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Can a shadow travel faster than the speed of light?
Yes, but since no information is transmitted this way, it does not violate general relativity.
Can shadows travel faster than speed of light?
> If shadows can travel faster than light then won't I be able to transfer information at rate faster than speed of light? darkness would represent 0, light 1... The entire message would travel at light speed to the receiver, who would then receive a very fast transition rate between the zeroes and ones. Each individual bit traveled at light speed. You can make them come in arbitrarily quick succession (or at least within whatever physical limits of your device, so in practice not close to the speed of light at all) but that won't let you actually get those changes to cross the spatial distance any faster.
If AIDS is a lack of white blood cells and leukaemia too many, can't we use HIV as a leukaemia cure?
HIV attacks one specific type of T cell, called a CD4 T cell. Leukemia can be caused by T-cells but can also be caused by other white blood cells. If you had a leukemia stemming from your B-cells (another type of white blood cell), HIV unfortunately wouldn't help you. Also, both HIV and cancers can mutate frequently. If you introduced HIV to kill a leukemia, even a leukemia that was caused by CD4 T-cells, the cancer could mutate away and be immune to the virus. Some other people have been talking about the recent news of a young girl with acute lymphoblastic leukemia who was cured using HIV. In this case, HIV wasn't used to actually kill the cancer cells, it was used to import a man-made gene into the girl's immune cells to better fight the cancer. This is a form of gene therapy that is starting to get into the news as the results from trials get published. Here is a link to a paper from the group that is doing these trials. _URL_0_ I think you should be able to open that... Hope this helped!
If a star's light reaching us today is 10 billion years old, how far was the star from us 10 billion years ago?
The answer depends on exactly what the universe is made of, because that determines how it expands over time. [This website lets you plug in various parameters and calculate its current distance](_URL_0_) for a variety of [distance measures](_URL_1_) (turns out "how far away is it?" can have multiple meanings depending on how you ask it.). If I plug in a redshift of z=1.815 and use the default universe parameters, I get the 10 billion year travel time and it says the current "comoving distance" (the distance of a ruler stretching from here to there right now) is 16.17 billion light years. If I then use the redshift to know how much the universe has expanded since then (a factor of 2.815), then I find that it was 5.74 billion light years away when the light was emitted.
If salt and sugar molecules disassociate into ions when they dissolve in a solvent (water, for example), how can we still taste their flavor?
The flavor that you think of as "salty" is the flavor of "Na+ and Cl- ions". It is the dissociated ions that you have receptors for. Also, sugars don't dissociate into ions in water. They do dissociate into smaller sugars, if possible, but those are still neutral.
How did the US develop so many different accents/dialects so quickly after leaving England?
There are a ton of factors at play here. In the years before the automobile and interstates (or planes and trains), distance was a big deal. You can drive across the country in a couple days, but by wagon it would take weeks. Months even depending on weather. So, people were more isolated. That's one part. For a second, interaction with various cultures that were regional. The accent in Louisiana is heavily influenced by the interactions with the French who used New Orleans as a major hub. Beyond just French and Spaniards, there is also the varying degrees of interactions between the various native american tribes, who all spoke different languages as well. There are probably more factors, but these are the low hanging fruit.
When and why did the English accent in early America fade away, and the American accents come about?
English as it was spoken during colonial times isn't the same as it is spoken now in either Britain or America. Both populations have had their pronunciation drift over time. So Americans never lost their British accent, because they never had what we would now call a British accent to begin with. You can look up "Original Pronunciation" (Called OP English) to get an idea of how pronunciation has changed over time since the early 17th century. There are people who do Shakespeare productions using reconstructed OP to recreate what Shakespeare's plays would have originally sounded like. P.S. I should have linked [this](_URL_0_) from the beginning. P.P.S. [Here](_URL_1_) is another video talking about a modern accent that many think is close to what colonial-era English sounded like.
When a star goes supernova, isnt the shrinking process faster than the speed of light?
