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Ask HN: What is a science fact that blew your mind when you learned it?
76 points by itronitron 9 months ago | hide | past | favorite | 184 comments
I'll go first. When I read that it takes a photon over 100,000 years to exit the Sun as visible light, I was completely astounded. Curious what other insights from science people have learned that were completely unexpected to them.

Electrons in a copper wire do not travel at the speed of electricity. Not even close.

Electricity travels at nearly the speed of light.

Electrons themselves travel like molasses:

"In the case of a 12 gauge copper wire carrying 10 amperes of current (typical of home wiring), the individual electrons only move about 0.02 cm per sec or 1.2 inches per minute (in science this is called the drift velocity of the electrons.). If this is the situation in nature, why do the lights come on so quickly [when you flip the switch]? At this speed it would take the electrons hours to get to the lights."

This completely caught me by surprise, but it makes sense once it's pointed out. Imagine a pipe filled with solid balls that just fit in it, with little friction. If you push a ball in one end, a ball pops out the other almost immediately. But not the same ball! Even if you keep pushing balls in, that first one you pushed will take a while to get the other end.

Update: as rrobukef notes in a reply, this would be the case for direct current (DC). With the usual household alternating current (AC), the electrons barely move at all!



For AC, they move back as well. Thus electrons move less than 4µm from their original place (excluding Brownian movement, I assume)

Another mindblower: you can chuck a capacitor at the other end and you’ve got a circuit with one wire!

Hence you can get shocks from one live wire.

A ripple in a pool simply displaces the ball up and down vertically. The ball doesn't move along the wave

Yeah, Physics 2 blew my mind with things like this.

The electrons actually move mostly side to side as they bump into each other. Pushing power just pushes the ones further down the pipe into the light, not the ones entering the copper cable.

Actually, 0.02 cm * 60 seconds = 1.2 cm (not inches).

Movement of charge is more like wave in water than flow of water. Also, the current tends to travel on the outside (skin) of the conductor.

Now what if a wire was moving backwards in the opposite direction faster than the drift velocity? Could current still flow?

Science doesn't "prove" hypotheses - it disproves them.

For this to work, a hypothesis must be falsifiable. Most pseudoscience (and religion) makes non-falsifiable claims, meaning they are incompatible with scientific discourse.

This simple observation is a powerful tool in any bullshit-detection kit.

Once produced, a scientific hypothesis of any merit will be attacked vigorously with experiments until enough parties are convinced that disproof is sufficiently unlikely. The process isn't always pleasant for those making the falsifiable claims.

Sadly, this is not how science is taught in most schools. There, students are given the "truth" and, on a good day, asked to verify it experimentally. We are now living with the terrible consequences of generations of youth who think science is about "proving" the truth.

This isn't really true if you use Bayesian science.

The idea you're stating is something like "It's not possible to prove hypothesis H1 if H1 is 'all swans are white' by sampling swans and only getting white ones, but it's possible to disprove hypothesis H1 by finding a black swan."

However, you can prove hypothesis H2 that "there exists a black swan" by finding such a swan. And you can't really disprove it for the same reason that you can't prove H1.

Neither of theses hypotheses are proven/disproven in the mathematical sense of "proof" but finding sufficient evidence in favor or against a hypothesis is practically the same in the case of science.

The Bayesian version of this is that the hypothesis is something like "the probability of a swan being white is P" and the value of P will by updated based on the strength of the evidence. It will never be exactly 0 or 1, but it will be close enough in many cases. Then you avoid the argument about whether a hypothesis is a binary "true" or "false" but discuss a continuous probability. E.g, the a hypothesis about the Earth being round has a probability close to one for most people while Russell's teapot and Sagan's invisible dragon have probabilities close to zero until evidence shows otherwise.

You're correct in the sense that this is what science is per Popperian definition. However, I would be careful to assume that many scientists are actually so busy refuting hypotheses. The Popperian ideal and the reality in science are two different things.

To cite a paper I currently read:

> "As Chalmers (1999) remarks, when we turn to history of science, the idea that falsificationism is actively undertaken borders on laughable (see especially Feyerabend, 1975). Not only do scientists exhibit extreme reluctance to falsify their work (Woodward and Goodstein, 1996), but if falsification had been followed as a scientific method, then many of the theories generated by some of the best minds in science would never have progressed beyond their earliest explications (see Chalmers, 1999: ch. 7; Feyerabend, 1975; Kuhn, 1970c: 234; 1970d: 13; Von Dietz, 2001: 22)." (https://journals.sagepub.com/doi/10.1177/1350508409104504)

I haven't read all the sources. But it's worth mentioning that your point is controversial.

I would like to add to your remarks that science actually says something along the lines of "Grant me these assumptions/axioms/miracles(i.e. space-time, causation, etc.)" and then I can give you a theory of gravity, statistics, etc. This I think is hinted at in Gödel's core insight from his incompleteness theorem.

I had Alan Chalmers as a teacher once. An interesting man.

Ironically, of course, Popperian falsifiability is itself unfalsafiable, and therefore does not meet its own definition as a member of itself.

Popper knew this, by the way. So the inverse to the trap you point out here is the claims that "if not falsifiable, then unable to be true/useful", which is just as specious.

