Gravity versus electromagnetism ...

https://news.ycombinator.com/item?id=900382

38 25 17 years ago

news.ycombinator.com

The item about the size of the atom reminded me of something that helped me visualise the difference in the size of the forces of gravity and electromagnetism.

It is often said that gravity is an incredibly weak force. But how weak?

Consider ...

Suppose you fall from a tall building. It takes ages to accelerate up to even a moderate speed, and yet you stop effectively in an instant. It's gravity that's making you fall, but electromagnetism that makes you stop.

It's visualisations like that that help me keep otherwise apparently random facts straight.

Hope you find that interesting, enlightening, or otherwise diverting.

mickster1997• 17 years ago

um i thik it is interesting!

graphene8• 17 years ago

this doesn't really comapre the two forces qualitatively I think..

Suppose you did the same experiment on Jupiter; you would fall a lot faster but the speed of decelleration wouldn't be affected. Perhaps on something like a neutron star the two times would even be similar, leading you to conclude the two forces are similar in magnitude...

Incidentally, it's for a large part statistical mechanics, not electromagnetism which would halt you fall - the hardness of solid material is due in a significant part to the electrons not wanting to share orbitals, and not just to them repelling each other.

PieSquared• 17 years ago

And why do electrons not want to share orbitals?

graphene8• 17 years ago

Because that's just the way they are :-)

More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion

btilly5• 17 years ago

Because they are fermions, not bosons. Which has to do with their spin, not their charge.

See http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.

btilly5• 17 years ago

Obviously the structure of an atom is affected by electrons not wanting to share orbitals. But isn't it electromagnetism that keeps atoms from passing through each other? And isn't it also electromagnetism that causes groups of atoms to clump in molecules, then the molecules to stick together in a crystalline pattern? (Update, the answers respectively seem to be "not entirely" and "yes".)

graphene8• 17 years ago

If that were true, why doesn't all matter stick together in one giant gob?

If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference?

In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.

RiderOfGiraffes• 17 years ago

The Pauli exclusion principle pertains, does it not, to electrons (etc) not wanting to duplicate QM states, but is it not still the case that one atom not passing through another atom is becuase of the repulsion of the orbitals?

Electrons in a single atom is a different question from electrons in different atoms.

Of course, I am probably wrong, and look forward to your pointers to further reading to help me understand more.

lucifer• 17 years ago

Not just electrons. Take a look at Fermions: http://en.wikipedia.org/wiki/Fermion

slackenerny• 17 years ago

Most detailed explanation in the blogosphere is "Why Stuff is Hard" by Jim Pivarski,

http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-har...

http://cornellmath.wordpress.com/2007/08/11/why-everything-i...

Complete argument is the Dyson, Lieb and Thirring's proof of stability of matter, http://projecteuclid.org/euclid.bams/1183555452 .

graphene8• 17 years ago

But if one atom "tries" to pass through another, the electrons from both atoms will be spatially very close to each other, and consequently, they will be occupying very similar states. Coulombic repulsion certainly also plays a role, but it's not the only force at work here. I don't know the relative magnitudes for single atoms bumping into each other, but for bulk material, I believe it's about 40% exclusion, 60% coulomb.

Why would electrons in different atoms interact differently than electrons in the same atom? They don't, as far as I know. Furthermore, if electrons from different atoms only interacted Coulombically (repelling each other), molecules and solids would never get formed.

These are difficult questions, not addressed in the clear-cut cases often presented in introductory QM textbooks. I can't think of any must-read references right now, but if you're interested in this kind of thing I know of two books on molecular quantum mechanics; one by McWeeney (methods of molecular quantum mechanics) and one by Atkins (molecular quantum mechanics), which I haven't read myself, but I hear it's good. I don't know of any free online sources for this kind of stuff, but it's probably out there, I just haven't really looked yet.

NickM• 17 years ago

If you did the same experiment on Jupiter, then the speed of deceleration absolutely would be affected. When you hit the ground, you stop because there is a large force accelerating you upwards. On Jupiter, there is still the same upwards force on you, but a stronger downwards force from gravity, and therefore you will decelerate somewhat more slowly. Not a lot more slowly, since the upward force from the ground is much larger than the gravitational force, but it still makes a difference.

On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.

graphene8• 17 years ago

Well, I don't know...

If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform).

If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.

jodrellblank2• 17 years ago

It's visualisations like that that help me keep otherwise apparently random facts straight.

Are there any similarly nice visualisations for biological features? How do bacteria, viruses, cells, DNA, mitochondria, molecules, proteins, fatty and amino acids compare in size? (Without resorting to rote SI unit memorisation, I mean)

jodrellblank2• 17 years ago

Yes, there are!

http://news.ycombinator.com/item?id=909618

archgrove• 17 years ago

It's a nice example. Another one I like is "It takes the whole earth to attract this small iron object, but only this small magnet to overcome that pull"

bwoodacre• 17 years ago

The easiest way to get a sense for the difference between the laws is to look at their mathematical statements. From the sheer magnitude of the forces induced, electrostatic repulsion of like charges (Coloumb's law) is ~10^23 times more strong between two electrons than the attractive force between their mass as given by the law of gravity; for protons (which have more mass & opposite but equal charge) you lose a few orders of magnitude, but the margin is still ~10^20. But gravity always attracts, while electrostatics may attract or repulse, so when you consider a real situation then things like your falling example get more complicated. I prefer just to remember the difference in their magnitudes.

Gravity versus electromagnetism ...

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