Gravity - The weakest force?

[Uranium235]

A question for the eggheads, but Im guessing I will get the standard scientific answer "we dont know".

I was watching a promo trailer of a new documentary series for the BBC which airs on sunday where the presenter describes gravity as the weakest force in the universe. And in my mind, I just couldnt see that.

I know there are other forces (ie: Atomic and Radiation, im sure there are more). But if gravity is so weak, then why dont (normal) stars fly apart? (where gravitational forces are equal to nuclear forces) And why black holes? (where gravity is king).

Is gravity is related to mass? (thats how I see it), as im sure if we all lived on Jupiter we wouldnt be saying gravity is so weak because we wouldnt be able to move for being squished by it!

[astoc]

There are four natural forces; weak nuclear, strong nuclear, electromagnetic, and gravitational. Weak and 'strong' have very short ranges of influence (subatomic), so although we encounter the effects of their existence on a day to day basis they remain 'unobservable'. Of the two remaining, electromagnetism is by far the strongest. It, like gravity, has an infinite range with fields decaying non linearly over space, but you get a lot more bang for your buck than you do with gravity. Compare the pull of a fridge magnet with the gravitational field of a similarly massed object.

The reason gravity does so well is that as you say yourself, the effects cumulate with added mass. Electromagnetism depends on the dynamics of a particular physical setup within your material, while gravity just needs 'more stuff'. The more massive an object the stronger the magnetic field. On the scale of planets, stars, and in particular black holes the masses are enormous.

Of the four forces gravity is by far the least understood. We can model its behavior at large scales, but we cant easily tie it in to theory on the quantum level. Electromagnetism, by contrast, can be modelled to an accuracy of 10^-12.

John

[brianb]

Actually gravity can be overwhelmingly strong, it's what stops stars flying apart under the pressure of their radiation ... and when a stellar mass shrinks to a few kilometers across, it's gravity that crushes the neutrons causing a collapse into a black hole.

The formula for the gravitational force between two (point) objects with masses M,m a distance r apart is GMm/r^2 - so the bigger the masses, and the smaller the distances, the bigger the force is.

[Matthew.Blake]

As someone has already said the strong and weak forces are really short range. The electromagnetic force is much stronger than gravity but it can be both attractive and repulsive so the net effect over large distances tends to be zero. Gravity on the other hand, despite being the weakest of them all, is always attractive so ends up dominating. An example of how much stronger electromagnetism is than gravity is if you took about 300kg worth of electrons from the Earth (leaving the Earth with a net positive charge) to the Moon, the attractive force due to electrostatic attraction would equal the attractive force due to gravity. This many electrons can be had from about 7000 Tons of coal. Not much really.

[astoc]

Do you mean 300kg of electrons would be attracted to Earth by the same force as the gravitational force of the Moon to the Earth? Bit confused.

[acey]

A fridge magnet is sufficient to demonstrate that electromagnetic force is a lot stronger than gravitational force. All the gravity of the entire Earth can't budge a single little bit of magnetised metal.

[George Jones]

Do you mean 300kg of electrons would be attracted to Earth by the same force as the gravitational force of the Moon to the Earth? Bit confused.

I agree that there is an ambiguity; here is my take on what Mathew.Blake means.

Remove 300kg of electrons form the Earth.

Find the net (negative) charge of this amount of electrons. First find the number of electrons by dividing 300kg by the mass of one electron. Multiply the number of electrons by the charge of one electron.

Since negatively charged electrons were removed from the Earth, the Earth now has the same magnitude positive charge.

Write down Coulomb's force for the electrostatic force between the negatively charged 300kg of electrons and the positively charged Earth.

Write down Newton's law of gravity using the mass of the Earth and 300kg for the two masses.

Set the electrostatic force equal to the gravitational force, and solve for the distance at which this happen. This distance is about the same distance as the distance between the earth and the Moon.\

I haven't run the numbers.

[Matthew.Blake]

Do you mean 300kg of electrons would be attracted to Earth by the same force as the gravitational force of the Moon to the Earth? Bit confused.

That is indeed what it means. Gravity is, basically, Rubbish.

[Matthew.Blake]

A fridge magnet is sufficient to demonstrate that electromagnetic force is a lot stronger than gravitational force. All the gravity of the entire Earth can't budge a single little bit of magnetised metal.

But the magnet is alot closer to the fridge than it is too the Earth. Orders of magnitude closer in fact...

[acey]

Alright, try rubbing a balloon and lifting little bits of paper, or combing your hair and making it stand up. Same idea - though obviously it can be expressed in more formal terms in dimensionless units etc etc.

