# Common false beliefs in Physics [closed]

Well, in Mathematics there are somethings, which appear true but they aren't true. Naive students often get fooled by these results.

Let me consider a very simple example. As a child one learns this formula $$(a+b)^{2} =a^{2}+ 2 \cdot a \cdot b + b^{2}$$ But as one mature's he applies this same formula for Matrices. That is given any two $n \times n$ square matrices, one believes that this result is true: $$(A+B)^{2} = A^{2} + 2 \cdot A \cdot B +B^{2}$$ But eventually this is false as Matrices aren't necessarily commutative.

I would like to know whether there any such things happening with physics students as well. My motivation came from the following MO thread, which many of you might take a look into:

• Community wiki? – Marek Nov 17 '10 at 23:10
• @MArek: i didnt find the option. if anyone can do it they are welcome – crskhr Nov 17 '10 at 23:15
• @Chandru: AFAIK StackExchange recently changed its rules about this matter so that only moderators can make a question community wiki (the rationale being that the CW option is being misused at StackOverflow). – Marek Nov 17 '10 at 23:26
• While i perceive the question interesting, I think the example with matrices is rather 'a common silly mistake' than 'a common false belief'. – Piotr Migdal Nov 17 '10 at 23:48
• It seems a bit odd to be accepting a single answer to a soft question... – David Z Dec 1 '10 at 0:21

The concept that quantum mechanics undermines determinism. The Schrodinger wave equation evolution is completely deterministic. The results of measurements are probabilistic, but this does not mean that the various superposed states do not have causes. This is not the same thing as a hidden variable theory. The probabilities are deterministic. T'Hooft has some interesting ideas on a determinism underlying QM (not the same thing as saying the wave equation is deterministic). I am not arguing that qm is in all senses deterministic, but it isn't completely non-deterministic either.

Notion of simultaneity. Because of speed of light is so big, it looks true in our day to day affairs.

But it really is a non existent thing [due to special relativity]. 2 people in 2 different places can't say "at the same instant".

I find people think that "a bullet and a ball shot and dropped repectively from the same height will not hit the ground at the same time".

• ...because that's a true statement. A shot bullet is going to start out with quite a bit higher vertical velocity than a dropped ball. – BlueRaja - Danny Pflughoeft Jan 12 '11 at 22:45

A common misconception is that motion of massive objects with superluminal velocity is prohibited in General Relativity. This is not true as only local superluminal motion is prohibited.

The big bang theory tells us the universe started from a point

Since Einstein we know all is relative

The speed of light is a fundamental constant

This is also a misconception of science in general, but I've heard many people say, "Physics (science) has proven. . ." or "Can we use Physics (science) to prove this?" The misconception is that the scientific method can prove something with 100% certainty. This is certainly not the case; experiments only validate a law in settings similar to that in which the experiments were conducted. Granted, we can often reasonably generalize our results far beyond particular settings, but we can only hold laws with certainty where they have been tested and verified.

The constancy of the speed of light postulated by Albert Einstein in 1905 was motivated by the null result of the Michelson-Morley experiment.

This is wrong. See

Einstein was mainly motivated by the results of Fizeau's experiment measuring the speed of light in moving water

• I agree that Einstein wasn't motivated by Michaelson morley at all. The reason people say this is because Michaelson's interfermometry is a fundamental tool, and students should pay attention. But Einstein wasn't really motivated by Fizeau either--- although this was a major unexplained experiment that convinced him the addition of velocities was right. His biggest outside motivation was probably Poincare, who was struggling to understand frame invariance in 1902, almost discovered relativity, but rejected it at the last minute because he started to sense the implausible consequences. – Ron Maimon Sep 23 '11 at 17:02
• Poincaré, you really have to read him if you haven't. A well known name, but not as well known as he should be. Especially from high-ranking people. – CHM May 14 '12 at 23:38

The greenhouse effect (not the atmospherical one) :

One common misbelieve is, that the reason for this effect is, that sunlight comes in and is transformed to infra red radiation that can't go out.

But the main reason is a lack of air exchange (see the section "Real greenhouses" in http://en.wikipedia.org/wiki/Greenhouse_effect).

• Took me a second there, but you have my agreement. The wording, however, is a hangup. You use the term greenhouse effect in reference to the normal (man-made) greenhouses. We are so used to the term being used in climate science, it seems a bit foreign to me. – Alan Rominger May 15 '12 at 2:41

A common misconception is that in a double slit experiment, the electrons or the photons go through both slits at the same time and interfere with themselves.

Nothing of the sort ever happens and that idea doesn’t come from quantum mechanics. It is just meaningless to ask the question (from which slit did the particle pass?) in the context of the particular experiment and even more meaningless to say that it went through both. The particle always passes from the one or the other slit if you setup an experiment that asks the question and in the case that you shoot the particles one at a time you always get one hit on the screen and not an interference of portions of the particle.