Isn't this like saying if X is an axiom and you can prove Y via X, you shouldn't trust Y is true because you can't prove X is true?

What systems for determining the truth exist that aren't built on top of some fundamental axioms?

That’s where Gödel’s theorem comes in. There’s always axioms unprovable without expanding the system

Goedel's completeness and incompleteness only apply if the initial set of axioms is powerful enough to express addition over all the integers.

none; you have to at least claim as an axiom "axioms are things that can be claimed" -- but that's not what's under discussion here: logic and math are neither of them empirical sciences, after all.

my point is that "we define science as empirical falsificationism" is itself nonscientific by its own definition, and that that's OK, still capable of being true, and is a handy way to get stuff done.

Aha! Now that's why some people say that science is a religion, too.

I always allow for the possibility that science is a faith (not religion) simply because there is no way to prove that the universe actually obeys laws that we can discover. Just because an apple fell down in the same way in each of a million experiments, it's not necessary that the relationship we discovered is a "law" that defines the universe. It could just as well be that the next time we try it, the apple falls differently.

This is called the problem of induction and is indeed fundamental to the scientific method.


it helps to translate “faith” as “trust in something, in which we can ground actions taken”, instead of “[intellectually] believing what we know ain’t so” and in that framing, yes, any endeavor requires an act of faith.

for much more, https://philpapers.org/rec/HAASSO

but at this point i would say that "religion" is a very squirmy thing to define meaningfully without a lot of "well, but except when ..." clauses tacked onto the end.

according to Karl Popper, yes. I prefer John Von Neumann's take on it... "“The sciences do not try to explain, they hardly even try to interpret, they mainly make models. ... The justification of such a mathematical construct is solely and precisely that it is expected to work - that is correctly to describe phenomena from a reasonably wide area."

these days, we adopt models. discard them if they fail and start all over again. seems to be a better and more elegant way.

My favorite course in college was Sociology of Science, the majority of which was basically reading Thomas Kuhn's Structure of Scientific Revolutions.

Very interesting

A compressed spring weighs more than the same spring uncompressed. The moon and earth weigh less together than if you weighed them separately and added the values together. Most of the weight of solid objects is due to the high speeds and binding energies of the elementary particles within them rather than the rest mass of the constituent particles. The number of particles that exist is relative to how fast you are accelerating. Everything that has energy (which is everything we know of) affects the gravitational field; this means that even photons are "attracted" to each other.

A non-relativistic quantum state will return arbitrarily close to its initial state an infinite number of times. There is such a thing as interaction-free measurements: you can take photos of things without ever letting light hit a detector and you can tell whether a bomb is "active" without actually interacting with the detonator.

Energy is just a number that is calculated as a function of the state of a closed system — that this number is a constant results from the time transitional invariance of the laws of physics. Similarly, conservation of momentum is due to the spatial invariance of the laws of physics, and conservation of angular momentum is due to rotational invariance. Also, conservation of energy does not hold under general relativity.

Similarly, a fully charged Tesla (or other electric) vehicle weighs more than an uncharged one. The difference is roughly the weight of 10 human hairs, so it's absolutely negligible, but that application of E=MC^2 blew my mind the first time I saw it.

> The number of particles that exist is relative to how fast you are moving.

I thought the existence of particles was relative to how fast you're accelerating? If it's relative to how fast you're moving, wouldn't that imply that absolute velocity exists, i.e. that given two objects with known relative velocity to each other, we could potentially establish which one was "really" moving and which one was "really" still?

Should have said how fast you are accelerating. Edited now. See http://www.scholarpedia.org/article/Unruh_effect

> There is such a thing as interaction-free measurements: you can take photos of things without ever letting light hit a detector and you can tell whether a bomb is "active" without actually interacting with the detonator.

Can you provide a link that talks about that? It goes against my understanding of why there's an uncertainty principle at all

>Most of the weight of solid objects is due to the high speeds of the elementary particles within them rather than the rest mass of the constituent particles.

Wait i agree energy is mass but is this specifically correct? The rest mass is always much higher when i've seen it in equations.

Whoops, I forgot binding energy. But yes, rest mass of quarks is only about 1% of a proton or neutron’s mass.

>A compressed spring weighs more than the same spring uncompressed.

This is false. The dilemma is that you can't "just weigh" a compressed spring without assistance. However, you can get around it by weighing a compression device with the spring, both when it's compressing and not compressing the spring. The weight will be the same.

You could not practically ever measure the difference in a lab, but it’s still a real effect. Look up the stress-energy tensor. I could equivalently say that a spinning ball weighs more than the same ball when stationary.


Weight is mass times gravity. It is the mass of the object. As affected by a single force - gravity. Gravity is a force.

A spinning ball has ANOTHER force acting on it, and therefore cannot be defined by "weight".

In short, this is an absurd statement that tries to redefine a scientific definition for...I don't even know? Sensationalism?

You are not considering the effect of general relativity. See theoretical physicist Professor Matt Strassler's page where he gives the exact same example of how energy can be stored that I did: https://profmattstrassler.com/articles-and-posts/particle-ph....

All forms of energy, including potential energy in the form of a compressed/stretched spring, couple to the gravitational field. Again, read about the stress-energy tensor for how this works.

> In short, this is an absurd statement that tries to redefine a scientific definition for...I don't even know? Sensationalism?

