Questions about time

[Tubbythin]

You could say that, but in a slightly different way. Light is always moving through space at c. And see pair production.

You are obsessed with pair production.

 

[edd]

I don't see any particular value in this argument from pointing out that if you can't see anything that has changed you can't tell what time has passed - this is surely trivially obvious from the fact that the laws of physics are symmetric under time translation and doesn't have any bearing on whether you travel through time or it's a dimension or anything else.

After all, the laws of physics are symmetric under space translation too.

 

[Farsight]

This sends us round in circles. How do you define "change observed" without reference to the time dimension along which the change is observed?

You don't define it. You observe it. You qualify it using the space dimension and the time dimension, but there's a circularity there. Let's say you're watching a spaceship move through space. You might say it's moving at 10000 m/s, but look closely at how you define the second and the metre.

The second is "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". In the NIST fountain Caesium clock hyperfine transitions emit microwaves, and the resonant frequency is found and measured by the detector. But note that frequency is in Hertz, which is cycles per second, and the second isn't defined yet. So frequency goes out of the window, and what's really going on is that we count incoming microwave peaks. When we get to 9,192,631,770, we say that's a second. The metre is then derivative, being the distance travelled by light in 1⁄299,792,458th of a second. In both cases it comes back to the motion of light.

So when you say your rocket is moving at 10000m/s, all you're really saying is how fast it's going in relation to the motion of light. Think it through, it's kind of scarey once you see it.

 

[Farsight]

The muon has no internal structure that moves, but it is spinning, right? Would this not be its motion?

Yes. See what I was saying to Beerina, and check out the Einstein-de Haas effect. See this bit: "the Einstein–de Haas effect demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics." It's really obvious when you think about it. A muon typically decays into an electron, an electron-antineutrino, and a muon-neutrino. The neutrinos depart at the speed of light, or so close to the speed of light that we can't tell the difference. It's going to be a bit tricky for them to do that from a standing start.

 

[edd]

Yes. See what I was saying to Beerina, and check out the Einstein-de Haas effect. See this bit: "the Einstein–de Haas effect demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics." It's really obvious when you think about it. A muon typically decays into an electron, an electron-antineutrino, and a muon-neutrino. The neutrinos depart at the speed of light, or so close to the speed of light that we can't tell the difference. It's going to be a bit tricky for them to do that from a standing start.

I think it's a little bit more complicated than that. Certainly the electron spin should be counted towards angular momentum but that doesn't make it clear the electrons are rotating in any useful sense.

As for the muon example you give - I'm afraid I just don't see the relevance. What's 'a bit tricky' about something at rest disintegrating into various components which individually are moving?

 

[anglolawyer]

Someone, I forget who, once said that time was needed to prevent everything happening at once.

That reminds me of Kant saying the universe cannot have existed for all time because an infinite amount of time would have to pass before the happening of an event.

I thought time had a direction because of entropy.

If things didn't tend to move from ordered states to less ordered ones, would we have a sense of time (I say yes, but probably only because I can't imagine what an absence of time would be like) and would time exist?

 

[edd]

I thought time had a direction because of entropy.

Once you have a universe that starts with low entropy that does give you a direction. But it might leave you wondering why it had a low entropy to start with...

 

[Tubbythin]

A muon typically decays into an electron, an electron-antineutrino, and a muon-neutrino. The neutrinos depart at the speed of light, or so close to the speed of light that we can't tell the difference. It's going to be a bit tricky for them to do that from a standing start.

Huh? If you give any body an amount of energy that is many times its rest mass energy it will end up travelling at close to the speed of light regardless of what speed it was travelling at to begin with.

 

[Farsight]

I think it's a little bit more complicated than that. Certainly the electron spin should be counted towards angular momentum but that doesn't make it clear the electrons are rotating in any useful sense. As for the muon example you give - I'm afraid I just don't see the relevance. What's 'a bit tricky' about something at rest disintegrating into various components which individually are moving?

What's tricky is going from a speed of zero to a speed of c in no time flat. It takes infinite acceleration to do that, which takes infinite energy, and it's a no-can-do.

The reality is really simple. The muon decays, things fly away at c, and you're left with an electron. Chuck a positron at the electron, annihilation occurs, things fly away at c, and you're left with nothing. And like the electron the muon is something you can diffract. It's described by a wave equation. Check out the Kauffman book that comes up 4th in the list, see the mention of knots and spherical harmonics and loopforms. Things like electrons and muons are just light tied in knots. Only the muon is a rubbish knot that shakes itself undone in about a microsecond. Whoosh! The neutrinos fly away at c, because there was always something going at c in there. Ditto for the photons flying away at c after electron-positron annihilation. Hence the c in the Dirac equation. It isn't there for nothing. Nor is the c in E=mc².

This might not be in your textbooks yet, but rest assured, it will be.

 

[edd]

What's tricky is going from a speed of zero to a speed of c in no time flat. It takes infinite acceleration to do that, which takes infinite energy, and it's a no-can-do.

