Gravity? It's Only A Theory

What is a theory - using Gravity as an example


A few people still dismiss the idea that evolution has occurred and is still happening with the remark, yes, but "it's only a theory".

This is a summary of the scientific method using something very familiar and less controversial than evolution, the Theory of Gravity.

This theory is most associated with the English mathematician and physicist, Isaac Newton. He published his theory (although strictly speaking it was a still a hypothesis until it had been tested) back in 1687. Before we evaluate what he said, I think it is very instructive to look at what he didn't say.

Newton did not say:

"I think gravity keeps the Moon going around the Earth and you can't prove otherwise".

This is a pseudo-scientific statement used by people like astrologers, believers in UFOs and creationists. Statements like these form no part of science. If you postulate the existence of something or an explanation for an observation, it is up to you to provide the evidence for it not for others to disprove it. Science would make no progress if the onus was on everyone to disprove every suggestion made whether reasonable or outlandish.

Newton did not say:

"A deity has told me that gravity keeps the Moon going around the Earth".

This is a religious statement and these types of statements form no part of science. They, too cannot be tested.

Newton also did not say:

"Gravity keeps the Moon going around the Earth".

This is speculation. This also cannot be tested. There is nothing to test. It's just an idea, a guess. There is nothing wrong with speculation but it is not part of the scientific method. It may play a part during the formation of a hypothesis and in informal discussions. You cannot publish a speculation in a scientific journal.

What Newton did say was (and I paraphrase here):

"Gravity keeps the Moon going around the Earth and these are the mathematical equations that describe how it works".

This is a hypothesis.

It is an idea, an explanation for something in nature but it also provides a means of testing it. The equations can make calculations that can then be compared to nature itself.

So how did this hypothesis become elevated to a theory?

A good theory must do two things. Firstly, it must explain existing observations accurately. Newton's ideas about gravity certainly did that.

The equations Newton published explained the Moon's motion around the Earth better than any other model before then had. His equations also explained the motions of the planets around the Sun. It explained why planets travel in elliptical orbits and why they travel faster when they are closer to the Sun. The theory allows tables of the Moon and planets to be created that helped in marine navigation - something that eventually led to the British Empire. It also explained why objects of different weights all fall at the same speed on the Earth's surface if you ignore air resistance.

A good theory must also make predictions of things that are not known.

The Theory of Gravity certainly did that.

Before the 1870s comets had been thought of unpredictable objects that came and went under unknown rules. Newton's contemporary and friend, Edmund Halley used the Theory of Gravity to show that a series of historical comets was actually a single periodic comet that had been returning every 76 years or so. He predicted its return in 1758. He never lived to see this prediction fulfilled. The comet became known as Halley's Comet and has been observed on every return since then.

In 1781, the planet Uranus was discovered. This was the first new planet to be discovered since the five naked eye planets which had been known from ancient times. The planet's orbit was calculated using the Theory of Gravity and it was observed for the next fifty years as it orbited the Sun.

When the orbit was calculated, allowances had to be made for the gravitational pulls of the giant planets Jupiter and Saturn. However, by the 1840s it was clear that Uranus was not quite following its calculated orbit. Did this mean that the Theory of Gravity was incorrect?

It might.

Another explanation was that there was another planet, further from the Sun that was pulling Uranus from its predicted orbit. Two mathematicians tackled the problem and made predictions based on the Theory of Gravity of where the new planet, if it existed, should be in the sky.

In 1846, the planet Neptune was discovered just a few degrees from its predicted position in the sky. This was a stunning confirmation of the Theory of Gravity.

In the late 1780s, binary stars were discovered. These are stars that appear to be single to the naked eye but are actually two stars in orbit around each other when viewed through a telescope. When these orbits were studied it was found they, too, followed the same laws from Newton's Theory of Gravity. The theory applied not just in the Solar System but also in the realm of the stars.

By the beginning of the 1800s, the Theory of Gravity was established as one of the great theories of science.

Science, unlike other disciplines, is always provisional. If evidence appears that cannot be explained by a theory, then the theory may have to be modified or, in extreme cases, even abandoned.

In the 1850s, the orbit of the planet Mercury was found to be differing very slightly from what the Theory of Gravity predicted, even when the gravitational effects of the other planets was taken into account.

The difference between theory and observation was tiny. Let me give you an idea of how tiny. The Moon in the sky has an apparent diameter of half a degree. A degree is divided into 60 minutes. A minutes angle is about the size of a crater on the Moon, barely visible to the naked eye. The minute itself is made up of 60 seconds.

The error in Mercury's orbit around the Sun as predicted by the Theory of Gravity amounted to just 43 seconds angle per century. Not per year; per century. This is a tiny difference. In science, these differences are important however small they are. And they must be explained and accounted for.

One way of explaining Mercury's orbit was by postulating a planet closer to the Sun that was pulling on Mercury. This had worked in the case of Uranus and had led to the discovery of Neptune. The calculations were made but this time no planet was discovered.

The Theory of Gravity survived this anomaly because it was so useful elsewhere but scientists knew that something was not quite right.

In 1916, Albert Einstein developed his famous Theory of Relativity. Part of this theory was a replacement for the Theory of Gravity. Like the good theory that it was, it explained all the observations that were known at the time.

The Theory of Relativity duplicated the explanations for the planetary orbits around the Sun and the Moon's orbit around the Earth. It also duplicated the explanation of the orbits of comets and the motions of binary stars.

Significantly, it explained the anomaly in the orbit of Mercury.

Remember that a good theory must also make predictions of things that are not known. The Theory of Relativity predicted that light would be bent by a large gravitational field. This strange prediction was verified during a total eclipse of the Sun in 1919.

The Theory of Relativity also predicted that the Universe as a whole would not be static - it must be expanding or contracting. In 1929 it was found to be expanding.

The Theory of Relativity predicted (and this is another strange one) that time itself would be affected by gravity. Modern atomic clocks verify this every day. Indeed, all GPS systems must allow for the theory if they are to report accurate locations.

Finally, the Theory of Relativity predicted the existence of Black Holes, which are now part of the life cycle of stars and galaxies.

Does that mean that the Theory of Gravity is wrong?

Well, yes and no.

The Theory of Gravity is really an approximation for the Theory of Relativity.

Under certain conditions, the mathematics for the two theories gives the same results. In other cases, the Theory of Relativity gives different but more accurate results. Another way of looking at it is that the Theory of Relativity is an extension of the Theory of Gravity.

The mathematics of the Theory of Gravity is far simpler. It can be understood by any child at school who has studied algebra. The mathematics of the Theory of Relativity, in contrast, is too complex for most university undergraduates. This means that scientists will use the Theory of Gravity if they can and only use the Theory of Relativity if they must (and if they know how to).

For building a bridge, predicting an eclipse of the Sun, or sending a probe to Mars, the simpler mathematics of the Theory of Gravity can be used. This approximation is sufficient for these purposes.

For looking at the behaviour of a Black Hole, setting up a GPS system or studying the properties of the entire Universe then you have no choice but to utilise the complex mathematics of the Theory of Relativity.

The history of the two theories of gravity is an excellent example of the way that science works. When something in science is described as a theory it means three things:

A Theory in science is not a guess to be dismissed or demeaned. It is an idea that has been tested against observation and has made predictions about nature that have turned out to be true. This applies to anything described as a Theory in science: the Theory of Plate Tectonics, the Theory of Quantum Mechanics, the Theory of Evolution, the Theory of Chemical Combinations, etc.

So let's not hear anymore of "it's only a theory".

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