In a core-collapse supernova, only the core of the star collapses. The outer layers don't have any idea what's going on until the supernova shockwave reaches them. Stars going supernova actually take of order a few hours to completely destroy themselves (100 R_Sun/10,000 km/s ~ 2 hours), even though the core completes its initial collapse in less than a second. Answering your second question is difficult. Destructive to what? Supernova shockwaves can travel many tens or hundreds of light years through the interstellar medium, but a supernova would not harm life on Earth at all if it were than 50-100 light years away.
How can a rocket in space be propelled without any matter to push against?
It's the third law. Equal and opposite reaction. Imagine a gun. You shoot it, there's recoil. Mediated by pressurized gas created by the gunpowder reaction, the gun and bullet push against each other. Take out the bullet and the gun is essentially a rocket with a very fast burning solid fuel engine. Momentum gets conserved, and it's a vector. Consider a rocket in space, at rest in your chosen frame of reference. It fires its engine, emitting a puff of gas. The mass of that gas multiplied by its velocity is opposite the remaining mass of the rocket multiplied by its new velocity.
How can rockets propel themselves in space without anything to push against?
Rocket work because they expel mass in one direction, which provides a force in the opposite direction. "Pushing on something" does not play a part in this reaction. As an example, sit on a rotating bar stool. Hold one hand straight out, and then quickly move it to the right. If the stool has good bearings, you will find yourself rotating to the left, to counteract you hand's movement.
How do we know what the digits of Pi are?
One method is to think about areas. Pi is by definition the ratio of a circle's area to its radius^2 (A = pi*r^2). So we draw a regular polygon outside a circle (with radius 1 unit for simplicity) and one inside the circle, calculate the areas of the polygons, and we know that the area of our circle is between the two values, and we can calculate a max and min approximation for pi. Add more sides to each of our polygons and their area becomes closer to that of the circle. As the areas of the two polygons start I match up, we know we've found some digits of pi. An illustrate explanation of this method can be found here _URL_0_
What is that feeling we get when we want to sneeze but can't. What exactly is going on in our noses at that time?
Usually, a person sneezes due to irritation in the nose, mucus, hair or any irritants that the body (or the nose) wants to expel explosively (hence the sneeze). But as the body is preparing to sneeze, you are inhaling air, inflating your lungs and, in general, doing the "aah..aaaaahhh..." part. If the irritant is moved or is dislodged from its place - the need and the urge to sneeze vanishes. Hence no sneeze. TL;DR: the preparation to fight the enemy causes the enemy to go away.
What is the minimum mass required for a celestial object to become spherical in shape?
AFAIK the smallest *round* moon in the solar system is Mimas (about 3.8 x 10^19 kg), although it slightly tidally distorted ovoid by Saturn's gravity. > Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment." [source](_URL_0_)
What is limiting the speed in which quantum computers can run? For instance, besides security issues, why can’t quantum computers guess a password by trial and error? Is the bottle neck just our interpretation of the information?
Quantum computers aren't any better at guessing passwords than regular computers are. There's a harmful misconception that the way quantum computers work they "try all alternatives simultaneously" and then magically "pick the one that works" as though they were a real fulfillment of a nondeterministic finite automata. Quantum computers don't work that way. Instead, they create a quantum superposition of states that, when measured, approximates the true solution well enough to hopefully shortcut a significant amount of traditional computing work. Scott Aaronson is an MIT researcher who works heavily in quantum computing, so I'll let him explain it better than I ever could: _URL_0_
Why can't dark matter just be ordinary matter that doesn't happen to be in a star?
Most astronomical observations have quite big error bars, so I prefer to mention the CMB, which has been observed in detail and foregrounds/backgrounds are understood very well. The CMB alone (in combination with oru standard cosmological model) tells you that there has to be an energy component like dark matter. Without it we would never be able to reproduce the measured power spectrum. No amount of "unseen normal matter" can change that
Why do whales have horizontal tails where as fish have vertical back fins?
It's a mammalian adaptation. Spines that flex up and down lend themselves to the ease of movement of terrestrial limbs.
Why are fish tail fins vertical while whale/dolphin tail fins are horizontal?
