Albert Einstein's theories
A discussion of outer space and the Universe would be incomplete without at least mentioning Albert Einstein. Perhaps his most famous contributions to the exploration of the Universe are his theories of special and general relativity.
Special Relativity
This theory, which Einstein developed in 1905, is responsible for one of the most famous scientific equations in history: E = mc2. To put it simply, this theory established that time is an illusion; it can vary for different observers depending on one’s speed through space. According to Jesse Emspak, a Live Science contributor, there are three main, simple points to understand about this theory:
First, there is no "absolute" frame of reference. Every time you measure an object's velocity, or its momentum, or how it experiences time, it's always in relation to something else. Second, the speed of light is the same no matter who measures it or how fast the person measuring it is going. Third, nothing can go faster than light. [9]
In accordance with this theory, if an astronaut were able to move very fast relative to the Earth—say, almost as fast as the speed of light—time would pass more slowly for her than for an observer on Earth, a concept known as time dilation. But what does that mean? Consider two twins, one of whom decides to travel to another planet some amount of lightyears away. Traveling close to the speed of light, she returns to Earth younger than her twin, who stayed home. Writing for Scientific American, Ronald C. Lasky explains this phenomenon, the Twin Paradox, with some simple math. Below is a video explanation.
Many people wonder if humans will ever be able to travel at the speed of light. The answer, unfortunately, is no. The more massive an object is, the more energy it requires to speed up. And because mass and energy are interchangeable, an accelerating object’s mass will increase. Near the speed of light, an object’s mass would reach infinity, and would therefore require infinite energy to keep moving. The only reason that light can move at, well, the speed of light, is that light particles, or photons, have no mass.
General Relativity
In between the publication of these two theories, Einstein’s former professor Hermann Minkowski developed the concept of space-time, which recognizes time as the fourth dimension, as well as the union of space and time. Einstein incorporated this idea into his theory of special relativity, eventually publishing his theory of general relativity in 1915, which says that matter weighs down the fabric of space-time, curving it and creating the sensation of gravity. Earth, then, does not orbit around the Sun, but instead is falling in a straight line “...along a space-time surface that is itself curved—warped by the mass of the Sun" [10].
A representation of how Earth weighs down the fabric of space-time.
If some adventurer with a reliable car and a lot of fuel were to travel in a straight line along the surface of the Earth, that “line” would eventually look a whole lot like a circle [11]. So, why doesn’t the Earth crash into the Sun? It’s moving too fast. Because the Earth is in free-fall and moving at the perfect constant speed, it remains in “orbit” around the Sun. It’s the same with the International Space Station. Robert Frost, an instructor and flight controller at NASA, explains it this way: “The ISS doesn’t fall to Earth because it is moving forward at exactly the right speed that when combined with the rate it is falling...produces a curved path that matches the curvature of the Earth" [12].
9. Medium, "A Mathematical Proof That The Universe Could Have Formed Spontaneously From Nothing," The Physics arXiv Blog, last modified April 11, 2014, accessed December 6, 2020, https://medium.com/the-physics-arxiv-blog/a-mathematical-proof-that-the-universe-could-have-formed-spontaneously-from-nothing-ed7ed0f304a3.
10. Jesse Emspak, "8 Ways You Can See Einstein's Theory of Relativity in Real Life," Live Science, last modified March 14, 2017, accessed December 7, 2020, https://www.livescience.com/58245-theory-of-relativity-in-real-life.html.
11. Oakland Ross, "Einstein discovered that gravity is not a force but a curvature," The Star, last modified August 18, 2012, accessed December 7, 2020, https://www.thestar.com/news/insight/2012/08/18/einstein_discovered_that_gravity_is_not_a_force_but_a_curvature.html#:~:text=Based%20on%20this%20thought%20experiment,surface%20that%20gives%20us%20weight.
12. Ross, "Einstein discovered," The Star.