ELI5: Why are astronaut’s movements in space seemingly slow motion when there’s no air/water resistance to slow them down?

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There are a few reasons. Their suits are pressurized. The joints of these suits want to fully extend due to this pressure. This makes it difficult to move and do things, and reduces precision substantially. Not only this, but the suits are quite heavy, and have inertia to slow movements. Lastly, the astronauts do not have anything to stand on. Fast movements will make their bodies start to rotate and further reduce the precision of their motions.


It’s precisely because there is nothing to slow them down. They move deliberately because if you flip something it’s not going to be slowed down by air. Move your arm too fast, and you’ll rap your wrist on something. “Every action has an equal an opposite reaction” said Newton, and without gravity to stick you to the ground that reaction has to come from your other hand or some Velcro gizmo.


Your actions on earth are made safe by gravity keeping you anchored and effectively eating most of the energy your muscles produce.

Your muscles are incredibly powerful. They can lift your entire body from a sitting to standing position with very little effort, a dead lift of 80lbs is so easy that it does not even register to you as a meaningful expenditure of energy.

In micro-gravity standing up with that same force would send you painfully crashing into the surface directly above you.

Any movement of your arms would add rotational energy that you wouldn’t be able to counteract without holding on to something that would anchor you and eat that energy. When you throw a ball if you lean into the throw then you might have to extend a leg behind you to act as an anchor and keep you from falling forward. In microgravity you’d just start spinning, possibly with a leg extended in a goofy way.


Since there is no resistance there is no way to slow down. It allows them to move around with that huge spacesuit. Fast moves can cause a big response. Any bump in the wrong direction may cause them to lose control. Therefore everything is in slow motion.


Good answers, but one factor hasn’t been mentioned yet that has nothing to do with reality: in movies and TV, it’s really difficult to capture the look of low- or zero-gravity on Earth, So many TV shows and movies will literally film the astronauts in slow motion to make things look a bit more “floaty”. This was especially true for older and cheaper stuff that didn’t have access to good CGI or a “vomit comet” aircraft.


ELI5: Serotonin is the hormone what makes us happy, so why aren’t we just injecting it into our body/taking it as tablets to get out of misery on command?

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Hormones like serotonin are like ingredients in a recipe. You need them to be in the right balance for a dish to work.

Imagine serotonin as sugar, if used sparingly, you would enjoy it as a glaze or sauce in your entrees and finish your meal with a sweeter dessert. If you dumped a container of sugar on every dish of your meal, none of it would be enjoyable.

People’s bodies, like their tastes, differ as well. Some people like their dishes sweeter while others do not. Therefore a set sweetness for one person would not be enjoyable for others. This is why it is difficult to find the right medicine to treat neurological diseases like depression because every person is different.

The body also has limited tools to communicate with other organs. Serotonin affects mood in the brain but also appetite and digestion in your digestive system. Some of the side effects from drugs increasing or decreasing one hormone is due to the body reusing the hormone for many functions.

Lastly, the body is naturally ~~lazy~~ efficient. If it’s getting a hormone from an injection or pill, it is less likely to make it. Therefore, when a person stops taking the injections or pills the body goes into withdrawal because it stopped making that hormone, causing other symptoms. This can cause a dependence on the substance, otherwise known as an addiction.


Too much serotonin is a very, very bad thing. Serotonin syndrome has a ton of very nasty symptoms, and allowing people to take the stuff whenever they feel a little down would end in massive tragedy.


Your brain is surrounded by a barrier that is very selective about what can pass through it. Serotonin is manufactured by your body inside this barrier. Injecting or ingesting sarotonin won’t do anything because it cannot cross that barrier.

However, the chemicals from which serotonin is manufactured (such as tryptophan) *can* pass through that barrier and, indeed, you can purchase purified tryptophan for this purpose. But, as with most things, it has side effects.


The simple reason is that we actually have no fucking idea of what cause us to be happy on a regular basis.

The neurotrasmitters-imbalance theory of depression is getting smashed a bit more every day.

Take for example serotonin. We cannot simply drink it, because as other said it cannot pass the blood-brain barrier.
So we use drugs that *can* pass that barrier, and cause your brain to re-absorb less serotonin. By reabsorbing less of it, you have more serotonin available in your brain.

The problem is that you won’t feel better quickly. You will feel better after months of taking the drug.

But the drug is acting every single day! You have more serotonine in your brain every single day!

