Reason why the chair can support me

[masudr]

Some will tell you this is merely because of our computational limits, and you should accept that the way you accept someone telling you they could jump the moon, if only they had really bouncy shoes.

It is merely computational limits that prevent us from predicting macroscopic properties; though statistical mechanics gets us quite far in getting from the quantum mechanics to thermodynamic properties.

Incidentally, if you DID have really bouncy shoes, you could get to the moon. Although I think they call these really bouncy shoes by a different name: rockets.

 

[fargoth]

there is something right about the OP, but I am not sure that this is the dominant force in such a low pressure.

whats this physicist was talking about is caled the electron degenaracy pressure, that's the dominant force that keeps white dwarves from collapsing into neuron stars.
in such a high pressure pauli's exclusion principle which prevents the electon's wave function from overlapping thus giving the electron's wave function less space and and more range of momentum.
the electro-magnetic force is only $$\frac{1}{r^2}$$
while this force is much stronger at low distances, and by experimental results is estimated to be $$\frac{1}{r^{15}}$$.
we used this aproximation in fluid thermo-dynamics.

A form of degenerate matter in which the weight of overlying material tries to force all of the electrons surrounding the atomic nucleus into the lowest energy quantum state. The electrons resist, because of the Paul exclusion principle, and so exert a pressure, known as degenerate electron pressure or electron degeneracy pressure, that halts further collapse. This pressure is sufficient up to the Chandrasekar limit of about 1.4 Msun. However, beyond this critical mass, gravity overwhelms the degeneracy pressure and results in further collapse. See also baryon degenerate matter.

also see:
http://www.answers.com/topic/degenerate-matter

 

[Locrian]

Locrian, I see what you are getting at and for all practical purposes, your answer is the best. But I made the thread because I was curious between the relationship between fundalmental physics and everyday life.

Which is an excellent thing to be curious about! One important goal I have with my questioning is to make someone - anyone - start asking more specific questions. You've made a massive jump from quantum mechanics to a chair, and there are many steps in between. Unfortunately, people mostly seem to be repeating the same droll nonsense over and over, which has made this thread bulkier than it should have been.

When I first started making my point about the material in the chair, the appropriate question that one of these reductionists should have moved the subject to is this: how do you get classical properties from quantum phenomena? This would help them bridge the gap they are so shy about. The reason people have been so tentative about moving to that question is probably that they don't know the answer. This issue does not get easier when looked at more closely, and this is central to my argument.

Even though you say that 'Neither of these properties is something you can predict from electrons, protons and EM fields.' I am sure all the fundalmental laws of physics contribute to the properties of strength and hardness. If they contribute a little bit.

They certainly do! And their final role is very important. You see, one thing to always keep in mind when dealing with the reductionism vs emergence arguments is that those who say "the whole is more than the sum of its parts" are not claiming the whole is entirely independant of its parts. They are claiming that believing that the whole is only the sum of its parts leaves you impotent.

Note that the past fifty years have been very much the hayday of the emergence crew. Almost every great scientific advance in that time has occurred at a higher level than can be predicted using sumatomic particles, and meanwhile "important" advances at the subatomic level (such as most of the standard model) have produced insignificant philosophical, technological and scientific results.

Everyone is welcome to whatever philosophical view they like, but as far as science goes (and this is a science forum), being a reductionist has recently been a very hard, unrewarding and unscientific road for a scientist.

 

[Locrian]

It is merely computational limits that prevent us from predicting macroscopic properties; though statistical mechanics gets us quite far in getting from the quantum mechanics to thermodynamic properties.

Well it's cute to say that, but it is unprovable, unfalsifiable and unscientific. You can repeat it ad nauseum, but the evidence for the statement will still be missing.

 

[Locrian]

If there is one thing to be learned from this, its that many times scientific dogma is less scientific than you might expect. People might repeat themselves a lot, and people like Haelfix might make swipes at your education without knowing what it is, but when asked to produce hard evidence, everyone becomes strangely quiet.

 

[Son Goku]

If there is one thing to be learned from this, its that many times scientific dogma is less scientific than you might expect. People might repeat themselves a lot, and people like Haelfix might make swipes at your education without knowing what it is, but when asked to produce hard evidence, everyone becomes strangely quiet.

