We wouldn't be here but for the solstices. Be thankful!

December 21 was the winter solstice in the Northern Hemisphere - the shortest day in the year. The solstices occur on most planets because they do not spin upright, or perpendicular to their orbits.

The Earth, for example, slouches 23.5 degrees on a tilted axis (inclination). This leaves the planet’s North Pole pointed toward the North Star over relatively long periods of time, even as Earth makes its year long migration around the sun. That means the Northern Hemisphere will spend half the year tilted slightly toward the sun, bathing in direct sunlight during summer’s long, blissful days, and half the year cooling off as it leans slightly away from the sun during winter’s short, frigid days. December 21 marks the day when the North Pole is most tilted away from the sun.

But every planet slouches at different angles.

The axial tilt of Venus, for example, is so extreme — 177 degrees — that the planet is essentially flipped upside down with its South Pole pointing up. Perhaps counter-intuitively, that means that there’s very little tilt to its upside-down spin and its hemispheres will never dramatically point toward or away from the sun. As such, the sun’s dance across the sky will remain relatively stable — shifting by a mere six degrees over the course of a Venusian year.

The axial tilt of Venus is so extreme that the planet is essentially flipped upside down with its South Pole pointing up

Had we evolved on Venus, it’s likely that we would not have noticed solstices or seasons at all.

The same can’t be said for Uranus!

An axial tilt of 98 degrees causes the ice giant to spin on its side. So, whereas one of Earth’s poles leans slightly toward the sun at solstice, one of Uranus’s poles points almost directly toward the sun at solstice — as though poised to make a perfect bulls eye. That means that one hemisphere will bask under the sun both day and night, while the other will experience a frigid and dark winter and not catch a glimpse of the sun for that entire season.

Two images of the same view of Uranus made by NASA’s Voyager 2 in 1986. The false-color image on the right shows how Uranus’s pole points towards the sun, its axis tilting at 98 degrees

Such a tilt on Earth would mean that the Arctic Circle didn’t begin 66 degrees north of the Equator, but at the Equator itself. All of North America, Europe, Asia and half of Africa would spend winters in permanent darkness and summers under constant sunlight. And on Uranus, which takes 84 Earth years to orbit the sun, these seasons last for decades.

But the king of extreme seasons is Pluto.

When NASA’s New Horizons spacecraft arrived at the dwarf planet in 2015, scientists discovered a unique world overflowing with surface features that look like networks of drainage channels and even a frozen lake. But given Pluto’s low atmospheric pressure and chilly surface temperature, liquids cannot flow across the surface — at least not today.

Scientists now have an explanation: seasons in Pluto’s past pushed atmospheric pressure high enough to allow liquids of methane and nitrogen to flow and pool on the surface.

A changing axial tilt is the biggest driver of wildly varying seasons on Pluto. Over the course of 4 million years, Pluto’s inclination shifts back and forth between 102 and 126 degrees, causing its equivalent of an Arctic Circle to grow and shrink. That occasionally creates seasons where the atmospheric pressure is high enough that liquid methane and nitrogen can flow.

Although, astronomers remain uncertain how a planet’s seasons might affect its likelihood to host life, it is believed that such dramatic swings — like those on Pluto — are likely a hindrance because they can make a planet unfit to live on for long stretches of time. Life needs a continuously habitable zone to thrive. Similarly, astronomers have long suspected that life would likely not survive on Earth should it have an axial tilt more akin to Uranus.

So, as the sun reaches its farthest point in the sky on December 21, be grateful. Never will the sun dip so far below the horizon that it plunges half of the globe into a months long night and the other half into an equally long summer. Nor does Earth’s tilt change drastically over millions of years, thanks to the influence of the moon. Instead, the sun appears to trot back and forth between the extremes, like the pendulum of a great clock, keeping the planet cozy while steadily counting off its years.

Source - The New York Times

Aldebaran – the fiery eye of the Bull, the red 'Rohini' or the red giant

The reddish star Aldebaran – the fiery eye of the Bull in the constellation Taurus – is an aging star and a huge star! The computed diameter is between 35 and 40 solar diameters. If Aldebaran were placed where the sun is now, its surface would extend almost to the orbit of Mercury. Follow the links below to learn more about this prominent and fascinating star.

