I am blogging my way through Stephen Hawking’s new book “The Grand Design”. I am not so much interested in presenting a detailed authoritative critique of the book, but a journal of my thoughts as I read it. I may have to go back again and re-read it another time to really be able to appreciate all the arguments and theories he puts forward but for now here is my raw response to chapter 4. So far it’s a great read and very stimulating but things are about to get a bit tricky for me to understand.
Two concepts from quantum theory that I will apparently need:
1) Wave particle duality – even individual photons or particles interferes with themselves like waves.
2) The Heisenburg uncertainty principle – the uncertainty of position multiplied by uncertainly of momentum (mass times velocity) is never less than Planck’s’ constant (which is very small, even smaller than plankton!). ie you can’t measure accurately both velocity and position. The more certain you are of one the less you can be of the other, oh and the total amount of uncertainty increases as the object gets smaller. (I think! Help me out if you are a physicist!)
“According to quantum physics each particle has some probability of being found anywhere in the universe.” P73
“Our use of probabilistic terms to describe the outcome of events in everyday life is therefore a reflection not of the intrinsic nature of the process but only of our ignorance of certain aspects of it”. P74
I don’t really follow that. Is that the deterministic presupposition appearing again or is that provable?
“now that we have a feeling for Feynman’s approach to quantum physics, it’s time to examine another key quantum principle that we will use later”. p80.
Oh dear. I haven’t got a feeling for it at all. Or rather I have but it’s not a good one. Less of a “Got that, what’s next” one and more of a “run that by me again, but I don’t think it will help much” one. I’ll just keep quiet at the back of the class and look like I’m understanding. Maybe I’ll pick it up later…
I have strained my brain through another few pages so let me see if I can regurgitate what is being said.
1) Fire a small particle at a barrier with two slits and it seems to know about both slits by the time it has been detected on a screen on the other side. Feymann models it as having taken every path simultaneously and the probability of it ending up somewhere is a function of all the paths to that point.
2) If you look at the particle as it passes through a slit to see which one it goes through, you will find that when it is detected on the other side it only “knew” about that one slit and not the other – The interference effect collapses or disappears so looking at a particle effects it’s future.
3) But it also effects it past! If we check which slit the particle went through after it has gone through a slit (not sure how you do that but apparently you can) when it hits the screen it has no knowledge of the other slit. Inspecting the particle for slit information after it has gone through a slit has stopped it taking any paths through the other slit. There is therefore no objective past, just a subjective one that is fixed as you look at it. Please someone who is a physicist tell me if I am wildly off track here.
The next chapter but one is going to show how the universe as a whole also has no objective past but first we have a crash course in the laws of nature. Even the pictures look scary. It’s a long chapter but it will take me past the half way point of the book. Surely its down hill from there on.