Although I agree with you about the reductionism viewpoint I think you may be exaggerating the context of the statement.
Although electromagnetic repulsion isn't the dominant reason you don't fall through the chair at the macroscopic scale, I don't think Jean Dalibard was "preaching dogma" or anything similar.
Without electromagnetic repulsion you would fall through the chair, that’s probably what he was trying to convey.
A simple "wow, electromagnetic repulsion on the atomic scale has some effect on every day life" kind of a thing. He never even claimed it was the reason. You have to factor in that it was a popular market CD, not "Emergent Properties of Wood for Material Scientists, 10th Edition".

Well it's cute to say that, but it is unprovable, unfalsifiable and unscientific. You can repeat it ad nauseum, but the evidence for the statement will still be missing.

It might unproven at the moment, but is it really unprovable?
For instance there is similar claim that low energy non-perturbative QCD can produce the proton, but this has yet to be shown. The reason given is similar, that we lack the computational power, however it is still being pursued.
If we gain enough computational power and QM doesn't predict macroscopic properties then the above statement is falsified.*

 

[Gokul43201]

The question of why water, which contains molecules under the same laws of E&M as wood, doesn't support someone and wood may, is the obvious counter argument to the backwards statement that E&M is the reason you don't fall through the chair!

I disagree. When someone says that the answer lies in the electrostatic interaction between the constituents, they clearly mean that the nature of the interactions is what determines whether or not you will be supported by a certain material. The nature of interactions between atoms (and "free" electrons) in a chunk of iron and those in a pool of water are different (at room temperature) because the constituent atoms are different.

 

[nbo10]

I didn't read the entire thread. But the correct question that the OP shouyld be asking is "why do some materials have a bulk modulus?"

 

[masudr]

Well it's cute to say that, but it is unprovable, unfalsifiable and unscientific. You can repeat it ad nauseum, but the evidence for the statement will still be missing.

If you do indeed think that the quantum mechanics of the situation alone will not get us to the macroscopically observed quantities, even in theory, then it means that you don't believe quantum mechanics correctly describes the system even on the micro scale.

Have I inferred this correctly?

 

[nrqed]

Well it's cute to say that, but it is unprovable, unfalsifiable and unscientific. You can repeat it ad nauseum, but the evidence for the statement will still be missing.

But it is *as much* unprovable, unfalsifiable and unscientific to say that the reductionnist approach is incorrect and that emergent phenomena (I am using it in the sense of phenomena that could not be predicted from the laws obeyed by fundamental particles) are required to understand the world. The truth is that the jury is still out.

Pat

 

[nrqed]

A physicist said that the reason why the chair is able to hold my weight is because of Heisenberg's Uncertainty principle. His reasoning was that when the atoms of my body are in contact with that of the chair, the electrons in the my body push against the electrons in the chair. Thereby forcing the electrons to a smaller region of space. Due to HU princple the more accurately we can determine the position of particles, the more wide ranging their momentum. Therefore when I sit on a chair, the electrons in the chair due to their high momentum push against my electrons hence providing pressure to hold me up. That is the reason why the chair is able to support me.
I am not fully comfortable with this explanation but that is due only to my basic understanding of QM
My inital reaction would be simply that when I sit on a chair, the electrons in my body will repel that in the chair due to the electromagnetic force. It is this repelsion that provides the reason why the chair is able to support me. The protons in my body and that of the chair are on average too far away to exert any attractive force.
It seems that my idea and that of the physicist is completely different. Can someone offer some insight here?

I know your question was posted a long time ago and the thread wnet into all sorts of directions but I would like to say a few words about your original question, in the hope that it could maybe address it directly.
You are right that the electric repulsion between the electrons of your body and of the chair play a major role (btw, electron degeneracy pressure does not play a role in that context, as others have suggested, because the average separation between the elctrons is far greater than their typical deBroglie wavelength. The pressure is simply not large enough).
However, your answer should lead you to another obvious question: but the electrons in the atoms should be strongly attracted to the nuvclei, which have positive charges. So what keeps all the electrons from collapsing to the nuclei in the first place? The answer is Heisenberg's uncertainty principle. *This* keeps the electrons away from the nuclei,which is why there is a chair to support you. Even if you assume that the electrons around nuclei have some angular momentum keeping them stable on orbits (like the planets around the Sun), once you would sit on the chair you would expect that the push created by your electrons on the electrons on the chair (and vice versa) would provide an additional force that *would* make the elctrons fall toward the nuclei (as would happen if some giant external force would suddenly give a push toward the Sun to the planets). The elctrons cloud *do* get distorted a bit and "squeezed" but, once again, the HUP comes into play and keeps the electrons from falling toward the nuclei.
(there is a few complications to this admittedly crude explanation. But I won't get into those unless there are questions).
I am pretty sure this is what that physicist had in mind in introducing the HUP in that context. That's my guess, anyway.
Pat

