ELI5: Why does sound travel faster in helium that is lighter than air but also travels faster in water that is more dense than air?

Read the Story

Show Top Comments

If you think about it this way. Air is mostly nitrogen and oxygen which are both heavier elements than helium. So getting air to move a certain way is going to take more energy to get things going and will also be slower in doing it. It’s exactly like if you push on a car, it’s not going to move much but if you push on a ball it’ll roll away from you easily.

For water, the atoms are all packed in together and all but touching. The moment you hit one water molecule, the energy quickly transfers to the next because it doesn’t have far to go to do it.

For another example, think about a game of billiards. Except at the start you line all the balls up in a straight row touching each other down the length of the table. Hit one end of the row and the ball at the other end pops off almost immediately. That’s how water do.

Now put an inch or two between each of the balls in that same kind of line and when you smack one end it takes a few seconds before the ball at the other end starts to roll. That’s how gas do. (In this case water vapor)

If you replaced the billiard balls with say squash balls or tennis balls, it should take a little less time for the far end to get moving because there’s less momentum to overcome for each ball in the chain. That’s how it looks for any gas lighter than water in gas form. Which is similar to how helium is “lighter” (less massive) than air.


The speed of sound (c) in gases is depends on two things:

– density (rho)
– bulk modulus (K)

by the equation:

c = sqrt(K/rho)

The bulk modulus tells you how much pressure you need to compress a gas – or vice versa, how much compression you need to transmit pressure. I think its kind of intuitive that a gas that only needs to compress a little bit transmit pressure faster thab one that has to compress a lot.


The speed of sound is related to the speed at which the molecules move, as it is the molecules bumping into each other that allows the pressure wave to propagate. The speed of the molecules depends on the temperature and the mass of the molecule. Temperature is the energy the molecules have, and for a given amount of energy, something lighter moves faster than something heavier.

Air is mostly nitrogen and oxygen, which are formed of diatomic molecules, so one nitrogen molecule has two atoms, same for oxygen. Oxygen has a mass of 16 and nitrogen 14 per atom, so air is composed of molecules of mass 28 (a bit above 70%) or 32 (most of the rest). Helium atoms have a mass of 4, and don’t form molecules. Water is 1 oxygen and two hydrogen (mass 1 each), so a water molecule has a mass of 18.


Sound is a physical pressure wave of particles. They act like little balls – one knocks the one beside it, which hits the one beside it, on and on, until one hits your ear drum. It’s like ripples in a pond. The “speed of sound” is how fast these ripples move along. It has to do with how heavy the particles are, and how close together they are.

**Case 1: helium vs air** – As long as they’re at the same pressure and temp, all gases have the same density of particles. If you have a balloon of helium and a balloon of air, the He atoms in one are the same distance apart as the “air atoms” (N2 and O2) in the other. All things are equal in this comparison *except the mass of each single particle*. He is MUCH lighter than O2 and N2, so when you give them the same push they’re able to move along knocking into each other more quickly.

**Case 2: Air vs water** – Now the biggest difference is the distance between particles. In liquids (and solids), the particles are packed MUCH closer together than they are in gases. The speed of sound is much higher in liquids and solids because of this. Each ball has a very very short distance to go before hitting the one next to it so the wave moves quickly. Imagine pool balls lined up touching each other. Hit one end and one flies off the other end of the line almost immediately. Also single water molecules are somewhat lighter than air molecules, so it has a double-advantage.


ELI5: Why are you cold and sweat at the same time when you are nervous?

Read the Story

Show Top Comments

you get an adrenaline rush, adrenaline makes your body go into fight or flight mode.
it pulls blood to more important places. you become cold in your hands and feet where it narrows arteries. your glucose processing goes into overdrive to create more energy for the muscles. adrenaline also makes your sweat glands go into overdrive. makes your face become red from the extra blood, makes it very hard to sleep and makes it very hard to eat because blood has been pulled away from your digestion.
afterwards you get to have a nice low blood sugar crash.


Don’t forget that sweating is the body’s way of cooling itself down. Water is pushed onto the skin’s surface where it can evaporate, taking some of the body’s heat as it moves from the skin to the air. Your body can sometimes tell the skin to produce sweat even if it’s already at a good temperature. If too much sweat is produced, you will feel cold.


ELI5: When you have a cold, why does your nose stop running while asleep?

Read the Story

Show Top Comments

Sometimes it doesn’t, people have been known to wake up with snot all over the pillow. When it does stop it is normally because the nasal passages swell which prevent both snot and air from going through them. The nasal passages tend to swell completely shut when you are laying down. When you treat a cold with advil + sudafed you are dramatically reducing the swelling in the nose so you can blow the crud out of your nose more easily.


