Tag: James Webb Space Telescope

JWST and the sorites paradox

The team operating NASA’s James Webb Space Telescope (JWST) released its first full-colour image early on July 12, and has promised some more from the same set in the evening. The image is a near-infrared shot of the SMACS 0723 galaxy cluster some 4.6 billion lightyears away. According to a press release accompanying the image’s release, the field of view – which shows scores of galaxies as well as several signs of gravitational lensing (which is evident only when very large distances are involved) – is equivalent to the area occupied by a grain of sand held at arm’s length from the eyes.

I’m personally looking forward to the telescope’s shot of the Carina Nebula: the Hubble space telescope’s images of this emission nebula were themselves stunning, so the JWST’s shot should be more so!

Gazing at the JWST’s first image brought to my mind the sorites paradox. Its underlying thought-experiment might resonate with you were you to ponder the classical limit of quantum physics or the concept of emergence as Philip Warren Anderson elucidated it as well. Imagine a small heap of sand before you. You pick up a single grain from the heap and toss it away. Is the sand before you still in a heap? Yes. You put away another grain and check. Still a heap. So you keep going, and a few thousand checks later, you find that you have before you a single grain of sand. Is it still a heap? If your answer is ‘yes’, the follow-up question arises: how can a single grain of sand be a heap? If ‘no’, then when did the heap stop being a heap?

Another way to conjure the same paradox is to start with one grain of sand and which is evidently not a heap. Then you add one more grain, which is also not a heap, and one more and one more and so forth. Using modus ponens supplies the following line of reasoning: “One mote isn’t a heap. And if one mote isn’t a heap, then two motes don’t make a heap either. And three motes don’t make a heap either. And so on until: if 9,999 motes don’t make a heap, then 10,000 motes don’t make a heap either.” But while straightforward logic has led you to this conclusion, your sense-experience is clear: what lies before you is in fact a heap.

The paradox came to mind because it’s hard not to contemplate the fact that both the photograph and the goings-on in India at the moment – from the vitriolic bigotry that’s constantly being mainstreamed to the arrest and harassment of journalists, activists and other civilians, both by the ruling dispensation – are the product of human endeavour. I’m not interested in banal expressions of the form “we’re all in this together” (we’re not) or “human intelligence and ingenuity can always be put to better use” (this is useless knowledge); instead, I wonder what the spectrum of human actions – which personal experience has indicated repeatedly to be continuous and ultimately ergodic – looks like that encompasses, at two extremes, actions of such beauty and of such ugliness. When does beauty turn to ugliness?

Or are these terms discernible only in absolutes – that is, that there is no lesser or higher beauty (or ugliness) but only one ultimate form, and that like the qubits of a quantum computer, between ultimate beauty and ultimate ugliness there are some indeterminate combinations of each attribute for which we have no name or understanding?

I use ‘beauty’ here to mean that which is deemed worthy of preservation and ‘ugliness’, of erasure. The sorites paradox is a paradox because of the vague predicates: ‘heap’, for example, has no quantitative definition. Similarly, I realise I’m setting up vague, as well as subjective, predicates when I set up beauty and preservation in the way that I have, so let me simplify the question: how do I, how do you, how do we reconcile the heap of sand that is the awesome deep-field shot of a distant galaxy cluster with the single grain of sand that is the contemporary political reality of India? Is a reconciliation even possible – that is, is there still a continuous path of thought, aspiration and action that could take a people seeped in hate and violence to a place of peaceability, tolerance and openness? Or have we fundamentally and irredeemably lost a part of ourselves that has turned us non-ergodic, that will keep us now and for ever from experiencing certain forms of beauty?