What I've said is not even remotely controversial in the physics community. Don't take my word for it; ask anyone who has taken a basic graduate course in general physics and they will agree that a compressed spring or a spinning ball weighs an imperceptibly (i.e., immeasurably) small amount more than the uncompressed spring or stationary ball, should you have a hypothetical scale that was so precise as to be able to measure the difference.

Will read and come back, maybe I am missing something. Thanks.

You’re assuming the Newtonian theory of gravity, the parent poster uses Einstein’s general relativity. These theories disagree on quite a lot; e.g. general relativity predicts that light is affected by gravity, while Newton’s theory would not since photons are massless particles. Einstein was proven right here too.

Note however that the quoted effect – difference in weight between a compressed and uncompressed spring – will realistically be so small that I doubt you could measure that in a lab. To get a sense of how small that effect is, try comparing the E=mc^2 mass-energy of a spring to its E=kx^2 compression energy...

Will look into all of this, thanks.

I think "mind blown" is an accurate description when I first saw the Hubble Deep Field image:


I already knew that "the universe is incomprehensibly large", but seeing how many entire galaxies there were in a random dark patch of sky was eye opening to me.

Don't know exactly why, but thinking about the vastness of the universe always cheers me up. Thanks for sharing :)

It makes me sad.

Born too late to explore the world; born too early to live forever and explore the entire universe; born just in time to make dank memes.

Relatedly, my dad always told me that thinking about the vastness of the universe "weirded him out."

My hypothesis is that he has narcissistic tendencies so pondering the vastness of the universe only highlights his own insignificance. I've noticed similar reaction from people who visit a large city for the first time and get a sense of how small they are in the grand scheme.

Reminds me of the Total Perspective Vortex from Hitchhikers Guide to the Galaxy, which exposed an individual's insignificance in the universe.

As a kid, it used to weird me out to the point where I would forget what I was doing, get the chills, and enter a sort of trance. It never impacted me negatively, but pondering the scale of everything and how little of an impact we have on the grand scheme of the universe gave me a weird sensation.

I've been getting into astronomy a bit and found this image to be awe inspiring: https://upload.wikimedia.org/wikipedia/commons/4/4a/Location...

Not science, but Mathematics - That there are more ways to arrange a deck of 52 cards than seconds that have elapsed since the big bang took place.

Thousands of similar questions on reddit for anyone interested- https://duckduckgo.com/?t=ffab&q=science+fact+site%3Areddit....

Also chances are that no one has shuffled a deck in the same order as someone else has, ever in history.

This one gets me too. It's so counterintuitive to me.

(Caveat: this is assuming people shuffle well, which of course most people don't, therefore in fact people probably have shuffled to the same state).

From last few days I have been watching cards videos from numberphile. It's so fun and interesting.

A 2016 poll of 1,500 scientists reported that 70% of them had failed to reproduce at least one other scientist's experiment (50% had failed to reproduce one of their own experiments).

Reminds me of doing chemistry labs at uni. You follow the experiment plan and get nothing like the expected result so you add big error bars and your result matches.

i remember getting a clear linear relationship once that as supposed to be a curve. Solution was error bars and a curve to match expectation. Pointless

I would love to see this number for computational scientists.

While anecdotal, its easy to run into issues like this[0] where scientist with little CS background assume certain behaviors in code that don't actually exist.

[0] https://www.vice.com/en_us/article/zmjwda/a-code-glitch-may-...

I don't have a clear reference to point you to but in many CS research fields these number are equally bad. Often code is not even published and is a complete mess when it is.

When i could "see" my visual blindspot. https://lasikofnv.com/try-these-3-fun-tests-to-find-your-vis....

OMG me too. I was having an eye exam. The technician had a black field with a bunch of lines. Holding a rod with a white dot, and told to let them know when it disappeared.

I laughed, "Well that will only happen if you put it behind your back."

They laughed, "Everyone has an astigmatism where the nerves enter the eye. We are looking for that or any other issues."

I was so shocked when the dot disappeared.

Also happy to hear it was the normal blind spot.

There is one driving your car too. :)

Not if your mirrors are set correctly. Lean your head to the side window, set the mirror so that it just misses the tail end of your car. Position your head over the centre console, set the mirror. There is now approximately no blind spot. You can fine-tune by careful observation on a multi-lane highway as people pass/you pass others. Done correctly, there is no blind spot.

There are always blind spots. You covered a blind spot, but not all of them. Some of them are surprising.


Upvoted but wanted to add that this also requires the right mirrors. If you try to tow some trailers with a vehicle that doesn't have tow mirrors, then you could have a blind spot, especially behind the trailer if it blocks your regular rear view mirror.

Holy shit... anyone reading that doesn't understand, follow that link and follow along. That was... terrifying when you think of the ramifications. What's more interesting with glasses, my blindspot is larger than without my glasses.

Wow... just... mind blown. I always read about it, but never experienced it... wow...

somewhat related, I was so glad when I found out the reason behind a dim bulb that's just turned off appearing brighter when _you look away_.

The answer is in the density of rod cells (which are photon receptors of the eye that specialize in low-light) are concentrated on the outer edges of the retina. So if you look straight at a dim bulb - the photons hit the cone cells, which aren't sensitive enough, but if you look away, the photons hit the rod cells and you can see the bulb!