Because if you’re laying on your back, you are now drinking your own snot and boogers all night.


It doesn’t stop. For most people it just runs down the back of their throat into either the stomach or lungs where it lodges unless you are able to cough it up in the morning. If you can’t cough it up it may become pneumonia.


It dont. The mucus drips backwards to pharynx and you swallow it like swallowing the saliva produced in your mouth. It stops when the virus is under control again.


It doesn’t. But when you’re lying down, the mucus tends to fall back into your throat or fill up different sinuses.

Most people sleep on their back or their sides. In each case, the natural way for the mucus to flow following gravity is deeper in to your head. That’s why you’ll often wake up with one nostril totally blocked, or with a lot of catarrh in your throat. During the day, the snot runs out of your nose because that’s the way gravity takes it.


ELI5: What technical limitations caused the ‘2.5D era’ of video games instead of directly moving from 2D to 3D?

Read the Story

Show Top Comments

Well, what it mainly comes down to is that PCs of the time didn’t have dedicated 3D hardware the way they do now, or the way that the 1994 Playstation did–it wasn’t until the release of the 3Dfx Voodoo 3D card in 1996 that this capability really started to become mainstream in gaming PCs. So, any 3D in a game had to be rendered entirely using the machine’s CPU, and ray casting engines like the ones used in Doom were simply a far more efficient way of rendering 3D, albeit they took some shortcuts in order to do that.

I actually remember playing through the whole of the original Tomb Raider (1996) on the PC in software rendering only, and the huge difference it made once I got a 3D card and ran it via that instead.


Doing true 3D polygons requires a lot of floating point operations (the PS1 did use intermediate integer shortcuts on top of dedicated floating point hardware which is why PS1 games have that distinctive warped perspective and shimmering) but in 1995 the target for PC games was still the 486. The fact that the makers of Dark Forces demanded a whole 8MB of RAM when there were still many people with only 4MB drew some interesting complaints at the time.

Unfortunately the big performance difference between the existing base of 486 users and the new Pentium adopters was the processor’s floating point performance. That’s why Quake (1996) required a 75Mhz Pentium and even on a 100Mhz 486 would run at about 10fps (I know because I only had the 486 at home so it was easy to compare just how big a difference the Pentium’s floating point unit made). So the two advances that allowed for 3D games where processors with strong floating point performance (Pentiums) and dedicated accelerators cards that could do many floating point operations in parallel (even better than a Pentium).

By comparison 2.5D games required no floating point operations and could use small integer based lookup tables. 2.5D games are based on a very simple ray casting. For each vertical line on the screen the game projects a ray along the 2D overhead grid until it hits a wall. It then takes the distance to the wall to determine which scale lookup table to use and draws an appropriate vertical strip of pixels taken from that walls texture, which unlike 3D texturing aren’t from arbitrary points but rather a simple vertical line using that lookup table to know when the duplicate (stretch) or omit pixels to draw it at the correct scale.


Wait this is 2.5D? I thought 2.5D was strictly games rendered in 3D but with 2D movement like platformers. Wikipedia says it is both, but it seems confusing to me since they’re very different and only one of them is still commonly used.

I always call it original Doom or Wolfenstein 3D type graphics but that is not concise.


Marathon (Bungie Software) for Mac in 1995 was fully 3D, as were Marathon 2: Durandal and Marathon Infinity (96/97?)

Better graphics, better gameplay, better storyline than pretty much all of the other 3D games of the era. Better than Quake.

Mac only though, other than Durandal, and the PC port of it wasn’t as polished and didn’t run nearly as well as on a Mac of the same era in my opinion.

Most people weren’t introduced to Bungie until Microsoft bought them out to make Halo into the Xbox exclusive killer game everyone HAD to have.

That said, if you ever get an old Mac emulator running and want to play some of the best gameplay of the 90’s, Pathways into Darkness and the Marathon series are both really great gaming experiences.

Pathways shows it’s age, but the story and puzzles and figuring it all out is incredible.


It’s a nightmare to code both and you end up making compromised that lead to less than desirable behavior. Command and Conquer Tiberian Sun and Red Alert 2 used voxels for vehicles and 2D sprites for terrain, buildings and vehicles.


ELI5 : How does gravity cause time distortion ?

Read the Story

Show Top Comments

Picture this: the space around you is a massive piece of fabric and that gravity are balls of different weights placed in random places of that fabric. By putting a heavy ball on the fabric, it creates a little “crater” and if you put another lighter ball in this crater, it basically rolls toward the heavier ball. This rolling of the lighter ball to the heavier ball is gravitational pull.

