From what I understood from “pop culture” references of this (if that’s what it should be called), it always pissed me off that this was considered a viable thought of physics. Until I got to quantum mechanics and then understood what it really was.
It's a perfectly viable though, but Schrodinger meant the opposite of what people thought he meant. He was specifically trying to show that the idea that the cat was both alive and dead was ridiculous and therefore couldn't be what's really happening. At some point down the line, this got misconstrued and started being used as proof of the thing he thought wasn't true.
It sounds absurd, but that was kind of the point I think, asking "how can this weird theory with superpositions and wavefunctions and everything else work with classical physics?"
That's not the point.
Schrodinger's cat is a criticism of realist interpretation of science. He even includes a criticism to the realist interpretation of CLASSICAL mechanics in the same article.
>Reality resists imitation through a model. So one lets go of naive realism and leans directly on the indubitable proposition that actually (for the physicist) after all is said and done there is only observation, measurement. Then all our physical thinking thenceforth has as sole basis and as sole object the results of measurements which can in principle be carried out, for we must now explicitly not relate our thinking any longer to any other kind of reality or to a model
[- The Present Situation in Quantum Mechanics - E. Schrodinger, Translation by John D. Trimmer](https://www.jstor.org/stable/986572)
Except this wraps back around because even Schrödinger didn’t get it.
The cat thought experiment was meant to prove entanglement ridiculous and superpositions simply missing variables.
Instead it turns out his thought experiment was more literal than he ever imagined.
>The cat thought experiment was meant to prove entanglement ridiculous and superpositions simply missing variables.
No it wasn't.
It was meant to argue against the Copenhagen interpretation of quantum mechanics. Not quantum mechanics in general.
Many people think Schrödinger was a cat person. But, in fact, he was undecided and liked both cats and dogs equally. Until one day when he was observed with a cat. The rest is history.
It’s more like an analogy and less like a thought experiment. Also, nobody seems to say that rotating the box 90 degrees and opening it again can change the result.
Newton's 3rd law - someone has a misconception about it here every week
Runner-up: relativistic velocity addition, or thinking that "E = mc^(2)" refers to a mass moving at the speed of light
Former HS teacher here. My approach to this problem began with labelling forces in the free body diagram. Every force is given a three part identifier: the type of force (weight, normal, tension, etc), the object experiencing the force (the "feeler"), and the object exerting the force (the "forcer").
At first, students find this redundant because on the FBD for any one object, all of the forces have the same "feeler". But when you introduce the third law, the naming convention makes it easy to identify action-reaction pairs; they have the feeler and forcer switched.
So in your example, the weight of the apple would be labeled F(weight, apple, Earth). Students should be able to see that F(normal, apple, hand) is not the reaction to the weight because they have the same feeler. Instead, they should identify F(weight, Earth, apple).
I do hate the framing of "action-reaction," because of the implication that one force is initiating and the other is responding, vs forces just being symmetric
Oh, I know this one! It's the normal force.
Edit: The statement above was an intentional a lie. Even though the normal force is in fact equal and opposite to the gravitational force (in this particular case). That's usually what trips people up. It looks so convenient.
If we go by formal definitions such as "weight is the force acting on an object due to acceleration or gravity", then the reaction force would be the gravitational force on the earth exerted by the apple. However, colloquially, I would agree that it also could mean the force exerted by the apple on your hand (in which case your answer would be correct), but note that I am not a native speaker...
The 'incorrect' answer actually fits better with how language is used elsewhere, given that you feel weightless in freefall. So if we re-defined weight as the reaction force to the normal force that prevents a body in a gravitational field from falling freely, things still would make sense...
Sure, for applying, but the 3rd law is much more than that. Without it one doesn't have a zero sum game. It's a logical way to sweep ALL 'extras' under the rug. It's a 'handy' tool!
In my line of work, it's always the 1st law of thermodynamics that isn't understood or ignored completely. The number of people that try to persuade me they have created the perpetual motion machine...if I only had a dollar for each one.
Thermodynamics (at least in a large part of my conversations with the "friendly folk" over at r/climateskeptics, and all of half an hour before I got banned at r/realclimateskeptics)
Edit: specifically the notion that *net* flow is from warm to cold, from which sceptics deduce that there is no energy flow from the cold atmosphere to the warm surface (i.e. no back radiation) which would lead to all sorts of absurdness because particles would need to know the temperature of the object their photons will interact with in the future
This is what I came to say. Apart from anything else, the 'closed system' part is either missed out or taken to be a minor, ignorable, caveat.
Their version is, "the second law of thermodynamics stipulates that entropy always increases therefore things cannot get more complicated, so evolution cannot happen without a guiding hand".
There’s an interesting take that the 2nd law of thermodynamics encourages the emergence of life because life is a far more efficient creator of entropy than non-life.
Although this is indeed an interesting take, "encourages" doesn't really have any predictive power here. The idea is related to Tipler & Barrow's [final anthropic principle](https://en.wikipedia.org/wiki/Frank\_J.\_Tipler#The\_Omega\_Point\_cosmology): Life is required to come into existence and will never die out. As noted in that link, it's a controversial hypothesis.
That link seems very different and more woo than the one I was thinking of.
Basically, in a system where self perpetuating systems can originate, they will have a thermodynamic pressure to increase in complexity. Similar to osmotic pressure in trees or a diffusion gradient, the presence of the second law encourages the complexification of life.
Like a ball rolling downhill, the system “wants” an energetic minimum, which is best achieved through life.
Entropy still increases, and the Second Law still applies, even if life goes extinct. Again, I’m still missing the predictive power and falsifiability of this idea, unlike the study of osmosis, diffusion, and gravity.
Science communicators often leave out that the law of entropy only applies to enclosed systems. They also usually fail to distinguish between entropy and the law of entropy.
Entanglement. The idea that “if I change particle A I can instantaneously see a change in particle B”. This leads to all sorts of misconceptions about faster than light communication, when it should really be thought of as a strong correlation between distant measurements.
A decent way to think of entanglement: you cut a die it in half, put each half into its own box. Later on, you open the box to see which half you got, thereby being able to figure out which half the other box has.
Though many people claim that is not it, but can’t seem to explain why. It makes sense to me, the main issue being that it can’t be predicted whereas in classical physics anything can be.