Science of star Aldebaran

This star glows with the orangish color of a K5 giant star. In visible light, it is about 153 times brighter than the sun, although its surface temperature is lower (roughly 4000 kelvin compared to 5800 kelvin for the sun).

Aldebaran is about 65 light-years away, much closer than the stars of the Hyades with which it misleadingly seems associated. The Hyades are about 150 light-years away.

Aldebaran is an erratic variable with minor variations too small to be noticed by the eye. It also has a small, faint companion star, an M-type red dwarf, some 3.5 light-days away. In other words, light from Aldebaran would need to travel for 3.5 days to reach the companion, in contrast to light from our sun, which requires 8 minutes to travel to Earth.

How to see Aldebaran

Aldebaran is easy to find. Frequently imagined as the fiery eye of Taurus the Bull, Aldebaran is part of a V-shaped star grouping that forms the face of the Bull. This pattern is called the Hyades.

We can also locate Aldebaran using the famous constellation Orion as a guide. Simply locate the three stars of Orion’s Belt. Then draw an imaginary line through the belt to the right. The first bright star we come to will be Aldebaran with its distinctive reddish-orange glow.

Aldebaran is the 14th brightest star, but five of those that outshine it are only barely visible or not visible at all from much of the Northern Hemisphere. Aldebaran is primarily a winter and spring star. At least, that is when this red star is most easily visible in the evening sky. By early December, it rises shortly after sunset and is visible all night. Three months later it is high to the south at sunset, and sets at around midnight. By early May, it hangs low about the western sunset glow – and before the end of the month, it’s lost altogether. It returns to the predawn sky around late June.

Although it appears among them, Aldebaran is not actually a member of the V-shaped Hyades cluster. It is actually much closer to us in space than the actual Hyades stars.

History and mythology of Aldebaran

Aldebaran is often depicted as the fiery eye of Taurus the Bull. Because it is bright and prominent, Aldebaran was honored as one of the Four Royal Stars in ancient Persia, the other three Royal Stars being Regulus, Antares and Fomalhaut.

The name Aldebaran is from the Arabic for “The Follower,” presumably as a hunter following prey, which here likely was the star cluster we call the Pleiades. The latter was often viewed as a flock of birds, perhaps doves. According to Richard Hinckley Allen in his classic book Star Names, the name Aldebaran once was applied to the entire Hyades star cluster, a large loose collection of faint stars.

In Hindu myth, Aldebaran was sometimes identified with a beautiful young woman named Rohini, disguised as an antelope and pursued by her lecherous father, disguised as a deer, Mriga. Apparently several ancient peoples associated the star with rain. In another Sioux story, Aldebaran is associated with the formation of the Mississippi river in America.

Aldebaran is the name of one of the chariot horses in the movie Ben Hur

On a different note, astronomer Jack Eddy has suggested a connection with the Big Horn Medicine Wheel, an ancient circle of stones atop a mountain in Wyoming, USA. He wrote that the ancient Americans may have used this site as a sort of observatory to view the rising of Aldebaran just before the sun in June to predict the June solstice.

Interestingly, in about two million years, the American space probe Pioneer 10, now heading out into deep space, will pass Aldebaran.

Source - EarthSky.org

Can You Hear Something That Doesn’t Make a Sound?

Recently, University of Glasgow psychologist and researcher Lisa DeBruine created a mini-sensation on social media when she tweeted a playful animated GIF in which an electrical transmission tower appears to be jumping rope and asked, "Does anyone in visual perception know why you can hear this gif?" In a subsequent non-scientific poll of more than 315,000 Twitter users, 67 percent said they heard "a thudding sound" when they watched the animation, and another 3 percent said they heard "something else." Only 20 percent said they heard nothing at all.

That's seven out of 10 people who think they heard a sound accompanying a silent image. So what's up with that?