So why the effect doesn’t start on day 1?

We don’t know. We have some hypotesis, but we don’t know.

This is why having a lot of serotonine in your brain will not make you Happy on a bad day.


Neither injecting nor swallowing serotonin allows your brain to use it because it won’t cross from the blood to the brain.

Some drugs effectively increase the level of serotonin you experience by reducing how much of it your brain clears (eg Prozac) or forcing your brain to dump the serotonin it already has stored (eg MDMA).


ELI5: Why does caffeine stop working on the brain/not make people feel awake anymore after adaption but still trigger physical side effects like shaky hands?

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Caffeine works by blocking the detectors that your brain uses to figure out if you are tired. If they get block too often, your brain decides the detectors are not working and simply grows more until their are more detectors than the caffeine can block. This means you can still feel tired now, but that doesn’t stop anything else the chemical may do.


When you’re awake, a chemical called adenosine slowly accumulates in your brain. Adenosine binds to receptors which slow down brain activity, making you feel more tired at the end of the day. When you sleep, the level of adenosine declines and promotes wakefulness.

Caffeine is very similar to adenosine and binds with the receptors. As it’s not adenosine, no sleepiness is felt. With long-term use of caffeine, your brain responds by creating more adenosine receptors so more caffeine is required for the same effect. Quitting caffeine cold turkey means you would feel more tired than you would have before you drank it. Eventually everything turns back to normal once you stop drinking coffee.


One crucial factor in a lot of your body’s reactions is something called “Homeostasis” This is basically a body’s tendency to try to stay the same. The word pretty much means “Same Same”.

The best way to describe it is say you and I push our hands together. The harder I push, the harder you push back to stay where you are. I am the drug, you are your body trying to stay the same. Your body is not very fast at this, so if I suddenly stop pushing, you stagger toward me. The OPPOSITE thing that you wanted the drug or whatever to do happens.

Pain killers are a good example of this. If you take them for a long time, you build a tolerance (your body makes you more sensitive to counteract it), then you stop taking them and you feel like you’re on fire, or you ache, etc.


Is it only me after drinking coffee and makes anxiety worse ?


We build tolerance over time to all it’s effects, but with various speed. I don’t even want to pee as much as I wanted when drank coffee for the first weeks. I experienced shaking 5 years ago, now only feel slightest jitter if drank really a lot.

About the mechanism: I don’t know exactly, but assume that there are several receptors that caffeine binds to (Adenosine A1 and Adenosine A2 as well as other minor targets). These receptors are located in various areas of the brain/nervous system and/so their response to habitual caffeine exposure may be different. For example Adenosine A1 will still cause exactly the same heart palpitation, tremor effect, while A2 waking effects will dissappear. It’s because Adenosine A2 receptors antagonism (blockade) will cause changes which in turn will cause number of A2 receptors to increase (which is called downregulation) which in turn will decrease actions of caffeine to this area of the brain. So A2 tolerance may develop much faster than A1 tolerance or something like that. Btw A2 is not so directly mentally stimulating itself. A2 antagonism causes striatal dopamine release (which we call “want” chemical, but it’s more complicated, it causes some mental arousal as well), acetylcholine release, etc. Dopamine release may be decreased independently from the above mechanism too. You see, brain is quite complicated as a system and it’s research is limited in measures and difficult. BTW I may be wrong in my statements.


ELI5: Why do all vitamin supplements say that their claimed benefits aren’t verified. Do we really not have any confirmation of what vitamins do?

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We have confirmation of what vitamins do.

What vitamins do and what vitamin supplement sellers want you to believe they do, however, are two entirely different things.

Stops your teeth from falling out? That’s something vitamins do.

Treats the common cold? That’s not something vitamins do.

For people who’s teeth aren’t falling out as it is, the second one sells better.


They are required by law to put that disclaimer on there. Supplements don’t have to go through FDA approval before being offered for sale, but at the same time there are certain sorts of health claims they cannot make, and they need to state explicitly that the claims they do make have not been evaluated by the FDA and are not intended to treat any disease.


Because the evidence required to support real health or medical benefits are regulated extremely stringently. Supplements however, are not well regulated. If they want to say that their product DOES something medically related, they need to prove it in scientific studies which are expensive and time consuming. However, they can advertise it saying it MAY do something while avoiding the cost of actually providing evidence.
Vitamins have very little evidence to suggest they provide a health benefit outside of people with deficient diets. This is why they can’t say they do anything much at all!