Language and the words that we use about ourselves will play a very important part here – the adjectives we save for ourselves versus those for the people or ideas that offend us, the terms in which we conceive of and describe our actions, everything from the order of words of our shortest poems to that of jargon of our courts’ longest judgments. Our words help us to convince ourselves, and others, that there is beauty in something even if it isn’t readily apparent. A bhakt might find in the annals of OpIndia and The Organiser the same solace and inspiration, and therefore the virtue of preserving what he finds to be beautiful, that a rational progressivist might find in Salvage or Viewpoint. This is among other things because language is how we map meaning to experience – the first point of contact between the material realm and human judgment, an interaction that will forever colour every moral, ethic and justicial conclusion to come after.

This act of meaning-making is also visible in physics, where there are overlapping names for different parts of the electromagnetic spectrum because the names matter more for the frequencies’ effects on the human body. Similarly, in the book trade, genre definitions can be overlapping – The Three-Body Problem by Cixin Liu is both sci-fi and fantasy, for example – because they matter largely for marketing.

One way or another, I’m eager, but not yet desperate, for an answer that will keep the door open for some measure of reversibility – and not for the bhakts but for those engaged in pushing back against their ilk. (The bhakts can go to hell.) The cognitive dissonance otherwise – of a world that creates things and ideas worth preserving and of a world that creates things and ideas worth erasing – might just break my ability to be optimistic about the human condition.

Featured image: The JWST’s image of the SMACS 0723 galaxy cluster. Credit: NASA, ESA, CSA and STScI.

Ways of seeing

A lot of the physics of 2015 was about how the ways in which we study the natural world had been improved or were improving.

The pitfalls of thinking that ASTROSAT will be ‘India’s Hubble’

The Hubble Space Telescope needs no introduction. It’s become well known for its stunning images of nebulae and star-fields, and it wouldn’t be amiss to say the telescope has even become synonymous with images of strange beauty often from distant cosmic shores. No doubt saying something is like the Hubble Space Telescope simplifies the task of communicating that object’s potential and significance, especially in astronomy, and also places the object in stellar company and effortlessly elevates its public perception.

It’s for the latter reason that the comparison shouldn’t be made lightly. Not all telescopes are or can be like the Hubble Space Telescope, which sports some of the more cutting-edge engineering at play in modern telescopy, undoubtedly necessary to produce some of the images it produces (here’s a list of stunners). The telescope also highlighted the role of aestheticism in science: humans may be how the universe realises itself but the scope of that realisation has been expanded by the Hubble Space Telescope. At the same time, it has become so famous for its discoveries that we often pay no heed to the sophisticated physics at play in its photographic capabilities, in return for images so improbable that the photography has become irrelevant to our realisation of their truth.

ASTROSAT, on the other hand, is an orbiting telescope whose launch on September 28 will place India in the small cohort of countries that have a space-borne observatory. That’s insufficient to claim ASTROSAT will be akin to the Hubble as much as it will be India’s debut on the road toward developing “Hubble-class” telescopes. ASTROSAT’s primary science objectives are:

  • Understand high-energy processes in binary systems
  • Search for black hole sources in the galaxy
  • Measure magnetic fields of neutron stars
  • Study high-energy processes in extra-galactic systems
  • Detect new transient X-ray sources
  • Perform limited high angular-resolution deep field survey in UV

The repeated mentions of high-energy are synonymous with the parts of the electromagnetic spectrum ASTROSAT will study – X-ray and ultraviolet emissions have higher frequencies and thus higher energies. In fact, its LAXPC (Large Area X-ray Proportional Counter) instrument will be superior to the NASA NuSTAR X-ray telescope: both will be logging X-ray emissions corresponding to the 6-79 keV* energy range but LAXPC’s collecting area will be almost 10x the collecting area of NuSTAR’s. Similarly, ASTROSAT’s UV instrument, the Ultraviolet Imaging Telescope, studies wavelengths of radiation from 130 nm to 320 nm, like the Cosmic Origins Spectrograph on board the Hubble spans 115-320 nm. COS has a better angular and spectral resolution but UVIT, as well as the Scanning Sky Monitor that looks for transient X-ray sources, tops with a higher field of view. The UVIT and LAXPC double up as visible-wavelength detectors as well.

