# Modern vs Classical Physics

by joeboo
Tags: classical, modern, physics
P: 37
Hoping for some input on the following:

In the development of what is now known as classical physics, there was a tacit assumption that the universe was governed by laws, which we did not know, but attempted to understand.

In modern physics, most notably with Relativity and Quantum Mechanics, it seems that this philosophy changed. Instead of assuming that the universe acted in a certain way, regardless of how we observed it, we began treating the universe as acting in a way consistant with how it is measured.

To more aptly illustrate my point consider the following:
Werner Heisenberg proposed that we could not measure both the position and momentum of a particle simoultaneously.
Classically speaking, one might say, a particle has a definite position and momentum at a given moment, but any attempt to measure one will skew the measurement of the other.
Modern Physics, specifically Quantum Mechanics, seems to insist that the particle does NOT have a definite position and momentum at a given moment.
Instead, the view became that a particle is actually a "likelihood of positions and momentums."

In an attempt to simplify my point, I will abuse the language a bit and say:

Classical View:
 Measurement of the universe must be consistent with the reality of the universe
Modern View:
 The reality of the universe is consistent with our measurement of the universe
I don't mean to suggest that either view is invalid. Certainly, physics is a pragmatic science, so we should expect that both of the above statements should hold true. But, if I am correct, it does seem to represent a philisophical difference in regards to the approach.

Am I mistaking something about the interpretation of modern physics, or overlooking something in the development of classical physics? In any case, input is appreciated.
 P: 309 In fact it seems to me you're speaking about 1) classical QM 2) modern QM I will first do just classical and quantum mechanics : a) Well I think in QM you can measure p and q simultaneously in the sense : if you measure them simultaneously, then you get an uncertainty. Interprete this as : taking the same configuration (wf in qm) if you measure position p', then momentum can have several values q' (uncertainty) in any case (there exist no wf for this is not true) b) Classically : if you take the same configuration (same initial conditions, same forces), then at a certain time at a given position you have only one p', there are no other possibility. The remarks are : 1) However if you take commutating observable in QM, you can deduce the value of one knowing the other measurement value. So that in fact the philosophical position of classical mechanics is a special case of the possibilities given by QM.... 2) the time specifically comes into classical mechanics to distinguish them. This should have something to do with the special nature of time in QM 3) I think QM gives no indication about "classical" interpretation (one measurement disturbs the other, but particle had a precise value for both before measurement). QM just affirms : generally before measurement the particle is in a superposition of states. I think there is somewhere in this forum an indication about Feynman Lectures III that gives indication about why in QM a physical quantity has no meaning considered independtly from a measurement (or something equivalent)
 P: 90 I think that the real difference between quantum mechanics and general relativity is that quantum mechanics is not only abstract but also deals with the interactions of absolute particles. Also quantum mechanics rejects the existence of physical space, existing seperately from absolute matter. I think that's the physical error in relativity. Space cannot exist seperately from matter. The relative is abstract (in our head) and depends on the absolute. Still not convinced? Check this site... The contradiction is when one introduces a time dimension, the possibility of motion is automically prevented. There is no change/motion in the time axis because time is an invariant by definition.
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Modern vs Classical Physics

 Quote by Starship Still not convinced? Check this site...
That guy is obviously a crackpot. From his site:

 Since time is defined in physics as a parameter for denoting change (evolution), the equation for velocity along the time axis must be given as v = dt/dt which is self-referential.
First of all, the velocity is not $\frac{dt}{dt}$. It's $\frac{dt}{d\tau }$, which is not 1. Second, the expression $\frac{dt}{dt}$ is not "self referential". That's just stupid.

 The self-reference comes from having to divide dt by itself. dt/dt always equals 1 because the units cancel out. This is of course meaningless as far as velocity is concerned.
Well, he's right about one thing: The units do cancel out when you compute the speed along the time axis. But that's only because we're working in natural units, in which speed is dimensionless! If you prefer to work in SI units, then the speed in the time direction is $\frac{d(ct)}{d\tau}$.
P: 309
 I think that the real difference between quantum mechanics and general relativity is...
If you look at the equations, let say Schroedinger for QM : it's an equation for $$\Psi$$ an abstract mathematical wavefunction which has a probabilistic interpretation....of course it can be defined on space-time.

