Black holes are very interesting things, aren’t they? There’s something fascinating about those things which are so hard to understand. Black holes are one of the most asked about objects at almost any public space talk. Certainly school children seem to be obsessed with them!

So a question I was asked this week (by both John and Margaret) was ‘what is the smallest size of a black hole?’. If a star can collapse into a small space and become a black hole, can a planet or even a proton? Well yes, they can, so long as you squash them down hard enough.

There is no theoretical limit on the sizes of black holes. A black hole is determined by how much mass is being contained within a radius. If you put enough material inside a small enough space, you get an object which is so dense that even light cannot escape: a black hole.

Put another way, if you were to squash down any object far enough you eventually get a black hole. This critical size is called the Schwarzschild radius. For example, the Schwarzschild radius for the Sun would be 3km and for the Earth it would be about 9mm. Just imagine compressing the Earth down to just 18mm across!

The follow-up question from John was a great one:

Newton’s olde cannon fired objects toward the horizon at faster and faster speeds until they reached escape velocity and ‘fell’ into orbit. Fair enough, but this idea was never applied to light because light was just too damn fast and also no one believed it was affected by gravity anyway. Now we know better. So does this mean that if a beam of light approached a black hole at precisely the right glancing angle to match its speed that it would not fall in but form a kind of ‘light in orbit’… like saturns rings but made only of light?   

I had never though of this before - which is why I love people sending in questions. So I went to find out from some people who should know, here at the university, whether light could orbit a black hole. The answer surprised me: ‘yes’.

It turns out that light can orbit a black hole, but not for very long. Just beyond the event horizon of a black hole (the distance at which nothing can escape), there is a short distance in which an incident photon is deflected into a circular ‘photon orbit’. It won’t stay very long in this state though. These orbits are highly unstable and soon the photon would either spiral into the black hole’s event horizon or be ejected outward again. I just really like that this can happen, even it happens for a very short time. I wonder what it looks like?

I hope that this is just as interesting for everyone to learn about. If anyone out there knows more on this topic, I’d love to hear from you.

Black Hole Hunter is an online game. This relates to a post of mine from last week about the sounds of gravity waves. If you like listening to white noise and looking at graphs then this is the game for you! If not, I’d give it a miss. I started getting a headache after a couple of minutes.

Black Hole Hunter

Yesterday there was brief moment when I thought that they had announced the first detection of a gravitational wave by LIGO. Needless to say, this turned out to not be the case. If it were then you would have heard about it - most likely from a newsreader doing their very best.

The paper that caused this trouble describes how LIGO has been used to place a low-limit on some properties of the Crab Nebula pulsar (you can read it here if you like). The way this paper was announced at the AAS meeting in St. Louis made it sound like they had a detection. But they didn’t and don’t (yet). Interestingly, I wasn’t all that excited though. For a few moments I was quite convinced they had finally made a detection and although it would have been historic I could only think of one thing: so what now?

The scientific community has been thinking about gravity waves for a very long time. They are a part of general relativity - although they were conceived of before that. To the non-astronomer, they basically answer the question, ‘what would happen if the Sun suddenly vanished?’. Would the Earth instantly fall out of orbit or wold it take time? Would we see it happened before or after we felt it happen?

The answer is that gravity is ‘transmitted’ via waves and that it travels at the speed of light - so we’d see and feel it at the same time. Gravity is thought to propagate through spacetime, much as light propogates through the electromagnetic field. The ripples it creates in spacetime are very tiny though and so they are extremely hard to detect.

So if you could ’see’ gravitationally energetic events, then what would you do? Well it would allow you to perform a new kind of astronomy. It would open a new spectrum of analysis and viewing on the universe. This new spectrum would range from highly energetic events (e.g. coalescing black holes) to lower energy events (e.g. accretion disks). Mainly it it interesting in the way it would let us look into the physics of black holes.

Huge amounts of money have been ploughed into gravity wave physics. Interestingly the gravity waves groups around the UK always seem to have a lot of money! They use it to meet up and discuss theoretical results. They create lavish PR campaigns and recruit PhD students. They take data with LIGO and its contemporaries. What they do not do, and have not yet ever done, is detect a single gravity wave!

I realise this is political, but it always irks me slightly. Gravity waves have lots of money but no results. ESA is potentially going to bump another project in favour of contributing to LISA, a gravitational wave detector in space.

So I am asking for anyone that knows something about gravity waves to give me some reasons to like the idea of studying them again. I used to when I was a kid. They are very cool, they are high-tech, but they are  - for now - undetected and very, very expensive.

So any general relativity enthusiasts/experts out there who can offer me something to work with? Anyone?

