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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:

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Community wiki? –  Marek Nov 17 '10 at 23:10
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@MArek: i didnt find the option. if anyone can do it they are welcome –  Chandrasekhar Nov 17 '10 at 23:15
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@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
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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
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It seems a bit odd to be accepting a single answer to a soft question... –  David Z Dec 1 '10 at 0:21
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49 Answers 49

Amazingly, Wikipedia has an article titled "List of common misconceptions". There is a (short) section dedicated to Physics, which mentions:

  • The role of the Coriolis effect in bathtubs and sink drains
  • The role of angular momentum in bicycle stability
  • The "equal time" fallacy in explaining the lift developed by an airfoil
  • Glass isn't actually a high viscosity fluid
  • Composition of air
  • "Lightning never strikes twice"

The Astronomy section has some good ones too:

  • When a star collapses into a black hole, its gravitational pull does not actually increase.
  • Meteorites are not actually hot when they land; usually they are cold. (I would add: the heating of meteors is more due to the compression of the air in front of them than to 'friction with the air' as commonly believed.)

Some that I would add:

  • "Once something is in orbit it is free from Earth's gravity." Even educated people get tripped up on this one; the internet is rife with people suggesting we just "nudge" the International Space Station into lunar orbit. At a much more basic level of misunderstanding, there is the idea that astronauts are "weightless" because they are far away from the earth.

  • "There is a high tide on the opposite side of the earth from the moon/sun because the earth 'shields' the ocean from the gravitational pull."

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Especially the equal time fallacy. Nobody believes you when you tell them it's not true. –  dan_waterworth Dec 15 '10 at 7:24
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Just to clarify the 6 physics examples are things that aren't true (which people believe) - the 2 astronomy ones are things that are true (but people don't believe) –  Martin Beckett Mar 19 '11 at 5:51
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A mistake that I often come across and that is so easy to make: people somehow have a visceral belief that heavy objects fall faster than light ones. Setting aside of course problems of air resistance, this is obviously false, but it seems to be so counterintuitive and I think it is somehow tied to our intuitive understanding of mass as inertia. Since higher mass means higher inertia. People understand this intuitively as it takes more force to push a fat guy than a thin one. But they don't see that gravity is a force proportional to mass, so that more inertia is paralleled by more gravity as well. With as a result the same gravitational acceleration for light and heavy bodies.

I've even noticed the mistake being made by professional physicists in colloquial conversations.

EDIT: I just found this article today, shedding a new light on why misconceptions in physics or science in general are so common and so hard to get rid of.

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+1 for "setting aside air resistance"... it always bugs me when people point out how things fall at the same rate, while neglecting to mention that this assumes air resistance is negligible. Leaving out this clarification I think leads to more confusion, as all someone has to do is drop a rock next to a feather to (incorrectly) conclude that Galileo was wrong. –  Tim Goodman Nov 18 '10 at 3:22
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If you are assuming a fixed earth, then you are correct. But if you let the earth move, then the Earth moves more rapidly when you drop a bigger mass than when you drop a smaller one. In that sense, bigger masses drop faster. This has nothing to do with the equivalence principle of course. –  Vagelford Nov 18 '10 at 17:49
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Seems important to mention here Galileo's famous gedankenexperiment. If you follow the line of thought that heavy objects fall faster, and go split a heavy falling object in two, large one and big one, both should be falling slower than the original. The upper part would be pulling lower part up, wanting to go even slower, hence a contradiction. –  Pavel Radzivilovsky Dec 1 '10 at 9:44
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There is also the complication of buoyancy - a dense object would be heavier at the same mass, and thus fall faster even if air was friction-less. –  romkyns Jan 12 '11 at 1:22
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@Pavel You're ignoring tidal effects. Splitting something in two doesn't really change anything, because the two halves, being in close proximity, experience different tidal forces than those between two objects farther apart. –  ErikE Jan 21 '11 at 18:21
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"Summer is when the Earth is closest to the sun, and winter is when it's furthest away."

It's true that the Earth's orbit is slightly elliptical, but the effect of this, as far as seasons, is very small. For one thing, this wouldn't explain why the sun rises and sets at different times in different seasons, and if this were true, the whole planet would have summer at the same time.

