Does this reduces experimental physics to theoretical physics?

samalkhaiat

An experimantal study is possible provided that;

1) nature is invariant under space-time translations.
(so that it is possible to reproduce the results)

2) nature is invariant under Lorentz's group.
(and this establishes a possible causal connection between parts of experimental setup)

If you agree with me (I hope) on the above, can I say that Poincare' invariance is the reason why we are able to do experiments?
If yes, does this reduces experimental physics to theoretical physics? But then, what makes theoretical physics possible?

Please note that these are not philosophical questions, so don't be philosophical about them.

pervect

If the universe happened to be invariant under Galilean transformations, we could still study it with experiments. So condition 2 is too restrictive.

I think the general situation is that we need to know what matters and doesn't matter when we do experiments, so we need to know what to control for to do really good experiments. But, we can find out what matters and what doesn't by experiment, if we use inductive reasoning. So the first phase of any experiment is figuring out what is important to control, the next phase is to see how the results vary when you change the important experimental quantities.

samalkhaiat

Not at all, exprimental study possible provided that we choose a class of "equivalent" frames of reference to work with. Regardless of how the equivalence of frames is realized in practice, the equivalence relation has the structure of a "group". Thus, any experimental study must assume some symmetry principle defined by a group of transformations.In a class of equivalent frames with respect to a given group of transformations, the symmetry group can be used to translate the "observations" made in one frame to any other frame reached by the transformations.This defines a "principle of relativity" which asserts that the laws of physics are the same for all equivalent (Galilian or Lorentzian) observers. This, in turn, determines the results which do not depend on the choice of the frame.As well as relativity principle, the laws of nature "here" at "this time" is the same as the laws "over there" at "some other time", this translation group makes the reproduction of experimental result possible.Since the Galilean algebra and group can be obtained as a low-velocity limit of the Poincare algebra and group,I see Poincare invariance as more fundamental.

regards

sam

samalkhaiat

[QUOTE=pervect]
I forgot to mention one very important point about your Galilean-invariant universe and that is; Such world will have no electromagnetic signals to communicate with (to dark to do our expriments ), unless you make maxwell theory invariant under Galile group, I think, one cann't do this, can you? In a world without light! well we, certainly, would not be around to do any thing.


sam

pervect

If one believes that Newton and Galileo were doing physics and physical experiments, they did so in ignorance of the principle of Lorentz invariance.

So I think you're barking up the wrong tree when you make Lorentz invariance a fundamental requirement to do experimental physics.

That's my opinion, and your arguments have not convinced me otherwise.

samalkhaiat

The very fact that newton & Galileo were able to do experiments, proves the finiteness of speed of light which,in turn, shows that Galileo's principle of relativity is an aproximation to the real world.ok!

pervect

What assumptions and logic led you to that conclusion?

samalkhaiat

[QUOTE=pervect]
My only "assumption" is that phenomena and experiments, designed to study them, are not arbitrary, but that they evolve according to a definite laws. The finite speed of light signal is one of these laws.
We know that all interactions in nature (the very thing which make experiments function) propagate with finite speed.This is impossible according to Galileo's relativity.
Galileo would say; if interactions propagate with finite speed, then this speed would be different in different inertial frames.Therefore, it is possible to distinguish one inertial frame from another, and this contradicts the very relativity principle of Galileo.

Can we now move to the main issue of my thead?

sam

lightgrav

Theoretical physics is distinct from mathematics in that
it has been selected to be "useful" - in most cases,
interpretable to describe the results of experiment.

It is experimental physics that makes theoretical physics possible.
- - - - - - -
I thought Pervect's first post was right-on-target,
directed at the "main issue" of the thread.
You derailed it by prefering "fundamental" to "necessary".

Now nobody knows WHAT you wanted as the "main issue".
- - - - - - -

Being a little bit careful about your starting points:

An experimental study would be possible even if the results
differed from place-to-place , and even from time-to-time.
The comparison of the results would just be done differently.