In contrast, the Hubble makes observations in the infrared, visible and UV parts of the spectrum. Its defining feature is a 2.4-m wide hyperbolic mirror that serves to ‘collect’ photons from a wide field of view onto a secondary hyperbolic mirror, which in turn focuses into the various instruments (the Ritchey-Chrétien design). ASTROSAT also has a primary collecting mirror; it is 30 cm wide.

Design of a Ritchey–Chrétien telescope. Credit: HHahn/Wikimedia Commons, CC BY-SA 3.0
Design of a Ritchey–Chrétien telescope. Credit: HHahn/Wikimedia Commons, CC BY-SA 3.0

But it’s quite wrong to think ASTROSAT could be like Hubble when you consider two kinds of gaps between the instruments. The first is the technical-maturity gap. Calling ASTROSAT “India’s Hubble” will imply that ISRO has reached that level of engineering capability when it has not. And making that reference repeatedly (here, here, here and here) will only foster complacency about defining the scale and scope of future missions. One of ISRO’s principal limitations is payload mass: the PSLV rocket has been the more reliable launch vehicle at our disposal and it can lift 3,250 kg to the low-Earth orbit. The GSLV rocket can lift 5,000 kg to the low-Earth orbit (10,000 kg if an upper cryogenic stage is used) but is less reliable, although promising. So, the ASTROSAT weighs 1,500 kg while the Hubble weighs 11,110 kg – the heaviest scientific satellite launched till date.

A major consequence of having such a limitation is that the technology gets to define what satellite is launched when instead of astronomers laying out what they want to find out and technology setting out to achieve it, which could be a useful impetus for innovation. These are still early days for ISRO but it’s useful to keep in mind even this component of the Hubble’s Hubbleness. In 1974, NASA and ESA began collaborating to build the Hubble. But before it was launched in 1990, planning for the James Webb Space Telescope (JWST) – conceived from the beginning to be Hubble’s successor – began in the 1980s. In 1986, an engineer named Pierre Bely published a paper outlining how the successor will have to have a 10-m primary mirror (more than 4x the width of the Hubble’s primary mirror) and be placed in the geostationary orbit so Earth doesn’t occlude its view of space, like it does for the Hubble. But even four years later, NASA didn’t have a launch vehicle that could heft 6,500 kg (JWST’s weight) to the geostationary transfer orbit. In 2018, Europe’s Ariane 5 (ECA) will be doing the honours.

The other is the public-outreach gap. As historian Patrick McCray has repeatedly noted, telescopes are astronomers’ central research tools and the quality of astronomy research is a reflection of how good the telescopes are. This doesn’t just mean large reflecting mirrors, powerful lenses and – as it happens – heavy-lift launch vehicles but also the publication of raw data in an accessible and searchable format, regular public engagement and, most importantly, effective communication of discoveries and their significance. There was a hint of ISRO pulling off good public outreach before the Mars Orbiter Mission launched in November 2013 but that evaporated soon after. Such communication is important to secure public support, political consensus and priority funding for future missions that can expand an existing telescope’s work. For the perfect example of what a lack of public support can do, look no further than the India-based Neutrino Observatory. NASA, on the other hand, has been celebrated for its social media efforts.

And for it, NASA’s missions are more readily recognisable than ISRO’s missions, at least among people who’ve not been following ISRO’s launches closely since the 1960s. Not only that, while it was easier for NASA’s scientists to keep the JWST project from being cancelled, due to multiple cost overruns, thanks to how much its ‘predecessor’ the Hubble had redefined the images of modern astronomy since the late 1990s, the Hubble’s infamous spherical aberration fault in its first years actually delayed the approval of the JWST. McCray writes in a 2009 essay titled ‘Early Development of the Next Generation Space Telescope‘ (the name of JWST before it was changed in 2002),

