What are the two principles that prevent walking through a wall?

[pivoxa15]

I can think of two principles which explains why I can't lesuirely walk through a concrete wall. The HUP and Pauli's exclusion principle. Which one contributes more?

 

[malawi_glenn]

well you can't walk through a wall because you need all your atoms to simultanious tunneling through the entire potential barrier.. And that is a pretty low probabilty =P

 

[genneth]

It's kinda hard to say which contributes more... They're part of a single theory. You can try constructing theories which do not require one of them, and then calculating the probability of tunneling, but I doubt anyone has bothered. You appear to be getting hung up on "understanding" the uncertainty principle and the fermionic exclusion. My suggestion is to learn to calculate things with quantum mechanics. For example, the wave function of a particle in 1D hitting a potential barrier. Then you will understand what these principles mean for physics. Remember that physics is about numbers, not words.

 

[Demystifier]

I do not see how HUP (Heisenberg uncertainty principle) forbids walking through the wall.
Can someone explain me that?

 

[Billygoat]

If you are talking about walking through the wall and maintaining your identity then you are assuming no chemical interactions - your molecules are not changed. This means that Pauli exclusion, and HUP aren't important. The reason you can't walk through the wall is due to the electrostatic repulsion between your electrons and the wall's. To get through unchanged means that you are looking at yourself as a single entity which is repelled by the potential barrier of the wall as an entity. You can actually calculate the probability of a 'particle' of your mass and momentum tunneling through a barrier the size of a wall and the result is astronomically small.

Another way to look at it is as follows. The wave function of a particle is usually represented by a Gaussian shaped curve (a 'bell-curve'). You are a particle of several kilograms mass. Your bell -curve is very peaked - not much in the wings. (The width of the bell curve is related to the HUP, so it does enter into it here.) The probably of tunneling is related directly to how much the wings of your bell curve reach to the other side of the wall. Very, very little of your bell curve wings extend six or eight inches away from you.

 

[DaveC426913]

I do not see how HUP (Heisenberg uncertainty principle) forbids walking through the wall.
Can someone explain me that?

It doesn't per se.

The default state of our universe (at least to macroscopic eyes) is that physical objects can't pass through each other. However, if we look at subatomic objects, we see a "loophole" in the rule wherein physical objects sometimes can pass through each other (i.e an electrons through a barrier) because of their uncertainty. As we look at larger objects, that uncertainty loophole closes rapidly.

 

[mgb_phys]

I do not see how HUP (Heisenberg uncertainty principle) forbids walking through the wall.

It doesn't, it's what PERMITS you to walk through a wall, for sufficently small values of wall.

 

[dextercioby]

If i was to be launched through a cannon at 100 Km/hr there would be parts of me passing through a 10 cm concrete wall found 10 m away from the cannon. Too bad i'd die without picking up the glory.

What does passing through a wall have to do with quantum physics and the "default state of the universe" ??

PS I remember the times when there was much more quantum $\mbox{physics}$ in this forum than we have today.

 

[malawi_glenn]

If i was to be launched through a cannon at 100 Km/hr there would be parts of me passing through a 10 cm brick wall found 10 m away from the cannon. Too bad i'd die without picking up the glory.

What does passing through a wall have to do with quantum physics ??

That is a very good question.

 

[DaveC426913]

What does passing through a wall have to do with quantum physics and the "default state of the universe" ??

I'm not sure if this is seriously questioning the statement.


By "default state of the universe" I simply mean that the OP's question was phrased as asking why he "can't lesuirely walk through a concrete wall". In fact, that question doesn't really need to be answered, as it is usually the case. The question he should be asking is, "under what circumstance would I be able to walk through a wall?"

As for quantum physics and walking through walls, well of course that's about quantum tunnelling.

When the phenomenon of quantum tunnelling was first discovered and people came to realize that matter was not made up of hard little balls called atoms and electrons that bounced off one another like so many billiard balls, the idea of matter being able to pass through solid objects captured the imagination of the public. Ever since, people have speculated about the theoretical ability to scale that up to macroscopic sizes.

It caused as profound a shaking of the bedrock that is the understanding of our universe as Einstein's declaration that time was not a constant.


Now, we all do know that it simply doesn't practically apply on large scales, but the principle is fascinating to the uninitiated.

