Most of Relativity Isn't Common Sense... Except This Bit - Relativity... Relatively Quickly Ep. 1

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Welcome to Relativity... Relatively Quickly - a series looking at the most basic concepts of three theories of relativity. This is the first episode - what is relativity?

In this video, we start from the beginning. We look at what relativity means, and how it discusses the relative motion between objects. From the perspective, or reference frame, of one observer, they are stationary and everything else around them can be either stationary or in motion. In this video we see how an asteroid moving past a spaceship has motion according to people in the spaceship. However from the perspective of the asteroid, the spaceship is moving and the asteroid itself is stationary.

Both reference frames are correct from their own perspectives. Since we are studying constant speed motion in a single direction, we won't look into too much detail about inertial reference frames, though this will be discussed later.

On Earth, we can think of walking along the ground as being due to us remaining stationary, and the Earth moving beneath our feet. However this is trickier to think about because we always use the Earth as our "stationary" frame of reference. The point is, though, that the Earth moves around the Sun (in the Sun's reference frame), and the Sun moves around the center of the galaxy, and this moves around a common center of mass. There is no one special stationary reference frame.

In this series, we'll study Galilean relativity, special relativity, and general relativity. The first of these is based on "common sense" Newtonian physics, that makes intuitive sense to us. However our intuitions don't work in scenarios that we do not commonly experience as humans. One example of this is travelling at very high speeds (close to the speed of light).

When studying objects moving relative to each other in Galilean relativity, we can use what is known as standard configuration. This is when one reference frame moves relative to another along their shared x (or x') direction with a constant speed v. In addition to this, it is agreed that both reference frames agree on a time coordinate of zero, meaning t = t' = 0, and this occurs when the frames overlap so x = x' = 0.

We jump back and forth between two comoving reference frames and look at how each one would perceive the motion of a third object. We do this by plotting a distance-time graph for each frame. One frame could see the object as stationary, while another could see it as moving! And this is all true without accounting for the spacetime bending weirdness of special and general relativity.

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Timestamps:
0:00 - The basic concepts of relativity: what is it?
2:32 - All the theories of relativity.
3:27 - Big thanks to Wren for sponsoring this video!
5:13 - Galilean relativity and reference frames
7:41 - Standard configuration
8:32 - Weirdness in other relativity theories

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