What makes you think it starts from zero??

edit to add: on top of which, you've no reason to think it goes from a standing start and does so in zero time (how would you accelerate a neutrino anyway?), and even if it did why would it take infinite energy rather than the energy the neutrino actually ends up with? I really don't think you've thought it through as well as you might think.

This might not be in your textbooks yet, but rest assured, it will be.

I think I might be waiting a little while.

 

[Farsight]

I thought time had a direction because of entropy.

Sure. But it's like the direction of a chemical reaction. Or a counting direction. You can count 1 2 3 4 5 6 7 8 9 10 forwards, and you can even provide a running total divided up into hours minutes and seconds called "the time". But there's no sense in which you're travelling forwards.

 

[RobDegraves]

It isn't my model. Now go and read A World Without Time: The Forgotten Legacy of Godel and Einstein. And if you have evidence of travelling through time, please do present it. After that you can show us all your time machine.

So... you read one book by a philosopher and you think you know all about physics do you? Nicely balanced approach...

I do have a time machine. It's my chair and it moves me forward in time. See.. I wasted a bit of it writing this.

Quick question for you... if you take all the matter out of the universe completely, how do you measure space? Does it still exist?

Is there still time without matter? Is there still space without matter? I think the question can be answered the same way for both.

 

[anglolawyer]

Sure. But it's like the direction of a chemical reaction. Or a counting direction. You can count 1 2 3 4 5 6 7 8 9 10 forwards, and you can even provide a running total divided up into hours minutes and seconds called "the time". But there's no sense in which you're travelling forwards.

But aren't I getting older, remembering the past but having no experience of the future? It sure as heck feels as though I am travelling in a direction.

I can see I am always actually in the present, not travelling through time, but the present itself seems to be travelling forward. Or it always has this quality of past and future being distinguishable, not like red and black on the roulette table but like a fully formed and unchangeable past state, stripped of all potential on one hand, and an infinite range of possible futures on the other. In the past Babe Ruth hit so many home runs and that cannot change but in the future any number of possible home runs can be hit by any number of people, the possible variations being consumed or condensed into certainty by the advance of the present.

Probably all bull, but I am interested to learn a little bit.

 

[Farsight]

What makes you think it starts from zero??

I don't. You do. You said "What's 'a bit tricky' about something at rest disintegrating into various components which individually are moving?" I told you, and it's very simple. As for the textbook thing, it beats me why guys like you are so negative about something that isn't in your current textbook or bible or whatever. Especially considering where we are.

 

[anglolawyer]

Once you have a universe that starts with low entropy that does give you a direction. But it might leave you wondering why it had a low entropy to start with...

Er, I am hoping low entropy = highly ordered. If the universe started with a big bang, surely that was very disordered wasn't it? There were no streets or houses or complicated structures waiting to be brought into a less ordered state.

I have read somewhere that life may have the property of reversing entropy, with beavers and such building things, which nature then has to go to all the trouble of breaking up again.

 

[edd]

I don't. You do. You said "What's 'a bit tricky' about something at rest disintegrating into various components which individually are moving?" I told you, and it's very simple. As for the textbook thing, it beats me why guys like you are so negative about something that isn't in your current textbook or bible or whatever. Especially considering where we are.

The muon is what starts at rest - why do you think that means the neutrino needs to accelerate from rest?

I'm not particularly bothered about textbooks - you were the one who said your ideas will end up in a future one. I don't think they will.

 

[Tubbythin]

I don't. You do. You said "What's 'a bit tricky' about something at rest disintegrating into various components which individually are moving?" I told you, and it's very simple.

Huh? You seem to have got confused. Edd never said the neutrinos started at rest.

As for the textbook thing, it beats me why guys like you are so negative about something that isn't in your current textbook or bible or whatever. Especially considering where we are.

Where are we?

 

[edd]

Er, I am hoping low entropy = highly ordered. If the universe started with a big bang, surely that was very disordered wasn't it? There were no streets or houses or complicated structures waiting to be brought into a less ordered state.

No, it might superficially look disordered and lacking in structure, but that's not the same thing as actually being high entropy. I'll see if I can think up a good way to explain more clearly (or someone might jump in and do so for me!)

I have read somewhere that life may have the property of reversing entropy, with beavers and such building things, which nature then has to go to all the trouble of breaking up again.

No, life leads to a net increase in entropy. It might lower it in one place, but it'll increase it generally.

 

[Bodhi Dharma Zen]

Time is the scenario where change happens, it is a mental model that allows us to order observations. As any other model, it is useful as long as our theoretical framework requires it (to make some calculations that lead to predictions for instance).

 

[Tubbythin]

Er, I am hoping low entropy = highly ordered. If the universe started with a big bang, surely that was very disordered wasn't it? There were no streets or houses or complicated structures waiting to be brought into a less ordered state.

No, everything was very smooth and uniform, the temperature didn't vary much, everywhere looked the same as everywhere else. The latter is still true of the Universe on a large scale (probably) but it certainly isn't true locally. I'm sure Sol Invictus or someone will be along shortly to explain this better.