It never fails to amuse me that any pressure or force that we intuitively feel is "pulled" by a vacuum, is only caused by air pressure on the other sides of an object.

When you see something hanging from a suction cup in your kitchen or bathroom, it's fun to imagine that the air around you is hammering the suction cup enough to keep it stuck there. With quite some force you'll notice, if you try to pull it straight out! But let some air in through a small gap, and it will help even things out.

A lack of air by itself does nothing. It's just about the net forces.

Wow, I never thought about that before. So, if you could draw a vacuum in your whole kitchen or bathroom, would the suction cup fall down?


Ok, so maybe not a "fact" but the hypothesis of the False Vacuum:


This quote blew my mind:

"The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated."

I immediately went out and bought Greg Egan's "Schild's Ladder" after find out he'd used this as a plot device.

You should also read the Three Body Problem trilogy, specifically the last book, although I won't spoil it. It uses a plot device very similar to a false vacuum.

Not a fact per se, but: the sheer amount of living things that came before me, in the sense of my direct ancestors and their direct ancestors etc. etc – not only my human ancestors, but the whole of my ancestors all the way back to the origins of life itself – and I would not be here today if even one of them would have died before reproducing... that manages to blow my mind every single time I think about it.

And it can be reeeeally depressing to think about for those that aren't having children.

Try counting the total number of your ancestors: you have 2 parents, they have 4 parents in total etc., i.e. 2^N where N is the number of generations. Going just 40 generations back (about 1000 years), you had 2^40 = 1,099,511,627,776 ancestors.

But not necessarily 2^40 unique ancestors. That is, the same person may be your ancestor by more than one route through the subsequent generations.

The sun orbits the center of the galaxy every 250 million years, so when the dinosaurs were around, they were on the other side of the galaxy, compared to where we are now.

Gravity travels at the speed of light (the speed of causality). If the sun suddenly disappeared, the earth would continue orbiting around where it used to be, for about 8 minutes.

Nothing can travel faster than light through space, in a vacuum. However, if you pick two points in space that are far enough apart (e.g. at opposite sides of the observable universe), these points will be moving apart faster than light, because space itself is expanding.

The expansion of space isn’t coming from a single point outward, like an explosion. It’s expanding by the same amount at every point in the universe. People analogize this in lower dimensions to stretching fabric or blowing up a balloon.

How do we know that the space is expanding if there is nothing to compare it to?

The farther away a galaxy is, the more red-shifted the light from that galaxy is, when it reaches us.

Also, really far-away galaxies are receding from us faster than light.

And, the existence of the cosmological event horizon seems to support this too, I think. Things beyond that horizon will emit light in our direction, but that light will never reach us. In fact, without this horizon, I believe that the night sky would be much brighter.

I find it kind of sad that, as time passes, more of the universe becomes unobservable. Eventually the only stars visible from our POV will be those in our own galaxy (and galaxies that ours has merged with, in the interim).

More info at https://en.wikipedia.org/wiki/Expansion_of_the_universe

The selective attention test, aka the "invisible gorilla" experiment, and the finding that 50% of people didn't see the gorilla.

I was pretty amazed by the Casimir Effect when I first learned of it.


I think I need to get an advanced degree just to be able to read that wikipedia entry.

A gallon of gasoline creates 20 pounds of carbon dioxide.

So my trip to the grocery store could be adding as much carbon dioxide to the air as the groceries I pick up!

That is fascinating. A gallon of gasoline weighs 6 pounds, so it probably must be using 14 pounds of oxygen.

Let me add on. Your average big tree (oak, pine, maple, etc) after about 10 years of age, collect about 40 pounds of carbon dioxide every year. Again, averages, there are plenty of factors.

I use that as a way to visualize the problem with "a solution".

The dual slit experiment. Run the experiment with no one watching it, it produces result A. Run it again and watch it, it produces result B. Watching the experiment changes the result. WTF????

It's false information. It's debunked multiple times. "Watching" means "placing or turning on an additional detector", which ruins interference pattern. To develop intuition, look at double slit experiment made at macro scale using walking droplet.

I'd really appreciate a few links to this. I am on a youtube binge lately watching quantum dynamics lectures for lay people. The constant talk about "watching" and "observers" drives me crazy. I refuse to believe anything has anything to do with whether or not anybody is "watching" anything. Bouncing photons off of it? Sure I can see how that would affect all sorts of things. But if the bouncing photons alter what's going on (and it seems they cause as much quantum decoherence as anything else might) then that's not the same thing as just "watching". That word implies to me "there are ambient photons or some other field or background that does not alter the experiment, and I am merely observing that background to get an idea what's going on within the experiment". If the experiment cannot be conducted without interference within an ambient background of photons flying around, or within some other medium we passively non-destructively-to-the-experiment observe, then we cannot currently be said to be "observing" anything at all with respect to that experiment. Let's just say that, can't we? If we can't not alter the relevant circumstances of what's going on, then let's just admit that, without using faulty and improper language to "describe" what's happening. Can somebody help me out with this?

You make a good point. The problem is interacting with the particles, e.g. the photons bouncing off them. Don't poke the particle -- it only interferes with itself. Poke the particle -- now it is entangled with your measurement device, that means zillions of other particles, which changes the interference patterns.