1) nature is not invariant under space-time translations.
for example, "g" in PA is not the same as "g" in NJ, and
the solar intensity is different now than it was last night.
Please recognize that re-writing this as "the LAWS of nature"
makes it obvious that it's about theory, not about experiment.

2) While some experiments have been designed based on
considerations of causality, others were motivated by
correlations that were not understood in a causal sense.
The same is true of some theories.
Please recognize that re-wording this as
"this allows US TO EXPECT a causal connection between different parts"
makes it obvious that it is about theories, not experiments.

It appears to me that you are arguing
a) nature behaves in a way that theory describes/explains
b) in experiments, nature obeys theoretical rules about this behavior
c) therefore theory is necessary for experiments.
this argument confuses our understanding (theory) with the real behavior.
Regarding Galileo, you seem to say:
a) We understand nature to act in manner "x"
b) all experiments observe nature
c) all experiments prove "x", our current understanding.

- - - - - -

Now, if you're wondering whether experiments are possible,
they are.

If you're wondering whether they might still be possible,
even if our set of theoretical physics was slightly different,
they were possible in the 1800's when we were wrong.

If you're wondering whether they might still be possible,
even if our universe worked differently than we understand,
it does.

If you're wondering whether they might still be possible,
even if the universe worked differently than it DOES,
you can't stifle others' viewpoints by arguing
that any interaction "proves" current theory.

- - - - -

I'd say that experiments whose results are meaningful are possible
only if the influence of one object on another decreases with distance.
I think this is relied on before either of your two invariances.

can I say that "1/r^2" is the reason we can do experiments?
No, but some sort of general proximity effect is needed.

Even if I could say something like that, it would no further
"reduce experimental physics to theoretical physics"
than the other way around.

samalkhaiat

[QUOTE=lightgrav]I vever meant to upset experimental physicists.My thread was not about theory versus experiment. If that was the case I would have started the thread by saying that theories have predictive power, experiments have non of that, or by saying; the value of a theory lies not just in its ability to explain experimental results,but also to pave the way for doing new experiment.Nothing in my posts indicate that I mean to make experimental study redundant.This would be garbage. the final say about a theory will always be left to experiment.
My thread meant to let people examine the following;
What is it about nature that makes physics possible?and what role,if any, do invariance principles play init?

The argument against;(if the world "happened to be" Galilean invariant) is simple.
Before we talk about experiment in this Galilean world, we need an answer to; what kind of world is it? We reject such world simply because light, either doesn't exist at all, or it travels with infinite speed.
The world,in 10000bc was Poincare invariance, it is Poicare invariance now, and it will be Poicare invariance in 2000000000ad.

Since we understand nature through her LAWS, when I say "nature" I always mean the LAWS.It is the law, which is same everywhere at anytime, that gives "g" its local value.Well today in London we measured the speed of light, the proton charge and London's "g". We got the numbers; "c","e", and our "g"=GM/r^2.
I wonder, if you repeat our experiments in your home town tomorow, will they give you:"4.9c","3.8e" and your "g"=GM/r^8?
Sir invariance under translations means; WE SEE THE SAME PHYSICS. If our world is not invariant under translations, you will not be able to repeat our experiments in a meaningful way,i.e. a way that REPRODUCES our results.
Were it not for the invariance, maps in spacetime would be as bad & diverse as the political maps on our earth.
It seem to me that you confuse laws with theories!
Theories are mental constracts (we invent them).
The laws are part of the world (we discover them).
Theories can be false.The laws are always true.

Forget about instruments and other experimental arrangements,without causal connection even our brains would not function.So, even thought experiment would be impossible.
Phenomena and experiment evolve according to the true laws of nature.
I do not confuse laws with theories.
Experimental setup, including the system under investigation,can not violate the laws.
The LAWS are necessary and sufficient for experiment.
What is the real behavior?

The differences between me and you are;
1) I raised the question; what makes experimental study possible?
You do not seem to be bothered about such questions. You think that experiments are possible full stop.You never said what makes them possible.
2) I presented an argument which seems to show that experimental study is possible because of Poicare invariance. You made "political" speech about theoretical versus experimental physics.

