Matter bends spacetime. Spacetime tells matter how to move.
Using this logic, you can imagine that above a certain threshold, this can become a feedback loop. These locations are black holes, where enough matter located in a small enough volume of spacetime can create enough distortion to further force more matter into the same volume of spacetime.
It’s possible that time causes gravity.
I once came across information that said natural forces never pull. They only push. If that’s true then magnetism and gravity are even more confusing than I had initially estimated.
I’m not following the logic.
Massive things cause gravity. We experience this, of course, as the earth pulling us in towards its center of mass.
But if gravity pushes, then what is it that is getting on the other side of me and pushing me towards the earth?
Where is the physical origin of that pushing force?
How does it know to get on the far side of me and push me “down” toward the giant mass that is the earth?
None of it makes sense to me. I hope it’s true and things are just that weird.
I once came across information that said natural forces never pull
That’s absolute nonsense. Discard that thought and never repeat it again.
Bending spacetime.
True. But that correction still doesn’t unbend my mind
Nor mine!!
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.
Why do you assume they’re hostile?
Maybe they want to help but respect our free will as a species.
An object will always follow the shortest path between two points in spacetime.
When it’s sitting alone in the universe, the shortest path is to move through time from A to B.
When other things are present to also curve spacetime the shortest path can entail accelerating in space and slowing in time (from the viewpoint of us, the omniscient massless observer floating nearby pointedly not having any casual interactions).
They call it the “fabric of reality” because that’s a good metaphor to describe how gravity works. (Or at least I assume that’s where it came from, I could very well be wrong.)
When you stretch a fabric thin, and place something heavy in it, it’s going to sink and stretch the fabric down with it. Then, if you place a smaller object next to the larger one, it’s going to roll around the larger one, gradually moving closer as it goes down the slope created by the larger object.
That might be hard to visualize, so here’s a neat video I found.
Can spacetime be ripped or torn? Or is the fabric metaphor simply a way of understanding and visualizing it?
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.
What if it is getting ripped/torn but there’s just more space ‘underneath’ that instantly fills the gaps as they are created? I guess at that point it’s indistinguishable from stretching but it’s interesting to think about
One could be picky and say you’re explaining gravity with gravity. But for the sake of simplicity that’s OK.
I’ve once read an article where someone complained about that and tried to explain it with the actual cause, curvature of space time, like using a model car with glue attached to the wheels. But that was not really intuitive and simple to understand.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.
Yes, but the smaller object is dragged into the valley formed by a heavier object due to gravity (of the earth), not due to following the curvature of the blanket.
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.
Here, probably this one. The only good explanation I’ve seen.
Exactly that one. Thanks, I’m glad I said something. It wasn’t anything new, just a new way to present it, and when he did the warped version and the straight line, I was like, okay, makes sense. Then he returned it to our “viewpoint” on the warped space seeing things straight, and even though it was the same lines, it was amazing to see those paths go precisely where we expect. All because of a warped graph. I think it was more incredulous because it wasn’t some animation, but a physical demonstration.
I find the metaphor frustrating because of that.
Or, if you prefer, here is “Einstein” explaining it.
Not gonna lie I really thought it was gonna be Neil DeGrasse Tyson
I think that explains the “how” more than the “why”.
What I meant was that’s “why” we use the term “fabric of reality”— because of “how” gravity works.
Indeed, it’s a neat way to visualize gravity, but that’s it. It lacks any sort of explanation of why masses appear to be pulled towards one another. (I will point to the other person in this thread saying it “explains gravity with gravity”.) This is why I think the metaphor you mentioned detracts from the original video.
One mind-altering fact that I love is that there’s no “acceleration due to gravity,” once you’re in free fall, until you hit the ground. Hop in a space ship with no windows and fly off straight in some direction. Turn off the engines and watch an accelerometer. It’ll never read anything until you run into something.
You could fly past a planet, a massive star, even a black hole. Your path through space could be full of curves and loops but you’ll never feel it. It’s popular to think of those things as like crazy high G turns but they’re not. You’re just flying in a straight line through space time.
On the flip side, say someone knocks you out and puts you on that ship. You wake up and instead of being weightless, you can walk around the ship like normal on earth. Are you on earth or is the ship in space accelerating at a constant rate? Again, there’s no way to tell. They are, physically, the same.
It’s popular to think of those things as like crazy high G turns but they’re not. You’re just flying in a straight line through space time.
Soooo… Interstellar was wrong with all the shaking of the camera?
Are you on earth or is the ship in space accelerating at a constant rate? Again, there’s no way to tell. They are, physically, the same.
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?
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.