But it is more complicated that this. Consider https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_t... -- if you only poke the particle in a "parallel universe", that also changes its pattern in this one.

At the end, quantum physics says there are multiple "versions" of how the particles move, and those versions interfere with each other. By interacting with each other, particles become entangled, which means that the "versions" of their states are no longer calculated independently, but together.

There are disagreements of what exactly this means: some people believe that reality only has multiple "versions" on a microscopic level, but when the entangled configuration becomes large enough (how large? no one knows), the parallel computation collapses, one of these "versions" is randomly selected to become the actual reality and the remaining ones disappear. Other people believe that multiple "versions" is the whole story; that observing the outcome means that you (being composed of particles that follow the laws of physics) also become entangled with the particles in the experiment, and now there are multiple "versions" of you, each observing a different outcome.

So, I'd say you got it half-right. Yes, it is about "observation = interaction"; and "observer" is just a shortcut for "the thing that poked the experimental particle, optionally also a display connected to that thing, optionally also a person observing the display". (That is, you could also have a completely impersonal "observer", e.g. a machine that measures the particle but no one is looking at its display.) But quantum physics is a different thing than mere classical physics where you correct for photons being actual things that hit the measured particle. It means there actually are multiple outcomes, which then interfere with each other, at least on the microscopic level.

If you want to develop intuition, look at this video: https://www.youtube.com/watch?v=nsaUX48t0w8 .

Please note that physics as science in general is advancing constantly, but mainstream physics advances in sprints. Currently, walking droplets and Pilot Wave Theory are not good enough to be included into mainstream physics.

Exactly, there's no such thing as passively 'watching' a system at the quantum level.

In order to obtain the information you wish to 'observe', you need to interact with it.

That interaction 'collapses the wave function', or rather, you become part the wave function, but seeing it 'from the inside' looks different compared to from the outside.

I love the “bouncing droplet” experiments. There are some cool videos of it on YouTube for those that want an intuitive macroscopic analogy to how quantum mechanics work.

The dual slit experiment produces different results? I must be thinking of something else - I thought that electrons passing through a dual slit always produce an interference pattern.

Anyone got a link?

There is a long Wikipedia article at


The most mind boggling aspect of this for me is the single photon version, which is under the "Interference of individual particles" section of the above article. The point is that you get an interference pattern even when only a single photon at a time is sent through the slits!

Yeah this is the point where you stare down into the rabbit hole.

I absolutely cannot psychologically deal with the Monty Haul paradox. https://en.wikipedia.org/wiki/Monty_Hall_problem I've coded it multiple times in multiple programming languages as a cathartic exercise. Despite seeing the results obviously printed out in a console in front of me, I just can't handle it. I lose SAN just thinking about it.

The explanation that makes the most sense to me, from wiki:

Yet another insight is that your chance of winning by switching doors is directly related to your chance of choosing the winning door in the first place: if you choose the correct door on your first try, then switching loses; if you choose a wrong door on your first try, then switching wins; your chance of choosing the correct door on your first try is 1/3, and the chance of choosing a wrong door is 2/3.

The bit that gets me... If he opens one door, which is a goat, so there is 1 goat and 1 prize remaining... then the chance should be 50% staying put or switching, right? That's the part that gets me. The whole bit about 3 doors and opening one of them and "do you want to switch" is all a ruse, it seems to me. At the end of the day there are two doors, one with a goat and one with a prize, and however you choose between 2 doors, you will win or lose 50% of the time. Right? Aargh the SAN loss...

It would only be 50-50 if you could only pick from 2 options, but your initial choice is between 3 options.

The explanation that makes it more visceral for me is to increase the number of doors:

  - You pick a door (let's say 17)
  - I open 98 doors (let's say 1 - 16, 18 - 78, and 80 - 100)
  - Only doors 17 and 79 are still closed
Does that help make it more intuitive?

A bit late to the party (came from Hacker Newsletter) but it seems obvious to me:

- If you happen to choose a door with a goat (⅔ of the time), the host has to choose a goat door, leaving "switch" a guaranteed win

- If you happen choose the door with the car (⅓ of the time), the host opens a random goat door and you have 50/50 between staying and switching

So always switching is the correct move - if you do that you'll win the ⅔ of the time (every time you open a goat door).

The only explanation that 'felt' acceptable to me is to imagine that you're really playing two games instead of one. In the first game, you're right 1/3rd of the time. Then you get to play a new game where you're right half the time, but you can also choose to not play that game and stick with the results of the first game. Playing the second game (picking another door) is more obviously the right thing to do in this case.

If it makes you feel better, famous Hungarian mathematician Paul Erdös didn't understand it either. One of his colleagues had to write a computer program simulating it before he accepted it to be true.

Not only him, the publication of the paradox in a column in Science (if memory serves me right) dragged thousands of condescending answers from top mathematicians (some of them apologized afterwards, including the one who called the journalist the goat of the paradox)

That when we look up at distant stars, nebulas, & galaxies and the like...we are looking back in time. What we see could have been gone for decades, centuries, or longer.

“longer” is an understatement. The Andromeda galaxy, which is viewable by the naked eye, is 2.5 million lightyears away. And that’s the closest galaxy to us.


One of the closest, anyway. Yeah, most distant objects observed are over 10 billion years "in the past".

