Well, Actually – Physics Answers
backreaction.blogspot.com
backreaction.blogspot.com
We primarily say "Well, actually..." to each other, and it's pretty much unavoidable. Physics is a diverse, complex, and difficult field. People working in any given niche have to make a plethora of simplifications and assumptions about peripheral stuff just to make it manageable. It's a bit like how computer scientists usually don't worry about the solid-state physics of a logic gate, how a CPU is designed, or how compilers are written when they're writing a program. Yes, it's the foundation their work is built upon but, most of the time, they can safely ignore it.
When we say "Well, actually...", some of us are letting our insecurities show by trying to one-up everyone else. Some of us (probably a lot of us actually) are obsessive about certain things that fall within our bailiwick. We have difficulty staying silent when somebody smooths over something wonderfully crinkly that we can't resist talking about.
Mostly, we're just pointing out things that we find genuinely interesting. This serves a valuable purpose. Sometimes the simplifications and assumptions we make bite us in our posteriors. When we talk to other physicists, mathematicians, etc. outside (or within) our own specialties, sometimes the "Well, actually..."'s lead to important realizations.
Most of the time, it doesn't really matter. Well, actually, sometimes it really does matter!
I've never felt so understood.
Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted, which causes an effective slowdown at a larger scale. A gas is just vacuum with particles floating around in it. When photons travel between those particles, they are traveling at c. The effective slowdown is determined by the lattice/gas, and we denote this slowdown with the refractive index...
> 7. “Light is both a particle and a wave.” Well, actually, it’s neither. Light, as everything else, is described by a wave-function in quantum mechanics. A wave-function is a mathematical object, that can both be sharply focused and look pretty much like a particle.
This is a great example of how quantum mechanical analogies don't work if over-analyzed. A great professor of mine once told me that you can't look too far into classical analogies for quantum mechanics -- you will end up coming to the wrong conclusion. At the end of the day, understand the math.
As someone else in the comments said, most of these are pedantic gotcha's. I thought the one about water being blue was the only one that actually surprised me as something people thought was true.
Well, actually the speed of light, specifically the rate of propagation or phase velocity, is not slower in mediums because of absorption and re-emittance. Instead, a photon is affected by feedback loops between its oscillating electric field and the charges of atoms it is passing near, without being actually absorbed by the atoms.
Paraphrased as best I could from: https://en.wikipedia.org/wiki/Refractive_index#Microscopic_e...
The only assertion of which I'm aware that the apparent speed of light in a medium is what really matters for relativity was Ron Mallet's work: https://www.phys.uconn.edu/~mallett/main/papers.htm . I don't believe there's been any experimental confirmation of it, though.
Besides, electromagnetic interaction is modeled as emission and absorption of photons on a small enough scale. You are just using a population simplification to dismiss the individual based analysis (what would be pretty reasonable because the later one is meaningless when you can't observe individuals, except that the entire point is pedantic, and pedantism can't be subject to dismissal by meaninglessness).
This is wrong. Two sources [0][1].
Now I want to know why that's wrong but don't want to spend ten minutes getting refraction explained to me beforehand...
In a sense, video is dimensionally reduced compared to an article. The information is serialized onto time axis. Whereas with articles, the information is serialized onto a spatial axis (usually vertical). A spatial axis supports rapid access, and viewing multiple point simultaneously, in a way that temporal axis doesn't.
Because the electric field only has a single value at any point in space. The equations that describe this are linear (ignoring quantum stuff irerelevant at these scales), so the waves combine by simple addition.
Why can't they just coexist?
In a way, they do continue to coexist (again due to the linearity): For example, if two waves pass each other by, you get back the original waves once you leave the region of interference.
And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible?
Rather, the electrons around the atoms start wiggling around, making new waves. I wonder if one could set up a mechanical analogue by putting some floating balls in a pool and attaching them to the bottom so that they breach the surface if a wave passes through, creating new waves...
The "waves" you mention add up because each photon sent in through the glass takes all the possible paths, and you need to add up the contributions from all these paths to see where this single photon wants to go. This looks like wave propagation mathematically. The reason the waves add up is because they are all the same photon.
If you send in two unrelated photons, they get their own waves and in that case they do coexist without interference, for all practical field strengths (there are light-on-light scattering effects that come into play at immense field strengths, just now starting to get probed).
1) He's saying the light moves the electrons and the electrons set up a 2nd wave that interferes with the original to produce a lower frequency. That's simple enough. But then he says the lower frequency wave travels slower. Eh, what? We are try to explain is why light travels slower? Ergo, you can use "because it travels slower" in your explanation.
