What is the theory of relativity?

Before the birth of the theory of relativity, it was thought that light, being a wave, required a medium to propagate, and that medium was known as the ether. Furthermore, the theory describing light phenomena seemed to take on a different form if observers were considered to be moving at a certain speed relative to the ether. In 1887, American physicists Albert Michelson and Edward Morley conducted an experiment to measure the speed of the Earth's motion relative to the ether. The result indicated that light propagated at the same speed in all directions on the Earth's surface, which called into question the very existence of the ether. This experiment baffled the scientific community until, in 1905, Einstein developed the Special Theory of Relativity , which is based on two postulates about the invariance of physical laws—that is, how nature is described in a similar way regardless of who observes it.

The first postulate or principle of relativity establishes that the laws of physics (not including gravitation for the moment) are the same as those seen by inertial observers, that is, observers moving at a constant relative velocity. Although the validity of Newton's laws for different observers of this type was known, if light required a medium to propagate, this ether could be considered an absolute reference system, and light phenomena could allow us to determine the propagation speed of an inertial observer with respect to the ether.

Reflecting on the fact that the ether had become unnecessary, Einstein concluded that the laws of electromagnetism (which describe light) must also hold for all observers. Furthermore, the second postulate states that the speed of light in a vacuum is the same as measured by any inertial observer. This implies that it is impossible for an inertial observer to travel at that speed. If a spacecraft were traveling at the speed of light relative to another observer and emitted a pulse of light with a laser in the same direction of motion, that observer would see that the spacecraft and the light remain in the same place, moving at the same speed, but from the spacecraft, they would see that the light is moving away from it, which is a contradiction.

The combination of these postulates leads to some counterintuitive results. Two events that are simultaneous for one observer are not simultaneous for another observer moving relative to her because the time elapsed between events dilates when viewed by a moving observer relative to another observer for whom they occur at the same location, while the length of objects contracts when viewed by an observer moving relative to the object. Obviously, these effects are negligible when the speeds involved are much lower than the propagation speed of light, but they radically change our understanding of space and time. Although spatial and temporal intervals may have different values depending on who measures them, a space-time interval can be defined that is the same for all observers. Physics takes place in the space-time we define through these intervals, and this has profound implications.

But does all physics take place in space-time? The theory of special relativity was not compatible with the Newtonian description of gravity. The great conceptual leap of the Theory of General Relativity , formulated by Einstein in 1915 , consisted in understanding that gravity is not a phenomenon mediated by another force contained in space-time, but is due precisely to the curvature of space-time produced by the masses it contains. Space-time is promoted from an inert structure in which physics occurs to a physical quantity in itself.

The principles underlying the General Theory of Relativity are the general principle of covariance and the equivalence principle. The former generalizes the principle of relativity, as it considers the equivalence of the laws of physics for all observers and in the presence of gravity. The equivalence principle was introduced in Newtonian physics and establishes the equivalence of inertial mass, which measures how an object resists a change in its motion, and gravitational mass, which describes how a body experiences the gravitational field. This relationship is a consequence of the universality of free fall, that is, that all bodies accelerate equally in the presence of gravity and the absence of other forces.

However, Einstein went further by reflecting that the physics in a uniformly accelerated laboratory in the absence of gravity should be the same as the physics in another laboratory immersed in a uniform gravitational field. Following this reflection, one can attempt to understand gravitational phenomena in terms of the quantities used to describe motion in space, which suggests an understanding of gravity as a geometric phenomenon.

The equivalence principle in the relativistic framework has several formulations. The so-called Einsteinian equivalence principle states that non-gravitational physical phenomena (and those with negligible effects on space-time) are not affected, in a small region of space and at any point within it, by the presence of a gravitational field.

This implies that at any point in space and at any time, within a sufficiently small region, the description of nature given by special relativity can be recovered. Thus, the theory of general relativity generalizes the theory of special relativity in the presence of gravity.

Prado Martín Moruno holds a PhD in Physics, is a researcher and professor at the Department of Theoretical Physics at the Complutense University of Madrid .

Coordination and writing: Victoria Toro .

Question submitted by José Ortega Carrascal .

Nosotras Respondemos is a weekly science consultation, sponsored by the L'Oréal-Unesco 'For Women in Science' program and Bristol Myers Squibb , which answers readers' questions about science and technology. These questions are answered by scientists and technologists, members of AMIT (Association of Women Researchers and Technologists). Send your questions to [email protected] or via X #nosotrasrespondemos.