Sort of similar to tree rings and even... dirt. We could probably dig in our backyards and find artifacts of 10,000 year old life. We are transient.

Growing up, I was taught that the main reason why it's bad to drink is because it kills brain cells and brain cells don't grow back. Apparently that is not the case anymore:


It's exercise that causes the brain growth.

(the famous rat experiment with the "stimulating environment" and brain grown was later attributed to the exercise wheel)

What level of drinking would cause this, though?

Come over to my house and I'll show you...

That's why haven't had a drink for the last 5 years

Well, i just meant i had not heard this - so i was curious if a glass of wine a night causes damage.

Not a science fact per se: Until recently I haven't realised the amount of damage inflicted by religion or in the name of religion if you will, to our species. Take Heliocentrism[^1] for example, it was alluded in the 5th century BC that the Earth revolves around the sun and that stars are other "suns". Then religion came along. Galileo nearly died for re-iterating a theory that was accepted by part of the scientific community nearly 2000 years before him.

In similar fashion, years ago while studying the circulatory system for the anatomy class I came across a wikipedia article. This particular part of the circulatory system was documented in detail by Egyptians in 200 BC. Knowledge came from the mummification process. The next breakthrough in this area was made in the 19th century.

Looks like our species could have a colony in Mars by now, if science were allowed to breakthrough linearly.

[^1]: https://en.wikipedia.org/wiki/Heliocentrism

I'm not personally religious but I'd be very careful before assuming religion, an institution that's been with humans for thousands of years, was a net negative.

I think humans probably have some innate desire for institutions with religious-like properties. And as we've become more secular over the past 60 years, there's been an increase in people that seem to be making politics the replacement, which may end up being more destructive than theistic religion ever was.

> I think humans probably have some innate desire for institutions with religious-like properties.

Hm, but we grow in a religious setting, the larger part of the population. So, is it an innate desire or a designed reality for population control?

Religion is an ambiguous term, which can be why arguments of this sort can get out of hand.

To some, religion = faith. And I think you'll find that most faith has little to say about science (and in the case of your example says nothing about the earth/sun relationship).

More commonly though, when referencing Religion you are really talking about Politics. Heliocentrasim, the crusades, Muslim terrorism, are all about politics, not faith.

When you make an argument using the word Religion in a political sense, but others read it as attacking their faith, then things get very ugly.

In other words you can be a devout Christian, and still consider the crusades to be awful. And you can be a good Muslim and consider 9/11 to be an act of war.

Most anytime someone talks about religion as being the cause of war and conflict, the root cause is politics, not faith.

I'm sorry if I offended you, that was not my intention. Other than that, your reply doesn't make any sense, at all.

Only 10% of the cells in a human body are human, but that 90% is only 2-3% of human body weight. https://www.nih.gov/news-events/news-releases/nih-human-micr...

So if your body was a democracy, you would lose.

How prion diseases work and spread. They're basically infectious lego. You may not thank me if you read about them...

I read somewhere they survive even 1000°C cleaning of stuff.

Maybe dental implements.

I get what you are saying, but it just occurred to me "survive" might not be fitting for these ... "things"? They are not quite "alive", are they?

The physics & biology behind eyesight.

Our eyes collect light that is "left over"(not reflected) from other surfaces.

Can't explain why but I had always had a sense that objects somehow emanated their own "image". Learning that colors manifest themselves because every other wavelength was absorbed was fascinating.

Why do you say not reflected? It seems like the light that enters our eyes often has been reflected?

I'm thinking the parent commenter meant to say "not absorbed"?

Obvious when you think about it, but I didn't realise for ages that all the planets orbit in the same plane.

Related: I was mind blown when I realized this is true for the pretty obvious reason that the various orbits "averaged" out.

Pluto does not orbit in the same plane - it is about 17° off. HOWEVER! Pluto is no longer considered a planet, making this science fact true. (It is now considered a dwarf planet instead).

Is the obvious reason because the sun is spinning? Or some other not so obvious reason I do not know of?

Well this may not be correct, but I'm assuming the plane they spin in is the same as the equator of the Sun and in the same direction as the Sun's rotation. That would make sense if they were made of "stuff" that was thrown out from the centre and clumped together.

The gold in my wedding ring was formed inside a star. I kinda sorta knew all matter was formed inside stars but it never really clicked until I thought about something as simple as a lump of gold and where those atoms really originated.

Related, most of the atoms in a new born baby are 13.7bn years old.

as Moby would say, we are all made of stars. It was a bit mindblowing to me to realize this was true and not just some hippie-ish slogan.

[0] https://en.wikipedia.org/wiki/We_Are_All_Made_of_Stars

BTW, "it takes a photon over 100,000 years to exit the Sun" is misleading - it sounds like there's a single photon that's generated deep inside the Sun's core and winds its way through like a cicada until it emerges at the surface.

In reality, any photon traveling inside the Sun will almost instantly collide with some other particle, which may emit zero, one, or more photons as a result. When we say "100,000 years", I believe we're summing up the total of these photons' expected lifetimes, basically following the flux of energy rather than individual photons.

That there is such a thing as deterministic chaos (aka the butterfly effect), ie. infinitesimal perturbations can quickly produce macroscopic changes, and all the philosophical implications that result from it.

