A short post on the James Webb Space Telescope, and the problem of looking for light from over thirteen billion years ago
With an aside on light more generally, and an aside from that aside on very large radio telescopes that get moved around on trains
My daughter Sophie gave me the idea for this post. (She has been helping me edit my Rabbit & Bear books since she was five: she is now seventeen, so at this point she’s pretty good at noticing where I have screwed up.) In an early draft of another post, I had thrown away the description of the James Webb Space Telescope, and the problem it was designed to solve, in a couple of lines, because I was fairly familiar with that stuff. Sophie argued that it was too complicated, and important, and interesting, for that treatment; that most non-scientists don’t REALLY understand this stuff, even if they think they do; and that it was therefore worth describing in more detail.
So, here we go. If you already know all about the James Webb Space Telescope, and the problems involved in trying to see redshifted light from the early universe, no problem, just skip this post.
Everybody else: hi!
SENDING BACK PICTURES OF THE EARLY UNIVERSE… WAIT, WHAT?!
As I’ve mentioned, I’ve spent the last decade researching a book that redescribes the universe.
And for that decade, I’ve worked on it in private.
Well, clearly I'm not working on it in private anymore.
The James Webb Space Telescope launched on Christmas Day 2021, and is now a million miles away from Earth, cooling down and calibrating its instruments. Very soon, probably on July 12th, it will begin sending back pictures of the early universe. And I need to get some predictions in ahead of that.
Sending back pictures of the early universe… Yeah, that’s the line that my daughter said needed, um, a little expansion. And she’s right. That’s an achievement so stunning, so crucial to the predictions, and so hard to fully take in, that we should probably unpack it a little. (And again, you can skip this post if this stuff is already familiar to you.)
So… let’s go back.
BORNE BACK CEASELESSLY INTO THE PAST
Because nothing can travel faster than the speed of light – not even, um, light – all telescopes are always looking into the past.
If you're just looking around our neighbourhood – ie, all the places that humans have ever been – that's not a big deal. Light travels at 300,000 km every second, and the Earth is only 40,000 kilometres around, so you can see anything on Earth pretty much in real time (even if you have to bounce the signal off a satellite).
Even light from the moon, about 385,000 km away, gets here in not much more than a second.
But light from our sun takes eight minutes to get here.
Light from the next-nearest star (Proxima Centauri, absolutely rubbish little star unfortunately) will take four years to get here.
Our galaxy is a complex, swirling spiral, containing one hundred thousand million stars, and light from the far side of it takes 100,000 years to get here. (Put another way, our galaxy is 1,000,000,000,000,000,000 kilometres across: any light reaching us from the far side of it started out before the last ice age.)
Light from the next nearest spiral galaxy, Andromeda, started its journey shortly after our ancestors in Africa started using stone tools, two and a half million years ago.
And the edge of the visible universe is roughly 20,000 times further away than THAT.
So light from the early universe is still arriving on earth today, after an incredibly long journey.
But looking at things that are further away isn't simply a matter of building a bigger telescope to magnify and see them. Visible light from the early universe – light in the frequency range that human eyes can detect – is no longer visible to human eyes by the time it arrives here. Why?
Because, as you look further and further away (and thus further back in time), something else starts to become significant: all the light you see is not just dimmer, but also redder, as its wavelength stretches out and gets longer. That is, there is not just less light, but that light has less energy.
To understand what’s going on here, it might be useful to have a quick aside on light…
A QUICK ASIDE ON LIGHT
Remember, humans can see light in an astonishingly narrow range – from .4 to .7 nanometers. So, from just under half of a billionth of a meter, to just over two thirds of a billionth of a meter. (Why did we evolve to detect those particular wavelengths? Because that’s the frequency range in which our sun produces the most light.)
In other words, we can see the thinnest possible slice of an enormously (almost infinitely) broad potential spectrum, that extends away in both directions. Light exists, to all intents and purposes, at all wavelengths. There is light millions of times shorter than the light you can see – and billions of times longer.
Light with a very short wavelength can punch right through solid matter with so much energy, it’s dangerous. That is how x-rays work: light punches right into you, some of it is stopped by your bones, and the remaining light, as it exits you, is then captured to make a picture of your insides.
And if you think that is pretty hardcore behaviour for a beam of light, well, gamma rays are a thousand times shorter, and thus punchier, than that. In fact, gamma rays can be shorter than a trillionth of a meter in length: they are so short in wavelength, so compact, so punchy, so small – that they pass between the atoms of detectors. (To detect gamma rays, you need to use a big, solid block of some densely packed crystal, and hope they hit something on the way through.)
Indeed, a gamma ray (the Dracula of photons!) is not reflected by a mirror, because the gamma ray is so small it doesn’t see the mirror as a flat surface, but as lots of atoms, far apart. And so the gamma ray just whizzes through the gaps.
Put another way, it takes a thousand visible light waves to cross the width of a single human hair. But you could fit a million gamma waves into each of those thousand visible light waves…
By contrast, light with a very long wavelength, like the light we call radio waves, is so low in energy, it’s completely harmless. The radio waves we listen to on an FM radio are a couple of meters long; the radio waves on a crackly old AM radio can be hundreds of meters long.
AN ASIDE TO THE ASIDE – THE WEIRDNESS OF RADIO TELESCOPES
The reason radio telescopes are so damn big, by the way, is because radio waves are so damn big. To get a clear picture of what you’re looking at, when each photon – each dot of light making the picture – is several meters long, you need a REALLY big telescope. (It’s like painting with a giant brush: to get any detail, you need a REALLY big sheet of paper.)