Sir, I am interested in whether nature herself can provide us with hints about the best method of investigation, whether nature prefers the pure theoretical (deductive) method of Einstein over the empirical (inductive) approach of Newton-Locke.
Since invariance principles restrict not just the form of the laws, but also the class of possible experiments(read post#3), I want to know whether invariance principles, being sort of SUPER LAWS, are the only thing that nature can offer us for doing physics.

sam

ZapperZ

Can you show me where, in physics, that this is an accepted definition? Are you saying that "Newton's Law of motion" is more valid than Quantum Theory or The Special Theory of Relativity, JUST because those two have the word "theory" associated with it? Have you seen a "theory" graduating to a "law"? The BCS theory, which is generally accepted as the MOST tested theory in physics so much so that by 1986, the whole field of superconductivity was thought to be absolutely matured, never was "promoted" to "laws" even when B,C, and S, received their nobel prizes in 1972. These things just never happen.

And since when is a "law" are always true? Are you going to stand by the Wiedemann-Franz Law just because the word "law" is attach to it?

Zz.

lightgrav

You should re-read your original post before you make claims like this.
Now, this could be discussed. But you did not want to discuss whether
"the influence of one object on another decreases with distance"
is one of the features about nature that makes physics possible.
Instead, you complained that I read your words, not your (hidden) intent.

Maybe you should edit the title of the thread.
I think you have confused Laws of Nature
(which physicists WILL understand as you are meaning it to be)
(even though there are plenty of poorly-named theories/models)
with theories.
It is theory that invariance principles are applied to, not the Law.
Any particular invariance principle is our our understanding of nature's behavior, so the principle itself is a mental construct which we've invented.
So it is not a Law of Nature. It is certainly not a "Super-Law"!
You MUST be careful with your wording if you want to avoid ambiguity ...
especially those worst ambiguities which even fool the speaker.

ps- you have NO idea what questions I contemplate, nor what attitude I retain regarding some question. Enumerating differences is confrontational.

krab

Like the other respondents, I think you go way too far with this. I would say the only requirement for experimentation to be useful is time invariance. If someone in Kolkata does an experiment and gets a different result from someone in New York, then it is still a viable hypothesis that space is not invariant. However, if the results of all experiments change day to day, or worse, moment to moment, then there is no longer any point in doing experiments. The difference is that one can go back and forth in space, but one can only go one direction in time.

Locrian

How amusing. I wonder what you think those questions are, then?

samalkhaiat

No, of course it is not about having the word "theory" or "law".
Was I that shallow?
Throughout history, the progress of physics rested on "great" theories. A theory becomes a "great" theory, when one or more of its postulates turns out to be universally true.
In the theory of special relativity, the constancy of speed of light is a postulate.Einstein was lucky,clever or both.His special relativity became a great theory because nature works in the way stated by the postulate.
In general relativity, equivalence principle is a postulate, and it is also a law of nature.
In the theory of electromagnetism, Maxwell's displacement current was a postulate. It led to Maxwell's prediction that light was an electromagnetic wave.It is a law of nature.The incosistency between the steady-state laws of Coulomb and Ampere on the one hand and the dynamical law of Faraday on the other hand, required the genius of Maxwell.
In the theory of QM, Schrodinger's equation is a postulate.The general form of the equation(without specific Hamiltonian) is a law of nature.
In relativistic field theories, Poicare invariance is a postulate.The classic work of E.P. Wigner(1937), I blieve, made Poicare invariance a member in the set of all physical laws.
However, superpoincare invariance is still a postulate.Unless superpartners show up, supersymmetric field theories will not become "great" theories.

