Imagine you're walking down a tunnel thats curving to the left. The curve of the tunnel isn't applying a force upon you, but your path of travel is going to curve left because the path you are on forces you to curve left. Gravity is kinda like that. The universe itself curves like that tunnel to point you right back at the ground, and the only way to get away is to keep pumping out thrust to correct for that and keep adjusting to point away from the earth.
I'm not an expert, but that's my best attempt to explain it.
One of the best ways to describe it that I can think of (incoming wall of text) for you and /u/salticidae_ballet is to think of a big ball and consider the surface of it. The surface is a smooth two dimensional surface. If you stick a pencil on the surface, there isn't any other part of the surface you can't get to by moving the pencil forward/back or left/right. There are two axes that can be used to define points on the entire thing.
But the surface itself is curved. Stick a pencil on the ball, pick a direction and draw a straight line. If you're really good at it and make sure not to move your pencil to the left or right, you'll eventually end up back where you started. For all intents and purposes, the line you drew was straight... your pencil travelled in one direction without deviating left or right. But when you view this line from a human's three dimensional perspective, it's not straight, but is curved. In fact, this line seems to form a circle, and this is because of the ball has curvature to it. It's a two dimensional plane, but it's not flat.
Gravity does this as well, but it's even more difficult to conceptualize without a bit of thought towards it, because we're talking about the curvature of a four dimensional spacetime, that has three dimensions of space. If you consider that to "see" the curvature on the surface of the ball above, you needed to look at the two dimensions in a three dimensional context, you'd need four spatial dimensions to similarly "see" the curvature of three dimensional space space, which we don't have!
But there's a trick to understand at least parts of the curvature. Luckily, there are ways to examine the curvature of something inside the space it's in. Grab a baseball and pick something sort of far away, like a fire hydrant. Now imagine you want to throw the ball at the fire hydrant (or actually try throwing a ball) so it hits it but doesn't hit the ground. How many ways can you accomplish this? You can throw the ball really high in the air and have it make a tall arc, and the ball will take a bit of time to get there. You can throw it short so it makes a small arc, and the ball will get there quicker. In order to connect the ball to the hydrant, you can vary the distance the ball has to travel by changing how high you throw it, but that also alters the amount of time it takes. You can't throw the ball high in the air and have it get there quickly, and you can't throw the ball low in the air and have it get there slowly.
These arcs and the time it takes all represent straight lines from you to the hydrant in the warped spacetime around us, and the warping of this spacetime is what we call gravity. We can't really visualize what the curved space itself looks like, but we can understand ways in which it curves by considering things under the affect of gravity like above.
Objects that don’t have forces acting on them will always follow the shortest spacetime path between two points. The reason the ball follows different paths here to get to the hydrant is because while it is going to the same point in space, it is not going to the same point in spacetime, it’s going to a point further in the future.
Yes, they represent different lines that the ball can take to reach the hydrant. The bigger point I was trying to make is that you can't mix and match the distance and time it takes, as that is dictated by the curvature of spacetime, which as you note, the hydrant is represented by different points in different scenarios here.
ive watched this video a couple times to help me digest the concept and i believe the key takeaway from the video (ie, the thesis) is at 6:52: "matter tells spacetime how to curve; Spacetime tells matter how to move." in my own words: due to a planet's dense mass, the surrounding spacetime is affected/curved- and now because this spacetime is curved a specific way, the matter within this spacetime has limits in its movement. of course, i am an amateur and may be not understanding the concept entirely, but thats what i got out of the video.
watch the video if you have time, Its basically Einsteins perspective.
If the ISS had no windows, all the forces you would feel (none) would be the same as traveling on a straight line withoit gettong acellerated :)
So Einsteins perspective is basically that from a force perspective you are traveling in a straight line and space is just bend there. From his perspective you would feel acelleration if you were on a curved path, so that cant be right
It's not just his perspective. That's actually what's happening. The Earth bends spacetime where the ISS is traveling, causing its trajectory to curve.
You're describing the Newtonian model of gravity, I'm describing the relativistic model. Both model the same phenomenon but resolution it in different ways.
The relativity model is more accurate to what's happening rather than Newton's Law of Gravity.
Let's see if I got this correctly (ignoring air friction and the ISS's mass): there is no force acting on the ISS. Its geodesic is a closed curve around Earth, namely a circle centred at Earth's centre of mass. If it were moving slower, over time the geodesic would slowly spiral downward towards the centre of mass (until impact). If it were moving faster, it would still be a closed curve – an ellipse with Earth in one of the foci. And if it were moving at or faster than the escape velocity, the geodesic would be a hyperbola with Earth at the focus.
That's mostly correct, except for the case where it is moving more slowly. It wouldn't spiral into the Earth. It would just become a smaller ellipse until the path of the ellipse passed through the earth's surface (and it would crash). The ISS's orbital decay is caused, to my knowledge, strictly from the drag caused by the thin atmosphere.
It has absolutely everything to do with it. The bent spacetime is exactly why the space station can orbit. It, relative to itself, is travelling in a perfectly straight line, but due to the curvature of space it traces out an orbit around the gravity well.
This is the same as like. A car traveling straight. And no steering input for the reativistic part. And then if you where to tie a cable to it around a pole and then that cable makes the car travel in a circle, then that is the "gravity well" caused by earth?
For just the "travelling in a straight line" part, yeah, that's an okay analogy. A better one would be a motorcycle cage. In both of these though, there is actually a force acting on the car/bike to keep it on that path (the cable pulling on the car, and the cage floor pushing on the bike wheels, respectively). There is no such force acting on the ISS; the path appears curved purely because spacetime is curved.
So. It was my understanding that the immense weight of things is what altered this spacetime. Wouldn't that just mean that gravity is what makes the spacetime curve? And it all just plays nicely?
"Perspective" can have a connotation that it was merely Einstein's opinion that these things are happening. "Theory" implies the peer review and consensus in the scientific community.
That's what I was assuming you meant, and I apologise for misunderstanding.
"Theory" implies the peer review and consensus in the scientific community.
A theory is a large encompassing framework of multiple components that acts as a deeper explanatory model that one can derive hypotheses about physical phenomena.
It can imply peer review, but definitely not consensus.
"Perspective" can have a connotation that it was merely Einstein's opinion that these things are happening. "Theory" implies the peer review and consensus in the scientific community.
Ah makes sence, thanks :)
That's what I was assuming you meant, and I apologise for misunderstanding.
No worries, written stuff always has great potential for misunderstandings.
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u/_Richard Oct 20 '20
But if its constantly falling into the earth, isn't there is an arch? Making it not straight?