The explanation, according to research, is that while we think of sound as being generated by the world around us, the experience of hearing sounds actually happens in the auditory cortex, which is located in the temporal lobe of the brain. When something actually occurs — for example, the honk of an automobile horn — that creates sound waves in the air, it causes our eardrums to vibrate, which transfers the information through a complex anatomical path. That eventually generates an electrical signal, which the auditory nerve carries to the auditory cortex, which processes the information and tells us that we're hearing a loud noise.

Interestingly, though, in the absence of sound waves in the air, our brain will try to fill in the silence. In a study, published in Nature Scientific Reports, researchers showed subjects hundreds of different still images, such as a man playing a saxophone or using a power saw, and also images that suggested silence, such as a woman sitting on a sofa reading a book. When the scientists measured the electrical activity in the subjects' brains, they found that the brain's auditory cortex was stimulated by pictures associated with sounds, in less than 200 milliseconds.

Back in 2008, Radiolab's Jad Abumrad spent time in an aneochoic chamber, a space designed to be super-quiet. He discovered that in the absence of actual sound, his brain soon began imagining sounds, ranging from the buzz of a swarm of bees to the vocals from a Fleetwood Mac song.

In yet another study published in Consciousness and Cognition, University College London researchers found that 21 percent of subjects reported being able to hear faint sounds when viewing flashes of light, a phenomenon known as visually-evoked auditory response.

Source -

1. Patrick J. Kiger "Can You Hear Something That Doesn’t Make a Sound?" 19 December 2017. HowStuffWorks.com. 24 December 2017
2. Nature Scientific Reports "When a photograph can be heard: Vision activates the auditory cortex in 110 ms"
3. Consciousness and Cognition "Hearing through your eyes"

Newton's apple: The real story

We've all heard the story.

A young Isaac Newton is sitting beneath an apple tree contemplating the mysterious universe. Suddenly... an apple hits him on the head. "Aha!" he shouts, or perhaps, "Eureka!" In a flash he understands that the very same force that brought the apple crashing toward the ground also keeps the moon falling toward the Earth and the Earth falling toward the sun: gravity.

Or something like that. The apocryphal story is one of the most famous in the history of science and now we can see for yourself what Newton actually said. Squirreled away in the archives of London's Royal Society was a manuscript containing the truth about the apple.

It is the manuscript for what would become a biography of Newton entitled Memoirs of Sir Isaac Newton's Life written by William Stukeley, an archaeologist and one of Newton's first biographers, and published in 1752. Newton told the apple story to Stukeley, who relayed it as such -

"After dinner, the weather being warm, we went into the garden and drank thea, under the shade of some apple trees...he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion'd by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself..."

So it turns out the apple story is true - for the most part. The apple may not have hit Newton in the head, but we'll still picture it that way. Meanwhile, three and a half centuries and an Albert Einstein later, physicists still don't really understand gravity. We're gonna need a bigger apple.

Source - Newton's apple: The real story by Amanda Gefter

Zero is older than we thought it was

Carbon dating finds Bakhshali manuscript contains oldest recorded origins of the symbol 'zero'

The origin of the symbol zero has long been one of the world's greatest mathematical mysteries. Today, new carbon dating research commissioned by the University of Oxford's Bodleian Libraries into the ancient Indian Bakhshali manuscript, held at the Bodleian, has revealed it to be hundreds of years older than initially thought, making it the world’s oldest recorded origin of the zero symbol that we use today.

The surprising results of the first ever radiocarbon dating conducted on the Bakhshali manuscript, a seminal mathematical text which contains hundreds of zeroes, reveal that it dates from as early as the 3rd or 4th century - approximately five centuries older than scholars previously believed. This means that the manuscript in fact predates a 9th-century inscription of zero on the wall of a temple in Gwalior, Madhya Pradesh, which was previously considered to be the oldest recorded example of a zero used as a placeholder in India. The findings are highly significant for the study of the early history of mathematics.

The zero symbol that we use today evolved from a dot that was used in ancient India and can be seen throughout the Bakhshali manuscript. The dot was originally used as a 'placeholder', meaning it was used to indicate orders of magnitude in a number system – for example, denoting 10s, 100s and 1000s.