Vitamins are good for treating vitamin deficiencies. If you do not have a vitamin deficiency, supplemental vitamins are just expensive urine.


One thing to keep in mind is that the supplement industry is a very dark industry. Vendors have gotten dinged for things found in supplements they sell. Also whenever studies are done and reports written there is money behind all of that more geared toward sales to consumers than scientific observation. We are reaching a time when the one thing we can depend upon is we can’t depend upon anyone giving us the honest truth.


ELI5: How does 4k video transfer work

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If you were transferring 30 unrelated images per second, that would be the case. But videos are not a bunch of unrelated pictures. Videos are s sequence of lightly changed pictures.

So, what we do is compare the first picture to the next one, and get the difference between them.

That’s what we send. Look up compression algorithms for video


There is this mathemthical technique used on the video to compress it. if for example you have 1 second of exactly the same black picture you could just as easily say “do this black picture 30 times” which would cost way less bandwidth than in detail describing the same black picture over and over and over again 30 times. this is basically how they do it, you can do an lot more advanced stuff such as if an black square was moving over an white background you could mathematically describe “move black frame 2 pixels to the left every frame” instead of having to detail every pixel individually again.

All the above would be considered “lossless” compression, because you can recreate the exact origin from the compressed data. but an near perfect picture is good enough for almost everyone, so with a lot more of clever mathmathics we can make files even smaller at the loss of details. some mathmethical algorithms for example focus on detail in faces and hair because people watch that the most, while the background is a lot more blurry because its just not as important to the scene. this is also why bluray is visually superior to any streaming, it almost always has much more MB per second to work with and can thus use an compression that loses less of the original image.


I’m no expert, but a lot has to do with compression. For one, a 24/30/60 fps video isn’t sending a full image for every frame, more like a full image every few frames (an i frame) and several ‘move these pixels around a bit and leave the rest alone’ frames (p frames – note that I may have gotten those backward).

That, and there’s data compression to shrink the images from a straight up 8 million pixels with individual info down to a much more manageable file with less accurate data. Difference between a .RAW file and a .PNG.

That’s why blacks and dark grays look like shit on streaming shows and movies: Most compression handles those shades very, very poorly.


Imagine you made a picture out of pieces of colored paper, every time something changes you only move the pieces of paper you have to, this means that most of the pieces of paper don’t have to be downloaded 30 times a second.

This is sorta how video compression works, another thing is where there isn’t detail the pieces of paper can be bigger, because it’s all the same color, so instead of cutting out 10 pieces of blue it sends “blue x10” instead of blue blue blue blue blue blue blue blue blue blue. This is similar to how image compression works.

Hope this helps.


These answers are correct but I’d like to add that although the image is 4k in size (the amount of pixels), Netflix does deliver it compressed (jpeg or mpeg compression as explained by the other). So technically you are seeing a degraded version of the RAW uncompressed master but compression techniques are so good these days it’s hard to notice the image quality loss with the naked eye.


ELI5: Whats the functional difference on a group of muscles between taking a muscle relaxer, stretching and working out?

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Muscle Relaxer – Prevents muscle constriction. Keeps them loose and relaxed. Helps with some muscle diseases.

Stretching – Makes the muscle longer and stretches it out. Useful for increasing your flexibility which can increase performance in some ways. (Help you get into a proper squat stance, increase ability in certain sports)

Working out – You’re putting your muscles through stress which forces them to adapt. The muscle cells are damaged by the stress and they respond by growing bigger in size and in number.


So would it be possible too take muscle relaxers before static stretching to increase speed of muscle stretching.


Muscle relaxers (most popular of which is Flexeril/cyclobenzaprine) help cramped muscles relax by inhibiting the signals to the muscles that start in the spinal cord. They don’t actually work on the muscle tissue itself, they work on the spinal cord/brain stem signals that are the ones telling the muscles to cramp. These are specifically meant to help muscle cramps that are due to injury or dysfunction. They will not improve muscle length or flexibility, they simply reduce the painful contraction associated with cramping/spasm that occurs with, for example, spraining your neck or tearing your rotator cuff.

Stretching (sometimes) improves the actual length and range of motion of a muscle, i.e. flexibility. By holding a stretch for a certain length of time (static stretching), the muscle will literally add more structural units to its end to grow longer. Longer muscles with a greater range of motion are generally healthier because they don’t get injured as easily, and they also don’t put as much tension on the structures they are attached to. For example, tight hamstrings put tension on your pelvis which can cause low back pain.

