How was it proved that mass exerts the gravitic forces?

July 6, 2009 11:00 AM

How was it proved that gravitation was a function of mass?

If we can estimate planetary masses from the strength of their gravity, including that of the earth, then we must be pretty confident that we've nailed the relationship between the two.

I know that the force of gravity is proportional to the mass of an object and inversely proportional to the distance. And certainly when we look at the universe there's evidence of strong gravity wherever we see a lot of mass. But I also know that science is robust and does not just accept obvious observations without testing them to destruction. How do we know that there's a causal relationship rather than it being circumstantial? And that it exists at all scales, not just with planetary masses?

It seems to me that you'd have to test known masses and be able to measure that object's gravity in order to prove anything. Is that what they did, and if so, how was it done? Is there a gravitational equivalent of Rutherford's gold foil and Robert Millikan's oil-drops?

posted by Lorc to science & nature (10 comments total)

How was it proved that gravitation was a function of mass?

Empirical studies by Newton, followed by a significant refinement by Einstein.
posted by Blazecock Pileon at 11:06 AM on July 6

But I also know that science is robust and does not just accept obvious observations without testing them to destruction

That's not really true. Your conclusion doesnt need a proof for each galaxy or whatever. If this theory outperforms the competing theory then its good enough. Considering how well this theory fits into the existing framework its accepted.

I guess you can have the scenario where in a galaxy everything is backwards, but thats a claim that someone needs to prove. In the meantime you assume the current theory is correct until evidence appears to contradict it.
posted by damn dirty ape at 11:06 AM on July 6

What you're looking for is the Cavendish Experiment as the first of its kind.
posted by adipocere at 11:08 AM on July 6

How do we know that there's a causal relationship rather than it being circumstantial?

We don't know for sure. It hasn't been "proved". Science doesn't operate on that level. What we have (in General Relativity) is a description of how we think it works, which has been used to create surprising predictions, which were then confirmed by observation or experimentation. That lends credence to the idea that the theory is a very close model of reality.

For example, Newton's famous equation describing gravity doesn't do a good job predicting Mercury's orbit around the Sun. General Relativity, on the other hand, nails it.

But that doesn't prove that it's right. It isn't possible in science to ever prove that a theory is right. Something could come along tomorrow which doesn't fit the theory, and then we'd have to reexamine it.

We don't know that gravity is caused by mass. (Leaving aside the fact that under GR gravity isn't a force.) What we know is that they correlate very closely. But it could turn out that both are created by something else. And once we understand it at that level, it could turn out that they can be decoupled.

I don't expect that, but there's no way to know for sure. And that's the point: science never knows anything for sure. That's not how science works.

What we have is an extremely high degree of confidence that General Relativity accurately describes how gravity works. And "an extremely high degree of confidence" is the best science can ever have.
posted by Chocolate Pickle at 11:27 AM on July 6 [2 favorites has favorites]

Not proven, but measured...
For details on how the Gravitational constant was first measured check out the Wikipedia entry: Gravitational constant - Wikipedia
Basically, just measuring the effect that different sized lead balls had on another lead ball.
posted by crenquis at 1:47 PM on July 6

Is what you're talking about the presumed equivalence between gravitational mass and inertial mass?

If so, you might be interested in a truly wonderful 1995 book devoted entirely to this very deep question: Gravitation and Inertia by Ignazio Ciufolini, and John Archibald Wheeler.

I own this book, but have yet to do more than glance through it several times. Here is a review.

I was left with the impression that establishing the identity of gravitational mass and inertial mass is very much a work in progress, and that the work goes forward on a footing of just the right experimentation, but that ontological and philosophical considerations can never be left behind.
posted by jamjam at 2:08 PM on July 6

Kudos to the Cavendish Experiment. That's it.

But there is something more that is being debated as we write. Gravitation shares some similarities with the electromagnetic force. The EM force is really one force with two manifestations. One is the electrostatic force and is pretty strong. It is the force that makes your hair stand on end in an electric storm. The other, of course, is the magnetic force and is much weaker. It results not from the presence of charge but the motion of charge hence we have electromagnets.

The gravitation we are most familiar with is gravitation due to the presence of mass. But Einstein showed that a gravitational force can arise from the motion of mass as well (the right side of the equation contains a tensor "T" which is used to fully describe the twists and contortions of space-time. The left side contains the mass that causes those twists and warps and contains a "static" term and a "dynamic" term. . .one for stationary mass and one for moving mass--containing G and Lamda). This is called "frame dragging" and like magnetism is much weaker than conventional gravitation. And because gravitation is so much weaker than electromagnetism detecting frame dragging is pretty difficult.

