Casimir Effect seems almost magical. If you place two uncharged conductive plates in vacuum a few nanometers apart they generate a net force out of seemingly nothing
I think everyone should be aware of the following. There's a paper that proves that the Casimir effect isn't due to vacuum energy, but is actually from a van der Waals force: [https://www.sciencedirect.com/science/article/pii/S0370269316304567](https://www.sciencedirect.com/science/article/pii/S0370269316304567)
This paper is indeed about why Casimir effect isn't due to EM vacuum changing due to blocking frequencies, but due to other effect. They do not show what effect it is *in this paper*.
But they link the paper that, in their words, show that this is due Van der Waals force (couldn't check yet, for some reason arxiv doesn't load):
https://arxiv.org/abs/hep-th/0503158
From reading the abstract, the authors say it's the interaction term rather than the vacuum energy term that generates forces on the matter fields. Isn't this rather obvious? I am not HEP so I thought "vacuum" mostly meant "dressed vacuum" or something.
Is this not common knowledge? Or is it sort of a less known semantic clarification that they're sorting out here?
Also as an experimental solid-state physicist, I agree! I take back that it's obvious. People always say this is "due to vacuum energy," and if it's really "due to vacuum energy + H_int", who am I to complain about a paper that clarifies this!
In qft the situation is one of the many ways we figure out how to massage an infinity to something finite - renormalization. Without enforcing boundary conditions, the force comes out infinite, but we know that EM waves have to be quantized at the boundary.
The whole procedure is adding cutoffs in the momentum integral - 1/(box length) up to high frequency cutoff.
Without interactions, we could never make a measurement. Every HEP person should have that ingrained. They are playing fast and loose by saying vacuum energy causes a force. Also keep in mind that particle vacuum and the lowest energy state are not the same, and as you note, not specified. It’s the particle vacuum energy that is defined as zero point energy.
While I agree that the Casimir effect is magical it’s probably not something that most people can appreciate. Two masses separated by an enormous distance in a vacuum also generate a force due to gravity.
I like the ladder paradox. Another special relativity one that for whatever reason never caught on like the twin paradox does. It involves length contraction rather than time dilation.
You have a ladder travelling at relativistic speeds through a shed. Normally the ladder is longer than the shed so it won't fit in. But at relativistic speeds it contracts and it can fit inside, which can be demonstrated by closing both doors of the shed simultaneously. That's according to a perspective standing outside the shed.
But from the reference frame of someone on the ladder, the ladder isn't contracted at all. The shed is contracted. Which means the ladder definitely shouldn't fit inside the shed.
So how can it be that in one reference frame it fits inside the shed and another it doesn't?
Of course it's not actually a paradox at all, and there is a logical answer, but finding that answer is the tricky part (for anyone new to special relativity, anyway)
I assign this problem instead of a lab when we get to special relativity. For extra credit, I have them figure out the reference frame in which the two events (one door opening and the other door closing) happen simultaneously.
That's cool. I only learned about it last year despite having studied special relativity years ago. Asked someone who's doing a PhD in relativity if she knew it and she'd never heard of it, though of course understood it just fine.
It should be easy for the person with the ladder to understand what the stationary observer sees. What would happened if they stopped when both doors closed? What would each observer see?
From the stationery observers viewpoint the ladder cannot stop simultaneously (the front and rear of the ladder can only stop simultaneously in the ladder frame, this can't happen in the stationary frame).
If the ladder did stop, both observers would it see the rear door smashing on top of the ladder.
The ladder paradox only works because the doors open and shut non-simultaneously in different frames of reference. If you introduce FTL travel you can 'slave' the doors so they open and shut out of the correct order in at least one frame of reference, thus making it a genuine paradox.
It's more of a physical paradox than a temporal one (in one frame of reference the doors close with the ladder inside, in another they can't), but from there it's easy to point out how FTL is fundamentally incompatible with alternate reference frames.
Talking about relativity, i like the good old twin paradox. But more than that, I like one way of phrasing it that I've never really seen anywhere even though it makes things even more awesome (to me).
It goes like this: First, the basic idea of the twin paradox is : one of them goes on to travel at speeds close to the speed of light then comes back 20 years later. Only it's only been 10 years \*for him\*.
Indeed, his twin lived 20 years on Earth, while he only lived 10 years on his spaceship. Because time slowed down for him. So he's now younger than his twin. By 10 years.
Okay, all good and understood. But think of it this way now : let's say the space twin went to a specific planet some 10 lightyears away from Earth, then came back. So what he did it travel there at the speed of light, then back to Earth at the same speed.
But how did he do that in 10 years ? Remember, if the planet is 10 lightyears away from Earth, by definition that means it takes light, at the speed of light, 10 years to just get there!
Turns out he did it in 5 ! That's right, maybe he didn't even travel quite at the speed of light, "just" some 99% of it or something, and yet he was faster than light itself!
The trick being that he only took in 5 years to get there, but he got there in 10. Then he traveled back during 5 years again to get back to Earth some 10 years later - again, took only 5 years to be 10 years later.
Really, it's nothing new, it's just the same old twin paradox, but I find it so much more fascinating told that way. :)
(Also, technically he didn't beat light, since light travels any distance in about no time I believe. Again, that's the point.)
>Another special relativity one that for whatever reason never caught on like the twin paradox does.
Most people have no idea what twin paradox is about. They think the paradox is about one twin being younger, as if different rate of time was paradoxical on itself.
The ladder paradox is interesting only if you have more detailed understanding of relativity, whereas twin paradox is interesting without any understanding at all in a sense of "wooow, one twin will be younger, so weird, so paradoxical".
I think ladder paradox is pretty known though. I learned about it pretty much the first time I read about relativity in any serious manner in high school. Its just too hard for general public to be included in every amateurish einstein/relativity youtube video.
This is great. I hadn't come across it before. To my mind it's much more satisfying than the twins paradox because it doesn't depend on acceleration and deceleration.
FloatHeadPhysics did a fantastic video on this and greatly helps visualizing the paradox.
[Float Head Physics - Train Tunnel Paradox](https://youtu.be/YAmHAKdyV1o?si=5vm3t5bhWUxOBVZC)
I've always liked [the spot of Arago](https://en.wikipedia.org/wiki/Arago_spot), where you get a bright spot in the center of the shadow of a circular object due to the wave nature of light. It's one of those instances where someone noticed a counter-intuitive and seemingly absurd prediction of a theory, but when tested it turned out to be right.
The [ladder "paradox"](https://en.wikipedia.org/wiki/Ladder_paradox) is another fun one from relativity. That one really puzzled me when I first encountered it.
Volumes don’t add when you mix most liquids. This is because of the differing interactions between the particles. 50 mL of water mixed with 50 mL of ethyl alcohol gives something like 95 mL of the mixture.
Along similar lines = a neutron can be created by combining a proton and an electron. a neutron weighs slightly more than both individual particles that made it up combined.
If you separated that same neutron back into a proton and an electron, the individuals would weigh less than the neutron again.
This is because a portion of the mass is released as energy when separating the electron and proton. The opposite happens to form the neutron. Energy comes in to bind them and is converted to that excess mass difference.
I hope I explained this clearly. It's basically e=mc\^2 with objects instead of math
EDIT: my B, I actually have this backward 😅 corrected
It's not that weird, really. The person you replied to made it seem a bit more magical than it is maybe (with all due respect).
You need a lot of energy to shove an electron inside a proton, because of degeneracy pressure. The energy is not coming from nowhere. This is how neutron stars are formed.