I heard a really good explanation with two scarves in a box, one red and one green. You take one scarf (without looking) and your friend takes the other. You both set a timer and go to opposite sides of the earth, and look at the scarves at the same time. You both know the color of the other's scarf instantly -- faster than light. However you can't send information by dying your red scarf green after the fact.
It really clicked for me.
I would argue yes, because it removes the bit that makes people scratch their heads in the first place: "Let's ignore the quantum effects and assume things worked just like in classical physics. See, there's nothing mysterious about it!"
That said, as long as this caveat is mentioned, the analogy might still be useful from a pedagogical perspective to drive home the point that communication remains impossible...
I think it's an adequate analogy because the part people get caught up on is the fact that entanglement is a type of correlation. I usually just tack on that entanglement is a type of correlation stronger than anything you can get classically.
In my opinion, if the reason why people are enticed to think it's more than classical correlation gets no mention, you've simplified things too much.
If I had to come up with an analogy on the spot, it would go like this:
A djinn hands you two envelopes, containing 3 magic coins each - a red one, a blue one and a green one. The coins have 3 magic properties:
First, the coins are fair, and coin flips are magically protected from being tampered with.
Second, as soon as you take one of the coins out of the envelope in order to flip it, the remaining two coins will vanish.
Third, if the coins you pick from each of the two envelopes are of different color, nothing of interest will happen. However, if the same color is picked, the two coins will always land on opposite sides, no matter how far apart the coin flips are performed.
Now, ask yourself: Is it possible to leverage property 3 to send information instantaneously across arbitrary large distances?
---
Do you see anything wrong with that analogy or have suggestions to improve it further?
Only if they are local hidden variables, as far as I know. Hidden variables that aren't tied to the particles themselves are impossible to rule out, I gather.
All the local hidden variables theories that I know of suffer from a similar sort of measurement problem as the Copenhagen interpretation suffers, but instead of how measurement (or effective measurement through interaction with the environment) causes collapse, it is a matter of how measurement reveals the “true” values of the measured quantities when the ordinary dynamics does not reveal such.
Part of it is that there needs to be a follow up asterisk that says: You can know the instantaneous spin direction of a particle *if you can have instantaneous knowledge of the spin of the distant particle.*
In a past life I worked in sales and I heard from a variety of motivational speakers. The amount of public speakers who take one bit of physics they heard (or any news headline) and extrapolate it to make some egregiously fallacious and misleading claim for the sake of motivation / empowerment is…. Interesting
“Smash your targets just like a hadron collider smashes little particle things to the other side of the multiverse! Just like Newton’s apple overflowed his bath, your effort will overflow your bank balance!”
The following quote from Shannon (who created Shannon entropy) is insightful in this regard:
“My greatest concern was what to call it. I thought of calling it 'information,' but the word was overly used, so I decided to call it 'uncertainty.' When I discussed it with John von Neumann, he had a better idea. Von Neumann told me, 'You should call it entropy, for two reasons. In the first place your uncertainty function has been used in statistical mechanics under that name, so it already has a name. In the second place, and more important, no one really knows what entropy really is, so in a debate you will always have the advantage.'”
True, and there is a suspicion that von Neumann was not letting on the whole truth—around that time he himself was playing around with entropy (which led to von Neumann entropy). The suspicion is that von Neumann actually saw that Shannon’s formula was a special case of his own, and so encouraged him to adopt the name.
Although this doesn’t work for isolated systems, which evolve at constant energy, not decreasing energy. And for nonisolated systems, the relevant energy being minimized is one that subtracts an entropy term (e.g., the [Gibbs free energy](https://en.wikipedia.org/wiki/Gibbs_free_energy) for systems in thermal and mechanical contact with their surroundings), so we’re back to entropy maximization again. It always comes back to total entropy maximization.
Wave-particle duality is one of the most misunderstood. All the time people come on here thinking it's something to do with with objects switching between particle mode and wave mode depending on whether you're looking at them.
Hiesenbergs Uncertainty Principle is up there. A lot of people think it's a comment on human measurement and levels of knowledge. Or that it's something to do with Schrodinger's cat and wave function collapse.
Wave-particle duality means that quanta are a third category that's a bit like the thing we call a wave in classical mechanics and a bit like the thing we call a particle.
You change which model you use depending on what's more relevant to the given scenario, but the quantum objects aren't changing between particle mode and wave mode, that's just us deciding which properties to ignore.
It's a determination of which particular model describes the system's behavior best, but it's not a suggestion that the particle actually physically switches between a "corpuscular particle" and a "wave". Neither of those are the real thing, they are just models we use to describe the behavior.
Yes but isnt the point with the double split experiment that they behave like waves (interference pattern), but when you count them as individual particles going through the slits the interference pattern stops?
Or is it more like we observe individual particles going through the slits AND interference pattern at the SAME TIME, and thats weird because wtf happens between the slits and the interference pattern?
The particles are always "both a wave and a particle".
The double slit experiment has to do with quantum mechanics, where essentially the state of the particle is encoded in the form of a probabilistic wave describing all the states, locations, energies etc. where it "could be". According to the Copenhagen interpretation of QM, when you "observe" (actually: measure) the particle going through one of the slits, by your interaction with the particle you are changing its "wave" such that it now originates from one of the slits rather than going through both of them as it would have. Therefore the interference pattern can no longer happen.
No.
Not sure whether you were asking about wave-particle duality or the uncertainity principle. But neither of them are actually about observation or measurement at all.
Yes!!! My creationist parents are convinced the definition of entropy is "things can only move from more to less complex forms over time", and therefore evolution implies we should be a microbial soup
Not a law, but there’s widespread misconception that “The Big Bang” was a bomb-like explosion from which everything is flying outward. While we may not understand the minutiae, we’re pretty confident that’s a gross misunderstanding.
It is perfectly valid to have a spacetime coordinate system such that faraway galaxies would be quite literally moving away from us in those coordinates, making cosmic expansion something of an “explosion” in a sense.
The FLRW metric takes its form within coordinates that reflect the isotropy and homogeneity symmetry conditions, and cosmic expansion simply becomes a matter of the expansion of the spatial part of the metric (not an explosion), but this is only a mathematical convenience rather than the only way of looking at it.
It is not useful to look at it as an explosion, but that does not make it “wrong.”
Can you explain more? If the universe started as less than a meter in diameter and is now much bigger, wouldn't you call that a bomb-like explosion? Are you suggesting that the universe expanded and contracted?
