The nucleus of the thorium-229 isotope has a special property: it has an excited state that’s incredibly close in energy to its ground state. The existence of such an isomer is remarkable because when nuclei normally get excited, they need enormous amounts of energy — hundreds of thousands or even millions of electron volts (eV). But the Th-229 nucleus’s excited state is only about 8.4 eV above its ground state. This is really small by nuclear standards and, importantly, it means light can excite the nucleus into this energy level.
This in turn matters because scientists have developed very precise atomic clocks over the last few decades that work by using lasers to excite electrons in atoms and measure the frequency of the light required to do this. These clocks are so accurate that they’re used for GPS, keeping time on the internet, and in fundamental physics experiments. But they also have a limitation: electrons are relatively easy to disturb, so a stray external electric or magnetic field can shift their energy levels slightly but enough to make the entire clock less stable.
Nuclei on the other hand are much smaller and are buried deep inside the atom, shielded by the electron cloud from the world beyond. So a nuclear clock based on a nuclear transition would potentially be much more stable and accurate than even the best atomic clocks.
The Th-229 isomer is the only nuclear transition that’s low enough in energy for scientists to realistically build a laser to make happen. In fact they have been trying to make a nuclear clock based on this transition for years now. Recently, two research groups finally managed to create this transition using lasers and they determined that the wavelength of light needed is 148.4 nm. This is in the vacuum ultraviolet range — i.e. ultraviolet light with a very short wavelength. Such light gets absorbed by air so they need to operate in a vacuum. Thus the name.
But here’s the catch: the laser sources that these research groups used to excite the transition were pulsed lasers, which means they only produced light in very short bursts, lasting just a few nanoseconds each.
When you have such short pulses, the light inherently has a broad range of frequencies mixed together. Scientists say the linewidth is several gigahertz wide. But the natural linewidth of the Th-229 isomer transition is very narrow, only about 60 microhertz. That’s a difference of several orders of magnitude. It’s like trying to measure something with a 1-m-long stick when you need precision down to the width of a single atom. Nuclear clocks demand a much more stable laser with a really narrow linewidth — ideally continuous rather than pulsed.
In a paper published in Physical Review Applied on February 11, researchers from Tsinghua University and the Chinese Academy of Sciences have proposed a way to generate a continuous-wave vacuum ultraviolet laser light at exactly 148.4 nm, with a very narrow linewidth, using a process called four-wave mixing.
Four-wave mixing is a nonlinear optical process. Normally, when light passes through a material, it just passes through without the different colours of light affecting each other. But if you have intense enough light and the right kind of material, you can get nonlinear effects, i.e. where multiple photons of light interact with atoms in the material to create new photons at other frequencies.
In four-wave mixing, you take three laser beams and send them through such a special medium. If everything is set up just right, they will combine to create a fourth beam at a new frequency. And the frequency of this new beam will be the sum of the frequencies of the three input beams.
The authors have proposed using cadmium vapour as the mixing medium. Cadmium because it has many properties that make it perfect for this job. First, it has electronic transitions that can be exploited to make the nonlinear process very efficient. Specifically, the team plans to use a two-photon resonance, meaning two of the input laser beams will have frequencies that, when added together, will exactly match the energy needed to excite cadmium atoms to a particular excited state. This resonance will greatly enhance the efficiency of the process. Second, the wavelengths of the lasers required to produce the desired output are readily available (of wavelengths 375 nm and 710 nm).
The two previous studies also used four-wave mixing but ended up with pulsed laser light because they used xenon as the mixing medium. Xenon is a generic choice because it results in light of a wide range of wavelengths. If researchers are exploring and don’t know exactly what wavelength they need or if they do want to use light of different wavelengths, xenon is great. On the flip side, it isn’t particularly suited to generating 148.4 nm light. Rather, it can if researchers can supply the input light at enormous power.
