Explain General and Special Relativity to Me Like I am 12 years Old

It may surprise you all that I never learned about general or special relativity in secondary school or in university. My earliest exposure to both concepts was from my physics textbook, which I blissfully ignored, since it wasn’t covered in class. I am no physicist, but its quite clear that both ideas, especially general relativity, are important to the foundations of modern physics.

With that brief historical detour, I would very much appreciate some serious dumbing down of the concepts. Maths is not my forte, so if you are including any, you are most likely doing so at my own peril. Thanks.


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It isn’t that hard to get the basics. Beyond the basics is much more difficult of course :slight_smile: .

I suggest starting out by understanding the experimental puzzles that lead us to relativity as the solution. Perhaps start with this experiment:

My undergrad physics text was The Feynman Lectures, which I found very intuitive and with not much math. Maybe try that?

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Yeah. We get reduced to laymen when the situation escalates :laughing: .

I am downloading a sort of documentary presented by a physicist, called Jim. This experiment was mentioned, so I will be getting a vocal elaboration of the “experimental puzzles” and why relativity solves them. Thanks.

You must be quite old John for this to have been your textbook.

I had it before but lost it a few years ago. Going over it again, your observations are correct and the conversation-style format already makes it seem like an enjoyable read. Thanks.

Older than some, younger than others.

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A couple resources that are really great:

Einstein for Everyone (online course materials covering SR and GR, contains the best intro to GR I’ve ever come across)

Minutephysics Intro to Special Relativity (youtube series)

Bonus: The Meaning of Einstein’s Equations (gives a one-line statement of the equations of GR in plain english)


Thanks for the resources everyone, but its pretty obvious I could have gotten some of them myself (and I indeed have). I opened this thread to get feedback on how physicists conceptualize both ideas in their heads, and how these ideas are linked to other phenomena like time travel.

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I am not sure there is an easy answer here other then spending time with reading relevant books and viewing videos like the ones Mathew posted. Both SR and GR are pretty counter intuitive.

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I agree, that’s why I want the take of the experts (devoid of much maths as possible). I don’t think understanding relativity is as hopeless as understanding quantum mechanics, so maybe @PdotdQ or @dga471 could help out here.

Try it anyway, because Feynman!

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If students that the hypotenuse of a right triangle has to be longer than the two other sides then this thought experiment might work.


The path of the light is longer for the person standing next to the moving train compared to the path observed by the person on the train. Light always travels at the same speed (in a vacuum) in all frames of reference, so the explanation for the observations is that clocks tick at different rates in different frames of reference.

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NOT Relatively, but a really fun exploration of basic physics is Brian Beckman’s Physics of Racing series, which is available from various sources around the Internet.

Yes I know this is off-topic, but I consider it one of the hidden gems of the Internet, and try to pass it along. :smiley:

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SR can be condensed to a single sentence

The result of any physical experiment is the same in all inertial reference frames

“Inertial reference frames” is just a fancy phrase to describe a bunch of laboratories that are moving relative to each other (at constant velocities). So, it doesn’t matter if you do an experiment on Earth or on a spaceship, the result is the same.

Any other things that people often attribute to SR, such as the constancy of the speed of light, time dilation, red/blueshifts, or the conjoining of space and time are just corollaries to that one sentence.

GR can be condensed to a single sentence

As stated above, one of the corollaries that can be derived from the “SR sentence” is that space and time are conjoined to form what’s called spacetime. GR is just the following statement:

Spacetime is curved by the presence of energy

Any other things that people often attribute to GR, such as gravity, black holes, time travel, or the expansion of the universe are just corollaries to that one sentence.

You might find that these two sentences generate many questions in your head. If so, welcome to the club!


Oh my, that’s straight to the point and really simple.

This is probably why I remained confused about SR and GR. I never got the condensed statement you provided. Other sources I consulted would talk about these corollaries instead of focusing on the core ideas.

How is this a corollary to SR?

I thought spacetime is curved by matter(which is condensed energy)?

It certainly got me thinking. What puzzles from GR have not been solved by physicists?

There are many ways to get there, this one is probably the easiest:

  1. From the “SR Sentence”, it follows that the speed of light is the same in all reference frames (because the speed of light is measured by a physical experiment)
  2. Then you can perform a “light clock” type of thought experiment, like what @T_aquaticus wrote; this gives you time dilation and length contraction – particularly, whenever time is dilated, length is contracted, and vice versa
  3. Thus, space and time is related with each other

Matter is just a type of energy.

The most important is this: what happens at singularities? (“Singularity” is just a term for locations where spacetime ends or disappears)


Aha, now I see why Brian Cox always uses the “light clock” to explain GR. Makes more sense now.


Alright. Thanks for the clarification.

This may be an amateur question, but does the train run both ways - can energy then be regarded as a manifestation of curved spacetime?

One of my favorites is the thought experiment of shining a flashlight out the front of a moving car. Is the speed of light 3E8 m/s plus the speed of the car? If you were driving at someone what would they observe, and how would it be different to what the driver observes?

No: energy is a property that a thing carries/possesses, which does many things, among which is curving spacetime. Energy does many other things beside just curving spacetime! Fun fact, spacetime itself can also carry energy.