r/AskPhysics • u/Rehynman-001 • 2d ago
Question?
What's the best way to understand and explain the theory of relativity
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 2d ago
Surely it depends on the target audience. The explanation of special or general relativity to a 5-year-old would be different from the explanation you would give to a Professor of Mathematics.
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u/YuuTheBlue 2d ago
Relativity is not a singular theory. It's more of a vague concept. You know 'left'? Like, the direction? It works in a funny way most take for granted.
Let's say that a car drives down a road. I ask a guy on the side of the road which way it went. "It went left!" He said. I then walk to the other side of the road and ask someone there which way it went. That person says "It went right!"
Now, these 2 people gave completely different answers. Opposite answers. But, despite that, they didn't actually disagree about any of the facts of the matter. And the reason this can happen is because the exact definition of 'left' and 'right' depends on who you're talking to. One man's left is another man's right. We intuitively understand this about the concepts of left and right. Thus, we call left and right "relative properties". This is in comparison to "Invariant" properties such as "How many fingers am I holding up", which there won't be much ambiguity on.
A Theory of Relativity, in so many words, separates the world into invariant quantities and relative quantities. In practice, this is usually a question of "What is the geometry of the universe". Classical relativity, for example, posits that the universe is a 3-dimensional space. This is a fancy term for how most people already assume the universe to work. 3d space has certain relative properties like right and left and up and down, but it also has some invariant properties like "How far apart are 2 objects", and Classical relativity tells you the math that relates relative concepts like "Right/left" to invariant concepts like "Distance".
The Special Theory of Relativity then came along and challenged the idea that the universe was as simple as a 3d space. It instead posits that it is a "4d Lorentzian Space", aka Spacetime, and this comes with a different set of invariant and relative properties. And then General Relativity comes in positing something even more complicated.
Ultimately, to understand Einstein's Special and General theories of relativity, you need to learn a new type of geometry and how it differs from 3d geometry. That's the gist here.
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u/nanpossomas 2d ago
Best for whom?
Special or general?