The seasons are actually caused by the tilt of the Earth relative to its orbit around the Sun.

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I would say that for most people, the quadratic scaling of kinetic energy with speed is somewhat of a mystery.

People don't understand how if you go twice as fast, a car accident actually four times as energetic, hence the high number of reckless drivers and deadly accidents.

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Classical mechanics is boring and mostly solved

...especially in case of fluid dynamics (-;

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I never met a person who would think this. If you don't know anything about mechanics then you obviously can't think it is boring. And if you do know it, just double pendulum or three-body problem should convince you that it's far from simple. I wouldn't call fluid dynamics classical mechanics though. While it is classical, it is certainly not mechanics but rather a field theory. And that is the main reason it is hard. –  Marek Dec 3 '10 at 12:46
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I would say most physics undergrads think this is so. This is closely related to "anything that can be solved analytically, already has" –  Pete Dec 3 '10 at 16:01
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I will give some meta-false beliefs: these are beliefs held by the general public, which happen to be true, which are hyper-corrected by many physicists with bogus corrections based on the urge to appear smart:

Electrons move slowly down a wire

  • The belief: the electrons move lightning fast down a wire.

  • the hypercorrection: in the completely obsolete Drude model, electrons move slowly. In this model, you imagine the current is carried by a classical gas of electrons, and you divide the total current by the density of all electronic charge to get the drift velocity. This predicts a completely bogus drift velocity of a few cm/s, which is total nonsense, because only electrons near the Fermi surface contribute to the conductivity. Nevertheless, you see this hypercorrection repeated endlessly (it appears here too).

  • The best answer: the electronic wavefunctions are spread out in a metal. The correct notion of electron velocity is the Fermi velocity, which is enormous typically, because the wavelength is about 1 atomic radius. While it isn't the same as the speed of electricity going down the wire (which is the speed of the field perturbations, some significant fraction of the speed of light), it is enormously high. Impurities which can scatter electrons will alter this speed, but not as much as the naive hypercorrection says.

The atom is mostly empty space

  • The belief: the atom is full of stuff, that's why stuff is hard when you push against it.

  • The hypercorrection: In the totally obsolete Rutherford-Bohr model, the atom is mostly empty space, the tiny pointlike electron orbiting a nucleus which contains most of the mass.

  • The best answer: But it is the electrons' wavefunction which tells you whether something is empty space or not. A region filled with electronic wavefunction feels hard to the touch, because two electrons can't be compressed into the same space without squeezing their wavefunction to have very high spatial variations, by the exclusion principle. Atoms are full of electronic wavefunction, and are therefore not empty space, at least not by any reasonable definition.

There is nothing mystical about measurement in quantum mechanics

  • The belief: the measurement problem in the standard quantum mechanics suggests that consciousness is somehow involved in measurements

  • hypercorrection: decoherence explains all that! Quantum mechanics is no different than determinism as far as enlightenment values are concerned.

  • The best answer: Decoherence tells you why you don't have interference between classically different worlds, or histories, and its an important part of the story if quantum mechanics is exact. But it does not tell you why you "percieve" one consistent history as a world. You need a dictionary between physics and perception. Since this dictionary is fundamental, and weird, and philosophical, it is important to explain that this is not an output of physics, but an input, which links mathematical theory with explicit sense-data.

There is no such thing as a centrifugal force

  • The belief: when things are rotating, they are pushed out by a centrifugal force.
  • The hypercorrection: There is no centrifugal force. There is a centripetal force (centripetal was a made up word to replace centrifugal) which pulls you in.
  • The best answer: This is obviously true from the point of view of the inertial frame, there is no centrifugal force, but if you are looking at it from the point of view of the rotating object, then there is. It all depends on your choice of reference frame.