Sport stretching is different from static stretching, because generally for a sport warm up we don’t hold the stretches long enough to stimulate the muscles to actually grow longer. But sport stretching is still beneficial because it puts the muscle through its full range of motion prior to your workout or sport, which helps it get ready to work and makes it less likely to cramp or get pulled.

Regarding your last topic, “work out,” that’s hard to answer because that is a very broad term. Sticking with the theme of muscle health, I’ll assume we’re talking about muscle. Strengthening your muscles is helpful for a lot of reasons, the most important of which is that strength provides stability for your whole body. For example, having strength in the muscles in between your shoulder blades is important for posture. Muscles that are too weak to do their jobs become irritable. A lot of people get knots and cramps in between the shoulder blades because those muscles are too weak to stabilize the shoulder blades, so the shoulder blades drift apart and pull on the muscles.

TLDR: muscle relaxers work on the spinal cord signals to stop muscle cramps, stretching for a long period makes muscles grow longer, stretching for a short period helps you avoid cramps during sport, and strengthening muscles makes them less mad because they’re better stabilized.


On another note, how does massaging effect muscles? If I recall from A&P, it has something to do with stimulating the over-contracting reflex? (Can’t remember what the exact term is.)


Another thing would be the difference between a anti-spasm and anti-spasticity. Or is that outside the scope?


ELI5 Why does it feel like parts of your body are twitching randomly and you can’t control it? Often referred to as a “nerve jumping”.

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Twitches/cramps can be caused by, low vitamin levels, dehydration, stress, over working a muscle, phsycial damage, stress, or even some serious medical conditions.

One time, for about a week, i could feel a muslce on the right side of my back twitch a couple times every second whenever i sat down. It didnt hurt, just kind of annoying. It went away on its own.


Every muscle is supplied by a nerve that tells it how and when to move. Every nerve is made up of many many nerve cells (neurons) running in parallel together. Each neuron’s body lives in the spinal cord, and it’s “tail” reaches out along the nerve to the muscle. At the muscle, each individual neuron “tail” splits off into a bunch of branches to make contact with a certain number of individual muscle cells. This combination of a single neuron with the specific number of muscle cells it touches is called a “motor unit.” Muscles are able to contract very hard or very light depending on the number of motor units that fire. A repetitive, involuntary muscle “twitch” is called a fasciculation. These happen when the neuron of one or a few motor units get irritated or excited, causing them to fire repeatedly without your permission. Since it’s only a few motor units, the resulting contraction is very small and localized.


ELI5: How does a pulse oximeter measure the blood oxygen levels without actually taking blood?

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It’s a colorimeter!

ELI5: it looks at the color of the blood in your finger and can see how much of it has oxygen based on what color it is.


I’m a nurse. I’m not necessarily an expert on exactly how it works but I have a basic understanding. Basically the little red light that the finger clip lets off goes through your finger and hits the sensor on the other side. Using the power of science the sensor can tell how much of the light was absorbed by the blood going through the finger, and it uses this to math out what percent of the hemoglobin has something bound with it. Hemoglobin (the part of the blood that carries oxygen) absorbs different amounts of light depending on if it has something on it or not and the computer can use that to figure out what percentage of the hemoglobin is bound with other compounds.

It’s a handy tool for a quick reading but it isn’t perfect and has its drawbacks. If someone has fingernail polish on the light won’t go through the finger properly and you won’t get a good reading. Similarly, if you’re really cold or have poor finger circulation for some other reason there won’t be enough blood going through to get a reading. In that case, you need a special probe that can be stuck to the forehead or somewhere else to get a reading.

Another problem is that it can only tell you how much oxygen is bound to your hemoglobin as a percent of total hemoglobin. So if you’re bleeding out or your hemoglobins low from something like anemia, the sensor will read as though you are properly oxygenated even if your cells aren’t getting enough oxygen. For example, say your hemoglobin is crazy low, like… 2 (normal is roughly 12-16 for females and 14-18 for males). If you’re still breathing ok the sensor will read 100%. But that doesn’t matter because 2 hemoglobin isn’t enough to give your body the oxygenation it needs even if every bit of it is bound with oxygen.

