Author: Viktorie Navarova
Background
Anybody who come in touch with theory of relativity (general or special, does not matter) for the first time, he must be overwhelmed by that concept, in my opinion. It is a quite strange idea, which you have to understand at first to accept it. So I was thinking about the time, when Einstein came with his work for the first time and introduced it to the academic society of his time. I supposed they could have not believe it for some time and I understand that. It was a very revolutionary idea, stating that Newtonian point of view is not true, if you consider velocity close enough to speed of light. But nowadays, almost exactly 100 years after publishing STR (in 1916), scientists take it for granted. There are maybe few, who deny relativity, but they are for sure in minority. So what did happen during this period of time? Why vast majority of academics accepted it in, in my opinion, such a short time?
From the book (R. Wolfson): ‘Behind Einstein’s theories is a profoundly simple principle that can be stated in a single statement: The laws of physics are the same for all observers, regardless of their state of motion.’
In my opinion, in physics hold the principle that the most simple explanation is most probably the right one. If something is complicated for us, it could be just because we do not know enough about it to understand it correctly. A nice example is Maxwell’s ‘second great unification in physics’.
Selected problem
How was the general theory of relativity proved? Which experiments supported this concept?
My own explanation
Actually I do not have any concrete idea… I am sure that some irreversible proofs (e. g. many different proofs with low probability of being incorrect) were made, because otherwise there will be a lot more discussion until today about it.
I think it would be related to the domains of physics which were developing a lot in 20th century, which are mainly ‘the very large scale physics’ - astrophysics - and ‘the very small scale physics’ - quantum physics. Because these two spheres were created or developed so much, they cannot be inconsistent with the theory of relativity. So at least they do not disprove the theory of relativity and probably some of the experiments or observations support and prove the theory of relativity.
Searched results
I would like to start with Einstein himself. When he published his theory in 1916, he probably already had some evidences for his statements or he had the ideas how to prove them.
http://www.pitt.edu/~jdnorton/homepage/research/hist_SR.html
From this source it seems that Einstein was using a process which can be called ‘thought experiment’, basically he was just thinking about the problems but he had no proofs.
http://www.quora.com/Was-Einsteins-theory-of-relativity-proved-right-while-he-was-alive
Einstein along with his general theory of relativity announced three predictions, which can be observed and be in accordance with his theory. They are the perihelion precession of Mercury, the bending of light and gravitational redshift of light.
http://physics.ucr.edu/~wudka/Physics7/Notes_www/node98.html
A long-term problem in the study of the Solar System was that the orbit of Mercury did not behave as required by Newton's equations.
To understand what the problem is, let me describe the way Mercury's orbit looks. As it orbits the Sun, this planet follows an approximate ellipse: it is found that the point of closest approach of Mercury to the Sun does not always occur at the same place but that it slowly moves around the Sun. This rotation of the orbit is called a precession.
The precession of the orbit is not peculiar to Mercury, all the orbits of planets precess. In fact, Newton's theory predicts these effects, as being produced by the pull of the planets on one another. The question is whether Newton's predictions agree with the amount an orbit precesses it is not enough to understand qualitatively what is the origin of an effect, such arguments must be backed by hard numbers to give them credence.
The precession of the orbits of all planets except for Mercury's orbit can be understood using Newton’s equations. But Mercury seemed to be an exception. As seen from Earth the precession of Mercury's orbit is measured to be 5600 seconds of arc per century (one second of arc = 1/3600 degrees). Newton's equations, taking into account all the effects from the other planets (as well as a very slight deformation of the sun due to its rotation) and the fact that the Earth is not an inertial frame of reference, predicts a precession of 5557 seconds of arc per century. There is a difference of 43 seconds of arc per century. Maybe it does not look so big difference, but from scientific point of view it is significant and explanation was needed.
This difference cannot be accounted for using Newton's method. Many complementary explanations were invented (such as assuming there was a certain amount of dust between the Sun and Mercury) but none were consistent with other observations (for example, no evidence of dust was found when the region between Mercury and the Sun was carefully examined). In contrast, Einstein was able to predict, without any made-up adjustments, that the orbit of Mercury should precess by an extra 43 seconds of arc per century should the general theory of relativity be correct.
http://simonsingh.net/media/articles/maths-and-science/1919-eclipse-and-general-relativity/
‘The eclipse has been the subject of scientific interest ever since the time of the Ancient Babylonian astronomers, but of all the thousands of eclipses studied by scientists, the most important one was the eclipse of 1919, which was able to provide the clinching evidence in favour of Einstein’s theory of general relativity.’
So an observation of solar eclipse in 1919 can be considered as the first proof. Inconsistency of Mercury’s orbit was already known before Einstein came up with his theory. The predictions based on general relativity were in good agreement with the observed data, but technically speaking it was not a prediction, as the data were known ahead of time. So I do not consider it as a proof.
Although general relativity was a radically new formulation of gravity, its predictions were largely consistent with Newton’s highly successful theory of gravity. While calculating only with ‘common distances’ and ‘common speeds’, Newton and Einstein would get basically the same results, differing only in small decimal places. That was the reason why scientists had to consider larger scale and speed of light, during the process of proving theory of relativity.
Based on Einstein’s theory physicists were able to make one prediction which was quite different from the one based on Newton’s theory. Einstein himself predicted that any gravitational field should bend rays of light much more than was expected by Newton’s theory of gravity. Although this effect was too small to be observed in the laboratory, Einstein calculated that the immense gravity of the sun would deflect a ray of light by 1.75 seconds of arc – less than one thousandth of a degree, but twice as large as the deflection according to Newton. This is significant enough to be measured.