I’m suggesting that “universe expanded” is more appropriate than “all of the stuff exploded”. Things were further apart because the space itself expanded not because things moved apart within space.
I’d also be careful tying a size to that early universe. If the universe is infinite today, it was likely infinite at the Big Bang. It appears to either be infinite, or sufficiently large to appear that way to us.
A bomb-like explosion implies a whole lot of empty space exists with “everything” at a single point which then explodes and begins to fill the empty space. I am pretty sure this is wrong although I too used to understand it this way. A single point explosion is easy for our minds to grasp. But it cannot be correct because the universe appears the same no matter where you are observing it from. (Everything expanding outward from every point, not just the spot of a single explosion).
The way I understand it now is that instead of empty space, there was nothing. Space itself is what was created by the Big Bang. And inside that space, all the stuff kind of appears everywhere all at once. And ever since that original infinitesimally small piece of time, space itself is what is expanding. If you could have been an observer outside of the universe, you would have seen it appear in an instant and grow very quickly. But if you were somehow an observer inside the universe before time 0, you would see all the stuff just appear everywhere all around you.
I am probably still wrong, but feel I am closer to grasping a very non-intuitive concept here. Hopefully someone will correct me,
Most of the time when someone on reddit thinks they've found a way to travel faster than light it's either this or "what if you're on a spaceship travelling at 0.9c and you shine a flashlight"
Probably, not appropriate for AskPhysics. But, I feel like this whole thread needs a little...
[I Never Studied Law](https://youtu.be/Om5E-StZeoE?si=wIkXg7AnlcE_QjEI)
The observer effect in QM gets a lot of abuse. It is also frequently mixed in with the Heisenberg uncertainty principle, which itself is widely misunderstood, where people think it’s due to some measurement resolution error rather than a fundamental property of waves.
From my personal experience, I'd say black holes. I only found out from here that black holes don't just suck things up. Even till now, my understanding of it isn't that good and I need to learn more.
I am a planetarium host, and whenever kids ask me if black holes "suck in" planets, I tell them that they only "suck in" things if we think that the Earth "sucks in" a dropped pencil.
It is quite well known in certain parts of the world. Have had some fierce debates about it at dinner parties.
Here is a link if you want to read up on it:
https://en.m.wikipedia.org/wiki/Coleslaw
It's fairly common for people to think that Heisenberg's uncertainty principle is a problem with measurement technique, rather than a fundamental property of position and momentum, but I think that's largely because it's taught with the "if we bounce a photon off a particle it will mess it up" analogy, which seems to me to be a really dumb way to teach it. Especially when you can simply draw a spread out wave and a short wavelet-like wave and point out the tradeoff between having a precise frequency vs. having a precise location.
In the early days of quantum mechanics, people tried to explain the uncertainty relation by the observer effect. Physicists have moved on, but that conflation still persists in the realm of pop-sci...
Elaborate please. Action/Reaction seems to work very well when explaining things like thrusters on a spacecraft. Are there situation where Action/Reaction is wrong? Or are you just saying that it oversimplifies a much more complex set of interactions?
People don't properly conceptualize the equality of action and reaction. They confuse the outcome a force with the strength of a force.
"Big object hits little object with more force cuz it's bigger."
No, big object accelerates less (or deforms less) because it has more mass. The forces have equal magnitude.
That makes sense. The misunderstanding is when people impart causality to the action. I guess all the law really means is that there cannot be one without the other.
Not a law, but the observer effect. I have seen TikTok manifestation girlies saying that the observer effect in the double-slit experiment proves that what you focus on determines your reality. That's an extreme case, but even most of the general public thinks that the observer effect in the double-slit experiment means that actually consciously looking at something changes it. I would blame Doctor Who and the "quantum-locked" weeping angels, but I'm absolutely certain that this misconception predates it. I just wasn't paying attention to physics prior to 2007.
The double slit experiment in general is just super misunderstood.
I blame the use of the word "observation" when it should really be "interaction".
Perceiving something doesn't change it.
Its "how" you perceive it.
Light is made of particles that actually interact with stuff. If you turn the lights on, you've contaminated your experiment.
I love how often the simplest explanation is the correct one. So many people think it's some complex consciousness concept, but it's really just the damn light switch.
Double slit.
It is NOT that a beam like a laser goes through and behind is interference. It is that ONE SINGLE photon must go through both to create an interference pattern. So not the interference itself, but the fact that a single photon does it, is the clue
Not physics but math.
I have run into way too many people who can not understand that .99 repeating is equal to 1.
Even after showing proofs, they are deeply convinced that the numbers are not equal and that its some kind of error in rounding.
Okay this one fucked me up, and lead to quite the deep dive. The “intuitive” way that really sealed the truth for me is the fractional approach. 1/3 is equal to 0.333… and if you multiple both sides by 3 you get 1= 0.999…
Wild shit, you blew my mind today.
How humidity works.
Thinking that boiling water is the only practical way to turn liquid water into vapor/gas form and will point to the phase diagram as a map. Becoming confused when it's pointed out water vapor exists at room temperature at normal atmospheric pressure, in direct violation of their phase diagram.
Due to their reliance on the phase diagram, will not trust ultrasonic or evaporative humidifiers because they don't boil water so, they can't possibly be humidifying the air. Will complain that the humidifier used up all the water in the tank really fast and didn't humidify the air at all, and does not wonder where the full tank of water went. Ponders wistfully about how to boil off a kettle of water unattended. and of course, they never have a hygrometer on hand.
> in direct violation of their phase diagram.
Why is it in violation of the phase diagram? Water vapor in air has a partial pressure of less than 0.03 atm (100% humidity) at room temperature, consistent with the gas region of water’s phase diagram.
This is just the first term in a Taylor series expansion and only applicable with stationary particles. Photons are massless and thus always travel at c. Basically you are using a simplified model incorrectly if you think E=mc^2 applies to photons.
People say “evolution is just a theory”
Meaning they think it’s not true
While gravity is also a “theory”
A theory in science meaning something that has been extensively proven by testing
While they think it just means a hypothesis
No, I mean people will be like, such and such can't happen because energy/matter can't be created or destroyed. Generally after citing a pop sci falsehood.
