I understand the concept of spacetime being warped (I’ve seen the rubber sheet analogy a thousand times), but does that mean that everything is *technically* always traveling in a straight—albeit warped—line?
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Everything that is operating solely under the influence of gravity is moving in a straight* line. The Moon orbiting around the Earth, for example. However, if there is anything else acting on it, it is no longer moving in a straight line. If you are sitting in a chair, the upward force from the chair causes your worldline to curve relative to spacetime.
*We use the term ‘autoparallel’, because ‘straight’ can have different meanings when the geometry is curved
Basically yes, as long as there is only gravity involved. Since the spacetime itself is curved “straight” isn’t exactly the best term. Instead we call it a geodesic, which is the shortest path between points.
The little curvy sheet pictures tend to have a grid of “straight” lines on them to show the curvature of the sheet visually but the grid doesn’t have much relevance to the math. Being able to lay a grid down presumes an absolute reference point to draw the grid from which doesn’t really exist in relativity.
Asking why is beyond science and into philosophy. Nothing about general relativity explains “why”, just “how”. General relativity is a geometric theory. Whether it has to be is a different question.
We also know that quantum field theory and general relativity are fundamentally incompatible.
Yes. If you could somehow ask a fundamental particle the speed and direction that it was traveling in, *from its own perspective*, the answer would always be “straight forward at the speed of light.”
What the different forces in the universe do is to alter what that particle’s definition of “straight forward at the speed of light” looks like *from the perspective of every other fundamental particle in the universe*.
Yes. Without outside forces, things move in a “straight line”. I put that in quotes because it is like if I asked you to paint a straight line on a curvy boulder. It’s a little bit ambiguous what straight means when the world is curved.
Try not to think of gravity as something that forces the universe to curve. Think of gravity as evidence that the universe is curved.
Imagine that the Earth was perfectly smooth and that you and a friend were standing side by side along the equator. Look around. The world looks flat doesn’t it?
Now both of you face north and start walking. If the world was truly flat, you guys would walk side by side forever. Instead, you two will eventually come together as you approach the North Pole. Nothing is pushing you together. You naturally come together because the world you’re in is curved even though it appears flat. How soon you come together depend on how curved your world is. E.g. the Moon has a smaller radius so it is more curved and you and your friend wouldn’t have to walk as far to see yourselves come together.
That’s what gravity is. It’s not a force that pulls things together. It is evidence that the true shape of the world is curved.
No one commenting has answered the question. I sure as hell can’t answer it either. I dunno if it’s even answerable, but I’d sure like to know!
If you can answer the title question, bravo, it isn’t straightforward.
The second question is easier – no, things don’t normally move in straight lines (on earthly scales) because we usually have other forces exerted, often friction in some way. Pretty much all human-derived motion is “unnatural” from a gravitational point of view – cars go up hills, trains go huge distances without slowing down, and elevators are frankly just mocking the universe.
On a grander, cosmic scale yeah, everything goes in close to straight lines (technically geodesics because you’re not in flat geometry any more). The exceptions are really high energy electromagnetic stuff like accretion disks for black holes which are just maelstroms of energy of all kinds, plus pulsars, magnetars, that kind of oddness. Basically if you look at an object and have to start calculating precisely which way your spaceship hull will melt, you probably shouldn’t trust things will move along local geodesics.
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