Do you think that Albert Einstein, one of the greatest physicists of all time, whose contributions to physics paved the way to open up new wonders of our universe can be wrong?
The proposal of the theory of general relativity in 1915 was a milestone in the history of science. It is said to be the most successful and widely accepted theory of all time explaining the behavior of almost everything in our universe. There are several pieces of observational evidence proving Einstein’s theory of relativity. The ideas that Einstein put forth about gravity and the curved spacetime could successfully explain most of the celestial activities that couldn’t be explained using Newton’s theory of gravity. For example, scientists couldn’t explain the precession of Mercury’s orbit when it was first recognized in 1859. Einstein, using his equations of gravity explicated this riddle in 1916. As the spacetime is curved around massive objects, the light should bend around huge masses according to the general relativity and this observation was also successfully carried out by the physicist Arthur Eddington and his collaborators in 1919 during a total solar eclipse.
Gravity Probe experiments
In 1976, NASA conducted a space-based experiment Gravity probe A to test the equivalence principle or the gravitational effects on time by sending a hydrogen maser into space. This maser clock rate was compared with the rates of another maser kept on earth. The experiment confirmed that time runs slower when the gravitational effects are large, which was a prediction of Einstein’s theory. This was the first relativity experiment in space by NASA.
The second gravity experiment was named Gravity probe B, which was proposed by scientists at Stanford University, and funded by NASA. This was to test another two predictions of general relativity, the geodetic effect, and frame-dragging, or in simple words, to test the idea that space-time curves around massive objects. It was launched in 2004. Four gyroscopes were sent to space attached to a satellite and the shift in the direction of the rotation of the gyroscopes was measured. And it could prove that space-time is a physical entity that curves like a fabric. Another proof of Einstein’s theory!
A similar experiment to test the gravitational effects on time was also performed by the scientists of the European space agency using the Galileo satellite launched in 2014, and the results were published in 2018. Scientists decided to use Galileo satellites 5 and 6, which were launched to be a part of the navigation system, for the experiment as they were stranded in incorrect orbits. This experiment also concluded with more precision, that the gravitational effects on time are in accordance with the predictions of Einstein’s theory.
Over the past years, we have observed its other predictions like the gravitational waves, the expansion of the universe, gravitational lensing, gravitational redshift, etc. Also, we are using the aid of Einstein’s theory of relativity in the working of GPS and in launching satellites to space. Obviously, the role that the theory of relativity has in physics and astronomy is inevitable. But, does that mean the theory is adequate by all means? No. Though nobody until today has proven the other way, there is something wrong with the theory of relativity.
Scientists across the globe are trying to prove the inadequacy of relativity and to come up with an improved version of the theory or a completely new theory that can explain the working of our universe. Now, here are a few reasons why Einstein’s theory needs improvisations.
QM and Relativity
Einstein explained the curvature of spacetime and came up with the complex equations with an assumption that spacetime is flat at infinitesimally small distances. If the spacetime is flat at infinitesimally small distances, how can we explain the effects of gravity at this small scale! Quantum mechanics is very accurate in describing the physical properties of nature at very small scales and is very effective. The primary issue with the theory of general relativity is that it doesn’t really fit with the postulates of quantum mechanics. This is a reason why we are still puzzled by the idea of singularity. And this is the point where people started thinking about quantum gravity and string theory. String theory adds extra dimensions to gravity and tries to explain the behaviour of gravity at smaller scales. But still, it is not a satisfactory theory to explain our complex universe as general relativity does.
Cosmology
Another major problem that general relativity has is when dealing with cosmology. The observations we have in cosmology are not really following the predictions of the general theory of relativity. Also, when it comes to the interpretation of dark energy and dark matter, general relativity is not helping us. Physicists have tried to develop new theories to explain these concepts. A significant theory among them is the Modified Newtonian Dynamics (MOND), first published in 1983 by Israeli physicist Mordehai Milgrom. This theory was developed as a modification to the Newtonian theory of gravity, in order to explain dark matter. Although it could deal with the dark matter to a certain extent, one of its predictions that the gravitational wave travels not with the speed of light is not true as we have observational evidence for that. Hence the theory failed.
There were also attempts in the scientific community to modify the theory of relativity to make it the best fit, by adding extra terms to gravity. But the problem with adding extra terms and dimensions is that when it succeeds in explaining one thing, it fails in another area. In short, no new theory could replace Einstein’s general theory of relativity to this day. But, the attempts are still going on.
Einstein’s theory of general relativity has survived all the experiments to prove its predictions and is still the best theory that fits well in explaining our universe. That doesn’t mean it is a complete one. We still don’t have a perfect theory to explain everything in the universe. What we can hope is that something miraculous will happen soon and will open another window to the mysteries of our universe.
Check out: “Einstein Was Right Again“
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