快猫短视频

IN SPACE, no one can hear you scream. There鈥檚 nothing out there for sound to
travel through. Puncture the skin of a space station, and鈥攁s the
cosmonauts on Mir found last year鈥攜ou鈥檇 better seal it off pretty quickly
or you鈥檒l soon be breathing a lot of nothing, in a pressure so low your blood
will boil.

And yet, you only have to look at the Hubble Space Telescope鈥檚 spectacular
pictures of gas clouds in space to realise that emptiness is relative. Outer
space is not a perfect vacuum, nor are all cosmic vacuums equal. In some places
space is teeming with atoms, relatively speaking, while neighbouring regions are
much emptier. Over the past few years astronomers armed with radio telescopes on
the ground and a battery of other kinds of telescope in orbit have been mapping
the different vacuums in space. The question is, can they find anywhere in the
Universe where there really is nothing at all?

It鈥檚 not far from the Earth鈥檚 cocooned surface to the beginning of the cosmic
vacuum. As British astrophysicist Fred Hoyle once remarked, 鈥淪pace is only a
two-hour drive away, if your car could go vertically upwards.鈥� And during those
first two hours, you would pass through more gas than in the trillions of years
it would take you to travel the remaining distance to the edge of the
Universe.

For human beings, the region where space shuttles and the Mir space station
orbit seems as near a true vacuum as makes no difference. You wouldn鈥檛 last long
there without an air supply and pressure suit. Yet the vacuum is not
particularly high on a cosmic scale. At 350 kilometres above sea level, the air
is still dense enough for astronauts to see a cosmic St Elmo鈥檚 fire surrounding
the shuttle as it rips through the tenuous gas at hypersonic speed.

The residual atmosphere up there creates a slight but potentially lethal wind
drag on an orbiting spacecraft. In a typical two-week flight, the shuttle is not
affected much. But one famous casualty was the US space station Skylab. Launched
in 1973, Skylab felt a strong enough braking force from the tenuous gas it was
passing through to bring it spiralling down. It reached the lower atmosphere and
burnt up six years later. Mir is continually boosted to save it from a similar
fate.

Earth orbit is sometimes touted as a natural laboratory for studying how
things behave when both gravity and atmosphere are absent. In fact, there are
residues of both. On a crewed spacecraft, thrusters and moving astronauts
produce accelerations equivalent to at least several millionths of the Earth鈥檚
gravity. And the 鈥渧acuum鈥� outside is nowhere near as good as can be achieved by
the best laboratory pumps back on Earth.

Let鈥檚 put this into figures. The range of gas densities across the Universe,
from the Earth鈥檚 atmosphere to the most tenuous cosmic gas clouds, encompasses
so many orders of magnitude that we鈥檇 soon get lost in the trillions and
billionths. Instead, think in terms of how far apart the atoms or molecules are:
the higher the vacuum, the larger the average separation of the gas
particles.

At sea level on Earth, air molecules are jostling so closely that they are
typically just a millionth of a millimetre apart鈥攐nly a few times the size
of the molecules themselves. The world鈥檚 best pumps can achieve an impressive
vacuum that stretches the separation of the molecules to an average of a tenth
of a millimetre.

In Earth orbit, nature falls short of this by a factor of 10. That is one
reason why few physicists have bothered to do vacuum experiments from the
shuttle or a space station. What鈥檚 more, orbiting vehicles are moving so fast
that what the surrounding atoms lack in number they make up in speed, as they
crash into the experiment at 28 000 kilometres per hour.

Brushed aside

But there is a way of putting this speed to use to create a high vacuum. It鈥檚
called Wake Shield鈥攁 simple satellite that has smashed all records for a
human-made vacuum. Wake Shield is essentially a disc of stainless steel, almost
four metres across, shaped rather like a saucepan lid. The shield flies on its
own several kilometres away from the space shuttle, its convex side forward. As
Wake Shield tears through the surrounding gas atoms, it pushes them aside so
rapidly that they don鈥檛 have time to diffuse around the back of the satellite.
The result is a 鈥渨ake鈥� of gas atoms behind the steel shield, and a high vacuum
at its centre.

