BRIAN SCHMIDT: So Paul, you can see that the distance
ladder is a little complicated.
And we have this nasty stuff, like dust, that is spread through the cosmos,
causing us problems.
PAUL FRANCIS: Oh, dust isn't nasty.
Dust is really pretty.
I mean just look images like this taken at the Anglo Australian Ttelescope.
What you're seeing here is stars.
And these dark patches here, it's not gaps in the stars.
It's dust clouds, interstellar dust clouds,
blocking the light from the background things.
This is just gorgeous.
You can even see it with a naked eye.
Here's a picture I took of the 74-inch telescope at Mount Stromlo.
We're showing the center of the Milky Way in the background.
And what you can see up here is the dark bits, the dark lanes
in the middle of the Milky Way, is where the dust is blocking the stars.
But also, off to the slide, you can see it's got a somewhat reddish color here.
And that's where there's not the really thick dust.
There's a little bit of thin dust that's blocking some of the light.
Now, this is just really pretty.
And you can see it in other galaxies as well.
And once again, it makes things gorgeous.
This sort of spiral pattern you're seeing here
is not actually where the stars are.
The stars are pretty uniformly distributed.
The spiral pattern is mostly due to dust blocking out certain areas
and making it all look so gorgeous.
So how can anyone not like dust?
It's just so gorgeous.
BRIAN SCHMIDT: Pretty, it might be Paul.
But, unfortunately, in terms of what I like to do,
which is to measure distances, dust is everywhere.
And the fact that it helped create the Earth,
is no solace to me when I try to measure the distance to this galaxy.
So this galaxy, for example, is full of Cepheids.
The Cepheid variable stars are scattered around it.
But most of them end up landing, with some amount of dust, unknown,
in front of them.
And that means when I try to measure the distance, I get too far of a distance
whenever there's dust in front of it.
And I get about the right answer when there isn't.
PAUL FRANCIS: And you have to bear in mind,
you see, for example, really strong dust here.
But there's probably dust there, and there, and there as well.
It's just a bit fainter.
But almost every line of sight, through almost any spiral galaxy,
is going to go through at least some dust.
There are no real dust-free views.
BRIAN SCHMIDT: That's right.
Only if they're way up on the top side, sort of out of the plane of the galaxy.
And these big, dark areas, we don't worry too much about.
Because there's so much dust there, you just can't even find the stars there.
So that's not the problem.
It's the insidious stuff that's everywhere.
It's like-- well, it's like the dust around your house.
It's always there.
And there's nothing you can do to get right of it.
PAUL FRANCIS: Well, is there a dust that's actually not
very much like the dust around your house?
I mean the dust around your house is actually
mostly flakes of skin, human skin, which is definitely
not the major contributor to the interstellar dust.
In fact, it's probably more like smoke on earth.
The particles are very much smaller than dust.
You couldn't see them with the naked eye.
They're probably microns, or even nanometers,
across in size, very tiny grains.
And this means, because they're so small, they're comparable in size
to the wavelength of light very often.
And so they have a really strong effect on things
about the same size as they are, which typically means blue light.
Those that have less and less effects at longer and longer wavelengths.
So what we're doing here is I'm plotting the wavelengths,
starting with very blue, and going out to the side infrared.
And we're plotting what fraction of light can get through a cloud of dust.
Now, if you have a thin cloud of interstellar dust,
it'll looks something like this curve over here.
So the infrared light mostly gets through.
Maybe about 99% of it's getting through.
But as you get to shorter and shorter wavelengths,
the fraction getting through gets less, and less, and less.
And so you're using maybe 40% of the light in the blue.
Now, if you ran up the amount of dust, and have more,
you might get something like the second curve.
In the infrared, you're now losing 2%.
By the time you're down here, in the green or the blue,
you're losing nearly all the light.
And at even more dust, it gets worse still.
But it gets worse primarily at the blue.
So this, it occurs, is also a way to find out how bad the dust is
and to correct for it.
So what you could do is instead of just taking a picture at one wavelength,
you could, say, pick two wavelengths, say a blue and a red wavelength.
And you measure how bright your Cepheid variables, or your supernovae,
or whatever they are, at these two wavelengths.
Now, you have to know what color they are if there's no dust.
You probably just take the bluest thing you can find and treat
that as it's got no dust.
It's probably still has small amounts of dust.
But you can mainly ignore that.
And then for another one, you can say, well, it sits down at 10% in the blue,
compared to the red.
That probably means it's on this curve.
Whereas if it's down by twice as much in the blue and the red,
it's probably this curve.
If it's down near 90 times as much in the blue as the red,
it might be that curve.
So by looking at two colors and seeing-- you don't how much it's down in either,
but you could look at the ratio.
And if it's a very big ratio of red to blue,
that means there's lot of dust going on.
It looks red to the eye and to the telescope.
So surely, you could measure the color and say, well, that one looks very red.
Therefore, it must be on this curve.
And that will then read out from the curve, how much of a correction
to make and look at how far it really should be.