Pulsed lasers help with this requirement using a trick. Imagine you’ve a water hose: if water flows out continuously at a steady rate, you might get a gentle stream, but if you put your thumb over the end and suddenly release it, you get a powerful jet that can spray much farther even when the total amount of water per minute is the same. Pulsed lasers work like this: at the brief moment when the laser emits light, the intensity is very high even though the average power is low. And four-wave mixing is much more efficient with this intense light — enough to generate enough vacuum ultraviolet light to detect the nuclear transition.
To this end, the paper went into considerable technical detail about calculating how efficient using cadmium vapour would be, including assessing the element’s atomic structure. The authors also calculated something called the nonlinear susceptibility, which said how strongly the cadmium atoms would respond to the light.
They also had to worry about phase-matching. For the four-wave mixing process to work efficiently, the different light waves need to stay synchronised as they travel through the medium. This is tricky because different wavelengths of light travel at slightly different speeds through cadmium vapour (a phenomenon called dispersion). However, the authors showed that carefully controlling the temperature of the vapour and tightly focusing the laser beams could result in good phase-matching.
Overall, their calculations suggested that with input laser powers of 3 W at 375 nm and 6 W at 710 nm — both very achievable using current technology — they could generate more than 30 µW of vacuum ultraviolet light at 148.4 nm. While 30 µW may not sound like much, it’s actually a lot for spectroscopy experiments. More importantly, because this is a continuous-wave process rather than a pulsed process, and because it’s essentially just a frequency multiplication of stable input lasers, the output light should have a very narrow linewidth. The team estimated it could be below 1 kHz, which is orders of magnitude better than the pulsed sources currently in use.
A narrow linewidth is so important because then scientists can observe something called Rabi oscillations in the nuclear transition. This is when you can coherently drive the nucleus back and forth between its ground state and excited state, which is essential to build a nuclear clock. The researchers showed that with their proposed laser system, the linewidth would be narrow enough to observe these oscillations, opening the door to much more precise measurements of the Th-229 transition and eventually to building an actual working nuclear clock.
Such a clock could have applications beyond just timekeeping. The Th-229 transition is particularly sensitive to changes in fundamental constants of nature, so it could be used to test whether these constants actually stay constant over time; scientists could also use it to search for certain types of dark matter. The proposed laser system thus represents a crucial technological step towards all these applications.
Rubidium isn’t respectable. It isn’t iron, whose strength built railways and bridges and it isn’t silicon, whose valley became a dubious shrine to progress. Rubidium explodes in water. It tarnishes in air. It’s awkward, soft, and unfit for the neat categories by which schoolteachers tell their students how the world is made. And yet, precisely because of this unruly character, it insinuates itself into the deepest places of science, where precision, control, and prediction are supposed to reign.
For centuries astronomers counted the stars, then engineers counted pendulums and springs — all good and respectable. But when humankind’s machines demanded nanosecond accuracy, it was rubidium, a soft metal that no practical mind would have chosen, that became the metronome of the world. In its hyperfine transitions, coaxed by lasers and microwave cavities, the second is carved more finely than human senses can comprehend. Without rubidium’s unstable grace, GPS collapses, financial markets fall into confusion, trains and planes drift out of sync. The fragile and the explosive have become the custodians of order.
What does this say about the hierarchies of knowledge? Textbooks present a suspiciously orderly picture: noble gases are inert, alkali metals are reactive, and their properties can be arranged neatly in columns of the periodic table, they say. Thus rubidium is placed there like a botanical specimen. But in practice, scientists turned to it not because of its box in a table but because of accidents, conveniences, and contingencies. Its resonance lines happen to fall where lasers can reach them easily. Its isotopes are abundant enough to trap, cool, and measure. The entire edifice of atomic clocks and exotic Bose-Einstein condensates rests not on an inevitable logic of discovery but on this convenient accident. Had rubidium’s levels been slightly different, perhaps caesium or potassium would have played the starring role. Rational reconstruction will never admit this. It prefers tidy sequences and noble inevitabilities. Rubidium, however, laughs at such tidiness.