GR is an equation for space-time itself (the metric)....no prob. of find anything here.

So the Graal would be : how to find an equation which governs space-time AND the wave-function depending on this space-time....with of course self-interaction possible....
P: 90
 Quote by Tom Mattson First of all, the velocity is not . It's , which is not 1. Second, the expression is not "self referential". That's just stupid.
What is the difference between $v = \frac{dt}{dt}$ and $v = \frac{dt}{d\tau}$?

Proper time is time measured when the clock is at rest relative to the observer.

$$d\tau = \frac{dt}{\gamma}$$

or

$$d\tau = dt\sqrt{1-[\frac{v(t)}{c}]^{2}}$$

He writes that tau is also an invariant evolution parameter that is used in physics as a calculational tool with which to describe a rate of motion or change (see time dilation).

 Quote by kleinwolf If you look at the equations, let say Schroedinger for QM : it's an equation for an abstract mathematical wavefunction which has a probabilistic interpretation....of course it can be defined on space-time.
The difference is that spaces in (non-relativstic) quantum mechanics are also abstract. In relativity i'm not sure.
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 Quote by Starship What is the difference between $v = \frac{dt}{dt}$ and $v = \frac{dt}{d\tau}$?
The first one equals 1 and the other doesn't.

 He writes that (snip)
Pardon my bluntness, but I could not care less about what he writes. He doesn't know what he's talking about. I strongly recommend you switch to a site such as the following:

http://farside.ph.utexas.edu/teachin...es/node15.html
P: 90
 Quote by Tom Mattson Pardon my bluntness, but I could not care less about what he writes. He doesn't know what he's talking about. I strongly recommend you switch to a site such as the following: http://farside.ph.utexas.edu/teachin...es/node15.html
Sorry and we could care less about you write. What he writes is not only logical but is also supported by evidence from real life. 4-velocity is not even a physical velocity so why do we call it velocity.

There is no evidence for the physical existence of spacetime and definitely not for motion in spacetime. Get a grip man.
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P: 5,532
 Quote by Starship Sorry and we could care less about you write. What he writes is not only logical but is also supported by evidence from real life.
The guy obviously does not understand relativity, which is what he's critiquing.
P: 90
 Quote by Tom Mattson The guy obviously does not understand relativity, which is what he's critiquing.
He doesn't have to. All he has is to understand the meaning of time, which you don't.
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P: 5,532
 Quote by Starship He doesn't have to.
Of course, one does have to understand a theory in order to make a sensible critique of it.

 All he has is to understand the meaning of time,
And it remains to be seen that he does.

 which you don't.
It's remarkable that you could reach that conclusion, because I have not posted my thoughts on the subject.

Starship, you are going to find yourself banned from here very quickly if you do not stop posting bald assertions and ad hominem argumentation. Stop it.
P: 90
 Quote by Tom Mattson Of course, one does have to understand a theory in order to make a sensible critique of it.
And he almost surely understands it better than you. Relativity does not allow motion in space-time. Why not? Because time is an abstract invariant.

 And it remains to be seen that he does.
It remains to be seen whether time travel is possible, as you say it is.

 Starship, you are going to find yourself banned from here very quickly if you do not stop posting bald assertions and ad hominem argumentation. Stop it.
How can you say that something is possible if the physical evidence does not support such thing? Everything works on energy, can't you see that?

The laws of physics (especially thermodynamics) do not even distinguish between our past, present and future. They're all abstract.
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 Quote by Starship And he almost surely understands it better than you. Relativity does not allow motion in space-time. Why not? Because time is an abstract invariant.
If you mean that it is not possible to define 4-velocity in relativity, then you are mistaken. I've already explained two of the fundamental errors in the website you posted, and I posted a website that treats the subject correctly. I can lead you to water, but I can't make you drink.

