While Isaac Newton’s laws of motion and gravity still have validity as a working model for basic astronomy, physicists rely on Albert Einstein’s complex relativity theories in their advanced work.
The special theory of relativity, or STR for short, is the physical theory of measurement in inertial frames of reference proposed in 1905.
In Einstein’s special theory of relativity, also known as special relativity, an event doesn’t necessarily occur at the same time for all those involved.
Simultaneity is relative, as Isaac and Albert show us.
A bolt of lightning hits the front of the first car of a three-car train and the back of the last car of the train. Isaac, who is standing by the railroad tracks, sees the lightning strike the front and back cars simultaneously as the train’s middle car passes him. Albert sits on top of the train’s middle car with an angled mirror that allows him to see both the front and back cars at the same time. He sees the front car get hit by the lightning first, and then the back car.
Both Isaac and Albert are in the middle of the train when the lightning strikes. Both know that the speed of light is constant. But Isaac saw that the lightning struck simultaneously, whereas Albert saw it hit at different times.
The point of the experiment is that while the speed of light remains constant, time moves more slowly for an observer in motion (Albert) than for an observer at rest (Isaac).
This theory goes on to say that a speeding object gets shorter in length and gains mass as it accelerates toward the speed of light. If an object traveled at the speed of light, which is theoretically impossible, it would shrink to nothing in length, have incalculable mass, and its time would stand still.
Einstein’s “general” theory of relativity dealt with the “principle of equivalence,” or the idea that gravitational force (a function of space) and acceleration (a function of time) are, in fact, indistinguishable from each other. They can’t be separated; they are equivalents. We’ll call it “spacetime.”
Isaac and Albert blindfold each other and get on an “up” escalator in a local subway station to test this theory. They discover that they can’t tell whether they are being pulled by the acceleration of the escalator or by a force of gravity attracting them.
Remember, gravity is not always a downward fall, as we usually experience it here on Earth.
Now, imagine starlight traveling (great acceleration) near a large planet (great gravitational force). The light bends as a result of gravity/ acceleration, or “spacetime.” Not only are space and time one concept; “spacetime” is curved.
If all this baffles you, don’t worry. It can take a light-year of “spacetime” to figure out Einstein’s theories. And not everyone agrees with them.