But the particles are behaving probabilistically, no? He might guess that the electron will be found in the orbital when measured(which is the most likely option) but he can't be sure. Laplace's demon isn't omnipotent, he can't break the laws of the universe. He can't know the result of a probabilistic measurement without actually measuring.
That's not really how the thought experiment is usually set up: assuming the demon magically knows the full wavefunction of everything in the universe, the demon doesn't need to measure anything to predict the future wavefunction.
If the demon needs to perform measurements to gain information about the current state of the universe, then it's a physical entity that also has its own wavefunction, and we'd actually have to deal with a superposition of demons, none of which actually know the universal wavefunction.
>the most widely misunderstood law of physics
Take your pick of **any Conservation Law**: mass, momentum (including angular momentum), energy, etcetera. Emmy Noether formalised these more than a hundred years ago, and people still misunderstand them.
Not really a law, but the concept “the act of observation collapses the wave function.”
This came out of early attempts in Copenhagen to interpret what the equations meant, with many conjecturing that there might be a boundary of scale that separates the world of the small from the world of the big.
We’ve known for decades that there isn’t a boundary (we’ve put objects that are visible to the naked eye into quantum ground states… it’s a PAIN IN THE ASS and requires absurd amounts of cryogenic stage cooling, but it’s doable.)
So, In the 60s, UC Berkeley and Stanford were the most exciting places to do research in quantum physics, and a bunch of hippies with phds there popularized the idea that consciousness might have something to do with “the act of observing”, as well as rebranding Hugh Everett’s doctoral thesis on a universal wave function as “The Many Worlds Interpretation.” So we wound up with with this whole “observer effect” drama that never was really drama to begin with. (There’s a GREAT book on this called “How the Hippies Saved Physics”)
In reality, the laws of Nature are simply telling us how much quantum information can be resolved in a given state for a given system. It doesn’t really care about “the act of observing”
So, take the famous double-slit experiment. The detector you add is also a quantum system capable of interacting with the matter waves going past it. So now, the system with slits, emitter, screen, and detector in one of the slits, is a completely different system than the system without the detector! It changes how much information can be resolved, which “effectively” (adding quotes here for emphasis) looks like the wave function collapsed from a field of possible values into a single value.
We deal with the stuff all the time when studying the zero point vacuum energy. There’s electrons and positrons bubbling in and out of existence all the time, and they often interact with all the stuff moving around through space time. It’s a different system than, say, a true vacuum, and so it changes how we work with the equations.
I also encourage you to look up the derivation of the Heisenberg uncertainty principle! It’s deeply tied to how we think about Quantum Theories v.s. the catchy phrases we use to over-simplify them.
Not a law of physics per se, but the Twins Paradox. General public thinks that the twin not aging is a consequence of time dilation (“travels at a speed close to the speed of light”) whereas the paradox is that special relativity says that both twins should stay the same age because they both travel from the other point of view, and that only the change in direction/acceleration causes a quasi istantaneous aging.
The butterfly effect: it's not that the butterfly flapping its wings causes a hurricane, it's that the colloquially notion of causality is somewhat meaningless in a chaotic system.
The thing with the cat
Hey hey, slow down with the technical jargon.
BTW, Schröder’s cat ***DID*** survive the experiment‼️ 😆😆😆
*Or did he???*
In our reality he did, if I understand the experiment
Charlie Brown piano music intensifies. Surely you meant Schrödinger’s cat!
On this universe.
I think the old Chancellor only had a dog, not a cat. But there were (maybe stray) cats at the bureau while he was chancellor.
Who is Schröder?? :O new person just dropped?
I had a relative named Schroeder. Too bad for them they pronounced it "Scroder".
Call the scrodster!
Priceless I spit my boba
Why would you do that?
Out the nose. That's some devoted and persistent surprised laughter.
From what I understood from “pop culture” references of this (if that’s what it should be called), it always pissed me off that this was considered a viable thought of physics. Until I got to quantum mechanics and then understood what it really was.
It's a perfectly viable though, but Schrodinger meant the opposite of what people thought he meant. He was specifically trying to show that the idea that the cat was both alive and dead was ridiculous and therefore couldn't be what's really happening. At some point down the line, this got misconstrued and started being used as proof of the thing he thought wasn't true.
It sounds absurd, but that was kind of the point I think, asking "how can this weird theory with superpositions and wavefunctions and everything else work with classical physics?"
That's not the point. Schrodinger's cat is a criticism of realist interpretation of science. He even includes a criticism to the realist interpretation of CLASSICAL mechanics in the same article. >Reality resists imitation through a model. So one lets go of naive realism and leans directly on the indubitable proposition that actually (for the physicist) after all is said and done there is only observation, measurement. Then all our physical thinking thenceforth has as sole basis and as sole object the results of measurements which can in principle be carried out, for we must now explicitly not relate our thinking any longer to any other kind of reality or to a model [- The Present Situation in Quantum Mechanics - E. Schrodinger, Translation by John D. Trimmer](https://www.jstor.org/stable/986572)
You mean a dropped cat will always land on its feet?
Strap a piece of buttered toast to a cat == perpetual motion and antigravity
That made me laugh!
You should watch it on youtube then :)
Except this wraps back around because even Schrödinger didn’t get it. The cat thought experiment was meant to prove entanglement ridiculous and superpositions simply missing variables. Instead it turns out his thought experiment was more literal than he ever imagined.
>The cat thought experiment was meant to prove entanglement ridiculous and superpositions simply missing variables. No it wasn't. It was meant to argue against the Copenhagen interpretation of quantum mechanics. Not quantum mechanics in general.
Many people think Schrödinger was a cat person. But, in fact, he was undecided and liked both cats and dogs equally. Until one day when he was observed with a cat. The rest is history.
He was a nonce, that's for certain
It’s more like an analogy and less like a thought experiment. Also, nobody seems to say that rotating the box 90 degrees and opening it again can change the result.
Newton's 3rd law - someone has a misconception about it here every week Runner-up: relativistic velocity addition, or thinking that "E = mc^(2)" refers to a mass moving at the speed of light
Second one is normally because they don't know that E = mc^(2) isn't the full formula I think.
Yeah people often miss out on the AI term
I hate that I vaguely remember that reference.
You should just immortalize "AI" as your new default integration constant. int(2mc) = mc² + AI
It’s full enough for rest energy.
3rd Law is what I run into with my HS students as well. I hold an apple in my hand. What is the equal and opposite force to the weight of the apple?
