In 1932 a telephone engineer in a New Jersey field listened to the static on a rotating antenna. Most of it was thunder. One faint hiss was not: it came back four minutes earlier every day.
Chapter 10 · Beyond the visible
The Size of a Dish
Light with waves as long as a bus. Where Jansky’s hiss came from, why a radio telescope has to be so enormous, and how astronomers built one as wide as the Earth.
Look first
A hiss that keeps its own time
Karl Jansky worked for Bell Telephone Laboratories, which ran radio-telephone calls across the Atlantic and wanted to know what made them crackle. In a field at Holmdel he built an antenna thirty metres long, a lattice of brass pipe and wood, and set it on four wheels from a Ford Model T so it turned a full circle every twenty minutes. Whichever way it pointed, a pen recorded the static at a wavelength of 14.6 metres.
Through 1932 he sorted the static into kinds. Thunderstorms near and far made sharp crashes. Under them lay a faint, steady hiss that swelled and faded once a day. Below are two of its days, a month apart.
The hiss on two days thirty days apart, with the time of its peak marked. The thunderstorm spikes are random; the swell is not.
Commit to a prediction
Four minutes a day
Day after day, the peak of the hiss came about four minutes earlier: in a month, two hours; in six months, twelve.
What does that tell you about where it comes from?
Time of the daily peak through a year, for a source tied to the Sun and for one fixed among the stars.
The Earth turns once relative to the stars in 23 hours 56 minutes 4 seconds. The other four minutes of a solar day are what it takes to catch up with the Sun, which drifts a degree a day eastward against the stars as we go round it. Anything that keeps time with the stars arrives four minutes earlier by the clock every day. Jansky found his hiss strongest toward Sagittarius: the centre of the Milky Way.
Why the first guesses made sense
A sky you cannot see sharply
If you chose the Sun, you picked the most obvious thing in the sky, and the one Jansky himself suspected at first. But the Sun keeps solar time: its peak would come at the same hour every day. Storms keep no time at all, and the upper air keeps solar time too.
Jansky could say the hiss came from the direction of Sagittarius, and not much more. His antenna was thirty metres across, and his waves were 14.6 metres long. In chapter 2 an objective could resolve detail only down to about the wavelength divided by the size of its opening. A lens 3 mm across, like the pupil of your eye, against green light half a micrometre long, gives about 46 seconds of arc. Jansky’s antenna, against waves fifteen metres long, gave him a blur about thirty degrees wide: sixty full Moons.
Commit to a prediction
A dish as sharp as an eye
Hydrogen gas, the commonest stuff in the universe, glows faintly at a wavelength of 21 centimetres. Radio astronomers map whole galaxies by it.
To see the sky at 21 cm as sharply as your eye sees it in green light, how wide must the dish be?
The rule is the same for every wavelength: sharpness is set by the size of the opening measured in wavelengths. Your pupil is about five and a half thousand wavelengths of green light across. A dish five and a half thousand wavelengths across at 21 cm is more than a kilometre wide. The largest single dish ever built, FAST in China, is 500 metres across and uses 300 metres of it at a time; at 21 cm it sees about as sharply as an eye with its pupil narrowed to a pinprick.
The bench
The Moon through a radio telescope
Every telescope blurs each point of the sky into a spot about one wavelength divided by its width across. Below is the Moon, schematically, as seen through that spot. Choose a wavelength and a size, or one of the real instruments.
The last two presets are not single dishes. In chapter 3, Michelson put two mirrors six metres apart on the Mount Wilson telescope and measured a star as if with a mirror six metres wide. Radio astronomers took the same idea much further: combine the signals from dishes kilometres apart, let the Earth’s turning swing them around one another, and the result sees as sharply as one dish as wide as the farthest pair. Martin Ryle called it aperture synthesis, and it won him a Nobel Prize in 1974.