 

[pivoxa15]

I know your question was posted a long time ago and the thread wnet into all sorts of directions but I would like to say a few words about your original question, in the hope that it could maybe address it directly.
You are right that the electric repulsion between the electrons of your body and of the chair play a major role (btw, electron degeneracy pressure does not play a role in that context, as others have suggested, because the average separation between the elctrons is far greater than their typical deBroglie wavelength. The pressure is simply not large enough).
However, your answer should lead you to another obvious question: but the electrons in the atoms should be strongly attracted to the nuvclei, which have positive charges. So what keeps all the electrons from collapsing to the nuclei in the first place? The answer is Heisenberg's uncertainty principle. *This* keeps the electrons away from the nuclei,which is why there is a chair to support you. Even if you assume that the electrons around nuclei have some angular momentum keeping them stable on orbits (like the planets around the Sun), once you would sit on the chair you would expect that the push created by your electrons on the electrons on the chair (and vice versa) would provide an additional force that *would* make the elctrons fall toward the nuclei (as would happen if some giant external force would suddenly give a push toward the Sun to the planets). The elctrons cloud *do* get distorted a bit and "squeezed" but, once again, the HUP comes into play and keeps the electrons from falling toward the nuclei.
(there is a few complications to this admittedly crude explanation. But I won't get into those unless there are questions).
I am pretty sure this is what that physicist had in mind in introducing the HUP in that context. That's my guess, anyway.
Pat

Your explanation sounds reasonable. But I have a question with the HUP. Dosen't this principle come into existence only when I try to measure the particle (in other words, when I shine a photons onto it)? When I do not interfere with the system, the electrons could be anywhere including all sticking with the nuclei?

 

[LURCH]

There is a sort of measurement implicit in the act of sitting (though this type of measurement does not utilise the advanced sort of measuring devices physicists generally prefer). The pressure from your "measuring device" tries to limit the range of motion, and therefore the possible locations of the ellectrons.

 

[pivoxa15]

There is a sort of measurement implicit in the act of sitting (though this type of measurement does not utilise the advanced sort of measuring devices physicists generally prefer). The pressure from your "measuring device" tries to limit the range of motion, and therefore the possible locations of the ellectrons.

What about before I sit onto the chair? Could you use the HUP to explain what prevents the electrons in the chair from completely collapsing into the nuclei (off couse I wouldn't know if the electrons did collapse because that would involve a measurement hence HUP would automatically come into it)?

 

[nrqed]

Your explanation sounds reasonable. But I have a question with the HUP. Dosen't this principle come into existence only when I try to measure the particle (in other words, when I shine a photons onto it)? When I do not interfere with the system, the electrons could be anywhere including all sticking with the nuclei?

No, it is "in effect" even before a measurement is made. Any wavefunction must satisfy it. It basically says that the "spread" in position times the "spread" in momentum of any wavefunction is larger or equal to hbar/2 (where the "spread" has a well-defined meaning). This applies to any wavefunction. If you make a measurement which, say, restricts the position of a particle to be known within a small volume, you change the wavefunction in such a way that the uncertainty in the momentum will still satisfy the HUP. But the HUP was obeyed before you made your measurement.