While the use of zero as a placeholder was seen in several different ancient cultures, such as among the ancient Mayans and Babylonians, the symbol in the Bakhshali manuscript is particularly significant for two reasons. Firstly, it is this dot that evolved to have a hollow centre and became the symbol that we use as zero today. Secondly, it was only in India that this zero developed into a number in its own right, hence creating the concept and the number zero that we understand today - this happened in 628 AD, just a few centuries after the Bakhshali manuscript was produced, when the Indian astronomer and mathematician Brahmagupta wrote a text called Brahmasphutasiddhanta, which is the first document to discuss zero as a number.

Although the Bakhshali manuscript is widely acknowledged as the oldest Indian mathematical text, the exact age of the manuscript has long been the subject of academic debate. The most authoritative academic study on the manuscript, conducted by Japanese scholar Dr Hayashi Takao, asserted that it probably dated from between the 8th and the 12th century, based on factors such as the style of writing and the literary and mathematical content. The new carbon dating reveals that the reason why it was previously so difficult for scholars to pinpoint the Bakhshali manuscript’s date is because the manuscript, which consists of 70 fragile leaves of birch bark, is in fact composed of material from at least three different periods.

The Bakhshali manuscript was found in 1881, buried in a field in a village called Bakhshali, near Peshawar, in what is now a region of Pakistan. It was found by a local farmer and was acquired by the Indologist AFR Hoernle, who presented it to the Bodleian Library in 1902, where it has been kept since.

Source - Bodleian Libraries, University of Oxford

Will Earth undergo 15 days of darkness in November 2017?

The days-of-darkness online hoax is back! Do these hoaxes ever die? No, Earth will not experience 15 days of darkness in November, 2017.

Did NASA announce it? No.

Will it happen? No.

YouTube videos are suggesting the event will be caused by “another astronomical event, between Venus and Jupiter.” Yes, Jupiter and Venus – the sky’s two brightest planets – are having a conjunction low in the east before dawn this month. It’ll be beautiful! It’s just so wrong to use this conjunction – which has happened countless times in Earth history, to the wonderment of all privileged to observe it – to perpetuate a hoax.

As for the idea that NASA has issued a “1,000-page document” on the event for the White House. Well. That’s just entirely fake.

Think about it. What would have to happen for Earth to experience 15 days of darkness? Our day-night cycle stems from Earth’s rotation on its axis around our local star, the sun. The sun shines on half of Earth for part of its 24-hour period; that’s daytime. Nighttime is simultaneously occurring on the opposite side of Earth.

For the whole Earth to undergo 15 days of darkness … what would have to happen? The sun would have to go out for 15 days? Or something would have to shroud the sun? Or pass between us and the sun?

All of those scenarios are unlikely to the point of ridiculousness, when you consider the vast size of our sun. That’s why zero days of all-Earth darkness have occurred in human history so far.

It’s never happened. It’s not going to happen.

This same hoax has been rearing its head every few years, since at least 2011, when the erstwhile Comet Elenin was supposed to pass between us and the sun and cause three days of darkness. In 2014, the number of supposed “dark days” increased to six.

In 2015, an article at Newswatch33 suggested NASA confirmation for 15 days of darkness between November 15 and November 29 of that year. The article said that – according to NASA – such an event hadn’t occurred in over 1 million years.

There were zero days of all-Earth darkness in November, 2015.

So. It didn’t go dark in 2011, 2014, or 2015, and it’s not going to go dark for 15 days in November this year.

It’s interesting that these “days of darkness” rumors all spring up around November and December, when the northern half of Earth is edging toward its winter solstice and shortest day of the year. Let’s face it, it’s darker out there now for us on this half of the globe. Just remember … it’s a natural kind of darkness, a resting kind of darkness.

In fact, for the Northern Hemisphere, the earliest sunsets of the year come in early December. After the solstice, for sure by early January, the longer days will be returning very noticeably as we move toward spring and rebirth.

That’s nature’s cycle, and we can depend on it!

