Most misunderstood physics concept

[Greg Bernhardt]

What do you think is the most misunderstood concept in physics and why? I'm guessing it's something in QM or relativity, but maybe somewhere else?

 

[George Jones]

Misunderstood by whom? By the general public? By physics students? By physicists? Examples from any of the previous categories?

 

[Greg Bernhardt]

Misunderstood by whom? By the general public? By physics students? By physicists? Examples from any of the previous categories?

I was thinking joe public but both might be interesting!

 

[Nugatory]

I'm guessing it's something in QM or relativity, but maybe somewhere else?

Well, those certainly are target-rich domains. Relativity has "Time slows down as you move faster" and "infaller never crosses the event horizon". Quantum mechanics has the endless misunderstandings generated by what I sometimes call the Copenpop Interpretation: "particle went through both slits", dead and alive cats, conscious observers collapsing wave functions, and the like.

Classical physics has its own conceptual hard spots: Newton's third law and the horse-cart problem; fictional forces and rotating motion; come to mind. However most people get through these fairly quickly once they see a competent explanation.

 

[OmCheeto]

...the Copenpop Interpretation...

O.M.G.

Now I have to buy 3 more t-shirts...

ps. I don't even try to understand QM anymore, so my personal latest "most misunderstood" concept is entropy: "It's like a jigglyness per cubic meter, or something like that..."

 

[fresh_42]

O.M.G.

Now I have to buy 3 more t-shirts...

(Att., opinion) I had to laugh, too, but please don't. It's a bit unfair with respect to the time it was created. Everything was in development and people tried to grasp the new ideas. To make fun of it almost a century later is a bit mean.

ps. I don't even try to understand QM anymore, so my personal latest "most misunderstood" concept is entropy: "It's like a jigglyness per cubic meter, or something like that..."

Yeah. Something with $- n \log n$, wasn't it.

 

[phyzguy]

I'm not sure that you have to go to QM or special relativity to find widely misunderstood concepts. Two simple concepts that I find are widely misunderstood by the general public are:

(1) Newton's first law. Many people think that I have to keep pushing on an object to keep it moving.
(2) The difference between energy and power. I find even people working in the power industry don't get this right. Look at this link, for example. It says,

"On Thursday, the California Public Utilities Commission unanimously approved its proposed mandate (PDF) that will require the state’s big three investor-owned utilities to add 1.3 gigawatts of energy storage to their grids by decade’s end."

Say, what? Gigawatts is a measure of power, not energy. Storing 1.3 gigawatts is easy - I can probably hold a capacitor in my hand that will do that. Of course, it won't do it for very long...

 

[Rive]

The most popular misunderstanging might be something else, but I think the most common will be something around mass <> weight.

 

[rootone]

That 'a singularity', such as the one referred to in the case a black hole, is a physical entity with defined properties.

 

[Andy Resnick]

What do you think is the most misunderstood concept in physics and why? I'm guessing it's something in QM or relativity, but maybe somewhere else?

Force. The overwhelming majority of people have a pre-Newtonian view of inertia and regularly confuse force with velocity.

 

[TurtleMeister]

My favorite has to be the concepts of inertial mass, passive gravitational mass, and active gravitational mass. Here's a thread about it:
https://www.physicsforums.com/threads/difference-between-gravitational-and-inertial-mass.689095/

And also just general misunderstandings about Newtonian gravity such as the universality of free-fall and the equivalence principle.

 

[Nugatory]

(Att., opinion) I had to laugh, too, but please don't. It's a bit unfair with respect to the time it was created. Everything was in development and people tried to grasp the new ideas. To make fun of it almost a century later is a bit mean.

I'm not making fun of the Copenhagen interpretation, I'm making fun of the popularizers who to this day continue to misrepresent quantum mechanics.

 

[Devils]

Something simple that students have trouble with is how rockets can move in space ie speed up or change direction, when there is nothing to "push" against. At this learning stage they probably know about Newton' laws, but need to understand conservation of momentum.

 

[RogueOne]

I saw this "brain teaser" on facebook and was appalled by the amount of people who got the wrong answer. I am not sure where to begin when describing the misconceptions that I heard, but it was saddening. This is very obvious to anybody who understands any physics

Here is the riddle.

 

[Bandersnatch]

This is very obvious to anybody who understands any physics

I don't think it's that trivial.

 

[OmCheeto]

(Att., opinion) I had to laugh, too, but please don't. It's a bit unfair with respect to the time it was created. Everything was in development and people tried to grasp the new ideas. To make fun of it almost a century later is a bit mean.