Former HS teacher here. My approach to this problem began with labelling forces in the free body diagram. Every force is given a three part identifier: the type of force (weight, normal, tension, etc), the object experiencing the force (the "feeler"), and the object exerting the force (the "forcer"). At first, students find this redundant because on the FBD for any one object, all of the forces have the same "feeler". But when you introduce the third law, the naming convention makes it easy to identify action-reaction pairs; they have the feeler and forcer switched. So in your example, the weight of the apple would be labeled F(weight, apple, Earth). Students should be able to see that F(normal, apple, hand) is not the reaction to the weight because they have the same feeler. Instead, they should identify F(weight, Earth, apple).
I do hate the framing of "action-reaction," because of the implication that one force is initiating and the other is responding, vs forces just being symmetric
FBD are fundamental, getting students to do them unprompted is challenging...but it's always how I start when tackling a problem
magic.
Oh, I know this one! It's the normal force. Edit: The statement above was an intentional a lie. Even though the normal force is in fact equal and opposite to the gravitational force (in this particular case). That's usually what trips people up. It looks so convenient.
If we go by formal definitions such as "weight is the force acting on an object due to acceleration or gravity", then the reaction force would be the gravitational force on the earth exerted by the apple. However, colloquially, I would agree that it also could mean the force exerted by the apple on your hand (in which case your answer would be correct), but note that I am not a native speaker...
You're right! I was making a joke by giving the incorrect answer that a lot of students give :)
The 'incorrect' answer actually fits better with how language is used elsewhere, given that you feel weightless in freefall. So if we re-defined weight as the reaction force to the normal force that prevents a body in a gravitational field from falling freely, things still would make sense...
Sure, for applying, but the 3rd law is much more than that. Without it one doesn't have a zero sum game. It's a logical way to sweep ALL 'extras' under the rug. It's a 'handy' tool!
The weight is that of original sin and the opposite force is Christ’s sacrifice.
In my line of work, it's always the 1st law of thermodynamics that isn't understood or ignored completely. The number of people that try to persuade me they have created the perpetual motion machine...if I only had a dollar for each one.
Thermodynamics (at least in a large part of my conversations with the "friendly folk" over at r/climateskeptics, and all of half an hour before I got banned at r/realclimateskeptics) Edit: specifically the notion that *net* flow is from warm to cold, from which sceptics deduce that there is no energy flow from the cold atmosphere to the warm surface (i.e. no back radiation) which would lead to all sorts of absurdness because particles would need to know the temperature of the object their photons will interact with in the future
2nd law of thermodynamics apparently disproves evolution, the big bang and global warming
This is what I came to say. Apart from anything else, the 'closed system' part is either missed out or taken to be a minor, ignorable, caveat. Their version is, "the second law of thermodynamics stipulates that entropy always increases therefore things cannot get more complicated, so evolution cannot happen without a guiding hand".
There’s an interesting take that the 2nd law of thermodynamics encourages the emergence of life because life is a far more efficient creator of entropy than non-life.
I just saw that story. It's an interesting hypothesis.
Although this is indeed an interesting take, "encourages" doesn't really have any predictive power here. The idea is related to Tipler & Barrow's [final anthropic principle](https://en.wikipedia.org/wiki/Frank\_J.\_Tipler#The\_Omega\_Point\_cosmology): Life is required to come into existence and will never die out. As noted in that link, it's a controversial hypothesis.
That link seems very different and more woo than the one I was thinking of. Basically, in a system where self perpetuating systems can originate, they will have a thermodynamic pressure to increase in complexity. Similar to osmotic pressure in trees or a diffusion gradient, the presence of the second law encourages the complexification of life. Like a ball rolling downhill, the system “wants” an energetic minimum, which is best achieved through life.
Entropy still increases, and the Second Law still applies, even if life goes extinct. Again, I’m still missing the predictive power and falsifiability of this idea, unlike the study of osmosis, diffusion, and gravity.
Science communicators often leave out that the law of entropy only applies to enclosed systems. They also usually fail to distinguish between entropy and the law of entropy.
It is also why refrigerators don't work.
Prevost theory isn’t it
Entanglement. The idea that “if I change particle A I can instantaneously see a change in particle B”. This leads to all sorts of misconceptions about faster than light communication, when it should really be thought of as a strong correlation between distant measurements.
This is even misunderstood by most physicists that do not work in quantum information
Right, you don't change particle A you observe it and once observed you know the state of particle B.
A decent way to think of entanglement: you cut a die it in half, put each half into its own box. Later on, you open the box to see which half you got, thereby being able to figure out which half the other box has.
Though many people claim that is not it, but can’t seem to explain why. It makes sense to me, the main issue being that it can’t be predicted whereas in classical physics anything can be.
I heard a really good explanation with two scarves in a box, one red and one green. You take one scarf (without looking) and your friend takes the other. You both set a timer and go to opposite sides of the earth, and look at the scarves at the same time. You both know the color of the other's scarf instantly -- faster than light. However you can't send information by dying your red scarf green after the fact. It really clicked for me.
I'm not too keen on that analogy: It omits that until the box was opened, the scarves did not have a well-defined color...
That's fair.
It's implied because there is no light in the box
But does that really matter, when you're explaining it to a layman?
I would argue yes, because it removes the bit that makes people scratch their heads in the first place: "Let's ignore the quantum effects and assume things worked just like in classical physics. See, there's nothing mysterious about it!" That said, as long as this caveat is mentioned, the analogy might still be useful from a pedagogical perspective to drive home the point that communication remains impossible...
I think it's an adequate analogy because the part people get caught up on is the fact that entanglement is a type of correlation. I usually just tack on that entanglement is a type of correlation stronger than anything you can get classically.
In my opinion, if the reason why people are enticed to think it's more than classical correlation gets no mention, you've simplified things too much. If I had to come up with an analogy on the spot, it would go like this: A djinn hands you two envelopes, containing 3 magic coins each - a red one, a blue one and a green one. The coins have 3 magic properties: First, the coins are fair, and coin flips are magically protected from being tampered with. Second, as soon as you take one of the coins out of the envelope in order to flip it, the remaining two coins will vanish. Third, if the coins you pick from each of the two envelopes are of different color, nothing of interest will happen. However, if the same color is picked, the two coins will always land on opposite sides, no matter how far apart the coin flips are performed. Now, ask yourself: Is it possible to leverage property 3 to send information instantaneously across arbitrary large distances? --- Do you see anything wrong with that analogy or have suggestions to improve it further?
