In the 19th century, the scientific consensus was that the realm of matter was separate from the realm of energy. That changed in 1905 when Albert Einstein introduced his theory of special relativity. This theory shows the relationship between energy and matter, and is generally known as E = MC^2. In other words, Einstein showed that matter was a form of energy, and energy was a form of matter.
So, how did Einstein arrive at this conclusion?
Einstein tried to imagine what he would see if he could run after a light wave - at the speed of light. Now, light moves at about 670 million meters per second. At that speed, it would take less than 1/5th of a second to travel from Lagos in Nigeria to London in the UK! Through his thought experiment, Einstein realized that it will never be possible to outrun a light wave. No matter how fast you try to run, light waves will always be faster than you.
To take the idea further, Einstein imagined what would happen to the person who was already approaching the speed of light but still wanted to travel faster. Well, as the person approaches the speed of light, any effort exerted to make him outrun the light wave will not make the individual any faster. Why? Because nothing can outrun light. What would happen instead is that the energy exerted will be converted to mass. An external observer that could see things moving as they approached the speed of light would actually see the person’s mass become bigger and bigger as more effort was exerted. Of course, from the running person’s point of view, everything remains the same - hence the term “relativity”.
The ideas from Einstein’s thought experiments have revolutionized our understanding of the universe. What is remarkable is the fact that Einstein was a theoretical physicist. He did not have any labs or run experiments. Yet, by harnessing the power of his imagination, Einstein gave the world an equation that has been proven to have numerous practical applications, from nuclear energy, to radiocarbon dating, to PET scans among others.
The question today is fairly straightforward: What are you doing with your own imagination?