# What Makes Laws Definitly Right No Matter What?

1. Jul 24, 2004

### Mk

On physics forums it has been stated thousands of times something like:

Same person:
So, is it just that its a law because it's survived hundreds of years without being disproven?

2. Jul 24, 2004

### Mk

Sorry, Check didn't post all of those, I just copied a random quote html/ubb/php or something to get the quote box thing.

3. Jul 24, 2004

### arildno

What do you mean by "disproven"?

For example, the theory of relativity did not disprove any of Newton's laws, but limited their validity.

Newton's 3.law, for example, will have the same degree of validity that it always has had forever.

4. Jul 24, 2004

### Galileo

We can never know whether a law or theory is really 'true'. But it can always
(rather, it should always) be falsifiable.
We cannot PROVE whether Newton's laws are true, but we believe in it because it is in agreement with experimental results and phenomona which it describes.

Strictly speaking: To say something is wrong because it contradicts a law of Newton or Thermodynamics is wrong. To say something is wrong because it contradicts experimental results is valid.
But usually contradiction with a physical law implies phenomona which don't really occur.

5. Jul 24, 2004

### pmb_phy

SR didn't disprove Newton's second law nor did it limit its validity. Newton's second law has always been and always will be

$$\bold{F} = \frac{ d\bold{p} }{dt}$$

and this is 100% correct in SR. To be more precise, in modern physics F = dp/dt is no longer a law of physics, it is a definition of force.

However SR did disprove the general validity of Newton's 3rd law. In SR Newton's 3rd law, which states

$$\bold{F}_{12} = -\bold{F}_{21}$$

only works for contact forces.

Pete

Last edited: Jul 24, 2004
6. Jul 24, 2004

### arildno

"However SR did disprove the general validity of Newton's 3rd law. In SR Newton's 3rd law, which states only works for contact forces."

It didn't disprove the general validity, it restricted/limited the extent to which it is valid..
("Disprove" is a verb I only find meaningful in a strictly mathematical/logical context.
It might well be that it can be used in English in a more loose sense; if that is so, then I apologize for my lack of knowledge in English)

7. Jul 25, 2004

yes it did. Lets say hypothetically you have a spaceship travelling at 0.9c. If you apply a force on it, you do not measure the force using F = ma, you have to take into account the relativistic mass of the rocketship, (using SR). Thus making newtons laws valid only for a small range of speeds. Once we get into the upper end, then Newtons laws have to be modified.

8. Jul 25, 2004

### pmb_phy

I explained that SR didn't disprove Newton's second law nor did it limit its validity. Nenad said yes it did.. I, of course, disagree with that claim.
As I explained in my post above, Newton's second law has always been and always will be
$$\bold{F} = \frac{ d\bold{p} }{dt}$$
Newton's law has never been F = ma. That is only a relationship between mass and force when the mass is constant. In this case the mass increases with speed and is therefore not constant. Newton himself asserted that his second law was F = dp/dt (See page 124 in Concepts of Force, by Max Jammer, Dover Pub. and The Feynman Lectures on Physics - Volume I, by Richard Feynman et al, page 12-2)
To see how the force is constant and how F = dp/dt gives the correct result for such a rocket please see

http://www.geocities.com/physics_world/sr/uniform_accel.htm
http://www.geocities.com/physics_world/sr/force_trans.htm

Pete

Last edited: Jul 25, 2004
9. Jul 25, 2004

### HallsofIvy

Staff Emeritus
Any statement that is "definitely right no matter what" is, by definition, a tautology. There are NO tautologies in science because science is based on imperfect observation and experimentation.

ALL theorems in mathematics are tautologies because they always have the (hidden) hypotheses "If all stated axioms are true" and follow logically from those axioms. Of course, if the stated axioms are NOT true, then the statement becomes "if FALSE then ..." which is logically a true statement no matter what the conclusion is.

10. Jul 25, 2004

### pmb_phy

That is not how the term tautology is defined. tautology is defined as

1 a : needless repetition of an idea, statement, or word b : an instance of tautology
2 : a tautologous statement

where a tautologous is defined as

1 : involving or containing rhetorical tautology : REDUNDANT
2 : true by virtue of its logical form alone

Something may be considered "definitely right no matter what" by some people and not be a redundant statement. I.e. I hold that 1 + 1 = 2 definitely right no matter what. But its not a tautology.
I'm not quite sure what you mean by this. Can you please clarify? E.g. The statement
I believe (but am not 100% sure) that statement is a tautology because its just another way of saying that the frame is accelerated.

Pete

11. Jul 25, 2004

### pnaj

Poor old MK,

Comes on trying to get a handle on how physics allows for such definite statements such as Newton's Laws to be taken as axioms in some situations, and not in others, and ends up with a argument about the word 'tautology'.

Isn't there a qualititive difference between something like Hooke's Law, which is akin to a 'handy' tool in very particular circumstances and say, Newton's Third Law, which, I thought, has never been contradicted by experiments?

It seems to me that there are principles, conservation of energy say, which are so powerful that, if an experiment is found to contradict them, it is the experiment that is considered to be faulty, and flaws are looked for, and usually found.

Is that right? Do all the conservation laws survive quantum mechanics, for example?

12. Jul 25, 2004

### pmb_phy

That was not my goal to be sure.

