Einstein says otherwise. No experiment can distinguish between being in a gravitational field and/or experiencing uniform acceleration. See: Equivalence Principle.
Put another way…gravity feels exactly like uniform acceleration. The two cannot be distinguished.
Does this mean that a thing always moves in a straight line (apart from other externally-applied forces) but GR identifies the curvature of spacetime due to gravity as, essentially, the redefinition of what a straight line is?
Yes, that’s essentially correct. To avoid confusion, the term usually used is “geodesic” rather than “straight line”.
Right. To extend that, a geodesic is the shortest possible line on a curved surface. That makes it the most energy efficient pathway and the natural way a massed body would move in space (without exterior forces).
On the surface of a globe, the geodesic connects two points with the shortest travel distance. Airplanes normally travel along these routes, which is why so many routes cross arctic territories. And why flatearthers have to flatly deny that southern hemisphere flights exist. That sounds insane because it is, but no other out is available on a flat earth.
That’s why the correct answer is c.
You’ve got that exactly backwards. When you’re falling is when you’re not accelerating. It’s when something is stopping you from falling, like the floor or a chair, that you’re accelerating.
What? You can’t be serious.
(bolding mine for emphasis)
Einstein’s first step towards that theory was the realization that, even in a gravitational field, there are reference frames in which gravity is nearly absent; in consequence, physics is governed by the laws of gravity-free special relativity – at least to a certain approximation, and only if one confines any observations to a sufficiently small region of space and time. This follows from what Einstein formulated as his equivalence principle which, in turn, is inspired by the consequences of free fall. - SOURCE
Sit in a chair holding an accelerometer. The accelerometer reads 1 g. Jump out of a plane holding an accelerometer. The accelerometer reads 0. The equivalence principle is basically just the statement that the accelerometer is correct.
My instinctive guess is that a “force” and a geometric explanation of acceleration should be inter-equivalent. We just haven’t come up with convincing geometric interpretations of the other three forces, and we haven’t come up with a consistent interpretation of gravity in terms of quantum force bosons.
ISTM that’s just two different reference frames. One where you “feel” gravity and one where you do not. The Equivalence Principle still applies in both though. You cannot do an experiment to determine whether you are in a gravitational field or are uniformly accelerating. Astronauts in the ISS do not “feel” gravity but the Equivalence Principle still applies to them (see my cite above).
Yes, it applies to them. They feel nothing, therefore they are not accelerating.
What? They are in orbit. They are constantly accelerating. See: Centripetal Acceleration
OK, picture a spacecraft in the depths of one of the great intergalactic voids, with its engines off. We can agree that that spacecraft is not accelerating, right?
Well, an astronaut in that spacecraft would feel the same thing, and make all the same measurements, as one in the ISS. If they’re making the same measurements, then they have the same acceleration.
If an astronaut at the ISS were accelerating, their path would deviate away from a geodesic. But that only happens if there’s some real force acting on them. But there isn’t any real force, only gravity.
With the caveat of course that uniform acceleration corresponds to a gravity source infinitely far away i.e. no tidal forces. Or as you said “if one confines any observations to a sufficiently small region of space and time.”
Parachutists are subject to a form of this. They exit the aircraft being thrown forward by its momentum and don’t feel the drop. By the time this force is spent they’ve reached terminal velocity and don’t experience acceleration. Lacking visual reference they may become disoriented during the sensation of floating. This was the cause of the 1997 Antarctic parachute disaster. The participants who’d used Automatic Activation Devices, mostly Norwegians, survived. Norwegians seem to have the best outcomes at the South Pole
Except for that whole base that disappeared mysteriously in 1982…
Are you suggesting the Equivalence Principle only applies in a few, particular circumstances?
And in a shocking change of tone, I discovered yesterday that seminal rocker Little Richard was discovered by Sister Rosetta Tharpe. What?? I am incredulous that I didn’t know this.
Little Richard referred to her as his favorite singer when he was a child. In 1947, she heard Richard sing before her concert at the Macon City Auditorium and later invited him on stage to sing with her; it was Richard’s first public performance outside of the church. Following the show, she paid him for his performance, which inspired him to become a performer.
Jeez, they even made an album together…
(If you don’t know who Sister Rosetta Tharpe is, time to fight some ignorance.The Godmother of Rock And Roll.
Was she good? Was she ever!
BTW the weather’s pretty typical for Manchester. That is all.)
j
No, it applies in all circumstances. That’s why we can absolutely definitely say that objects in orbit aren’t accelerating.
See the post above. Everything in orbit is under constant acceleration.
Maybe we are talking past each other.