General Relativity

Published in 1916, the general theory of relativity is often regarded as Albert Einstein's greatest achievement and one of the most remarkable scientific contributions of the 20th century. It is known for redefining gravity beyond the previous Newtonian interpretation and describing it as a geometric property of space-time. General relativity is viewed as an expansion of the special theory of relativity and it differs from the special case because it takes into account the motion due to gravitational fields and other accelerating reference frames, thought to be similar according to the equivalence principle. Along with quantum theory, general relativity is considered to be a central pillar of modern physics and is currently accepted as the leading theory for gravitation.


Earth's mass warps space-time and shapes the Moon's geodesic orbit (Image: NASA).

General relativity describes the attractive force of gravity as being the result of an acceleration produced when something interacts with curved space-time, usually there because of an object with a very large mass. The more mass an object has, the more it warps the space-time around it and the larger its gravitational field is. Any object with some mass or just energy is thought to have a gravitational influence as well but will always experience an attractive pull when it is close enough to another object with a sufficient amount of gravity. Even rays of light that approach a gravitational field will bend towards it in a phenomenon known as gravitational lensing. This effect was confirmed to exist when the light from stars behind the Sun was observed to travel around it during the total solar eclipse of 1919.

Other tests of general relativity include the observation of a gravitational redshift in visible light emerging from a gravitational field, which loses energy and increases its wavelength, shifting towards a redder color. When entering a gravitational field, it behaves in the opposite way by gaining a shorter wavelength and a bluer appearance. Gravitational redshift was first measured in the light emitted by white dwarf star Sirius B in 1925. When a very massive object rotates on its axis, it will also twist the space-time around it as it spins. This is known as frame-dragging and it too was confirmed to exist by observing the orbital shifts that emerged from satellites traveling around our planet. General relativity predicts the existence of gravitational waves, which are ripples in space-time that propagate at the speed of light and are caused by sudden changes in an object's gravitational field, but their direct detection is currently a subject of on-going research.

From the smallest interaction of an elementary particle to the behavior of the cosmos itself, Albert Einstein's ideas have altered the way we view our physical surroundings on many scales. While general relativity accurately describes the gravitational attraction between celestial objects, it is additionally useful for describing other processes such as the expansion or contraction of a universe. Exotic objects such as black holes, white holes, and wormholes are also predicted to exist according to the equations of general relativity. Einstein's dream was to find a theory of everything that could combine the fundamental interactions of nature, specifically gravity and electromagnetism, to show that they were all facets of the same unified force. The search for a theory of everything continues to this day with the goal of bringing together Einstein's laws of general relativity with those of the quantum world in order to develop a consistent theory for quantum gravity.

Special Relativity

Albert Einstein, a German-born physicist and mathematician, is revered as a genius and one of history's greatest thinkers. In 1921, he was awarded the Nobel prize in Physics for accurately describing the photoelectric effect as being a result of light's discreet quantized nature. His special theory of relativity was introduced in 1905, and it greatly increased our understanding of motion, atomic energy, and space and time. It was originally devised as a way to explain how electromagnetic waves could propagate without the need for a medium, known at the time as a luminiferous ether. The ideas that followed revolutionized the way people think about our universe.

Special relativity builds upon Galileo Galilei's work that describes how all uniform, or steady, motion is viewed as being relative to a specific frame of reference. In other words, an object that appears to be moving from one observer's perspective may also appear to be motionless to another observer situated on that very same object. There is no absolute, or fixed, reference frame from which to define objects that are in motion. Special relativity adds to this that while many of the basic properties of motion may vary relatively between any two observers, the speed of light will always be invariant. This is because it is the universal speed limit for matter and energy in motion. Additionally, the physical laws by which these systems undergo any changes are experienced the same in every frame of reference, and the arena in which they happen is referred to as space-time: a unified version of space and time with three dimensions for space and one dimension for time.


Einstein's most widely recognized equation sets forth the mass-energy equivalence (Image: Me).

One of the most important revelations of the special theory of relativity is the mass-energy equivalence. It allows the mass of an object to be seen as a measure of its energy content. Einstein used this now famous relation to describe how matter is able to transform into radiated energy.

Peculiar differences in measurements regarding the passage of time, when an event happens, or even the apparent length of an object may arise if two observers are traveling through space-time with different frames of reference relative to each other. For example, a relative increase in an object's apparent velocity and/or its proximity to a gravitational source causes it to experience time more slowly. This phenomenon is known as as time dilation, and in everyday conditions its influence is very small. However, it can be verified to exist experimentally through the use of separate atomic clocks, one placed on the ground and the other traveling in orbit around the Earth. Varying frames of reference for different observers can also make it difficult to know whether two spatially separated events happen at exactly the same time. Relativity of simultaneity allows for one such event to appear to precede the other from one frame of reference, while in the opposite order from a different perspective, except when one event is the direct cause of the other. Lastly, an observer watching an object approaching an extremely high relative velocity may see the object's apparent length decreasing in the direction of its motion. Length contraction is an effect only noticeable for objects moving at velocities close to the speed of light and is not apparent at everyday speeds.

With the arrival of the special theory of relativity, the constancy of the speed of light was established and the need to account for space and time as being absolute and independent of one another had disappeared. Albert Einstein was able to show that perceived time, along with distances, varied between observers in different circumstances. Matter and energy also emerged as two versions of the same substance, linked together by a relation which is now considered fundamental for explaining processes in nuclear and particle physics. Special relativity has created an impression in modern day science that few other theories have been able to match by quickly gaining widespread acceptance and exciting the imagination of many through its extraordinary implications.