Tag: interferometry

Sauron’s singularity: Sucking in light but lighting up the universe

This composite image shows the central region of the spiral galaxy NGC 4151, dubbed the 'Eye of Sauron' by astronomers for its similarity to the eye of the malevolent character in 'The Lord of the Rings'.
This composite image shows the central region of the spiral galaxy NGC 4151, dubbed the ‘Eye of Sauron’ by astronomers for its similarity to the eye of the malevolent character in ‘The Lord of the Rings’. Image: X-ray: NASA/CXC/CfA/J.Wang et al.; Optical: Isaac Newton Group of Telescopes, La Palma/Jacobus Kapteyn Telescope, Radio: NSF/NRAO/VLA

When heavier stars run out of hydrogen to fuse into helium, the fusion reactions that keep the stars from imploding due to their own gravity become more difficult (as they infeasibly fuse helium into heavier elements) and eventually stop happening. At this stage, they blow away their outermost layer of gases and collapse into neutron stars (If the parent star is heavy enough, the neutron star collapses into a black hole).

The neutron star is an extremely dense, rapidly rotating body composed mostly of neutrons and ridden with powerful magnetic fields. These magnetic fields accelerate particles on and around some neutron stars and eject them in beams from the poles. Because the star is spinning, these beams periodically point toward and away from Earth, making them look like flashing points of light in the night sky.

For this reason, such neutron stars are called pulsars, and pulsars are used as ‘cosmic candlesticks’, relatively fixed points of light that astronomers use to gauge distances in the universe. Pulsars can remain stable for 10-100 million years, making them reliable on par with atomic clocks when it comes to keeping time as well.

The keys to their relevance for human observations are the stability and distinctness of the beams. Astronomers would use any other natural object like pulsars if only they emitted radiation that was long-lasting and distinguishable from the other light in the universe. Now, they might have a new kind of candidates starting with the ‘Eye of Sauron’.

That’s the common name of the galaxy NGC 4151, located about 40 million light-years from Earth. A group of Danish astrophysicists have measured the distance between the supermassive black hole at the heart of this galaxy and Earth by studying how it is heating up gas clouds and a ring of dust that surround it.

The clouds are heated as they’re compressed by the black hole’s intense gravitational pull. In the process, they emit ultraviolet radiation. The UV radiation then heats up a ring of dust orbiting the black hole at a large distance, and in turn the ring emits infrared radiation. Effectively, thanks to the thermal cascade, there are two concentric ‘zones’ of radiation around the singularity.

Astronomers from the Niels Bohr Institute at the University of Copenhagen used the twin Keck Telescopes in Hawaii and this effect to their advantage when trying to measure how far the black hole is from Earth. Darach Watson, an associate professor at the institute, explained,

Using telescopes on Earth, we [measured] the time delay between the ultraviolet light from the black hole and the subsequent infrared radiation emitted from the dust cloud.

Keeping in mind the speed of light, Watson’s team calculated the delay to be 30 light-days, corresponding to a distance of about 777 million km between the cloud of irradiated gas and the ring of dust.

If this weren’t cool enough, the astronomers then used a technique from 19th century (a.k.a. high school) optics to measure the distance between the black hole itself and Earth.

The most powerful astronomical telescopes are not built to observe electromagnetic radiation at all wavelengths because their resolution depends on the wavelength of the radiation they’re observing. Specifically, a telescope with a fixed lens diameter will have lower angular resolution (which is good) when observing radiation of lower wavelengths. So each of the 10-meter-wide Keck Telescopes will have an angular resolution of 8.75 arc-seconds when observing infrared emissions but 1.6 arc-seconds when observing UV light – an increase in resolution by 5.4-times.

But what makes Keck much better is a technique called interferometry. The two telescopes are separated by 85 meters, which makes their collective effective lens diameter 85 meters. The resultant interference pattern due to the difference in the time at which light reaches each of the lenses is then corrected for by computers, giving rise to an image of the object as if it were observed by an 85-meter-wide telescope.

Related: What is Very Long Baseline Interferometry?

Using interferometry, Watson and his colleagues were able to measure the diameter of the entire dust ring. As a result, they had two fixed distances in the night sky: the distance between the ring and the cloud of gas, and the width of the ring. The only thing left to find out the black hole’s distance from Earth was simple trigonometry, and a simple trigonometric calculation later, the astronomers had their answer: 62 million light-years.

Clouds of gas and rings of dust are common around supermassive black holes, which often reside at the center of large galaxies (the one at the Milky Way’s center is called Sagittarius A*). This means the ‘Eye of Sauron’ needn’t be an uncommon occurrence and could instead join pulsars in holding up candles in space’s dark for astronomers.

And coolness wasn’t the only outcome of the Niels Bohr Institute group’s experiment. Their work heralds a long-sought element of precision missing until now in measuring the masses of black holes. As Watson explained, again,

The calculations of the mass of the supermassive black holes at the heart of galaxies depends on two main factors: the rotational speed of the stars in the galaxy and how far it is from the black hole to the stars. The rotational speed can be observed and the distance from the black hole out to the rotating disc of stars can now be calculated precisely using the new method.