Take condensed matter. In the 1990s and 2000s, solar researchers sought efficiency in perovskite crystals. These crystals were fragile, prone to decomposition, but again rubidium slipped in: a small ion among larger ones, it stabilised the lattice. A substitution here, a tweak there, and suddenly the efficiency curve rose. Was this progress inevitable? No; it was bricolage: chemists trying one ion after another until the thing worked. And the journals now describe rubidium as if it were always destined to “enhance stability”. But destiny is hindsight dressed as foresight. What actually happened was messy. Rubidium’s success was contingent, not planned.
Then there’s the theatre of optics. Rubidium’s spectral lines at 780 nm and 795 nm became the experimentalist’s playground. When lasers cooled atoms to microkelvin temperatures and clouds of rubidium atoms became motionless, they merged into collective wavefunctions and formed the first Bose-Einstein condensates. The textbooks now call this a triumph of theory, the “inevitable” confirmation of quantum statistics. Nonsense! The condensates weren’t predicted as practical realities — they were curiosities, dismissed by many as impossible in the laboratory. What made them possible was a melange of techniques: magnetic traps, optical molasses, sympathetic cooling. And rubidium, again, happened to be convenient, its transitions accessible, its abundance generous, its behaviour forgiving. Out of this messiness came a Nobel Prize and an entire field. Rubidium teaches us that progress comes not from the logical unfolding of ideas but from playing with elements that allegedly don’t belong.
Rubidium rebukes dogma. It’s neither grand nor noble, yet it controls time, stabilises matter, and demonstrates the strangest predictions of quantum theory. It shows science doesn’t march forward by method alone. It stumbles, it improvises, it tries what happens to be at hand. Philosophers of science prefer to speak of method and rigour yet their laboratories tell a story of messy rooms where equipment is tuned until something works, where grad students swap parts until the resonance reveals itself, where fragile metals are pressed into service because they happen to fit the laser’s reach.
Rubidium teaches us that knowledge is anarchic. It isn’t carved from the heavens by pure reason but coaxed from matter through accidents, failures, and improvised victories. Explosive in one setting, stabilising in another; useless in industry, indispensable in physics — the properties of rubidium are contradictory and it’s precisely this contradiction that makes it valuable. To force it into the straitjacket of predictable science is to rewrite history as propaganda. The truth is less comfortable: rubidium has triumphed where theory has faltered.
And yet, here we are. Our planes and phones rely on rubidium clocks. Our visions of renewable futures lean on rubidium’s quiet strengthening of perovskite cells. Our quantum dreams — of condensates, simulations, computers, and entanglement — are staged with rubidium atoms as actors. An element kings never counted and merchants never valued has become the silent arbiter of our age. Science itself couldn’t have planned it better; indeed, it didn’t plan at all.
Rubidium is the fragment in the mosaic that refuses to fit yet holds the pattern together. It’s the soft yet explosive, fragile yet enduring accident that becomes indispensable. Its lesson is simple: science also needs disorder, risk, and the unruliness of matter to thrive.
Featured image: A sample of rubidium metal. Credit: Dnn87 (CC BY).
You’re familiar with clocks. There’s probably one if you look up just a little, at the upper corner of your laptop or smartphone screen, showing you what time of day it is, allowing you to quickly grasp the number of daytime or nighttime hours, depending on your needs.
There some other clocks that are less concerned about displaying ‘clock time’ and more about measuring the passage of. These devices are useful for applications designed to understand this dimension in a deeper sense. The usefulness of these clocks also depends more strongly on the timekeeping techniques they employ.
For example, consider the caesium atomic clock. Like all clocks, it is a combination of three things: an oscillator, a resonator and a detector. The oscillator is a finely tuned laser that shines on an ultra-cold gas of caesium atoms in a series of pulses. If the laser has the right frequency, an electron in a caesium atom will absorb a corresponding photon, jump to a higher energy level before then jumping back to its original place by emitting radiation of exactly 9,192,631,770 Hz. This radiation is the resonator.
