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 travelling faster than the speed of light.
Almost five times faster, in fact, as measured by the Hubble Space Telescope. This feat was first observed in 1995 in galaxy M87, and has been seen in many other galaxies since. It might have you questioning your entire reality. Nothing can break the cosmic speed limit, right? You can’t just flaunt the laws of physics… can you?
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 that travel more quickly than the speed of 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’ head 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 travelling at ludicrous speed.
One of several
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.
It is difficult for faraway objects like galaxies to change noticeably over a short period of time, but jets like the one in M87 do so by appearing to move faster than light. This enables astronomers to calculate with great accuracy how quickly the plasma is travelling and, consequently, how powerful the process is. M87 is unique because, in comparison to other galaxies, it is quite close and simple to examine. Astronomers were able to observe this plasma ripple in 1999 using Hubble images of the jet acquired over a four-year period. Meyer extended that to 13 years of photos in 2013, and as if things weren't difficult enough, it appeared that the plasma may also be flowing in spirals resembling corkscrews.
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.
She still pauses occasionally to acknowledge the faster-than-light effect even though she is familiar with it. The majority of celestial objects we observe moving across the sky, such planets and comets, are in our vicinity. M87 is millions of light years away, though.
“We can see, over a human lifetime, things moving,” she says. “Which is crazy.”
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