That despite the apparent complexity of the weather many atmospheric phenomena can be explained from first principles with pen and paper calculations.

That the existence of elementary particles can be derived from simple symmetry considerations (That one blows my mind every time.)

That when we look out into the universe we see elements roughly in the same proportions as they appear on earth. (We are all made out of star dust!)

Isn't that just probability distributions?

we can see our nose all the time, but our brain filters it out.

if you close one eye and keep the other open you'll suddenly see one side of your nose.

We also have a blind spot in each eye where our optic nerve passes through our retina, which the brain constantly edits out.

Also while turning your head you don’t really see, but your brain pieces things together and plays an image of what it thinks you should see.

I can see my nose now

Got your nose!

> Can explain Quantum gravity

> Can explain DNA


> Can see his own nose

Average density of the universe is about 1 proton mass per cubic meter

That's including dark matter, I think? With just the matter we can see, it's not even that.

For me it's material science misconceptions, corrections to what a kid / the over-simplified public conception are. As I'm not a material science expert, if I'm wrong or slightly incorrect please explain what actually happens.

"diamonds are forever"

It's appealing to believe that a crystalline structure, a pretty one too, might exist until actively changed.

Diamonds are not forever. What 'blew my mind' is that diamonds are the result of compression and very slowly decompressing. That slowly over time the outer layer leaves that state and turns to dust.

Glass is a 'liquid'

I'm much less sure about glass, I'm not even sure that science is sure about glass. Apparently one process for making glass in the old days involved something blowing and spinning discs of it to produce nearly flat segments. When installed the artisan making the window would place the thicker end down for stability or some other reason. I'm not positive if glass is a liquid or not, but the reason many people might think it's a liquid is that intentional selection bias when fitting the panes of glass.

The whole concept of glass possibly being a liquid though changed the way I perceive solid, liquid, and gas states. Those labels better reflect much more temporally localized potential change and interaction than they do to uniquely describe matter.

BTW, if there is an expert, is glass actually a liquid or a solid?

May be wrong, but I think it correct to say glass is a 'fluid' (it flows - very slowly), gases and liquids are generally fluids. But glass is not a liquid.

Information has mass (and thus energy). As in a full hard drive weighs more than an empty one. https://aip.scitation.org/doi/10.1063/1.5123794

In Physics 2 (Light, Electricity, and Electromagnetism), I could not derive the correct answer for calculating the flux of an infinite sheet.

TA: The flux is S for an infinite sheet.

Me: How? We have S for this side plus S for the other which yields 2S!

TA: For an infinite sheet, there is no other side.

Me: O_O

To me that sounds mostly like a philosophical distinction. In most physics calculations, “infinite” just means “finite but so large that we can neglect edge effects”.

The 100-step rule in neuroscience, that states that no primary brain operation (e.g., face recognition) can take more than 100 neuron firing “steps.” (Feldman & Ballard, 1982)

This is a bit misleading since there are way more steps / neuron firings involved, the rule just says that these operations happen in less than 500ms and 1 firing per 5ms is the maximum throughput of a single neuron. These brain operations aren't implemented as a sequential chain of neuron activations though.

Creating something out of nothing: The Banach - Tarski Paradox


That's a consequence of the axiom of choice, which has historically been a highly controversial axiom.

That had interactions between protons and neutrons been but a tiny bit weaker or stronger than they are, the universe would probably contain a lot less carbon, nitrogen and oxygen (https://en.wikipedia.org/wiki/Triple-alpha_process#Resonance...), perhaps precluding existence of life as we know it.

Gravity is not really a force or objects "attracting" themselves but a consequence of the curvature of spacetime.

At least that's my pedestrian interpretation :)

What releases energy then, when objects are accelerated towards each other?

I usually ask people to imagine they are putting a rope tight around the earth, following the equator (idealized sphere).

Then they add 1 meter to the rope (it is now longer by a meter) and spread it around the earth uniformly (it hovers above the earth, everywhere at the same distance from the earth)

Estimate that hovering distance.

Try yourself to estimate it, then make the calculation.

Exactly anything you can think of can be represented as a segment of pi's infinite decimal expansion.

That's assuming pi is a normal number, which has not been proven.

That the Amazon soil is actually very poor in nutrients and the dust from the Sahara desert is actually what keeps the Amazon rainforest alive.

Long ago the South American and African continents were one. To think of it, there is still a major connection!

A man can fast for more than a year without die https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast

The life cycle of the lancet river fluke: https://www.damninteresting.com/a-fluke-of-nature/

> which states that entropy, or a loss of the energy available to do work, must rise over time.[2] Stars will eventually exhaust their supply of hydrogen fuel and burn out.

Since entropy is a statistical quantity, the second law of thermodynamics is a statistical statement. And because modern physics essentially deals with Hamiltonian systems, it can be proven that a time exists when entropy goes down again. The intuitive proof is really nice. Since energy is conserved, one can imagine the global state (position, momentum) as position in a park covered in snow. So someone walking through it will eventually walk over his own footsteps. Not precisely but it's arbitrarily close, the longer one waits.


I really like that theorem

That 'normal' matter only makes up 5% of the universe. That some omnipresent dark energy field makes up the majority (63%) of the universe.