The detector will be looking for radiation of this frequency – and the moment it has detected 9,192,631,770 waves (from crest to trough), it will signal that one second has passed. This is also why, technically, a caesium clock can be used to measure out a nine-billionth of a second.
Scientists have need for even more precise clocks, clocks that use extremely stable resonators and, increasingly of late, clocks that combine both advantages. This is why scientists developed optical atomic clocks. The caesium atomic clock has a resonant frequency of 9,192,631,770 Hz, which lies in the microwave part of the electromagnetic spectrum. Optical atomic clocks use resonators that have a frequency in the optical part. This is much higher.
For example, physicists at the Inter-University Centre for Astronomy and Astrophysics and the Indian Institute of Science Education and Research, both Pune, are building clocks that use ytterbium and strontium ions, respectively, with resonator frequencies of 642,121,496,772,645 Hz and 429,228,066,418,009 Hz. So technically, these clocks can measure out 600-trillionths and 400-trillionths of a second, allowing scientists ultra-precise insights into how long very short-lived events really last or how closely theoretical predictions and experimental observations match up.
In fact, because we have not managed to measure 400-trillionths of a kilogram, of a metre or in fact of any other SI unit, time is currently the most precisely measured physical quantity ever.
Sometimes, scientists need to use multiple atomic clocks in the course of an experiment or to ascertain how synchronised they are. This is not a trivial exercise.
For example, say you have two clocks whose performance you need to compare. If they are simple digital clocks, you could check how precisely each one of them records the amount of time between, say, astronomical dawn and astronomical dusk (the moments when the Sun is 18º below the horizon before sunrise and after sunset, respectively). Here, you take the act of looking at each clock face for granted. If the clocks are right in front of you, light travels nearly instantaneously between your eye and the display. And because the clocks tick one second at a time, you can repeat the task of checking their synchronisation as often as you need to just by looking.
What do you do if you need to know how well two optical atomic clocks are matched up continuously and if they are separated by, say, a thousand kilometres? Scientists in Europe demonstrated one solution to this problem in 2015.
They had optical clocks in Paris and Braunschweig connected with fibre optic cables to a processing station in Strasbourg. The resonant frequency of each clock was encoded in a ‘transfer laser’ that was then beamed through the cables to Strasbourg, where a detector measured the two laser pulses to decode the relative beat of each clock in real-time. The total length of the fibre optic cables in this case was 1,415 km. With this “all-optical” setup plus signal processing techniques, the research team reported a precision of three parts in 10-19 after an averaging time of just 1,000 seconds – a cutting-edge feat.
But scientists are likely to need one step better, if only because they also anticipate that the advent of optical atomic clocks at facilities around the world is likely to lead to a redefinition of the SI unit of time. The second’s current definition – “the time duration of 9,192,631,770 periods of the radiation” emitted by electrons transitioning between two particular energy levels of a caesium-133 atom – originated in 1967, when microwave atomic clocks were the state of the art.
Today, optical atomic clocks have this honour – and because they are more stable and use a higher resonator frequency than their microwave counterparts, it only makes sense to update the definition of a second. When this happens, optical clocks around the world will have to speak to each other constantly to make sure what each of them is measuring to be one second is the same everywhere.
Some of these clocks will be a few hundred kilometres apart, and others a lot more. In fact, scientists have figured it would be useful to have a way for two optical atomic clocks located on different continents to be able to work with each other. This represents the current version of the coordination problem, and scientists in Europe and Japan recently demonstrated a solution. It involves astronomy, because astronomy has a similar problem.
Everything in the universe is constantly in motion, which means telling the position of one moving object from another – like that of Venus from Earth – is bound to be more complicated from the start than knowing where your friend lives in a different city.
But astronomers have still figured out a way to establish a fixed reference frame that provides useful information about the location of different cosmic objects through space and time. They call it the International Celestial Reference Frame (ICRF). Its centre is located at the barycentre of the Solar System – the point around which all the planets in the Solar System orbit. Each of its three axes points in the direction of groups of objects called defining sources.
