difference between free fall and zero gravity?

[Bodhi Dharma Zen]

Im wondering. We can simulate a zero gravity environment by letting an object to fall without being restrained by anything (eg, air).

But what a subject is experiencing in that situation is precisely the effect of gravity!!

So, why is this the same as, for example, being suspended in between two planets?

Im sure is a dumb question, but Im not ashamed to ask.

 

[drkitten]

Im wondering. We can simulate a zero gravity environment by letting an object to fall without being restrained by anything (eg, air).

But what a subject is experiencing in that situation is precisely the effect of gravity!!

So, why is this the same as, for example, being suspended in between two planets?

Im sure is a dumb question, but Im not ashamed to ask.

There is no such thing as "zero gravity." The Earth attracts every other body in the universe -- every other body -- according to the usual laws of physics. No matter where you go, you can't get away from gravity.

However, the felt effect of gravity is negligible for an object in free-fall, so people call it "zero gravity." They're wrong, but they're wrong about so many other things, its not worth getting ones knickers in a twist about.....

 

[noryl]

Its all free-fall or microgravity or zero g. Like was posted, you can't escape gravity so there is no such thing as zero g. You would always be in orbit of something, the biggest thing nearest to you. Since you and everything in your ship would be moving at the same speed, you can float around.

 

[tracer]

Would there be a difference, as far as General Relativity is concerned, between being in free-fall in a decidedly non-zero gravitational field, and being so far away from all other massive objects in the universe that the net gravitational field is as close to zero as you're going to get?

 

[JoeTheJuggler]

However, the felt effect of gravity is negligible for an object in free-fall, so people call it "zero gravity." They're wrong, but they're wrong about so many other things, its not worth getting ones knickers in a twist about.....

This isn't what the OP asked, but what about "zero g" where "g" is just a unit of measure for acceleration (defined by the acceleration due to gravity at the Earth's surface)?

The acceleration, or lack thereof, doesn't necessarily have to be due to gravity.
The Blue Angel pilots experience several gs of acceleration, and gravity is insignificant as a cause of that acceleration.

So if by any reasonable measure a body is experiencing no acceleration, it can be said to be zero g (which does not mean the absence of gravity).

 

[Bodhi Dharma Zen]

This isn't what the OP asked, but what about "zero g" where "g" is just a unit of measure for acceleration (defined by the acceleration due to gravity at the Earth's surface)?

The acceleration, or lack thereof, doesn't necessarily have to be due to gravity.
The Blue Angel pilots experience several gs of acceleration, and gravity is insignificant as a cause of that acceleration.

So if by any reasonable measure a body is experiencing no acceleration, it can be said to be zero g (which does not mean the absence of gravity).

Thanks Joe. Yes. My question is related to the conceptual or physical differences between both states. I guess is more related to the one about the relativity of movement (without any other objects around and without being able to notice any acceleration... are you moving or not?).

So, rephrasing, which is the difference between a state of no acceleration and the absence of gravity? Or, to put it in other words, is there a way to differenciate between acceleration and gravity?

 

[NobbyNobbs]

Thanks Joe. Yes. My question is related to the conceptual or physical differences between both states. I guess is more related to the one about the relativity of movement (without any other objects around and without being able to notice any acceleration... are you moving or not?).

So, rephrasing, which is the difference between a state of no acceleration and the absence of gravity? Or, to put it in other words, is there a way to differenciate between acceleration and gravity?

If I recall correctly (and I may not be), Einstein said there is no discernable difference between feeling the acceleration due to gravity and feeling acceleration due to any other reason. The laws of physics are upheld the same way in either case. (Isn't this the "relative" part of the Theory of Relativity?)

 

[Ocelot]

Indeed if you were in a sealed capsule experiencing an acceleration you would have no way of telling if it were due to being in a gravitational field or due to the capsule accelerating. Einsteins hypothesis for the General Theory of Relativity was that if you can't tell any difference then there is no difference. What follows from that is not only bizarre in theory but thoroughly mind bending when it was subsequently demonstrated experimentally to be true.

