Why was Einstein's theory of relativity such a monumental discovery?
The theory of relativity includes both the Special Theory of Relativity and the General Theory of Relativity. Both are credited to Albert Einstein. What made these discoveries so monumental was that they completely changed what we intuitively believed about the physics of our world and also added new dimensions to our understanding of physics.
The Special Theory of Relativity changed our concept of Euclidian, Newtonian geometry from a concept of an absolute fixed frame of reference to understanding that nothing in space can be considered at rest and fixed. The concept was changed to relative inertial reference frames that extend without limit. The discovery that the speed of light was always measured to be the same speed regardless of the speed of the source of light or the speed of the observer or instrument measuring it. This gelled the relative reference frame concept since absolute, fixed space requires that velocities of all entities can be added as vectors. The new speed of light discovery verified that light did not meet that requirement. By using Lorenz transform equations and the newly discovered fact that the speed of light (in a vacuum) was measured the same in any reference frame and the fact that the laws of physics are the same in any reference frame, the Special Theory of Relativity was born. This began a whole new paradigm in the world of physics.
Probably the most profound scientific advancement resulting from special relativity was the equation of E=mc2 which provided the relationship between mass and energy. This equation led to the development of the atomic bomb. Other very profound discoveries resulting from special relativity include many strange things that happen as we observe something approaching the speed of light and the violation of what we think of as simultaneity of time. As we observe something approach the speed of light, the length gets shorter, the mass gets greater and our time moves faster than time does on the object in motion.
The General Theory of Relativity adds the effect of large masses or the gravity of large masses on light and time. In this more general concept of physics, the reference frames of special relativity had to be limited to local areas where large masses do not distort them. Although small masses like our earth have a significant effect on our weight, the effect on light and time is very small. However, with large masses the effect can be very large. This concept actually predicts black holes that have been verified through astronomy. In addition to the largeness of the mass causing this very strong gravitational field, when the gravity is sufficiently high it causes the atoms to collapse from their normal "almost all space" low density to a "no extra space" high nuclear density form. The combination of this huge collapse and the fact that the force able to act on light or anything else goes up as the inverse square of the distance from the center of the mass increases the effect on light many orders of magnitude. A non-mathematical conceptual explanation of why the light is bent by a gravitational field is explained under the general relativity article.
Another significant area where the relativity theory has provided a better understanding of our universe is in quantum mechanics and small particle motions. Here it was identified that radioactive particles appear to have a higher half-life when approaching the speed of light because a clock on the particle appears to be running slower from our observation reference frame. The General Theory of Relativity is one of the two main pillars of the Big Bang Theory.
When Einstein introduced the cosmological constant in his theory of general relativity, he did so because he was guided by the paradigm of the day that the universe was static (i.e. neither contracting nor expanding). The cosmological constant provided a way of balancing the gravitational contraction caused by matter. It was later discovered by Edwin Hubble that other galaxies appear to be receding away from us, that the universe was actually expanding. When Einstein heard and fully appreciated these observations, he declared that the inclusion of the cosmological constant was his "biggest blunder."
Although relativity has made great strides toward a better understanding of our universe, not only Einstein questioned his theoretical cosmology and our understanding of it, but also many other voices are questioning it today. We cannot always be certain of everything. However, one implication of both special relativity and general relativity is that it is easier to understand how a transcendent God could exist who is beyond space and time.
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