2) That said, the lower frequency does travel slower because light gets split into a spectrum by the glass prism. To me it seems a satisfying answer would not only explain why it got slower, but why slowdown depends on frequency.
https://qr.ae/pNSdj9 (quora answer with sixty symbols video link in it; explains reasons for every debunked version of why)
For example, you can use a convention where the speed of light is infinite for infalling light rays, and twice as slow than normal for outgoing ones. It's a bit weird (when you look into the mirror, light from the sun reaches you instantaneously, 'slowly' moves towards the mirror, and then jumps back to your retina instantaneously), but predictions will work out the same. Of course, it'll affect more than just travel time of light, you'll also mess with anything that inludes a gamma factor, eg making inertia anisotropic (the force you need will depend on the direction you push in).
Ultimately, it doesn't help the creationists, because things like time passed since the big bang for an observer following the Hubble flow does not depend on such conventions. All you'll achive is changing the big bang from an instantaneous thing (which in itself is already a bit of a dicy concept in relativity) that happened everywhere all at one into a continuous thing that started 13.8 billion years ago and has been going on ever since.
As to 8, well actually when referring to objects one would by default use a reference frame where those objects are approximately at rest. E.g. a reasonable reference frame would be one where the aforementioned center of mass is at rest. So it's reasonable to say the Earth and Sun are 8 light minutes apart.
Unfortunately material like this also makes up a substantial fraction of what passes for science education in the US.
"Today I will tell you how to be just as annoying as a real physicist. And the easiest way to do that is to insist correcting people when it really doesn’t matter."
If you still don't get that, I might have bad news for you.
To take #1, Earth orbiting the Sun. It's a fine shorthand, assuming you remember that all matter attracts all other matter gravitationally. If you do, you'll not be surprised by learning about how black holes work, or figuring out why planets are spherical, etc. If you don't, you'll probably end up considering gravity a magic glue between objects - which may also point out towards critical epistemological problems, like thinking objects (like "Earth" or "dog") are first-class in reality (vs. being just convenient boundaries around matter that humans draw, boundary itself not being a real thing). Knowledge is deeply interconnected; getting one bit wrong can make you wrong about a lot of other things. Conversely, fixing one wrong may fix a bunch of others too.
Also, in my experience, response to such "pedantry" varies. I've known people who were grateful for such corrections, and used them as springboards to read up some more. These people ended up being the ones having any kind of clue about how the world (both physical and social) functions, unlike general population. A difference in attitude, perhaps?
The video is pretty nice in that it teaches interesting facts and at the same time criticizing the behaviour you mentioned (I agree with you).
The video itself is named "How to talk like a physicist", so the blog post linked here could be a bit misleading in that regard if one does not know Sabine Hossenfelder already.
It is my interpretation that these are the situations the video actually parodies. NOT situations in which a related oversimplification is corrected.
To be more precise myself I'd update my agreement with the OP comment to "I agree that the general sentiment of being pedantic in inappropriate situations is bad, I still enjoyed the facts presented in the video". Which is what I wanted to convey with my comment. This would also mean that I only partially agree with the OP comment, because I enjoy the facts themselves.
Well, actually weight is a force that depends on the gravitational pull of the planet you are on, and it’s also a vector, meaning it has a direction. You probably meant you lost mass.
Most people expect to not leave the earth, so for all purposes, losing mass is the same as reducing the earth's effect on one's body, which is what people feel. We don't feel mass, we feel weight, so if we feel lighter, we've lost weight.
Well, actually this is redundant. All forces are vectors.
It may be annoying[1] to correct people on such stuff, but it is important. Misinformation established at social/cultural level affects us all. If you ever wonder where these people afraid of microwave ovens or infrared thermometers come from[2], it's this: many misconceptions adding up to an invalid mental model of the world, which makes them susceptible to further scares[3] they hear or read.
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[0] - https://en.wikipedia.org/wiki/List_of_common_misconceptions
[1] - Probably as annoying as hearing someone spout these misconceptions once you know the correct answer.
[2] - And behind each such person there's dozens if not hundreds people who harbor similar beliefs, but won't admit to them.
[3] - It's not just conspiracy theories, either. There is plenty of established businesses that just sell this kind of bullshit, usually as a part of an ad delivery vector. For instance, all the health and lifestyle magazines general population reads are a major source of annoying health-related misinformation.
5. One reason we might not see quantum effects at a large scale is that they are small. Another reason might be that the effects are large, but we are entangled with the system we are observing.
Assuming you're not travelling very far, a "bumpy Flat Earth" geometry works quite well, and didn't your lecturer tell you physics about using "models that work quite well"?
That said, one of the things I've enjoyed about travelling a decent way North-South is how the night sky feels different. I do wonder how many Flat Earthers have had that opportunity.