 

[Ziggurat]

A falling reference frame is an inertial reference frame. This means that if you're in free fall, you cannot tell based on any local measurement that you are falling and not floating in deep space.

The trick, though, is that in practice you aren't actually confined to local measurements. Real gravitational fields are not uniform, and this nonuniformity leads to tidal forces (the side of an object nearest the gravity source will feel a pull relative to the side farthest from the gravity source) even for free-falling objects.

 

[JoeTheJuggler]

Yes--my point is that "zero g" (where g is a unit of acceleration) is not the same thing as "zero gravity"--right? So the following statement isn't really true:

Like was posted, you can't escape gravity so there is no such thing as zero g.

Again "g" is a unit of measure for acceleration--from any cause--and not an abbreviation for "gravity".

I'm not really addressing the OP's question--I'm only pointing out that "zero g" and "zero gravity" aren't the same thing.

 

[elgarak]

A falling reference frame is an inertial reference frame. This means that if you're in free fall, you cannot tell based on any local measurement that you are falling and not floating in deep space.

The trick, though, is that in practice you aren't actually confined to local measurements. Real gravitational fields are not uniform, and this nonuniformity leads to tidal forces (the side of an object nearest the gravity source will feel a pull relative to the side farthest from the gravity source) even for free-falling objects.

No. A falling reference frame is NOT an inertial reference frame. It is an accelerated reference frame, with gravity accelerating it. It can be treated as an inertial reference frame since it is usually small enough so that the acceleration is nearly constant across it.

To Bodhi Dharma Zen: It's not a dumb question really. The problem is, both situations are strictly speaking not the same. If you're in deep space (no gravitation), or in between two planets at the center of gravity, the net forces are ZERO. In a free-falling frame, the net force is not zero, but the gravitation of the biggest nearby body (which is the ONLY force acting on the reference. Both situations can be treated similarly within the reference frame if the free-fall reference frame is small in comparison to the nearby body so that the net force acting on it can be called constant.

 

[Bodhi Dharma Zen]

Indeed if you were in a sealed capsule experiencing an acceleration you would have no way of telling if it were due to being in a gravitational field or due to the capsule accelerating.

Ok, that makes sense, its like the example of relative movement. It might be paradoxical to the common sense, but its logical.

Einsteins hypothesis for the General Theory of Relativity was that if you can't tell any difference then there is no difference. What follows from that is not only bizarre in theory but thoroughly mind bending when it was subsequently demonstrated experimentally to be true.

Here is where I would disagree. Even without really understanding why do I disagree. Acceleration is perceived/measured as changes of inertia, but its very different (from the physical point of view) if the cause of that acceleration is the gravity "force" of a large object or momentum gained from the inside of the vehicle/body by other means.

What was demonstrated?

 

[Bodhi Dharma Zen]

To Bodhi Dharma Zen: It's not a dumb question really. The problem is, both situations are strictly speaking not the same. If you're in deep space (no gravitation), or in between two planets at the center of gravity, the net forces are ZERO. In a free-falling frame, the net force is not zero, but the gravitation of the biggest nearby body (which is the ONLY force acting on the reference. Both situations can be treated similarly within the reference frame if the free-fall reference frame is small in comparison to the nearby body so that the net force acting on it can be called constant.

Ok, not sure if I understood. The net acceleration forces would be zero in both instances, but... only in relation to an object or its absense. Say, a spaceship is "lost in space" and the navigation is off and there is no way to tell where the spaceship is. Nor astronauts nor instruments pick any kind of inertial changes.

Still, in one example, the spaceship could be headed to burn in a star. There is a force going on (or at least thats the common explanation). But where is it?

 

[Ziggurat]

No. A falling reference frame is NOT an inertial reference frame.

It is locally.

It is an accelerated reference frame, with gravity accelerating it.

No. A frame that is stationary in a gravitational field is an accelerating frame.

