Celebrating 100 years of Einstein's theory of relativity

We will mark a rather important anniversary in our overall understanding of the universe around us Wednesday, Dec. 2,. It was  exactly 100 years ago on Dec. 2, 1915, that the German physicist Albert Einstein published what is now known as his General Theory of Relativity.

Einstein already had a remarkable string of accomplishments to his credit by this time. A decade earlier he had proposed that a beam of light, instead of being composed of waves, was instead composed of individual packets of energy called photons, as a means of explaining a phenomenon involving light called the photoelectric effect which up until that point had eluded explanation.

Einstein’s explanation was soon verified, and he was awarded the 1921 Nobel Prize in physics for his work in this area.

During the same year as his work on the photoelectric effect Einstein also published what has become known as his Special Theory of Relativity. He started with the assumption that all observers, regardless of how fast they are moving, would measure the speed of light to be the same, and worked out the consequences from there. His calculations led to such phenomena as time dilation, i.e., the slowing down of time as one approaches the speed of light, and the equivalence of mass and energy, represented by the now-famous equation E = mc2. All of Einstein’s conclusions resulting from Special Relativity have been experimentally verified in the decades that have elapsed since then.

With General Relativity Einstein tackled the issue of gravity, which was difficult to reconcile with the more recent developments in physics. The equations of gravity had been worked out in the late 17th century by the British physicist Isaac Newton, and although these had worked remarkably well in explaining the behavior of objects on Earth and elsewhere in the universe, these were beginning to seem incompatible with the nature of matter as was being revealed by the emerging science of quantum mechanics.

Newton had proposed that gravity was a force acting between two objects in space separated by a distance, although the exact nature of this force and how it was transferred from one object to another remained a mystery. Einstein proposed instead that the universe could be considered as being composed of spacetime, wherein time is a separate dimension of space although multiplied by the speed of light, and that the behavior of objects within a gravitational field could actually be explained by their motions through this four-dimensional spacetime.

Much of General Relativity seems counter-intuitive to our normal way of viewing things. For example, the behavior of a ball that has been tossed up by one hand which then arcs down to the other hand seems quite explainable as being influenced by the Earth’s gravitational field. However, in General Relativity the ball has actually traveled in a straight line through spacetime; the person who tossed the ball has traveled on a slightly different path through spacetime, and thus perceives the ball’s motion as being curved.

General Relativity was successful in explaining a previously-unexplained slight drift in the orbit of the planet Mercury. Meanwhile, one of the predictions made by General Relativity is that light will seem to travel a curved path when passing through the gravitational field of a massive object. This was experimentally verified during the total solar eclipse on May 29, 1919, when photographs taken during totality showed that the apparent locations of the background stars beyond the sun had shifted exactly as had been predicted by General Relativity.

This same concept has led to the phenomenon of gravitational lenses, wherein close-by massive objects will bend the light of more distant objects and make them more visible. Numerous examples of this are now known, including distant galaxies that have been made more visible – sometimes even with multiple images – and which have provided evidence for the so-called dark matter that pervades the universe. The existence of planets orbiting relatively nearby stars have been revealed via this phenomenon when the parent stars — and the planets — have passed directly in front of more distant stars.

General Relativity also predicts the existence of gravitational waves, ripples in spacetime created by extremely energetic events. These would be very weak, and at this time no conclusive evidence for these waves has ever been produced, but that may soon change. Extremely sensitive detectors at gravitational wave observatories at locations in the U.S. and elsewhere will soon be operational, and meanwhile next Wednesday – on the exact anniversary of Einstein’s paper — the European Space Agency will be launching its Laser Interferometer Space Antenna (LISA) Pathfinder mission, which is designed to be a test bed for a three-spacecraft space-based gravitational wave detection mission scheduled for launch in the 2030s.

While performing his calculations for General Relativity Einstein realized that they predicted an expanding universe. The general consensus at that time was that the universe was static, so Einstein arbitrarily inserted a cosmological constant into his equations to keep things that way. A little over a decade later the American astronomer Edwin Hubble demonstrate that the universe is indeed expanding, and Einstein retracted his cosmological constant, calling it “the greatest blunder of my life.” However, the discovery of dark energy in the late 1990s, wherein the expansion of the universe is actually accelerating, suggests that there might be some kind of cosmological constant embedded within the fabric of spacetime after all.

There are in fact some competing theories to General Relativity that have been proposed over the years, and the upcoming efforts should hopefully reveal which of these is most correct in explaining the goings-on in our universe. Meanwhile, although for practical purposes all of our activities, including space missions such as the New Horizons mission which flew by Pluto in July 2015, can be calculated from Newton’s law of gravity, General Relativity and its cousins continue to lead our quest to determine the very nature of reality in this universe in which we live.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://earthriseinstitute.org