# Can't walk through a wall.

1. Aug 23, 2007

### 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?

2. Aug 23, 2007

### malawi_glenn

well you cant 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

3. Aug 23, 2007

### 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.

4. Aug 23, 2007

### Demystifier

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

5. Aug 23, 2007

### 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.

6. Aug 23, 2007

### DaveC426913

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.

7. Aug 23, 2007

### mgb_phys

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

8. Aug 23, 2007

### 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.

Last edited: Aug 23, 2007
9. Aug 23, 2007

### malawi_glenn

That is a very good question.

10. Aug 23, 2007

### DaveC426913

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.

Last edited: Aug 23, 2007
11. Aug 23, 2007

### cesiumfrog

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?

12. Aug 23, 2007

### Gokul43201

Staff Emeritus
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.

13. Aug 24, 2007

### JDługosz

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.

14. Aug 25, 2007

### ueit

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.

15. Aug 26, 2007

### JDługosz

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".

16. Aug 26, 2007

### ueit

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.

I missed that segment.

17. Aug 28, 2007

### JDługosz

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

18. Aug 28, 2007

### Alistair Maxwel

Thank you for allowing me to express my thoughts.
Alistair.

19. Aug 28, 2007

### quetzalcoatl9

and lets not forget our good friend electrostatics..

20. Aug 29, 2007

### JDługosz

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

21. Aug 29, 2007

### quetzalcoatl9

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

22. Aug 31, 2007

### 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.

23. Aug 31, 2007

### quetzalcoatl9

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 seperated 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 seperation 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 seperation due to kT, so the PEP will (in general) not be a factor there.

Last edited: Aug 31, 2007
24. Sep 1, 2007

### 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 [Broken], pages 29-30.

Last edited by a moderator: May 3, 2017
25. Sep 2, 2007

### JDługosz

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

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