There is no doubt that within the ‘Space’ topic, students struggle most with Einstein’s Special Relativity. It is perfectly normal to be confused by it and definitely expected! It is a complex theory with conclusions that seem to contradict our basic and intuitive understanding of Physics. On top of that, there are no real ways to observe Einstein’s predictions in the classroom! Here is a brief summary of what you need to know. Einstein’s key idea was that the speed of light is constant no matter where you are or how fast you are moving i.e. in all frames of reference. Already this might confuse some as it contradicts our basic understanding of relativity. For example, if someone throws a ball forward out of a moving car, the ball will be moving faster for a bystander than say the person throwing the ball. The ball has different velocities in different frames of reference. This is something we all experience daily, but it does not apply to light. Light always travels at 3×10^8 m/s. Einstein explained that because light does not change, everything else which physicists thought to be constant in the universe change to accommodate. Mass, length and time which we have always been taught to be concrete values can actually change relative to their observer! For example, if a spaceship flies by the Earth at a speed close to the speed of light, a scientist on Earth will observe time within the ship to move slower, the mass of the ship to be larger and its length shorter than the values measured by the ship’s pilot. This is because to the pilot, the ship is not moving at all since he is moving at the exact same velocity. But for the scientist on Earth, the ship is moving very fast, and Einstein’s predictions come into play. These effects are known as ‘time dilation’, ‘mass dilation’ and ‘length contraction’ (dilation meaning to get larger and contraction meaning to get smaller). The Physics syllabus requires the use of three formulas provided to calculate differences in these measured values. Interestingly, these three effects have major impacts on hypothetical space travel at such velocities:
If a spacecraft can travel at a relativistic velocity then its pilots’ time will run significantly slower and they will age much less compared to people on Earth. The extremely lengthy space travel as observed by the people on Earth will be reduced considerably according to the pilots; this allows the pilots to make prolonged space travel within their lifetime. Also, when they return, they will see their children and probably grandchildren to be older than they are; they are able to ‘see the future’!
An increase in mass as the speed of a spacecraft approaches c means that it would become more difficult to further accelerate the spacecraft once its velocity becomes relativistic. This factor limits the speed of the spacecraft with the maximum speed being one that is slightly under the speed of light even in an ideal situation
When a spacecraft is moving through space, relative to the pilots on the spacecraft, the space in front of the spacecraft is moving towards them. The distance of the journey will appear shorter to the pilots than that being measured by people on Earth.
Hopefully this article has helped you understand Einstein’s complex Theory of Special Relativity. If you don’t believe me about these seemingly crazy effects, remember that they have been scientifically proven and research the Hafele-Keating experiment. If you still don’t quite understand why Einstein thought the speed of light is constant in all frames of reference, research his mirror and light bouncing thought experiments. And if you think you understand everything I’ve said but still don’t get how Einstein could come up with these conclusions, keep in mind that physicists work through very complex mathematics. Einstein didn’t come up with his theories out of nowhere, the numbers told him they were true!