experi-
ment, conducted in 1887. Albert Michelson argued that if Earth
plowed through an ether, then a beam of light traveling in the same
direction should get a speed boost and ref lect back faster than a sim-
ilar light beam aimed at right angles to it. With the help of Edward
Morley, Michelson used an apparatus attached to a stable concrete
platform floating atop a pool of liquid mercury. The multiple-mirror
device rotated easily without introducing unwanted tilt. The results
were incontrovertible: The light that traveled back and forth across
the “ether stream” accomplished the journey in exactly the same
time as light going the same distance forward in our planet’s travel
direction. Either Earth had stalled in its orbit around the Sun, or the
ether didn’t exist.
Albert Einstein settled the matter a few years later. In 1905, his
first relativity theory showed that light travels happily through a vac-
uum. Nothing need convey its waves of electric and magnetic pulses.
This was good news. It hadn’t really made sense for the planets to
be passing through a substance without the slightest resistance. It
was time to ax the ether with a good riddance. Now fashion totally
swung the other way, and “nothing” pleased everyone. Even the
church was no longer anti-vacuum.Dust in the wind
Ah, but not so fast. Light from distant stars showed the spectro-
scopic absorption lines of an intervening material. Some skimpy
stuff — mostly hydrogen atoms, as well as atomic fragments like
protons and electrons — must be occupying space after all. Simple
calculations revealed that, on average, one atom f loats within each
cubic centimeter of space.
The degree of vacuum depends on the neighborhood. Around
here, the Sun sends out a constant stream of disembodied atom frag-
ments. This “solar wind” — the term created by physicist Gene
Parker in the 1950s and confirmed during the first satellite launches
— has an average density of three to six atoms per sugar cube vol-
ume. It’s substantive enough to push comet tails backward like air-
port wind socks and make them always point away from the Sun.While we can infer the presence of dark matter from watching how galaxies
interact, this simulation of the dark matter distribution in the universe lets
us visualize the vast networks of invisible material shaping the cosmos.
THE EAGLE COLLABORATION/J. SCHAYE/R. BOWER/J. BORROW
Power of 10
These pictures highlight the objects that fill space,
zooming in by factors of 100 or 1,000 with every step
(the scale shown here is in meters). Yet at nearly every
level, from the largest galaxy clusters to the smallest
atoms, the distance between objects dwarfs the size of
the objects themselves.
10
26
1023
1021
Observable universe Galaxy cluster Galaxy