I think that the misconception has its roots in the wave analogy of the wave function description of quantum mechanics, where you must have a wave passing from both slits in order to have interference on the other side. Of course that picture doesn’t translate to having a particle passing from both slits, but it only states that there exists interference between the wave functions of the two independent events that translates to a distribution of probability for finding a particle on a particular point on the screen. That is a statement of Born’s interpretation that has been recently tested with a triple slit experiment.

One could ask at this point, “Ok, I get it for the electrons, but light behaves like a wave in everyday experience. What happens in that case?”, and the answer is that light behaves like a wave only when you have a large flux of photons and you can go to the continuous field approximation. It is then that the wave-like properties become apparent both for photons and electrons.

Update3: The Feynman Lectures on Physics vol3: Scattering from a crystal (neutrons).

... Let’s review the physics of this experiment. If we could, in principle, distinguish the alternative final states (even though you do not bother to do so), the total, final probability is obtained by calculating the probability for each state (not the amplitude) and then adding them together. If you cannot distinguish the final states even in principle, then the probability amplitudes must be summed before taking the absolute square to find the actual probability. The thing you should notice particularly is that if you were to try to represent the neutron by a wave alone, you would get the same kind of distribution for the scattering of a down spinning neutron as for an up spinning neutron. You would have to say that the “wave” would come from all the different atoms and interfere just as for the up spinning one with the same amplitude. But we know that this is not the way it works. So as we stated earlier, we must be careful not to attribute too much reality to the waves in space. They are useful for certain problems but not for all.

• Actually, what you said is not true. First, electron go through both slits at once (if you want to say something contrary, it's much more about taste or interpretation, that 'fighting with a false belief'). Second, while properties of a single photon are interesting, I would not say it does not behave like a wave (it certainly have many wave-like properties). – Piotr Migdal Nov 20 '10 at 9:56
• Very strange answer. Actually, it only depends on interpretation whether you think that electron passes through both slits. So this means that it is not a misconception at all. Actually, the most attractive interpretation to me (Feynman's path integral) tells you that the electron travels through all paths and all of them are equally important in determining the final amplitude. In particular, the paths through left and right slits interfere. – Marek Dec 1 '10 at 0:04
• It seems to me like you are trying to propose ancient preconceptions (like the ones Einstein had) that quantum effects appear just as statistics after letting many electrons through the slits. Well, this is obviously not the case. Every single electron behaves quantum mechanically and travels through all the paths (in path integral view) or is a wave that passes through both slits (in Schroedingerian view). – Marek Dec 1 '10 at 0:08
• Sorry to step in abruptly, but QM does not tell us ANYTHING about the path of the electron. The ONLY objective, measurable and sustainable thing that QM tells us is the OUTCOME of the MEASUREMENTS. I.e. that electrons will hit the screen in some pattern. All the rest is (non-physical) interpretation and subjective. – Sklivvz Dec 3 '10 at 21:16
• @Sklivvz: but you are implicitly using one such interpretation yourself. Namely, one that tells you that it's unphysical to ask anything except measurements. You have arbitrarily chosen to make measurement the basis of what is physical (and seems you are not even aware of it). The point is, I can take another interpretation, one that tells me where the electrons went and it also perfectly reproduces all the measurements, so there is no problem with it. It's as valid (and physical) an interpretation as yours is ;-) – Marek Dec 4 '10 at 16:12

Here's a false belief. Everything your physics teacher says is true. Because physics teachers would never mislead you over how many states of matter there are or how lift is generated on a wing.

I know the experiment(s) showing parity violation. However, I guess what I still don't understand is the following, rotational symmetry is fundamental. And parity is just rotating 180 degrees. So if we say that all rotational symmetry is fundametal, why would a subset (rotating with a very specific angle - 180 degrees) not being fundamental? Isn't that self contradicting?

• That's only the case in two dimensions. In 3D space, a parity transformation is not equivalent to any rotation. – David Z Jan 13 '11 at 22:59
• @David: in 2d parity is defined by reflecting 1 axis, not both, same in any even dimension. – Ron Maimon Sep 23 '11 at 16:55
• @Ron: true, guess I made a mistake. – David Z Sep 23 '11 at 18:51

I highly recommend reading the article "Quantum mechanics: Myths and facts" by H. Nikolic http://arxiv.org/abs/quant-ph/0609163

Some topics include wave-particle duality, time-energy uncertainty relation and fundamental randomness.

Misconception - The uncertainty principle is a statement about "our" ability to make measurements.

Correction - The uncertainty principle is a result of the nature of the particles themselves and refers to the ability of anything to "make the relevant measurements". It's not just us that can't determine the simultaneous values of incompatable observables, God can't either.

• The point is that if we can't measure it, then it doesn't exist. The two are synonyms. This is an important philosphical position. – Ron Maimon Sep 23 '11 at 17:05
• I think it is important to take the "we" out of the statement. "If it can't be measured, it doesn't exist" is closer to the point. – AdamRedwine Sep 23 '11 at 19:51
• yes, you are right. – Ron Maimon Sep 23 '11 at 22:31

Earth revolves around the Sun. It is wrong to say that the Sun revolves around the Earth.