BRIAN SCHMIDT: Oh, Paul, if only things were so easy.
So there's a number of problems.
The first problem is-- let's just take Cepheid variable stars,
our venerable tool for measuring distances.
It turns out that Cepheid variable stars have a range of colors.
So when I do this ratio, I don't know what to compare it to.
Is it the blue part of the Cepheid or the red part?
So that means I'm going to have uncertainty
when I apply it to a given Cepheid.
But that's just the beginning problem.
So a type Ia supernova, for example, I could try to use it.
Now, type Ia supernovae are pretty well-behaved,
we think, in their color, although probably not perfectly.
But then we look, and not all dust is the same.
You plotted these curves like they're the only ones.
But, in fact, there can be different dust laws
because the dust has different sizes.
And we see this with the type Ia supernovae all the time.
There seems to be bigger dust and littler dust.
And we don't know which it is, unless we get huge amounts of data, which
is just simply not practical in most cases,
PAUL FRANCIS: Yeah.
I've got two different dust extinction curves.
And they both have the same ratio of red to blue.
But there are actually quite different amounts of absorption,
twice absorbed as the other.
And we know this is a problem.
I mean this is actually some work I do because I like dust.
I actually try to measure properties of dust in distant quasars.
And you can find, particularly at blue and ultraviolet wavelengths,
they're also just features.
The dust is probably produced in winds of red giant stars mostly.
But then some of it is destroyed, particularly through hard radiation,
like a supernovae or a quasar is nearby.
That can destroy some of the dust grains in the other ones.
There can be chemical reactions that change them, catalysis on the surface.
That has to be really complicated.
But different sorts of dust properties and different ratios
of big dust to small dust, you can get very different curves.
So this color method may work as an average.
But it's not that great.
BRIAN SCHMIDT: It's true.
So the obvious thing to do is just not to work here, but to work over here,
where the effects are much smaller.
PAUL FRANCIS: And indeed, here's an optical picture of galaxy NGC 253.
And you can see it's very broken up by dust.
If you look at the same thing out at the wavelength of two microns
in the near-infrared, from the two-micron sky survey--
BRIAN SCHMIDT: It's almost a completely different looking galaxy.
And you can see the stars are the same.
So it gives it away.
But, yeah, all those stars.
It has a big bar across the thing.
It looks completely different.
PAUL FRANCIS: There's still a bit of dust you can see in here.
But it's got very much better.
So working at the infrared works well.
I mean some work I did-- this is what's called
a blob, a giant-forming cloud of gas in the early universe, Blob 6,
this one is.
Look at the optical, you don't see very much.
At the infrared, it's a whopping great collection
of massively star-forming galaxies.
You wouldn't even see this over here.
So it makes a huge difference going out to the infrared.
BRIAN SCHMIDT: So the infrared is clearly where we want to work.
The problem is observing in infrared is really, really hard from the ground.
It requires, to do accurately, very big telescopes, very good sites.
And it turns out that our detectors are really problematic in the infrared.
PAUL FRANCIS: Yes.
We normally use charged-coupled devices, the silicon
chips the same as you've probably got in your cell phone or digital camera.
The trouble is silicon has a band-gap energy that
corresponds to a photon with a wavelength of about 1.1 microns.
What that means is to detect a light falling on it,
the photon has to have enough energy to knock an electron away
from its lattice position.
And anything in the infrared can't do that.
BRIAN SCHMIDT: Yup.
They just sort of go right through the silicon.
It's transparent, effectively.
PAUL FRANCIS: So you start needing to use exotic semiconductors, instead
of silicon, which you can pick up on any beach.
You start using indium antimonide or mercury-cadmium-telluride.
And that puts the price though the roof, a,
because the elements are rare and expensive.
But secondly, because they're not being manufactured
in millions of camera phones all over the place.
It's a very complicated, difficult manufacturing process,
which puts the price through the roof.
BRIAN SCHMIDT: And the other problem is these same detectors
are used by the defense industry, which means it's actually really hard to even
get them at all because they're considered to be "top secret."
PAUL FRANCIS: So many countries are not allowed
to import these things at all because of the military export rules.
But even if you've got these wonderful detectors,
then you've still got a problem.
And that's at most infrared wavelengths, you're
close to the wavelengths at which the black body of Earth's temperature
emits.
So try to work at, say, 5 micron wavelengths,
your telescope is going to be glowing.
Because at 5 microns, you and I, and the telescope,
and the dust in the Earth's atmosphere are all glowing like crazy.
So it's like trying to work in the dome with the lights on and the telescope
made of fluorescent tubes.
BRIAN SCHMIDT: Yeah.
So you have that background problem.
And then you have another problem, which is the diffraction of light.
When you're working in the infrared, the telescope
is closer to the wavelength of light.
And so your resolution shrinks down.
So we want to look at something like a Cepheid,
normally, if you're looking at it in the blue light,
you can see it because it's a nice, sharp image.
When you look at the infrared, it's blurry.
So all these things make it really hard to observe in the infrared.