Animals that move from land to water tend to swim in the same way they walked or ran. Most non-avian reptiles walk using an [undulating side to side motion](_URL_0_). That's also how the earliest tetrapods walked. They walked that way because fish swim that way. When reptiles that use undulatory locomotion readapt to water they swim using side to side movements and hence need a laterally flattened tail like an Ichthyosaur or a sea snake. Conversely the ancestors of whales were mammals that ran by flexing the spine up and down, rather than side to side so they swim that way too (as do Otters, Manatees, etc). A dorsoventrally flattened tail is better for that type of swimming. The difference is a result of the contingency of evolution and shows how organisms carry the baggage of their ancestors.
Why are rainbows curved?
So [rainbows](_URL_0_) aren't a physical thing. They're [reflection and refraction](_URL_0_#Explanation) of light within the drops of water. And since each color has some principle angle that it exits the drop at, you can take that angle and project a circle in front of you of constant angle for which you should see one specific wavelength. Or a range of angles for the range of colors we can see, thus leading to a thin circular ring of colors. If you do this with a garden hose or at a waterfall, for instance, you may see the whole circle. Otherwise, part of the circle is intersected by the earth and thus just forms an arc.
How is it possible to become lactose intolerant later in life?
Not being lactose intolerant later in life is a Neotenous trait. Your body doesn't require milk naturally after a certain stage, and that is nature's way of weaning you off it
Why did animals get smaller during the past million years?
There's a discovery bias... you're more likely to notice a fossil shark if it's the size of a whale than if it's the size of a minnow. Also, 99% of the species that ever lived have gone extinct, so the expectation would be that the largest shark species would probably be an extinct one (the smallest species is probably extinct too). There is [some evidence](_URL_0_) that large species go arise and go extinct faster than small ones, and of course in recent times, hunting and habitat loss by humans has affected large species disproportionately. Otherwise, I'm not aware of any comprehensive evolutionary bias towards smaller size during the past million years. In fact, the largest animal known in the history of Earth, the blue whale, is still living.
How do forest fires start in the wild?
Lightning is the usual culprit. In the western US, dry lightning strikes (storms develop, produce lots of lightning, but not much rain) are the primary cause for forest fires (after man made sources). In the eastern US, most lightning comes with heavy rain so fires started by lightning get extinguished pretty quickly. There's also the difference of fuels and topography that could affect a fire's spread.
How do forest fires start 'naturally'?
Lava flows/volcanic sources, spontaneous combustion from organic material overheating (think compost piles or certain types of coal), and rockfalls producing sparks are all potential other sources but lighting is by far the most common. Rainstorms are often isolated sporadic events. For example it can rain on one side of a stream and not the other and lighting doesn't have to strike under the rain, it can hit a tree on the other side of the stream so the rain will not effect any fire produced. There is also a lot of energy in lighting a good strike on a tree can set the whole tree ablaze even in a rainstorm. While the rain may put out the exposed fire, sheltered embers in the core of the tree can continue to smolder until well after the rain passes and then flare up. There are many areas where organic material is very deep and a fire may smolder underground. Fire can burn on the bottom of a log or under the shelter of a larger tree's limbs protected from direct rain.
What was the size of the very early universe?
Particles aren't traveling faster than the speed of light--- *space* is expanding faster than the speed of light. Which is fine, there is no inertia frame for space according to GR, it isn't bound to the speed of light.
The Moon illusion makes the Moon appear bigger when lower on the horizon. Does this mean our vision is actually capable of better resolution than usually?
Think of a a more traditional optical illusion, [like this one.](_URL_0_) In that image, the two center circles are the same size, but the left one looks bigger because the context it's in tricks our brain. The moon illusion is like that: it's not that you're getting a magnified view of the moon with more detail, but that you are not judging the moon's size correctly. For an in-depth description of the moon illusion and how it works, [see this webpage by a professor at the University of Wisconsin.](_URL_1_) The current thinking seems to be that the moon illusion is really an angular size illusion, [which is described in this Nature paper.](_URL_2_)
How does radiation cause mutations?