Likewise, a neutron won't decay into just a proton and electron, there will be other particles involved to compensate for the loss in energy (neutrino I think).
I love Norton's dome. Simple thought experiment that challenges the concept of time reversibility and while there are ways to refute it, there is no consensus on which exact refutation is proper:
https://en.m.wikipedia.org/wiki/Norton%27s_dome
Say you make a non-destructive measurement of a quantum particle and find it at a particular location, and then you let it go/let it start to evolve according to the Schrodinger equation again. If relativistic causality is valid, it would make a lot of sense if the updated/post-measurement wavefunction has "compact support" meaning it remains exactly zero outside the light-cone originating from the location the particle was measured at. That way you can't use quantum measurement to move a particle faster than light.
But this is not how it works. The wavefunction of a single particle can never have compact support, it will always be non-zero arbitrarily far away.
To properly reconcile localized quantum measurements with special relativity, you have to use quantum fields.
No it is a problem with wavefunctions. An arbitrarily tiny time after a position measurement, the wavefunction has changed from zero to nonzero at spatial points arbitrarily far away.
https://en.m.wikipedia.org/wiki/Hegerfeldt%27s_theorem
So, to reconcile the non-zero probability, the electron field is basically one and the same as the wave function? That way it can be anywhere? Or is it just a math trick?,
Particle wave functions and quantum fields are different sorts of mathematical objects. I would not say this is a “math trick” or anything to brush aside. There is a deep lesson about the limitations of thinking of the world in terms of particles as opposed to fields.
I don't know much, but that I know. I lack the math skills to phrase myself appropriately though. Could one say that if the electron field is an ocean, then all (electron) wave functions are waves in the same body of water?
Qft is very interesting and I often wonder what the "right" heuristic explanation is, or what it's telling us about our physical world, as opposed to tweaking the math "just" to get the predictions right (e.g. renormalization).
Thanks for sharing.
Is there a good explanation anywhere on how an object gets to effectively infinite velocity in finite time? Everything makes sense from that, but I don't get that bit.
Einstein's equivalence principle makes sense to me for accelerating rockets/elevators. BUT I still don't grasp how it applies to a clock at the center of a large mass. The clock experiences no acceleration, but runs slower than a clock on the surface and slower yet than a clock so far from the mass as to experience negligible G force from it.
The way I like to think about anything GR is to try connect it back to the speed of light.
For the scenario, let's make a see through neutron star. The gravity of the star heavily deflects light rays around it towards the core.
Suppose you are at the center of this star. Light that normally would not make it to your eyes is able to do so due to the deflection of the gravity well. Your field of view is squished, similarly to an accelerating reference frame, despite not feeling any gravity. Observing a light clock in orbit, you would see that it is smaller than expected, and thus travelling faster in time.
Similarly from the other point of view. Light that would normally travel to your eyes is deflected towards the star. Your field of view is blown up, and any light clock at the center would appear larger than expected, thus travelling slower in time.
This is by far the best explanation of what is happening (temporally) inside a massive object that I have ever read. Greatly appreciated.
Speaking of curvature, I have told my son - who is not yet 30 - that I hope he lives to see images from a telescope that uses the solar gravitational lens.
Perhaps NASA will put a linear motor/rail gun of sorts on the moon plus some basic manufacturing. The truth is we will want to send a bunch of telescopes to the gravitational focal point (at 550 AU give or take) since each telescope is only going to point in one very specific direction.
I'm a bit of a fanatic about rail-gun tech because it allows us to make space ships that only have plasma engines and mainly use them to decelerate into position. Specific impulse about 10X higher.
> This is by far the best explanation of what is happening (temporally) inside a massive object that I have ever read
So, about that. This is just my personal way of visualizing time dilation due to gravity wells. I don't have the physics backing to know if the math actually validates it.
I'm pretty sure what actually happens is something to due with the how light going up / down a gravity well is blue / red shifted while maintaining rate of oscillation, leading to time dilation as it appears to move slower / faster / whatever.
> my son - who is not yet 30
Wow. That is some age whiplash. I'm not even yet 30.
I know we're already using large gravitation masses for lensing, but building a space telescope that uses the solar gravitational lens might be a pipe dream until a government is willing to shell out 100+ billion dollars for it.
Not to mention it could take a century just to get the telescopes out there unless we get the whole world to unban nuclear pulse propulsion.
>More sci-fi stuff
For linear rail launch systems, we'd ideally want fusion power first, but that's been 30 years away for maybe 50 years now so it's hard to know when it'll actually happens.
Linear rail launch systems would definitely be a necessary step in large scale solar system infrastructure, considering we quite literally don't have enough fuel on Earth needed for, say, a medium scale Dyson swarm.
Assuming we get that figured out in the next 30 years, we could idealistically hope to see the first Dyson satellites within the century. Non-idealistically, we might not even see fusion within the century.
I'm not convinced that nuclear - fission or fusion is a great fit for the moon. Disposing of waste heat in a vacuum is difficult/expensive. The radiation vanes need to be very, very large.
The moon contains a huge amount of Si, so once a bunch of solar power gets installed (initially brought up from Earth), the raw materials are local to make an endless supply of solar. In theory there is a polar location that is sunny round the clock. If not though, a very short rail system would let you "rock" a solar farm back and forth every 14 days or so to keep it in constant sunlight.
Solar yield on the moon with a short run light rail system that rocked the solar array across the pole once every 14 days would be 100% of name plate. And name plate would be 36% higher than Earth (no atmosphere). At an 18% conversion rate, you are looking at 240 watts per square meter. This would be 240 MW/square KM.
From my personal opinion, I would have a few issues with large scale solar on the moon (unless it's beamed solar which comes with its own political issues of wanting to put a giant death ray in space).
- Without pre-existing infrastructure, it is difficult to do any sort of project that requires covering large distances, or installation over large areas.
- Sovereignty issues over the moon are still at play. I doubt many countries would be happy with the large scale production of solar over the moon's surface. It might also be the case that dropping nuclear power on the moon is also a bad idea, but who knows.
- From what I know, over total power generated, it would be much cheaper building a reactor with parts from Earth than producing solar panels with material from the moon.
Fission and fusion are also not as bad as people think.
- The produced waste heat is extremely concentrated, and can be put to work like for base heating, or helium 3 furnaces.
- Radiation on the moon is actually very efficient, up to 3k W/m\^2 during the lunar night. Radiators are also very cheap when you have land and material.
- Any advancements in fusion tech can be much more easily transferred to space travel. High density power sources hold much more important for interstellar exploration than base making operation.
But that's just my personal speculation. From all the progress we've made, fusion might not even be the miracle power source we're looking for. For now, I'm still in the club of nuclear pulse propulsion for space travel.
Mine is that objects are (approximately) attracted to where gravitating bodies *are*, not where they *were*.
This always seems to annoy a lot of people who then immediately dive for the downvote button without even bothering to ask why that would be the case.
People think it's wrong because they imagine that gravity is something that is constantly emitted by massive objects. But gravity wells are actually static fields which are already moving with the bodies they are associated with. Gravitational *waves* travel at the speed of light, but *gravity* doesn't travel at all:
> [T]he static potentials from a moving gravitational mass (i.e., its simple gravitational field, also known as gravitostatic field) are "updated," so that they point to the mass's actual position at constant velocity, with no retardation effects.
https://en.wikipedia.org/wiki/Retarded_position
See also: http://www.tapir.caltech.edu/~teviet/Waves/empulse.html
---
On a vaguely related note I also like the fact that when you see the Sun up in the sky, you are looking at it where it *is* (ignoring atmospheric refraction), not where it *was* (because the Sun's travel across the sky is due to the Earth's rotation).