FACT: Motion is relative. There is nothing wrong in saying the Sun revolves around the Earth. The former is more frequently mentioned because the Sun (or more accurately, barycenter) is a better inertial frame, and other planets revolve in a near circle around the Sun as well, but in a bizarre way around the Earth.

By traveling faster than light in the vacuum one can go back to the past.

FACT: No explanation needed.

• In the Earth-centered frame, the light from distant stars also abberates once a year around the earth, because all the distant stars, and the light, are also wobbling. It's a nonsense frame for cosmology, but in principle, and in principle only, it works. – Ron Maimon Sep 23 '11 at 17:05

The misconception: Naive people consider an object will float if it is placed in a vacuum container regardless of the existence of gravity.

The correct fact: The absence of gravity makes the object float.

It's a common false belief among cynical physicists that there is no physical meaning in asking "why is there something rather than nothing". The question of whether there is an inevitable, self-consistent, self-referencial mathematical law that mandates the universe to exist is a real and legitimate one, a unified theory would be a step in the right direction. Research in number theory, prime numbers, infinity, etc, also plays into this.

I know it's an old question, but it's too much fun to pass this one up. There are quite a few examples of wrong ideas that have taken hold, but for the centrality to physics and the degree to which it is wrong and misleading, it is hard to find a better example than the Bohr atom. When it came out, it electrified the physics world on account of its revolutionary viewpoint and stunning success. But eleven years later it became completely obsolete. Nevertheless, in that short time it gripped the imagination of both the physics world and the general public to the point where it remains today the iconic picture of the atom in the minds of just about everyone.

The misleading influences of this model are extremely far reaching. One can begin with the idea of the "quantum leap", which begat wave function collapse, which begat multiple universes et cetera. But perhaps the most persistent holdover of the Bohr atom is the notion that the ordinary laws of electromagnetics must be suspended at the atomic level, otherwise the atom would be unstable. Of course, nothing of the sort is true in modern quantum mechanics, least of all for the hydrogen atom. The greatest triumph of the Schroedinger equation was to show that the motion of electric charge could be tracked through time, and that the stable atomic configurations were precisely those with no accelerating charge distributions. This was an immediate and obvious consequence of Schroedinger's solution for the hydrogen atom.

I would claim that it is not merely true that the stable configurations in QM are those with no accelerating charges. I would go further and suggest that in those cases where the charges do accelerate, the rate at which the kinetic energy of motion is converted into electromagnetic energy is exactly in agreement with that which would be calculated in the ordinary way by applying Maxwell's equations. So, for example, in the case of a black body radiator, if one simply takes the vibrating atoms, accounting for the accelerating motion of the charges, and applies classical antenna theory, you will get the correct black-body spectrum.

• -1: no, this is not correct. The Bohr model with its quantum jumps is more accurate than the 1926 Schrodinger description, because the wavefunction cannot be a classical field, because for two electrons its in 6 dimensions. For more particles, you get many decohered worlds and wavefunction collapse, and the charges are accelerating in all solutions, including the ground state of Hydrogen. – Ron Maimon Sep 24 '11 at 6:01
• It's hard for me to understand what you are trying to tell me. In what interpretation are the charges accelerating in the ground state of hydrogen? More to the point, when you say "this is not correct" I'm wondering if you can identify one specific error in what I said? – Marty Green Sep 24 '11 at 13:59
• The acceleration of a quantum particle is given by the operator $dp\over dt$, which is only zero if the particle is in a plane wave. The hydrogen ground state is accelerating. Everything you said is wrong, so it is hard to find one error. – Ron Maimon Sep 24 '11 at 18:57
• Oh come on now. There is no accelerating charge in the ground state of the hydrogen atom and everyone knows that. – Marty Green Sep 24 '11 at 20:18
• Everyone who knows, knows the opposite. – Ron Maimon Sep 24 '11 at 23:45

This is tongue-in-cheek!

That an emptying tank with a nozzle pointing downwards would actually experience a force!

:-)

Something cannot come from nothing

Yes, it can: In quantum field theories, the vacuum is not empty.

• In QFT vacuum is certainly not nothing (it just happens to be a state vector annihilated by some operators) so this has nothing to do with your proposed misconception. – Marek Dec 3 '10 at 9:34
• Watch "Universe from Nothing" by Lawrence Krauss. youtube.com/watch?v=7ImvlS8PLIo – Lagerbaer Dec 3 '10 at 16:13
• I don't have time to watch it. If it has a punchline then just tell me. Anyway, nothing either has a precise mathematical sense in some theory (and then it's certainly not nothing) or else it is just philosophical concept (and therefore not physics). Either way, your answer is not good. – Marek Dec 3 '10 at 20:41