Simple answer: Ionizing radiation causes formation of reactive oxygen species. These are molecules and ions that can react with things in your cells - namely DNA. Usually, the changes to DNA can be repaired. Sometimes the changes to DNA are permanent, and can: - Be such a drastic change that the cell cannot survive, and will either straight-up die, or will commit suicide. - Be a genetic change (mutation) of no significance. - Be a mutation that actually matters. Some types of radiation (namely UV) can cause direct DNA damage that is not mediated by reactive oxygen species (i.e. no middle man).
Do colors of the pixels of our mobile phones affect its battery life (say black pixels consume less energy than white)?
In short - no if you have LCD display, yes - if you have OLED display. Here's why: LCD uses a white backlight is always on, and a layer of liquid crystals above it is used to block part of the light. Thus, the brightness of an individual pixel doesn't change the power consumption at all. Completely black image on LCD display leads to the same power consumption as the completely white one. The only thing that changes the power consumption is the dimming of the whole display. That reduces the backlight, and thus the brightness of each colour. With OLED, each subpixel emits light by itself. The brighter that subpixel is, the more energy it uses. So black OLED pixel uses almost no power, even while there can be extremely bright white pixels on screen at the same time. That's why there are always on displays on some OLED smartphones, dimly showing time and some other info in part of the screen. It takes very little power to show those, as most of the screen is kept completely black.
With helium becoming more scarce, are there any feasible replacements for helium in scientific work?
Helium is simply alpha radiation. Tons of elements give it off. It won't totally go away for billions of years, but it will definitely become extremely valuable
Will nebula from exploding stars such as the crab nebula be visibly different over the course of a lifetime?
Yep, people have been observing this sort of thing for quite some time. _URL_0_
Why don't we dispose radioactive waste in space ?
Because that's prohibitively expensive and there's a chance the rocket could fail and spread radioactive waste over a much larger area.
Is it feasible to send nuclear waste into space?
It is possible, yes. However, it's usually perceived as being too risky. In the event of catastrophic failure during launch, the radioactive waste will be dispersed rather more widely than it was before. For context, the *Columbia* disaster spread debris across parts of Louisiana, Arkansas, and Texas, creating more than a dozen separate debris fields, including several which traversed populated areas. Imagine the potential heath hazards involved if a significant portion of that debris was radioactive.
Is there a well established link between the use of antibiotics in livestock and antibiotic resistant bacteria in humans?
Short answer: Yes there is a well-established link. [Here is a list of scientific studies on this topic.](_URL_0_)
Would a "base-prime", number system be useful in any way?
It would make prime factorization trivial, multiplication easier, and addition extremely difficult. You'd basically have to work out the numbers in a more useful format, add them, and then factor that. But that last step would get really difficult with large numbers. Also, as others have noted one is not prime. If you have a space for one, you could stick any number there and it wouldn't matter.
Why does tickeling make us laugh?
The areas of the body that tend to be ticklish are often vulnerable spots, like the neck, sides and stomach. When tickled, we tend to laugh, yet there is something almost unpleasant about it that compels us to protect that area of the body and stop the stimulus. This is believed to come from an impulse of mammalian children to "play fight" and it helps to reinforce the need to protect vulnerable areas from attackers.
How big would a helium tank have to be to float?
What's the tank made of and what pressure's the helium under? A balloon could be considered a "tank".
Do computers inevitably slow down over time? Is there anything inherent about the way computers work that will cause them to eventually slow down, even after a fresh install of the original operating system?
Short version is no. If you timed certain operations like boot, launching a program, doing a benchmark, etc., and compared the results from today and 3 years ago, they would be the same. Your experience is subjective. This is assuming that your system is now in the original state with proper drivers and there are no hardware malfunctions and correct bios configuration. There are a few common hardware failures that can impact performance, but the impact is dramatic. Broken fans may throttle your CPU, bad sectors on the drive may cause retries, and very bad hard drive cables may cause ecc errors on the ata bus. If any of that was going on, it would not be subtle, it would be dramatic. Multiple seconds of unresponsive mouse, for example. Nearly every part of the computer has parity or error correction and complains loudly if something is wrong that isn't transparently recoverable.
Why does bruised fruit taste weird?