Watson & co. were able to find that the ‘Eye of Sauron’ was 40% heavier than expected.

So, not just coolness…

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… but also awesomeness.

Feeling the pulse of the space-time continuum

The Copernican
April 17, 2014

Haaaaaave you met PSR B1913+16? The first three letters of its name indicate it’s a pulsating radio source, an object in the universe that gives off energy as radio waves at very specific periods. More commonly, such sources are known as pulsars, a portmanteau of pulsating stars.

When heavy stars run out of hydrogen to fuse into helium, they undergo a series of processes that sees them stripped off their once-splendid upper layers, leaving behind a core of matter called a neutron star. It is extremely dense, extremely hot, and spinning very fast. When it emits electromagnetic radiation in flashes, it is called a pulsar. PSR B1913+16 is one such pulsar, discovered in 1974, located in the constellation Aquila some 21,000 light-years from Earth.

Finding PSR B1913+16 earned its discoverers the Nobel Prize for physics in 1993 because this was no ordinary pulsar, and it was the first to be discovered of its kind: of binary stars. As the ‘B’ in its name indicates, it is locked in an epic pirouette with a nearby neutron star, the two spinning around each other with the orbit’s total diameter spanning one to five times that of our Sun.

Losing energy but how?

The discoverers were Americans Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst, and their prize-winning discovery didn’t culminate with just spotting the binary pulsar that has come to be named after them. Further, they found that the pulsar’s orbit was shrinking, meaning the system as a whole was losing energy. They found that they could also predict the rate at which the orbit was shrinking using the general theory of relativity.

In other words, PSR B1913+16 was losing energy as gravitational energy while proving a direct (natural) experiment to verify Albert Einstein’s monumental theory from a century ago. (That a human was able to intuit how two neutron stars orbiting each other trillions of miles away could lose energy is homage to the uniformity of the laws of physics. Through the vast darkness of space, we can strip away with our minds any strangeness of its farthest reaches because what is available on a speck of blue is what is available there, too.)

While gravitational energy, and gravitational waves with it, might seem like an esoteric concept, it is easily intuited as the gravitational analogue of electromagnetic energy (and electromagnetic waves). Electromagnetism and gravitation are the two most accessible of the four fundamental forces of nature. When a system of charged particles moves, it lets off electromagnetic energy and so becomes less energetic over time. Similarly, when a system of massive objects moves, it lets off gravitational energy… right?

“Yeah. Think of mass as charge,” says Tarun Souradeep, a professor at the Inter-University Centre for Astronomy and Astrophysics, Pune, India. “Electromagnetic waves come with two charges that can make up a dipole. But the conservation of momentum prevents gravitational radiation from having dipoles.”

According to Albert Einstein and his general theory of relativity, gravitation is a force born due to the curvature, or roundedness, of the space-time continuum: space-time bends around massive objects (an effect very noticeable during gravitational lensing). When massive objects accelerate through the continuum, they set off waves in it that travel at the speed of light. These are called gravitational waves.

“The efficiency of energy conversion – from the bodies into gravitational waves – is very high,” Prof. Souradeep clarifies. “But they’re difficult to detect because they don’t interact with matter.”

Albie’s still got it

In 2004, Joseph Taylor, Jr., and Joel Weisberg published a paper analysing 30 years of observations of PSR B1913+16, and found that general relativity was able to explain the rate of orbit contraction within an error of 0.2 per cent. Should you argue that the binary system could be losing its energy in many different ways, that the theory of general relativity is able to so accurately explain it means that the theory is involved, and in the form of gravitational waves.

Prof. Souradeep says, “According to Newtonian gravity, the gravitational pull of the Sun on Earth was instantaneous action at a distance. But now we know light takes eight minutes to come from the Sun to Earth, which means the star’s gravitational pull must also take eight minutes to affect Earth. This is why we have causality, with gravitational waves in a radiative mode.”

And this is proof that the waves exist, at least definitely in theory. They provide a simple, coherent explanation for a well-defined problem – like a hole in a giant jigsaw puzzle that we know only a certain kind of piece can fill. The fundamental particles called neutrinos were discovered through a similar process.

These particles, like gravitational waves, hardly interact with matter and are tenaciously elusive. Their discovery was predicted by the physicist Wolfgang Pauli in 1930. He needed such a particle to explain how the heavier neutron could decay into the lighter proton, the remaining mass (or energy) being carried away by an electron and a neutrino antiparticle. And the team that first observed neutrinos in an experiment, in 1942, did find it under these circumstances.

Waiting for a direct detection

On March 17, radio-astronomers from the Harvard-Smithsonian Centre for Astrophysics (CfA) announced a more recent finding that points to the existence of gravitational waves, albeit in a more powerful and ancient avatar. Using a telescope called BICEP2 located at the South Pole, they found the waves’ unique signature imprinted on the cosmic microwave background, a dim field of energy leftover from the Big Bang and visible to this day.