Now, if you consider, let's say, measuring the position of the electron in a hydrogen atom (in a ceratin state, let's say the ground state), then sure, you do have some chance of finding the electron pretty much anywhere, including right on top of the nuclei. But that does not violate the HUP, which is a statistical relation. It only says that if you take a gazillion hydrogen atoms all in the same state and you measure the position for half a gazillion of the states and the momenta for the other half gazillion states and calculate the spread of the results (again, this is given by a well-defined equation), the product will satisfy the HUP. So this does not say anything about any single measurement (it does not say, for example, that the electron could never be observed near the nucleus). What it *does* say is that if you have a state where the spread of positions (say) is very small (that is, the distribution in positions is peaked in a certain region of space, let's say at a certain radius), then the spread of momenta will be large.
Of course, *after* your measurement of the position (say), the wavefunction will have changed and it will have to obey the HUP,so that if your measurement of the position makes it certain that the electron is in a small volume, its new wavefunction will have a large spread of momenta. But even before making your measurement the wavefunction obeyed the HUP.


Hope this makes sense.

Patrick

 

[nrqed]

What about before I sit onto the chair? Could you use the HUP to explain what prevents the electrons in the chair from completely collapsing into the nuclei (off couse I wouldn't know if the electrons did collapse because that would involve a measurement hence HUP would automatically come into it)?

As I mentioned in my other post, the HUP is obeyed by the wavefunction even before you make any measurement.
And yes, even before you sit on the chair the HUP can be invoked to explain why the chair does not collapse (or why your own body does not collapse!), no matter if you take a measurement or not.

Pat

 

[alfredblase]

Bah you guys call yourselves physicists and can't come up with an all round satisfactory answer to why you don't fall through a chair?! Let's us assume we have a diamond crystal large enough to sit on. This will be our chair. The question then becomes: why am I unable to compress a diamond crystal by sitting on it, by an amount noticeable to the naked eye? The answer of course is that diamond has a crystal structure that is resistant enough. This crystal structure is held together by the electro-magnetic forces exerted on each atom by each atom. And these atoms do not collapse because of Pauli's exclusion principle, the EM force and the quantized nature of electron orbitals. And what about the HUP? Well, yeah, that works too, but the HUP alone is not enough because we would not conserve atomic structure. And why do the nuclei not fall apart? Well that's because of the strong force, the quantized nature of nucleon orbitals (see the shell model) and Pauli's principle (again HUP is not enough on its own as we must conserve nuclear structure) . And why do the nucleons not tear apart? Well that's QCD for you. Nuff said... (dear oh dear).

 

[inha]

Bah you guys call yourselves physicists and can't come up with an all round satisfactory answer to why you don't fall through a chair?! Let's us assume we have a diamond crystal large enough to sit on. This will be our chair. The question then becomes: why am I unable to compress a diamond crystal by sitting on it, by an amount noticeable to the naked eye? The answer of course is that diamond has a crystal structure that is resistant enough. This crystal structure is held together by the electro-magnetic forces exerted on each carbon atom by the electrons in each carbon atom. And these carbon atoms do not collapse because of Pauli's exclusion principle. Nuff said... (dear oh dear)

Not all chairs are made out of crystalline materials.

 

[alfredblase]

Not all chairs are made out of crystalline materials.

That's obvious... I used the diamond chair example beacuse it was the simplest example I could think of. Skipping the carpintery, ironmongery, smithery, etc, etc step the arguments hold for all materials resistant enought to be made into a chair, meaning that plastics for example, without having a crystal structure, are still held together by the forces and principles mentioned in my previous post. Nuff said

P.S.I modified my previous post to make it more complete and satisfactory.

 

[Gokul43201]

Bah you guys call yourselves physicists and can't come up with an all round satisfactory answer to why you don't fall through a chair?!

That, sir, is your opinion.

Lets us assume we have a diamond crystal large enough to sit on. This will be our chair. The question then becomes: why am I unable to compress a diamond crystal by sitting on it, by an amount noticeable to the naked eye?

No, that is not the question. Nuff said !

 

[jhe1984]

The reason that the chair supports you is because angels are holding it up.

I think we can all agree on this.

 

[alfredblase]

No, that is not the question. Nuff said !

Hehehe. How is that not the question? I wish people would stop replying to my posts in this thread, with one sentenced answers lacking in depth.

In the case of a chair made form a block of material (e.g. a very large diamond crystal), the block undergoes compression when I sit on it. The fact that it does not compress too much means that it is a good chair. Hence if you engage your brain for more than a few seconds I think you will find that the answer given in my post before last, answers the original question in a satisfactory manner...

