Out Of The Syllabus
Neutron stars collision and gravitational waves
"If you want to make an apple pie from scratch, you must first create the universe." - Carl Sagan (A very smart guy).
From hand-drawn maps of the night sky to powerful telescopic images, astronomy has evolved over the past thousands of years. On 16th October 2017, scientists announced the first confirmed detection of ripples in space-time called gravitational waves caused by neutron stars colliding.
Around 130 million light-years away, two neutron stars collided and caused a travelling ripple through space-time, marking the first time that astronomers have detected both light and gravitational waves from the same event in space.
What are neutron stars?
Neutron Stars - city-sized stellar objects with 1.4times the solar mass are ancient remnants of stars that have reached the end of their evolutionary journey through space-time. Born from the explosion of once-large stars, they are the smallest and densest stars known to exist, and possess an enormous gravitational pull.
When formed, neutron stars rotate in space. With gradual compression and shrinking, the spinning speeds up due to the conservation of angular momentum.
The two neutron stars rotate around each other and the closer they get, the faster they spin ultimately leading to the collision and the merging. The spinning and the collision released energy in the form of gravitational waves or the ripples in space-time.
Here's an impression of the collision:
Woah, what are gravitational waves?
Briefly, gravitational waves are ripples in the space-time continuum generated by certain gravitational interactions, exploding stars, merging of black holes or collision of neutron stars, propagated as wave in the outward direction from the source at the speed of light.
It is known that the sound waves disturb air to make noise, similarly the gravitational waves disturb the continuum of spacetime as if it existed in a funhouse mirror. If one such wave were to pass through us, we'd see one of our arms grow longer than the other and if we were wearing a watch on each wrist, we'd see them get out of sync.
In 1916, Albert Einstein predicted the gravitational waves on the basis of his theory of general relativity. The gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation.
By the time, these waves reach us from the source, they distort space-time by an extremely miniscule amount. Gravitational waves have been washing over earth all this while, but our technologies have not been sensitive enough to detect them until recent times.
How are the gravitational waves detected?
During the 1970s, while observing a pair of pulsars orbiting one another, scientists indirectly detected gravitational waves. Measuring the orbit of the two pulsars, the scientists observed that the pulsars were moving close together and determined that the system must have been radiating gravitational waves. These measurements proved for the first time that such waves exist, and earned Russell Hulse and Joseph Taylor, the 1993 Nobel Prize in Physics.
On 17 August 2017, due to the event, the ripples which reached earth were detected by the LIGO team and the Virgo team. The LIGO uses two identical L shaped interferometers in Louisiana and Washington state, each of which uses lasers and mirrors to detect the tiny changes in the spacetime made by the gravitational waves. The VIRGO team in Italy uses a similar interferometer. With three working observatories, scientists and researchers can now identify particularly the region on the sky where the gravitational source is located. This time, scientists were able to locate the event to a 28-square-degree patch of sky, which is about 20 times smaller than the localization of LIGO's first detection.
For the past two years, LIGO (Laser Interferometer Gravitational-Wave Observatory) has been detecting gravitational waves generated by black holes that had crashed into one another. But this detection was different than the rest as this event occurred much closer - 130 million light years away instead of a billion light years away and thus it resulted in a stronger signal.
Source: National Geographic | An all-sky map showing the detected gravitational waves till date. The numbers show the date of detection - GW170817 - the latest detection
How exactly does it help the scientists or the humanity as a whole?
Vicky Kalogera, a Northwestern astrophysicist and LIGO collaborator, explains "Imagine if we lived in a windowless room and all we could hear is thunder and never see the lightning, and then imagine we put ourselves in a room with a window. And not only do we hear the thunder but we see the lightning. Seeing the lightning gives you a whole new opportunity to study a thunderstorm and understand what is really going on."
The gravitational waves are the thunder, while the telescopic observations are the lightning. Since the first detection we've gained a deep insight into the cosmos. We can now detect events that would have otherwise left little to no observable light, letting us understand a valuable amount of information regarding black holes. Also we can now find out the breaking point for the general relativity theory.
We can now see further back in time. With Ligo and the other interferometers, we could potentially detect gravitational waves emerging from the early universe, the big-bang or the beginning.
We can find the source of dark matter. It is approximated that dark-matters occupy 27% of all the matter in the universe but we haven't seen it. Matter creates gravity, and using gravitational we may find their source.
We might even find some other weird or interesting celestial objects, the 'weird-objects' after-all the universe isn't a small place.
Well, thanks to the gravitational waves, astronomers are now able to scrutinize the remnants of these neutron stars and among other things, help solve long standing questions about the origin of precious metals such as gold, silver as well as other heavy metals. Previously it was understood that the supernovae(explosive death of giant stars) were responsible for the majority of universe's gold but now the scientists have found out that the kilonova (merger of neutron stars) is responsible for the production of 16000 worth of Earth's heavy metals.
The last few weeks have been quite remarkable for the gravitational waves. Astronomers recently unveiled the fourth confirmed detection of the waves, produced as a result of colliding black holes. And on 3rd October, LIGO founders Rai Weiss, Kip Thorne, and Barry Barish were awarded the 2017 Nobel Prize in Physics for their work on detecting gravitational waves.
The true achievement was the detection of gravitational waves and electromagnetic signals from the same source which now enables an entirely new set of cosmological tests. We are now entering a new and a much anticipated era of "multimessenger" astronomy where we will observe a source through both gravitational waves and photons. And keeping the history on the record, new tools led to better ground-breaking achievements no one would have imagined. So let's celebrate what we have achieved uptill now and keep our fingers crossed.
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