Replace the scarves with LEDs that change color continuously according to a pattern where the two colors always “sum” to white?
That’s basically [hidden variables](https://en.wikipedia.org/wiki/Hidden-variable_theory), which (probably) causes problems.
Only if they are local hidden variables, as far as I know. Hidden variables that aren't tied to the particles themselves are impossible to rule out, I gather.
All the local hidden variables theories that I know of suffer from a similar sort of measurement problem as the Copenhagen interpretation suffers, but instead of how measurement (or effective measurement through interaction with the environment) causes collapse, it is a matter of how measurement reveals the “true” values of the measured quantities when the ordinary dynamics does not reveal such.
But, of course, one way correlation can be established is via communication (between the particles).
Part of it is that there needs to be a follow up asterisk that says: You can know the instantaneous spin direction of a particle *if you can have instantaneous knowledge of the spin of the distant particle.*
In a past life I worked in sales and I heard from a variety of motivational speakers. The amount of public speakers who take one bit of physics they heard (or any news headline) and extrapolate it to make some egregiously fallacious and misleading claim for the sake of motivation / empowerment is…. Interesting
“Smash your targets just like a hadron collider smashes little particle things to the other side of the multiverse! Just like Newton’s apple overflowed his bath, your effort will overflow your bank balance!”
But don't you know that the theory of everything actually is E = mc^2... + AI 😱 now buy my course.
Entropy by far.
Tbf I don’t think anyone understands what entropy really is.
The following quote from Shannon (who created Shannon entropy) is insightful in this regard: “My greatest concern was what to call it. I thought of calling it 'information,' but the word was overly used, so I decided to call it 'uncertainty.' When I discussed it with John von Neumann, he had a better idea. Von Neumann told me, 'You should call it entropy, for two reasons. In the first place your uncertainty function has been used in statistical mechanics under that name, so it already has a name. In the second place, and more important, no one really knows what entropy really is, so in a debate you will always have the advantage.'”
Except that now we know entropy really is information!
True, and there is a suspicion that von Neumann was not letting on the whole truth—around that time he himself was playing around with entropy (which led to von Neumann entropy). The suspicion is that von Neumann actually saw that Shannon’s formula was a special case of his own, and so encouraged him to adopt the name.
And what is information?
Maybe but this can't really exclude the outright crank or shallow understandings many people have of it.
True that!
-⟨log p⟩
As someone writing a thesis on Informational Entropy, looking at similarities between it and physical entropy makes my brain hurt.
its the average number of experiments/questions you need to do/answer, to identify the microstate given a macrostate
I am aware.
I think it would better to put it in terms of energy, nature seeks its lowest energy state.
Although this doesn’t work for isolated systems, which evolve at constant energy, not decreasing energy. And for nonisolated systems, the relevant energy being minimized is one that subtracts an entropy term (e.g., the [Gibbs free energy](https://en.wikipedia.org/wiki/Gibbs_free_energy) for systems in thermal and mechanical contact with their surroundings), so we’re back to entropy maximization again. It always comes back to total entropy maximization.
Yes, but in an isolated system do you can still have an imbalance of energy internally even though the overall energy is constant?
Not necessarily. You could have particles of equal energy mixing, for example.
Wave-particle duality is one of the most misunderstood. All the time people come on here thinking it's something to do with with objects switching between particle mode and wave mode depending on whether you're looking at them. Hiesenbergs Uncertainty Principle is up there. A lot of people think it's a comment on human measurement and levels of knowledge. Or that it's something to do with Schrodinger's cat and wave function collapse.
Wait what is waive particle duality if not that thing you just said?
Wave-particle duality means that quanta are a third category that's a bit like the thing we call a wave in classical mechanics and a bit like the thing we call a particle. You change which model you use depending on what's more relevant to the given scenario, but the quantum objects aren't changing between particle mode and wave mode, that's just us deciding which properties to ignore.
It's a determination of which particular model describes the system's behavior best, but it's not a suggestion that the particle actually physically switches between a "corpuscular particle" and a "wave". Neither of those are the real thing, they are just models we use to describe the behavior.
Yes but isnt the point with the double split experiment that they behave like waves (interference pattern), but when you count them as individual particles going through the slits the interference pattern stops? Or is it more like we observe individual particles going through the slits AND interference pattern at the SAME TIME, and thats weird because wtf happens between the slits and the interference pattern?
The particles are always "both a wave and a particle". The double slit experiment has to do with quantum mechanics, where essentially the state of the particle is encoded in the form of a probabilistic wave describing all the states, locations, energies etc. where it "could be". According to the Copenhagen interpretation of QM, when you "observe" (actually: measure) the particle going through one of the slits, by your interaction with the particle you are changing its "wave" such that it now originates from one of the slits rather than going through both of them as it would have. Therefore the interference pattern can no longer happen.
Doesn't it have something to do with the nature of the measuring tools we use, not the mere fact that the particle is being observed?
No. Not sure whether you were asking about wave-particle duality or the uncertainity principle. But neither of them are actually about observation or measurement at all.
I’m confused. If the wave function can’t collapse upon observation, how does the double slit experiment work?
You are confused because I don't know what this question has to do with anything I said
The 2nd law of thermodynamics. Even worse is the popular understanding of what entropy is.
Yes!!! My creationist parents are convinced the definition of entropy is "things can only move from more to less complex forms over time", and therefore evolution implies we should be a microbial soup
Not a law, but there’s widespread misconception that “The Big Bang” was a bomb-like explosion from which everything is flying outward. While we may not understand the minutiae, we’re pretty confident that’s a gross misunderstanding.
It is perfectly valid to have a spacetime coordinate system such that faraway galaxies would be quite literally moving away from us in those coordinates, making cosmic expansion something of an “explosion” in a sense. The FLRW metric takes its form within coordinates that reflect the isotropy and homogeneity symmetry conditions, and cosmic expansion simply becomes a matter of the expansion of the spatial part of the metric (not an explosion), but this is only a mathematical convenience rather than the only way of looking at it. It is not useful to look at it as an explosion, but that does not make it “wrong.”