Many of these objects are quasars. ‘Quasar’ is a portmanteau of ‘quasi-stellar’, and is the name of the region at the centre of a galaxy where there is a supermassive black hole surrounded by a highly energised disk of gas and dust. Quasars are as such extremely bright. Astronomers spotted the first of them because they showed up in radio-telescope data as previously unknown star-like sources of radio waves. Because each galaxy can technically have only one quasar each, the number of quasars in the sky is not very high (relatively speaking) and most quasars are also located at such great distances that the radio waves they emit become very weak by the time they reach Earth’s radio telescopes.
Different views of the antennae of the Giant Metre-wave Radio Telescope at Khodad, Maharashtra. Credit: NCRA-TIFR
So on Earth, physicists either use very powerful telescopes to detect them or a collection of telescopes that work together using a technique called very-long baseline interferometry (VLBI). The idea is elegant but the execution is complicated.
Say some process in the accretion disk around the black hole at the Milky Way’s centre emits radio waves into space. These waves propagate through the universe. At some point, after many thousands of years, they reach radio telescopes on Earth. Because the telescopes are located at vastly different locations, in Maharashtra, Canary Islands and Hawaii, say, they will each detect and measure the radio wave signals at slightly different points of time. There may also be slight differences in the waves’ characteristics because they are likely to have moved through different forms and densities of matter in their journey through space.
Computers combine the exact times at which the signals arrive at each telescope and the signals’ physical properties (like frequency, phase, etc.) with a sophisticated technique called cross-correlation to produce a better-resolved picture of the source that emitted them than if they had used data from only one telescope.
In fact, the resolving power of a radio telescope is proportional to the telescope’s baseline. If scientists are using only one telescope to make an observation, the baseline is equal to the dish’s diameter. But with VLBI radio astronomy, the baseline is equal to the longest distancebetween two telescopes in the array. This is why this technique is so powerful.
For example, to capture the first direct image of the black hole at the Milky Way’s centre, some 52,000 lightyears away, astronomers combined an array of eight telescopes located in North America, South America, Hawaii, Europe and the South Pole to form the Event Horizon Telescope. At any given time, the baseline would be determined by two telescopes that can observe the black hole simultaneously. And as Earth rotated, different pairs of telescopes would work together to keep observing the black hole even as their own view of the black hole would change.
Each telescope would record a signal together with a very precise timestamp, provided by an atomic clock installed at the same facility or nearby, in a hard-drive. Once an observing run ended, all the hard-drives would be shipped to a processing facility, where computers would combine the signal and time data from them to create an image of the source.
Credit: EHT/Harvard University
As it happens, the image of the black hole the Event Horizon collaboration released in 2019 could have been available sooner if not for the fact that there are no flights from April to October from the South Pole. So astrophysics also has some coordination problems, but astrophysicists have been able to figure them out thanks to tools like VLBI. Perhaps it’s not surprising then that scientists have thought to use VLBI to solve optical atomic clocks’ coordination problem as well.
According to a paper published in July 2020, the current version of ICRF is the third iteration, was adopted on January 1, 2019, and uses 4,588 sources. Of these, the positions of exactly 500 sources – including some quasars – are known with “extreme accuracy”. Using this information, the European-Japanese team reversed the purpose of VLBI to serve atomic clocks.
Using VLBI to measure the positions and features of distant astronomical objects is called VLBI astrometry. Doing the same to measure distances on Earth, like the European-Japanese team has done, is called VLBI geodesy. In the former, astronomers use VLBI to reduce uncertainties about distant sources of radio waves by being as certain as possible about the distance between the telescopes (and other mitigating factors like atmospheric distortion). Flip this: if you are as certain as possible about the distance from Earth to a particular quasar, you can use VLBI to reduce uncertainties about the distance between two atomic clocks instead.
And the science and technologies we have available today have allowed astronomers to resolve details down to a few billionths of a degree in astrometry – and to a few millimetres in geodesy.