Years before the Hubble Space Telescope was launched in 1990 a number of astronomers and engineers in the US and Europe were thinking hard about a possible successor to the HST as well as working to engage a broad community of researchers in the design of such a new observatory. That the launch of any such successor was likely to be many years away was also widely accepted. However, the fiasco of Hubble’s spherical aberration had a serious effect on the pace at which plans were advancing for the Next Generation Space Telescope. Thus crucially for the dynamics of building the “Next Big Machine,” the fate of the offspring was intimately tied to that of the parent. In fact, … it was only when in the mid-1990s that the NGST planning was remade by the incorporation of a series of technology developments in infrared astronomy that NASA threw its institutional weight and money behind the development of a Next Generation Space Telescope.

But even for all the aestheticism at play, ISRO can’t be said to have launched instruments capable of transcending their technical specifications, either: most of them have been weather- and resource-monitoring probes and not crafted for the purpose of uncovering elegance as much as keeping an eye out. But that doesn’t mean, say, the technical specifications of the ASTROSAT payload shouldn’t be readily available, that there shouldn’t be one single page on which one can find all info. on ISRO missions (segregated by type: telecom, weather-monitoring, meteorology, resource-monitoring, astronomy, commercial), that there shouldn’t be a channel through which to access the raw data from its science missions**, or that ISRO continue to languish in its misguided conflation of autonomy and opacity. It enjoys a relative abundance of the former, and does not have to fight for resources in order to actualise missions it designs based on internal priorities. On the other hand, it’s also on the cusp of making a habit of celebrating frugality***, which could in principle provide the political administration with an excuse to deny increased funding in the future, and surely make for a bad idea in such an industry that mandates thoroughness to the point of redundancy as space. So, the day ought to come when the bright minds of ISRO are forced to fight and missions are chosen based on a contentious process.

There are multiple ways to claim to be the Hubble – but ASTROSAT is definitely not “India’s Hubble”. ISRO could in fact banish this impression by advertising ASTROSAT’s raw specs instead of letting people abide by inadequate metaphors: an amazing UV imager, a top-notch X-rays detector, a first class optical observer. A comparison with the Hubble also diminishes the ASTROSAT by exposing itself to be not like the Hubble at all and, next, by excluding from conversation the dozens of other space-borne observatories that it has already bested. It is more exciting to think that with ASTROSAT, ISRO is just getting started, not finished.

*LAXPC will actually be logging in the range 3-79 keV.

**There appears to be one under construction.

***How long before someone compares ASTROSAT’s Rs.178 crore to the Hubble’s $2.5 billion?

Money for science

Spending money on science has been tied to evaluating the value of spin-offs, assessing the link between technological advancement and GDP, and dissecting the metrics of productivity, but the debate won’t ever settle no matter how convincingly each time it is resolved.

For a piece titled The Telescope of the 2030s, Dennis Overbye writes in The New York Times,

I used to think $10 billion was a lot of money before TARP, the Troubled Asset Relief Program, the $700 billion bailout that saved the banks in 2008 and apparently has brought happy days back to Wall Street. Compared with this, the science budget is chump change, lunch money at a place like Goldman Sachs. But if you think this is not a bargain, you need look only as far as your pocket. Companies like Google and Apple have leveraged modest investments in computer science in the 1960s into trillions of dollars of economic activity. Not even Arthur C. Clarke, the vaunted author and space-age prophet, saw that coming.

Which is to say that all that NASA money — whether for planetary probes or space station trips — is spent on Earth, on things that we like to say we want more of: high technology, education, a more skilled work force, jobs, pride in American and human innovation, not to mention greater cosmic awareness, a dose of perspective on our situation here among the stars.

And this is a letter from Todd Huffman, a particle physicist at Oxford, to The Guardian:

Simon Jenkins parrots a cry that I have heard a few times during my career as a research scientist in high-energy physics (Pluto trumps prisons when we spend public money, 17 July). He is unimaginatively concerned that the £34m a year spent by the UK at Cern (and a similar amount per year would have been spent on the New Horizons probe to Pluto) is not actually money well spent.

