My Physics Professor says Newton's laws are wrong?

[DylanC.]

My Physics Professor from my Physics 1 class says Newtons laws are wrong? Don't get me wrong, but he is a amazing physics professor! One that does not talk to you as if you are dumb or as if he is better than anyone!

I love physics and I'm hoping to go into physics engineering, but that's not important. He explains how einstein's theory of relativity 'proves' Newtons laws to be wrong. But I've read about this and I've heard that Newtons laws were incomplete. There is a big difference between being wrong and being incomplete.

For example, he explains that the closer one gets to the speed of light the object's mass increases which increases affects the Newtons equation F=ma. Which makes sense, I know the basics of Special Relativity very well. But he then explains that this "proves" that Newtons laws are wrong and then he went and said something about how quantum mechanics also proves Newtons laws wrong?

I'm pretty sure he only knows classical physics. The reason why is because quantum mechanics and general relativity is need when distances are small and the mass is huge. Which Quantum gravity will solve most of the mysteries of black holes and possibly how particles interact at the subatomic level when the mass is great and distance is small.

He said that at low velocity's Newtons laws work, but close to the speed of light the answers are wrong because of the effects of quantum mechanic and Special Relativity. This makes NO sense to me and I think he's wrong. But is he right to say Newtons laws are wrong or am I right to say Newtons laws are incomplete?

 

[Choppy]

I think you're getting yourself into a semantics argument. Meaning that you're in disagreement about the wording used, but you agree on the underlying concepts.

One could arge for example that all theories are wrong at some point, because when you get to some kind of extreme situation that breaks the underlying assumptions, they are going to give you a prediction that is inconsistent with observation.

I'm pretty sure he only knows classical physics.

It would be near impossible for someone to become a physics professor and only know classical physics.

It's quite possible your professor is presenting the work in this way simply as a means of drawing your attention to conditions under which their Newton's laws break down. Whether you feel that the word "incomplete" is a better way of phrasing it than "wrong" doesn't change the basic issue. It's probably best just to move on.

 

[DylanC.]

Thank You so much! I guess I didn't see it that way! I just wanted to be sure. Don't like being fed with the wrong information!

 

[nasu]

Thank You so much! I guess I didn't see it that way! I just wanted to be sure. Don't like being fed with the wrong information!

It's not wrong information. Just incomplete. :)

Newton did not write F=ma but rather put it in terms of momentum. And in this form is valid in relativistic mechanics too.

 

[ZapperZ]

Since we are dealing with "semantics" here, I would prefer to say that Newton's Laws are an "approximation".

Once can see this clearly since we can derive them under "terrestrial" situation, i.e. v<<c from special relativity, and n approaches a large number in quantum mechanics. This is the definition of an approximation, a special case, etc.

Things that are an approximation are NOT wrong. They're just an approximation, and by definition, does not work all the time. From very far away, I can approximate a cow as a sphere. Almost everything that I can measure, I lose no accuracy by making this approximation. But as I get closer and closer to the cow, certain approximation will start to be beyond the accuracy that I desire, meaning that the cow starts to look less and less like a sphere.

Zz.

 

[QuantumQuest]

Putting things in a historical context, what was the wrong assumption regarding two different observer frameworks in Newtonian physics, was that they thought that time was the same (t' = t) for two observers moving with different velocities watching a light beam, because this was breaking what was Einstein's axiom - and this was confirmed by experiments, that speed of light is the same and an upper bound for all observers. So definitely, in a relativistic context, Newtonian laws broke down but regarding small velocities as we experience them in everyday life, there is no such problem and of course given the instruments and theoretical framework back in Newton's era, he really did very well, so the word "incomplete" for his laws, was something inevitable to happen back then . On the other side of spectrum of Physics i.e. QM, there is the fundamental issue of duality and measurement, an issue that macroscopic objects and measurements, do not suffer from. So, totally agreeing to Choppy, it's best to move on and over the course of your learning, you'll put things at the right scale and context, yourself.

 

[Hercuflea]

Taken at face value, Newton's laws are wrong in the context of Einstein's theories of Special and General Relativity. But they are a good enough approximation to everyday happenings that we can use them to model objects that are moving at slow velocities. (Much less than, say 10% of the speed of light).

 

[DrStupid]

For example, he explains that the closer one gets to the speed of light the object's mass increases

It is quite surprising to hear that from a physics professor.

which increases affects the Newtons equation F=ma.

This equation is not from Newton but from Euler. Newton's F=dp/dt still holds close to the speed of light.

 

[rcgldr]

Is Newtons' third law (forces only exist in equal and opposing pairs) ever violated in inertial frames of reference?

 

[CWatters]

Just curious but did Newton ever know his laws were incomplete/an approximation? Did he have data that didn't fit or did that only come later?

 

[Orodruin]

For example, he explains that the closer one gets to the speed of light the object's mass increases which increases affects the Newtons equation F=ma.

The concept of relativistic mass which increases with an object's velocity is rather archaic and not in use by most professional physicists today. When physicists talk about mass they are usually referring to the invariant mass of an object. There are several problems with using relativistic mass, some of them are explained in the Insight: https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/
You are already touching upon one of the key points, if you want mass to be defined as the $m$ in $F = ma$, then the mass would depend on whether the force was applied parallel or perpendicular to the direction of motion. Historically, these concepts were referred to as transversal and longitudinal relativistic masses.

 

[vanhees71]

Just curious but did Newton ever know his laws were incomplete/an approximation? Did he have data that didn't fit or did that only come later?

No, for almost 300 years his mechanics was considered an exact description of nature. It's the paradigmatic example for a physical theory in the modern sense, and this is not so surprising since we don't experience relativistic effects in everyday mechanics, and not even the astronomers dealing with the calm immediate neighborhood of the solar system find deviations easily. What was known for quite a while was an unexplained part of Merkur's perihelion shift. The astronomers where so precise in their measurements and the mathematical theory (based in Newton's physics) of the orbits of the bodies in the solar system, that they could sometimes find new planets, comets etc. by just measuring perturbations of other objects' orbits, but for Mercury they couldn't find such a body. It was only in 1915 that Einstein could explain the perihelion shift with his General Relativity.

 

[jeffery_winkle]

Well, Newton's laws are obviously wrong because they don't take into account relativity or quantum mechanics. You yourself admit that they are "incomplete" which means you admit that they are therefore wrong. With macroscopic objects moving at slow speeds, in weak gravitational fields, you can make the subjective judgement that in your opinion, the error intrinsic to Newtonian mechanics is small enough, under those circumstances, that you can choose to ignore it, under those circumstances, but the error is still there nonetheless. The error is always there. I think part of the problem with your way of thinking is the way science is usually taught, and I give your teacher credit for not teaching it that way. The way science is usually taught, is that the teacher stands at the front of the classroom, and tells the kids, "This is answer is right" or "That answer is wrong". The teacher might write on the board "F = ma", and if a student writes in their homework, "F = ma", they get it right. If they write "F = mv", they get it wrong. The kids end up walking away with the false impression that Newton's mechanics is true, when of course, Newtonian mechanics, while a useful approximation, is obviously wrong, since it does not take into account relativity or quantum mechanics. If Newtonian mechanics wasn't wrong, which we now know it is, physics would have ended with Newton.

There is nothing wrong with using the word "wrong" the way your teacher does. If it makes you feel better to say "incomplete", you can do that, as long as you don't think that "incomplete" means "not wrong" because if it's incomplete, it is therefore wrong. I also think part of the problem comes from the fact that usually in the classroom, when the teacher says something is "wrong", they mean you gave the wrong answer, and would get a bad grade, while here, your teacher is using the word "wrong" to mean "not the real universe". This is a different usage of the word "wrong" which I think is leading to confusion. You might say "Why would I want to learn something that's wrong?" The answer is you have to learn a more wrong thing now is order to be able to learn a less wrong thing later. You should also distinguish between something that is wrong, but a step in the right direction towards a better understanding, versus something that is wrong and also not a step in the right direction towards a better understanding.

 

[mpresic]

I side closer with you. Your physics prof should know Newton introduced his second law as force is proportional to the time derivative of the momentum; not F = m a. Nasu and several others are correct in bringing this to your attention. Many times educators say provocative statements to grab the student's attention. To that purpose, you (the student) are thinking for yourself and this is a good thing.

I seem to remember Newton used a concept of vis viva, which is not the way we think of these concepts today. The concepts of the 17th century have been sharpened and restated by now.

Make sure not to deteriorate relations with your physics prof by challenging him/her inappropriately or impolitely.
I made the mistake of correcting my physics teacher in high school when he said the nearest star was alpha centuri, I told him it was the Sun.

 

[lightarrow]

So definitely, in a relativistic context, Newtonian laws broke down but regarding small velocities as we experience them in everyday life

Ok. But I would like to take advantage of this post to say that textbooks usually don't mention the fact Newtonian mechanics, namely, absolute time, fails to be true even at low velocities, if we consider systems very far apart: considering the usual boost transformation laws between two frames of reference :

x' = γ(x-vt)
t' = γ(t-xv/c²)

you see that the term xv/c² is significantly different from 0 even at low velocities v, if x is great enough.

--
lightarrow

 

[nuuskur]

Newton's laws are inaccurate at relative velocities. For our every day use, we don't travel at a third, figuratively speaking, of the speed of light, so Newton's estimates are accurate enough.

 

[vanhees71]

Make sure not to deteriorate relations with your physics prof by challenging him/her inappropriately or impolitely.
I made the mistake of correcting my physics teacher in high school when he said the nearest star was alpha centuri, I told him it was the Sun.

Mh, why was this a mistake. You were right! I'd be delighted about such a comment. Teachers are sometimes strange people...

 

[Orodruin]

Mh, why was this a mistake. You were right! I'd be delighted about such a comment. Teachers are sometimes strange people...

Unfortunately, there are bad teachers, just as there are bad practitioners of nearly any profession. I can imagine some teachers enjoy being an authority and dislike being challenged.

 

[jeffery_winkle]

Actually, Alpha Centauri isn't even the closest star aside from the Sun. The closest star aside from the Sun is not Alpha Centauri but Proxima Centauri.

 

[Orodruin]

Actually, Alpha Centauri isn't even the closest star aside from the Sun. The closest star aside from the Sun is not Alpha Centauri but Proxima Centauri.

Alpha Centauri is not a star, it is a star system with at least two stars Alpha Centauri A and Alpha Centauri B. It is likely that a third star is gravitationally bound to the system, which would make it a ternary system. The third star is sometimes designated as Alpha Centauri C, but perhaps more commonly referred to as Proxima Centauri. In other words, Proxima Centauri is likely a part of the Alpha Centauri star system.

 

[mpresic]

It would have been more polite for me to mention the teacher's mistake in private and not in front of the class. -MP. Orodruin also brings up a point that I was aware of even back then (in 1974). But because th Sun is closest, I did not have to quibble about the nearest star excluding the Sun. It may have been a mistake considering my teacher called on me often to try to trip me up in a little good natured retaliation. Make no mistake about this though, he was not a bad physics teacher, and he was very knowledgeable, and had 20-30 years teaching experience. I have no complaints.

 

[Orodruin]

Orodruin also brings up a point that I was aware of even back then (in 1974).

As the orbital period of Proxima Centauri in the ternary system would be larger than 100000 years, knowledge from 1974 on this matter is still pretty well updated.

 

[newjerseyrunner]

Just curious but did Newton ever know his laws were incomplete/an approximation? Did he have data that didn't fit or did that only come later?

Yes, he knew that his theory was incomplete. He solved the equation, he could not explain why. If you look at the relationship between mass, distance, and force you realize that the units don't match up and you have to add a really awkward constant with the units m³kg^-1s^-2. He knew that this probably wasn't natural.

Physics is at a similar position with the standard model, there are 19 constants that seem to come out of no where.

 

[nasu]

This is really wrong and misleading. The units have nothing to do with it.

By the way, the units m and kg were not even invented during Newton's time. Are you aware of this?

 

[CrackerMcGinger]

He explains how Einstein's theory of relativity 'proves' Newtons laws to be wrong.

Just saying something I noticed immediately, if I'm missing some info on laws and theories please remind/refresh me, but can a law be proven false by a theory when that theory is still being debated (certain aspects, not all of it) and that law is considered completely true? I would like to remind some that, even though a large number support it, Special Relativity is not a law. I've researched this. And, you can't argue with that fact. When we talk about relativity, we usually say theory of relativity. When we talk about Newton's Laws, well I mean it says Newton's Laws you can't even talk about the concept without bringing up the key word Law. And it's not incomplete because we know more now than 300 years ago and it still is true on the aspects it covers until a certain point, but that is even debatable for reasons I have already said.

 

[nasu]

Newton's mechanics is a theory too. Just historical reasons make people to refer to it more often as "Newton's laws" rather than Newton's theory.
And the theory of relativity is based on some "laws". Again, it is called a theory mostly from historical reasons, people got used to the term.
In physics "law" is not something stronger or more valid than a "theory".

 

[jeffery_winkle]

You are confusing the non-science usage of the words "theory" and "law" with the science usage of the words "theory" and "law". In non-science usage, the word "law" makes something more true or certain than the word "theory", and people think it sounds more impressive to say something is a law than a theory. In science, it's the opposite. The word "law" refers to a single isolated simple fact that you can write down in one sentence or a simple equation, while the word "theory" refers to the total body of knowledge on the subject, which involves a lot more information and evidence than a single isolated fact. You often see creationists getting tripped over the use of the word "theory" in the phrase "theory of evolution".

In addition to all that, you notice that today, physicists only use the word "law" when talking about things that were discovered/invented a long time ago, and for historical reasons were called a "law". Things discovered/invented more recently are not called "laws". For example, they could have easily legitimately used the phrase "Einstein's laws" to refer to a set of equations including the definition of the Lorenz factor, time dilation, length contraction for special relativity, Einstein's field equations for general relativity, and you would probably throw in E = mc^2. So why don't we call these "Einstein's laws"? I think the reason has nothing to do with physics, but is instead because of history, psychology, and sociology. In the past, the government was the king, and the law was whatever the king decided for it to be. Asking why the law was what it was would be questioning the wisdom of the king, which was not allowed. They would just say "Because it's the law!" Around that same time, they thought of God as like the King of the Universe. Natural law was whatever the King of the Universe, A.K.A. God, decided for it to be. Asking why the law was what it was would be questioning the wisdom of God, which was not allowed. They would just say "Because it's the law!" Because they had that way of thinking, they would use the word "law" to refer to the rules of nature. Well, later, they got away from that way of thinking, and embraced democratic government, where laws are created as the result of the legislative process, which involves endless debate over why the laws should be the way they are. In a democracy, you are supposed to ask why the law is what it is. Carrying that way of thinking over into science, you can now ask why the so-called natural laws are what they are. You could now ask "why" in regards to things they couldn't before. Evolution explained why all the species of plants and animals had the forms they do. When you are allowed, and encouraged, to ask, "why" in regards to everything in science, you are not going to refer to something as a "law". Many examples of what we, for historical reasons, still call "laws", we now know are not true in general. Ohm's law breaks down when talking about a quantum dot which electrons can enter or leave one at a time. Something isn't true just because you call it a "law". Newton's laws are not true even though we call them "laws". Einstein's equations are more true than Newton's laws even though we don't call them "laws".

 

[vanhees71]

Just saying something I noticed immediately, if I'm missing some info on laws and theories please remind/refresh me, but can a law be proven false by a theory when that theory is still being debated (certain aspects, not all of it) and that law is considered completely true? I would like to remind some that, even though a large number support it, Special Relativity is not a law. I've researched this. And, you can't argue with that fact. When we talk about relativity, we usually say theory of relativity. When we talk about Newton's Laws, well I mean it says Newton's Laws you can't even talk about the concept without bringing up the key word Law. And it's not incomplete because we know more now than 300 years ago and it still is true on the aspects it covers until a certain point, but that is even debatable for reasons I have already said.

This is obviously wrong. In physics you cannot prove a theory, i.e., a description of phenomena observable in nature, but you can test it by making observations in nature. Special relativity has been discovered, because it was found that the Newtonian spacetime description is inconsistent with observations with regard to electromagnetic phenomena, because the Maxwell equations are not Galileo-invariant, and Maxwell's equations were the offspring of some centuries research on electromagnetic phenomena, including the great investigations by Faraday (including the concept of fields, which is the most important concept discovered since Newton's concept of particles). Nowadays, special (and also general) relativity are known to be the best description of spacetime, and Newton's spacetime model is an approximation, for which the limits of validity are well known. It may well be that also the relativistic spacetime model is only an approximation for an even better description, but up to now, we have only hints about its boundary of validity, which are likely to be given by a full understanding of the quantum description of gravity and a consistent general-relativistic quantum theory.