[themos]

But the magnet is alot closer to the fridge than it is too the Earth. Orders of magnitude closer in fact...

Put the magnet on the floor. Pick it up with another magnet.

[George Jones]

Of the four forces gravity is by far the least understood.

I can't resist playing the role of a contrarian.

As honest mathematics, gravity (general relativity) is far better understood than the other three forces (quantum field theory).

I don't mean problems with interpretation of the meaning of quantum theory, I mean just the mathematics, and I don't mean the level of difficulty or complexity of the mathematics.

[Matthew.Blake]

Put the magnet on the floor. Pick it up with another magnet.

You have to measure the distance to the centre of mass of the Earth so even on the floor it's a lot further from the Earth than the other magnet. Bwuhaha, pedantry for teh win!

[themos]

Come on George, general relativity has singularities that we know are wrong AND we can't quantise it. The quantum field theories for the other three are known to be well-defined (aren't they?).

If you forget about theories and just deal with the phenomena, gravity has not been measured that well. We keep finding strange things like the Pioneer Anomaly and Dark Energy.

[themos]

You have to measure the distance to the centre of mass of the Earth

Ok, get a huge magnet the size of another Earth and pick the small magnet up with THAT.

[George Jones]

Come on George, general relativity has singularities that we know are wrong AND we can't quantise it.

Yup!

The quantum field theories for the other three are known to be well-defined (aren't they?).

Nope!!!

If you forget about theories and just deal with the phenomena, gravity has not been measured that well. We keep finding strange things like the Pioneer Anomaly and Dark Energy.

So much to say; so little time.

[brianb]

general relativity has singularities that we know are wrong AND we can't quantise it.

Singularities - what's wrong with those?

There exist quantum theories of gravitation, they fall fairly naturally out of most variants of string & M theory ... what we don't have is direct experimental or observational evidence (but the LHC may rectify this shortcoming).

We keep finding strange things like the Pioneer Anomaly and Dark Energy.

The so-called "Pioneer Anomaly" is well understood in terms of venting of unspent fuel from the attitude control system.

As for "dark energy" - the jury is still out on this - when we understand the way in which quantum gravity works a bit better, we'll have a better understanding of what goes on in relativistic degenerate environments (core collapse of supernovae), it's more than possible that we'll find that the luminosity of type 1a supernovae depends in some critical way on the proportion of heavy elements in the accreting material, such a finding could remove the "recent" apparent acceleration of the expansion of the universe and the need for "dark energy" along with it. Or perhaps another supernova mechanism (pair production?) which occurs preferentially in very massive stars can be mistaken for type 1a SN at large distances but in fact the luminosity is very different. We do know that supermassive stars were much more common in the early universe - the dust comprised of heavy elements ejected from SN didn't exist then, and its optical thickness limits gravitational infall, resulting in supermassive stars being much less likely to form now.

[astoc]

This many electrons can be had from about 7000 Tons of coal. Not much really.

Might call it nitpicking, but I think it's worth noting that the electrons in 7000 Tons of coal are in electrostatic equilibrium with their parent nuclei. That's why I asked earlier, we cant have people thinking 7000 Tons of coal will move the moon

John

[Space Bat]

Interesting ....

Ok so if you have a a large magnet in a near vacuum of space- where no interacting gravitaional forces can be applied locally to the event.

If we have a ferrous bar object come within say X Astronmical units of the magnet - what interacts with the bar first, magnetic forces or gravitational forces ?

Chris

[themos]

George, this is no time to get coy.

brianb, "M-theory" and "string theory" are very clever mathematics but they are not physical theories.

As for Pioneer Anomaly and Dark Energy, we will know more in the future. I think they are real phenomena.

[themos]

what interacts with the bar first, magnetic forces or gravitational forces ?

According to current theories, the two forces travel at the speed of light.

[Space Bat]

Thermos - are you saying that both interact equally at the same time ?

[themos]

at the same time, yes, but the strengths will be different.

[Uranium235]

Wow... thanks for the interesting and informative answers to my query, was able to follow most of it well enough, even though physics inst really one of my strong points. Its interesting that electromagnetism has a repulsive and attractive nature (so over a great enough distance, its effect is zero), and that gravity is only attractive. So, what about the cumulative effect of time?

Assuming gravity eventually wins over a massive timescale (like over googolplexes of years) - then doesnt that mean the universe will eventually disappear up its own backside into one massive black hole? (and then possibly start all over again)

Its not as if black holes think "hmmm ive had enough to eat for today, time for a diet", nope... its just going to get bigger and bigger. Of course, im just speculating

[themos]

Gravity doesn't have to win, especially if it's true that space is being torn apart at an accelerating rate as dark energy suggests.