Einstein's equivalence principle is the most common way of applying this kind of postulate linked to general relativity.
Equivalence principle is the name that identifies a postulate connected to the theory of general relativity and other conjectures associated with gravity .
Albert Einstein used this foundation when he noted that the metric structure of the context surrounding particles in free fall within the framework of gravitational fields is decisive in marking the trajectory that said particles will have.
In order to provide information of interest on this topic and contribute to refreshing concepts or adding knowledge, below we will describe the varieties, applications, consequences and verifications of the principle of equivalence .
Types of equivalence principle
In the development of the laws focused on the movement of a body, three variants of the equivalence principle are considered.
One of the possibilities has been called the principle of weak equivalence . Galileo Galilei knew how to allude to this, which is reflected in the second of Newton's laws of motion referring to inertial mass . The weak formulation states that a test particle that is in free fall exhibits a movement that is independent of its structure and composition.
Taking into account the law that speaks of universal gravitation , it is noted that the principle of weak equivalence results in an equality between gravitational and inertial mass .
There is, on the other hand, the principle of strong equivalence , which aims to exalt the geometric essence of gravity . In this case, the statement indicates that a test body has a gravitational movement that does not depend on its constitution but only on the initial position it has in space-time . In experiments carried out in the laboratory, the movement, considered in an inertial reference system scenario, ends up being independent of its location in space-time and the speed of the laboratory.
When the equivalence principle related to Galilei and the idea of special relativity are combined, the so-called Einstein equivalence principle gains prominence. Here it is alluded to that the results of non-gravitational experiments carried out in a laboratory, with a displacement contextualized in an inertial reference system , are independent in relation to their location in space-time and the speed of the laboratory .
According to the principle of equivalence, in a free fall within a reference system the effects of gravity are nullified, and the effect of special relativity comes into play in that context.
Consequences
Different consequences arise based on the principle of equivalence . One of them involves geodesics and establishes, mathematically, that the trajectories marked by masses in gravitational fields are geodesic lines in space-time .
It is also interesting to note that general relativity is supported by both the principle of special relativity and the principle of equivalence .
The aforementioned principle of equivalence also influences when calculating the Lagrangian of the gravitational field (it is necessary to master, in this case, the notions of scalar curvature , metric tensor , Christoffel symbols and scalar functions ) and in the punctual cancellation that involves the gravitational field , which cannot be detected in accelerated systems.
Within the framework of the GRAVITY project, a group of physicists partially demonstrated Einstein's equivalence principle in the vicinity of a supermassive black hole.
The equivalence principle in practice
Research and testing to apply and demonstrate the principle of equivalence does not stop and becomes increasingly more precise thanks to accumulated knowledge and the advancement of technology .
A few years ago, for example, a group of physicists carried out a study where the principle of local position invariance (LPI) was of great relevance. The researchers, as they officially announced, managed to demonstrate a significant part of Einstein's equivalence principle in the vicinity of a supermassive black hole whose location exceeds a distance of twenty thousand light years.
Using a minisatellite, meanwhile, experts from entities in the United States and France documented in space the universal nature of the principle of equivalence fulfilled in the free fall of elements. According to the conclusions published in a scientific journal, it was discovered that a pair of objects with different masses fall into space territory at a very similar rate, registering a margin of difference of just two billionths of a percentage when comparing one and the other. Scientists involved in an investigation of international significance also found compliance with the principle of equivalence in a system made up of a pair of white dwarfs and a pulsar, which they observed for a period of six years to, among other aspects evaluated, establish its acceleration .
Members of the University of Granada and the Institute of Astrophysics of the Canary Islands , to detail another case useful as a reference, gave impetus to a revolutionary experimental method that was made known as a test of the equivalence principle . With it, it has been possible to test and validate Einstein's equivalence principle , thus achieving a better understanding of what gravity consists of, on cosmological scales. In this framework, specialists precisely and clearly measured the redshift in a large number of quasars . This information is essential when it comes to calculating the mass of each of the detected black holes .