The big bang happened everywhere at once

  • The belief: the big bang means that the universe started at some definite point and got bigger from there.
  • The hypercorrection: the big bang happened everywhere at once, and it is just wrong to think of it as happening at some single point in an extended model of space-time. If the universe is open, the big-bang was infinite in extent.
  • The best answer: There are three important caveats: 1. In FRW models, the bang-point is a singularity, so its outside space and time, and it is impossible to determine if it is "really" a single point or "really" everywhere at once, so it's just a meaningless question. 2. In the Newtonian "big bang" model, where you imagine the universe is now filled with particles which have a speed away from you linearly proportional to the distance from you, everything does come out from a single point! All the newtonian world lines converge on your current position. That's true even though the universe is spatially homogenous (the reason its not a paradox is that Galilean boosts are nontrivially mixed with translations). 3. the best picture, in my view, is the holographic picture, where you are surrounded by a horizon which was smaller in the past. This view is similar to the Newtonian big bang, in that everything came from a small region bounded by a dS cosmological horizon. This is mathematically equivalent to everything else, except throwing away the stuff outside the horizon that can't be observed.

I would like to admit that I was a little flabbergasted when a lay-person told me that everything in a Newtonian big-bang comes from a single place. That was completely counterintuitive.

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You should be teaching, Ron. –  Mike Dunlavey Nov 2 '11 at 2:36
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Nice answer, but think of looking at your "best answer" concerning quantum decoherence - it could be clearer. –  CHM May 14 '12 at 22:43
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Quantum mechanics is way too strange so it can't possibly be a correct description of the real world. Right? I think nothing else needs to be said about this.

Or maybe on a second thought, some more concrete beliefs and their solution are in order:

  1. The physical world has to be deterministic (it doesn't).
  2. Every possible question that can occur to you must have a precise answer by measurement (we observe only what we can, not what we want to).
  3. The collapse of the wavefunction is in contradiction with finite speed of light (no information is being transmitted).
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@Noldorin: Bohr? Really? You wouldn't find a more stringent advocate of quantum mechanics than Bohr (who I think can rightly be called its father). Einstein on the other is a different story. He wasn't able to let go of his belief that physics must be complete and answer everything we want to know. But still, this led to nice results like EPR paradox. So his inquisitive mind arrived at interesting physics even though his prejudices didn't let him accept it :-) –  Marek Nov 17 '10 at 23:33
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@Marek: That's why I put the question mark, silly. :P Who can blame Einstein in any case? I certainly don't. To call him prejudiced is not only arrogant but hugely ironic! I do not wish to continue with this debate, thank you. –  Noldorin Nov 17 '10 at 23:39
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@Noldorin: why would it be arrogant? Just consider that Einstein himself called the inclusion of cosmological constant his biggest mistake, so he himself admitted that he was too prejudiced about stationary universe and unwilling to admit its expansion (but he eventually let go because of Hubble's experimental evidence). Even physicists (especially older ones) can be prejudiced. I am not saying this with any contempt, Einstein was one of the best minds of the human kind. Just that everyone has some prejudice or other. Although in this case it's quite ironic because he helped to create QM :-) –  Marek Nov 17 '10 at 23:47
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@Marek: I think it is not only about lack of experience, but also about teaching QM in wrong way (how often do you hear "No-one knows if electric field really exists or only is our tool to describe how electron moves"? or "If someone says s(he) understand classical probability, s(he) must be lying!") –  Piotr Migdal Nov 18 '10 at 0:23
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@Marek: Yes, it is ironic, as he was one of the great figures in very early quantum mechanics. Still, I don't think that even this day we can say he's wrong! There's nothing to stop some other more fundamental theory superseding quantum mechanics and proving Einstein right. In any case, fair enough, I just suggest you keep a slightly more open mind. :) (I know too many close-minded physicists.) –  Noldorin Nov 19 '10 at 21:39
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A couple of space and sci-fi derived misconceptions:

  • An orbiting satellite needs propulsion and that orbiting is different from free fall.
  • You can actually see a laser beam in free space! I've seen this in my experimental physics class a few years back, before keychain lasers were common.
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And the misconception that orbiting satellites and other space vessels make cool whooshing sounds as they go by at 1/1000th of their actual speed by the pretend-stationary camera. –  romkyns Jan 12 '11 at 1:22
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Seeing a laser beam in free space? Aren't all photons supposed to go straight in free space instead of some of them changing path to land in my eyes? –  Kim Kim Mar 17 '11 at 16:21
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@KimKim I didn't express myself properly: I've seen people believing that. –  Sklivvz Sep 23 '11 at 12:40
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That if an object is moving, there must be a force propelling it in that direction. Students very commonly think that forces cause objects to have velocity, rather than the fact that forces cause objects to change velocity.