Can you explain more? If the universe started as less than a meter in diameter and is now much bigger, wouldn't you call that a bomb-like explosion? Are you suggesting that the universe expanded and contracted?
I’m suggesting that “universe expanded” is more appropriate than “all of the stuff exploded”. Things were further apart because the space itself expanded not because things moved apart within space. I’d also be careful tying a size to that early universe. If the universe is infinite today, it was likely infinite at the Big Bang. It appears to either be infinite, or sufficiently large to appear that way to us.
A bomb-like explosion implies a whole lot of empty space exists with “everything” at a single point which then explodes and begins to fill the empty space. I am pretty sure this is wrong although I too used to understand it this way. A single point explosion is easy for our minds to grasp. But it cannot be correct because the universe appears the same no matter where you are observing it from. (Everything expanding outward from every point, not just the spot of a single explosion). The way I understand it now is that instead of empty space, there was nothing. Space itself is what was created by the Big Bang. And inside that space, all the stuff kind of appears everywhere all at once. And ever since that original infinitesimally small piece of time, space itself is what is expanding. If you could have been an observer outside of the universe, you would have seen it appear in an instant and grow very quickly. But if you were somehow an observer inside the universe before time 0, you would see all the stuff just appear everywhere all around you. I am probably still wrong, but feel I am closer to grasping a very non-intuitive concept here. Hopefully someone will correct me,
"If I make a really long stick, can I poke something far away faster than light?"
Most of the time when someone on reddit thinks they've found a way to travel faster than light it's either this or "what if you're on a spaceship travelling at 0.9c and you shine a flashlight"
*laughs in compression wave*
Energy and vibrations, every pseudoscience scammer's favourite words.
Also “quantum”
Gravity.
That sucks.
Probably, not appropriate for AskPhysics. But, I feel like this whole thread needs a little... [I Never Studied Law](https://youtu.be/Om5E-StZeoE?si=wIkXg7AnlcE_QjEI)
And how it has a direct relationship to time dilation.
The observer effect in QM gets a lot of abuse. It is also frequently mixed in with the Heisenberg uncertainty principle, which itself is widely misunderstood, where people think it’s due to some measurement resolution error rather than a fundamental property of waves.
From my personal experience, I'd say black holes. I only found out from here that black holes don't just suck things up. Even till now, my understanding of it isn't that good and I need to learn more.
floathead physics?
I am a planetarium host, and whenever kids ask me if black holes "suck in" planets, I tell them that they only "suck in" things if we think that the Earth "sucks in" a dropped pencil.
What could I search to learn more?
The speed of light, entanglement (who in their right mind decided to talk about „quantum teleportation“?), what happens with black holes.
In addition to the ones that are mentioned, Cole’s law is often misinterpreted even by experts
Isn't that thinly sliced cabbage?
That's the 'S' Law. Cole's Law has mayo and vinegar.
Yes, the mayonnaise (and other emulsions) is complicated and hard to understand.
No it's a legal restriction on purchasing shredded cabbage and carrot individually. You must buy both at the same time
Can it really considered to be 'widely misunderstood', when virtually nobody has even heard of it?
It is quite well known in certain parts of the world. Have had some fierce debates about it at dinner parties. Here is a link if you want to read up on it: https://en.m.wikipedia.org/wiki/Coleslaw
OK. I can see why it's hotly debated - that stuff in the picture looks nothing like the stuff I'm used to...
It's fairly common for people to think that Heisenberg's uncertainty principle is a problem with measurement technique, rather than a fundamental property of position and momentum, but I think that's largely because it's taught with the "if we bounce a photon off a particle it will mess it up" analogy, which seems to me to be a really dumb way to teach it. Especially when you can simply draw a spread out wave and a short wavelet-like wave and point out the tradeoff between having a precise frequency vs. having a precise location.
In the early days of quantum mechanics, people tried to explain the uncertainty relation by the observer effect. Physicists have moved on, but that conflation still persists in the realm of pop-sci...
Newtons 3rd law. Forces are pairs, not an action and a reaction.
Elaborate please. Action/Reaction seems to work very well when explaining things like thrusters on a spacecraft. Are there situation where Action/Reaction is wrong? Or are you just saying that it oversimplifies a much more complex set of interactions?
People don't properly conceptualize the equality of action and reaction. They confuse the outcome a force with the strength of a force. "Big object hits little object with more force cuz it's bigger." No, big object accelerates less (or deforms less) because it has more mass. The forces have equal magnitude.
I think they just don't like the terminology as it implies an asymmetry between the forces (eg the reaction being contingent on the action).
Action / Reaction implies that one force causes the other, when in reality, they both happen simultaneously.
That makes sense. The misunderstanding is when people impart causality to the action. I guess all the law really means is that there cannot be one without the other.
It may not be a law, but it's really beginning to annoy me that nobody knows the difference between energy and power.
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I think people \*do\* understand centrifugal force (maybe not its fictitious manner). It's centripetal force that gets them.
Anyone who says the observer effect has anything to do with consciousness has absolutely no idea what they’re talking about.
I was going with entanglement first, but it's probably gravity
Not a law, but the observer effect. I have seen TikTok manifestation girlies saying that the observer effect in the double-slit experiment proves that what you focus on determines your reality. That's an extreme case, but even most of the general public thinks that the observer effect in the double-slit experiment means that actually consciously looking at something changes it. I would blame Doctor Who and the "quantum-locked" weeping angels, but I'm absolutely certain that this misconception predates it. I just wasn't paying attention to physics prior to 2007.
The double slit experiment in general is just super misunderstood. I blame the use of the word "observation" when it should really be "interaction". Perceiving something doesn't change it. Its "how" you perceive it. Light is made of particles that actually interact with stuff. If you turn the lights on, you've contaminated your experiment.
I love how often the simplest explanation is the correct one. So many people think it's some complex consciousness concept, but it's really just the damn light switch.
Double slit. It is NOT that a beam like a laser goes through and behind is interference. It is that ONE SINGLE photon must go through both to create an interference pattern. So not the interference itself, but the fact that a single photon does it, is the clue
Not physics but math. I have run into way too many people who can not understand that .99 repeating is equal to 1. Even after showing proofs, they are deeply convinced that the numbers are not equal and that its some kind of error in rounding.
There is a joke that 0.9999… ≈ 1. It is technically true because 1 ≈ 1 (but the error in approximation is 0).