I love NASA’s Astronomy Picture of the Day, known to its friends as APOD. So to honour that Great website, which has been running for more than a decade, I here present my own personal top ten list of APODs. They may not be quite the same as anyone else Top Ten but they represent some of the best things about the APOD site as well as being some of the most impressive images.

10: Mars Then and Now

Two images of Mars taken over 100 years apart. The first was drawn by Eugene Antoniadi in 1894, the second is from Hubble during the close approach of Mars in 2003. Its interesting to see what they got right and what they got wrong. Notably the extensive system of Martian Canals, which Percival Lowell was so adamant existed and proved the presence of Martians.

Credit: Tom Ruen, Eugene Antoniadi, Lowell Hess, Roy A. Gallant, HST, NASA
More Historical APODs - Yuri Gagarin, Voynich Manuscript

9: The Surface of Europa

Back in January 1998, when this image appeared on APOD, the Galileo Mission was still sending back amazing new pictures from Jupiter and it’s Moons. This was around the time I first discovered APOD and have been checking it ever since.

Credit: Galileo Project, JPL, NASA
More Groundbreaking APODs - Shadow of Saturn, Pluto’s True Colours

8: Bubble vs Cloud

This 2005 APOD captured my imagination for some reason. You can really see a three-dimensional effect in this beautiful picture. The actual title of the image is ‘ NGC 7635: The Bubble Nebula’ but the phrase ‘Bubble vs Cloud’ stuck with me somehow. It just got me thinking about how dynamic all of these giant objects are, even if we will never see it for ourselves.

Credit: Russell Croman
More APOD Nebulae - The Eagle Nebula, Orion Nebula, Carina Nebula

7: Close Up of the Face on Mars

After years of being told by people that there was a giant human face on Mars that an ancient civilisation had once created it was satisfying to see it for real and notice that ‘hey, there is no face’.

Credit: Malin Space Science Systems, MGS, JPL, NASA
More Debunking APODs - Equinox Eggs, Green Flash, The Moon Illusion

6: Binary Black Hole

This composite image shows the x-ray in blue and the radio in pink. You can see two black holes in the middle, which are each streaming out relativistic particles. These two objects are 300 million light years away! This APOD from April 2006 was the first time I had ’seen’ a black hole in any convincing sense.

Credit: X-Ray: NASA/CXC/D.Hudson, T.Reiprich et al. (AIfA); Radio: NRAO/VLA/NRL
More Unseeable APODs - Dark Matter, Neutrinos

5: A Sun Pillar

One of the things APOD does best is show not only planets, nebulae and other distant objects, but also some cool, Earth-bound astronomical tit-bits. This Sun Pillar is created by ice crystals in the atmosphere. They are best seen during the colder months.

Credit: Stan Richard
More Earthbound APODs - Giant Machine, Lenticular Clouds, Aurora

4: Earthrise

This is a Christmas Eve APOD from 2004. The site has never been afraid to put up historical pictures or to be a little poetic when it feels like it. This image was taken around ChristmasEve when the three Apollo 8 astronauts were orbiting the Moon. They returned safely on December 27th, in time to see the 1967 Sunrise back on the Earth.

Credit: Apollo 8, NASA
More Poetic APODs - Martian Love, Huygens Discovery, Liberty Bell

3: Hyperion

Like a 250km sponge, Saturn’s Moon Hyperion is covered in odd craters and has a density so low that it has led researchers to assume it made up of vast deep caverns. This image shows the moon in excellent detail. The images on APOD have steadily improved with improving technology over the years and this amazing Hyperion picture is a great example.

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
More High Resolution APOD - Annual Eclipse, Mars HiRISE, ISS Silhouette

2: Blue Lagoon

Sometimes you see a picture that just takes your breath away. This APOD was just such a picture. This wonderful Lagoon Nebula picture, which to me, looks like a painting, was not taken by Hubble as many might think but by a 20″ telescope on Earth.

Credit: Russell Crowman
More Artistic APODs - Moon from Space, Sombrero in IR

1: Venus Near the Moon

Not a grand finale in one sense, but a stunning picture never-the-less. What APOD has always done best are images like these, taken not by massive telescopes but by normal observers with affordable equipment. The stuff you can see with your eyes and appreciate instantly without explanation. This crisp, detailed image of Venus near the Moon sums up what so many of us like about astronomy, and why the subject remains accessible to just about anyone.

Credit: Jay Ouellet
More Backyard APOD - Moon Animation, Station and Shuttle, Blue Moon

So thank you to Astronomy Picture of the Day for many years of service to the internet. May you long continue to wow us on a daily basis.

APOD is also available as a Google Homepage Gadget, a Netvibes Module, an Apple Dashboard Widget, a Wordpress Widget, a Yahoo Widget, a Windows Vista Gadget , as a Twitter Feed and probably loads more.

If you have other APODs you think deserve attention, leave a link in the comments thread so we can all see.