Source - EarthSky.org

Warm air helped make 2017 ozone hole smallest since 1988

At its peak on Sept. 11, 2016, the ozone hole extended across an area nearly two and a half times the size of the continental United States. The purple and blue colors are areas with the least ozone.

Measurements from satellites this year showed the hole in Earth's ozone layer that forms over Antarctica each September was the smallest observed since 1988.

NOAA and NASA collaborate to monitor the growth and recovery of the ozone hole every year. According to NASA, the ozone hole reached its peak extent on Sept. 11, covering an area about five times the size of India - 7.6 million square miles in extent - and then declined through the remainder of September and into October. NOAA ground- and balloon-based measurements also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988.

The smaller ozone hole in 2017 was strongly influenced by an unstable and warmer Antarctic vortex - the stratospheric low pressure system that rotates clockwise in the atmosphere above Antarctica. This helped minimize polar stratospheric cloud formation in the lower stratosphere. The formation and persistence of these clouds are important first steps leading to the chlorine- and bromine-catalyzed reactions that destroy ozone. These Antarctic conditions resemble those found in the Arctic, where ozone depletion is much less severe.

In 2016, warmer stratospheric temperatures also constrained the growth of the ozone hole. Last year, the ozone hole reached a maximum 8.9 million square miles, 2 million square miles less than in 2015. The average area of these daily ozone hole maximums observed since 1991 has been roughly 10 million square miles.

Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.

Scientists said the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing.

First detected in 1985, the Antarctic ozone hole forms during the Southern Hemisphere's late winter as the returning sun's rays catalyze reactions involving man-made, chemically active forms of chlorine and bromine. These reactions destroy ozone molecules.

Thirty years ago, the international community signed the Montreal Protocol on Substances that Deplete the Ozone Layer and began regulating ozone-depleting compounds. The ozone hole over Antarctica is expected to gradually become less severe as chlorofluorocarbons—chlorine-containing synthetic compounds once frequently used as refrigerants - continue to decline. Scientists expect the Antarctic ozone hole to recover back to 1980 levels around 2070.

Ozone is a molecule comprised of three oxygen atoms that occurs naturally in small amounts. In the stratosphere, roughly 7 to 25 miles above Earth's surface, the ozone layer acts like sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants. Closer to the ground, ozone can also be created by photochemical reactions between the sun and pollution from vehicle emissions and other sources, forming harmful smog.

Ozone depletion occurs in cold temperatures, so the ozone hole reaches its annual maximum in September or October, at the end of winter in the Southern Hemisphere.

Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large compared to the 1980s, when the depletion of the ozone layer above Antarctica was first detected. This is because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.

NASA and NOAA monitor the ozone hole via three complementary instrumental methods. Satellites, like NASA's Aura satellite and NASA-NOAA Suomi National Polar-orbiting Partnership satellite measure ozone from space. The Aura satellite's Microwave Limb Sounder also measures certain chlorine-containing gases, providing estimates of total chlorine levels.

NOAA scientists monitor the thickness of the ozone layer and its vertical distribution above the South Pole station by regularly releasing weather balloons carrying ozone-measuring "sondes" up to 21 miles in altitude, and with a ground-based instrument called a Dobson spectrophotometer.

The Dobson spectrophotometer measures the total amount of ozone in a column extending from Earth's surface to the edge of space in Dobson Units, defined as the number of ozone molecules that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 32 degrees Fahrenheit at an atmospheric pressure equivalent to Earth's surface.

This year, the ozone concentration reached a minimum over the South Pole of 136 Dobson Units on September 25 — the highest minimum seen since 1988. During the 1960s, before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 250 to 350 Dobson units. Earth's ozone layeraverages 300 to 500 Dobson units, which is equivalent to about 3 millimeters, or about the same as two pennies stacked one on top of the other.

10 Things You Must Know about Homi Bhabha – Pioneer of Nuclear Program in India

Homi Jehangir Bhabha said

I know quite clearly what I want out of my life. Life and my emotions are the only things I am conscious of. I love the consciousness of life and I want as much of it as I can get 

And he did it. Also known as the father of the Indian nuclear programme, Homi J. Bhabha made the most of his eventful life with his knowledge and intellect. The Indian nuclear physicist was the founding director of two institutions – Tata Institute of Fundamental Research (TIFR) and Bhabha Atomic Research Centre, both of which led to immense growth and development in the field of research.

