Far out cosmology: Einstein's relativity explained (Introduction)

by David Turell , Monday, November 14, 2022, 16:28 (529 days ago) @ David Turell

In simple language:

https://www.quantamagazine.org/

"But this mismatch between intuition and theory makes the leap to a space-time perspective somewhat intimidating. What’s worse, presentations of relativity often take a bottom-up approach — they start with our everyday conceptions of space and time and alter them in the new context of relativity.

"We’re going to be a little different. Our route into special relativity might be thought of as top-down, taking the idea of a unified space-time seriously from the get-go and seeing what that implies. We’ll have to stretch our brains a bit, but the result will be a much deeper understanding of the relativistic perspective on our universe.

***

"Einstein’s theory came to be known as the special theory of relativity, or simply special relativity. In his foundational paper, “On the Electrodynamics of Moving Bodies,” he argued for new ways of thinking about length and duration. He explained the special role of the speed of light by positing that there is an absolute speed limit in the universe — a speed at which light just happens to travel when moving through empty space — and that everyone would measure that speed to be the same, no matter how they were moving. To make that work out, he had to alter our conventional notions of time and space.

"But he didn’t go quite so far as to advocate joining space and time into a single unified space-time. That step was left to his former university professor, Hermann Minkowski, in the early 20th century. The arena of special relativity is today known as Minkowski space-time.

"Once you have the idea of thinking of space-time as a unified four-dimensional continuum, you can start asking questions about its shape. Is space-time flat or curved, static or dynamic, finite or infinite? Minkowski space-time is flat, static and infinite.

"Einstein worked for a decade to understand how the force of gravity could be incorporated into his theory. His eventual breakthrough was to realize that space-time could be dynamic and curved, and that the effects of that curvature are what you and I experience as “gravity.” The fruits of this inspiration are what we now call general relativity.

"So special relativity is the theory of a fixed, flat space-time, without gravity; general relativity is the theory of dynamic space-time, in which curvature gives rise to gravity.

Comment: Sean Carroll then explains how to think about spacetime. Read it for clarity.