The Very Large Array in New Mexico, for instance, combines 27 individual dishes, arranged in a Y shape, to make one huge radio telescope, 36 kilometres across at its widest. Oh, and to change the focus of the telescope, they don’t just have to tilt all the dishes up and down; they have to physically move the dishes closer to each other, or further apart, on a train. (The Y of dishes are set along a Y of railroad tracks.) And not a small train, either: each dish is 25 meters across, and weighs over two hundred tons…
Does that mean we can, by using different size telescopes, see all the light the universe produces?
Some radio waves are millions of miles long. Have fun trying to focus that…
End of Aside to the Aside; back to the original aside…
THE ORIGINAL ASIDE, ON LIGHT, CONTINUED
Thing is, we’ve known since 1929 that, the further back you look, the weirder the light behaves. Light that should be visible to our eyes, for instance – the light of stars like our own sun – changes, as the source gets further away. Its wavelengths get lower, its energy falls, and it becomes harder and harder to see, until it drops below the frequency/energy range our eyes can detect at all.
OH, THAT’S WHY IT’S CALLED REDSHIFT!
Yes, the process is called redshift, because it shifts visible light towards the red end of the spectrum.
For example, the wavelength of red light is about twice as long as the wavelength of blue light: so, stretch blue lightwaves to twice their original length and they will look red. Stretch red light, and it will become infrared: the wavelengths are now too long for our eyes to detect, and we won’t be able to see it at all.
To understand why this happens, you need to understand what's been happening to spacetime, and thus to the light travelling through it, since the Big Bang.
THE BIG BANG WAS NOT A NORMAL EXPLOSION
There is a very human tendency to think of the Big Bang as an explosion, but that’s not quite right. An ordinary explosion expands out into the surrounding spacetime – if a bomb goes off in a street, the debris is scattered all over the street. But in the Big Bang, spacetime itself is expanding, which stretches everything in the universe further and further apart as time goes on. It’s more as though the street itself were getting longer, and the gaps between the buildings growing larger…
To imagine how this is happening to the entire universe, draw some galaxies on a deflated balloon. Now blow it up, and watch the space between the galaxies get bigger, even though the ink (=matter) of the galaxies isn’t moving relative to the rubber surface (=spacetime) of the balloon. The expansion of spacetime is a bit like that.
That doesn’t mean everything is receding from everything else. Things that are close enough to be bound together by gravity will remain together – you will stay on the surface of the earth; the earth will continue to orbit around the sun; the stars in our galaxy will continue to rotate together, and the local cluster of galaxies will stay in touch with each other. (Indeed, our neighbouring spiral galaxy, Andromeda, is so gravitationally attracted to our own Milky Way galaxy that we are due to collide in about another four or five billion years.)
But, as time goes by, our cluster will continue to recede from all the other gravitationally-bound galaxy clusters, as they all recede from each other, until (eventually, after many tens of billions of years) nothing in the universe will be reachable by us, even at the speed of light, except our local cluster…
SO, HOW MUCH HAS THE EXPANSION OF SPACETIME SINCE THE BIG BANG STRETCHED LIGHT?
Because the universe has been expanding for the 13.8 billion years since the Big Bang, the light waves travelling through that space-time have been stretched along with it.
Stretched a lot.
Light emitted very soon after the Big Bang would have been stretched out to dozens of times its original length by the time it finally gets to earth.
That means visible light from the early universe has been stretched so much, it has dropped deep into the infrared by the time it reaches us, and we see it. So, we don't see it (or at least we never have, up till now). Because infrared light is basically heat; the kind of light everything warm gives off.
This is a huge problem for telescopes, because everything on earth, including our telescopes, including the air itself, is warmer than the incredibly faint, extremely redshifted light of the early universe – and our warmth drowns out its faint signal. Imagine using infrared goggles, which are on fire, to look through a nearby burning forest, for a glimpse of a distant icecube.
INTRODUCING OUR HERO, THE JAMES WEBB SPACE TELESCOPE
So to see that far across the universe, and thus that far into the past, the James Webb Space Telescope (which is designed specifically to see that infrared light) has had to travel more than a million and a half kilometres away from the hot, noisy earth; has blocked the hot, noisy sun, with a heat shield five layers thick and the size of a tennis court; and has refrigerated its most sensitive infra-red detector down to minus 266 degrees Celsius… which is only 7 degrees above absolute zero.
It’s the astonishing, triumphant result of deep collaboration between intellectual thought and physical craftsmanship across multiple decades: the technological equivalent of Chartres Cathedral. When I complain about the limits of reductionist materialism, I am complaining about the limits – and our failure to do other stuff that needs doing beyond those limits – I am not complaining about the approach itself, because, holy crap, modern reductionist science can launch a cathedral full of technological miracles into space.
Now we will finally be able to see light that started out well over 13 billion years ago, light that has been stretched to invisibility on its voyage, finally arriving from an early universe we’ve never seen before.
If the argument I laid out in my last post is right – that our universe is the result of an evolutionary process at the level of universes – I can make some specific predictions about that early universe.
But of course, predictions only count if you make them before the experimental data comes in.
So I am breaking cover, and starting this Substack for the book now, in order to make some strong, simple, predictions in public.
Starting next post…
(Yes, I am putting off laying out my actual predictions because I am filled with anxiety about committing. Anxiety? EXISTENTIAL TERROR! But OK, next week, my first predictions.)