Today I Learned

I found a video once where the guy built a device to demonstrate curving based on mass, to avoid the gravity simulating gravity problem, but I failed to find it again when searching. It was something he'd bend to show larger mass, and you'd see the effect with the bands along it or something. Even that isn't accurate, but visualizing 4D can be challenging, especially if you then have to put it in 2D media.

I think of it as a 2d cross section of the experiment (it’s happening in every direction possible tangent to the ball), which necessarily breaks into a third dimension. In our 3-spatial-dimension reality that’s the best we can do.

I find the metaphor frustrating because of that.

That's a really interesting perspective. I don't think I've ever seen it described that way before, but it's very easy for me to grasp. Do you have any resources I can look into for more information? Does that concept have a name I can look up?

What I meant was that's "why" we use the term "fabric of reality"— because of "how" gravity works.

If cosmic inflation is correct, probably not. Inflation is our best theory of the beginning of the universe.

According to inflation, spacetime expanded exponentially from an infinitesimal point to many billions of light-years across. As far as we can tell, the universe is expanding again but at a much slower rate, due to dark energy.

Spacetime survived the inflationary period, so it looks like it doesn't have a "tear" mechanism.

Another way to think of it, is to assume once torn, what is it tearing "into". If you rip a bit of fabric, you look through to the other side, nothing special. If you tear our 4-dimentional spacetime, what are you looking at when you look "through" the ripped portion? This implies that out 4D spacetime is somehow existing in a higher dimensional reality.

Not gonna lie I really thought it was gonna be Neil DeGrasse Tyson

What's really freaky is one of the theories we can't see other life, is to our perspective (due to expansion of the universe) their rate of expansion is faster than the speed of light, and we won't know till they're pretty much at our doorstep.

There might be a bunch of them all over, jumping around the universe and claiming everything they see like old school European countries.

If that happens the best result is they make us a "colony" but honestly a civilization that scale doesn't need resources or anything. The most likely explanation for that behavior would be just to eliminate any future competition if we advance unchecked.

Rather than colonize, they might just want to wipe out our planet, which would be practically effortless for a civilization that advanced. If something crashed into Earth at those speeds, the planet itself would be gone, no way for us to survive or even stop it.

The only defense is to spread out from Earth, which makes the hypothetical advanced space race afraid of our expansion kind of right

Any civilization in a universe like that needs to constantly expand just to ensure it's survival. Because the only reason we'd be preserved was if they cared about the novelty of life.

If they've spent millions of years expanding their space, alien life probably isn't that novel to them anymore. So really, our only bet is we're entertaining, like a zoo.

Nor mine!!

Only a few really dry textbooks I'm afraid, it's a subject that's extremely difficult to explain in lay terms as the mathematics is so complicated.

That said if you're feeling masochistic, Schultz's first course in GR is the most approachable that I know.

Oh god the case for a photon is super hard to talk about in any meaningful way, photons "see" every point in their journey as happening at the same instant of time and at the same place, null geodesics are nuts.

But yeah, the underlying mathematics that causes this can (kinda) just be pinned on the normalisation of the four velocity, which I think is what you're describing.

Soooo… Interstellar was wrong with all the shaking of the camera?

All for the cinematography :) I will say that there's a small caveat in really extreme situations like close to a black hole. Spacetime gets so warped there that your head and your feet take very divergent paths through spacetime, enough to stretch you out and even break you apart at the atomic level. You'd definitely notice that...

In case of accelerating ship, I wonder what would happen in local frame once you hit/get really close to c. You’d get decelerated out of nowhere? Just as if you hit something?

Oh boy, special relativity is another fun one. So here's the thing: there's no "universal speed" that you're moving so you're never any closer to c no longer how long you accelerate for. To accelerate is to change your reference frame and there are no special reference frames.

Which is to say that any physical test you could run inside your ship will give you the same result, always. Accelerate for 13 billion years at any rate and check the the how fast light moves within your ship, the answer is always c.

This is where the name relativity comes in. You have to think in terms of relative speed. Your speed relative to earth will indeed advance closer and closer to c but never reach it. There's a bunch of really wild and crazy implications behind this.

Like that acceleration doesn't change the relative speeds of things uniformly. Keep accelerating at 1 meter per second per second and every second Earth's relative speed changes by less than 1m/s. And look up relativity of simultaneity, another consequence of special relativity. It's fascinating stuff.