It can be treated as an inertial reference frame since it is usually small enough so that the acceleration is nearly constant across it.

Which is basically what I said: falling is locally inertial, but nonuniformity of gravity creates tidal forces.

 

[Ziggurat]

What was demonstrated?

Quite a few of the effects of general relativity, such as gravitational lensing and the orbital precession of Mercury, and a full report confirming frame dragging (the GR-predicted effect of a rotating gravitational body) is expected shortly from Stanford's Gravity Probe B.

 

[Bodhi Dharma Zen]

Which is basically what I said: falling is locally inertial, but nonuniformity of gravity creates tidal forces.

AFAIK, those tidal forces can only be evaluated over very long periods of time, but are irrelevant for measurements from within the object in question. But would you assume that they might be one of the differences between the possibilities presented before?

 

[Bodhi Dharma Zen]

Quite a few of the effects of general relativity, such as gravitational lensing and the orbital precession of Mercury, and a full report confirming frame dragging (the GR-predicted effect of a rotating gravitational body) is expected shortly from Stanford's Gravity Probe B.

Oh yes, I have read about that. I still fail to see a relation with the current "problem"

 

[Ziggurat]

AFAIK, those tidal forces can only be evaluated over very long periods of time, but are irrelevant for measurements from within the object in question.

It's purely a matter of sensitivity. It's easier to obtain high sensitivity over long periods of time, as a practical matter, but that's about it.

But would you assume that they might be one of the differences between the possibilities presented before?

Basically, tidal forces are the only way to distinguish between free falling (in a real, non-uniform gravitational field) and being in a true "zero gravity" environment if you shield your system to any other influences (no light, heat, sound, etc. from outside). So if those tidal forces are smaller than the sensitivity of whatever it is you want to do, then yes, you can simulate "zero gravity" by merely dropping whatever it is you want to look at.

 

[Schneibster]

Zig's got it essentially right, elgarak, sorry to disappoint you. The equivalence principle implies that an accelerated frame of reference is equivalent to a motionless frame of reference in a gravity field, and that there is no local experiment that you can perform that will tell the difference between acceleration and a gravity field.

The actual statement is that you can perform a local experiment in any inertial frame of reference and you can't tell the state of motion of the frame. In other words, it might be floating freely in space, at any speed, or it might be accelerating under the influence of a gravity field, and you can't tell the difference. A frame accelerating under the force of gravity is inertial just as the freely-floating one in space is; it is therefore from this point of view correct to refer to free fall as "zero g," although it is not from other points of view.

In practice, because of the tidal effects that Zig mentions (and another effect he doesn't, hang loose, I'll get there in a minute), you can measure the difference- but it is subtle.

The other difference is that because a gravity field generated by a massive object is spherical, the vector of the forces on objects that are separated angularly with respect to the center of mass of the massive object are different. Each points toward the center of mass, and as a result, if (for example) you were free-falling toward the Moon's surface along with two balls on either side of you, you would see the balls slowly move toward you. This of course would not happen under the influence of a "plane gravity field," but we don't know of any physical way to generate such a field in the real world (short of actually making a massive plane and suspending some objects over it, or allowing them to fall toward it, a major engineering project to say the least).

ETA: Zig would no doubt have me point out (correctly) that both the stretching along the force vector, and the compression orthogonally to it, are tidal effects, although only the first is generally mentioned in discussions of the actual tides created on the Earth by the Moon and Sun.

 

[autumn1971]

All the physics books have the example of a little lab in both a truly zero gravity frame and in a freely falling frame, and given that the sensitivity could not detect tidal differences, the two labs would give identical results for any experiment conducted in them. What is neat, is the relativistic implications of a freely falling lab near a supermassive (so tidal forces are not pronounced) black hole. The tiny lab freely falling is going to be accelerated to near the speed of light, yet still gives the same results as a zero gravity one. The implications as to how this would appear to an observer outside the relativisticly accelerated frame are very interesting.