# How come we can't walk through walls?

1. Dec 5, 2006

### Wellsi

Hey all......
im still at school and doing the IB diploma, and for Theory of Knowledge subject i have to write an essay, the topic being
"Our sense say that a table is solid, yet science tells us it is mainly empty space, so how come we cant put our hand through it?"
I have been looking and it seems it has something to do with electron repulsion and the Heisenburg Uncertainty Principle can someone help me please! Why cant we put our hands through tables or walk through walls?

2. Dec 5, 2006

3. Dec 5, 2006

### marlon

Because tunneling is a QM phenomenon and the physics that describes humands colliding onto a wall is not valid on atmic scale : ie classical physics.

marlon

4. Dec 5, 2006

### Gokul43201

Staff Emeritus
"...science tells us it is mainly empty space..." ! Does it, really? I think not, and I wish high school textbooks would stop propagating this ill-formed idea.

5. Dec 5, 2006

### Jheriko

Why is this idea ill-formed? Because the probability distributions of quantum particles fill all space? Or is there some classical reason why the idea is flawed?

6. Dec 5, 2006

### Nexus555

Think of it like this. When your ceiling fan is off, you can easily put your hand through it and touch the ceiling. Now when it's on full blast, the blades start looking almost like 1 object, and if you put your hand in there, it's going to stop you.

Now think about atoms and electrons and such orbiting at light speeds. It may give the appereance of being 100% solid, but I doubt that's even possible. Anyways, I hardly know what I'm talking about, just referring to an example my chemistry teacher gave me way back in highschool, maybe it'll help.

7. Dec 5, 2006

### Claude Bile

Agreed 100%.

To the OP, you should consider challenging this statement in your essay, provided of course that you are able to use arguments grounded in physics.

Claude.

8. Dec 5, 2006

### Wellsi

so what grounds do i challenge it on? The fact that "science" in general does not state that atoms are mostly free space so therefore the hypothesis is wrong?
thanks for the fan analagy that helped alot :)
check this url for some deductive reasoning about the subject:
http://everything2.com/index.pl?node_id=919472&lastnode_id=0
i mean, theoretically, it must be possible to walk through a wall?

9. Dec 5, 2006

### regent

Theoretically yes, the probability is just really low...

10. Dec 5, 2006

### regent

Electrons 'repel' other things with the EM force. This makes it so your particles and a table's particles repel each other to a point where you cannot go through the table itself.

11. Dec 5, 2006

### Wellsi

ok so the outer most electrons repel each other with the EM force..... thats all good. So it means that we never actually touch something? its only the force between the outer most electrons? how can we then tell something is rough or smooth, and why do different things "feel" different?
wasnt there an experiment before done about quantum tunnelling and proved it worked?

12. Dec 5, 2006

### Epicurus

Then how do you explain Rutherfords experiment where he was able to shoot helium through a sheet of gold. QM definitely tells us that what was previously thought as solid i.e. a smooth rigid continuum is not entirely true. In fact, QM can show us that our previous notion of the phases of matter, primarily say Solid, liquid and gas, although macroscopically having very different properties, on the microscopic scale they look almost identical (there are many ways to characterise this difference, correlation lengths, order parameters etc), however this is a very different notion than what was previously believed about matter.

13. Dec 5, 2006

### Wellsi

things are all the same on a microscopic level? How come a helium atom can go through when say an atom on my hand cant?

14. Dec 5, 2006

### Epicurus

The gold foil was pretty thin, but the point of the experiment was that if the 'size' of nuclei were what they were supposed, the scattering angle would have been much larger. Instead of you hand lets talk about making the gold thicker and thicker. Forget all that crap about tunnelling through walls and the like, its one of those popular physics notions which frustrates the hell out me and others I know. If a solid was a continuum then if I was to take an infinitesimal slice, nothing would be able to pass through it without breaking it. However if the slice is made out say a single sheet of atoms (think of filling a plane with equilateral triangles, at each vertex is placed an atom), then there will be 'space' between the atoms (regions where there is most likely little or nothing to impede the motion of a projectile) and an object will pass through. Let us say that statistically if we randomly shoot projectiles at this sheet, the probabilty that it will pass through is

$$Pr(passing through 1 sheet)=\frac{1}{q}$$

Now if I build up a 3d solid by adding layers to this sheet, the probability of passing through the solid will be a function of the number of layers

$$Pr(passing through n sheets)=\frac{1}{q^{n}}$$

Now say a sheet of gold is effectively 5.0 Angstrom thick (mean plane seperation in fcc structured gold), and the transition probability is nearly one, let's say $$\frac{99}{100}$$ for some particular momentum projectile (it will be different for a whole lot of parameters). Now take a slab of gold to be 1cm thick, that's approximately $$10^{9}$$ layers of gold, such that the transition probability will be

$$Pr(passing through 1cm gold)=\frac{99}{100}^{10^{9}}$$

which is an incredibly small number, although it is still theoretically possible.

Last edited: Dec 5, 2006
15. Dec 5, 2006

### Wellsi

"Forget all that crap about tunnelling through walls and the like, its one of those popular physics notions which frustrates the hell out me and others I know."

thanks for that, it makes sense there. These popular physics notions? They would help to know so i can use them in the essay about generalisations (see the original post)
Can u explain the one i just used (about the tunneling?) what really happens there?

16. Dec 5, 2006

### turbo

You might appeal to the Pauli Exclusion Principle. The reason that your hand stops when you slam it on the table-top is that the electrons in the molecules making up your hand refuse to be forced into the same quantum state as the same-spin electrons in the molecules that make up the table. Of course, the PEP could be entirely wrong, and then it would be your job to explain why you can't walk through walls.... ;)

17. Dec 5, 2006

### Wellsi

is PEP proved? Or is it one of those things we use to explain something that we dont quite understand properly?

18. Dec 5, 2006

### Epicurus

The Pauli exclusion principle is phenomenoligical. However the mathematics behind such mechanisms is well understood in that if we say

'If a type of particle has the propertu that only one particle is able to any particular state'

then we can go quite a long way. However no one really knows why some particles (half integer spin, fermions) can only singularly occupy states and others (integer spin, bosons)may all occupy the same state. This is one of the open problems in physics

19. Dec 5, 2006

### KleptoPanda

Because the "atoms" in your hand are molecules and even at the smallest size are 4-5 times bigger than a helium atom. Not to mention pseudo-surface tension in the phospholipid bilayer and the spacing between the individual units as described by the fluid mosaic model dictate what can/cannot pass through it.

20. Dec 6, 2006

### tim_lou

indeed, it is annoying but how else are quantum physicists gonna get funding support from the public?