 

[cesiumfrog]

the OP's question was phrased as asking why he "can't lesuirely walk through a concrete wall". In fact, that question doesn't really need to be answered, as it is usually the case. The question he should be asking is [..]

I had the impression that the OP sought to know the answer to the question that was actually asked. That is, since Pauli exclusion resists overlapping wave-functions, one could be forgiven for supposing it similarly prevents macroscopic objects from overlapping - but is it not actually Coulomb repulsion that is solely responsible?

 

[Gokul43201]

I can think of two principles which explains why I can't lesuirely walk through a concrete wall. The HUP and Pauli's exclusion principle. Which one contributes more?

Take a single alpha particle and send it flying into a 1cm thick slab of metal at a speed of about 0.9c. A rough estimate of the probability that it comes out the other side gives me a number smaller than 1 part in 10,000. Trying to get a single alpha particle through at a "leisurely walk" is itself less likely than a ppb, forget about a macroscopic network of atoms!

But even for the simple case above, you don't use either of the two principles to make this determination.

 

[JDługosz]

This means that Pauli exclusion, and HUP aren't important. The reason you can't walk through the wall is due to the electrostatic repulsion between your electrons and the wall's.

I believe that for a "hard solid" object, the solidness is due overwhelmingly to Pauli exclusion when the electron clouds start to overlap, not the electromagnetic force. The coulomb force is easily overwhelmed by momentum. But the exclusion principle is like hitting a wall.

 

[ueit]

I can think of two principles which explains why I can't lesuirely walk through a concrete wall. The HUP and Pauli's exclusion principle. Which one contributes more?

Well, by definition, a wall is a wall because you cannot pass through it. There are things we can pass through, like water. We don't call these things "walls".

Now, seriously, I think that all this talk about how classical physics cannot explain tunneling comes from a number of unjustified assumptions. For example, a solid wall doesn't look so solid at microscopic level. If you were a charged particle, the size of a nucleus, you would see 1 cm sized nuclei placed at about 1 km away of each other. A big number of electrons are flying (or maybe staying) around. There are places where the electric field becomes very small. The probability of "tunneling" can be thought of as the probability to find and pass through such a place. For a great number of particles (about 10^26 in your body), this probability is very small.

I think that HUP is irrelevant for the above question. HUP refers to what predictions we can make, not to the way the universe functions.

Pauli's exclusion principle does not "explain" the phenomenon, it's a restatement of it. You cannot place two fermions in the same state, therefore you cannot put your electrons very close to the wall's electrons.

Everything I said above is only my opinion about these facts. QM, as it is, does not offer any explanation. It enables you to calculate the probability to pass through a wall but doesn't tell you "how" and "why", it lacks a detailed mechanism.

 

[JDługosz]

It enables you to calculate the probability to pass through a wall but doesn't tell you "how" and "why", it lacks a detailed mechanism.

I thought it follows from fundamentals of QM. The probability of the electron being where it doesn't belong becomes zero. The wave function reflects that fact. The presence of the other electron affects the sum of all possible paths, and stores the potential energy by affecting the function that is confining the electron to its energy well. That is, the orbitals become distorted and smaller.

However, on the TV show "Braniac" they have a segment on "Things you can run through".

 

[ueit]

I thought it follows from fundamentals of QM. The probability of the electron being where it doesn't belong becomes zero. The wave function reflects that fact. The presence of the other electron affects the sum of all possible paths, and stores the potential energy by affecting the function that is confining the electron to its energy well. That is, the orbitals become distorted and smaller.

And how is this an explanation for tunneling? That's exactly what I was saying. You can calculate the probability to find an electron here or there, and that's it.

In fact, tunneling through a wall is a semi classical approximation. A full QM treatment would require a complete specification of the state of the wall (electrons, quarks, etc.). In this later treatment, "tunneling" would be a sort of chemical reaction. The incoming electron modifies the molecular orbitals of the wall as a whole, producing, maybe, an ionized species which then reverts to the neutral state releasing an electron on the other side. But even in this case we lack a clear mechanism because QM doesn't provide a dynamic for an electronic transition. In other words, you cannot show, step by step, how the electron is passing through the wall. It's here, then it's a part of the wall's electron cloud, then it's there.

However, on the TV show "Braniac" they have a segment on "Things you can run through".

I missed that segment.

