What do you mean by that? Mathematical models proves that Newton's laws must be true? — TheMadFool

...

Newton's laws being theorems from other assumptions doesn't mean they describe reality.

Get a narrow piece of plastic strip and bend into a u-shape measuring 5ft from end to end. Make sure that the plastic is lubricated so as to cut down on friction and wide enough for a ball-bearing. Place the ball-bearing at one end and let it drop (don't push or throw it). If the plastic track has minimal friction then the ball bearing should reach the same height, from whence you dropped it, at the end:



Repeat this for a plastic track measuring 10ft, then 15ft, then 20ft, then 25ft, and so on. If all plastic tracks and ball bearings have minimal friction then the ball bearing should again reach the same height at the end as the height at the beginning. From this we may inductively reason that if we don't allow the plastic track to bend back up at the end, then the ball-bearing could hypothetically go on forever at a constant velocity if no other net force is acted upon it, i.e. Newton's first law of motion.

Secondly, NO law of physics can ever be proved; because every law of physics is a historically contingent approximation, good to a few decimal places, to the results of the experiments we're capable of doing at any given level of technology. — fishfry

Well said!

The OP is not as concerned with the meaning of "proof" as he is concerned about the circularity of the proof. — TheMadFool

That circularity is gestured at as if it existed, but where and how exactly does it exist? As to deciding what constitutes a proof, you really cannot do without that. Without it, how do you know that anything is proved? Also, I think @fishfry has got the substance of the matter, above.

But let's take Newton's first law:

Every object in a state of uniform motion will remain in that state of motion unless an external force acts on it.

Immediately there's a problem: "every object." That's a lot of objects. You could modify the law to, every object we've tested exhibits these qualities, and we expect other objects not yet tested to behave similarly. With this formulation, I think we're with Hume among his "objections": if all you know is objects, then where and how do you get a law? Well, "law" is a mental construct, agree? As such it is about criteria, again agree? And once agreed, and criteria established, then you get to think in terms of laws, subject of course to verification through experiment whether by thought or in the field or laboratory.

And it's this last that led to relativity as (apparently) a refining of Newton's laws, but Newton's laws still work in many cases and are in fact those applied and used.

Every child who asks "Why?" sometimes gets caught in the whys and keeps asking long after the efficacy of the question has been exhausted. And even the best of us old and wise and smart adults can fall back into that. You ask above, "Is it a real/interesting question or is it a question borne out of ignorance?" Ignorance, I'd say, but not pejoratively, but just that ignorance that is the human condition and that we're all sometimes subject to.

While this isn't the only option, laws are basically a way of describing observations. Then you set up experiments to further test that the descriptions are on-target.

How can you prove Newton's laws? — Fernando Rios

Unfortunately, you cannot prove anything about the real, physical world, simply, because its construction logic is unknown. The term "proving" means that you demonstrate that a theorem necessarily follows from the construction logic of its world.

Still, Newton's laws were really good at resisting falsification for a long while. Only by looking at quite unusual situations, they were falsified:

Experimentally, Newton's law of gravitation has been falsified due to for example the anomalous perihelion precession of Mercury, which matches instead with Einstein's theory of gravitation.

Still, Einstein's theory is rather used as a refinement and not as a replacement for Newton's laws.

:up:

Newton's laws being theorems from other assumptions doesn't mean they describe reality.
5h — fdrake

This is puzzling. Laws are descriptions of reality, right?

@alcontali read the underlined.

Unfortunately, you cannot prove anything about the real, physical world, simply, because its construction logic is unknown. The term "proving" means that you demonstrate that a theorem necessarily follows from the construction logic of its world. — alcontali

Thank you all for your answers. They have somehow help me to think about the situation. Does someone just know what were the experiments Newton performed to prove his laws?

I would like to know how can you prove these laws, but not using devices that use the the same laws. — Fernando Rios

Physical laws can't be proven by science. All physical laws are subject to change, correction, or invalidation.

Does someone just know what were the experiments Newton performed to prove his laws? — Fernando Rios

He did not prove his laws, so he performed no experiments to prove his laws.

This was the easiest question to answer so far on the forums.

Sorry if I seem ignorant, but how can we know they are true then?

Sorry if I seem ignorant, but how can we know they are true then? — Fernando Rios

They seem to be true, but you can never be absolutely, 100% sure. Most people treat gravity and Newton's laws as true. It is practically very useful to think of them as true. But science is not capable of proving that they are true. Science has a lot of evidence that supports that Newton's laws are true, but science will never claim that Newton's laws are true.

