Is gravity wrong?

[claymore]

Watched a great show on tv Sunday night titled ..Most of our universe is missing. Scientists say that only 5% of the universe is made of ordinary atoms, the rest , they say, is so called dark matter and dark energy. The dark energy is the reason for the acceleration of the expansion of the universe. It went on to show how they are trying to find what dark energy is and how it works, so far without success. This got me thinking, if the best brains in the business don`t know what it is and can`t find it could it be that it doesn`t exist. Could it be that instead of a force pushing the universe apart ,there is a force beyond the observable universe that is pulling it apart.? This implies that maybe the law of gravity needs to be questioned.

[fo3nix]

It is being questioned, all the time.

That's the whole point of science.

You shouldn't care if some scientific theory is 'wrong' or 'right'. Just that you're applying scientific principles towards your ultimate goal of understanding.

Take gravity: I make an observation that if I let go of an apple in my hand, it falls to the ground.

Therefore, there is some invisible force that it pulling this apple to the ground. I call this force 'gravity'. I notice that everything is being pulled to the ground, including myself. My theory is that this is all the work of gravity. Perhaps I do some work and find that, mathematically, the strength of this force falls off as an inverse square law (1/r^2).

So, I'm extending my theory. I test it, and it seems true for everything. Thus, the theory is 'correct'. But there's no such thing as 'truth' in science, and you should never put your faith in a theory.

I jump to some amazing conclusion that the planets are being pulled to the Sun via gravity, this same force, and the Moon to the Earth. My theory, and its mathematics, fit with the observational evidence, so I say it is 'correct'.

Time goes on long enough, with my theory still seeming to be correct, that it becomes a 'law'.

But then along comes some other fellow with other ideas, who wants to talk about spacetime and metric tensors and other such things. He shows that my law is actually only part of the greater truth -- that it works for many cases, but it's far from describing everything it should.

[fo3nix]

Am I sad that my theory is now 'wrong'? No. It's not wrong, it's just that it's only partially true. Advancement of my theory (even though it means showing that my theory is not totally correct), is a great thing. We understand more about the universe and the ultimate truth now than we did before! This is great!

If you stop questioning science, then you may as well stop performing anything scientific at all.

As you probably know, the first guy in my story was Newton, and the second Einstein. If someone else comes along and shows that Einstein's theories were only part of the truth (and there are some other forces that we didn't know about, or other such new things), then that's great too. Advancement of science, through new obsevations. We see where the current theories take us, and if they fit describe these new observations, then we're happy. But if the current theories don't work out, and some new theory is needed, then that's great too. We know more than we did before. It can't possibly be bad.

[R1Geezer]

good answer above.

To expand on that, gravity is not right or wrong, it is the name we give to the phenomena we observe. Dark matter/energy is really the current name that is given to the "something" that has effect but is not locatable. Generally for Galactic sized clumps of ordinary matter, ie atoms and molecules, there is just not enough for cohesion of the structure, so the missing element is labelled dark matter as it clearly has gravitational effect but is undectable. In this labelling scientists seek to match their equations with what is observable. It is entirely possible that the way we visualise it it wrong. All we know is that something does exist. So the law of gravity is being questioned continually by "the best brains in the business".

[jake-the-peg]

Dark matter has been with us a long time. It was a big talking point when I was at college in the 80's. There are a number of candidates each of which could explain things none on which need a really serious revision of the way things work.

One or more of them might be right or perhaps something else - right now we don't have the experiments to tell. Massive neutrinos though are a very interesting development.

Dark Energy is much more perplexing.

Newton's laws of motion and Gravity were good enough to put a man on the moon but Einstein's modifications are needed for high precision work.

It may be that dark energy is telling us that a further modification to Gravity is required. It's a difficult area because our models of Gravity are awkward at the moment. They don't work well at the very small level either so it's ripe for refinement.

But there's a big difference between refinement and wrong.

[chakka35]

Excellent answers, all of those.

[kassiopeia]

I love the other theory of gravity.

I can't remember who it was now but I'm sure all of you do. I'm talking about the philosopher who theorised that everything just went back to it's natural place.

Water always flows back to the sea, fire always rises back to the sun......

I think that is a lovely theory. Some things can't be explained with science, that's why the world is so wonderful.

[fo3nix]

kassiopeia: I believe that's the kind of reason as to why Feynman didn't like philosophers :P

[Rev. Green]

I'll be glad when someone finds the graviton and proves that gravity is a real force. My money is on it being a pseudo force like Coriollis force and centrifugal force, because, like those, it is proportional to mass.

[chakka35]

Dear Rev Green,
Please explain why Coriolis and centrifugal forces are pseudo. Why are they such because they are proportional to mass?
And how would the discovery of the alleged graviton help?

[Rev. Green]

The four known forces are the electromagnetic force, the strong force which holds nuclei together, the weak force which mediates radioactivity, and gravitation.
The way that a force acts on a distant object is for "bosons" to pass across the intervening space. So if you rub a balloon it will stick to the ceiling because it becomes charged and the bosons of the electromagnetic force (which are in fact photons) pass between the balloon and the ceiling. Bosons have been discovered also for the strong force and for the weak force. The boson for gravitational force has been named the graviton. Some of its properties are known, if it exists, for example its spin must be 2, but it has never been seen. Until it is discovered we don't know that gravity is a real force.
By pseudo-force I mean an apparent force which can be explained in terms of other forces. For example, if you throw a stone due South it will land a bit East of due South. (Not much East. I haven't calculated how much but it is probably less than a millionth of a millimetre.) This can be explained by saying that there is a force, Coriollis force, pushing the stone Eastwards. The Eastwards motion can be explained without using Coriollis force by observing ...

[Rev. Green]

......that, before it is thrown, the stone is rotating with the Earth, once in 24 hours. At its highest point the stone has the same angular momentum as before it was thrown, but it is now further from the centre of the Earth and so its period must now be greater than 24 hours. Similarly, the Moon stays the same distance (approximately) from the Earth because centrifugal force balances gravity, but you can explain the motion of the Moon without using centrifugal force by observing that the Moon needs to "fall" away from its natural straight-line motion in order to orbit the Earth. So Coriollis force and centrifugal force are pseudo-forces.
The effects of gravity can also be explained without using a gravitational force by imagining that objects travel in straight lines in curved space as Einstein thought.
The point that I was making was that the force of gravity is exactly proportional to the mass of the object. The pseudo-forces are also exactly proportional to the mass of an object : the constant of proportionality must be 1 for the pseudo-force to explain the required effect.
For gravitation, the constant of proportionality could be any number. The fact that it is exactly 1.000000 (It has been measured to a large number of digits) makes it suspiciously look as though it is also a pseudo-force.

[Rev. Green]

(Correction: for most positions on the Earth, a stone thrown to the South will land to the South and almost 1 mm Westwards.)