Is Gravity a Theory or a Law?

Is Gravity a Theory or a Law?

This week's experiment comes from a recent question, wanting to know whether gravity is a law or a theory. That question brings up so many more questions that I thought it would be fun to explore. To try this, you will need:

- an object to drop.

OK, pick an object that will not break, dent the floor, cause a mess, or get either of us in trouble. Hold it out in front of you and release it. What happens? It falls, of course. The gravitational attraction between the Earth and the object pulls it towards the ground. But, when we do this experiment, should we be talking about the Law of Gravity or the Theory of Gravity?

Actually, we should be talking about both. To understand why, we need to understand the scientific meaning of the words "law" and "theory."

In the language of science, the word "law" describes an analytic statement. It gives us a formula that tells us what things will do. For example, Newton's Law of Universal Gravitation tells us that "Every point mass attracts every single point mass by a force pointing along the line intersecting both points. The force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses." That formula will let us calculate the gravitational pull between the Earth and the object you dropped, between the Sun and Mars, or between me and a bowl of ice cream.

We can use Newton's Law of Universal Gravitation to calculate how strong the gravitational pull is between the Earth and the object you dropped, which would let us calculate its acceleration as it falls, how long it will take to hit the ground, how fast it would be going at impact, how much energy it will take to pick it up again, etc.

While the law lets us calculate quite a bit about what happens, notice that it does not tell us anything about why it happens. That is what theories are for. In the language of science, the word "theory" is used to describe an explanation of why and how things happen. For gravity, we use Einstein's Theory of General Relativity to explain why things fall.

A theory starts as one or more hypotheses, untested ideas about why something happens. For example, I might propose a hypothesis that the object that you released fell because it was pulled by the Earth's magnetic field. Once we started testing, it would not take long to find out that my hypothesis was not supported by the evidence. Non-magnetic objects fall at the same rate as magnetic objects. Because it was not supported by the evidence, my hypothesis does not gain the status of being a theory. To become a scientific theory, an idea must be thoroughly tested, and must be an accurate and predictive description of the natural world.

While laws rarely change, theories change frequently as new evidence is discovered. Instead of being discarded due to new evidence, theories are often revised to include the new evidence in their explanation. The Theory of General Relativity has adapted as new technologies and new evidence have expanded our view of the universe.

So when we are scientifically discussing gravity, we can talk about the law that describes the attraction between two objects, and we can also talk about the theory that describes why the objects attract each other.

Have a wonder-filled week.

Learn more about Laws and Theories.

Home - Process of Science - What is Science?
Hypotheses, Laws, and Theories - What Do Scientists Do?

Anonymous wrote on Tue, 10/09/2012 - 19:57:

i actually want to know about why it is a law but not a theory?

rkrampf wrote on Sun, 10/14/2012 - 23:45:

There is a law of gravity AND a theory of gravity (actually more than one.) The law of gravity tells us how gravity affects things. The theories of gravity are explanations of why gravity affects things

Anonymous wrote on Sun, 04/22/2012 - 16:20:

when i let go of my object it balloons beat your theory.

Anonymous wrote on Tue, 06/26/2012 - 01:28:

It floated as it fell...

rkrampf wrote on Wed, 04/25/2012 - 15:29:

Ah, but why does it float? Because of gravity. The surrounding air is denser than the helium in the balloon, so it is pushed upwards. If you had a helium balloon on the International Space Station, it would not float upwards.

Anonymous wrote on Fri, 10/12/2012 - 09:29:

rkrampf, It would also not float in a vucuum, so gravity has nothing to do with balloons floathing.

rkrampf wrote on Sun, 10/14/2012 - 23:25:

The balloon floats in air because the helium inside is less dense than the air around it. In a vacuum, the helium would be more dense than the empty space around it, so it would sink instead of floating. Without gravity, everything would have neutral buoyancy, neither floating nor sinking.

Anonymous wrote on Thu, 10/11/2012 - 22:19:

That is technically correct. The balloon would not be floating because of lack of gravity, but because the international space station is falling at a constant rate back to Earth. It's falling so fast around the Earth's rotation that it appears to float. The balloon would float upwards, but it would not appear to be doing so. And I'm very much hoping that there is air in the International Space station for the sake of Astronauts attending it :)

rkrampf wrote on Sun, 10/14/2012 - 23:40:

There is not a lack of gravity on the international space station, but because everything is in free fall, everything has neutral buoyancy. That is why the astronauts, tools, etc. appear to be weightless. If they placed a helium balloon, a hammer, and a feather in the air in front of them, none of them would float or sink. They would all be falling at the same rate as the space station, so all three would remain in place relative to each other until some force pushed or pulled on them.

Anonymous wrote on Thu, 03/07/2013 - 02:11:

Beautifully articulated.