The European-Japanese team implemented the same idea. The team members used three radio telescopes. Two of them, located in Medicina (Italy) and Koganei (Japan), were small, with dishes of diameter 2.4 m, but with a total baseline of 8,700 km. The Medicina telescope was connected to a ytterbium optical atomic clock in Torino and the Koganei telescope to a strontium optical atomic clock in the same facility.
First, the Torino clock’s resonator frequency was converted from the optical part of the spectrum to the microwave part using a device called a frequency comb, like in the schematic shown below.
Credit: NIST
(To quote myself from an older article: “A frequency comb is an advanced laser whose output radiation lies in multiple, evenly-spaced frequencies. This output can be used to convert high-frequency optical signals into more easily countable lower-frequency microwave signals.”)
This microwave frequency is transferred to a laser that is beamed through a fibre optic cable to the Medicina telescope. Similarly, at Koganei, the strontium clock’s resonator frequency is converted using a frequency comb to a corresponding microwave counterpart. At this point, both telescopes have time readings from optical atomic clocks in the form of more easily counted microwave radiation.
In the second step, the scientists used VLBI to determine as accurately as possible the time difference between the two telescopes. For this, the telescopes observed a quasar whose position was known to a high degree of accuracy in the ICRF system.
Since quasars are inherently far away and the two telescopes are quite small (as radio telescopes go), they were able to detect the quasar signal only weakly. To adjust for this, the team connected both telescopes via high-speed internet links to a large 34-m radio telescope in Kashima, also in Japan. This way, the team writes in its paper published in October 2020,
“the delay observable between the transportable stations can be calculated as the difference of the two delays with the large antenna after applying a small correction factor”.
Once the scientists had a delay figure, they worked backwards to estimate when exactly the two telescopes ought to have recorded their respective signals, based on which they could calculate the ratio of the microwave frequencies, and finally based on which they could calculate the ratio of the two clocks’ optical frequencies – autonomously, in real-time. To quote once again from the team’s paper:
“One node was installed at NICT headquarters in Koganei (Japan) while the other was transported to the Radio Astronomical Observatory operated by INAF in Medicina (Italy), forming an intercontinental baseline of 8,700 km. Observational data at Medicina and Koganei were stored on hard-disk drives at each station and transferred over high-speed internet networks to the correlation centre in Kashima for analysis. Ten frequency measurements were performed via VLBI between October 2018 and February 2019, and from these we calculated the frequency difference between the reference clocks at the two stations: the local hydrogen masers in Medicina and Koganei. Each session lasted from 28 h to 36 h and included at least 400 scans observing between 16 and 25 radio sources in the ICRF list.”
This way, they reported the ability to determine the frequency ratio with an uncertainty of 10-16 after ten-thousand seconds, and perhaps as low as 10-17 after a longer averaging time of ten days.
This is very good, but more importantly it’s better than the uncertainty arising from directly comparing the frequencies of two optical atomic clocks by relaying data through satellites. An uncertainty of 10-17 also means physicists can use multiple optical atomic clocks to study extremely slow changes, and potentially be confident about the results down to 0.00000000000000001 seconds.
The architecture of the solution also presents some unique advantages, as well as food for thought.
The setup effectively requires optical atomic clocks to be connected to small, even portable, radio telescopes as long as these telescopes are then connected to a larger one located somewhere else through a high-speed internet connection. These small instruments “can be operated without the need for a radio transmission licence,” the team writes in the paper, and “where laboratories lack the facilities or sky coverage to house a VLBI station, they can be connected by local optical-fibre links” like the one between Medicina and Torino.
The scientists have effectively used existing methods to solve a new problem instead of finding an altogether new solution. This isn’t to say new solutions are disfavoured but only that the achievement, apart from being relatively low cost and well-understood, is ingenious, and keeps the use of optical atomic clocks for all the applications they portend from becoming too resource-intensive.