Yet I read his article online using the world wide web, which was developed initially by and for particle physicists. I did this using devices with integrated circuits partly perfected for the aerospace industry. The web caused the longest non-wartime economic boom in recorded history, during the 90s. The industries spawned by integrated circuits are simply too numerous to count and would have been impossible to predict when that first transistor was made in the 50s. It is a failure of society that funnels such economic largesse towards hedge-fund managers and not towards solving the social ills Mr Jenkins rightly exposes.

Conflict of interest? Not really. Science is being cornered from all sides and if anyone’s going to defend its practice, it’s going to be scientists. But we’re often so ready to confuse participation for investment, and at the first hint of any allegation of conflict, don’t wait to verify matters for ourselves.

I’m sure Yuri Milner’s investment of $100 million today to help the search for extra-terrestrial intelligence will be questioned, too, despite Stephen Hawking’s moving endorsement of it:

Somewhere in the cosmos, perhaps, intelligent life may be watching these lights of ours, aware of what they mean. Or do our lights wander a lifeless cosmos — unseen beacons, announcing that here, on one rock, the Universe discovered its existence. Either way, there is no bigger question. It’s time to commit to finding the answer – to search for life beyond Earth. We are alive. We are intelligent. We must know.

Pursuits like exploring the natural world around us are, I think, what we’re meant to do as humans, what we must do when we can, and what we must ultimately aspire to.

Why you should care about the New Horizons probe nearing Pluto

The Wire
May 29, 2015

Alex Parker is a planetary astronomer at the Southwest Research Institute, Texas, and he posted his tweet just as I started writing this piece. And not just for Parker – it’s an exciting time for everyone, an exhilarating period in the history of space exploration. In just under 48 days – on July 14, 2015 – the NASA New Horizons space-probe will make its first fly-by of our favourite dwarf planet Pluto. Until then, it will be relaying less and less grainy pictures to Earth, each of more interest than the last, of a cold and distant world discovered by Clyde Tombaugh in 1930. One batch of images taken from May 8 to May 12 has already added to old evidence that Pluto hosts icy polar caps, and variations in surface brightness suggest a more uneven composition. On May 28, New Horizons restarted another phase of imaging – and as each day takes the probe 1.2 million km closer to its target, this is Pluto finally emerging out of the blur.

What more could we stand to find out? Quite a lot, as it turns out, from three points of view:

1. Toward the outer limits

The engineers operating the Voyager 1 space-probe (currently the farthest human-made object from Earth) had exciting news in September 2013: they claimed that about a year earlier, the probe had entered the interstellar medium – the space between stars, where the Sun’s influence was no longer the dominant one but had to contend with particulate emissaries from other stars in the galaxy. At the time, V1 was running on what little remained of its battery, a feeble ingot of blinking lights 19 billion km from Earth, and the occasion was replete with symbolism: humankind (or a representative) had set foot into the universe.

Actually, that moment could’ve transpired earlier. The engineers said that, in February 2012, the readings to indicate if V1 had entered the interstellar medium were spotted by the probe. However, they couldn’t be verified because the instrument that could do that had run out of juice. Luckily for them, a solar flare that erupted in March 2012 set the region of space around the probe thrumming with energy, which V1’s weak were able to pick up on and settle the matter.

Pluto, now, is much closer to the Sun than the threshold of the interstellar medium – in fact, the distance between Pluto and the Sun is 3.7 times smaller than the distance between Pluto and the medium. However, it is still quite far, and any space-probe sent to study it will either have to use up as little of its battery as it can until the rendezvous or be able to make only perfunctory observations of the dwarf planet. New Horizons is of the former kind – its primary mission is the farthest till date, and unlike the Voyager and Pioneer probes, will be able to respond to its environment agilely and be less susceptible to the vagaries of a dying battery.