Then comes light. Imagine light to travel like a drop of water along the fabric at a constant speed that does not change. What you perceive as time is essentially the duration it takes for a drop of water (i.e. light) to reach your eyes from an object. Now, if you picture a large heavy ball in the middle of a big sheet of fabric versus a small light ball in placed in the the same position of this sheet of fabric – the distance of the balls to your eyes will seem the same. However, due to the weight of the heavier ball creating a larger “crater”, the actual amount of fabric between you an the heavier ball is actually larger. Going back to the water droplet that is light, light will take a longer time to travel from the larger heavier object to you because it needs to traverse a greater amount of fabric (and it’s speed remains constant). This longer time it takes is a simplified explanation how gravity warps time


Cause space and time are related concepts. Say it takes you 10 seconds to walk from point A to B. Now imagine the space between point A and B is stretched like a rubber band, now it will take you longer than 10 seconds.
Strength of gravity determines space distortion and hence time dilation.


Gravity is what we call the math that explains how objects move. We can predict a lot stuff using that math. We can tell you exactly where Venus will be in the night sky eight years from now using that math, it’s really good math.

Very clever people noticed that there were predictions that weren’t so accurate using our gravity math, and they worked on the problem a long time until they discovered that they could make predictions more accurate by distorting time.

Since the new math is more accurate we can say with as much certainty as anything that space and time are both variables of our universe that can be measurably effected.

We evolved to pick ticks off one another and spot predators in long grass. Don’t put so much pressure on that brain of yours to wrap itself around this. You’re doing fine.


>I guess my new question is what is gravity ? 🙂 and how can weight affect it ?

I’ll be honest here. While it’s possible to explain how gravity is a curve in spacetime by illustrating it as a ball on elastic fabric, you’re venturing into stuff that *can’t* be explained like you’re five because even modern day scientists aren’t sure of this stuff.

I am not a scientist or a physicist by any means, but I have a very (*very*) basic understanding of some of this stuff, so I will try to explain it in as simplified terms as I can, understanding that some of this may be over-simplified or factually inaccurate as a result. Some stuff can only be simplified so much.

Gravity is a curve in spacetime caused by the presence of mass. Objects with mass are essentially “accelerating” through spacetime. That’s why you’re pinned to Earth right now. Earth is accelerating through spacetime at a steady rate of 9.8m/s^2 and it’s essentially ‘pushing’ you along with it.

If you jump off of a building, you leave Earth’s frame of reference. You’re no longer accelerating through spacetime, but Earth still is, so you’re going to be there when it catches up and slams into you. Now, you’ll eventually reach terminal velocity if the building is tall enough, because as Earth accelerates through spacetime it’s pushing a big buffer of air in front of it, which is going to accelerate you in the same direction as Earth a little bit, though not fast enough to prevent the eventual impact.

We perceive and categorize that acceleration through spacetime as *gravity.*

To try and explain time dilation…spacetime is just that. It is space *and* time. When you move through one, you move through the other. Velocity (V) is a function of Distance (D) over Time (T). It can be written as V = D/T. For example, if D = 1 mile and T = 1 hour, then V = 1 mile per hour.

If you’re moving quickly relative to another person, then any action you take is going to be spread out across more spacetime relative to the other person. Since (D)istance in this case is static (the distance you moved relative to the other person), then when you alter (V)elocity, (T)ime *has* to change in order to keep the equation balanced. In essence, because you’re traversing spacetime faster than the other person, any action you take will be spread across more spacetime, which is perceived as time “passing more slowly” for you.

Taking what I said earlier about gravity being mass accelerating through spacetime, the reason gravity affects time is because an object in a gravity well is *traversing spacetime* at a different rate than objects outside of a gravity well.


Very simple intuition:

Velocity = Distance / Time

(you measure your car speed in miles per hour).

The speed of light, C, is constant in all reference frames (hint: this is part of what let us discover space & time warp).

Since gravity curves space, it makes Distances longer (think windy road is longer than straight road between two points).

If C is fixed, and mass/gravity elongates space, the only other thing that can change to make it work is time changing.

C = ⬆️ Distance / ⬆️ Time

i.e. time slows down for light (and actually all things) in high gravity/warped space

Einstein’s brilliance was showing how a constant speed of light in ANY reference frame (moving rocket, in curved space time, etc…) implies that time and space themselves warp.

Note: the actual relationships involving mass are what general relativity specifies and are more complicated. This example only explains the intuition on how time and distance are fundamentally related.


ELI5: How come we can’t feel our organs touching each other and moving around but we can feel pain in them when hurt?