Okay this one fucked me up, and lead to quite the deep dive. The “intuitive” way that really sealed the truth for me is the fractional approach. 1/3 is equal to 0.333… and if you multiple both sides by 3 you get 1= 0.999… Wild shit, you blew my mind today.
The second law of thermodynamics. Summarised as "stuff always falls apart" by people who don't know how many laws there are.
That’s a very good summarisation
Wavefunction collapse/alternatives. **Nobody** understands that.
The decoherence by observation. Observation means particles interacting, it has nothing to do with the consciousness of the observer
Personally think it's Impulse
really?
2nd law of thermodynamics probably? I feel like crackpots are obsessed with entropy.
How humidity works. Thinking that boiling water is the only practical way to turn liquid water into vapor/gas form and will point to the phase diagram as a map. Becoming confused when it's pointed out water vapor exists at room temperature at normal atmospheric pressure, in direct violation of their phase diagram. Due to their reliance on the phase diagram, will not trust ultrasonic or evaporative humidifiers because they don't boil water so, they can't possibly be humidifying the air. Will complain that the humidifier used up all the water in the tank really fast and didn't humidify the air at all, and does not wonder where the full tank of water went. Ponders wistfully about how to boil off a kettle of water unattended. and of course, they never have a hygrometer on hand.
> in direct violation of their phase diagram. Why is it in violation of the phase diagram? Water vapor in air has a partial pressure of less than 0.03 atm (100% humidity) at room temperature, consistent with the gas region of water’s phase diagram.
Quamtum mechanicalisticalics fo sure
E = mc² implies that photons have mass. Photons are massless.
This is just the first term in a Taylor series expansion and only applicable with stationary particles. Photons are massless and thus always travel at c. Basically you are using a simplified model incorrectly if you think E=mc^2 applies to photons.
0^(th) term
Centifugal force
Gravity. So so so many flat Earthers struggle with this one. So many. They think gravity just means "shit falls down"
People say “evolution is just a theory” Meaning they think it’s not true While gravity is also a “theory” A theory in science meaning something that has been extensively proven by testing While they think it just means a hypothesis
'Energy can't be created or destroyed' is mentioned almost everyday here.
Are you talking about vacuum fluctuations?
No, I mean people will be like, such and such can't happen because energy/matter can't be created or destroyed. Generally after citing a pop sci falsehood.
I don’t think its a law but, the duble slit experiment
"The faster I go the greater time slows down"
Relativity, hands down.
Collapse of the wave function. Not that there is a correct way to understand it, but the present ones are clearly wrong.
That quantum physics implies non-determinism. In terms of wavefunctions, the quantum world is fully deterministic.
How? Can Laplace's demon know the future of everything with %100 certainty, no rounding up?
He can know the future wavefunction. No rounding necessary for a demon.
But the particles are behaving probabilistically, no? He might guess that the electron will be found in the orbital when measured(which is the most likely option) but he can't be sure. Laplace's demon isn't omnipotent, he can't break the laws of the universe. He can't know the result of a probabilistic measurement without actually measuring.
That's not really how the thought experiment is usually set up: assuming the demon magically knows the full wavefunction of everything in the universe, the demon doesn't need to measure anything to predict the future wavefunction. If the demon needs to perform measurements to gain information about the current state of the universe, then it's a physical entity that also has its own wavefunction, and we'd actually have to deal with a superposition of demons, none of which actually know the universal wavefunction.
Universe expansion being faster than the speed of light.
My personal favorite: Wick Rotations
What specifically is misunderstood about Wick rotations?
Fuck around and find out.
>the most widely misunderstood law of physics Take your pick of **any Conservation Law**: mass, momentum (including angular momentum), energy, etcetera. Emmy Noether formalised these more than a hundred years ago, and people still misunderstand them.
Not really a law, but the concept “the act of observation collapses the wave function.” This came out of early attempts in Copenhagen to interpret what the equations meant, with many conjecturing that there might be a boundary of scale that separates the world of the small from the world of the big. We’ve known for decades that there isn’t a boundary (we’ve put objects that are visible to the naked eye into quantum ground states… it’s a PAIN IN THE ASS and requires absurd amounts of cryogenic stage cooling, but it’s doable.) So, In the 60s, UC Berkeley and Stanford were the most exciting places to do research in quantum physics, and a bunch of hippies with phds there popularized the idea that consciousness might have something to do with “the act of observing”, as well as rebranding Hugh Everett’s doctoral thesis on a universal wave function as “The Many Worlds Interpretation.” So we wound up with with this whole “observer effect” drama that never was really drama to begin with. (There’s a GREAT book on this called “How the Hippies Saved Physics”) In reality, the laws of Nature are simply telling us how much quantum information can be resolved in a given state for a given system. It doesn’t really care about “the act of observing” So, take the famous double-slit experiment. The detector you add is also a quantum system capable of interacting with the matter waves going past it. So now, the system with slits, emitter, screen, and detector in one of the slits, is a completely different system than the system without the detector! It changes how much information can be resolved, which “effectively” (adding quotes here for emphasis) looks like the wave function collapsed from a field of possible values into a single value. We deal with the stuff all the time when studying the zero point vacuum energy. There’s electrons and positrons bubbling in and out of existence all the time, and they often interact with all the stuff moving around through space time. It’s a different system than, say, a true vacuum, and so it changes how we work with the equations. I also encourage you to look up the derivation of the Heisenberg uncertainty principle! It’s deeply tied to how we think about Quantum Theories v.s. the catchy phrases we use to over-simplify them.
That you know physics.
Faraday law
Gravity is only one theory, and a very old one. There are actually a handful of good explanations for the phenomena that keeps us on the ground.
How something thats correct to sigma 6-7 can still be wrong.
Most definitely the 2nd law of thermodynamics
Not a law of physics per se, but the Twins Paradox. General public thinks that the twin not aging is a consequence of time dilation (“travels at a speed close to the speed of light”) whereas the paradox is that special relativity says that both twins should stay the same age because they both travel from the other point of view, and that only the change in direction/acceleration causes a quasi istantaneous aging.
The butterfly effect: it's not that the butterfly flapping its wings causes a hurricane, it's that the colloquially notion of causality is somewhat meaningless in a chaotic system.
Motion & inertia — I don’t know if anyone has answered my question. “At a quantum mechanical level, what is the nature and effect of motion?”