 It remains to be seen whether time travel is possible, as you say it is.
You need to stop putting words in my mouth. I never said that. What I will say is that 4-velocity is defined in SR.

 How can you say that something is possible if the physical evidence does not support such thing?
Show me precisely where I made which claim, and I will tell you why I made it. But don't keep blathering on about some claim you think I made.

 Everything works on energy, can't you see that?
This sentence makes absolutely no sense in the context of any known physical theory.

 The laws of physics (especially thermodynamics) do not even distinguish between our past, present and future. They're all abstract.
I wonder why there are irreversible processes then.

Kid, you need to bring up the quality of your posts. You aren't making any sense, you are quoting obvious cranks as authorities, and you are making one strawman argument after another. It's really getting boring.
P: 90
 Quote by Tom Mattson If you mean that it is not possible to define 4-velocity in relativity, then you are mistaken. I've already explained two of the fundamental errors in the website you posted, and I posted a website that treats the subject correctly. I can lead you to water, but I can't make you drink.
It's possible to define a 4-vector in special relativity but it's not a physical velocity:

$$g_{\mu\nu}\eta^{\mu}\eta_{\mu} = \gamma^2 c^2 (1-\frac{v^2}{c^2}) = c^2$$

 You need to stop putting words in my mouth. I never said that. What I will say is that 4-velocity is defined in SR.
If you don't believe in time travel then why are you are arguing with me?

 This sentence makes absolutely no sense in the context of any known physical theory.
Particle physics says that there are only particles and their interactions. So why does general relativity say otherwise?

 I wonder why there are irreversible processes then.
In the microscopic level almost all processes are time-symmetric. At the macroscopic level it's not so.

 Kid, you need to bring up the quality of your posts.
You need to stop dreaming about time travel.

 You aren't making any sense, you are quoting obvious cranks as authorities, and you are making one strawman argument after another. It's really getting boring.
These cranks obviously make more sense than you.
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 Quote by Starship It's possible to define a 4-vector in special relativity but it's not a physical velocity:
Why not? It's proportional to the 4-momentum, the conservation of which determines the dynamics of a moving body.

 If you don't believe in time travel then why are you are arguing with me?
I'm arguing with you because you put that crackpot website forward as a convincing argument, when the guy obviously does not know what he is talking about.

 Particle physics says that there are only particles and their interactions. So why does general relativity say otherwise?
Particle physics does not say that. Particle physics describes particles and their interactions yes, but it doesn't also include the statement "and nothing else exists". Furthermore, particle physics doesn't address gravity at all. So why on Earth would you expect that GR says nothing in addition to particle physics?

 In the microscopic level almost all processes are time-symmetric. At the macroscopic level it's not so.
And thermodynamics is macroscopic, which is why you are wrong about that.

 You need to stop dreaming about time travel.
There you go again. All I have said is that 4-velocity is a feature of SR. I don't even know what you mean when you say "time travel".

 These cranks obviously make more sense than you.
Perhaps you wouldn't mind explaining why then, just once.
P: 90
 Quote by Tom Mattson Why not? It's proportional to the 4-momentum, the conservation of which determines the dynamics of a moving body.
Like i said, the velocity 4-vector is not a physical velocity. I guess we just have to agree to disagree.

 I'm arguing with you because you put that crackpot website forward as a convincing argument, when the guy obviously does not know what he is talking about.
You are wrong. What the guy said is that space-time is a mathematical construct and he's definitely not wrong about that. There's no time axis, it's abstract (non-physical).

 Particle physics does not say that. Particle physics describes particles and their interactions yes, but it doesn't also include the statement "and nothing else exists".
Do you have any evidence that something else exists? Is it physical? I don't think so.

 Furthermore, particle physics doesn't address gravity at all. So why on Earth would you expect that GR says nothing in addition to particle physics?
The physical mechanism of gravity is still unknown but we'll find it out sooner or later. But in order to that we'll have to free ourselves from abstract concepts such as space and time.