At this altitude, the average distance between gas atoms is one-hundredth of
a millimetre. Wake Shield increases this to a full millimetre. After teething
problems during its first few flights, Wake Shield flew successfully on the
shuttle Columbia in November 1996. Its onboard automated lab grew several thin
films of semiconductor in the highest artificial vacuum ever
achieved鈥攐pening the way to ultra-pure chips of new semiconductors and
films of high-temperature superconductors.

Go beyond where the shuttle flies, and the Earth鈥檚 atmosphere eventually
peters out. Compared with planetary atmospheres, the gas in outer space is
indeed tenuous. But even so, it is far from empty. No sooner do you rise above
the Earth鈥檚 shroud of air than you enter the atmosphere of the Sun. The hot
gases in the Sun鈥檚 outer layer鈥攖he corona鈥攃onstantly boil away into
space in a solar wind that sweeps out past the planets.

The blustery solar wind is wracked by gusts from magnetic eruptions on the
Sun鈥檚 surface. Although they can light up our skies with magnificent displays of
auroras, and disrupt electricity supplies down on Earth, we are talking here
about storms in a vacuum. The average density of the solar wind is less even
than Wake Shield鈥檚 vacuum, with atoms here about a centimetre apart from each
other.

Somewhere well beyond the orbit of Pluto, the solar wind has thinned out so
far that it is matched by the tenuous gas that fills the space between the stars
in the Milky Way. Interstellar gas is invisible even to the best optical
telescopes, but it has an accomplice that gives it away. Scattered throughout
the gas are tiny dust particles that absorb light from anything lying behind.
Where the gas and dust are most concentrated, you see dark clouds in silhouette.
To the naked eye, the Coal Sack near the Southern Cross is one familiar example
of interstellar matter in bulk. More spectacularly, the Hubble Space Telescope
has revealed vast dusky pillars of gas and dust silhouetted against the luminous
gases of the Eagle Nebula.

Even though these pillars look dense鈥攁lmost solid鈥攖hey are more
tenuous than the best vacuum on Earth, more or less equalling the vacuum in the
Wake Shield experiment. The dust specks that make them dark are only the size of
a particle of cigarette smoke. They are spread so diffusely that you would find
only one, on average, in a volume the size of St Peter鈥檚 in Rome. It is only
because space is so huge that the dust particles amass to become an
all-obscuring fog.

Thick and thin

About half the gas in our Galaxy lies in relatively dense clouds like these.
The rest is spread more widely. Although it is invisible to ordinary telescopes,
its hydrogen atoms emit telltale radio energy at a wavelength of 21 centimetres.
For decades, astronomers thought this gas was spread smoothly, but as it turns
out this isn鈥檛 so. Even leaving aside the dense, dark clouds, the gas between
the stars is a tangle of dense strands and tenuous patches. If you travelled
through the interstellar medium from a tenuous region to a neighbouring denser
strand, the relative increase in density would be greater than diving from
Earth鈥檚 atmosphere into the sea.

Yet even the denser regions contain atoms a centimetre or more
apart鈥攚ay beyond what humans can achieve. In the tenuous patches, the
atoms are another ten times further apart. The difference between the two
becomes clear when nature unleashes its ultimate stellar cataclysm鈥攖he
death of a star in a supernova explosion. Supernovae send out a shock wave that
speeds through space in a fireball and shows up brilliantly to telescopes tuned
to radio waves and X-rays. The shock wave sweeps up the gas it meets like a
snowplough, to form a dense shell.

I have a particular affection for this phenomenon, as it provided my
introduction to the various vacuums of space. As a radio astronomer in Cambridge
in the 1970s, I was checking out the fine details of the fireball left over from
the supernova explosion that was seen from Earth in 1572. It was obvious that
the fireball isn鈥檛 spherical: the expanding shell has been pushed out of shape
as it encountered irregularities in the surrounding medium. Where the
surrounding gas is thinner, the shock travels further and faster; where the
shock hits a denser strand it slows down. Within the fireball, the expanding
shock wave leaves a 鈥渧acuum within a vacuum鈥�. The gas inside the expanding shell
is thousands of times less dense than even the tenuous interstellar gases
outside it. And the passage of the fireball has raised the temperature inside
the shell to millions of degrees.

The discovery of hot, tenuous gas within a supernova鈥檚 fireball was no
surprise. But astronomers in the 1970s were puzzled to find signs of similar hot
and extremely tenuous gas in parts of our Galaxy, the Milky Way, where there was
no sign of recent supernova explosions.

