Imagine sitting in a car and pressing the gas. You can tell you're moving since you feel the car's acceleration and see things moving around you. Once you're traveling at a constant velocity, you no longer feel the acceleration but see the outside world moving around you.
Let's take this thought experiment further: imagine yourself moving at a constant velocity inside a black box. Can you tell if you're moving? You no longer have outside clues that you're moving, such as trees whipping by. Perhaps you could perform some physical experiments to gain some insight. It turns out that there's no way for you to conclude that you're moving! Any experiment you perform inside the black box traveling at a constant velocity will give the same results as if the black box were at rest. The black box is what's known as an inertial frame of reference. The laws of physics are the same in all inertial frames of reference. Galileo was the first to notice this remarkable symmetry of nature, referred to today as Galilean relativity.
In the early 1900s, Albert Einstein proposed that if we're sitting inside a black box and can't tell whether we're at rest or moving with a constant velocity, why should light be able to tell either? More precisely, he postulated that the speed of light is the same in all inertial frames of reference. This notion constitutes Einstein's famous special theory of relativity and stands in stark contrast with Galilean relativity. Unsurprisingly, when you're at rest and shine a laser, you will always find that the emitted light travels at the speed of light. But even if you were moving near the speed of light relative to an outside observer and shining the same laser, you and the outside observer would find that the emitted light travels at the same rate—the speed of light.
As a consequence, space and time cannot be absolute. If you're traveling near the speed of light, you would appear flattened in your direction of motion to an outside observer at rest. Moreover, you would find that time passes slower for you than for the outside observer at rest. These phenomena are referred to as length contraction and time dilation, respectively, and highlight that space and time are, in fact, deeply connected entities, forming what is known as spacetime. Motivated by similar thought experiments, Einstein incorporated gravity into his theory of relativity and concluded that mass and energy warp ambient spacetime. Gravity then arises as the motion of objects through curved spacetime. General relativity has predicted several phenomena that are not justifiable with classical physics, including black holes.
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