An excerpt from Cosmic Vertigo, the show where Amanda Bauer and Alan Duffy discuss physics and all things beyond Earth. Here they discuss the immense speeds of galaxies, planets and other celestial bodies.
Robyn Williams: And so to end, some Cosmic Vertigo, from the second series of this popular podcast. Alan Duffy and Amanda Bauer. She is now at the University of Arizona at Tucson, she was formally at the telescope on Siding Spring Mountain in Coonabarabran. And Alan Duffy, Eureka Prize winner, is at Swinburne University in Melbourne. So what's fastest up there?
Amanda Bauer: So I'm sitting still in this room that I'm in, but what does that actually mean? How fast am I moving while I'm sitting still? We've talked about the Earth rotating around and we're on the surface and so we are barrelling along with the surface of the Earth, even though we don't really feel that. And the Earth itself moves around the Sun. We are about 10 million km away from the Sun and it only takes us 365 days to go all the way around it. So that means we are clipping along at a fairly impressive 30 km a second. So just sitting here doing nothing, the Earth is orbiting around the Sun at a speed of 30 km a second. At that speed you could get from Sydney to Perth, all the way across Australia, in about two minutes. Or, equivalently, all the way across the US from San Francisco to Washington DC in two minutes at that speed. It's pretty fast. And we're not even the fastest planet orbiting around the Sun, Mercury is going at 47 km a second.
Alan Duffy: But let's keep zooming on, let's consider bigger reference frames. Our Sun is moving around the Milky Way, orbiting an entire galactic year takes 230 million Earth years. That speed is 230 km/s. So this is the speed that we are all experiencing right now as we sit, not moving, we are actually barrelling around the Milky Way at 230 km/s.
Amanda Bauer: Even though we are all barrelling over at 230 km a second, most of the stars nearby us don't really look like they are moving that quickly relative to us. We are all moving within tens of kilometres a second of each other's speed.
Alan Duffy: So it's like our cars on a freeway; our neighbouring stars don't appear to move that much relative to us because we are all moving at incredible speeds but just in the same direction, give or take.
Amanda Bauer: With one famous exception which is called Barnard's Star. This is cruising around at 142 km a second relative to us, so it's not even that far from us but you can actually perceive within a human lifetime how much this star moves relative to some of the other stars in the sky, which is incredible. That really doesn't happen. The only things we can see moving relative to other stars are planets, so they're not stars at all, or the Moon moves across pretty quick. So this star, its position on the sky moves at something like half a Moon's width in 100 years or little bit less than that, that's a lot in terms of how quickly things actually move on the sky in our lifetimes.
Alan Duffy: But I guess that gives a sense of how far away this thing must be for a speed of 142 km/s, to only equate to that much on the sky, but yet trust us when we say in astronomy terms that is alarming, to see something move across the sky that fast.
Let's continue to grow our reference frame, and now we consider we are sitting still here on Earth, but yet the galaxy itself is moving. We are travelling towards the Great Attractor, a beautifully named, very apt description of something that lies very far away but there's a lot of it so its gravity is enormous and pulling us ever closer, ever faster and we are currently travelling at over 2.2 million km/h, that's 610 km/s towards this Great Attractor.
Amanda Bauer: And it's weird because we don't actually know what the Great Attractor is. We've measured this motion in a few different ways now, but it's something, it's big, it's got a lot of gravity, and we are flying towards it, and it's got a good name. That's all we know.
Alan Duffy: 610 km/s is obviously very fast, I'm not saying it's not, but in terms of collision speeds for galaxies or even larger objects, it's nowhere close to the record holder.
Amanda Bauer: The fastest collisions of objects in the universe, one of the fastest ever recorded was actually the merger of black holes. So depending on how you measure it, this is somewhere around two-thirds the speed of light. So we are getting so, so fast that they just zoom around each other so quickly that they really increase their speed to a ridiculous rate.
Alan Duffy: Yes, we sort of use phrases like relativistic speeds and two-thirds the speed of light, that is 200,000 km/s. That is monstrously quick, and these are huge objects, so when they eventually do collide, this is why you can have such extraordinary outbursts of energy because they are moving that quickly.
Amanda Bauer: We are starting to get into the realms where we are making observations of things in the universe that might appear to be close to the speed of light, actual matter, particles that are close to the speed of light. One of my favourites is something that we call a blazar. At the centre of galaxies, supermassive black holes live at almost every galaxy that we've measured, and mostly these black holes just kind of sit dark and quiet, they are not very active, but sometimes if gas gets very close to it it starts spinning around and heating up and forming what's called an accretion disk around that black hole, emitting lots and lots of energy, and these are called active galactic nuclei. But in some cases some fraction of the material that's falling into that black hole gets sent off away from the black hole, shooting up and down along the access of its rotation to form these things that we call jets, and these things are shooting out actual particles of gas that are moving with speeds close to the speed of light.
Robyn Williams: Alan Duffy and Amanda Bauer with a piece of Cosmic Vertigo. Hear the whole pod or the whole series by using the link on the Science Show website. Amanda Bauer is at the University of Arizona, and Alan Duffy at Swinburne University in Melbourne.