Bruising the fruit releases polyphenol oxidase which in turn produces quinones which turns into melanin. Melanization entraps the foreign bodies of infection. It may be a psychological effect or they might taste different than unoxidised polyphenols.
What is happening when I have a cold and my nostrils take turns being stuffed?
see info [here](_URL_0_). When combined with mucusal inflammation, you might experience blockage in alternating nostrils. Personally, one of the most fleeting and euphoric moments when fighting a bad cold is that 20-45 second window when both nostrils are clear. If only researchers could discover a drug that keeps them open all the time... It's thought to be under the control of the CNS, so I don't expect a fix any time soon.
How fast do smells travel?
From [this website ]( _URL_1_ ) > f we assume that the average mass of air (since it is a mixture of different gases) is 28.9 g/mol (or each gas particle is around 4.799*10^-26), and room- temperature is 27C or 300K, we find that the average velocity of a single air particle is around 500 m/s or 1100 miles per hour! For larger molecules like the aromatics in blue cheese, the molecular weight is obviously going to be higher and the velocity therefore lower. These molecules are also going to have to random walk through collisions with air gas molecules, but you're going to find it is still damned fast. Anecdotally, if I take blue or some other aromatic cheese out of the refrigerator in my house, it takes less than 5 seconds after I unwrap it before I hear my dog sleeping on the 2nd floor jump down from the bed and come rushing down the stairs.
Why isn't there an equivalent of "antibiotics" for viruses?
So to keep it short and sweet. Many antibiotics work by blocking the protein machinery of bacteria. This works because even though our cells have versions of the same machinery, they are different enough that the antibiotics don't affect our cells. This isn't universally true for viruses because they use our own cellular machinery to replicate themselves. So for many cases to block viral replication would require blocking our own cells. There are cases of viruses that make viral proteins unique to the virus. Those proteins we can target, with [anti-viral](_URL_2_) (or [anti-retroviral](_URL_1_)) drugs. [HIV intigrase inhibitors](_URL_0_) are an example of such drugs
How can we still see the Cosmic Microwave Background from the Big Bang but galaxies formed after the Big Bang are too far to see?
Since light travels at a finite speed and the universe hasn't existed forever, we can only see a finite-sized patch of the universe. From the CMB measurements we can deduce that the universe probably is much bigger than we patch we can observe, and there is no reason to assume that there are no galaxies in the unobservable parts (we assume that the universe looks approximately the same everywhere).
Is There A Rigorous Definition Of Infinity In Mathematics?
Infinity has many mathematical definitions that depend on the context. For example, if I say that The sum of N from N=1 to N=infinity is equal to infinity then I have used infinity in two contexts (and misused "equal to"). The statement means that for every value, X, there exists a value, Y, such that the sum of N from N = 1 to N = Y is greater than X. This form of the statement is logical and explicit and leaves no ambiguity that arises from treating infinity like a number. There are many other contexts in which infinity can be used, but they all follow about the same form--the original statement is just meant to mean some associated formal statement which removes the infinity. Thus Infinity itself never needs to be defined, you just define what you mean when you use it certain ways.
Why don't trees get cancer?
They do, it's caleld a [burl](_URL_0_). Tree's have different cell structure from animals, and they also have a very different vasculature. Because of this, cancer in plants can't invade nearby tissue or spread throughout the organism, and it is rarely fatal to the plant.
Do any trees contact cancer? If not, why don't they?
Cancer only becomes dangerous when cancer cells can move around the body and establish new tumours. Because plants cells have cell walls, they can't migrate around the body of the tree in the same way a metastatic cancer cell could in an animal. Trees can get tumours, in fact you've probably seen this before. Those random lumps you see on trees are often tumours, but since the tumour cells can't invade other tissues in the tree, they will never be fatal. [source](_URL_0_)
Has there ever been a documented case of 2 separate ape species that have been taught sign language, using it to communicate with each other?