Einstein pictured in his mind a scenario where the imaginary straight line of sight between a star and an observer on Earth would be just blocked by the edge of the Sun. Einstein was convinced that the star would be still visible because gravity bends the rays of light around the Sun and towards the Earth. The sighting of a star that should have been blocked by the sun would prove Einstein’s theory is right, but it is generally to impossible to see starlight that passes close to the Sun, because it is mixed with the light rays of the Sun itself. However, during an eclipse, the Sun is shield by Moon, and under such conditions a gravitationally distorted star should be visible.
Arthur Eddington compared his eclipse photos with images taken when the sun was not present, and announced that the sun had caused a deflection of roughly 1.61 seconds of arc, a result that was in agreement with Einstein’s prediction, therefore validating the theory of general relativity. Einstein’s model is describing the reality better than Newton’s model.
In recent years, scientists have questioned Eddington’s margin of error, arguing that his equipment was not sufficiently accurate to discriminate between the predicted effects of the rival gravitational theories. This is not very important in these days, because Einstein's prediction has since of course been repeatedly verified to much greater precision.
http://www.einstein-online.info/spotlights/redshift_white_dwarfs
Gravitational redshift is the process by which electromagnetic radiation originating from a source that is in a gravitational field is reduced in frequency when observed in a region of a weaker gravitational field. As an example can serve the light coming from the Sun (strong field) to the Earth (weak field).
The gravitational redshift was first measured on earth in 1960s at Harvard University. They examined gamma rays emitted and absorbed by atomic nuclei. The gravitational redshift of light coming to us from the Sun has also been observed, but the accuracy is not very good because of gas motions on the solar surface. Anytime when light is emitted by a moving source, there is a motion-dependent frequency shift called a Doppler shift, and in the case of the Sun, the Doppler shifts due to the moving gas are larger than the gravitational redshift due to the light having to climb out of the gravitational field of the Sun.
The expected amount of redshift for light from the surface of a massive object reaching a distant observer is proportional to the object's mass divided by its radius. This means that the stars astronomers call White Dwarfs are interesting candidates for observation. White dwarfs have masses close to that of the Sun, but radii smaller by factors near 100. The following illustration shows the relative sizes of our sun, the earth, and a White Dwarf star. The Sun is so large that we can only show some part of its disc here.
Consequently, the shift should be much larger for them than for our Sun. Perhaps the best known white dwarf is Sirius B, the white dwarf companion to the star Sirius you might have seen in the night sky. It is not an object visible with the naked eye, but it can be observed with telescopes - the following image was taken with the Hubble Space Telescope. Sirius B is the small object visible on the left-hand side; the cross-like structure and the small ring around Sirius B are artefacts caused by the telescope's optical systems.
The very small radius of Sirius B was recognized in the 1920s, and several efforts were made to measure the gravitational redshift of light reaching us from Sirius B. The results obtained was accepted but later scientists noticed that about half the light which was measured was actually scattered from the much brighter Sirius A.
As Sirius A and B orbit each other, their apparent separation in the night sky changes from year to year. From 1930 to 1950, the two stars were so close in their mutual orbit that no precise measurement was possible. The first correct determination was in the 1950s, and this was much improved later in 1970. The result agreed with general theory of relativity.
http://www.popularmechanics.com/space/deep-space/a6175/5-recent-tests-that-prove-einstein-right/
Einstein's general theory of relativity was exposed to many rigorous scientific testing. All the time new ways how to test it are inventing. Here are five recent tests of the theory. It still holds up.
New explanation
Einstein was a huge thinker, he made a lots of thought experiments. He also thought of three predictions which he postulated in the time of publishing general theory of relativity. The actual experiments proving theory of relativity were carried by other scientists.
The predictions based on general theory of relativity about Mercury’s orbit were fitting the data, but it was not a prediction, as the data were known before. So I do not consider it as a real proof.
The first experiment which validated Einstein’s theory was conducted in 1919, published 1920. From photographies, it was seen that light coming from a distant star is bended while passing around Sun, due to the gravitation field of Sun.
It took more time to observe correctly the gravitational redshift of light but after the measuring devices became accurate enough, this another approving phenomenon.
TR is being proved everyday. Ok, that is an exaggeration, but it is proved very often. Any astronomical measurement are in accordance with it (maybe our measuring devices still do not have enough accuracy to show to scientists that something is wrong/missing in our contemporain explanation). Also experiments in CERN or on other particle accelerators are in accordance with it.
Additional questions
https://einstein.stanford.edu/SPACETIME/spacetime3.html
Einstein's theory of general relativity has passed every test that it has ever been put to. Nevertheless there are at least four good reasons to think that the theory is incomplete and will eventually need to be overthrown in just the same way that Newton's was.
References
http://en.wikipedia.org/wiki/Criticism_of_the_theory_of_relativity
http://en.wikipedia.org/wiki/General_relativity
http://en.wikipedia.org/wiki/History_of_general_relativity
http://en.wikipedia.org/wiki/Tests_of_general_relativity
http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/
http://www.einstein-online.info/dictionary/tests-of-general-relativity-classical
http://www.astro.sunysb.edu/rosalba/astro2030/GeneralRelativity_tests.pdf
https://einstein.stanford.edu/RESOURCES/KACST_docs/KACSTlectures/KACST-IntroPhysicsInSpace..pdf