Read the Story

Show Top Comments

Oversimplified answer: There are different nerves that sense pressure, heat, and pain. Internal organs don’t have the pressure kind but they do have the pain kind.


In addition to what others have said, it’s also the same reason why you didn’t feel your socks touching your feet until I just mentioned it. Our brains are designed to filter out the vast majority of the stimulus that we receive so that we can focus on the important things.


You can hear your own body, it’s just never ever quiet enough no matter where you are. Watched a video only a few weeks ago and cannot for the life of me find it (Think it was a Tom Scott video but could he wrong).

The video was about precisely this and was only a few minutes long, but told the story of a man who was hiking through the desert and had been told before he left that he will reach an area so quiet he will be able to hear his body functioning, and he could. There is also a superficial room where you can experience this but this was a completely natural phenomenon. I hope somebody else can find the video or some information on it.


Most organs are not touching each other – everything has a place and most organs are held in place with other connective tissues.


You can once you learn what they feel like. It took me five years of fairly dedicated qigong and breathing practice before I could feel my kidneys.

The easiest organ to feel is the lungs. You can close your eyes and feel those when you breathe. After the lungs comes the heart, you can feel it beating in your chest. From there you get into the kidneys, liver, and other internal organs.


ELI5: How do games that are in development for several years catch up to the latest graphic technologies?

Read the Story

Show Top Comments

There is continuous improvement, but it’s not like these other games releasing in 2021 popped out of the ether, fully formed. Very rarely does a game have a cycle less than 2 years, often more, and in many cases the games will be built on legacy bases anyway. I’m not sure if it’s still the case, but until at least quite recently Call of Duty still had some of its Quake II code in there somewhere. So the idea of the start and end of game’s development cycle is a bit of an outdated idea.

You also have some engines which are developed independently of games – third party ones like Unreal are the obvious examples, but also many studios will have an internal engine that gets used across games, and a such represents iterative improvements over the years. There will come a point when they have to lock down which version they’re working with but that point isn’t going to be 6 years before the game is released.


Firstly, not all of development is actual coding. “Development” can include project planning, game design, story boarding, prototyping, asset creation, and more.

Secondly, including new graphics technologies isn’t always a drastic move. The art assets are usually created in high detail, and downscaled for adequate performance on each platform. Enabling HDR and anti-aliasing only impact the renderer; that’s one of the more complicated components of the game, but still only a fraction of the development effort. Most games will abstract away the rendering to the game engine, which is either developed by a different team or licensed from a third party. So those new technologies barely impact development of the core game.

Regarding DLSS as far as I know the actual work happens in the GPU, there’s no code change required besides adding the option to enable it. The devs do need to submit their game to Nvidia to train the upscaling AI but that would be done on the tail end of development or with individual assets/textures as they’re completed.

DirectX is probably the most significant change you could make to a game mid-development, but a) DirectX doesn’t update all that often, b) most/all of the changes will be in the game engine, and c) DirectX by and large is backwards compatible, if you built your game on DX11 and want to support DX12, there’s really only one or two additional rendering features the engine needs to add support for, if there happen to be any improvements in existing features they will be a bonus for little to no effort.

As CyclopsRock said, you can iteratively update your libraries, engines and other components during development, but at some point towards the end of the cycle the versions will be locked in.

Also, these new technologies don’t just pop up overnight by surprise, Nvidia or AMD aren’t going to announce new technologies without at least a handful of supported games to showcase it and drive sales, otherwise it’s a pretty useless feature. Graphics companies are working with developers to get these new features in the game, months or even years before they’re released.


Depends on the situation. Final Fantasy 15 was originally meant
to release much earlier for the PS3, under the name Final
Fantasy versus 13. But the console’s hardware limitations meant
the developers faced technical difficulties until the PS4
launched and they decided to restructure the project and
upgrade some technical parts.

Then there are games which were also delayed and launched in a
more or less obsolete state, with Duke Nukem Forever probably
being one of the most prominent examples as it disappointed not
only graphically but also in terms of level design and game
mechanics, thanks to its 14 years of development.

Another category are games like Dark Souls 2 which was later
upgraded to DX11 graphics and bundled with the DLCs to the
Scholar of the First Sin Edition.


Don’t worry, consoles hold technological advancement back, so AAA games in 5 or so years will still be made with PS5/XSX hardware as a base, with varying degrees of optimization for PC Hardware.

On the best cases, devs actually make an extra effort to push the version PC Gamers deserve, with much more visual fidelity or better technologies implemented, but those examples are very limited.


Just because something is released to public today doesn’t mean it hasn’t been released to insiders 2-3 years ago. PS5 coming out this month, the dev kits went out to devs months or even last year.