**An important caveat**: Evidence of non-human primates using "sign language" to communicate anything beyond rudimentary symbols is nonexistent; well-publicized cases of chimps and the like learning sign language are grossly bastardizing the meaning of sign language. Non-human primates who are taught signs use them to communicate mostly physical desires (i.e. "want" "apple"), completely without grammatical structure or attention to order. This is not sign language; it is not any language, it is just simple communication using manual symbols. That being said, I believe there have been documented instances of two nonhuman primates of the same species throwing these signs at one another when a researcher was not present to incite it. I don't know of any cross-species instances, however, as it would require different species living together, which doesn't generally happen in observational environments like zoos or laboratories, and isn't something that naturally occurs in the wild.
Have non-human Great apes ever been seen communicating with each other in sign language?
Most great apes don't get beyond using very, *very* simple vocabularies. Most of the exceptions are listed on [Wikipedia](_URL_0_). The wikipedia article is actually quite thorough (make sure you read the references for more info).
What is stickiness? What makes sugar water sticky while water isn't?
I think your guess is quite reasonable. Sugar molecules are much larger than water molecules, and it has a [chemical structure](_URL_0_) that can accommodate many more hydrogen bonds with other nearby molecules compared to water. Those extra hydrogen bonds (mediated by those -OH groups) raise the level of cohesion, and the net effect is more stickiness. The key consideration is the overall balance among the attractive and repulsive forces between pairs of molecules. Sometimes, it's possible to engineer new adhesives by considering the chemical groups that contribute to these forces and how they should be arranged.
Why does burping through your nose after drinking soda burn your nostrils?
Because soda contains high levels of CO2, which, when dissolved in the water covering the inside of your nostrils, turns them acidic (H2CO3). Of course, the same happens in your mouth, but your mouth is better protected against acids, as you need to be able to ingest various acidic things relatively often.
If twins both have sex with a woman, and she gets pregnant, would a paternity test be able to distinguish which one is the father?
Yes, with detailed enough test. While identical twins comes from a single egg, each cells group later split million times even before birth. The [copying between each split isn't perfect](_URL_1_) (hence, mutation). While we have good enough redundancy for decades of living, the imperfect copies are still there, isn't identical between twins, and transferred to their children via sperm/eggs). Now, standard paternity test doesn't check all part of someone's DNA. Only dozens or so of parts that usually differs between unrelated individual. So even in case of non-twin brothers, a standard test could fail to distinguish them. At least in one case in [2007](_URL_0_), a court-ordered parental test fail to conclusively distinguish the real parent between identical twin brothers. Thus, theoretically we may one day be able to distinguish the father, but at least for now the available technology for reasonable cost isn't enough for 100% accuracy
Two planets sharing one moon?
While you can find a [mathematical solution](_URL_0_) for such an orbit, it requires such precise positioning and starting momentum for every object in the system that even a tiny pertubation will shove it into an orbit of one of the two planets. I wouldn't expect to find any natural objects in such an orbit, and even an artificial object wouldn't stay on it for long. If you're willing to relax that requirement, what you could have is a moon orbiting the common barycenter of a pair of planets, similar to a planet orbiting a binary star system. In that case you'd probably expect the moon's orbital radius to be no closer than a bit over the separation distance between the planets. In this case, the moon would essentially behave as if it were orbiting one planet with a mass equal to the total mass of the two planets. For the period of that orbit you can refer to Kepler's 3rd law.
Is it possible that the dark matter is actually stars with Dyson spheres around them so we can't see them?
Nice idea, but it won't work. There are a few reasons: (1) We know dark matter does not feel the electromagnetic force. If the dark matter were Dyson spheres, from absorbing the energy of the stars they surround, they would radiate in the infrared, and so we'd detect this. In fact, people have looked, and [no Dyson spheres](_URL_0_) have been found. (2) Dyson spheres would be an example of MACHOs (massive compact halo objects), which more conventionally includes things like brown dwarves (stars that didn't quite ignite). Searches for MACHOs show that the vast majority of dark matter cannot be in MACHO form. (3) Studies of the cosmic microwave background radiation give us the ratio of ordinary to non-ordinary matter in the universe (there has to be a certain amount of matter that interacts with ordinary matter via gravitation but does not interact with photons). Without dark matter, this doesn't work.
We share 50% of our DNA with bananas. If the other 50% is enough to make us so radically different, what does the 50% we share do to make us the same?
