Let's say I give you a nice, solid shove, which sends you flying away at 0.9 - that is, 90% the speed of light. Here's how that concept plays out in practice: No matter what, no one can crack that speed. To get to the speed of light, something has to have infinite mass, so it would be impossible to push!" A fundamental aspect of the universe is that there's a universal speed limit: the same speed light goes. But what did Einstein mean by it?.Īccording to the most basic understanding of special relativity, "it's impossible to move at the speed of light, because the faster something goes, the more mass it has. That term isn't used so much nowadays because it causes so much head-scratching. Instead, when Einstein wrote down that equation, he meant something different, usually referred to as relativistic mass. People normally think of mass as something concrete and simple. The confusion comes from the m used in E = m. But where's the momentum in E = m? It looks like there aren't enough letters to account for it. But they do have momentum, which is how things like light sails (also called solar sails) get the oomph they need to glide around the solar system: Their propulsion comes from the sun's radiation pressure. What about light? Photons, or packets of light, don't have any mass, but they have lots of energy. (Image credit: Bettmann / Contributor) What is the full equation? If you define your units - for example, what a "meter" and a "second" are - you can say that the speed of light is around 300 million meters per second (or 670 million mph, although we haven't defined a mile and an hour).Īlbert Einstein first wrote a version of the famous equation in 1905. One way to understand what E= mc^2 means is to think about the speed of light as simply a number that can be expressed in terms of any arbitrary set of units. He later tweaked it to the now-famous formula, according to the American Museum of Natural History. Although Einstein didn't write exactly that E = mc^2 in that paper, he did write an equivalent expression that means the same thing, according to educational website. These insights were a part of his development of the theory of special relativity, which describes the relativity of motion, particularly at near light speed. He found that the conservation of mass (a famous and important law in physics) is the same as the conservation of energy, and vice versa. In a famous paper written in 1905, Albert Einstein discovered an equality between mass and energy.
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