2. Within the outer limits

Even if Pluto is among the outermost significant, planet-like bodies to orbit the Sun, it’s equally significant as being the largest body in the Kuiper Belt, a ring of asteroids like the one between the orbits of Mars and Jupiter. The belt starts from around the orbit of Neptune and extends to six AUs beyond the orbit of Pluto (AU is the astronomical unit, the distance between Earth and the Sun: 149.5 million km). It is also 200 times heavier than the Mars-Jupiter belt. Overall, both belts are important for two reasons in the context of New Horizons.

Before the Solar System took the form we now know – with a star at the centre, eight planets orbiting it, and two rings of asteroids – it comprised a young Sun at the centre of a massive disk of gases, dust and other materials called the protoplanetary disk. It is so named because it is out of this disk that the Solar System’s planets condensed, born as clumps of matter whose gravity accrued more matter, growing in size. And even as a planet formed, its gravitational pulls would ‘clean’ out a space in the protoplanetary disk, forming gaps. This phenomenon is visible among Saturn’s rings as well, with the space between rings having been cleaned out by the formation of small moons. The gaps in the disk survived to this day as the space between planets’ orbits. On the other hand, parts of the disk that didn’t get fully cleaned out formed the asteroid belts. So, they’re residues of the matter that the first planets were formed of, and studying them throws a lot of light on the history of the Solar System’s formation.

The second reason is that the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt are separated by 4.2 billion km – even on the cosmological scale, that’s a non-trivial gap. However, many objects in the two belts share chemical and physical properties as if they were once part of a common larger body. One logical explanation is that the belts were ‘mixed’ after they were formed. And to explain such mixing, astronomers have an awe-inspiring yet plausible explanation. According to them, as Jupiter was forming, its orbits moved closer to the Sun and then farther away, before shrinking down to place it between the inner asteroid belt and Saturn. The increase and decrease in the orbit’s size could’ve been due to the formation of other planets in the system, which would’ve disrupted the gravitational equilibrium. And while Jupiter moved, its prodigious gravity could’ve tugged a part of the inner asteroids out and vice versa, resulting in a mixed composition of asteroids in both belts. Since Pluto is the largest among Kuiper Belt objects, New Horizons studying it in detail could provide more clues about if such mixing could’ve happened.

3. Beyond the outer limits

Pluto is all of 2,300-km across – the distance between Kanyakumari in south Tamil Nadu and New Delhi – and it has five moons all to itself: Nix, Styx, Hydra, Charon and Kerberos. All of them are Kuiper Belt objects, too, and astronomers are curious to know if Pluto has a ring system as well, populated by smaller asteroids. The dwarf planet will also likely have smaller rocks orbiting it, and dust particles kicked up as a result of collisions between them. Such dust will be dangerous for New Horizons because they could impact the probe at some 50,000 km/hr and damage on-board systems. In January 2014, Simon Porter, one of the probe’s mission scientists, had told Wired that to protect against such collisions, his team had a contingency plan in mind: to turn the probe’s 2.1-metre-wide dish antenna into a shield.

If the probe does make it through the danger zone and get to within 12,500 km of the surface of Pluto, its observations of any rings as well as the dwarf planet’s surface, atmosphere and any craters/seismic activity will reveal more about the composition of Kuiper Belt objects, how they interact with each other, whether they sport any signs of violence from the past, and if at all they have atmospheres, what they’re composed of – information important to understand how and where the Solar System’s other planets could’ve formed. Astronomers also already know that Pluto’s surface has frozen methane and carbon monoxide.