 And thermodynamics is macroscopic, which is why you are wrong about that.
If the laws of thermodynamics are irreservable (like conservation of energy), what makes you think there is an arrow of time at all? In fact, irreversibility proves there is no arrow of time. It's abstract.

 Perhaps you wouldn't mind explaining why then, just once.
How was Savain wrong when he said that motion in space-time is impossible?

In fact, i think he was right in everything he said. There is no evidence for the physical existence of a time axis.
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 Quote by Starship Like i said, the velocity 4-vector is not a physical velocity. I guess we just have to agree to disagree.
You can say it all you want, but without one iota of valid argumentation to back it up all you've got is a bald assertion.

 You are wrong. What the guy said is that space-time is a mathematical construct and he's definitely not wrong about that.
He is wrong about the things I pointed out, which are the invalidity of the unitless velocity, motion along the time axis = dt/dt, and self-reference.

Yes, the space-time of SR is a mathematical construct. So is any other theoretical device used in physics. The question is, "How well does that mathematical construct map onto observable reality?" The answer is that SR maps very well onto it.

 There's no time axis, it's abstract (non-physical).
Guess what? The x-, y-, and z-axes are abstract, too. The time axis is no more or less real than the others.

 Do you have any evidence that something else exists? Is it physical? I don't think so.
Something else besides what is included in the standard model of particle physics? Of course I do. It's called "gravity".

 If the laws of thermodynamics are irreservable (like conservation of energy), what makes you think there is an arrow of time at all? In fact, irreversibility proves there is no arrow of time. It's abstract.
Oh for Pete's sake! Please look up "irreversible"! Irreversible procceses suggest that there is an arrow of time. And by the way, I'm not talking about abstract laws. I'm talking about actual physical process.

 How was Savain wrong when he said that motion in space-time is impossible?
Assuming Savain is the guy who wrote that website, I've already explained why his argument is faulty. I have neither the time nor the inclination to repeat it.

 In fact, i think he was right in everything he said. There is no evidence for the physical existence of a time axis.
If you think that he's right in everything he said, then you are either unwilling or unable to understand the obvious mistakes I pointed out, which is too bad.
P: 90
 Quote by Tom Mattson You can say it all you want, but without one iota of valid argumentation to back it up all you've got is a bald assertion.
Where is the valid experimental argumentation for space-time, time travel etc?

 He is wrong about the things I pointed out, which are the invalidity of the unitless velocity, motion along the time axis = dt/dt, and self-reference.
First of all velocity is a vector and a vector has both magnitude and direction but distance & direction are both abstract (non-physical).

 Yes, the space-time of SR is a mathematical construct.
Then you understand that gravity has nothing to do with the curvature of a physical space-time?

 So is any other theoretical device used in physics.
What do you mean by devices? The devices we use are absolute.

 The question is, "How well does that mathematical construct map onto observable reality?" The answer is that SR maps very well onto it.
The theory is mathematically accurate and makes correct predictions but the flaw in it is that it describes metric spaces as physical, instead of abstract.

 Guess what? The x-, y-, and z-axes are abstract, too. The time axis is no more or less real than the others.
Yes, because metric tensors are abstract.

 Something else besides what is included in the standard model of particle physics? Of course I do. It's called "gravity".
There is no problem i think. Gravity is an effect generated by a series of particles interaction with the CMBR. Particles on an elementary level have intrinsic vibration states, that is internal motion (change in location) capability. This is confirmed with the two hole diffraction experiments of the electrons.

 Oh for Pete's sake! Please look up "irreversible"! Irreversible procceses suggest that there is an arrow of time.
How so? In fact the irreversibility of physical processes prohibits a physical arrow of time.

 And by the way, I'm not talking about abstract laws. I'm talking about actual physical process.
And we cannot reverse physical processes right? Matter on earth cannot fall up, can it?

 Assuming Savain is the guy who wrote that website, I've already explained why his argument is faulty. I have neither the time nor the inclination to repeat it.
Louis is completely correct. Particles, their properties and their interactions is all that exists. All the rest is abstract (non-physical).

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