The latest attempt to measure frame dragging is Gravity Probe B which is a satellite around the Earth with an extremely accurate gyroscope on board. As the satellite orbits the Earth in a polar orbit the mission payload seeks to measure the precession of the gyro induced by frame dragging due to the rotation of the Earth. GP-B was successful in measuring frame dragging but with the surprise that the frame dragging effect is much stronger than anticipated. No one yet has an explanation for the discrepancy. So the debate continues.

The really cool thing is the fact that Einstein also equated mass with energy with E=mc^2. So. . .in his field equation above you can replace the mass in the left side with energy. And for this demo you could use, say, electromagnetic energy. And so you can imagine a contortion of space-time (that is "gravity") arising from a specific concentration of electromagnetic energy!
posted by Lord Fancy Pants at 2:17 PM on July 6 [1 favorite has favorites]

I did muddy the water a fair bit with my rambling question. Thanks for penetrating my incoherent waffle and pointing me at the Cavendish experiment. That was exactly the kind of thing I was looking for.

Though the other information is still interesting.
posted by Lorc at 2:36 PM on July 6

Note this from Newton himself:

I deduced that the forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.

In short, he had an insight (force of gravity must act like this) and checked it out against various data, and it seemed to work.

The groundwork for this was laid by people like Tycho Brahe and Johannes Kepler, who had developed good enough data about the various planets and their positions that it was possible for someone like Newton to actually check out his hunch to a reasonable degree of accuracy with actual data.
posted by flug at 4:27 PM on July 6

"If we can estimate planetary masses from the strength of their gravity, including that of the earth, then we must be pretty confident that we've nailed the relationship between the two."

"It seems to me that you'd have to test known masses and be able to measure that object's gravity in order to prove anything."

"How do we know that there's a causal relationship rather than it being circumstantial?"

When you ask questions like these you are actually getting into some fairly profound territory.

How do you know the mass of anything? Well, in a very real sense you only know it through the application of Newton's various laws. You can, for instance, put it on a scale but how does a scale work? It basically measures the force the earth's gravitational field puts on the object. In short you are not measuring the mass directly but you are measuring the force in a gravitational field--which, according to Newton's gravitational law, tells you something about the object's mass.

The other basic way to measure an object's mass is to use another of Newton's laws (f=ma): put it under a constance force and measure it's acceleration. So there you have measured a force and an acceleration and you have deduced a mass from those.

Yet another way is to use something like a balance to compare the mass of one thing to another--in that case you are assuming the gravitational field is the same for both and so that allows you to compare the mass of one thing directly against another.

Looking at the planets the situation is even more complex but the same general situation holds. We certainly don't have any way to measure a planet's mass independent of Newton's gravitational law--we can't bring them back to earth and put them on a scale or something. So all our knowledge about the mass of this or that planet or asteroid or star goes back to a bunch of measurements of positions and orbits and the like and a bunch of calculations using Newton's laws.

(Of course we can measure a planet's size and take a guess about its density, which would give at least a ballpark estimate of their likely mass. But that's more like an educated guess than an accurate measurement. If you really want to know their mass, though, you measure their effect on different objects--everything from their moons to satellites to other planets--and do a bunch of calculations using the law of gravity. Still, this gives some way to cross check your results--and of course the cross check works out. The gravity of Jupiter is indeed stronger than that of the Earth, yet weaker than that of the Sun, etc. etc. etc., as we can determine by the orbits of objects near them and other similar observations.)

Anyway, all that is a rather long-winded way of saying it it really doesn't work like some scientist says, "Well then, we know through our complicated mad-scientist mass-determination device that Pluto is exactly 120,304,304,030.43948293 kilograms. We will know double check the strength of its gravitational field to see if it matches. AHA! It does!!!!!11!!!"

It's rather that when you hypothesize that something like Newton's law of gravity is true it sets up a whole, complex, interconnected web of relationships among a whole host of things that can be measured in different ways.

When you carry out a whole bunch of those measurements--in many different ways over the space of centuries in this case, with literally millions of people working on it from various different angles--and they all check out pretty darn close then you start to suspect that particular "law" must be pretty darn close to the way things actually work.

And that is, of course, exactly what happened in the ensuing centuries after Newton published his theory.

And of course everything did check out--pretty darn close--and they still do to the point that the same laws are still used to work out things like spacecraft trajectories and orbits of satellites and asteroids--around earth, the moon, mars, jupiter, saturn, etc. etc. etc.--and they still work.

(Of course over time a few discrepancies emerge and that is what leads to refinements like the General Theory of Relativity. But because of all the measurement and cross-checking that has been done we know that any new theory must be a refinement of the old, not "chuck out the old and start with something completely new".)

In sum, you do check known masses and measure the object's gravity, at least in some lab situations as Cavendish did, but the whole picture is quite a lot more complicated that just that.
posted by flug at 8:39 PM on July 6