As a subscriber to the Copenpop interpretation, I can assure you, I was not making fun of Quantum Mechanics.

Yeah. Something with $- n \log n$, wasn't it.

Good lord. I just checked wiki, and it says that "Entropy" has the units of "energy/temperature".
So, this implies, that as I slow down in the winter, and my body temperature stays the same, my entropy goes up?*
I will never understand QM, nor entropy, as both seem to be described by incomprehensible maths.

(I spent the last two days trying to relearn how to derive an equation from a curve fit of a set of numbers. My spreadsheet gave me the answer; "It's a parabola, Om!", but I decided that I can no longer comprehend even the simplest of maths, to do it on my own. Fortunately, I can still add and subtract.)

*I promise not to start one of those whackadoodle threads: "Entropy is wrong!"

 

[TurtleMeister]

I don't think it's that trivial.

I think what makes it non-trivial is that it is badly worded. I think what is actually meant is that the conveyor will match the ground speed of the aircraft in the opposite direction. Of course the aircraft can take off regardless of how fast the conveyor is moving, provided the ratings of the tires are not exceeded, because an aircraft does not depend on it's wheels for propulsion.

 

[Bandersnatch]

I think what makes it non-trivial is that it is badly worded.

Yes. Specifically, the way it's worded, any non-zero ground speed would make the conveyor try and accelerate to infinity. Then the question becomes: 'for realistic belt accelerations, can the plane take off before its wheels explode/break off/create more friction than the engines produce thrust?'

And to stay on topic, for the most misunderstood concept, I nominate Big Bang. It seems especially prone to breeding misplaced sense of understanding both in laymen and in people who should know better. I blame the name.

 

[Devils]

I was reading about answers to the aircraft problem and the moving belt will interact with the air, and the air will move due to a surface effect. So the air will move a well as the belt. That's not simple.

 

[rootone]

It might give the plane a tiny amount of lift, but no more than an average gentel wind, the plane stays on the ground.

 

[Ben Niehoff]

No-one seems to have mentioned "relativistic mass" yet.

 

[Stavros Kiri]

Two often ones are also 1. Vacuum 2. Inertial frames [vs non-inertial frames] and inertial forces (centrifugal, centripetal and/or inherent internal force in a non-inertial frame ...) [a lot of confusion there by everyone!]
These are not easy, by the way.

Another common one is Big Bang theory and multidimensionality.

 

[jfoldbar]

i definitely fall under the category of joe public. i am not uni educated so most physics stuff goes over my head. the stuff i do know though interests me though and i wish i knew more.
for me, the misunderstood thing is probably "how can light supposedly be massless". i don't get how anything can have zero mass at all. and i don't think i'll ever "get" it.i like the topic of the plane taking off. is there are thread specific to that anywhere?

 

[Stavros Kiri]

for me, the misunderstood thing is probably "how can light supposedly be massless". i don't get how anything can have zero mass at all. and i don't think i'll ever "get" it.

The rest mass m₀ of photons is zero, but we can never see photons at rest. Photon traveling at velocity c does actually have mass [equals m=hf/c[SUP]2[/SUP], where f is the frequency of the photon]. So it's all consistent and understood!

 

[nuuskur]

Weight vs mass.

 

[mfb]

Photon traveling at velocity c does actually have mass

No they do not. "Mass" always refers to rest mass in physics.
The concept of a relativistic mass is not used any more. It just survived in bad pop-science descriptions.Energy-mass equivalence is misunderstood frequently.

 

[Stavros Kiri]

"Mass" always refers to rest mass in physics.

You mean as a quantum number?

 

[mfb]

Mass is not a quantum number.

 

[Stavros Kiri]

Mass is not a quantum number.

m₀, q, S, etc. ... standard particle physics descriptions ... that's what I mean

 

[Stavros Kiri]

+

No they do not. "Mass" always refers to rest mass in physics.
The concept of a relativistic mass is not used any more. It just survived in bad pop-science descriptions.Energy-mass equivalence is misunderstood frequently.

Isn't for a photon E=mc²=hf ?
What's wrong with that?

 

[mfb]

+

Isn't for a photon E=mc²=hf ?
What's wrong with that?

That is correct. But it has nothing to do with mass.

Please start a new thread if you want to discuss this further. Or use the search function, we had many threads about that topic already.

 

[Stavros Kiri]

That is correct. But it has nothing to do with mass.

Please start a new thread if you want to discuss this further. Or use the search function, we had many threads about that topic already.

Thanks. I will look at the threads, time permitted.

 

[papercace]

There is no gravity in space.