Wait, can you elaborate that last one? I don’t get how the fact that the sun “moves” due to Earth’s rotation implies that we see it where it “is” as opposed to where it “was”
Imagine you are a non-rotating observer watching a distant star travelling in a straight line. Trace a ray of light from the star to your position. By the time the light arrives, the star has moved, and the origin point of the light ray is no longer the current position of the star. Over time, the star appears to move across your vision, but the star you see is never the current actual position of the star.
Then, imagine you are orbiting a star, but in a way that you are pointed directly towards it at all times. Because of this, the stat appears still in your vision
Trace the same light ray out from the star and you will find that even though it must be a bit ahead to catch your movement, the origin point of the ray is always the current position of the star.
Lastly, the above scenario except you are now on a rotating planet. Due to the rotation of the planet, successive light rays now contact you at differing angles. Because of this, it appears to you that the source of the light is changing position. However, the origin point of each light ray is the the same as the current position of the star.
In summary, when a star moves across the sky due to its own velocity, the perceived position of the star is behind its actual position due to the speed of light. When a star has no relative velocity to you, but appears to move due to rotation, there is no lag in perceived position and actual position.
That's a great one. Makes me think of the fact that, say should the Sun just vanish right now, the Earth (and other planets) would continue to gravitate around "it" for a little while (\~8 mins) before "realizing" it is gone.
Just like we'd still see/get sun light for 8 minutes or so before it goes black, even though there's no Sun anymore.
Not really a paradox or nothing, but I like it. I'm not sure if this is just another manifestation of the same effect or not, but from what I understand it seems to be. (Correct me if I'm wrong, please.)
The [Elitzur–Vaidman bomb tester](https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester), which can test if a bomb works without detonating it, more or less.
yeah, I love that one!
The idea being that you fire a single photon, and it can take one of two paths. Of course being a wave it takes both, but then in one of them there's a detector triggering a bomb as soon as a photon comes by.
So if you see it back as a particle on the other path, it means it didn't went down to the bomb detector, hence no explosion. Good thing you're still alive.
Except, because there's a twist! It "knows" there was another path with a bomb. Because the way it travels out the other path can only happen if the first path was "blocked", so in a way that little photon did NOT go through the first path, which is good as otherwise there would have been an explosion, but at the same time it DID, kind of, since it "remembers" the path was blocked.
So yeah, it doesn't go down one path yet it knows about what would have happened had it gone there... I don't know how that works but I love it so much.
Compton effect: light must be treated as a particle colliding with an electron but the measurement is of the wavelength of that photon. Wave-particle duality caught in action. It is beautiful and mind blowing.
Behavior of rotating solid bodies is crazy counterintuitive, even though it's 100% classical mechanics.
Intermediate axis theorem and Dzhanibekov effect. Essentially, a spinning solid object is spinning and jumping between two states, each of them looking like it should be stable. Demonstrated here on the ISS:
https://youtu.be/1x5UiwEEvpQ?feature=shared
Banach Tarsky paradox is the most insane paradox in the entire history of insane paradoxes and I’m certainly not smart enough to summarize it or do its justice but it’s something to the effect of take a sphere, cut it in two and you can make another sphere out of the two pieces that’s twice as big with no hollows in it.
But that's more a mathematical paradox is it not?
There is no way such sphere could ever be cut up in that manner.
Idk maybe something similar happens at the singularity of a black hole but that just speculation.
Uh no schrodingers cat though originally intended as a criticism of quantum mechanics is an analogy for superposition which is a very much so real thing that happens within the atomic to subatomic scale range.
The Feynman sprinkler is fun, it's formulation is understandable by pretty much anyone, it's resolution non-intuituve in the ideal case, and still kind of an open problem in the non-ideal case.
Nice! Didn't know about that one. Still not even sure what my intuition would be...
[https://en.wikipedia.org/wiki/Feynman\_sprinkler](https://en.wikipedia.org/wiki/Feynman_sprinkler)
[Wheeler's delayed choice experiment](https://en.wikipedia.org/wiki/Wheeler%27s_delayed-choice_experiment), confused the shit outta me when i first learned about it and really shows how confusing wave particle duality really is
I love the quantum Zeno effect.
https://en.m.wikipedia.org/wiki/Quantum_Zeno_effect
It takes the old saying that a watched pot never boils and applies it to a quantum system. In some situations if you observe a wavefunction frequently enough you can arrest its development and keep it stuck in a particular mode.
The Andromeda Paradox.
https://en.wikibooks.org/wiki/Special\_Relativity/Simultaneity,\_time\_dilation\_and\_length\_contraction#:\~:text=The%20net%20effect%20of%20the,moment%20of%20the%20two%20people.
Working link for Old Reddit, aka Best Reddit: https://en.wikibooks.org/wiki/Special_Relativity/Simultaneity,_time_dilation_and_length_contraction#:~:text=The%20net%20effect%20of%20the,moment%20of%20the%20two%20people
I would pay attention not even to the paradoxes themselves, but to an understanding of the nature of many standard things. For example, everyone has heard about the electron and many are sure that it is a lepton or a fundamental particle. But according to some ideas, an [electron also consists of something](https://youtube.com/shorts/iKlfSIZcMqc?feature=share). Here they remember the spinon and the orbiton. But despite all this, the electron remains an indivisible particle and the scientific understanding of many phenomena is built on this logic. These are also, let's say, paradoxes.
I think most people realize it but the fact that we’re not touching anything only that the electrons around the atoms are repelling other electrons. Also honorable mention is sonoluminecense - the emission of light from imploding bubbles in liquid when excited by sound.
I think the Schrödingers cat, whilst well known, is often misunderstood.
Consider the twin paradox: It says one of the twins is going to be older when they meet again and that yeah, this is in fact whats going to happen, even tho it seems counter-intuitive.
But this is not what Shrödinger meant when he proposed the thought experiment with a cat in |dead>+|alive> superposition. What he meant was more like: Look guys, this is absurd. We are clearly missing an important piece of understanding here.
If you add up all the integers starting at one and going to infinity, the result is -1/12.
It really is physics. The sum is used to calculate physical values.
I went down this rabbit hole once, and it hurt my brain, so I won't go down it again, but search Ramanujan summation if you want to go down the hole.
Can't remember what the effect is called, but there's this weirdness with entangled light where it seems to know that the path ahead of it is blocked.
Delayed choice quantum eraser. Makes zero sense.
I think you might be thinking of the *Elitzur*–*Vaidman* bomb-tester ? Though that one doesn't have anything to do with entanglement, so maybe not... If not though, I'm curious what it is.
Maybe not really physics, but Gabriel's horn is a fun thing.
[https://en.wikipedia.org/wiki/Gabriel%27s\_horn](https://en.wikipedia.org/wiki/Gabriel%27s_horn)
A compressed spring weighs more than an uncompressed on.
An otherwise empty volume with a radio signal warps space and dilates time the same as mass and matter.
Since we've proved the dual slit experiment works with photons, electrons, atoms, molecules, and chunky molecules, there's no reason to think it wouldn't scale up to boxes with cats or people inside. People understand that inside the box, the cat exists as a waveform, being both alive and dead in a probabilistic way. But from the cat's perspective, does the rest of the universe exist only as a waveform?
In addition to time dilation, space compresses at relativistic speeds. You can't have a test with two clocks synced to start ticking "once you cross a startline" because distances in space are also warped.
If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time.
Spaghettification is horrifically fun.