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Related to this, it really seems to confuse a lot of intro-level physics students when acceleration and velocity vectors are pointed in opposite directions. One of my standard quizzes when I taught freshman physics was to toss a ball straight up and down in the air, and then hand out a position vs. time plot and ask them to sketch the acceleration and velocity vectors at several key points. (neglecting air resistance). Always gets an interesting mix of answers. –  Tim Goodman Feb 21 '11 at 17:24
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Some I have heard:

  • Heavier objects fall faster (this is just plain wrong.) However, bigger and smaller objects in a typical Earth environment would fall at different rates due to air resistance, but actual mass has no influence.

  • Two cars colliding at 60 mph is the same as one car colliding into a wall at 120 mph. I think MythBusters did something on this.

  • Electrons travel really fast around a circuit: actually, they travel really slow but it is similar to Newton's cradle in that a small movement in one ball can transfer the energy to the last one almost instaneously.

  • The laws of thermodynamics have been broken by some guy in a garage with some magnets. Well, no, they still seem to be intact, and a lot of modern science depends on them!

  • That an oscilloscope trace travels faster than the speed of light on expensive high speed analog scopes (~1-2GHz.) This is not quite true: although the beam may sweep the surface of the CRT faster than c (due to the relatively small movement at the neck of the CRT), the trace cannot communicate information faster than c.

  • More related to chemistry, but the fact that water can have "memory" and all that homeopathic nonsense that con arti^H^H^H^H^H^H^H^H homeopaths spurt out.

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It's important to make the distinction about the speed of electrons, but electrons traveling slowly is only half the story. Electrons have a slow drift velocity, but they have a high thermal velocity in most situations. –  Mark Eichenlaub Dec 3 '10 at 9:19
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@Mark Eichenlaub: you can say that they move fast, but they don't travel fast. It's probably a language-related thing, but traveling implies overall displacement. –  Sklivvz Dec 3 '10 at 21:11
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Wait, what's wrong with the colliding cars? Shouldn't it be the same in all frames of reference? It certainly is the same in the case of an elastic collision. –  Greg Graviton Dec 4 '10 at 17:49
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Ah, because the energy is used to deform both cars. So, that would be like a car with 120 mph against another car instead of a wall. Would it? Kinetic energy is quadratic in velocity, but something weird happens when you switch the frame of reference. –  Greg Graviton Dec 5 '10 at 13:09
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@Greg: here's an analysis of the crash that might interest you. Also see this question. –  David Z Dec 15 '10 at 0:06
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The misconceptions about special relativity and quantum mechanics are quite well-known. A lot of the posts above discuss them in detail. So rather than doing that I'll list some misconceptions from general(say high school) physics:

  1. When a body rests on a surface the upward contact force acting on it is reaction to its weight. This is obviously wrong as action and reaction act on different bodies.
  2. There's a lot of misconceptions about non-inertial(pseudo) force. My physics teacher once said that non-inertial forces arise only when the body is in contact with an accelerating frame.
  3. Nothing can move faster than light. Of course it's false unless you add the phrase "in vacuum". The Cherenkov radiation happens when some charged particle moves in a medium with speed greater than the speed of light in that medium.
  4. Friction always has to act in the opposite direction of overall motion. Actually friction provides the necessary force for rolling without which no vehicle would ever run. The correct formulation is friction opposes the instantaneous motion of the point of contact.
  5. Light always travel in straight lines. Even without gravitational bending if we simply have a medium with a variable refractive index light will follow a curve through it. It's a nice application of Snell's law.
  6. Newton's second law provides a definition of force. It's a very widespread misconception unfortunately even among professional physics students. This strips Newton's second law of any physical content and forces(pun intended) it to become a tautology. Of course the actual content of the law is that the force is given by some other law(say gravitational or em) and it equals ma. For a persuasive discussion on this see the first volume of Feynman's Lectures on Physics. (I am very sorry that I forgot the chapter or page number).
  7. Newton's first law is derivable from second law. The proof goes as follows : F=ma. If F=0 then a=0 since $m~{}\neq 0$ QED. The problem is without first law there is no notion of an inertial frame and the laws become pointless.
  8. In special relativity the hypothesis of the constancy of the speed of light in vacuum(c) with respect to all observers is redundant because it can be derived from the principle of relativity. Of course c may vary without contradicting the principle of relativity. In fact, in Newtonian mechanics c is observer dependent it respects the principle of relativity. The constancy of c hypothesis gives the Lorentz transformation whereas in Newtonian mechanics we have the Gallilean transformation. If you are still not convinced then look at this formulation of special relativity without the second hypothesis. Google doubly special relativity.
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Also, I don't think anyone believes 5, 6 and 7 are arguable at best, and 8 is only stated by people who believe Maxwell's equations, so that the principle of relativity plus the validity of Maxwell's equations implies the constancy of the speed of light. –  Ron Maimon Aug 16 '11 at 16:51
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If you somehow manage to BREAK a law of physics, the universe will vanish!