 

[JDługosz]

And how is this an explanation for tunneling? That's exactly what I was saying. You can calculate the probability to find an electron here or there, and that's it.

Sorry, I was thinking of how Pauli exclusion makes a wall "hard".

 

[Alistair Maxwel]

Remember that physics is about numbers, not words.

Does this sound wrong to anyone else? Isn't that like saying that carpentry is about tools, not wood? I get the fact that tools are important but the study of tools is not the same as carpentry which uses tools to shape wood into things that are useful for us. Perhaps it's my understanding of the word 'Physics' that is wrong, here I thought that mathematics was a separate science from physics, just like biology and chemistry. Plus, give words the credit they are due... If physics was about numbers not words then how would your teachers tell you that the pattern on your sweater is distracting them during their lectures? Or without strings of words (books, metaphors, short stories, etc.) how would you have become interested in physics in the first place? How would you tell your child to wear his seat belt because you understand F=ma and not your child? Or that a certain colour is soothing. Or, and most important to myself personally, without the words that were so kindly conveyed to me here on this forum, i would have never learned that Lisa Randall rocks. In fact, without words, you would not have been able to answer the OP because it could not have been asked. Furthermore, your answer, delivered in words, helped one physicist understand physics better because the mathematical language and/or physical concepts he was exposed to were unclear to him. Your words contributed, ever so slightly but still measurably (the OP was answered, at least one person knows more), to the global understanding of physics by the human species. But i think we're getting off topic so I'll stop.
Thank you for allowing me to express my thoughts.
Alistair.

 

[quetzalcoatl9]

Sorry, I was thinking of how Pauli exclusion makes a wall "hard".

and let's not forget our good friend electrostatics..

 

[JDługosz]

and let's not forget our good friend electrostatics..

See post #13. Sorry I can't find a reference.

 

[quetzalcoatl9]

See post #13. Sorry I can't find a reference.

the distinction is only relevant for extremely dense matter (such as a neutron star, etc.)

 

[JDługosz]

Do you have a reference? I was taught that Exclusion dominates for ordinary solids like bricks. The electron repulsion is easily overcome by momentum, and only starts to show when the atoms' shells start to overlap anyway. The Pauli exclusion then builds faster than the electric repulsion.

 

[quetzalcoatl9]

Do you have a reference? I was taught that Exclusion dominates for ordinary solids like bricks. The electron repulsion is easily overcome by momentum, and only starts to show when the atoms' shells start to overlap anyway. The Pauli exclusion then builds faster than the electric repulsion.

what you are talking about is referred to as "degeneracy pressure" and is only the dominant contribution for extremely dense matter. there IS a contribution toward the pressure of a material at room temperature due to the PEP (in the sense that the matter is stable because of it through bonding) but it is smaller than the dominant contribution due to electrostatics.

here is my argument in a nutshell:

calculate the electrostatic energy of two electrons separated by a distance of 1 angstrom. The energy is approx. 14.4 eV - yet kT at room temperature is only 0.02 eV. note that the energy drops off linearly, so to get on the order of kT we are talking about a separation that is over 2 orders of magnitude farther away (~720 A).

furthermore, the de Broglie thermal wavelength at 300 K (~40 A) is still an order of magnitude less than the electrostatic separation due to kT, so the PEP will (in general) not be a factor there.

 

[JDługosz]

But you don't have two free electrons an ångström apart. You have two neutral atoms on the surface of their respective bricks. The atoms' radii are about an ångström, but the electrons in one will see the other (before they "touch") as a symmetric negative charge surrounding a positive charge, totaling neutral. The van der Waals radius is several times the empirical radius, so interesting things happen even at a larger distance. "Two atoms which are not chemically bonded have a minimum distance between their centers, which is equal to the sum of their Van der Waals radii."

So, what happens now that makes them strongly repel each other, in the situation of atoms, not free electrons?

P.S. see http://books.google.com/books?id=tZu_5rEv9tkC&pg=PA30&lpg=PA30&dq=hard+solid+pauli+repulsion&source=web&ots=H0tLno6rI7&sig=MzJxruDImUc83RouVjF1gXo9Cz8#PPA29,M1 , pages 29-30.