This and other data gleaned from Pluto and its surroundings will take until late-2016 to be transmitted to Earth but the probe’s journey will continue – rather, has to continue because a probe that’s gone so far might as well just go farther because of the considerable time taken to travel such distances. Because the primary mission will almost exhaust its battery, the probe will subsequently become less manoeuvrable – like the Voyager and Pioneer probes did, yet still boast of a sophisticated suite of instruments. To take advantage, astronomers from the Southwest Research Institute, including Alex Parker, had spotted three other Kuiper Belt objects in New Horizons‘ path in late 2014 that would be interesting to study. All three objects are about 30-55 km across and located about 44 AU from the Sun, meaning the probe will reach them around 2020. This timeline is very interesting because NASA plans to launch the James Webb Space Telescope – successor to the Hubble and Spitzer space telescopes – in 2018. The JWST will be better equipped to study the Kuiper Belt objects than Hubble is, and its observations could be complemented by New Horizons‘.

It is probably from all these expectations that the probe draws its promising name. There are parallels to be drawn between its (impending) exploration of Pluto and the Kuiper Belt, and the space beyond, and how astronomers look into the older universe. The speed of light in vacuum is the highest possible speed in the universe, so when astronomers train their telescopes to look billions of lightyears in one direction, they’re simply looking billions of years into our past. The farther a part of the cosmos is from us, the older the light from it is – and the older the information it is carrying is. A parallel of this ingrained association between space and time can be drawn with the distance New Horizons is travelling and the more than four billions years into our past it will be able to see. Here’s waiting with bated breath…

Looking for life? Look for pollution.

Four-thousand years on Earth and we’ve a lot of dirt to show for it. Why would an advanced alien civilization be any different?

That’s the motivation that three astrophysicists from Harvard University have used to determine that powerful telescopes could look for signs of chlorofluorocarbons (CFCs) in alien atmospheres as signs of alien civilization.

“If the civilization reaches an industrial revolution similar to ours, then the chances are high” of finding CFCs in their atmosphere, Avi Loeb, a member of the study from the Harvard-Smithsonian Center for Astrophysics, told Is Nerd. “In our paper, we are demonstrating the detectability of the related signal if industrial pollution exists in the atmosphere of a planet.”

The team have estimated that NASA’s upcoming James Webb Space Telescope could look for signs of tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F) in alien atmospheres with a few days’ exposure. However, this is a high opportunity cost for such a powerful telescope, so the trio propose looking for these CFCs if biomarkers like molecular oxygen are found first.

While oxygen, alongside methane and nitrous oxide, points to the possible existence of primitive life, CFCs are almost exclusively anthropogenic.

Absorption spectroscopy

The space telescope will be studying starlight that has passed through an exoplanet’s atmosphere. Molecules of CF4 and CCl3F will absorb photons of light of specific wavelengths, casting a shadow. Astronomers then match the shadows with the molecules.

The team’s pre-print paper says their technique would be suitable for Earth-like exoplanets orbiting white-dwarfs. This is because photons of the wavelengths absorbed by CF4 and CCl3F are available in sufficient quantities from the star. On the downside, methane and nitrous oxide also absorb light along similar wavelengths as CF4, and oxygen and water along similar wavelengths as CCl3F.

Nevertheless, they find that the James Webb Space Telescope could detect the presence of high CF4 and CCl3F concentrations in 3 and 1.5 days respectively. An advantage of looking for CFCs like CF4 is, according to their pre-print paper, its longevity. “[Given] the half-life of CF4 in the atmosphere is [about] 50,000 years … it is not inconceivable that an alien civilization which industrialized many millennia ago might have detectable levels of CF4,” they write.

NASA plans to launch the space telescope, successor to the Hubble, in 2018. By then, it will be one of the next generation of telescopes (diameter in the range of 24-40 meters), each of which could look for signs of alien civilization using the Harvard team’s technique. “They include the Giant Magellan Telescope, the European Extremely Large Telescope, and the Thirty Meter telescope,” Dr. Loeb said.

Of them, the Giant Magellan is planned to a have a dedicated instrument called G-CLEF with exceptional spectroscopic capabilities, he added. Construction for the Extremely Large Telescope began last week in Chile.