Love the Hossenfelder reference (though she really needs to move to Bluesky...). That said, I'd love if someone could explain some physics to this noob:
If you jump out of a plane, at t=0 you'll not be moving (relative to the earth), at t=1 you'll be moving a bit (relative to the earth), and at t=2 you'll be moving even faster. How is that not acceleration? A quick wikipedia rabbit hole from "Accelerometer" --> "Inertial Reference Frame" --> "Fictitious Force" led me to this:
A pseudo force does not arise from any physical interaction between two objects, such as electromagnetism or contact forces. It is just a consequence of the acceleration a of the physical object the non-inertial reference frame is connected to, i.e. the vehicle in this case. From the viewpoint of the respective accelerating frame, an acceleration of the inert object appears to be present, apparently requiring a "force" for this to have happened.
They use the example of a passenger in an accelerating car, which makes sense. And there's some discussion of how the earth is rotating and thus has angular (?) acceleration, which also makes sense. I think this is what Hossenfelder is referencing by "the earth is accelerating you upwards". But the rotation discussion seems completely unrelated to gravity, no? An asteroid that isn't spinning would still exert gravitational (pseudo-)force, as all massive objects do of any size. Surely a person standing on a non-spinning asteroid in deep space isn't being accelerated upwards?
At the end of the day, I guess I'm missing why exactly "free fall" is the same thing as "no forces acting upon you". To my cynical arrogant brain, this reads like the physicists are harping on an unnecessary terminological thing, namely that a "force" is defined as "a physical interaction between two objects". In other words, its quantum bias, in the original sense of quantum; why can't objects interact with the continuous field of spacetime?
I'm commenting on your quote because her explanation especially "we have a machine with acceleration in the name, thus that's what acceleration is" set off a million alarm bells in my head, philosophically speaking!
The point is that, unlike velocity, acceleration is absolute in GR.
If we're both moving towards each other at constant speed, it's perfectly equivalent to say that I'm moving towards you and you are stationary, or to say that I'm stationary and you're moving towards me, or that we're both moving towards each other relative to some outside observer.
The same isn't true with acceleration. If we're in the same scenario and I start a rocket thruster, then I'm experiencing acceleration and you're not. Our relative velocity towards each other is increasing, but it would be wrong to say that I'm stationary and you're accelerating towards me.
So, if you fall from a plane, your relative speed towards the Earth's surface is increasing. But it's not you who is experiencing acceleration, it is the Earth, and the difference is measurable in principle.
This is similari concept to how when something is moving in a circle, it experiences an acceleration towards the center of the circle, but this is often experienced as a "centrifugal force".
Question: Two black holes that encircle each other are on geodesic orbits and thus should not feel acceleration. However, graviational waves are emitted during the orbits until they merge. How is this possible when there is no accelleration acting on the masses?
Well, with me standing where I am, and anti-me standing on the exact opposite side of the Earth, the ground must be accelerating in opposite directions at once!
It feels like taking a somewhat straightforward model and inverting it (in the x -> 1/x sense); that's how you get straight lines to split into pieces and curve away.
It's forced if you start from the perspective of "General relativity describes reality", and obviously so if you look back at the inspiration for relativity, one of which was "there is no way to differentiate between different free-falling rest frames from inside a box".
Of course it's not always the most convenient model, and there are ones in which gravity is indeed a force — the Newtonian approximation, for example — but the starting point of this article is "Here's how reality works if GR describes reality".
If you jump out of a plane, at t=0 you'll not be moving (relative to the earth), at t=1 you'll be moving a bit (relative to the earth), and at t=2 you'll be moving even faster. How is that not acceleration? A quick wikipedia rabbit hole from "Accelerometer" --> "Inertial Reference Frame" --> "Fictitious Force" led me to this:
They use the example of a passenger in an accelerating car, which makes sense. And there's some discussion of how the earth is rotating and thus has angular (?) acceleration, which also makes sense. I think this is what Hossenfelder is referencing by "the earth is accelerating you upwards". But the rotation discussion seems completely unrelated to gravity, no? An asteroid that isn't spinning would still exert gravitational (pseudo-)force, as all massive objects do of any size. Surely a person standing on a non-spinning asteroid in deep space isn't being accelerated upwards?At the end of the day, I guess I'm missing why exactly "free fall" is the same thing as "no forces acting upon you". To my cynical arrogant brain, this reads like the physicists are harping on an unnecessary terminological thing, namely that a "force" is defined as "a physical interaction between two objects". In other words, its quantum bias, in the original sense of quantum; why can't objects interact with the continuous field of spacetime?
I'm commenting on your quote because her explanation especially "we have a machine with acceleration in the name, thus that's what acceleration is" set off a million alarm bells in my head, philosophically speaking!