Gravity

[Zareon]

Hi, I have some questions. I hope you guys can help me out.

 

Gravity affects all objects in the same way right? The motion an object makes in freefall (no forces except gravity) is independent of its mass.

General relativity says an object follows it's natural path (straight line, or geodesic) through a curved spacetime if no forces are acting on it. That means nothing is really 'pulling' on an object right?

 

When I sit in a car making a strong turn I can feel the acceleration. I get pushed against the side of the car, the car pushes on me and makes me go into the other direction. My internal organs have a tendency to stay behind too, so because of the force by body exerts on them I can feel I`m accelerating with my eyes closed. Is that correct so far?

Now with gravity. All parts of my body are affected by it and accelerate in the same way, so I shouldn't be able to feel any acceleration. Is that also what the equivalence principle says? That an observer in freefall wouldn't be able to know whether he's in freefall by performing local experiments? So it's just like he's in an inertial frame.

[insane_alien]

yes as long as the acceleration is uniform throughout the entire space in which he has to perform the experiments. near a gravitational source such as a planet/sun/average american there will be a change in gravitational acceleration with a change in distance from the source. therefore with sensitive equipment it COULD be proven that he is in freefall.

[timo]

yes as long as the acceleration is uniform throughout the entire space in which he has to perform the experiments. near a gravitational source such as a planet/sun/average american there will be a change in gravitational acceleration with a change in distance from the source. therefore with sensitive equipment it COULD be proven that he is in freefall.

You should keep in mind that "locally" means "sufficient close to avoid any impact of a finite extension". So if you had something in mind like [description follows in terms of classical Newtonian mechanics] dropping two stones and seeing that during freefall their horizontal distance slightly decreases (because both are attracted to the center of earth), then such a test is explicitely ruled out by "locally". If you had something in mind like "the closer you get to earth, the bigger the attraction will be - I could measure that", then the question would be: How?

 

@Zareon: Apart from some nitpicking one could do (e.g. the motion of an object B is not independent of its mass if it gravitationally influences other bodies G around it which causes a change in the gravitational field caused by G), that all sounds fine.

[insane_alien]

@Athiest: time dilation effects possibly(GR) or (i can't remember the name of the effect so bear with me) the effect on an object with uneven mass distribution where it will (eventually) settle so the heavy side is pointing toward the source of gravitation(i know this happens to the spaceshuttle)

[timo]

@Athiest: time dilation effects possibly(GR) or (i can't remember the name of the effect so bear with me) the effect on an object with uneven mass distribution where it will (eventually) settle so the heavy side is pointing toward the source of gravitation(i know this happens to the spaceshuttle)

"Locally" rules out any effect of an [again clasically speaking] inhomogenous gravitational field within the experimentation apparatus. Time dilatation would be the same for all clocks positioned anywhere in it and the heavy side would not point towards earth since the net force on the heavy and the lighter side of the device would experience the same force (none in free fall). Of course it is just an idealization and theoretically, for any non-zero extension you have a finite inhomogenity in the gravitational field of a homogeneous massive sphere. Imagine the restriction "locally" as the statement as "you can continuously shrink the measuring apparatus down to a size where the effect caused by the inhomogeniousity of the gravitational field is below a measurable amount". How well locallity is true for an actual realitstic system is a practical issue.

[Zareon]

@Zareon: Apart from some nitpicking one could do (e.g. the motion of an object B is not independent of its mass if it gravitationally influences other bodies G around it which causes a change in the gravitational field caused by G), that all sounds fine.

I see. The reason I asked is that, if it's true, then why would I be able to tell if I`m falling. You get that really strange feeling in your stomach (I've never skydived, but I can imagine). But I realize that must be just what it feels like if gravity 'falls away'. So the astronauts in a spacestation like the ISS must be feeling like they're constantly falling down, even when they're sleeping. That must be so weird!

 

Hmm, another thing that got me thinking. In Newtonian gravity, an (rigid) object doesn't exert a net force on itself, just like you can't pull yourself up by your hair or something. But can an object influence it's own path in general relativity. That is, can the space-time distortion caused by an object be such that the region of spacetime in which the object is itself is curved due to its own mass?

[DaveC426913]

I see. The reason I asked is that, if it's true, then why would I be able to tell if I`m falling. You get that really strange feeling in your stomach (I've never skydived, but I can imagine). But I realize that must be just what it feels like if gravity 'falls away'. So the astronauts in a spacestation like the ISS must be feeling like they're constantly falling down, even when they're sleeping. That must be so weird!

Yes. Exactly. I've always thought it amusing when you see people in movies drifting into freefall, like they're suddenly floating.

 

Um, no. They're suddenly falling.

 

If you have ever flown in a passenger jet as it reaches cruising altitude, you know what it feels like (in a very small way) to have the plane drop out from underneath you. It is a ... disturbing feeling. Now imagine of the plane dropped right out from under you - and continued to drop such that, as you began to fall, it fell away just as fast.

 

I was vindicated recently with the most recent space tourist who went up to the ISS. She was sick for the whole first day from the feeling.