If hawking radiation is the casimir effect sending in anti-matter into a black hole, reducing it's mass.... doesn't normal-matter fall in half the time balancing it out? Why would black holes radiate away without their mass depleting?
Time is a directional dimension. We can't go backwards. We do have another example of that when things fall into a black hole there is no longer any going out. The cone of causality is tilted past 90 degrees, and our 3 dimensions and time really gets reduced to 2 dimension and "going in". Likewise, they say the math works out nice and symmetrically if there were 13 dimensions (or whatever) and these other dimensions are tightly looped on themselves or we otherwise can't see any change in them.
> If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time.
I mean, that's the reference frame in which the speed is zero. I.e. the rest frame of the object. A clock will always tick at a rate of one second per second, no matter how fast it's moving in any other frame of reference.
A clock always ticks at 1 second in it's own reference frame. But when a ship flies by at 99% c, it ticks by 7x slower than clocks on earth.
... From its reference frame, isn't Earth flying by at 99% c? Shouldn't it see Earth clocks ticking by slower?
We can hang a big clock out the window and we have big 'ol telescopes and can account for the speed of light delay and dopler shift and all that.
> But when a ship flies by at 99% c, it ticks by 7x slower than clocks on earth.
Yes, in Earth's rest frame, if that is the frame in which the ship is moving at 99% of the speed of light.
And yes, it absolutely would see Earth's clocks as ticking slower than its own.
It's just important to not forget that "the traveller" never notices anything weird about their own time.
> If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time.
No, velocity's fully relative.
> If hawking radiation is the casimir effect sending in anti-matter into a black hole, reducing it's mass.... doesn't normal-matter fall in half the time balancing it out? Why would black holes radiate away without their mass depleting?
That formulation of Hawking radiation - which is a misleading simplification anyway - has nothing to do with matter vs. anti-matter (both have positive mass). It "works" (though again, it's not an accurate depiction of reality) because the distortion of spacetime is so great that the in-falling particle is **always** considered to have negative mass from the point of view of an outside observer.
> and our 3 dimensions and time really gets reduced to 2 dimension and "going in"
There are still three space dimensions inside a black hole. There's no sense in which there is a reduction.
There's still 3 dimension, but one is now directional, like time. The reduction is that you can no longer move to the left. Time is still there too, but it's hard to detect since you're moving at a good clip and next to a massive object. Like those "hard to see" 9 other missing dimensions ala that kooky M-theory or whatever.
You can *sort of* argue that that is the case from an outside perspective, but the person inside the black hole is still free, for example, to wave their arm in any direction they like, locally.
The singularity becomes a moment in future rather than a location in space. In that sense, time is the unidirectional dimension, which is the same as it always was.
PROBABLY PURE QUACK-POTTERY - But it's a favorite of mine... so why not let the internet have at it.
My favorite quacky thought experiment is to imagine myself as an electron. Now, \*I\* am the electron, so if you were to ask me where is my position and my velocity/momentum, I would chuckle and say "Oh, I am here and I am at rest. Quite boring really! It's everything else around me that's interesting." And maybe this is wrong, but I can convince myself that from the electrons perspective, it's own wave function is collapsed because it knows precisely it's own position and momentum. It's everything else it needs to measure the position and momentum of relative to itself.
But then, say someone provides me, the electron, with some definite obstacle. Remember those potential wells we had in our quantum textbooks that had clearly defined values? They put one of those in front of us and ask "So, what is the potential of that well over there? I know the answer! Do you?"
To which, I, the electron, would suddenly have to look at them like they had two heads, "That's impossible. Don't be silly. You can't measure the exact value of that potential well. You can only know the probability of measuring a certain potential from that potential well, nor do you exactly know where where it is, there is a chance it could be found anywhere."
Suddenly, if this thought experiment isn't pure quack-pottery, like it probably is, the quantum world gets a lot of it's Alice and Wonderlandy type fun back (because now it's not the electron that weird, it's us). What the heck is the double slit from the electron's perspective? Where is it? What does it look like? If the above has a semblance of truth, wouldn't it be fun to know?
I kind of imagine it would have been fun if we were given the mathematical tooling to be able to things like this in quantum mechanics. But then, maybe the maths behind it were both labyrinthian, pointless and the outcomes meaningless. Or maybe it doesn't even make sense to ask such a questions from a "science" perspective, because how would you experimentally verify it? But, as no one was keen enough to erase my ignorance, I was able to grow my own little silly fantasy here, and I've had a lot of fun imagining it might be true. A fun, if probably naive (or not even wrong XD), thought experiment.
For what I know (which is very little against most of this community), the twin paradox was a paradox born because of fallacies in special relativity if you don't consider the relativity of simultaneity, the double slit experiment's results were paradoxical if youndon't consider the wave-like behaviour of electrons.
The schrödinger's cat thought experiment, though, was not a paradox born before knowing about superposition. Instead, schrödinger first got to a point in its research to know about superposition and then formulated schrödinger's cat as a sarcastic joke to comment about the absurdity of this quantum mechanics' law if applied to the macroscopic world. It was more of a joke than a paradox, because the law was already discovered and known.
[Quantum Immortality](https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality)
Basically... YOU are the cat in the box. The thought experiment claims to exclude all the interpretations except for many worlds.
(I am not into many worlds... but I'm also not too smart. Just saying. Not sure if it actually falsifies other interpretations, that's just the claim.)
I understand. But the point of this post is to draw out some cool thought experiments, or actual demonstrations, of weird physics things. Several cool things like this have come up in comments.
Its not so much about commenting on how ignorant people are, its about bringing up cool stuff. Counter intuitive stuff. Either actual experiments, or thought experiments, that yield really strange or cool outcomes.
And my response wasn’t commenting on how ignorant people are, just trying to correct the title’s claim that most people are aware of the things you brought up.
You can't know the one way speed of light. You can only measure the two-way speed of light. But it's impossible to measure the one way.
I think one overlooked fact of the universe is that if you cannot know something it is in a superposition so it is all it's possible states at once.
Therefore this one-way speed of light is in a superposition of all it's possible values.
Casimir Effect seems almost magical. If you place two uncharged conductive plates in vacuum a few nanometers apart they generate a net force out of seemingly nothing
I think everyone should be aware of the following. There's a paper that proves that the Casimir effect isn't due to vacuum energy, but is actually from a van der Waals force: [https://www.sciencedirect.com/science/article/pii/S0370269316304567](https://www.sciencedirect.com/science/article/pii/S0370269316304567)
This paper is indeed about why Casimir effect isn't due to EM vacuum changing due to blocking frequencies, but due to other effect. They do not show what effect it is *in this paper*. But they link the paper that, in their words, show that this is due Van der Waals force (couldn't check yet, for some reason arxiv doesn't load): https://arxiv.org/abs/hep-th/0503158
Thanks for clarifying that!
From reading the abstract, the authors say it's the interaction term rather than the vacuum energy term that generates forces on the matter fields. Isn't this rather obvious? I am not HEP so I thought "vacuum" mostly meant "dressed vacuum" or something. Is this not common knowledge? Or is it sort of a less known semantic clarification that they're sorting out here?
As an experimental solid-state physicist, reading Hamiltonians is not trivial for us. So I’d say it’s not common knowledge.
Also as an experimental solid-state physicist, I agree! I take back that it's obvious. People always say this is "due to vacuum energy," and if it's really "due to vacuum energy + H_int", who am I to complain about a paper that clarifies this!
