I have a question about gravity that has puzzled me for some time. I assume it’s based on my misunderstandings and I’m hoping someone can clarify things for me.
As I understand it, Newton’s view of the world is that an object in motion stays in motion and an object at rest stays at rest unless acted upon by an outside force. If I drop an object, it falls because gravity is a force and acts on it.
On the other hand, General Relativity says that gravity is not a force, but objects moving through space-time. If I drop an object, it moves to a lower energy level in space-time.
So in General Relatively how does an object “know” that I’m no longer holding it and that it needs to move? What’s the “force” that is moving it through space-time?
When you hold an object, your hand is exerting a force on it, opposing gravity (however defined or expressed), so it’s not moving. When you release the object, you’re no longer exerting that force; you’re no longer opposing gravity, so the object falls.
You just need to account for all the forces acting on the object … which can be tricky.
I think probably the simplest answer is that general relativity doesn’t really deny that gravity acts like a force, but rather, it provides a deeper explanation about the nature of this apparent force. Indeed, a basic consequence of GR is that you cannot tell the difference between the force felt in an accelerating frame of reference (say, one which is being accelerated by a rocket engine) and the force (measured at any given point) that is due to gravity.
It’s much like the several different classical explanations for why a satellite stays in orbit. Which one is right? They all are. The typical Newtonian explanation is that the satellite experiences a centrifugal force due to its circular motion which, if it’s travelling at the right speed, exactly balances the force of gravity. Whereas the GR explanation – equally valid but deeper and more general – is that due to the curvature of spacetime, the satellite’s path actually follows a geodesic. It is, IOW, simply following the shortest distance between any two points on its path. In non-curved space, a geodesic would be a straight line.
Let’s say that I’m holding a bucket. I’m putting an upwards force on it, through my arm and the handle of the bucket. If that’s the only force on it, the bucket accelerates upward.
When I let go of the bucket, it stops accelerating upwards, and instead continues on an unaccelerated trajectory.
All that GR adds to this explanation is that, in the vicinity of a mass, you need to be constantly accelerating to stay in place, because an unaccelerated trajectory will end up falling.
The object you are holding IS being acted upon by an outside force, you. And you are exerting a force on the object to counter gravity. Letting go of the object removes one of the forces acting on the object.
When Newton talks about an object at rest, or an object in motion, he is talking about the object in the complete absence of all other forces.
General relativity does not change the physics of how gravity works, only its nature. The object knows because there is a force attracting it to other mass. That’s the way it was in Newton’s time, and that’s the way it still is. We used to say objects with mass are attracted by gravity, but we don’t really know why. Then Einstein said it’s because mass affects the curvature of space time and objects with mass sort of roll through the distortion towards each other. But the math of how gravity behaves has not changed. We still model it as a force in the math.
Now there’s talk of hypothetical graviton particles, which is over my head.
In Newton’s view, everything stays moving in a straight line at a constant speed (I’m including not moving at all as a degenerate case of moving in a straight line at a constant speed) unless there’s a force on it. Newton also has this mysterious force called gravity that affects everything identically, causing the same deviation from straight line constant velocity motion for rocks, people, bullets, etc. Newton has no explanation for why this force exists - just that it seems to depend on the distance and mass of other objects.
Einstein says everything moves in a straight line at a constant speed unless a force acts on it - but what “straight line and constant velocity” mean depends on the shape of space-time, which in turn depends on the distribution of mass. Now there’s no mystery as to why gravity affects everything identically - because everything is following the shape of space-time, and there’s no need to describe gravity as a force - because gravity is really just objects moving in a straight line at a constant speed (under the new definition of straight and constant - meaning "along minimum length paths in space time), which objects do whether or not there’s a significant gravity field around.
P.S. In Newton’s view, accelerometers are weird. Attach an accelerometer to a car, and hit the accelerator - and the accelerometer measures the acceleration caused by the the wheels exerting a force on the ground and thus accelerating the car. Have an accelerometer attached to an iron object, and turn on an electromagnet, and the accelerometer measures the acceleration caused by magnetism. But toss an accelerometer off a building, and it reads zero all the way down.
In Einstein’s view, this makes perfect sense - accelerometers measure accelerations caused by forces and gravity isn’t a force.