Gas this thin is very hard to study:
with so little material, evidence becomes more and more difficult to find. The
first sign that it exists came from the imprint of its spectral lines on the
ultraviolet light from distant stars. Its existence was confirmed by the
discovery of faint X-rays given off by the gas. Astronomers have now concluded
that half the Galaxy鈥檚 volume is filled with million-degree gas whose individual
atoms鈥攊n fact ions, as they are stripped of their electrons鈥攁re
fully 10 centimetres apart.

This hot gas, it seems, has come from thousands of supernova explosions over
the aeons, each blowing its own bubble in the surrounding interstellar gas. And
as the bubbles grew, they coalesced to produce a Galaxy-sized 鈥淪wiss
cheese鈥濃�攁 network of holes surrounded by denser matter. The hot gas has
also burst out of its colder surroundings, and now envelops the Milky Way in a
faintly glowing halo.

Hot pools

It鈥檚 a good vacuum鈥攖he best in the Galaxy鈥攂ut it鈥檚 still not
perfect. How about the reaches of space beyond? In general, galaxies are not
spread uniformly through space. Most of them are gathered in giant clusters, and
here you might expect to find the denser parts of the intergalactic medium.
Indeed, X-ray telescopes have picked out pools of gas at temperatures of 100
million kelvin or more, bound by the gravity of the galaxies in a cluster. The
density of this pool is similar to that of the gas in the most tenuous parts of
our Galaxy, with a distance between ions of about 10 centimetres.

Astronomers are only beginning to probe the intergalactic regions where
matter is spread even more thinly. It鈥檚 too much to hope that the gas here will
emit any significant amount of radiation. But that does not mean it is
undetectable. The atoms may reveal themselves by absorbing light coming from
bright beacons beyond. And nature has provided astronomers with suitable light
sources: distant and extremely bright galaxies called quasars.

In the spectrum of radiation from the most remote quasars, astronomers can
see dark lines at certain wavelengths. The lines correspond to a hydrogen
wavelength called Lyman-alpha, and the spectra of some quasars are crossed by so
many dark lines that astronomers refer to them as the 鈥淟yman forest鈥�. The
鈥渢rees鈥� making up the forest represent clouds of hydrogen, each one at a
different distance and so 鈥渞ed shifted鈥� to a slightly different wavelength. Some
may be part of a galaxy; others are just clumps of intergalactic gas. In these
clouds, the gas ions are almost a metre apart from one another.

Between these clouds there is very little sign of any matter. This is a
serious conundrum鈥攖he vacuum here is actually too perfect to fit in with
theories of the Big Bang, and in particular the amount of hydrogen that should
have been produced when the Universe was born. If you subtract the amount
observed in galaxies, clusters and intergalactic clouds, there should still be
plenty left over鈥攁nd where else could it be but spread out thinly, filling
the gaps between the trees in the Lyman forest? Yet there is no sign of it.

Thinner and thinner

But theorists have come up with a possible explanation. In the early
expanding Universe, they argue, any small region in which the gas just happened
to be more tenuous than average would lose gas by gravitational attraction to
denser regions to either side. It would end up as the ultimate vacuum: a
鈥渕ini-void鈥� about ten million light years across, and one hundred times more
tenuous than the expected thin intergalactic medium.

Last August, astronomers using the Hubble Space Telescope and the European
Southern Observatory in Chile tracked down a possible mini-void. Observing a
quasar called HE 2347-4342, which lies roughly ten billion light years away,
they found the imprints of intergalactic gas lying in front of the quasar. Some,
as expected, was in relatively dense clouds鈥攖he trees of the Lyman forest.
But some of the gas was spread out in regions ten to twenty million light years
across. Here the density is the lowest yet measured in astronomy. In these
distant mini-voids, atoms and ions are on average some 10 metres distant from
one another.

So here, at the ends of the Universe, lies the physicists鈥� dream vacuum.
Absolute emptiness. Take an experimental chamber the size of a your living room
to one of these mini-voids and open the doors. When all the air has gone,
there鈥檒l be nothing at all inside鈥攐r possibly just a single atom, if
you鈥檙e very unlucky.