The Moon, half a degree across
A case you have not seen
The shadow of a black hole
At the centre of the galaxy M87, 55 million light years away, is a black hole six and a half billion times the mass of the Sun. The dark shadow it casts on the glowing gas around it should be about 40 microarcseconds across: the size of an orange on the Moon. Radio astronomers set out to see it at a wavelength of 1.3 millimetres.
How wide a telescope does that take?
So the Event Horizon Telescope is the Earth. Eight observatories from Hawaii to the South Pole, Spain to Chile, recorded the same millimetre waves on the same nights in April 2017, each stamping its data with an atomic clock. The disks were flown to two centres and combined, as if every dish were a patch of one mirror the size of the planet. In 2019 the result was published: a ring of light around a dark centre, 42 microarcseconds across.
What you can now say
A telescope’s sharpness is its width counted in wavelengths. Radio waves are so long that a dish must be enormous to see detail, so radio astronomers join dishes far apart and let them act as one.
The hiss Jansky heard keeps time with the stars, and came from the centre of our galaxy. The rule that blurred it is the same one that limited the pinhole, the microscope objective and Michelson’s star.
For the determined reader: the numbers and their limits
Sidereal time
In a tropical year of 365.2422 solar days the Earth turns 366.2422 times relative to the stars. One sidereal day is therefore 86,400 × 365.2422 / 366.2422 = 86,164.09 s, or 23 h 56 min 4.09 s, and a source fixed among the stars returns 235.9 s earlier each day by the clock.
Resolution
θRayleigh = 1.22 λ / D, FWHM of the Airy core = 1.029 λ / D
| Instrument | λ | D | Beam (FWHM) |
|---|---|---|---|
| Your eye, pupil 3 mm | 550 nm | 3 mm | |
| Jansky’s antenna (1932) | 14.6 m | 30 m | |
| Reber’s dish (1939) | 1.87 m | 9.5 m | |
| FAST, illuminated | 21 cm | 300 m | |
| VLA, widest spacing | 21 cm | 36.4 km | |
| Event Horizon Telescope | 1.3 mm | 12,700 km |
Where this stops
Jansky’s antenna was a long, low array, sharper in one direction than the other; a single number is a rough summary. An interferometer’s resolution is about λ/B for its longest baseline B, but its picture is built from the spacings it has, and gaps between them leave artefacts that have to be modelled away. The Moon on the bench is drawn schematically; real radio images of the Moon show its warmth, not its reflected light. The blur is a Gaussian stand-in for the Airy pattern’s core.
A hobby, a war, and a hydrogen line
Jansky published in 1933 and the New York Times put the news on its front page, but Bell Labs moved him to other work and professional astronomers mostly ignored it. The person who listened was Grote Reber, a radio engineer in Wheaton, Illinois, who in 1937 built a dish nine and a half metres across in his mother’s back yard. After failing at shorter wavelengths he found the galaxy at 1.87 metres in 1939, and made the first radio map of the sky.
The Second World War trained a generation in radar, and left behind receivers and engineers. In 1951 Harold Ewen and Edward Purcell at Harvard detected the 21-centimetre glow of hydrogen that Hendrik van de Hulst had predicted in occupied Holland. Within a few years it had traced the spiral arms of the Milky Way, which dust hides from optical telescopes.
Where this explanation stops
The same rule, everywhere
The pinhole in chapter 1, the objective in chapter 2, Michelson’s mirrors in chapter 3, and now a telescope the size of the planet: every one of them is limited by its width measured in wavelengths. Radio waves are the longest light there is, so they need the widest instruments. At the other end of the spectrum the waves are so short that the rule stops mattering, and something else about light takes over.
If the shortest light has waves smaller than an atom, what happens when it meets one?
End of chapter 10
Added to your notebook
A telescope’s sharpness is its width counted in wavelengths. Radio waves are so long that dishes far apart must be joined to act as one.
What happens when the shortest light meets an atom?
Chapter 11 · A New Kind of Ray