At the time, Chao-Lin Kuo, a co-leader of the BICEP2 collaboration, had said, “We have made the first direct image of gravitational waves, or ripples in space-time across the primordial sky, and verified a theory about the creation of the whole universe.”

Spotting the waves themselves, directly, in our human form is impossible. This is why the CfA discovery and the orbital characteristics of PSR B1913+16 are as direct detections as they get. In fact, finding one concise theory to explain actions and events in varied settings is a good way to surmise that such a theory could exist.

For instance, there is another experiment whose sole purpose has been to find gravitational waves, using laser. Its name is LIGO (Laser Interferometer Gravitational-wave Observatory). Its first phase operated from 2002 to 2010, and found no conclusive evidence of gravitational waves to report. Its second phase is due to start this year, in 2014, in an advanced form. On April 16, the LIGO collaboration put out a 20-minute documentary titled Passion for Understanding, about the “raw enthusiasm and excitement of those scientists and researchers who have dedicated their professional careers to this immense undertaking”.

The laser pendula

LIGO works like a pendulum to try and detect gravitational waves. With a pendulum, there is a suspended bob that goes back and forth between two points with a constant rhythm. Now, imagine there are two pendulums swinging parallel to each other but slightly out of phase, between two parallel lines 1 and 2. So when pendulum A reaches line 1, pendulum B hasn’t got there just yet, but it will soon enough.

When gravitational waves, comprising peaks and valleys of gravitational energy, surf through the space-time continuum, they induce corresponding crests and troughs that distort the metrics of space and passage of time in that area. When the two super-dense neutron stars that comprise PSR B1913+16 move around each other, they must be letting off gravitational waves in a similar manner, too.

When such a wave passes through the area where we are performing our pendulums experiment, they are likely to distort their arrival times to lines 1 and 2. Such a delay can be observed and recorded by sensitive instruments.

Analogously, LIGO uses beams of light generated by a laser at one point to bounce back and forth between mirrors for some time, and reconvene at a point. And instead of relying on the relatively clumsy mechanisms of swinging pendulums, scientists leverage the wave properties of light to make the measurement of a delay more precise.

At the beach, you’ll remember having seen waves forming in the distance, building up in height as they reach shallower depths, and then crashing in a spray of water on the shore. You might also have seen waves becoming bigger by combining. That is, when the crests of waves combine, they form a much bigger crest; when a crest and a trough combine, the effect is to cancel each other. (Of course this is an exaggeration. Matters are far less exact and pronounced on the beach.)

Similarly, the waves of laser light in LIGO are tuned such that, in the absence of a gravitational wave, what reaches the detector – an interferometer – is one crest and one trough, cancelling each other out and leaving no signal. In the presence of a gravitational wave, there is likely to be one crest and another crest, too, leaving behind a signal.

A blind spot

In an eight-year hunt for this signal, LIGO hasn’t found it. However, this isn’t the end because, like all waves, gravitational waves should also have a frequency, and it can be anywhere in a ginormous band if theoretical physicists are to be believed (and they are to be): between 10-7 and 1011 hertz. LIGO will help humankind figure out which frequency ranges can be ruled out.

In 2014, the observatory will also reawaken after four-years of being dormant and receiving upgrades to improve its sensitivity and accuracy. According to Prof. Souradeep, the latter now stands at 10-20 m. One more way in which LIGO is being equipped to find gravitational waves is by created a network of LIGO detectors around Earth. There are already two in the US, one in Europe, and one in Japan (although the Japanese LIGO uses a different technique).

But though the network improves our ability to detect gravitational waves, it presents another problem. “These detectors are on a single plane, making them blind to a few hundred degrees of the sky,” Prof. Souradeep says. This means the detectors will experience the effects of a gravitational wave but if it originated from a blind spot, they won’t be able to get a fix on its source: “It will be like trying to find MH370!” Fortunately, since 2010, there have been many ways proposed to solve this problem, and work on some of them is under way.

One of them is called eLISA, for Evolved Laser Interferometer Space Antenna. It will attempt to detect and measure gravitational waves by monitoring the locations of three spacecraft arranged in an equilateral triangle moving in a Sun-centric orbit. eLISA is expected to be launched only two decades from now, although a proof-of-concept mission has been planned by the European Space Agency for 2015.

Another solution is to install a LIGO detector on ground and outside the plane of the other three – such as in India. According to Prof. Souradeep, LIGO-India will reduce the size of the blind spot to a few tens of degrees – an order of magnitude improvement. The country’s Planning Commission has given its go-ahead for the project as a ‘mega-science project’ in the 12th Five Year Plan, and the Department of Atomic Energy, which is spearheading the project, has submitted a note to the Union Cabinet for approval. With the general elections going on in the country, physicists will have to wait until at least June or July to expect to get this final clearance.

Once cleared, of course, it will prove a big step forward not just for the Indian scientific community but also for the global one, marking the next big step – and possibly a more definitive one – in a journey that started with a strange pulsar 21,000 light-years away. As we get better at studying these waves, we have access to a universe visible not just in visible light, radio-waves, X-rays or neutrinos but also through its gravitational susurration – like feeling the pulse of the space-time continuum itself.