From Introduction to Elementary Particles by David Griffiths: a neutrino of moderate energy could easily penetrate a thousand lightyears of lead.

That mitochondria have their own DNA and used to be separate organisms. Or when I heard that trees feed each other and fungi through their roots.

That something trivial like seeing speckle patterns in my eyes when I'm tired is proof of the quantum nature of light.

Interesting. How does that work?

I'm sure I'll do a poor job articulating it. It's similar to bokeh that photographers are familiar with. You look at a light source and the light seems to bounce through the pupil and back out again. If there's a drop of liquid on your eyelashes you'll see a bokeh ring with interference ripples around the edge.

I just remember as a child I thought it was strange, then in high school physics I saw how laser light was always grainy, and even things like the solidity of matter can be thought of as optical illusion (caused by the wavelengths of visible light casting a well-defined shadow).

Then later I got into photography and realized what you can do with even a little knowledge about light and optics. I still have a sense of wonder about it even though I'm an average shooter at best.

Hope that all made sense.

This is what I'm taking about, when I was a kid before I got fitted with glasses, I remember the world looking like this...


OK, but that's proof of the wave nature of light, not quite of the quantum nature.

Let's try to work the problem in a way that avoids pedantry, just for a moment.

This is one of my favorite PBS clips from the 80's in which Richard Feynman explains how strange the nature of light and by extension all of electromagnetic radiation.


If you listen carefully, you notice that he never talks about wave/particle duality (because he wasn't giving a classroom lecture).

What he does talk about is an electric field that waves slosh around in, and that something "elaborate and complicated" is going on at a deeper level.

That's what I'm on about, if you were pressed to explain it so that a bright child would get it, how would you do that?

For this problem/phenomenon, I'd probably just talk about waves and interference. I might mumble something about something elaborate and complicated going on at a deeper level, or I might ignore that and just talk about waves.

You always weigh less in the morning compared to the previous night because you have exhaled carbon atoms from your body!

Isn't most of reason you weigh less in the morning than at night due to the amount of water weight you have lost while sleeping.

Trees form out of thin air.

Black hole singularity is infinitely dense and zero size

Hot water freezes faster than cold water (sometimes).

This question feels AskReddity

camels have three eyelids

look at the time-lines of the universe and some of the entries, especially the last few, are mind blowing ( https://en.wikipedia.org/wiki/Timeline_of_the_far_future )


"Due to the gradual slowing down of Earth's rotation, a day on Earth will be one hour longer than it is today"

"From its present position, the Solar System completes one full orbit of the Galactic Center"

"All the continents on Earth may fuse into a supercontinent (Pangaea Ultima, Novopangaea, or Amasia)"

"Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible."

"the Andromeda Galaxy will have collided with the Milky Way, which will thereafter merge to form a galaxy dubbed "Milkomeda" ... There is also a small chance of the Solar System being ejected. The planets of the Solar System will almost certainly not be disturbed by these events"

"time until stellar close encounters detach all planets in star systems (including the Solar System) from their orbits"

"time until those stars not ejected from galaxies (1–10%) fall into their galaxies' central supermassive black holes. By this point, with binary stars having fallen into each other, and planets into their stars"

"estimated time for rigid objects, from free-floating rocks in space to planets, to rearrange their atoms and molecules via quantum tunneling. On this timescale, any discrete body of matter "behaves like a liquid" and becomes a smooth sphere due to diffusion and gravity"

"... they [Positrons] find a distant electron to pair with and the two enter into a highly excited state of positronium, with a radius larger than the current universe. Over the next 10^141 years they will gradually spiral inwards until they finally annihilate"

"time until a supermassive black hole with a mass of 20 trillion solar masses decays by Hawking radiation ... marks the end of the Black Hole Era. Beyond this time, if protons do decay, the Universe enters the Dark Era, in which all physical objects have decayed to subatomic particles"

"... time for all nucleons in the observable universe to decay ..."

"... estimated time until all baryonic matter in stellar-mass objects has either fused together [into iron-56] ... or decayed from a higher mass element into iron-56 to form an iron star"

"Estimated time for a Boltzmann brain to appear in the vacuum via a spontaneous entropy decrease"

"estimate for the time until all iron stars collapse into black holes ... which then (on these timescales) instantaneously evaporate into subatomic particles ... Beyond this point, it is almost certain that Universe will contain no more baryonic matter and will be an almost pure vacuum until it reaches its final energy state ..."

"Because the total number of ways in which all the subatomic particles in the observable universe can be combined is <big number> a number which, when multiplied by <big number>, disappears into the rounding error, this is also the time required for a quantum-tunnelled and quantum fluctuation-generated Big Bang to produce a new universe identical to our own ..."


Most of the facts stated are inaccurate, or at least phrased incorrectly in a pop science manner. What you have written are technical subtleties, not the main ideas that undergird biology. Biology is not an absolute hard science, but it serves nobody to emphasize and blow up the details while rejecting the core ideas completely. For example, if I tell a novice programmer that JavaScript is a compiled language like C, that is a very inaccurate statement. The subtleties of JIT interpreters can be clarified when you already have a clear understanding of the differences between an AOT compiler and a high level interpreter. HN is predominantly filled with people of math and computer science background. Those 'facts' in your comment are at best misleading if you do not already have strong grasp of the central dogma.

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