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Fortunately if you do it will be replaced by a backup copy –  Martin Beckett Mar 19 '11 at 5:59
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As a tutor, I frequently have conversations like this:

"So we worked out that if when I toss my pen up at 2m/s, it will go 20cm high. How high will it go if I toss it up at 4m/s?"

"40 cm."

"Well, okay, let's check by working through that equation again..."

[We find out that the answer is 80 cm]

"So when I throw it up twice as fast, it goes four times as high, because it takes twice as long to get to the top, but is also going twice as fast."

"Okay."

"Now what if I throw it up three times as fast? How many times as high will it go?"

"Six."

This isn't a misconception about kinetic energy, so much as a lack of comprehension about what scaling is. When students are missing this concept, almost all of physics is more difficult to discuss.

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@Mark No, not really. They "know" the term is squared. They can instantly recite the formula KE = 1/2mv^2. But they don't really get what that means. They think that the only way to answer questions about what would happen if we throw the thing twice as fast is to plug in some numbers to the formula. The idea of looking at the exponents in formulas and gaining physical insight from that is alien to them. It's natural to you because you know basic physics and math quite well, but to students it is a weird and unusual trick. It's easy to forget how little you knew a long time ago. –  Mark Eichenlaub Nov 19 '10 at 22:25
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Yes - exactly. Building intuition about math is a slow process, but it's interesting to watch a student progress throughout the course of a year. –  Mark Eichenlaub Nov 19 '10 at 22:46
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You need something more to get something more

This is about emergent macroscopic properties of microscopical laws. Some people can't understand that statistics is powerful enough to make seemingly random heap of molecules suddenly show macroscopic properties like being solid or being magnetic and they think some hand of god is required to make this happen.

The best illustration of a contradiction to this principle are living organisms. They consist of nothing else than few physical laws all the way down and large number of molecules. All that was needed was statistics and natural selection.


I will agree though that it is quite amazing all of our nature can spontaneously emerge just from particles given enough space and time.

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@Ron: I am not saying it's obvious, only that it is possible (i.e. you don't need God to create life). And it is certainly the only scientific explanation we have, so what needs to be done is "only" better quantification of relevant processes. –  Marek Aug 16 '11 at 17:15
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The most common misconceptions are about gravity:

(1) Gravity turns off at space-shuttle orbit distance because the astronauts are weightless

Gravity is at about 80% strength compared to the surface of the Earth. The astronauts are weightless because the shuttle is in free-fall (orbit). If there was no gravity, the shuttle could not orbit.

(2) Gravity is generated by the spinning Earth. If the Earth stopped spinning, gravity would turn off.

Gravity is generated by virtue of the Earth's mass and the mass of the object; the two exert a mutual pull. There are some smaller effects associated with the spinning Earth (e.g. the Coriolis effect), but gravity would still work fine if the Earth stopped spinning.

There is another great misconception about the force of impact between a truck and a small car in a collision:

(3) The truck exerts a greater force of impact on the car, than the car on the truck

While the damage can be certainly unequal, the forces are.

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(3) is not really a misconception if force is understood in a colloquial way (normal people don't really use words like force and work in the physical sense). In particular, truck will have a lot more momentum so the situation is certainly asymmetric. But otherwise I like these. +1 –  Marek Dec 3 '10 at 16:25
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If you are riding a bicycle and you turn the front wheel to the left then the bicycle will steer to the left.