Here are 10 things about the man who enhanced the country with his impressive scientific ideas and outstanding administration.

1. He went against the wishes of his family to pursue physics

Homi Bhabha’s father and uncle wanted him to become an engineer, so he could eventually join the Tata Iron and Steel Company in Jamshedpur. However, at Cambridge, his interest shifted to theoretical physics and in a letter to his father, he wrote –

I seriously say to you that business or job as an engineer is not the thing for me. It is totally foreign to my nature and radically opposed to my temperament and opinions. Physics is my line. I know I shall do great things here. For, each man can do best and excel in only that thing of which he is passionately fond, in which he believes, as I do, that he has the ability to do it, that he is in fact born and destined to do it… I am burning with a desire to do physics. I will and must do it sometime. It is my only ambition.

Finally in 1933 he received a PhD in nuclear physics with his paper, “The Absorption of Cosmic Radiation.”

2. It was the second world war that kept him in India

In 1939, when the second World War broke out, Bhabha was in India for a short vacation. He had to go back to complete his research at Cambridge, but the war made him change his plans. Thus he joined the Indian Institute of Science (IISc) in Bangalore, as a reader.

3. It was Bhabha who convinced Nehru to establish the nuclear programme in India

While working at IISc, Bhabha did his best to convince notable political leaders, specially Jawaharlal Nehru, to start the nuclear program in the country. With a view of moving towards this goal, he established the Cosmic Ray Research Unit at IISc and also began to conduct independent research on nuclear weapons in 1944. Then in 1948, he wrote to Nehru, the then Prime Minister, and said –

The development of atomic energy should be entrusted to a very small and high powered body composed of say, three people with executive power, and answerable directly to the Prime Minister without any intervening link. For brevity, this body may be referred as the Atomic Energy Commission.

His proposal was accepted and the Atomic Energy Commission was established in 1948. Bhabha was appointed its first director.

4. He advocated for the peaceful use of atomic energy and was against manufacturing atomic bombs

5. He founded two world-class research institutions

TIFR main campus, Mumbai

In June 1954, Bhabha established the Tata Institute of Fundamental Research (TIFR) in the campus of IISc. It was later relocated to Mumbai, and gained international recognition in the fields of cosmic ray physics, theoretical physics and mathematics. On realising that technology development for the atomic energy programme could not be carried out within TIFR, Bhabha built a new laboratory dedicated for the same. It was started as Trombay Atomic Energy Establishment in 1954. The centre was renamed as Bhabha Atomic Research Centre after his death in 1966.

6. In 1954, he was honoured with the Padma Bhushan for his invaluable contributions to science and engineering

7. It was Bhabha who suggested the name ‘meson’, used for a class of elementary particles.

He also gained international recognition for deriving a correct expression for the probability of scattering positrons by electrons – a phenomenon that was named Bhabha scattering after him.

8. He was much more than the father of India’s nuclear programme

In 1950s Bhabha represented India in the International Atomic Energy Agency conferences. He was appointed the President of the United Nations Conference on the Peaceful Uses of Atomic Energy in Geneva, Switzerland in 1955. He served as the member of the Indian Cabinet’s Scientific Advisory Committee. He was also the President of the National Institute of Sciences of India in 1963 and President of the Indian Science Congress Association in 1951.

9. He had a deep love for Art and Music

Bhabha had a deep interest in both art and music. He learned to appreciate classical Western music because of the record collection of his grandfather and aunt. He also started painting when he was a student at Cambridge. He was a patron of contemporary art in India, and used to purchase paintings and sculptures.

10. On Jan. 24, 1966, he died in an air crash near Mount Blanc when he was on his way to Vienna to attend a meeting of the Scientific Advisory Committee of the International Atomic Energy Agency.

His death remains shrouded in mystery, sparking many conspiracy theories including one in which the Central Intelligence Agency (CIA) is involved in the crash to paralyze India’s nuclear program.

