“there is no objective past” (The grand design part 5)

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.

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2 thoughts on ““there is no objective past” (The grand design part 5)

  1. As a physicist (well, I have a BSc in Physics, it hardly makes me an expert) your summary is pretty accurate.

    Heisenberg uncertainty principle is correct, we cannot know both speed and position precisely. In fact, the very act of measuring one changes the other (e.g. in pinpointing where something is, you affect its motion).

    Wave/particle duality is broadly correct too. Particles behave as both. Shove a load of photons (the particles that carry radiation, e.g. light) through a couple of slits and you’ll see an interference pattern on the wall behind them from the photons ending up in different places. However, shove just one towards the slits and a pattern will still emerge, like it has some knowledge and interaction of both, even though by definition this small packet of energy could not be subdivided up. So it can also be modelled as a wave, which explains the interference but not some of the other aspects observed.

    The objective past bit is beyond me though, at least in the terms described. Sorry! I know I’ll end up mulling it over for the rest of the day though.

    On the subject of observations determining outcomes, are you familiar with the famous Schroedinger’s Cat argument?

  2. Thanks so much, thats really helpful! Yes, I do vaguely remember the Schroedinger’s Cat experiment so refreshed my memory via Wiki. In it a quantum state is amplified to kill a cat in a sealed box. Opening the box and observing the cat’s state was thought to effectively collapse the quantum state. However I am told that it doesn’t require a conscious observer to fix the state so the amplifier does this before the box is ever opened. It seems to me that the amplifier acts as a kind of sanity interface between the micro and macro worlds ensuring we don’t get cats that are both alive and dead at the same time. In the multiple universe model of quantum physics the cat is dead in one universe and alive in another. When we look at the cat we discover which universe we are in.

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