Anonymous wrote on Fri, 04/20/2012 - 09:15:

this is very good

Anonymous wrote on Sun, 04/01/2012 - 22:06:

There is a great deal of good material in this article. Im signing up to your rss feed.

Anonymous wrote on Sun, 04/01/2012 - 07:58:

So is gravity a law or a theory ?

Anonymous wrote on Tue, 10/09/2012 - 19:58:

i want to ask the same question

Anonymous wrote on Mon, 06/18/2012 - 23:40:

ITs both... the math that show you WHAT will happen are usually concise one liners... So Gausses LAws, Newtons Laws, JEans instability etc... they all explain mathimatically what is happening on front of you as the ball falls....

But all of those laws need to explain WHY the ball falls.... not just that it does fall...we know that anyway...the LAWS only give us a way of predicting how fast the ball will fall, how fast it will accelerate as it gets closer to the other object (the ground/earth) etc. They do nothing to tell us why this is so... thats what a 'Theory' is... its an explanation of all the laws that govern a phenomena. Inasmuch as we've tested them they are accurate.

However even Newtons laws of Gravitation are essentially 'estimates'.. Einsteins Law of reletivitty is MORE doubt there are other more accurate still.

Wikipedia does a pretty good job of explaining this.

Anonymous wrote on Tue, 08/09/2011 - 13:20:

A friend once explained why gravity is a theory not a law by saying that gravity can be fooled. Because the experience of gravity is relative to your inertial frame, there can be situations where gravity seems to be disproven. For example, a guy in an elevator. Say the elevator is in space and a guy is floating round in it, gravity as the guy would define it is absent. If we start to accelerate the elevator upward the man would experience "gravity" because be can now stand on the floor. But from an outside perspective what the man is feeling is a fictitious force created by the acceleration of the elevator.
Is this a valid explanation or just creative reasoning?

Anonymous wrote on Sat, 12/17/2011 - 23:45:

when you go in an elevator you feel as you are having more pressure on top of you (if you are going upwards) so you still have gravity pulling down on you (hence the feeling of pressure on top of you as you are going up.

rkrampf wrote on Fri, 09/16/2011 - 22:27:

Your friend was off on the definition of a theory and a law. While your perception of the pull of gravity is relative, the actual attraction between bodies (Law of Gravity) still works the same. The Theory of Gravity (part of the Theory of General Relativity) is an explanation of what we think causes that attraction.

Anonymous wrote on Fri, 09/16/2011 - 20:49:

no that wouldnt work beacause the average human falls at 85 mph so if the elevator would be going faster than that than basically the human would be free falling so he is being pulled down by gravity but can only free fall as fast as 85 mph so the elevator would be going faster than the human

Anonymous wrote on Sun, 09/04/2011 - 14:39:

that's an interesting argument, but i would say that what he's experiencing is not gravity and merely the force that the acceleration of the elevator is exerting on him. because gravity is an attraction between two objects, it can't be fooled as your friend suggests. for example, even though it is small, there is a gravitational attraction between the man and the elevator.

Anonymous wrote on Sun, 06/05/2011 - 18:19:

Thank you for this. Did you mean to say "scientific meaning" ?

rkrampf wrote on Sun, 06/05/2011 - 19:42:

Yes, thanks for catching that!

Anonymous wrote on Sun, 05/08/2011 - 15:15:

Couldn't we find out if gravity can be defied? I'm thinking NASA should fund a trip into space; the destination should not be near any large planet etc. We should take one large ball and a small ball. We should release them into space, and observe them. If the smaller ball orbits the large ball, we can presume that gravity cannot be defied. However, if it does, then we might need to worry about our information.

rkrampf wrote on Sun, 05/08/2011 - 16:54:

Good thinking. In fact, NASA just completed their latest experiment, confirming Einstein's theory about gravity. Check it out on the National Geographic site:

Anonymous wrote on Tue, 03/09/2010 - 17:20:

While we say that laws rarely change, they do on occasion. For example, when Albert Einstein came up with General and Specific Relativity, these slightly changed Newton's Law of Gravity so that, for example, the new law better described the specifics of Mercury's orbit--to the accuracy of 36 seconds of arc per year (I think) of the precession of the perihelion. Still, with a few exceptions, including the clocks on the GPS satellites, Newton's laws are quite good enough for most purposes. For example, nobody bothers to make the Relativistic corrections when lining up a billiards shot.

In a way, we could say that a new law often characterizes the limits of the previous law, buy showing where that law is inaccurate.

Also, laws and theories, to be good science, must make it possible to make specific predictions which can then be tested. That was often the complaint of Wolfgang Pauli--that modern "theories" like String theory cannot make useful predictions to test. As he liked to put it, "That's not even wrong!" In other words, if we cannot prove it wrong by testing, it's not science. Other scientists call such things "theology" as opposed to real "science".