Sorry, this is how it is.

You can show they are true by running the experiments. — StreetlightX

/thread

So many respondents missing the point regarding "proof". He clarified that by "proof", he means showing to be true. This can be done through the relevant experiments. If you disagree, then you need to revise your conception of truth.

Thank you for your answer. What are the experiments that Newton used to show their laws are true?

Thank you for your answer. What are the experiments that Newton used to show their laws are true? — Fernando Rios

You've already been given examples, and you can look them up. So why ask me that?

I just see the example shown by a drawing. What variables are being measured there? How are they measured?

I just see the example shown by a drawing. What variables are being measured there? How are they measured? — Fernando Rios

You know that logo at the top, what does it say? It doesn't say, "The Science Forum", does it?

That circularity is gestured at as if it existed, but where and how exactly does it exist? — tim wood

I'll offer an analogy. Imagine the color red = Newton's laws. The instruments used to "prove" Newton's laws can be considered as a pair of red-tinted glasses (obeying Newton's laws). Any and everything you see using these glasses will appear red i.e. obeying Newton's laws. This is what I think the OP is trying to say.

Newton's laws proving Newton's laws!

Now I think there's a distinction we have to make between a logical proof and a scientific proof. Circularity is a bad character in the former but is necessary in the latter.

For instance a classic case of circularity given in elementary logic books is the Bible proving God exists because it's the word of God. This is a vicious circularity and the argument is rejected.

In science the difference is laws discovered/proved are necessarily universal and must be obeyed by everything. So, the instruments, as of necessity, should obey the laws, in this case Newton's. I guess I'm saying in science there has to be circularity if unavoidable.

What say you?

There's an even bigger point here which the OP is hinting at - that the theory of relativity which has supplanted Newton's was proven by instruments that had to follow Newton's laws. — TheMadFool

Of course this is false. Newton looked through a telescope that he himself had made. He was a master lens grinder. That's peripheral to the discussion but just an interesting factoid. Now the point is that Feynman taught us exactly how light passes through lenses. The laws of optics were known to Newton or rather discovered by him. But optics are a quantum phenomenon and Feynman (and others) won the Nobel prize for elucidating this fact.

Everything in the world is a quantum phenomenon. A rock doesn't fly apart because of quantum theory. I think someone made this point earlier but it bears repeating. Newton didn't know his telescope worked on quantum principles, but eventually we discovered that it does.

If you've read my other posts then you know that the Theory of Relativity was derived from measurements from instruments that followed Newtonian laws. What do you have to say about that? — TheMadFool

Me? Wasn't sure about the quoting. Newton's instruments only followed Newton's laws to a certain degree of approximation. They follow Feynman's laws -- quantum laws -- to a far greater degree of approximation. Newton didn't happen to know that, but it was true nevertheless. How is this point not perfectly obvious to everyone?

You know that logo at the top, what does it say? It doesn't say, "The Science Forum", does it? — S

What's wrong with you?

Every child who asks "Why?" sometimes gets caught in the whys and keeps asking long after the efficacy of the question has been exhausted. — tim wood

Newton took a lot of flack at the time. His law of gravity told us what gravity does; but not what it is. He famously said that "I frame no hypotheses." Newton well understood that science is descriptive and not explanatory; a point that modern scientists and philosophers of science would do well to understand.

But at the time, Newton's contemporaries did NOT understand. Descartes had a theory of vortices that said what gravity WAS, not just how it acted. That was, by the scientific ethos of the time, better science than what Newton did. It took people a while to come around to Newton's point of view. We see this echoed today, when people want to "interpret" quantum mechanics rather than be satisfied that it describes what the universe does; and not necessarily why.

https://en.wikipedia.org/wiki/Hypotheses_non_fingo

I would like to know how can you prove these laws, but not using devices that use the the same laws. — Fernando Rios

The basic hypothetico-deductive method works like this:

1. Assume that the system under consideration is described by some theory - in other words, it follows some laws, such as Newton's laws.

2. On the basis of this assumption, make predictions about the system's behavior under certain conditions.

3. Prepare the system and perform an experiment. If your observations are in line with your hypothesis, then your hypothesis, and by extension your theory, are confirmed, otherwise it is disconfirmed (falsified).