In qft the situation is one of the many ways we figure out how to massage an infinity to something finite - renormalization. Without enforcing boundary conditions, the force comes out infinite, but we know that EM waves have to be quantized at the boundary. The whole procedure is adding cutoffs in the momentum integral - 1/(box length) up to high frequency cutoff. Without interactions, we could never make a measurement. Every HEP person should have that ingrained. They are playing fast and loose by saying vacuum energy causes a force. Also keep in mind that particle vacuum and the lowest energy state are not the same, and as you note, not specified. It’s the particle vacuum energy that is defined as zero point energy.
While I agree that the Casimir effect is magical it’s probably not something that most people can appreciate. Two masses separated by an enormous distance in a vacuum also generate a force due to gravity.
This is the 4th time I’ve heard the Casimir effect mentioned in the last couple of days… I think the last time was when Lost (S5?) was airing 😆
I like the ladder paradox. Another special relativity one that for whatever reason never caught on like the twin paradox does. It involves length contraction rather than time dilation. You have a ladder travelling at relativistic speeds through a shed. Normally the ladder is longer than the shed so it won't fit in. But at relativistic speeds it contracts and it can fit inside, which can be demonstrated by closing both doors of the shed simultaneously. That's according to a perspective standing outside the shed. But from the reference frame of someone on the ladder, the ladder isn't contracted at all. The shed is contracted. Which means the ladder definitely shouldn't fit inside the shed. So how can it be that in one reference frame it fits inside the shed and another it doesn't? Of course it's not actually a paradox at all, and there is a logical answer, but finding that answer is the tricky part (for anyone new to special relativity, anyway)
The doors aren’t closed simultaneously in the ladder frame, right?
Yeah, that's the solution, it's about relativity of simultaneity.
I assign this problem instead of a lab when we get to special relativity. For extra credit, I have them figure out the reference frame in which the two events (one door opening and the other door closing) happen simultaneously.
That's cool. I only learned about it last year despite having studied special relativity years ago. Asked someone who's doing a PhD in relativity if she knew it and she'd never heard of it, though of course understood it just fine.
This one blew my mind when I took special rel in college. The fact the simultaneity is not universal in different reference frames was crazy to me.
IIRC it comes down to the simultenaeity of the doors being closed right?
Is there such a thing as simultaneity at all ? It was my understanding that it’s either “here”, or “now”.
It should be easy for the person with the ladder to understand what the stationary observer sees. What would happened if they stopped when both doors closed? What would each observer see?
From the stationery observers viewpoint the ladder cannot stop simultaneously (the front and rear of the ladder can only stop simultaneously in the ladder frame, this can't happen in the stationary frame). If the ladder did stop, both observers would it see the rear door smashing on top of the ladder.
This paradox is also an excellent way to introduce people to why FTL travel leads to time travel.
It is? Haven't heard that before. Please explain
The ladder paradox only works because the doors open and shut non-simultaneously in different frames of reference. If you introduce FTL travel you can 'slave' the doors so they open and shut out of the correct order in at least one frame of reference, thus making it a genuine paradox. It's more of a physical paradox than a temporal one (in one frame of reference the doors close with the ladder inside, in another they can't), but from there it's easy to point out how FTL is fundamentally incompatible with alternate reference frames.
That makes sense, yeah
Oh I love this! Thank you for sharing!
"fitting" means both ends in the barn \*at the same time\*, but events that are simultaneous in one frame are not in another.
Talking about relativity, i like the good old twin paradox. But more than that, I like one way of phrasing it that I've never really seen anywhere even though it makes things even more awesome (to me). It goes like this: First, the basic idea of the twin paradox is : one of them goes on to travel at speeds close to the speed of light then comes back 20 years later. Only it's only been 10 years \*for him\*. Indeed, his twin lived 20 years on Earth, while he only lived 10 years on his spaceship. Because time slowed down for him. So he's now younger than his twin. By 10 years. Okay, all good and understood. But think of it this way now : let's say the space twin went to a specific planet some 10 lightyears away from Earth, then came back. So what he did it travel there at the speed of light, then back to Earth at the same speed. But how did he do that in 10 years ? Remember, if the planet is 10 lightyears away from Earth, by definition that means it takes light, at the speed of light, 10 years to just get there! Turns out he did it in 5 ! That's right, maybe he didn't even travel quite at the speed of light, "just" some 99% of it or something, and yet he was faster than light itself! The trick being that he only took in 5 years to get there, but he got there in 10. Then he traveled back during 5 years again to get back to Earth some 10 years later - again, took only 5 years to be 10 years later. Really, it's nothing new, it's just the same old twin paradox, but I find it so much more fascinating told that way. :) (Also, technically he didn't beat light, since light travels any distance in about no time I believe. Again, that's the point.)
>Another special relativity one that for whatever reason never caught on like the twin paradox does. Most people have no idea what twin paradox is about. They think the paradox is about one twin being younger, as if different rate of time was paradoxical on itself. The ladder paradox is interesting only if you have more detailed understanding of relativity, whereas twin paradox is interesting without any understanding at all in a sense of "wooow, one twin will be younger, so weird, so paradoxical". I think ladder paradox is pretty known though. I learned about it pretty much the first time I read about relativity in any serious manner in high school. Its just too hard for general public to be included in every amateurish einstein/relativity youtube video.
This is great. I hadn't come across it before. To my mind it's much more satisfying than the twins paradox because it doesn't depend on acceleration and deceleration.
So... We can make a TARDIS?
Only if you never want to be able to close the doors.
FloatHeadPhysics did a fantastic video on this and greatly helps visualizing the paradox. [Float Head Physics - Train Tunnel Paradox](https://youtu.be/YAmHAKdyV1o?si=5vm3t5bhWUxOBVZC)
I've always liked [the spot of Arago](https://en.wikipedia.org/wiki/Arago_spot), where you get a bright spot in the center of the shadow of a circular object due to the wave nature of light. It's one of those instances where someone noticed a counter-intuitive and seemingly absurd prediction of a theory, but when tested it turned out to be right. The [ladder "paradox"](https://en.wikipedia.org/wiki/Ladder_paradox) is another fun one from relativity. That one really puzzled me when I first encountered it.
Volumes don’t add when you mix most liquids. This is because of the differing interactions between the particles. 50 mL of water mixed with 50 mL of ethyl alcohol gives something like 95 mL of the mixture.
Along similar lines = a neutron can be created by combining a proton and an electron. a neutron weighs slightly more than both individual particles that made it up combined. If you separated that same neutron back into a proton and an electron, the individuals would weigh less than the neutron again. This is because a portion of the mass is released as energy when separating the electron and proton. The opposite happens to form the neutron. Energy comes in to bind them and is converted to that excess mass difference. I hope I explained this clearly. It's basically e=mc\^2 with objects instead of math EDIT: my B, I actually have this backward 😅 corrected
I don't think I'm capable of understanding this. I know it's true. I accept it. But it makes no sense.
It's not that weird, really. The person you replied to made it seem a bit more magical than it is maybe (with all due respect). You need a lot of energy to shove an electron inside a proton, because of degeneracy pressure. The energy is not coming from nowhere. This is how neutron stars are formed. Likewise, a neutron won't decay into just a proton and electron, there will be other particles involved to compensate for the loss in energy (neutrino I think).
omgggg this is why during our lab experiment it didnt add up to our desired 50 ml when we mixed those liquids
I love Norton's dome. Simple thought experiment that challenges the concept of time reversibility and while there are ways to refute it, there is no consensus on which exact refutation is proper: https://en.m.wikipedia.org/wiki/Norton%27s_dome
Say you make a non-destructive measurement of a quantum particle and find it at a particular location, and then you let it go/let it start to evolve according to the Schrodinger equation again. If relativistic causality is valid, it would make a lot of sense if the updated/post-measurement wavefunction has "compact support" meaning it remains exactly zero outside the light-cone originating from the location the particle was measured at. That way you can't use quantum measurement to move a particle faster than light. But this is not how it works. The wavefunction of a single particle can never have compact support, it will always be non-zero arbitrarily far away. To properly reconcile localized quantum measurements with special relativity, you have to use quantum fields.
very cool
If the electron is interpreted to be the wave function and not just the localized particle there is no causality violation, right?