I guess I never really directly answered the question What Makes Laws Definitly Right No Matter What? - The answer is that "no matter what" can't logically be applie to a law of physics since a law of physics can never be proven to be true in all concievable cases. The comments you quoted, e.g. ...which directly contradicts Newton's 1st law and is therefore wrong. are not logically correct. If something contradicts an established law then all that can be said, without further proof, is that the two things are incompatible. E.g. Experiment proved that Newton's third law was wrong in some cases and therefore it is not correct in all concievable cases and therefore it is not a true law.

Pete

13. Jul 25, 2004

### pnaj

Pete, I'm sure you didn't, but so many threads on pf end up with an argument about the use of English.

I thought MK was trying to gauge the validity of statements like 'something can't be true because it broke Newtons 3rd law'.

I have immense trouble knowing when to apply certain laws, usually because when I first learnt the law, I didn't realise the significance of the bounds within which it was valid.

Newton's laws are valid over such a large range of phenomena that it took 300 or so years of hard graft by hundreds of people, doing thousands of experiments, and loads of near-misses, before Einstein came up with a more accurate formulation.

So, MK, it sounds obvious, but in order to truly understand any physical laws, you've got to learn what bounds must be placed on them as well. Then statements like the ones you were talking about will be put in their proper context.

But, I'm asking, anyone, are there any laws that have survived everything thrown at them? The third law of thermodynamics? Surely that hasn't been contradicted yet, has it?

And Pete, I'd be interested to see what goes wrong with Newton's 3rd in SR. I did SR as part of my maths degree, but we only really skimmed some of the maths from it, and messed around with the usual stuff, length contraction etc.

Paul.

14. Jul 25, 2004

### pmb_phy

Yeah. I hate those type of arguements.
I tried to avoid this thread at first because it seemed like it was going to be a philosophical debate and I'd rather spend my time posting about physics. But on re-reading Mk's post and re-thinking it I decided that it was easy to answer. Hence my last response. In the meantime I took a crack at some definite statements about what SR has "proven/disproven."
Me too. Especially the one that says that I can't drive faster than 55 mph. :rofl:
It took the work of people like Faraday, Maxwell, etc. to bring Einstein to see that something fundamental needed to be changed.
Newton's Third Law is a statement about action and reaction between particles which may not be in contact. If they are seperated in space then the expression

$$\bold{F}_{12} = -\bold{F}_{21}$$

means that the force on particle #1 due to particle #2 has the exact same magnitude as the force on particle #2 due to particle 1 at the same time. However at the same time does not have an absolute meaning in relativity. Two events which happen at the same time in one frame of reference doesn't mean that they will happen at the same time in all other frames of reference (In general they won't, especially if they are not contact forces. If they are contact forces then they will always be equal and opposite). This means that the forces can't be equal and opposite when they are seperated by a finite distance.

Pete

Last edited: Jul 25, 2004
15. Jul 25, 2004

### Nereid

Staff Emeritus
At least two - QFT ('quantum mechanics') and GR (General Relativity). AFAIK, there are no experimental or observational results which are inconsistent with either of these (within their respective domains of applicability, of course).
and similar ... a qualified comment (I haven't seen the context) ... these may be shorthand. They could refer to either an experiment/observation or an idea/hypothesis/theory; the last may refer to something completely different

Idea: the writer is exploring the consistency of an idea with 'textbook physics', and have found something which is inconsistent. When such a thing happens, it's almost always a sign there's an error in the calculation, or that the idea is very likely to be inconsistent with observations. In most cases of the latter, it should be fairly straight-forward to construct a concrete case where the inconsistency would show, and since a wide body of experimental results are available, at most a quick check will show the inconsistency to be fatal to the idea (at least in the form it is being tested).

Experiment/observation: This is a bit trickier ... is the writer refering to an actual experimental result, or a thought experiment? I'll assume the latter (if the former, and the experiments stand up to scrutiny, it would be truly remarkable thing). A thought experiment is another kind of consistency test for an idea - you describe an experiment that may (in principle) be possible to do, and work out what the results would be, based on your idea. If those results are inconsistent with the results expected based on Newton's 3rd law you either have a problem or an excellent test of your idea!

16. Jul 25, 2004

### pnaj

God, I feel dumb ... I now 'fully' understand what you mean now by 'except contact forces', which I sort of missed in your earlier post. I was thinking about 'things hitting each other' when I asked the question.

By the way, we get upto 70 mph over here!

17. Jul 25, 2004

### pmb_phy

A Law of Nature is, by definition, something which holds true in all cases, no matter how hard you look, no matter what precision you use and no matter where in the universe you go. It doesn't mean "It works like this in some cases if you don't look too hard, if you don't look in too many places and if you don't look too closely." Any other use of the term will make me cringe and weep uncontrolably.

Literally there are few true laws of physics.

Pete

18. Jul 25, 2004

### arildno

In that case, we have not discovered any laws of nature at all, just a few contenders for the title.

19. Jul 25, 2004

### pmb_phy

If we take the term literally then - Yup.

20. Jul 26, 2004

### Staff: Mentor

Most of those were me and I must admit I am a little uncomfortable with the idea that they are called "laws." I have never gotten a good explanation of the difference between a "law" and "theory," and don't like the implication that a law is set in stone. But that answer you gave yourself is a good enough reason to accept things like Newton's laws and the laws of thermodynamics as true.

The one caveat I make is what pmb_phy said - the "Laws of Nature" (capitalized) are THE immutable laws that the universe operates by. Unless we find them on stone tablets written by God him(her?)self, we won't really know if our theories really do accurately represent the real Laws of Nature. Newton's law is probably close, but I wouldn't characterize it as a Law of Nature.