Transcript discussion Albrecht

July 20 17.45
Probability without time
Speaker: Andreas J. Albrecht
Floor speakers (in order of appearance):
Barbour
Saunders
Brown
Maudlin
Albert
Hartle

83

[questions only]

Barbour
1. I’m going to attack you head-on right at the start, I think you’ve got the totally wrong idea of what a clock is. I think the whole idea of an internal theory for a clock has been a total and utter disaster. You only have to look back over a century to see how astronomers solved the problem of determining what time is to see that the internal time idea is just nonsense. When the astronomers had a problem with the motion of the moon there were two alternatives: one, the theory of gravity was wrong, the other was that the Earth wasn’t rotating uniformly. And they guessed that it was the second. So then they said where do we find a reliable clock? And they said we are going to assume that the solar system is a closed dynamical system and they realised that there was only one single sensible time variable they could choose, it’s the time variable that conserves the energy of the solar system. And that actually in 1937?.was confirmed as the only sensible one and for two decades that was actually the official standard of time. Now, when you say that clock is a localised system that is completely true, but it is a very special localised system to make sure that it keeps time with that time that is defined by the solar system and for finding what is a good clock, the definition of a good clock is that you must have clocks that keep in step with each other, and it’s got to be, ultimately, the complete universe that is going to provide the standard of time. And in any individual degree of freedom or subset of it can never do it. Therefore I think the whole programme with internal time was misguided from the start, and Brice was I think the first person to introduce this with his paper in ’67, and I’ve twice asked Brice “What is a good clock?” and he said to me it’s what the National Bureau of Standards tells me keeps time to a good accuracy, and I said to Brice: it’s your job as a theoretician to tell the experimentalists what a good clock is, not vice versa.

Albrecht
2. But once I’ve got?..my future I’ve got clocks coming out of my ears so that’s not a problem. So the way to attack this is to find something wrong with the premises because this is what everyone does

Barbour
3. ……[short phrase]..

Albrecht
4. No, no, but poke a hole in it.

Barbour
5. I already have done because you’re not defining clocks properly.

Albrecht
6. What you’ve described is some…..co-ordinate of the physical state and then you can carry the rest with it

Barbour
7. …chosen very carefully..

Albrecht
8. I’m totally into choosing it carefully, that’s what half this talk is about.

Saunders
9. …choose it randomly!

Albrecht
10. Well, then it lucked out that I could choose it randomly and still get my field theory out. I didn’t even expect that. That’s a really remarkable thing. I thought I’d choose it randomly, get a bunch of crap and then say okay, well, but that’s such a terrible clock and I’m going to throw out most of the random things I see and then hunker down and choose some good ones and take it from there, but remarkably just random junk is giving a field theory as much as I seem - so I think that’s a remarkable result.

Brown
11. This reminds me of the issue………….[faint]……Julian’s saying that when you choose subsystems as clocks you choose them very carefully. If you don’t choose them carefully you can still do physics, it just becomes horribly complicated but that doesn’t threaten the nature of physics itself, it just means that you’ve chosen clocks that make the equations look very, very complicated. And what I’m trying to understand is: this threat that you raise when you show that there’s this ambiguity in the choice of clocks, is it any worse than the issue that …[name]….was raising?

Albrecht
12. Well, it’s worse because you name a perfect Hamiltonian that has a well-defined time and I can pull it out of this junk. And it can be completely different from the one everyone’s working hard with particle accelerators to find, and it’s on a completely equal basis to that one, and it’s just as good. So, yes, there’s lots of junk and the remarkable thing, a little bit distracting, is that is seems like it’s easy to find something interesting in almost all the junk. But that set aside, even if that’s too-naïve an account of what I’ve just shown you, even if most of the junk is just junk, there’s still all the good stuff that’s in there. You name good Hamiltonians, it’s all in there, apparently on an equal basis to the one we’re working so hard to construct in our attempts to understand the world around us.

Maudlin
13. I think this is the same question, let try to do it from a different angle. You said there was a big disaster…………….So, Galileo needed a good clock, right, for doing his inclined-plane experiments; I’ve heard different stories, sometimes it’s said that he used………..water clocks and stuff and when I hear the beginning part of your story it sounds like here’s a really horrible…………clock, suppose you chose a hamster and then you had a bunch of hamster states……when the hamster does this it will be time 1 and when the hamster does that it’ll be time 2 and you started rolling a ball down a plane and said, hey, I’ve got nothing here, right….

Albrecht
14. That’s the same question and I’ll give you the same answer

Maudlin
15. But I don’t understand the answer, it doesn’t seem like there was ever such a disaster, the only disaster would be, I mean, Einstein used to say it was a mistake, or incorrect, to believe you could have a theory in which measuring instruments, clocks and rods, were primitives…..Now it sounds like you’re saying, well, I’m not going to……….I’m going to choose a brain and see what happens, and then you get, of course some disaster….I guess the question I have is: that disaster seems no surprise at all, I thought there was a disaster I never thought of…….when you choose a …..you’ll get a mess

Albrecht
16. You will get good clocks when you choose, and you’ll have a chance to get good clocks; I’ve argued it’s a better chance than I thought it would be…..chance of getting very good clocks and, when you do so all possible good Hamiltonians, very accurate, well-behaved laws of physics are seen to be on an equal footing. All the garbage is in there too but the good stuff is good regardless of what results you get from CERN..

Maudlin
17. You’re telling me there’s some weird clock that if I use it I’ll get good-looking results, but I have no reason whatsoever for choosing.

Albrecht
18. Right, so you can take a very anthropic view and say: I’m just going to look at this Hamiltonian, look at this random process of defining my physical world and just throw things out unless they match exactly what I see, so you’re just fixing all the parameters based on data that you have and that’s that. I think you can do that but I think this is saying that..

Albert
19. Isn’t the name of that ‘…..from our experience’? …..

Albrecht
20. But you want predictability, you want……this is my Hamiltonian and then you say there’s this extra term, you say, no, it’s not that, it’s not my theory of physics but in fact………one of the extra terms waiting to be pulled out; it’s all sitting in there in the theory waiting to - they’re all on an equal footing, the question is: do you have any principles of physics that inform the kind of theory you can build and if you don’t then you’re just fitting data and it’s fine, everyone’s happy, but this undermines having principles of physics like field theories and so on that constrain how you build your model; it removes all constraint from how you build your model.

[new question]

Hartle
21. I just thought the usual definition in classical physics, the definition of a ……as something that is correlated with the time supply of my metric in spacetime. I would have thought the definition of a clock in quantum mechanics was………….mechanical system that was correlated with some……….This is somehow different from that?

Albrecht
22. Well, it’s saying that in your super-space, if you dig around for good clocks in that sense, no matter how hard you try to set up your super-space to reflect the laws of physics as we know it, with standard model, blah, blah, blah, the process of finding the clocks wouldn’t give any preference to that model that you put in. That you just as well get O3 model out or another model out. So that’s it.