There are variations and elaborations of the above (with some, for example, prioritizing falsification over confirmation), but this is the basis of the so-called scientific method. For this method to work, you absolutely need your instruments to be predictable, i.e. to follow laws - including, yes, the laws that you are testing. You just need to consider the instruments as part of the system that you are testing and make your predictions based on that assumption. If the test fails, you could blame all or any part of the system for the failure, including your instruments. You could then try to isolate the problem by performing further tests. And if the test succeeds, and so do other tests, then you will have a high confidence that both the experimental system and your instruments act in accordance with the laws. There is no vicious circularity here.

Of course, in practice we often choose to simplify and neglect many things when modeling an experiment. Thus, we may leave the instruments and our own actions out of consideration. But these simplifications are not made willy-nilly: ideally, we make them only when our theory predicts that they will not have much impact on the result of the experiment. And when the experiment doesn't show what we expect, then one of the explanations that we have to consider is that our assumptions were overly simplistic. We may then have to go back and take into account factors that we thought we could neglect, such as the behavior of the instruments.

What are the experiments that Newton used to show their laws are true? — Fernando Rios

This question worries me a little. I hope that you are not under the impression that Newton's laws are accepted solely on the authority of Newton himself, much like a religious teaching is accepted on the authority of a prophet or a sacred text. How Newton convinced himself that his theory was correct is a question for a historian or a biographer, but it is quite irrelevant to a 21st century scientist. Newton's own thinking could be deeply flawed and his methods wholly inadequate, for all we care - his laws have been tested so thoroughly since then that it no longer matters.

Thank you for your answer. What are the experiments that Newton used to show their laws are true? — Fernando Rios

Newton did not arrive at his theory by empirical methods (experimenting) but by deductive methods, formulating mathematical equations that closely followed his observation of natural phenomena.

They appear to be true because when you plug in values into the equations they predict a result that resembles the equivalent event in real life. For instance his basic acceleration formula predicts that if you subject a mass to a force, it will accelerate. It just happens that if you actually take the mass and apply a force to it, it will accelerate as the formula predicts.

If you've read my other posts then you know that the Theory of Relativity was derived from measurements from instruments that followed Newtonian laws. What do you have to say about that? — TheMadFool

Same with this, the theory of relativity was not developed empirically, it was completely formulated mathematically before physical evidence was observed. The predictions of the theory were so revolutionary that not even Einstein was convinced, until scientists were able to see the path of light bent by gravity, by measuring starlight distorting near the sun during a full eclipse.

The problem for the OP is how an instrument that is Newtonian can ever prove that some other event is NOT Newtonian in nature. — TheMadFool

I would like to know how can you prove these laws, but not using devices that use the the same laws. — Fernando Rios

If we know the devices (rulers, scales, telescope, etc) existed before Newton, and therefor before Newton's laws, what were the devices measuring? (so the device itself is not 'Newtonian', it existed before Newton!) Newton's laws described what the devices were measuring (predicted outcomes of measurements), more accurately than previous models.

I think you are putting the cart before the horse. Newton's laws are based on the measurements of those devices, not the other way around.

Also, Newton's Laws are not accepted as perfect in science. Relativity and Quantum Mechanics have introduced aspects to physics that are not in line with Newton's laws or pick up where Newton's laws stop working (as far as they can tell so far).

@TheMadFool@Fernando Rios

Sorry, I missed a whole page of responses before posting above. Ignore my oversimplified summary if you have already received better answers.

Thank you everyone for your responses. I would only like to know how Newton knew his laws are true. What were the experiments he performed?

So many respondents missing the point regarding "proof". He clarified that by "proof", he means showing to be true. — S

All proofs mean that. Proofs in physics mean that. Except there are no proofs in the science of physics.

This is preposterous. Any one counter-example that does not obey the law destroys that law's usefulness. Newton's laws have not had examples in real life that would nullify his laws, but CONCEPTUALLY they may happen. Therefore a billion experiments and observations that are thought to be showing Newton's laws to be true by S and by Fernando Rios, can be shown to be false by just one experiment or one single observation.

This is philosophy of physics. If you want to see the LOGIC that lead Newton to arrive at pronouncing his laws, then that's a different matter. If you want to see the process in chronological order that were the steps of Newton's developing his insights that lead to the formulation of his laws, that can be done. But to see what makes his laws true, is not possible.