“A fact is a simple statement that everyone believes. It is innocent, unless found guilty. A hypothesis is a novel suggestion that no one wants to believe. It is guilty, until found effective.” -Edward Teller
The idea of black holes has gone from a curious thought experiment to a theoretical likelihood to a near-certainty — with thousands of known candidates — in a very short time.
But never before have we been able to image a black hole directly; all we have are artists’ renditions of what they might look like.
And for the first time ever, all of that may be about to change.
Above is a series of images taken of a star that exploded — more or less — about 1,000 years ago. And you’ll notice that the infrared, visible light, and X-ray images are far superior in quality to the others (e.g., the radio). It isn’t just this object (the Crab Nebula), either. Take a look at the supernova remnant from Cassiopeia A.
Hmm… this looks a little different; this one doesn’t have way lower resolution in the radio than in the IR, visible, or X-ray. What’s going on? Let’s start by explaining the lower-resolution image.
- The infrared (from Spitzer), the visible light (from Hubble), and the X-ray (from Chandra) images are all taken from space, where there is no atmospheric distortion to contend with. Despite incredible recent advances in adaptive optics, observing from space will always be superior — all other things being equal — to observing from the ground. And…
- X-ray, visible, and even infrared light is much shorter in wavelength than radio waves.
Why should this second thing matter? Let’s take a look at a good radio telescope.
This telescope — in Arecibo, Puerto Rico — is huge. That is, in fact, a mountain that the primary mirror is built into. With a diameter of 1,000 feet (or over 300 meters), it is about thirty times the size of the world’s largest optical telescopes.
And yet, it is far less powerful. Why’s that? Because radio waves are huge! While visible light has a wavelength such that one million wavelengths could fit across you from shoulder to shoulder, you could fit about one radio wavelength in that same span.
And that’s what determines your resolution: how many wavelengths across your telescope’s primary mirror is.
But radio telescopes can do something special in a way that no other type of light can.
Instead of building one giant dish, we can build two (or more) small dishes a great distance apart. If their observations are synched up, and they observe the same object at the same time, we can use the technique of astronomical interferometry to “see” with a resolution equivalent to the distances between the telescopes!
In principle, building a huge dish that same size would still be better, because in addition to the resolution, you’d also get the huge increase in light-gathering power, which you don’t get for building multiple small dishes. So it’s like looking up at the sky with incredibly sharp eyes, but you’re wearing very dark sunglasses; that’s the trade-off. But this interferometry trick is true for all wavelengths of light. In the visible part of the spectrum, the twin Keck telescopes (below) have taken advantage of this for more than a decade.
But radio can go one step further. Due to difficulties synching these telescopes’ observations together, the optical telescopes (above) need to be relatively close together, as you can see. But thanks to a technique developed by Roger Jennison in the 1950s using atomic clocks, radio telescopes can be placed incredibly large distances apart.
And this gives us not just interferometry, but a special type known as VLBI, or Very Long Baseline Interferometry. What does the ability to do this mean? Let me show you.
In 2004, the European-VLBI Network took an image of a radio source, above. As you can see, the resolution isn’t so great. But using this VLBI technique, which can take data from telescopes on opposite sides of the Earth, i.e., with a baseline of thousands of kilometers, allowed us to obtain the highest-resolution image of all-time.
With a resolution many times superior to that of the Hubble Space Telescope already, radio astronomy using this VLBI technique is already the best in the world in terms of resolving bright, point-like radio sources.
Well, if you wanted to see a black hole, and you were living here on Earth, where would you look?
You’d go and look for the closest, largest, most radio-loud-and-active black hole to us. Which is in the heart of the nearest large cluster of galaxies to us: the Virgo Cluster. At “just” 53 million light years away, the galaxy M87, living deep inside the Virgo Cluster, has a black hole that’s more than a thousand times as massive as our own, and it is active.
Let’s take a look with the Hubble Space Telescope to see what I mean.
That’s pretty spectacular, isn’t it?! But using radio-VLBI techniques, we have already obtained resolution of this active black hole (and the emitted jet) that is fifty times greater than Hubble can give us. Want to see?
The “1 pc” bar indicates a distance of just about three light years, and remember this thing is over 50,000,000 light years away!
But if we wanted to image this black hole directly — in other words, to see with resolution so good that we could resolve the event horizon of the black hole — we’d need a much bigger interferometer than the Earth can give us.
How could we make that happen?
You send a giant radio telescope out into space. As long as you can send an atomic clock with it and synch it up with telescopes on the ground, you can use this same technique. Only, instead of thousands of kilometers, you can make the interferometer distance hundreds of thousands of kilometers!
Well, guess what launched on Monday?
The Russian Radio Telescope, RadioAstron, went up into space aboard this Zenit rocket, launched from Kazakhstan! (Story also at Wired and New Scientist.) And in order to get the longest possible baseline from this rocket, you know where it went?
Out as far as the Moon! Over its five-year mission, it will achieve a maximum distance of 390,000 km away, or about ten times the current distance record.
And although its science goals are very lofty, and include planet-finding, neutron-star imaging, and galaxy rotation, the most exciting one is obvious.
To image the event horizon of a black hole. With a resolution down to seven micro-arc-seconds, or 10,000 times that of Hubble (!), M87’s black hole is primed to be the first black hole ever to have its event horizon measured and imaged by us.
Personally, I can’t wait to see. Pics as soon as they’re released, I promise!
(And thanks to Alex at RealClearScience for recommending this story to me!)