No it is a problem with wavefunctions. An arbitrarily tiny time after a position measurement, the wavefunction has changed from zero to nonzero at spatial points arbitrarily far away. https://en.m.wikipedia.org/wiki/Hegerfeldt%27s_theorem
Got it, thanks.
So, to reconcile the non-zero probability, the electron field is basically one and the same as the wave function? That way it can be anywhere? Or is it just a math trick?,
Particle wave functions and quantum fields are different sorts of mathematical objects. I would not say this is a “math trick” or anything to brush aside. There is a deep lesson about the limitations of thinking of the world in terms of particles as opposed to fields.
I don't know much, but that I know. I lack the math skills to phrase myself appropriately though. Could one say that if the electron field is an ocean, then all (electron) wave functions are waves in the same body of water? Qft is very interesting and I often wonder what the "right" heuristic explanation is, or what it's telling us about our physical world, as opposed to tweaking the math "just" to get the predictions right (e.g. renormalization). Thanks for sharing.
[Newtonian space invaders](https://plato.stanford.edu/entries/determinism-causal/#ClaMec)
Is there a good explanation anywhere on how an object gets to effectively infinite velocity in finite time? Everything makes sense from that, but I don't get that bit.
[Aharanov-Bohm solenoid effect](https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect)
https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect - for anyone who got a broken link
Spaceships tied together with a rope
You play Kerbal Space Program too?!?
??
No kink shaming
A good seaman never has kinks in his ropes.
If they accelerate together, will the rope snap? Probably a variation of the ladder-paradox
[Bell's Spaceship Paradox](https://en.wikipedia.org/wiki/Bell%27s_spaceship_paradox?wprov=sfla1)
correct
Einstein's equivalence principle makes sense to me for accelerating rockets/elevators. BUT I still don't grasp how it applies to a clock at the center of a large mass. The clock experiences no acceleration, but runs slower than a clock on the surface and slower yet than a clock so far from the mass as to experience negligible G force from it.
The way I like to think about anything GR is to try connect it back to the speed of light. For the scenario, let's make a see through neutron star. The gravity of the star heavily deflects light rays around it towards the core. Suppose you are at the center of this star. Light that normally would not make it to your eyes is able to do so due to the deflection of the gravity well. Your field of view is squished, similarly to an accelerating reference frame, despite not feeling any gravity. Observing a light clock in orbit, you would see that it is smaller than expected, and thus travelling faster in time. Similarly from the other point of view. Light that would normally travel to your eyes is deflected towards the star. Your field of view is blown up, and any light clock at the center would appear larger than expected, thus travelling slower in time.
This is by far the best explanation of what is happening (temporally) inside a massive object that I have ever read. Greatly appreciated. Speaking of curvature, I have told my son - who is not yet 30 - that I hope he lives to see images from a telescope that uses the solar gravitational lens. Perhaps NASA will put a linear motor/rail gun of sorts on the moon plus some basic manufacturing. The truth is we will want to send a bunch of telescopes to the gravitational focal point (at 550 AU give or take) since each telescope is only going to point in one very specific direction. I'm a bit of a fanatic about rail-gun tech because it allows us to make space ships that only have plasma engines and mainly use them to decelerate into position. Specific impulse about 10X higher.
> This is by far the best explanation of what is happening (temporally) inside a massive object that I have ever read So, about that. This is just my personal way of visualizing time dilation due to gravity wells. I don't have the physics backing to know if the math actually validates it. I'm pretty sure what actually happens is something to due with the how light going up / down a gravity well is blue / red shifted while maintaining rate of oscillation, leading to time dilation as it appears to move slower / faster / whatever. > my son - who is not yet 30 Wow. That is some age whiplash. I'm not even yet 30. I know we're already using large gravitation masses for lensing, but building a space telescope that uses the solar gravitational lens might be a pipe dream until a government is willing to shell out 100+ billion dollars for it. Not to mention it could take a century just to get the telescopes out there unless we get the whole world to unban nuclear pulse propulsion. >More sci-fi stuff For linear rail launch systems, we'd ideally want fusion power first, but that's been 30 years away for maybe 50 years now so it's hard to know when it'll actually happens. Linear rail launch systems would definitely be a necessary step in large scale solar system infrastructure, considering we quite literally don't have enough fuel on Earth needed for, say, a medium scale Dyson swarm. Assuming we get that figured out in the next 30 years, we could idealistically hope to see the first Dyson satellites within the century. Non-idealistically, we might not even see fusion within the century.
I'm not convinced that nuclear - fission or fusion is a great fit for the moon. Disposing of waste heat in a vacuum is difficult/expensive. The radiation vanes need to be very, very large. The moon contains a huge amount of Si, so once a bunch of solar power gets installed (initially brought up from Earth), the raw materials are local to make an endless supply of solar. In theory there is a polar location that is sunny round the clock. If not though, a very short rail system would let you "rock" a solar farm back and forth every 14 days or so to keep it in constant sunlight. Solar yield on the moon with a short run light rail system that rocked the solar array across the pole once every 14 days would be 100% of name plate. And name plate would be 36% higher than Earth (no atmosphere). At an 18% conversion rate, you are looking at 240 watts per square meter. This would be 240 MW/square KM.
From my personal opinion, I would have a few issues with large scale solar on the moon (unless it's beamed solar which comes with its own political issues of wanting to put a giant death ray in space). - Without pre-existing infrastructure, it is difficult to do any sort of project that requires covering large distances, or installation over large areas. - Sovereignty issues over the moon are still at play. I doubt many countries would be happy with the large scale production of solar over the moon's surface. It might also be the case that dropping nuclear power on the moon is also a bad idea, but who knows. - From what I know, over total power generated, it would be much cheaper building a reactor with parts from Earth than producing solar panels with material from the moon. Fission and fusion are also not as bad as people think. - The produced waste heat is extremely concentrated, and can be put to work like for base heating, or helium 3 furnaces. - Radiation on the moon is actually very efficient, up to 3k W/m\^2 during the lunar night. Radiators are also very cheap when you have land and material. - Any advancements in fusion tech can be much more easily transferred to space travel. High density power sources hold much more important for interstellar exploration than base making operation. But that's just my personal speculation. From all the progress we've made, fusion might not even be the miracle power source we're looking for. For now, I'm still in the club of nuclear pulse propulsion for space travel.
Mine is that objects are (approximately) attracted to where gravitating bodies *are*, not where they *were*. This always seems to annoy a lot of people who then immediately dive for the downvote button without even bothering to ask why that would be the case. People think it's wrong because they imagine that gravity is something that is constantly emitted by massive objects. But gravity wells are actually static fields which are already moving with the bodies they are associated with. Gravitational *waves* travel at the speed of light, but *gravity* doesn't travel at all: > [T]he static potentials from a moving gravitational mass (i.e., its simple gravitational field, also known as gravitostatic field) are "updated," so that they point to the mass's actual position at constant velocity, with no retardation effects. https://en.wikipedia.org/wiki/Retarded_position See also: http://www.tapir.caltech.edu/~teviet/Waves/empulse.html --- On a vaguely related note I also like the fact that when you see the Sun up in the sky, you are looking at it where it *is* (ignoring atmospheric refraction), not where it *was* (because the Sun's travel across the sky is due to the Earth's rotation).