True enough, in a rotating frame of reference. But personally, I really dislike this explanation. It is much more intuitive to me to look at an object in orbit in an inertial frame of reference, in which there is a gravitational force acting on the orbiting object that is directed towards the center. This means that the orbiting object is constantly accelerating as it constantly changes direction. And yes, I fully admit I may be a [former] overzealous science teacher.
In the absence of air resistance, why do you think an accelerometer would not measure acceleration due to gravity?
The problem with that explanation is that it’s countered by the intuitive observation that an accelerometer aboard an orbiting satellite would register zero, since the satellite is in free fall.
Maybe I’m misunderstanding you, but the answer to a plain reading of that comment is that the accelerometer would not measure anything because within a freely falling body there is no acceleration to measure. A reference frame in free fall is locally an inertial reference frame. Such a reference frame is undergoing only coordinate acceleration relative to the earth; it is not undergoing proper acceleration. It is indistinguishable from a reference frame in outer space.
That’s a bit of a misleading description. Gravity is a property of spacetime that is altered by the presence of mass. It isn’t “radiation”. While it’s true that gravity can in a certain sense be described as propagating at the speed of light, It’s only because the propagation of causality is limited to the speed of light.
Despite their extreme weirdness, black holes nevertheless have a few specific, measurable properties, like mass, spin, and potentially electric charge. These properties have detectable effects on the BH’s environment and from that perspective BHs outside their event horizons are just ordinary but very massive objects. Spin, for instance, can be inferred from X-ray reflection spectroscopy that reveals the properties of the BH’s accretion disc, and potentially from evidence of frame-dragging.
Gravity is not “radiated” from the interior, it’s what creates the event horizon in the first place, and the gravitational effects outside the event horizon exist as a result of the way mass affects spacetime (the effects theoretically may be very different inside the black hole). Similarly, when two black holes collide, gravitational waves are created in the region around them; they don’t originate from the interior nor do they reveal any information about the interior.
This sounds a bit like the thought experiment where light is not propagated by photons, but rather, light is a manifestation of the absence of darkons - which is interesting, but doesn’t (I think) account for the direction of propagation being observable at the speed of light. If propagation were instantaneous, it would be harder to tell.
Because gravity causes the same motion for every object. An accelerometer detects acceleration by detecting the difference between the motion of the object you want to know the acceleration of, and the motion of a proof mass. When gravity is the only force in play, both the proof mass and the object do the same thing - so no measured acceleration.
I know wolfpup sort of addressed this, but Speed of Light is a bit of a misnomer. It’s the absolute speed limit of the universe. Light just happens to be a very familiar thing that obeys this speed limit in a vacuum.
We’ve observed gravity waves from cataclysmic events far away. They arrive along with light from the event. How would you define gravitons other than as the particle nature of gravity waves?
And really, in the previous examples, the accelerometer should have been measuring a constant 1G downward, and when it is falling from the building would be the only time it does not. Of course it will register an extremely high and short deceleration (assuming it’s very robust) when it hits the ground.
It also has an electric charge that can be detected outside the horizon.
The thing is, it’s not gravity waves that are being detected, no more than it’s electromagnetic waves. If two black holes collided while within a third, larger black hole, assuming that’s possible, they would not emit gravity waves that could be detected outside, but their mass contribution to the black hole would be.
The best visualization I’ve seen of GR upgrades the bowling ball on a trampoline to a sphere on a grid. The grid itself is being pulled in (or absorbed) by the sphere. Does that mean that gravitons are absorbed by matter? Maybe… but whatever it would mean, it’s not a property unique to black holes, they are just the most extreme examples.
I don’t know if this is the best GR explanation out there, but it does have the best visual representation I’ve come across.
While gravitational waves are, presumably, composed of gravitons in the same way that light waves are composed of photons, it’s worth mentioning that they’re composed of a whopping huge number of gravitons. Gravitons would have the same relationship between frequency and energy that photons do, and the highest-frequency gravitational waves that we expect to occur in the Universe are only a few kilohertz. Which means that each graviton of those waves carries only an absolutely minuscule amount of energy, and the events that produce those waves typically convert tens of solar masses into energy. It’s likely that we’ll literally never detect a single, individual graviton.
No, what @wolfpup said is perfectly valid. You can work in the inertial frame and say that gravity is the centripetal force acting on a satellite, or you can work in the co-rotating frame and say that gravity is counteracted by the centrifugal force acting on it due to rotation. Both descriptions give the same results, and which one is preferred depends on why and how you’re describing it.
