What are the notable uses of the General Theory of Relativity in astronomy?

In 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected space-time ripples after two black holes collided about 1.4 billion light-years from Earth. These space-time ripples are known as gravitational waves. LIGO first detected gravitational waves in 2015; 100 years after Einstein predicted their existence. The waves are a part of Einstein’s theory of general relativity.

The matter of Mercury’s orbit has been discussed earlier. It was general relativity which showed how Mercury’s motions were affected by the curvature of space-time. It is even possible for Mercury to be cast out of our solar system due to these changes after billions of years.

Gravitational Lensing is the phenomenon by which a massive object (like a galaxy cluster or a black hole) bends light around it. When astronomers observe that region through a telescope, they can see the objects directly behind the massive object, due to the light being bent. A commonly given example for this is Einstein’s Cross, a quasar in the constellation Pegasus. The light of the quasar was bent by a galaxy nearly 400 million light-years away in such a way that the former appears four times around the galaxy.

The first ever images of a black hole were shown by the Event Horizon telescope in April 2019. The photos once again gave confirmation of several facets of general relativity. It not only showed that black holes exist, but also the existence of a circular event horizon. This is a point at which nothing, including light, can escape.

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Are laser devices inspired by Einstein’s Theory of Stimulated Emission?

You might have come across laser pointers while attending a seminar or conference, or perhaps used it to play with your cat or dog. In the sixty years since physicists demonstrated the first laboratory prototype of a laser in 1960, it has been put to use in numerous ways from barcode readers to systems for hair removal.

The technology behind laser devices is based on Einstein’s Theory of Stimulated Emission. This theory came a year after the discovery of general relativity. Einstein imagined a bunch of atoms bathed in light. He had earlier discovered that atoms sitting in their lowest energy state can absorb photons and jump to a higher energy state. Similarly, higher energy atoms can emit photons and fall back to lower energies.

After sufficient time passes, the system attains equilibrium. Based on this assumption, he developed an equation that can be used to calculate what the radiation from such a system would look like. Unfortunately, Einstein’s calculations differed from the laboratory results. It was obvious that a key piece of the whole puzzle was missing.

Einstein resolved this by guessing that photons like to march in step. This would mean that the presence of a bunch of photons going in the same direction will increase the probability of a high-energy atom emitting another photon in that direction. Einstein labelled this process stimulated emission. He was able to rectify the disparity between his calculations and the observations by including this in his equations.

A laser is a device to harness this phenomenon. It excites a bunch of atoms with light or electrical energy. The photons released as a result are channelled precisely in one direction. Lasers are used in delicate surgery or industrial processes that require precision.

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How is the photoelectric effect connected to our day-to-day life?

Many of the everyday mechanisms  we take for granted, such as automatic lighting of street lamps as daylight fades, how the elevator doors remain open when there is someone in the way, the device that regulates printer toner, and breathalyser tests- all of these use photoelectric cells that are based on Einstein’s theories.

Photoelectric cells, originally used to detect light, used a vacuum tube containing a cathode (to emit electrons) and an anode (to gather the resulting current). Modern versions of these “phototubes” have advanced to semiconductor-based photodiodes. These find applications in solar cells and fibre optics telecommunications.

Photomultiplier tubes are a variation of the phototube. Devices like solar panels that turn light into electricity are possible because of the photoelectric effect.

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Why is it said that Albert Einstein had even contributed to the daily functioning of Wall Street?

Wall Street in New York is the home of the New York Stock Exchange. An army of mathematicians are employed there to analyze and predict the stock price variations. Their employers can potentially earn millions of dollars based on their predictions about which way the prices will jump.

Mathematicians however say that stock markets follow a random walk. This means that unless some spectacular event occurs, the prices have the same chances of decreasing and increasing at the end of any day. If patterns do exist, they will be elusive and difficult to find, which is why financial mathematicians are paid huge sums.

Some of the intricate mathematics used for stock market analyses can be traced back to Einstein. He developed the fluctuation-dissipation   theorem to explain the random movement of particles found in liquids or gases.

This movement called ‘Brownian motion’ was first observed by the Scottish biologist Robert Brown. Brownian motion is highly similar to the price fluctuations seen in stock markets. The similarity was observed in 1970 and since then it has been used on Wall Street. Einstein’s paper on Brownian motion is still used as the basis for certain stock market predictions.

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Does GPS primarily use the General Theory of Relativity?

Einstein’s General Theory of Relativity has predominantly found applications in astronomy through gravity waves, big bangs and black holes. One of its rather unexpected applications was in the multi-billion-dollar industry centred around the Global Positioning System (GPS).

All GPS navigators including Google Maps work by measuring the distance from one point on Earth to one of the satellites orbiting our planet. Though GPS was originally developed with military use in mind, it has since become an inherent part of everyday life.

GPS is based on a collection of 24 satellites, each carrying a precise atomic clock. A hand-held GPS receiver which detects radio emissions from any satellite overhead can find the latitude, longitude and altitude with accuracy up to 15 metres and local time to 50 billionths of a second. The clocks on satellites are ahead of those on Earth by 38,000 nanoseconds. The reason for this is explained by the General Theory of Relativity. Though it may appear as an inconsequential amount of time, if these nanoseconds are not taken into account, GPS systems would be highly inaccurate.

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