You can understand Einstein's
Theory of Special Relativity
{ within 5 minutes ! }

iou – I (Craig Rusbult) began writing this page in May 2022, and it's very incomplete.
But today you can see an explanation, in Sections 16.1 & 16.2 of the chapter
about Relativity in my 1989 textbook, Physics – Power Tools for Problem Solving.
Later, maybe in August, I'll revise my earlier explanation, and you'll see it in this page.

And today you can understand why it might be more logical to call it
Einstein's Theory of Constancy instead of his Theory of Relativity.

a disclaimer:  In 5 minutes you can understand WHY time must be relative,
IF two other things (speed of light, laws of physics) are not-relative, i.e. are invariant —
and all of our observations about nature have persuaded scientists that this "IF" is true —
although you won't understand everything about Special Relativity.  (and certainly not
about the General Relativity that is much more complex, and more difficult to understand)

      Here is the key idea in understanding relativity:  light is very strange.  Its behavior is not like anything in our human experience.
      normal behavior:  If you're on a train that's moving at 250 mph (an extremely fast train) and you shoot a bullet forward at 500 mph (an extremely slow bullet) you – on the train – will see it moving forward at 500 mph, but someone on the ground will see it moving forward at 750 mph, because 250+500 = 750.  This is normal behavior, it's what we observe in our everyday experience.
      But... by contrast, if you're on a rocket ship that's moving at half the speed of light (0.5c, which is MUCH faster than any vehicle that humans have constructed, so it isn't everyday experience) and you shoot a light beam forward, you will see it moving forward at the speed of light (1.0c) as expected, but an observer on the ground also will see it moving forward at 1.0c, even though it's shot forward from the fast rocket forward at 0.5c, so we predict 1.5c for the speed of the light beam.  Therefore this observation of 1.0c is unexpected.  It's very surprising, it isn't like anything in our normal experience.  When you hear “the speed of light is constant” you probably think “ok, of course, so what” but the claim actually is “the speed of light is always observed to be the same, for every observer” and you should not think “of course” about this claim, because it isn't what you should expect.
      In fact, every person – whether they're on the rocket, on the ground, or on another rocket ship – will see the light traveling at 1.0c in every possible situation, no matter what direction the light beam is aimed, no matter what the rocket speed is.  This “constant speed of light” is one of the two CONSTANCIES in Einstein's Theory of Constancy (Theory of Invariance);  and due to this very-strange behavior of light, it's easy to show – using a simple “thought experiment” I'll describe later (it's now in Section 16.2) – that IF the speed of light is constant, THEN other things (like time) must be relative, they must depend on who is observing.  (and it also depends on what event is being observed.)

      Here are some "extra ideas" to teach, or to ask students about.
      iou – It's in gray text to say "don't expect it to be coherent and polished" now, because I'll be developing it later.
      timings depend on the situation:  is time always "slower" for clock on train?  no, think about another experiment with shooter-mirror-detector on ground [ask "who sees the light go straight out and straight back, for shortest time?" & show two paths] -- time is shortest for observer who is at same location for initial (shooting) and final (detecting), this time is the Proper Time, as measured by a Proper Observer who is at same location for initial and final events.
      constancy produces relativity – so why don't we name his theory "...Constancy" instead of "...Relativity"?  in 1905, Einstein proposed that two things are constant (laws of physics, speed of light) and logically concluded, using simple mathematics, that some other things (like time, length, speed of non-light objects, acceleration, mass) are relative (--> other things being relative, being "measured different" by different observers) -- Einstein could have called it either constancy (the cause, with two things constant) or relativity (the effects, with other things depending on motion), and later he had regrets about calling his theory "relativity" instead of its foundation of constancy-invariance)
      the difficulty of thinking differently:  a main concept of relativity – proposing that light has constant velocity – was so strange that nobody thought-of-it and proposed it for __ years after Michaelson-Morley that showed (like before Roger Bannister's first 4-minute mile, when "going against the wind" and then "with the wind" leads to overall decrease of running speed, and the same math applies for light's speed) ---- M-M's experiment showed that light waves don't behave like familiar sound waves;  later, Lorentz worked out the mathematics (Lorentz Contraction) -- 7 years earlier, Einstein did a "thought experiment" about light speed, by imagining what would happen if he could run at light speed and observe the light, but (due to the strange-ness of the concept) he didn't propose "constant light speed" until 1905 when it was one of his 4 major papers – 2 on Special Relativity (the basics, plus e=mcc), Brownian Motion, Photoelectric Effect [that led to his eventual Nobel Prize, which he didn't get for Special Relativity or General Relativity] – in addition to his phd dissertation (re: a new way to derive Avagadro's Number).

i.o.u. – Later, maybe in late-August 2022, I'll finish writing this page.