Ask Slashdot: How Would You Explain Einstein's Theories To a Nine-Year-Old?

SiggyRadiation writes: A few days ago, my 9-year-old son asked me why Albert Einstein was so famous. I decided not just to start with the famous formula E=mc^2, because that just seemed to be the easy way out. So I tried to explain what mass and energy are. Then I asked him to try to explain gravity to me. The earth pulls at you because it has a lot of mass. But how can the earth influence your body, pull your feet to the ground, without actually touching you? Why is it that one thing (the earth) can influence something else (you) without actually being connected? Isn't that weird? Einstein figured out how energy, mass and gravity work and are related to each other. This is where our conversation ended.

Afterwards I thought: this might be a nice question to ask on Slashdot; how would I continue this discussion to explain it to him further? Of course, with the goal of further feeding his interest in physics.

Comments

Newton.

[msauve]

"Einstein figured out how energy, mass and gravity work and are related to each other. "

That would be Newton. Einstein tweaked Newton to cover the extremes.

To acquaint someone with Einstein, start with some of his thought experiments which break Newtonian physics.

Re:Newton.

[dfsmith]

Newton didn't get the energy part.

Einstein (as I understand it) and the rest of the physics world had a problem in that Maxwell's Equations did a really good job of describing electromagnetism. However, the wave equation that pops out does not account for the velocity of the observer. This implied that the speed of EM radiation (light) is constant for ANY observer: oh dear. Einstein (and Lorentz) hypothesized that time didn't have to be the same everywhere, and came up with Special Relativity to describe it. And, remarkably, SR was shown to be accurate. It's also how energy gets mixed in with mass.

A handful (nearly two hands full) of years later, Einstein published General Relativity as a description of why acceleration looks the same as gravity. (Inspired by Newton's F=(constant)*Ma=GMm/(rr).) He did this by hypothesizing that distance is not the same everywhere. And, remarkably, GR was shown to be accurate. (He needed some help from other mathematicians, because the math is hard for warped spacetime.)

Maybe the above is not quite kid-friendly, but Einstein challenged the ideas of classical physics (time and space being "flat"), and got it right. Or at least the next-level-of-right.

Re:I Wouldn't.

[ShanghaiBill]

Relativity, spacetime curvature, and mass-energy equivalence are not beyond a nine year old's ability to understand. They aren't going to be able to understand all the formulas, but they can get the gist of the concepts.

If you don't want to explain it to your kid, there are plenty of great Youtube videos you can point to that explain all this stuff really well in kid friendly terms.

Youtube and Wikipedia have made parenting much easier.

Re:If you think Special Relativity makes sense...

[Pfhorrest]

Two twins orbit around each other and then meet. They are both older than each other.

Sounds like you're the one who doesn't understand relativity.

Two twins are set into motion relative to each other, and then left to coast inertially like that. Time passes. Each twin thinks more time has passed for the other than for themselves since they were set into motion. Neither is objectively correct; a third observer could find the same amount of time to have passed for both, or any different ratio of time to have passed for either, depending on how that observer is moving.

But then the twins are set back into motion toward each other. Again, after being set into motion like that, they disagree about how much time is passing for each other, but then, so does every other observer, and about everything else in the universe too. Observers in different states of motion disagree about how much time is passing how quickly where.

The twins come back together and are brought to a stop relative to each other. They have definitive ages relative to each other that they both agree on, as does every other observer in the universe.

The trick is that when they're being set into motion apart, turned around and sent back together, and stopped at the end, time is also passing differently for each of them not just because of their different states of motion, which nobody can agree upon, but depending on whose motion is being changed how much, which is something that every observer can agree upon even if they can't agree on the absolute measure of that motion. (That is, while observers may disagree about which twin is stationary and which is moving, they will all agree that one twin is moving more [or less] now than it was before).

If one twin stayed in the same state of motion the whole time, while the other got sped away, turned around, and then stopped when he got back, then the one who stayed behind is older, and everybody agrees.

In your scenario, it sounds like they both underwent the same acceleration, just mirrored from each other, so both would be the same age when they came back together, and both would agree on that, as would every other observer in the universe.

Other observers moving differently than the twins would disagree on what age they are, but they'd all agree that they're the same age as each other, whatever that age is.