 

[JDługosz]

I found another reference on line: Surface Chemistry of Solid and Liquid Interfaces
By H. Yıldırım Erbil states that

All the different kinds of interactions we have discussed so far have been attractive forces. There must also be some repulsive force, otherwise molecules would collapse. Two types of repulsive force have been considered in the preceeding sections: the Coulomb repulsion between like-charged ions, and the repulsion between atoms and molecules brought too close together which are very short-range. When repulsion occurs between to ions it is generally called Born repulsion. For the second example, the repulsive forces increase very suddenly as two atoms or molecules approach each other very closely, this is due to the repulsion between electron clouds overlapping at very small separations. This repulsion, which increases very steeply with decreasing distance, is due to the Pauli principle, which forbids outer electrons of one molecule from entering occupied orbitals of the other. This repulsion is called hard core or Born repulsion. We will use the name hard core repulsion for the interactions between two uncharged molecules in order to discriminate them from ionic repulsions. These repulsion interactions are quantum mechanical in nature and there is no general expression for their distance dependence, but some empirical potential functions are derived. Ha4rd core repulsions are responsible for the magnitude of the densities of solids and liquids.

In the next section, he goes on to explain van der Walls radius as a sudden boundary, an incompressible hard sphere, in terms of the hard-core repulsion.

A little reading indicates that "hard core" repulsion or Pauli principle exclusion is why ionic solids space the way they do, when the atoms are attracting each other. The hard-core repulsion stops them.

Everything I find is consistent with the view that non-dipole or multipole molecules will ignore each other at a distance except for random electron density fluctuations which cause an attraction at short distances. And at the van der Waals distance they bounce off each other.

There is no mention of electrostatic repulsion (only attraction) and sketching them as negative spheres with positive cores would indicate that they can't repel until after they are overlapping.

Perhaps the effect varies with the exact nature of the solid-solid interface. But I can't find anything to account for electrostatic repulsion being the dominant force in "hardness".

--John

 

[quetzalcoatl9]

in computer simulations of most materials/fluids, the electrostatic energy is the dominant term (VDW energy calculated by using, for example, the Lennard-Jones potential is the weaker contribution).

 

[cesiumfrog]

Surface Chemistry of Solid and Liquid Interfaces By H. Yıldırım Erbil states that [.."..]This repulsion, which increases very steeply with decreasing distance, is due to the Pauli principle, which forbids outer electrons of one molecule from entering occupied orbitals of the other.[.."..]

Nice citation!

in computer simulations of most materials/fluids[..]

This point definitely went to John.
Q', do you have anything more solid (oops) to support your side (since so far it looks like I have to renounce my attributions to Coulomb force)?

 

[quetzalcoatl9]

Q', do you have anything more solid (oops) to support your side (since so far it looks like I have to renounce my attributions to Coulomb force)?

the hard-core VDW portion of the Lennard-Jones potential drops off like 1/r^12. Whereas the electrostatic potential drops off 1/r. I have shown you the back-of-the-envelope calculation showing how the VDW repulsion is important but is short-ranged. However, it is far more important how the long-range forces (i.e. electrostatics) are behaving since there is a very large scaling prefactor.

nearly all solids are held together via coulombic or covalent bonds, not VDW! it is these bonds that are being stressed when you "push" on the crystal. i haven't bothered referencing because this is general chemistry. to say that there is a book out there that mentions VDW as being short-range repulsive doesn't prove anything.

 

[quetzalcoatl9]

infact, i will offer you a counter-example, a material that has ONLY Pauli exclusion repulsion (well, and VDW attraction but that only strengthens my point) in it's interaction potential:

Nobel gases. which, I will point out, are not in the solid phase under STP.

 

[JDługosz]

So how about a specific example, different from the free electrons of post #23: Two rocks, with rough surfaces. When they "touch" so that a force pushing them together will not (only) move them closer together but will stress the mineral's structure, a particular location is found to have a Si atom on the surface of a mineral grain on one rock near to an O atom on the surface of a mineral grain on the other rock. These two atoms are not bound to each other, so it follows that they are separated by at least the "Van Der Waals Radius" of those atoms.

At this distance, the charge is seen as neutral excepting transient fluctuations. Why are the atoms repelling each other so strongly, via electrostatics?

 

[DaveC426913]

Nobel gases. which, I will point out, are not in the solid phase under STP.

Are those the ones that someone won the noble Prize for? :grin: