# My Physics Professor says Newton's laws are wrong?

1. Sep 18, 2015

### 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?

Last edited by a moderator: Sep 22, 2015
2. Sep 18, 2015

### 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.

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.

3. Sep 18, 2015

### 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!

4. Sep 19, 2015

### nasu

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.

Last edited: Sep 19, 2015
5. Sep 19, 2015

### ZapperZ

Staff Emeritus
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.

6. Sep 19, 2015

### 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.

7. Sep 19, 2015

### 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).

8. Sep 19, 2015

### DrStupid

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

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

9. Sep 19, 2015

### rcgldr

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

10. Sep 22, 2015

### 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?

11. Sep 23, 2015

### Orodruin

Staff Emeritus
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.

12. Sep 23, 2015

### vanhees71

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.

13. Sep 23, 2015

### 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.

Last edited: Sep 23, 2015
14. Sep 25, 2015

### 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.

15. Sep 26, 2015

### lightarrow

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

16. Sep 26, 2015

### 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.

17. Sep 26, 2015

### vanhees71

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

18. Sep 26, 2015

### Orodruin

Staff Emeritus
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.

19. Sep 26, 2015

### 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.

20. Sep 26, 2015

### Orodruin

Staff Emeritus
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.