One other issue with it is it only reads what % of your hemoglobin has *something* bound to it. Not just oxygen. Hemoglobin binds to other things besides oxygen. A common example is carbon monoxide. The reason for this is that if you breathe in both oxygen and carbon monoxide, Hemoglobin will bind to the carbon monoxide *before* it binds with the oxygen. This has to do with chemistry and whatnot but basically the hemoglobin has a greater affinity for the carbon monoxide than it does with the oxygen. So if you end up in an ambulance with carbon monoxide poisoning and they put a pulse ox on you, it will probably read as normal. This is because the machine just reads that the hemoglobin has something bound to it, it doesn’t realize it’s something that you can’t breathe.

Pulse ox’s are a good tool but their just machines that can be fooled and have problems. True low readings are bad and should be treated, but high readings don’t necessarily mean the patient is ok. That’s why there’s a common saying in healthcare to always treat the patient and not the machine. People get super caught up in treating numbers like O2%, blood pressure, etc. without looking to see if those numbers match what the person is experiencing. If your heart monitor says the person’s heart isn’t beating but they’re sitting on the edge of the bed talking to you, it’s usually wise to troubleshoot the equipment before trying to do CPR.


It shines a light through your finger, on the other side of an eye that’s really sensitive to color. The color of blood is different when there’s oxygen in then when it’s not in there. So the eye can see the color and determine how much oxygen is on the blood.


Oxygenated homoglobin really likes to aborb infrared light(specifically wavelenght = 940nm), but allows red light(660nm) to pass through. While, deoxygenated homoglobin allows more infrared light to be passed through, but aborbs red light. There are specific sets of lights and sensors that only work on those wavelenghts, and ignore the other. We fire off light and measure how much light gets on the sensor. We can now divide those amounts and get the ratio of saturated blood cells in our bloodstream.


Came back to all these fantastic explanations! Thanks, y’all!


ELI5: What is it that produces such an incredibly loud, sharp sound when you click your tongue against your palate?

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Vacuums filling with air are very loud.

Other examples include lightning, opening jars and bathtub drains.

Your tongue is a very powerful muscle and your pallet allows for a very strong seal.

That combination allows for quite a sizeable vacuum that can totally open very suddenly, this leads to a rapid influx of air that causes the noise.

This, combined with the shape of your mouth (which functions as a very effective echo chamber) amplifies an already loud sound.


When you pull a cork out of a bottle it makes a loud noise even though nothing is hitting anything. (actually the air molecules are hitting each other but the cork and bottle aren’t hitting).


Keep in mind your tongue is a large, long, powerful, multi-mobile muscle. Imagine building tension in your bicep or quadricep and slapping it on a wet surface.


Not a native English speaker here, just wondering if there’s a word describing this sound, or a verb for making this sound?


There’s actually two sounds. When your tongue leaves the palate. And when your tongue impacts the floor of your mouth. Click.click


ELI5: How do dryer sheets work?

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If you touch a dryer sheet, you’ll notice it’s oily or tacky, and basically it’s fabric softener spread on a sheet. The sticky feeling is from a surfactant. The word “surfactant” comes from “surface active agent”. They have molecules that attract and stick to both oils and water.

Surfactants are useful in detergents (they stick to oils and let them get rinsed away) and in salad dressings (they keep the oil and vinegar from separating).

Laundry detergent uses surfactants to do the cleaning. These are usually “anionic” surfactants, which means the water-liking side has a negative charge. These are great at pulling dirt and oils out of fabric and then forming a little cluster, making sure the oily dirt stays suspended in the water in a little bubble. This prevents your clothes from getting dirty in the laundry water.

A dryer sheet in your laundry spreads another kind of surfactant all through your clothes.
The surfactant used in dryer sheets has a positive electrical charge on the water-liking end, it is “cationic”, so it sticks to negatively charged clothing fibers like a magnet. It cancels out static electricity from the fabric rubbing together. It also adds a layer of oil to the outside so the fabric doesn’t easily rub together well enough to build up any more static charge.

The water-liking side sticks to your clothing fibers and the other oil-liking end points outward. The dryer sheet also contains a lot of oil or silicone to coat your clothes and make them feel really soft.

Because each molecule has a positive charge sticking outward, the fibers repel one another and feel extra fluffy when they’re dry.


So first of all, what is static? It’s a buildup of charge that causes our clothes to stick to us.

To get rid of that charge we need to balance it out with something charged the opposite of it.

Dryer sheets are coated with a material that is charged oppositely when heated to negate the charge of the clothes.