More than a hundred years after the publication of Einstein's seminal theory of relativity, scientists are starting to question its universal validity.
Did you know? Albert Einstein, often regarded as “The Father of Modern Physics” won the Nobel Prize for the discovery of the photoelectric effect (helping develop quantum theory and), not his influential theory of general relativity .
Einstein’s presence in almost every branch of physics marks his versatile genius, but is his standing unshakeable?
To date, general relativity is the most competent theory of gravity due to its unprecedented success in explaining a plethora of observations, for example:
So, should we question Einstein? First let’s look at where his theory shines.
He has revolutionised the concept of force and acceleration advocated by Newton.
It states that the presence of a massive object (e.g. stars) curves space-time around it and acceleration between two masses is due to motion in curved spacetime.
Thus, force is interpreted as the curvature of space-time which causes objects to accelerate. The ability of the object to curve space-time increases with an increase in its mass and the curvature is perceived closer to the source.
Imagine four people holding the corners of a carpet with a ball in the middle. The carpet is like space-time, and the ball is the star. A heavy ball dips’ the carpet more.
Meanwhile, far from the source where space-time is nearly flat, Einstein’s equations give way to Newton’s laws.
General relativity was proposed in 1915. The opportunity to test it came four years later with an ultra-long total solar eclipse on May 29, 1919.
It was visible closer to the Equator, so astronomers from Britain travelled to Africa and Brazil to measure the exact position of stars in the Hyades cluster in the constellation of Taurus when the sky briefly darkened during the eclipse.
Since light from the stars would bend while travelling near the Sun according to Einstein’s theory, the positions of the stars would be different from the predictions using Newton’s laws.
When images taken during the event were compared with the pictures of the same stars taken in the night sky without the Sun, the discrepancy clearly indicated the warping of spacetime by the Sun.
Further verifications of Einstein’s theory came from observations of the orbit of the planet Mercury, with its changing orbit - accurately predicted by general relativity.
The horizon represents the characteristic radius of any mass distribution, such that if the mass is compressed within the horizon it captures all signals emanated from it.
Such an object of gargantuan density is a remarkable prediction of general relativity and features its greatest feat and fiasco.
While the detection of black holes by X-ray telescopes adds to its success, the theory encounters a point inside the black hole where the curvature becomes infinite. This point, also known as the singularity, marks the breakdown of all known physical laws thereby providing room for a more improved theory of gravity, free from singularities.
Black holes can be indirectly detected by observing the motion of the objects they swallow.
As matter from the surroundings or from a binary companion (an ordinary star) falls into a black hole, often in the form of a gaseous accretion disk, electromagnetic waves are emitted. These can be picked up by X-ray telescopes.
By studying the spectrum emitted by the accretion disk one can study the nature of gravity near the black hole and also derive useful information about its mass, rotation etc.
Additionally, the techniques of Very Large Baseline Interferometry have made it possible to obtain the silhouette of a black hole also known as the black hole shadow.
The ‘shadow’ represents the set of directions in the observer’s sky from where no light is received.
Imagine light from a distant star reaching the observer on Earth, encountering a black hole in its path. When the light passes very close to the event horizon of the black hole, due to strong curvatures in the spacetime, it bends and encircles the horizon a few times causing “light rings”.
It then either falls inside the horizon or reaches the observer. These light rings projected onto the observer’s sky mark the boundary of the black hole shadow.
The shape of the black hole shadow can reveal useful information regarding the nature of strong gravity near the horizon. These can also be used to test alternative gravity models against general relativity.
Imagine two black holes spiralling and falling onto each other. When they come very close, they tidally deform themselves before the final merger and collapse.
Such a grand astronomical event distorts the fabric of spacetime and it manifests as ripples in spacetime in the form of gravitational waves.
When a gravitational wave passes an observer, the distortion in spacetime can be detected by a periodic increase and decrease of distances between objects, proportional to the frequency of the wave.
This strain in spacetime is inversely proportional to the distance from the source and hence gravitational wave signals received on Earth from distant black hole mergers is as weak as 1 part in 100 billion billion.
With the advent of the sensitive detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo, it has been possible to detect gravitational waves from binary black hole mergers.
This has opened a new window to witness these violent astrophysical events and the Noble Prize in Physics was awarded to Rainer Weiss, Barry C. Barish and Kip S. Thorne in 2017 for their contribution is revolutionising the gravitational wave astronomy. (Read a Sparrho Digest about it here).
The waveforms of gravitational waves can not only reveal useful information regarding the nature of strong gravity at the merger site but also unveil the state of our universe some billions of years ago when the event actually occurred.
Such signals can therefore be potential probes to test alternative gravity models against general relativity. The merger phase is believed to encode the maximum information regarding the nature of strong gravity.
While gravitational wave observations have by no means ruled out general relativity, one needs more evidence to confirm the validity of the theory in all length scales.
The opportunity to test the nature of strong gravity with gravitational wave astronomy will further enhance with more observations, improved sensitivity of the detectors (which will increase the signal-to-noise ratio) and the launch of the much-expected space based gravitational wave detector LISA (Laser Interferometer Space Antenna).So, should we modify GR?
Despite the unprecedented success of general relativity, there are several unresolved issues. If GR is correct at all length scales, we need to invoke dark matter to explain the flat rotation curves of galaxies.
Dark matter consists of particles that do not emit, absorb or reflect light, but interact only gravitationally. Despite arduous searches such particles have not been detected till date. (Read my Sparrho Digest on dark matter).
Similarly, to explain the accelerated expansion of the universe we need an exotic form of energy, the dark energy which behaves like a fluid with negative pressure, something that we have never encountered in our laboratories.
Additionally, the theory has singularities (e.g. inside black holes) where it loses its predictive power.
Alternative gravity models can be a possible rescue. Such models include extra dimensions, higher curvature gravity, scalar-tensor theories, to name a few.
Some of these models can naturally explain the flat rotation curves of galaxies and the accelerated expansion of the universe. The question of detecting dark matter particles therefore does not arise in this scenario. In addition they can also address the observations which general relativity predicts.
While recent observations of gravitational waves and black hole shadow have by no means ruled out GR, several alternate gravity models are equally considered by scientists.
However, models which significantly deviate from GR are ruled out. We therefore need to test the remaining models against various cosmological, astrophysical and solar system based observations, to gain a deeper insight into the nature of spacetime in the strong gravity regime.
Research Associate, School of Physical Sciences, Indian Association for the Cultivation of Science, Kolkata, India.