ELI5: Why do joints make a “Cracking” noise for certain activities such as walking up stairs, but have no actual pain associated with them?

Read the Story

Show Top Comments

The generally accepted reason is the diffusion of (and the resulting collapse of) gaseous bubbles in the fluids between your bones/cartilage.

The debate is still out on whether it’s the bubble forming that makes the sound or the bubble collapsing that makes the sound.

However you’ll notice you can only crack joints that you haven’t moved in a while, which allows the gasses to diffuse. Then you can pull the joint apart creating a void which is promptly filled by the gasses diffused in the fluids surrounding the joint as the pressure drops in that area. Eventually pressure returns to normal and the bubble collapses.

Again, generally it’s accepted that the collapse of the bubble is the cause of the sound but the debate is still out on that.


There are two types of cracking or popping. The one we do with fingers backs etc that’s cavitation of solute gasses in the synovial fluid.

The other one is tendons that have some sort of scaring on the sliding inside their fascia causing a snapping sound.

Sorry eli5s

Your fingers pop like by little bubbles of air bursting.

Your knees crack because tendons and ligaments snap like guitar strings.


Because they have seeds in them? Oh, that joint….


My jaw does that clicking sound too every time I open my mouth wide and I experience lock-jaw almost every morning. My left shoulder’s bone clicks too every time I raise my left hand up.. Is this normal?


There’s also something called Crepitus at least in your knee joints. It has to do with the cartilage being damaged so that it is no longer smooth. As you crouch down it makes a noise like you are crumpling paper, but it is not associated with continuous damage or pain.


Eli5: why does it seem like the unsubscribe button for emails does nothing?

Read the Story

Show Top Comments

One sneaky thing legit companies are doing all the time now is making the “unsubscribe” button unsubscribe you to those specific types of emails. Oh, you don’t want to subscribe to our daily deals? That’s ok, we’ll still send you the weekly deals and the special deals. Some sites have like 10 different lists that you have to unsub from individually


It often doesn’t. In fact for spam, clicking that just tells them your email is real and they can sell it to more spammers.


It depends on if the email you tried to unsubscribe from is a legitimate marketing list, or spam. If it’s a legitimate marketing list from a reputable company, the chances are pretty high that the unsubscribe link will work, but if it’s spam, it’ll either do nothing, or encourage more spam. Legitimate unsubscribe links can often take days to propagate, though, so maybe mark the message as spam too.


This is not a certified way but try using an European VPN.

Companies breaching GDPR get quite huge fines, so it is the case in general that in Europe that like actually works.

Or, even better, just mark the emails as spam so that your spam filter learns to block them.


It’s like the “close door” button on the elevator – just there to make you feel like you’re doing something.


ELI5: In the recent days in Atlanta, Trixie the whale shark dies at the Georgia Aquarium. With whale sharks being the largest fish species in the world, what do they do with the body once a fish this size passes away? It must to be a lot different that what we do with human bodies after death.

Read the Story

Show Top Comments

Speaking from experience with large animals in managed care, I can tell you that these institutions either have a way to cremate animals or work closely with a pathology facility that does.

After an animal’s death, a thorough post-mortem examination is performed and many samples are collected. Then the carcass is incinerated. If this is done off grounds, the transport is usually “hush hush” before the death is officially announced.


They would probably be doing a necropsy to figure out the cause of death. Tissue samples and DNA samples collected and stored for future research.

Sometimes the rest of the remains will be donated for educational purposes like to museums. Or if it’s deemed free of parasites or diseases, it might be returned to the ocean and serve as something akin to a “whale fall” site.

Or they would just cremate the remains once they’ve gathered all the data they need.


I can’t say from experience about large captive animals, but I assisted with necropsy and carcass preservation of a blue whale in Atlantic Canada.

After samples from as many tissues as possible were taken, the whale was essentially cut up with large knives. It was dragged further ashore using heavy machinery and veterinarians and pathologists handed it over to technicians and students. It was at a point where most connective tissues were broken down so bones weren’t a concern, and our goal was to preserve and display the skeleton eventually. The flesh was trucked away to be incinerated and the raw bones were trucked to an agricultural college to be composted under a big compost heap. They unearthed an almost completely clean and intact skeleton a few months later.

Now it is being handled by a university curator and an arts and design college to 3-D print any missing bones and articulate everything together.


People in here arent going in depth enough, like sure it will be cremated, therell be an investigation etc, but how tf do they get it out in the first place? Crane? Chop it up into smaller bits?


Hey I swam with them one time, if you have the chance do it, the prices are super reasonable and it’s a great experience.