 

[jfoldbar]

like i said. i will never "get" it

 

[OmCheeto]

Mass is not a quantum number.

Thank you.
I've been looking for that phrase; "quantum number", apparently, for many years.

Things are starting to make more sense now, and I haven't even started researching this yet.

{edit: I may have just given Tom Mattson the most belated "like" in PF history. [ref] }

 

[davenn]

The one I see over and over again in general physics and EE on here and on many other forums, dozens and dozens of times

" I can't use power supply that can supply more current than what the device is specified for, else the device will burn out "

to which the answer always is ... "No, the device will only draw what it needs (specified requirement)"

then they respond " so what happens to all the other amps ... eg a 1A device on a 10A PSU?"

Answer ... " it doesn't go anywhere, it wasn't there for a start "

Dave

 

[JoePhysics]

The Coriolis force and how some people think it has something to do with the direction in which water swirls as it drains in ordinary-sized objects (bathtubs, toilets, sinks, etc.) on either hemisphere of the planet.

 

[rootone]

like i said. i will never "get" it

A photon has zero REST mass, but a photon cannot be at rest, it is always moving at 'c' for every observer.
This lead to the misunderstanding that photons acquire 'relativistic mass', which idea is now discredited.
However photons still are 'things', and any thing has some momentum. depending on the observer.
For visible light, photons with more momentum are at the blue end of the spectrum, less energetic photons at the red end.

 

[DaveC426913]

Most misunderstood concept?

Science.

And getting worse every day.

 

[Stavros Kiri]

The one I see over and over again in general physics and EE on here and on many other forums, dozens and dozens of times

" I can't use power supply that can supply more current than what the device is specified for, else the device will burn out "

to which the answer always is ... "No, the device will only draw what it needs (specified requirement)"

then they respond " so what happens to all the other amps ... eg a 1A device on a 10A PSU?"

Answer ... " it doesn't go anywhere, it wasn't there for a start "

Dave

They confuse it with voltage. Is that it?
[Voltage and Resistance (or Impedance) give rise to current ... (that means they don't understand Ohm's law properly etc.)]

 

[davenn]

They confuse it with voltage. Is that it?
[Voltage and Resistance (or Impedance) give rise to current ... (that means they don't understand Ohm's law properly etc.)]

no, your second sentence ... a given load resistance will only draw xx amps ... according to Ohms law

 

[Stavros Kiri]

no, your second sentence ... a given load resistance will only draw xx amps ... according to Ohms law

Ok

 

[phinds]

That 'a singularity', such as the one referred to in the case a black hole, is a physical entity with defined properties.

Yeah, that's my candidate as well.

 

[Thecla]

What is the length of a photon? I have heard a wide range of answers .

 

[Richard]

no, your second sentence ... a given load resistance will only draw xx amps ... according to Ohms law

A given resistance will only drop so much voltage based upon the current at that point (E=I*R isn't it?). However total current use as a whole would be computed using Kirchoff's Law. ;)

BTW: What was the big deal with the airplane on the treadmill? If I understood the description right. It had zero net forward motion, therefore no lift. Aerodynamics of air movement above and below the wings makes a plane fly, not engine thrust, or wheel speed. Or am I wrong?

 

[davenn]

A given resistance will only drop so much voltage based upon the current at that point (E=I*R isn't it?). However total current use as a whole would be computed using Kirchoff's Law. ;)

that isn't the problem with the ongoing same question from so many people

they don't understand that a 10A capable PSU ISNT putting out 10A continuously ... rather it only puts out what is drawn by the load

 

[OmCheeto]

...
BTW: What was the big deal with the airplane on the treadmill? If I understood the description right. ...

bolding mine

IMHO, this is why most riddles, especially poorly constrained ones, should NOT be included in "misunderstood physics concepts", as they are open to interpretation.

747 nerds; "If there is a 181 mph headwind, then yes, it will start flying."
Pedants; "Define 'take off'."
Spork and friends; "Well, if you add a propeller to the top, and there's a tailwind...[14 threads and 2 bazillion posts later]...Still not convinced? Here, watch this video."
etc, etc, etc.

ps. I see no one mentioned "temperature" yet:

I guess I still don't understand what "temperature" means...

[edit] Just in case you are interested, and to prevent a thread hijack: Aeroplanes & Conveyor Belts [PF: 2003-2008]

 

[dkotschessaa]

I think hands down the most misunderstood concept is energy. It is used for all manner of whackadoodle new age nonsense.

 

[Nidum]

I'd say that the most misunderstood physics concept of all is just the principle of action and reaction .

 

[nuuskur]

There is no gravity in space.

in tears reading this :D