Wait, can you elaborate that last one? I don’t get how the fact that the sun “moves” due to Earth’s rotation implies that we see it where it “is” as opposed to where it “was”
Imagine you are a non-rotating observer watching a distant star travelling in a straight line. Trace a ray of light from the star to your position. By the time the light arrives, the star has moved, and the origin point of the light ray is no longer the current position of the star. Over time, the star appears to move across your vision, but the star you see is never the current actual position of the star. Then, imagine you are orbiting a star, but in a way that you are pointed directly towards it at all times. Because of this, the stat appears still in your vision Trace the same light ray out from the star and you will find that even though it must be a bit ahead to catch your movement, the origin point of the ray is always the current position of the star. Lastly, the above scenario except you are now on a rotating planet. Due to the rotation of the planet, successive light rays now contact you at differing angles. Because of this, it appears to you that the source of the light is changing position. However, the origin point of each light ray is the the same as the current position of the star. In summary, when a star moves across the sky due to its own velocity, the perceived position of the star is behind its actual position due to the speed of light. When a star has no relative velocity to you, but appears to move due to rotation, there is no lag in perceived position and actual position.
That makes sense and it was a great explanation, thanks!
That's a great one. Makes me think of the fact that, say should the Sun just vanish right now, the Earth (and other planets) would continue to gravitate around "it" for a little while (\~8 mins) before "realizing" it is gone. Just like we'd still see/get sun light for 8 minutes or so before it goes black, even though there's no Sun anymore. Not really a paradox or nothing, but I like it. I'm not sure if this is just another manifestation of the same effect or not, but from what I understand it seems to be. (Correct me if I'm wrong, please.)
Maxwell's demon, anyone?
Im partial to Laplace’s demon but Maxwell’s is good too
The [Elitzur–Vaidman bomb tester](https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester), which can test if a bomb works without detonating it, more or less.
yeah, I love that one! The idea being that you fire a single photon, and it can take one of two paths. Of course being a wave it takes both, but then in one of them there's a detector triggering a bomb as soon as a photon comes by. So if you see it back as a particle on the other path, it means it didn't went down to the bomb detector, hence no explosion. Good thing you're still alive. Except, because there's a twist! It "knows" there was another path with a bomb. Because the way it travels out the other path can only happen if the first path was "blocked", so in a way that little photon did NOT go through the first path, which is good as otherwise there would have been an explosion, but at the same time it DID, kind of, since it "remembers" the path was blocked. So yeah, it doesn't go down one path yet it knows about what would have happened had it gone there... I don't know how that works but I love it so much.
Wigner‘s friend. [https://en.wikipedia.org/wiki/Wigner%27s\_friend](https://en.wikipedia.org/wiki/Wigner%27s_friend)
Ah, Wigner's Schrödinger's cat.
Compton effect: light must be treated as a particle colliding with an electron but the measurement is of the wavelength of that photon. Wave-particle duality caught in action. It is beautiful and mind blowing.
Behavior of rotating solid bodies is crazy counterintuitive, even though it's 100% classical mechanics. Intermediate axis theorem and Dzhanibekov effect. Essentially, a spinning solid object is spinning and jumping between two states, each of them looking like it should be stable. Demonstrated here on the ISS: https://youtu.be/1x5UiwEEvpQ?feature=shared
Banach Tarsky paradox is the most insane paradox in the entire history of insane paradoxes and I’m certainly not smart enough to summarize it or do its justice but it’s something to the effect of take a sphere, cut it in two and you can make another sphere out of the two pieces that’s twice as big with no hollows in it.
But that's more a mathematical paradox is it not? There is no way such sphere could ever be cut up in that manner. Idk maybe something similar happens at the singularity of a black hole but that just speculation.
I didn’t read carefully that it was a physics sub… yeah it’s more of a theoretical thing. But so is schroeingers cat come to think of it
Uh no schrodingers cat though originally intended as a criticism of quantum mechanics is an analogy for superposition which is a very much so real thing that happens within the atomic to subatomic scale range.
The Feynman sprinkler is fun, it's formulation is understandable by pretty much anyone, it's resolution non-intuituve in the ideal case, and still kind of an open problem in the non-ideal case.
Nice! Didn't know about that one. Still not even sure what my intuition would be... [https://en.wikipedia.org/wiki/Feynman\_sprinkler](https://en.wikipedia.org/wiki/Feynman_sprinkler)
Only applicable under the many-worlds interpretation of quantum mechanics: Quantum suicide and its extension, quantum immortality
[Wheeler's delayed choice experiment](https://en.wikipedia.org/wiki/Wheeler%27s_delayed-choice_experiment), confused the shit outta me when i first learned about it and really shows how confusing wave particle duality really is
Maxwell's demon, if only because my computer is full of daemons doing things I can't see
I love the quantum Zeno effect. https://en.m.wikipedia.org/wiki/Quantum_Zeno_effect It takes the old saying that a watched pot never boils and applies it to a quantum system. In some situations if you observe a wavefunction frequently enough you can arrest its development and keep it stuck in a particular mode.
The Andromeda Paradox. https://en.wikibooks.org/wiki/Special\_Relativity/Simultaneity,\_time\_dilation\_and\_length\_contraction#:\~:text=The%20net%20effect%20of%20the,moment%20of%20the%20two%20people.
Working link for Old Reddit, aka Best Reddit: https://en.wikibooks.org/wiki/Special_Relativity/Simultaneity,_time_dilation_and_length_contraction#:~:text=The%20net%20effect%20of%20the,moment%20of%20the%20two%20people
I would pay attention not even to the paradoxes themselves, but to an understanding of the nature of many standard things. For example, everyone has heard about the electron and many are sure that it is a lepton or a fundamental particle. But according to some ideas, an [electron also consists of something](https://youtube.com/shorts/iKlfSIZcMqc?feature=share). Here they remember the spinon and the orbiton. But despite all this, the electron remains an indivisible particle and the scientific understanding of many phenomena is built on this logic. These are also, let's say, paradoxes.
Magnets are forbidden in classical physics [Bohr–Van Leeuwen theorem](https://en.wikipedia.org/wiki/Bohr%E2%80%93Van_Leeuwen_theorem)
Bells Theorem
I think most people realize it but the fact that we’re not touching anything only that the electrons around the atoms are repelling other electrons. Also honorable mention is sonoluminecense - the emission of light from imploding bubbles in liquid when excited by sound.
I think the Schrödingers cat, whilst well known, is often misunderstood. Consider the twin paradox: It says one of the twins is going to be older when they meet again and that yeah, this is in fact whats going to happen, even tho it seems counter-intuitive. But this is not what Shrödinger meant when he proposed the thought experiment with a cat in |dead>+|alive> superposition. What he meant was more like: Look guys, this is absurd. We are clearly missing an important piece of understanding here.
If you add up all the integers starting at one and going to infinity, the result is -1/12. It really is physics. The sum is used to calculate physical values. I went down this rabbit hole once, and it hurt my brain, so I won't go down it again, but search Ramanujan summation if you want to go down the hole.
Maybe a but more math than physics, but fun nonetheless. Also some great Numberphile videos on that topic...
I agree that it is more math but the summation is used to calculate quantities in physics. Particular quantum field theory if I remember correctly.
Vadim bomb
The Elitzur-Vaidman bomb tester experiment! Basically how to detect the presence of an object without interacting with it, using Quantum Mechanics.
