As we have discussed in this astronomy series, as well as the modern physics series, Albert
Einstein’s theory of general relativity is the model of gravitation that has enabled
us to better understand the universe.
Qualitatively speaking, general relativity describes the gravitational force exerted
by massive objects, like this galaxy, as resulting from the warping of spacetime, the four-dimensional
manifold that fuses the three spatial dimensions and the dimension of time.
This warping of three spatial dimensions around a fourth is impossible to accurately visualize,
let alone depict on a two-dimensional screen, so it is typically illustrated in this manner,
leaving us to analogize the 2D grid up to an omni-directional version in our minds as
best we can.
As abstract as this all sounds, the theory, which was published in 1915, was quickly corroborated
in 1919, when Einstein made predictions about how light will follow a curved path around
massive objects.
He proposed that the light from a star directly behind the sun would follow a curved path
around the sun due to the warping of spacetime, and if this was tested during a solar eclipse,
so that the star could actually be seen, the star would appear in a different and very
specific location.
This was indeed observed by Arthur Eddington on an expedition in Africa, so Einstein’s
prediction was correct.
But this was only the first of many victories for this theory.
What else have we observed in the universe in the past century since Eddington’s observation
that makes this model so powerful?
One line of observation has to do with black holes.
As we learned earlier in the series, these objects are remnants of dead stars with sufficiently
large mass and small volume such that the warping of spacetime is significant enough
that not even light can escape the gravitational pull of the object.
Years after black holes were postulated, we eventually built the technology required to
see strong evidence for their existence, including the supermassive black hole at the center
of our Milky Way galaxy, which we believe is a common feature of all galaxies.
Beyond direct evidence of the black hole itself, we can also monitor the motion of the stars
that are closest to the galactic center.
The orbits of these stars only make sense in the context of the existence of the supermassive
black hole, as they orbit around a point of seemingly empty space, accelerating to incredible
speeds as they near the black hole, and then slingshot around to complete another orbit.
Beyond this, we can even see stars exhibiting a perihelion precession in their orbit, just
like Mercury does around our sun.
This orbital behavior is exclusively predicted by the mathematical equations of general relativity,
and is thus another victory for the model.
So as we can see, both the existence of the supermassive black hole at galactic center
and the motion of stars around it are predicted immaculately by general relativity.
But in recent decades, telescopes like the Hubble space telescope, as well as large telescopes
on the surface of the Earth, have become so powerful, that we have been able to look not
just at objects in our own galaxy, but also at other, very distant galaxies.
In doing so, we regularly see examples of another phenomenon predicted by general relativity,
which is called gravitational lensing.
This is when our view of an object, like a distant galaxy, is obscured by another object
in between, like another, closer galaxy, and the light from the more distant object curves
around the object in front, due to the curvature in spacetime that is produced, often traveling
along multiple paths to do so.
We can gather all kinds of valuable information about both the lensed object and the object
doing the lensing when we observe this phenomenon.
This is essentially the same principle that enabled that very first observation by Eddington
in 1919, but when looking into intergalactic space, what we can visualize is far more fascinating.
As we said, when observing the light from a distant galaxy being distorted by the gravitational
effects of a foreground galaxy, the nearer object acts like a lens, distorting and magnifying
the image of the galaxy behind it.
In certain cases, this will result in rings of light called Einstein rings, where the
light is traveling around the object in the foreground in all directions.
Finally, because general relativity deals with spacetime essentially acting as sort
of a fabric, it predicts ripples in that fabric due to the motion of objects that exert gravity.
Because gravity is so weak, these are difficult to detect, but they were thought to be pronounced
when looking at certain exotic objects, like this binary system consisting of a neutron
star pulsar and white dwarf star.
For some time, gravitational waves had been theorized but not yet directly detected, until
2016, when direct evidence was finally collected by LIGO and other gravitational wave observatories.
This was done by observing ultra-rare events like black hole mergers, and later on, neutron
star mergers.
Neutron stars are so dense that the gravitational waves generated by this activity eventually
propagate to earth and can be detected by extremely sensitive equipment, such as the
equipment found at the LIGO observatory, with its two long perpendicular vacuum tubes, that
use light and mirrors to detect any warping of spacetime as the result of gravitational
waves that pass through.
Not only can we detect the gravitational waves produced by such an event, but we can also
observe the incredible kilonova that results, a spectacular explosion that is actually the
main source of production for some of the heavy elements on the periodic table, such
as gold and platinum.
The phenomena we have just examined, including supermassive black holes and the orbits of
nearby stars, gravitational lensing, and gravitational waves, represent overwhelming evidence in
support of general relativity.
The equations of this theory predict their existence and properties, and over the past
few decades, they have been observed and studied at great length, matching predictions immaculately.
Even without examining other phenomena like gravitational time dilation, which has to
be accounted for in GPS satellite clocks for them to function correctly, we can see just
how indispensable Einstein’s theory is.
It is still the reigning model of gravity over a century after its development.
There is more work to be done, as relativity must still be reconciled with quantum physics
in order to be able to describe things like singularities, and therefore the first few
instants of the existence of the universe.
But that will have to wait for another day, and another discovery, perhaps put forward
by another Einstein.