Source - The Better India

What are gravitational waves?

Computer simulation of two merging black holes producing gravitational waves.

Scientists working at the LIGO experiment in the US detected elusive ripples in the fabric of space and time known as gravitational waves. There is no doubt that the finding is one of the most groundbreaking physics discoveries of the past 100 years. But what are they?

To best understand the phenomenon, let’s go back in time a few hundred years. In 1687 when Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, he thought of the gravitational force as an attractive force between two masses – be it the Earth and the Moon or two peas on a table top. However the nature of how this force was transmitted was less well understood at the time. Indeed the law of gravitation itself was not tested until British scientist Henry Cavendish did so in 1798, while measuring the density of the Earth.

Fast forward to 1916, when Einstein presented physicists with a new way of thinking about space, time and gravity. Building on work published in 1905, the theory of general relativity tied together that what we commonly consider to be separate entities – space and time – into what is now called “space-time”.

Space-time can be considered to be the fabric of the universe. That means everything that moves, moves through it. In this model, anything with mass distorts the space-time fabric. The larger the mass, the larger the distortion. And since every moving object moves through space-time, it will also follow the distortions caused by objects with big mass.

One way of thinking about this is to consider two children, one heavier than the other, playing on a trampoline. If we treat the surface of the trampoline as the fabric then the more massive child distorts the fabric more than the other. If one child places a ball near the feet of the other then the ball will roll towards, or follow the distortion, towards their feet. Similarly, when the Earth goes around the sun, the huge mass of the sun distorts the space around it, leaving our comparatively tiny planet following as “straight” a path as it can, but in a curved space. This is why it ends up orbiting the sun.

Trampolines: fun and educational

If we accept this simple analogy, then we have the basics of gravity. Moving on to gravitational waves is a small, but very important, step. Let one of the children on the trampoline pull a heavy object across the surface. This creates a ripple on the surface that can be observed. Another way to visualise it is to consider moving your hand through water. The ripples or waves spread out from their origin but quickly decay.

Any object moving through the space-time fabric causes waves or ripples in that fabric. Unfortunately, these ripples also disappear fairly quickly and only the most violent events produce distortions big enough to be detected on Earth. To put this into perspective, two colliding black holes each with a mass of ten times that of our sun would result in a wave causing a distortion of 1% of the diameter of an atom when it reaches the Earth. On this scale, the distortion is of the order of a 0.0000000000001m change in the diameter of the Earth compared to the 1m change due to a tidal bulge.

What can gravitational waves be used for?

Given that these ripples are so small and so difficult to detect, why have we made such an effort to find them – and why should we care about spotting them? Two immediate reasons come to mind (I’ll leave aside my own interest in simply wanting to know). One is that they were predicted by Einstein 100 years ago. Confirming the existence of gravitational waves therefore provides further strong observational support for his general theory of relativity.

In addition, the confirmation could open up new areas of physics such as gravitational-wave astronomy. By studying gravitational waves from the processes that emitted them – in this case two merging black holes – we could see intimate details of violent events in the cosmos.

LISA, a planned space-based laser interferometer, could study astrophysical sources of gravitational waves in detail

However, to make the most of such astronomy, it is best to place the detector in space. The Earth-based LIGO managed to catch gravitational waves using laser interferometry. This technique works by splitting a laser beam in two perpendicular directions and sending each down a long vacuum tunnel. The two paths are then reflected back by mirrors to the point they started at, where a detector is placed. If the waves are disturbed by gravitational waves on their way, the recombined beams would be different from the original. However, space-based interferometers planned for the next decade will use laser arms spanning up to a million kilometres.

Now that we know that they exist, the hope is that gravitational waves could open up the door to answering some of the biggest mysteries in science, such as what the majority of the universe is made of. Only 5% of the universe is ordinary matter with 27% being dark matter and the remaining 68% being dark energy, with the latter two being called “dark” as we don’t understand what they are. Gravitational waves may now provide a tool with which to probe these mysteries in a similar way that X-rays and MRI have allowed us to probe the human body.

Source - The Conversation