To give an exact definition of the laws would be a "demand more than impossible to meet".
Discussing this problem sometime sounds like the conversation between King & Alice in Lewis Carroll's Through the Looking Glass:
"I see nobody on the road",said Alice.
"I only wish I had such eyes", the King remarked..."To be able to see Nobody!..."
However, the great men of physics, always had something to say about the laws. So we can learn from them.
"Nature is the realization of the simplest conceivable mathematical idea" Einstein
Kepler "Nature uses as little as possible of anything" and reveals herself by simple mathematical statements.
Enistein "the supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built by pure deduction".
The action of the laws can be understood only by conceiving them as being universally true.We found ourselves in a world where the laws aretrue. If they were not, then we would be very different from what we are.
They can be an approximations of more accurate laws.But, fundamental changes to the laws require changes in the very fabric of spacetime.So you could say;the laws are eternal.
They are independent of any event in spacetime.i.e. they are absolute.

Obviously, I have a very small knowledge of physics for I have never heard about Weidemann-Franz law. I think this "law", whatever it is, points to the point I was shouting about which is;
a set of mathematical relations, which seem to have explanatory power, need not be a law of nature unless it is,always, universally true.
We may call something as this or that person's law, but nature, frankly, does not give a damn unless it is true.

regards

sam

samalkhaiat

No, of course it is not about having the word "theory" or "law".
Was I that shallow?
Throughout history, the progress of physics rested on "great" theories. A theory becomes a "great" theory, when one or more of its postulates turns out to be universally true.
In the theory of special relativity, the constancy of speed of light is a postulate.Einstein was lucky,clever or both.His special relativity became a great theory because nature works in the way stated by the postulate.
In general relativity, equivalence principle is a postulate, and it is also a law of nature.
In the theory of electromagnetism, Maxwell's displacement current was a postulate. It led to Maxwell's prediction that light was an electromagnetic wave.It is a law of nature.The incosistency between the steady-state laws of Coulomb and Ampere on the one hand and the dynamical law of Faraday on the other hand, required the genius of Maxwell.
In the theory of QM, Schrodinger's equation is a postulate.The general form of the equation(without specific Hamiltonian) is a law of nature.
In relativistic field theories, Poicare invariance is a postulate.The classic work of E.P. Wigner(1937), I blieve, made Poicare invariance a member in the set of all physical laws.
However, superpoincare invariance is still a postulate.Unless superpartners show up, supersymmetric field theories will not become "great" theories.
To give an exact definition of the laws would be a "demand more than impossible to meet".
Discussing this problem sometime sounds like the conversation between King & Alice in Lewis Carroll's Through the Looking Glass:
"I see nobody on the road",said Alice.
"I only wish I had such eyes", the King remarked..."To be able to see Nobody!..."
However, the great men of physics, always had something to say about the laws. So we can learn from them.
"Nature is the realization of the simplest conceivable mathematical idea" Einstein
Kepler "Nature uses as little as possible of anything" and reveals herself by simple mathematical statements.
Enistein "the supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built by pure deduction".
The action of the laws can be understood only by conceiving them as being universally true.We found ourselves in a world where the laws aretrue. If they were not, then we would be very different from what we are.
The can be an approximations of more accurate laws.But, fundamental changes to the laws require changes in the very fabric of spacetime.So you could say;the laws are eternal.
They are independent of any event in spacetime.i.e. they are absolute.
Obviously, I have a very small knowledge of physics for I have never heard about Weidemann-Franz law. I think this "law", whatever it is, points to the point I was shouting about which is;
a set of mathematical relations, which seem to have explanatory power, need not be a law of nature unless it is,always, universally true.
We may call something as this or that person's law, but nature, frankly, does not give a damn unless it is true.
regards
sam

ZapperZ

Sorry, but this is gibberish.

Do not add bells and whistles to your answer. Show me where there are "laws can be understood only by conceiving them as being universally true". NONE of what you have described qualifies. If Einstein's postulate is considered to be "universally true", then why in heaven are physicists STILL testing them in more ways than one? And Maxwell equations? Really?!

Again, these are semantics. I have not seen why such a distinction has ever made any difference in the practice of physics. We NEVER (i) teach kids about the "differences" between laws, theories, principles, etc (ii) write papers on these (iii) put ANY care on such things.

So why are you?

Zz.