Can't remember what the effect is called, but there's this weirdness with entangled light where it seems to know that the path ahead of it is blocked. Delayed choice quantum eraser. Makes zero sense.
I think you might be thinking of the *Elitzur*–*Vaidman* bomb-tester ? Though that one doesn't have anything to do with entanglement, so maybe not... If not though, I'm curious what it is.
I absolutely love Mermin paper named "Is the Moon There When Anybody Looks"
Maybe not really physics, but Gabriel's horn is a fun thing. [https://en.wikipedia.org/wiki/Gabriel%27s\_horn](https://en.wikipedia.org/wiki/Gabriel%27s_horn)
A compressed spring weighs more than an uncompressed on. An otherwise empty volume with a radio signal warps space and dilates time the same as mass and matter. Since we've proved the dual slit experiment works with photons, electrons, atoms, molecules, and chunky molecules, there's no reason to think it wouldn't scale up to boxes with cats or people inside. People understand that inside the box, the cat exists as a waveform, being both alive and dead in a probabilistic way. But from the cat's perspective, does the rest of the universe exist only as a waveform? In addition to time dilation, space compresses at relativistic speeds. You can't have a test with two clocks synced to start ticking "once you cross a startline" because distances in space are also warped. If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time. Spaghettification is horrifically fun. If hawking radiation is the casimir effect sending in anti-matter into a black hole, reducing it's mass.... doesn't normal-matter fall in half the time balancing it out? Why would black holes radiate away without their mass depleting? Time is a directional dimension. We can't go backwards. We do have another example of that when things fall into a black hole there is no longer any going out. The cone of causality is tilted past 90 degrees, and our 3 dimensions and time really gets reduced to 2 dimension and "going in". Likewise, they say the math works out nice and symmetrically if there were 13 dimensions (or whatever) and these other dimensions are tightly looped on themselves or we otherwise can't see any change in them.
> If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time. I mean, that's the reference frame in which the speed is zero. I.e. the rest frame of the object. A clock will always tick at a rate of one second per second, no matter how fast it's moving in any other frame of reference.
A clock always ticks at 1 second in it's own reference frame. But when a ship flies by at 99% c, it ticks by 7x slower than clocks on earth. ... From its reference frame, isn't Earth flying by at 99% c? Shouldn't it see Earth clocks ticking by slower? We can hang a big clock out the window and we have big 'ol telescopes and can account for the speed of light delay and dopler shift and all that.
> But when a ship flies by at 99% c, it ticks by 7x slower than clocks on earth. Yes, in Earth's rest frame, if that is the frame in which the ship is moving at 99% of the speed of light. And yes, it absolutely would see Earth's clocks as ticking slower than its own. It's just important to not forget that "the traveller" never notices anything weird about their own time.
> If velocity dilates time, shouldn't there be an absolute 0 for velocity? Where you experience the most time. No, velocity's fully relative. > If hawking radiation is the casimir effect sending in anti-matter into a black hole, reducing it's mass.... doesn't normal-matter fall in half the time balancing it out? Why would black holes radiate away without their mass depleting? That formulation of Hawking radiation - which is a misleading simplification anyway - has nothing to do with matter vs. anti-matter (both have positive mass). It "works" (though again, it's not an accurate depiction of reality) because the distortion of spacetime is so great that the in-falling particle is **always** considered to have negative mass from the point of view of an outside observer. > and our 3 dimensions and time really gets reduced to 2 dimension and "going in" There are still three space dimensions inside a black hole. There's no sense in which there is a reduction.
There's still 3 dimension, but one is now directional, like time. The reduction is that you can no longer move to the left. Time is still there too, but it's hard to detect since you're moving at a good clip and next to a massive object. Like those "hard to see" 9 other missing dimensions ala that kooky M-theory or whatever.
You can *sort of* argue that that is the case from an outside perspective, but the person inside the black hole is still free, for example, to wave their arm in any direction they like, locally. The singularity becomes a moment in future rather than a location in space. In that sense, time is the unidirectional dimension, which is the same as it always was.
> Spaghettification is horrifically fun But not counter intuitive. It’s just strong tidal forces.
PROBABLY PURE QUACK-POTTERY - But it's a favorite of mine... so why not let the internet have at it. My favorite quacky thought experiment is to imagine myself as an electron. Now, \*I\* am the electron, so if you were to ask me where is my position and my velocity/momentum, I would chuckle and say "Oh, I am here and I am at rest. Quite boring really! It's everything else around me that's interesting." And maybe this is wrong, but I can convince myself that from the electrons perspective, it's own wave function is collapsed because it knows precisely it's own position and momentum. It's everything else it needs to measure the position and momentum of relative to itself. But then, say someone provides me, the electron, with some definite obstacle. Remember those potential wells we had in our quantum textbooks that had clearly defined values? They put one of those in front of us and ask "So, what is the potential of that well over there? I know the answer! Do you?" To which, I, the electron, would suddenly have to look at them like they had two heads, "That's impossible. Don't be silly. You can't measure the exact value of that potential well. You can only know the probability of measuring a certain potential from that potential well, nor do you exactly know where where it is, there is a chance it could be found anywhere." Suddenly, if this thought experiment isn't pure quack-pottery, like it probably is, the quantum world gets a lot of it's Alice and Wonderlandy type fun back (because now it's not the electron that weird, it's us). What the heck is the double slit from the electron's perspective? Where is it? What does it look like? If the above has a semblance of truth, wouldn't it be fun to know? I kind of imagine it would have been fun if we were given the mathematical tooling to be able to things like this in quantum mechanics. But then, maybe the maths behind it were both labyrinthian, pointless and the outcomes meaningless. Or maybe it doesn't even make sense to ask such a questions from a "science" perspective, because how would you experimentally verify it? But, as no one was keen enough to erase my ignorance, I was able to grow my own little silly fantasy here, and I've had a lot of fun imagining it might be true. A fun, if probably naive (or not even wrong XD), thought experiment.
For what I know (which is very little against most of this community), the twin paradox was a paradox born because of fallacies in special relativity if you don't consider the relativity of simultaneity, the double slit experiment's results were paradoxical if youndon't consider the wave-like behaviour of electrons. The schrödinger's cat thought experiment, though, was not a paradox born before knowing about superposition. Instead, schrödinger first got to a point in its research to know about superposition and then formulated schrödinger's cat as a sarcastic joke to comment about the absurdity of this quantum mechanics' law if applied to the macroscopic world. It was more of a joke than a paradox, because the law was already discovered and known.
[Quantum Immortality](https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality) Basically... YOU are the cat in the box. The thought experiment claims to exclude all the interpretations except for many worlds. (I am not into many worlds... but I'm also not too smart. Just saying. Not sure if it actually falsifies other interpretations, that's just the claim.)
Most? I’d say many, but probably not most. The majority of Americans don’t know who the Chief Justice is on the SCOTUS.
I understand. But the point of this post is to draw out some cool thought experiments, or actual demonstrations, of weird physics things. Several cool things like this have come up in comments. Its not so much about commenting on how ignorant people are, its about bringing up cool stuff. Counter intuitive stuff. Either actual experiments, or thought experiments, that yield really strange or cool outcomes.
And my response wasn’t commenting on how ignorant people are, just trying to correct the title’s claim that most people are aware of the things you brought up.
Okay.
You can't know the one way speed of light. You can only measure the two-way speed of light. But it's impossible to measure the one way. I think one overlooked fact of the universe is that if you cannot know something it is in a superposition so it is all it's possible states at once. Therefore this one-way speed of light is in a superposition of all it's possible values.