Here is a pretty heatmap showing the diffrence between geoid heights and oblate spheroid model:
https://en.wikipedia.org/wiki/Geoid#Undulation
All of that seems like it doesn't matter, and then (like me over the past month) you want to do something in the GIS space, and then suddenly it does. It is important for accurate mapping and measurement.
Of course, nobody has time dealing with this crap (and the shape of the geoid wasn't known until recently anyway), so we cheat - we standardized on a large bunch of region-specific coordinate systems, where one or more points are carefully referenced to geoid or ellipsoid (oblate spheroid) models, and everything else is measured in this local coordinate system.
I will say, though. I was surprised at how much you can do with a flat Earth model. It takes a few km to accumulate significant error. For some calculations, it's a valid model... but you do need that accurate reference point to start from.
As for the flat earthers, you assumed they would accept another concept that stars are very far, if they resides not far above us, that could kinda explain that.
For the record I'm not a flat earther, please do not downvote me because of the previous paragraph.
Oblate spheroids are round in the same way my dinner plate is round!
Would you call a golf ball round? Depends. If you want to calculate how a golf ball travels upon being hit by a golf club, then you’re going to get more predictive results if you consider the dimples in the golf ball.
If you only want to calculate the number of balls you can fit in a bag, by all means ignore the roughness.
Just as in the flat earth debate: it’s perfectly reasonable to assume flat earth when planning your bike trip, but the assumption breaks down when you want to fly large distances.
For me it is all about knowing which models and abstractions are useful in which contexts rather than philosophising what is “real”.
https://philosophynow.org/issues/114/Richard_Feynmans_Philos...
> For a black hole you keep falling towards the center, cross the horizon, and the gravitational pull continues to increase.
I've always struggled with "crossing the horizon".
- From the perspective of an observer falling into the black hole, I understood that the event horizon falls away from you, so you never reach it (maybe you get "closer", though - not sure).
- From the perspective of an observer away from the black hole, time appears to slow down for the object falling into the black hole, so it never crosses the event horizon.
Am I right? What does it mean to talk about crossing the event horizon if this never happens from either of the observers' perspectives?
In the abstract, in a 2d spacetime diagram (ie ignoring angular degrees of freedom), the event horizon is just a line, and an infalling timelike worldline will cross it.
Cf [1], a black hole spacetime diagram in Kruskal-Szekeres coordinates, via [2].
And yeah, I am aware we've watched things falling into black holes, although I'm not sure how much different the "it's frozen just before the horizon" scenario would look like.
What observations we have come from gravitational waves.
I see it as a property of nerds, which I say as a nerd myself with a PhD in physics. I still do it and feel the tug to, but I've worked hard to learn to listen and reflect with more empathy and generosity.
Seeing that people believe this was quite unexpected, at first - it's such an obvious phenomenon looking down into any slightly deep body of water. But on reflection, it makes sense: many (most?) people in the world might live most or all of their lives without going beyond the shorelines of a sea, ocean, or larger lake/river.
It's a good reminder to always be critical of the assumptions you make, and how much they depend on your own context.
(If you ever get the opportunity, visit Crater Lake. The drop from blue to midnight to pitch is beautiful.)
>...So the major reason the oceans look blue, if they do look blue, is indeed that they mirror the sky.
(And, of course, after it rains there is a good chance the water will actually be green, but it's a different timing and a different shade of green.)
But I think at the surface of the sun (if you could stand there naturally) the time/distance is still around 8min (happy if someone does the math) - Note that the statement says 8min, not 480.000s which would be metrologically wrong.
Well actually it's a Geoid, which is defined as something the shape of the earth and is approximately slightly pear shaped.
That being said I do GIS stuff professionally and 90% of the time just pretend the earth is spherical.
Well, actually, the difference is that all non-black-hole masses have spacial extent, so if you get 'too close' you are now inside the object and the gravitational attraction decreases if you continue to move closer to the center of the object, since you are now inside the 'newton shell' of some of the mass.
There is no place you can be relative to any non black hole where you can find a gravitational field as strong as what you would find near the event horizon of every black hole (which is always equal to any other black hole, and is basically by definition the strongest gravitational field that can be found anywhere).
Black holes do have stronger gravity than non black holes.
I didn't dislike the article though, that's not really what I mean to say. Was conveniently to the point and I can just lookup more about Lorentz contraction if I want to :)
The only one I didn't like was #6 - I think it may be wrong. Not just because of language use (words gain multiple meanings over time; dieting!weight != physics!weight), but also because, while weight is indeed a force (a vector), it's also proportional to mass - so if you lose mass, you also lose weight.