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I hear from time to time people highly educated and skilled in Physics (unlike those believing that heavy objects fall faster than light ones) making the following claim:

... quantum mechanics indicates that certain physical quantities can take only a countable set of discrete values. Consequently, many current approaches to foundational questions in physics and cosmology advocate novel discrete or 'digital' pictures of nature.

("Is Reality Digital or Analog" essay contest at FQXi)

The discrete spectra of some quantum observables do not imply/suggest that nature, in particular spacetime, is fundamentally discrete. The spectrum of a continuous operator acting on Hilbert spaces [which is a topological (vector) space, hence is continuous], often has a discrete part. This has nothing to do with spacetime being discrete. If it will (eventually) turn out to be discrete, it will be for other reasons.

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Historically the concept of absolute velocity was commonly believed until the time of Galileo Galilei in the early 17th century. As a naive child, before studying basic physics, it is surprisingly easy to believe in this even these days!

The idea of absolute velocity states that all velocities are fixed with respect to an absolute frame of reference. Galileo showed that velocity is relative to your frame of reference, a principle known as Galilean relativity. This was later fully quantified by Isaac Newton, who also proposed that acceleration is invariant with respect to inertial frames.

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Why do I have to learn this law when they change it every few years?

This has to do with a fact that some (actually, a lot of) people believe that the progress in physics is done in form of revolution and (in particular) that one day we might find laws that will contradict everything we knew up till then.

Well, if one looks closely on history of physics, it should become apparent that progress was always just evolutionary. Even when some idea needed a revolution in the way people think (as with SR and QM) it always turned out to be just a generalization of our previous ideas (so both SR and QM have nice classical limits which coincide with Newtonian mechanics).

Barring the useless philosophical views (like we might live in Matrix or we don't know whether the sun will rise tomorrow for sure) it's pretty certain that our universe is a comprehensible place and our theories are just better and better approximations to the reality. So it will always be useful to learn Newtonian mechanics, even one million years from now.

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"Burning coal for heating is more efficient than electrical due to thermodynamical losses at coal power plant"

This is false, even though it is true that converting electrical power to heat would be even worse. The correct way to heat with a given amount of heat source is this:

  • burn it at high temperature
  • make work with heat machine between Thigh and the environment
  • use the work to power an air conditioner to bring heat from Tenvironment into Troom.

This gives efficiency more than 1 (more heat brought into room than heat produced by burning), and net cooling of the surrounding environment.

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I think one common false belief is that a light mill rotates because photons deposit more momentum on the shiny side (where they are reflected) than on the black side (where they are absorbed).

I find it quite astonishing to see that many people think so despite the fact that a light mill spins to the opposide direction than predicted by that explanation.

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It depends on the vacuum you have. When there isn't a high enough vacuum, the black side is heated more than the shiny side and because of convection of air and some edge effect there is motion from the black side to the shiny side. When there is a high enough vacuum, then the radiation pressure is what moves the mill. So it depends on the light mill. –  Vagelford Nov 30 '10 at 23:04
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Someone should invent a light mill where the pressure inside can be changed enough to observe it working either way. It would be fun to show that to students and non-scientists and ask them to explain. –  DarenW Mar 5 '11 at 21:52
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Here's another list of false beliefs. These are held by science popularizers. Whether they actually believe these beliefs, or just utter them for the purpose of getting more viewers, is an unanswerable question:

The curved space near massive object can be pictured as a deformed rubber sheet

This one is due to Einstein, unfortunately. You put balls on a rubber sheet, and you see that they roll towards each other. The reason this is a terrible explanation is because you have the Earth's gravity doing the pulling, not the curved space. The actual geodesics on a curved space like the rubber sheet are repelled by the central mass. The reason things attract in relativity is because of the time-dilation factor, and this is the dominant effect. It is just as easy to explain things correctly, in terms of time slowing down near a massive object, and world-lines trying to maximize their proper time with given fixed endpoints, but popularizers never do this.

A variable speed of light can replace inflation.

