Please welcome a skilled illusionist to the stage. The ultimate magic trick is being performed by an energy beam that emerges from galaxy M87 like a toothpick in a martini olive: it appears to be traveling faster than the speed of light.
In reality, the Hubble Space Telescope has shown that it is almost
five times faster. This accomplishment was initially noted in 1995 in the
galaxy M87 and has subsequently been noted in several other galaxies. You can
start to doubt all you thought you knew. The speed limit of the cosmos cannot
be exceeded, right? You are unable to just ignore the rules of physics, are
you?
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If you want to just enjoy the illusion from your seat in the
audience, stop reading. Otherwise, I welcome you backstage for a look at how
the trick works – and how it’s helping astronomers to understand the fate of
entire galaxies.
Blobs faster than light?
We’ve known about the jet of plasma shooting from the core
of M87 since 1918 when astronomer Heber Curtis saw a ray of light connected to
the galaxy. To be visible from so far away, it had to be huge – about 6000
light-years long.
As modern astronomers now know, pretty much all galaxies
have a central black hole that periodically draws in stars and gas clouds. When
gas begins to swirl down the drain, it heats up and magnetic fields focus some
of it into jets of hot plasma. These jets shoot out at velocities near to – but
not faster than – the speed of light.
Cosmic uncertainty: Is the speed of light really constant?
If you were to aim a telescope into the sky towards M87, you
would see that this lance of plasma is askew. Instead of pointing exactly into
our line of sight, it’s angled a bit to the right. To understand the illusion,
picture a single glowing blob of plasma starting at the base of this path and
emitting a ray of light, both of which travel towards Earth. Now, wait 10
years. In that time, the blob has moved closer at a sizeable fraction of the
speed of light. That gives the rays emitted from that later position a few
light years’ heads start on the way to us.
If you compare the first and second images from Earth’s perspective,
it looks like the blob has just moved across the sky to the right. But because
the second position is also closer to us, its light has had less far to travel
than it appears. That means it seems to have arrived there faster than it
actually did – as if the blob spent those 10 years traveling at ludicrous
speed.
One among many
The jet from M87 is more than just a curiosity, says Eileen
Meyer at the University of Maryland, Baltimore County. All over the universe,
outflows of energy from massive black holes can stop or start the formation of
stars throughout galaxies. But it’s unclear how these outflows work and how
much energy they contain.
By appearing to move faster than light, jets such as the M87
one change visibly over just a few years, which is unusual for distant objects
like galaxies. That allows astronomers to make precise estimates of how fast
the plasma is moving and thus how powerful the process is.M87 is special
because it is relatively close compared to other galaxies, making it easy to
study. In 1999, astronomers used Hubble pictures of the jet taken over four
years to see that plasma ripple outwards. In 2013, Meyer lengthened that to 13
years of images, which seemed to show that the plasma might also be moving in
corkscrew-like spirals – as if it wasn’t complicated enough.
Fresh results from Meyer, now being prepared for
publication, extend that baseline again to a total of more than two decades and
may offer new surprises. “Over 20 years, you know, things go bump in the
night,” she says.
And although the faster-than-light effect is old hat to her,
she still stops to appreciate it sometimes. Most things we see traveling across
the sky, such as planets and comets, are close to us. But M87 is tens of
millions of light-years away. “We can see, over a human lifetime, things
moving,” she says. “Which is crazy.”