It’s also fascinating that the clocks participating in this exercise are effectively a group of machines translating between processes playing out at two vastly different scales – one of minuscule electrons emitting tiny amounts of radiation over short distances and the other of radiation of similar provenance emerging from the exceedingly unique neighbourhoods of colossal black holes, travelling for many millennia at the speed of light through the cosmos.
Perhaps this was to be expected, considering the idea of using a clock is fundamentally a quest for a foothold, a way to translate the order lying at the intersection of seemingly chaotic physical processes, all directed by the laws of nature, to a metronome that the human mind can tick to.
Featured image: A simulation of a black hole from the 2014 film ‘Interstellar’. Source: YouTube.
On June 25, scientists announced the discovery of a trio of supermassive black holes at the center of a galaxy 4.2 billion light years away. The find was credited to the European VLBI Network. A Space.com report stated that this network “could see details 50 times finer than is possible with the Hubble Space Telescope”. How is this achieved?
VLBI stands for Very-Long-Baseline Interferometry. It is a technique used in astrometry to obtain high resolution images of the sky using a network of telescopes instead of using one big telescope. VLBI is commonly used to image distant cosmic radio sources such as quasars.
This sophisticated technique has its roots in 18th century physics, specifically in Thomas Young’s famous double-slit interference experiment in the early 1800s. When Young placed a screen with two extremely narrow slits in front of a light source, such as a burning candle, the shadow cast on the other side was actually an alternating patchwork of bright and dull bands. This was the interference pattern. Young’s experiment was important to establish that light travels as a wave, overturning Newton’s conviction that light was composed of particles.
The interference pattern
When light passes through each slit, it diffracts, i.e. starts to spread out. At some point in front of the slits, the diffracted waves meet and interfere. Where crest met crest, there was constructive interference and that resulted in a bright band. Where crest met trough, there was a duller band. Where trough met trough, there was a dark band. If the position of the slits was changed, the interference pattern also shifted.
In VLBI, the candle is replaced by a distant source of radio waves, like a quasar. The slits are replaced by radio antennae on telescopes. Since the Earth is rotating, the antenna are in relative motion with the quasar. As a result, there is an interference between the signals being received by the two telescopes. This interference pattern is processed at a central location along with the time at which each signal was received at each antenna as recorded by a clock.
In the second stage of this colossal Young’s experiment, let’s talk some wave physics. Radio waves have greater wavelength than visible light. As a result, radio telescopes have an inherently low angular resolution than optical telescopes of the same size. Angular resolution is defined as the ratio of an emission’s wavelength to the diameter of the telescope receiving it. Qualitatively, it describes the smallest unit of distance the telescope can distinguish in the image it receives and that must be as low as possible. For example, a 50-meter wide radio telescope will have an angular resolution of 50/0.01 = ~41.2 arc-second. An optical telescope of the same size will have an angular resolution of 0.004 arc-second, 10,000-times better.
Baseline + Atomic clocks
VLBI resolves this issue (this isn’t really a pun). Because there are multiple telescopes receiving the radio signals, the angular resolution is redefined: it’s no longer the ratio between the wavelength and the diameter of the telescope. It’s the ratio between the wavelength and the baseline. The baseline is the maximum physical separation between two telescopes in the array. If, say, the baseline is 1,000 km, the angular resolution of an array of radio telescopes becomes 0.002 arc-second, 20,000-times better.
However, this technique couldn’t find wide implementation until the atomic clock was invented in the 1950s. Before they were around, a single metronome had to be connected to multiple telescopes with cables, which limited the baseline length. With atomic clocks, telescopes could be placed on different continents because the clocks were globally coordinated.
So, a telescope receives a radio signal, a computer sticks a timestamp on it and sends it to the receiver. The receiver collates such data from different telescopes and creates the fringe pattern characteristic of interference. A processor finally recreates the source of all the radio waves at different locations using the fringe pattern and the times at which each signal was received. Of course, there are many systems in between to stabilize and improve the quality of the signal, to coordinate observations by the telescopes, etc., but the basic principle is the same as in Young’s experiment of two centuries ago.
