The Book of Ashes

Young's double slit experiment. 3:49PM

I didn't really realise it until I saw it written down but I think I agree fully with this...

"[Richard] Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment."

For those that don't know the double slit experiment a fuller description of it can be found here. For those that don't care that much you can probably stop reading. This rest of this post contains a lot of physics stuff that may not interest some.

But essentially it is this. If you fire photons at a wall with two slits in it then you will get an interference pattern on the far side (photons going through either slit interfere with each other to create bright spots or cancel out). This kind of makes sense until you reduce the photons firing to one at a time. Then the single photon must go through one or the other slit and the interference pattern should be lost (if you block a slit so there is only 1 then you just get a spread of light with no interference bands). But this is not what happens. The interference bands still appear and it is as if the photon is travelling through both slits at once like a wave would rather than a particle. If you place a detector in one slit then the interference pattern disappears. There is much more to this but essentially the phenomenom lacks a solid explanation and I think that if it is explained satisfactorily it could lead to a re-writing of physics. However I lack any such explaination :)

This is a really good visual look at the double slit experiment. A lot of variations have been done on this experiment trying to "catch" out the photon to determine which slit it went through. However it seems that any attempt to measure where the photon is makes it "collapse" back to the particle model and it only travels through one slit and the interference pattern disappears.

Another part of physics that is not very well explained (at least to me) is the speed of light. This all ties into Einstein's theory of relativity and gets very complicated very fast but I think a few interesting questions are...

- Why is there a limit for how fast photons can travel? As you try to accelerate them faster, what is forcing them to gain mass rather than go faster than the speed of light?

- Why is the speed of light different depending on the medium it is in? E.g. it is slower in a medium such as glass than in a vacuum.

- Why does light refract when changing mediums, e.g. air to glass? What is the force that makes it change direction?

- If you travel away from light coming towards you (at a fast enough pace) then the light appears to change colour (it is red-shifted) rather than change speed. Changing colour makes sense but why does it not change speed?

Einsteins entire ideas are based on the starting premise that the speed of light is an absolute limit and cannot be surpassed. From there you can see that if you were on a rocket travelling at half the speed of light and you shone a torch then that light could only travel away from you at half the speed of light (from an outside reference point) otherwise it would break your starting premise that the speed of light cannot be broken. For this to make sense to any observer on the rocket we would have to slow down time to half speed (and this is what has been observed to happen, obviously not with a person on a rocket going at half the speed of light). This means that the person on the rocket sees the light slowly travelling away (at half the speed of light) but they are thinking slowly so it actually appears to still travel at the speed of light. Lots of tricky things then occur. The problem is that Einstien never explained why the speed of light is an absolute limit and why photons cannot travel faster than that.

For further reading on the speed of light click here.

Now there are two other things that don't make any sense to me and this may be due to a lack of understanding rather than there not being an answer.

The first is entanglement whereby two particles are entangled at a joint starting point and then when split up over large distances you can determine the state of one by finding out the state of another. E.g. what appears to be instantaneous action over a large distance. This doesn't seem to make sense and just seems that there states were predetermined from the start, e.g. one is up spin, the other down and when you find the up one you know the other must be down (due to conservation of spin or something).

The second is the Heisenberg Uncertainty Principle which states that you can't tell the momentum of a particle and the position at the same time (e.g. measuring one effects the other and so by measuring the postion you are now uncertain of the exact momentum). Now physicists take this to mean that the particle doesn't have an exact momentum or position (or other values) but instead exists in a probability state and collapses down into an exact position when measured. Now this just sounds like crap to me (but too many people believe it for me to dismiss it too quickly). It seems that particles are continuously interacting and there is no difference from particles that we use to measure something than from other random particles that it has bumped into on the way. Hence things have been "measured" without humans since the begining of time and should have exact locations and momentums. Now I agree with the Uncertainty principle in that you can't measure both the position and momentum at once. Obviously to measure an atom you must interact with it somehow, e.g. to fire a photon at it and measure the time taken to bounce back. This may give you the exact position but in doing so the momentum imparted by the photon has changed the particles momentum and you are now unsure about it. Similarly in measuring its momentum you will effect its position. Pretty much this is just saying that you cannot measure anything without interacting with it and that interacting will change it, hence things are uncertain to a degree.

I think there is something missing in physics that once known would help to explain all of the above experiments. It feels like it should be obvious to work it out. You can see that something is missing, but you can't find what.

Just had an after thought. I have a gut feeling that the refraction (bending of light when changing mediums) is related to the double slit experiment. Not sure how that helps.