Edit: inha I would be grateful if you would update the quote you made of my first post as I wrote it in a hurry and have since changed it a lot. =)

 

[Gokul43201]

Blase : (for starters) What does noticeability "by the human eye" have anything to do with the problem being discussed ?

I find that I do not pass through my hammock !

 

[alfredblase]

Gokul,

you refuse to think...

The crystal lattice of the diamond will undergo a small amount of compression, but not enough compression so we will notice it... Please use your brain. All I was saying is that this qualifies it as a chair that can support me. It was a simple example of a chair. But you are being very pedantic over this. I repeat: the forces and principles mentioned in my first post allow a block of suitable material to support a person as a good chair is supposed to do. You can get into an even more trivial and pedantic argument over carpintery and come up with infinite examples of chairs but the general arguments I made hold for all of them. If you disagree with the physics behind my arguments please tell me exactly which part of my arguments you disagree with... [please refer to the latest version of my first post (post 52) as due to having originally written in it a hurry i updated it quite a lot to make it better]

 

[pivoxa15]

Gokul,

you refuse to think...

The crystal lattice of the diamond will undergo a small amount of compression, but not enough compression so we will notice it... Please use your brain. All I was saying is that this qualifies it as a chair that can support me. It was a simple example of a chair. But you are being very pedantic over this. I repeat: the forces and principles mentioned in my first post allow a block of suitable material to support a person as a good chair is supposed to do. You can get into an even more trivial and pedantic argument over carpintery and come up with infinite examples of chairs but the general arguments I made hold for all of them. If you disagree with the physics behind my arguments please tell me exactly which part of my arguments you disagree with... [please refer to the latest version of my first post (post 52) as due to having originally written in it a hurry i updated it quite a lot to make it better]

So you have basically reduced all of the variables a normal chair might have for breaking such as its geometry and considered a pure physical substance and answered why it would not break after a force is applied onto on the quantum level. It makes sense to me.

 

[Gokul43201]

Bah you guys call yourselves physicists and can't come up with an all round satisfactory answer to why you don't fall through a chair?! Let's us assume we have a diamond crystal large enough to sit on. This will be our chair. The question then becomes: why am I unable to compress a diamond crystal by sitting on it, by an amount noticeable to the naked eye? The answer of course is that diamond has a crystal structure that is resistant enough.

Q : Why can the chair resist my weight.
A: Because the material of the chair is resistant enough.

This crystal structure is held together by the electro-magnetic forces exerted on each atom by each atom.

This (the role of EM interactions between atoms) has been stated at least thrice in this thread before your "pioneering" post. Besides, a wooden chair has no crystal structure.

And these atoms do not collapse because of Pauli's exclusion principle, the EM force and the quantized nature of electron orbitals.

That's a complete non-answer (and the question has nothing to do with why an atom is stable, so everything from this point on is just fluff).

Secondly, you can't just throw words around and call it an explanation. I contend that the reason for the stability of the atom (of the chair under the forces it seees from your butt ) comes from time-dependent perturbation theory (applied to the eigenstates of the electrons), and can not be explained by the Exclusion Principle.

Thirdly - and this is pedantry -there is no mention of any kind of "force" in the QM description of an atom.

And what about the HUP? Well, yeah, that works too, but the HUP alone is not enough because we would not conserve atomic structure.

You are not saying anything physically meaningful with this sentence. What do you mean by "that works too" ?

And why do the nuclei not fall apart? Well that's because of the strong force, the quantized nature of nucleon orbitals (see the shell model) and Pauli's principle (again HUP is not enough on its own as we must conserve nuclear structure). And why do the nucleons not tear apart? Well that's QCD for you. Nuff said... (dear oh dear).

All you've done is show that you know some key words. Please, show me how QCD ensures the stability of the nucleon. I can't recall how asymptotic freedom arises out of the color of virtual gluons...

Blase : Your "explanation" is not explaining anything more than for instance, if I said : the reason the chair supports me is that this outcome corresponds to a local free energy minimum in the system whose activation energy is extremely large compared to thermal energies. This at least, is correct.

The only part of your post that goes towards answering the question, is the part that has been covered more than a few times before you joined this thread.

Edit: PS : Nuff said.