This appeared in a recent popular show, and it is based on the following bogus idea: if light moved faster at early times, then all the universe could have been in communication! The reason this is false is because no matter how the speed of light is imagined to vary, one can recoordinatize space-time in terms of the intersections of light cones, and unless these lightcones split instead of merge, you get the same communication paradox--- new regions coming into causal contact are coming into causal contact for the first time.

Mesons and Baryons are made of quarks like atoms are made of protons, neutrons and electrons.

This is insidious, because its true for heavy mesons. But it's much more false than true for pions and protons and all the excitations at lower than 1GeV, because of the vacuum condensates. There is no reasonable model of light pions which does not take into account their Goldstone nature. This type of explanation also leaves out Nambu and Skyrme, both of whom were unjustly ignored for too long.

String theory is a theory of strings

This picture is not good for someone who doesn't already have a sense of string theory, because if you start out making home-made models of relativistic strings, you will never get anything like the correct string theory. The strings you naively picture would not have the special light-cone interactions that strings do in the Mandelstam picture, and they would not obey Dolen Horn Schmidt duality. They would just be conglomerations of point particles held together by rubber bands. They would have the wrong spectrum, and they would be full of ghosts.

The only proper way to say what strings are is to say right off the bat that they are S-matrix states, and that they are designed to be an S-matrix theory with linear Regge trajectories. They have a string picture, but the constraint that exchanging strings in the S-channel is dual to exchanging them in the T-channel is all-important, just as it was all-important historically. Without this, even with the Nambu action, you are at a loss for how to incorporate interactions. It is not obvious that the interactions are by topology unless you know Dolan Horn Schmidt.

It is also important for realizing that string interactions are somewhat holistic (that they become local on the light cone is the surprise, not the other way around). You add them order by order in perturbation theory by demanding unitarity, not by asking what happens when two strings collide in the usual sense. These "strings" are strange new things born of 1960s Chew-isms, and their closest cousins are flux lines in gauge theory, or fishnet Feynman diagrams, not a collection of point masses held together by spring-like forces.

This is also insidious, because Chew, Mandelstam, Dolen, Scherk, and all that generation developed the greatest physical theory the world will ever see, and their reward was: "You're fired". (in Scherk's case, "You're crazy"). Then they were heckled for thirty years, while their work was appropriated by a new generation, who described them as the deluded misguided Chew-ites who discovered something great by accident.

There is more than a snowball's chance in hell for large extra dimensions

The idea that there are large extra dimensions was very popular in 2000, but it's completely preposterous. Large extra dimensions bring the planck mass down to about a TeV, giving neutrinos generic majorana masses which are in the KeV-MeV range, so you need to fine tune. They lead to essentially instantaneous proton decay, and huge CP violations in strong interactions, so you need to fine tune some more. To avoid proton decay, there is a clever mechanism due to Arkani-Hamed and Schmalz which puts the quarks and leptons in different places in the extra dimensions. This idea is appealing only at a superficial first glance, because it requires that the SU(2) and U(1) of the standard model be extended in the extra dimensions, which affects their running immediately. The theory predicts unambiguously and model-independently that proton decay suppression requires huge electroweak running at around a TeV. That's a signal you haven't seen a hint of at 100GeV collisions. Come on. In addition, how do you stabilize large dimensions? It's the same fine-tuning as before, so the number of problems has gone up.

A low Planck scale would completely demolish the predictivity of string theory. You can squeeze a lot of stuff into large dimensions. In my opinion, it is this brand of string theory that the critics correctly criticize as fundamentally non-predictive.

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One widespread belief (I think due to popular books such as Hawking's) is that GTR can never ever, ever be quantized and you always obtain infinities and blah blah. Well, it can, in many situations and in many theories. What is actually meant is that GTR is not a renormalizable quantum field theory in the naive way. But this specification is never explicitly pointed out so people get a false impression that quantum gravity is something completely out of the realms of current physics. Well, surprise, surprise, it's not. We can quantize many gravitational effects (such as waves), we understand that black holes have entropy, we understand they produce Hawking radiation and eventually disappear, etc. And assuming that string theory is correct we can predict whole deal more about it.

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Some sport instructors would tell you:

Running on a treadmill is easier because you only have to jump, while on the street you also push forward

I suggested then that running in a train should be the easiest, by this line of thought. However, it is true that starting to run (or, accelerating) is indeed easier. Or the air resistance, unless there's a slowest forward wind. Or, the randomly changing slope. Also, lack of the air conditioner. That pretty much summarizes the difference.

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As an instructor, I have great difficulty teaching Newton's 3rd law: "For every action there's an opposite and equal reaction." This is very basic and very old physics but it's hard to teach. A typical example of the false reaction force answer is the following:

I hold an apple in my hand. The earth pulls down on the apple with a gravitational force. What is the reaction force to this?

The answer most of them will give is that the reaction force is "my hand pushing up on the apple." Arggghhhh! Of course the reaction force is "the apple pulling up on the earth."

Students fail to realize that the opposite and equal reactions have to be between the same pair of objects. That is, forces arise as pairs. I wish they'd just rename the law so that it makes it more clear that the reaction force has to operate between the same pair of objects.

I demonstrate the law by holding a long spring in my hands and telling them that forces are like this spring. When it applies a force on one end it applies a force on the other (assume massless spring). Next quarter I'm going to try some more extreme measures on this, clearly I'm failing.

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While the other answers are absolutely correct, they are very subtle in the way they appear. However one very large false-belief in electrical science is that current is taken to be "flowing" from the + terminal to the - terminal in DC current.

While it doesn't really matter which way you choose since you are dealing with close to light speed current, electrons are actually moving from - terminal to + terminal(do not get me wrong electrons are NOT moving anywhere near light speed while drifting. They are moving in the order of centimeters per second.). Hence the current actually moves from - to +.

And it would be interesting to note that not a soul in the professional area of electrical science considers the current from - to +, as it would cause inconsistency with his/her colleagues.

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If current always flows as electrons from - to +, then please explain how the Hall coefficient can be positive in many materials, such as p-type semiconductors. –  Keenan Pepper Nov 18 '10 at 0:02
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The best explanation I heard for this was when they came up with the diagrams they had no idea what an electron was. So they just picked a direction. Turns out that they had it backward and what's actually flowing from + to - is the absence of electrons (holes). –  jcollum Nov 27 '10 at 18:25
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"Long hair grows slower" is due to biological effects

In fact, this is a purely mathematical phenomenon. The bigger average length, the more decrease is caused by each hair fall. This leads to a differential equation $$\frac{dL}{dt} = K - \alpha L$$ that has a solutions which decay exponentially to an equilibrium at $$K/\alpha$$.

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@Pavel: sigh... are you intentionally misinterpreting my statements? I never said single hair is important. Just that your average doesn't account of single hairs (in particular, the long ones) and that the average is not important for this question. In reality what's important is that there is enough long hairs. The girl couldn't possibly distinguish whether she had 10000 30cm hairs or 5000 20cm hairs and 5000 40cm hairs. Your average would come out the same but obviously in the second case her hair would seem 10cm songer. Your model doesn't account for this at all and therefore it's useless. –  Marek Dec 3 '10 at 9:18
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@Pavel: sure, that's what I am saying. But there are many such characteristics and you used just one: the average. And this is the most crudest one and most irrelevant too. –  Marek Dec 5 '10 at 12:38
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the belief that cartoon-universe rules apply to falling objects:

as embodied by the statement that, "if you are trapped in a falling elevator, you can avoid destruction by jumping at the last moment, so that when the elevator hits, you are in the air, and only fall the last inch or two."

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It really bugs me. I've even heard from some quantum mechanics lecturers that they think quantum entanglement implies faster than light communication!

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Quantum theory is not non-local (and I am not really sure what you mean by that intergalactic invasions, @Pavel)! This could be put as another misconception stemming from EPR paradox. What EPR (or rather Bell's inequalities) say is that quantum theory is either non-local or incomplete (in a sense of absence of hidden parameters). We have good reasons to think it is local (e.g. QFT has to obey locality if it is to make any sense) so what these experiments actually concluded is that there are no hidden variables (i.e. QM is not just statistics). –  Marek Dec 5 '10 at 12:34
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@Pavel: okay, done. It's interesting but it's still just the good old correlation of entangled spins, nothing more. Calling it non-local (or second-best, or whatever) is just confusing and precisely the reason why people think there is superluminal communication going on. –  Marek Dec 5 '10 at 17:55
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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.

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