At the bottom level it encodes the way our DNA is processed: ribosomes, transfer RNAs etc. These are things we share with all known life. Then there are things like the cell cycle, which includes cell division. Different cell organelles like mitochondria, the way the cells are stabilized (the cytoskeleton). Probably a lot of genes that are involved in using sugars, fats and starch in our bodies.
Is the amount of Fluoride in drinking water large enough to cause serious side effects?
Water fluoridation is (mostly) good for dental health and overall health, the people who say water fluoridation is bad are conspiracy theorists. People with fluoridated drinking water have been scientifically proven to have healthier teeth than people with non-fluoridated well water, for example. The conspiracy theories about water fluoridation are very similar to the conspiracy theories about vaccines, they stem from a mistrust of the government and an unscientific fear of "chemicals". That said, too much fluoride during infancy and childhood can cause [dental fluorosis](_URL_1_), which results in bad teeth with streaks and discolorations. Also of note, all fluoride salts are inherently toxic (see [Wikipedia's article on fluoride toxicity](_URL_0_)). You can make yourself very sick by swallowing a bunch of fluoride mouth wash, you may even die. However, the dose in fluoridated drinking water is far too low, you would have to drink many gallons of tap water to get a substantial dose of fluoride.
Is it possible for a planet to spin upon multiple axis?
Not a planet, but [Hyperion (moon of Saturn)](_URL_0_) has chaotic rotation period and variable axial tilt; its orientation is unpredictable.
On another planetary system, could humans be giant?
Such hypothetical / speculative / open-ended questions are better suited for our new-ish sister sub /r/asksciencediscussion. Please consider reposting there instead.
What happens within an ear that causes a loud noise to damage hearing?
Within the inner ear there is a structure called the Cochlea. It's a spiral-shaped structure that's filled with fluid and lined with these long, hair like proteins called "cilia." When sound vibrations are transferred from the bones of your ear through the fluid, it causes these cilia to vibrate as well, which is transferred into a nerve impulse and sent to your brain to interpret. Damage can be caused when extremely intense vibrations through the fluid damage the cilia and make them either less sensitive to any vibrations after that, or will cause damage to the systems that create nerve pulses from sound, causing you to hear sounds that aren't actually there.
Are there Dark Matter "stars"?
Dark matter only interacts with other matter - dark and not dark - by gravity. Star formation requires more than just gravity, strong, weak and EM are required too at various points. It doesn't exists only in a halo around galaxies, there is actually dark matter within our solar system. It permeates through the entire galaxy. I usually imagine it distributed like dust through the galaxy (even though it's not really...)
How does a car determine its speed?
It is done by counting revolutions, of the wheel, axel, transmisson, ect. And yes, changing the tire size does change the calculations, which is why you are supposed to have it recalibrated if you do change tire sizes.
How does a car calculate its average speed?
I don't know if this is how cars do it, but a common technique is a "digital low-pass filter," which at it's simplest uses an equation like v_new = 0.9\*v_old + 0.1\*v_detected. This means that each time a speed value (v) is detected (likely many times per second), the saved value isn't set to the detected value itself, but rather to a weighted average of the detected value and the old saved value. This means that small, choppy changes in speed have little effect on the saved value, but if the average speed changes, the saved value will quickly approach the new average after a few cycles. There are also analog versions of the same idea that could have been used before cars had computers in them.
Are there any stars that orbit around rocky objects?
A single rocky object the mass of a star is not stable against gravitational collapse. A more complicated geometry might work. For example, a swarm of small rocks orbiting each other.
Why is our blinking synchronized between both eyes?
Additional question: Similarly, does synchronized blinking have anything to do with the way we have synchronized eyeball movement?
If a Ferrari can travel at 110 m/s max and nerve impulses travel at 100 m/s max, does this mean that the driver would be traveling forward mroe quickly than his brain could process the visual information from the road?
The brain has to process information from over 120m ahead, even at only 80mph. So the car has one second to reach the object, while the nerve impulse only needs to travel a couple of meters to the legs. [Stopping distance](_URL_0_) If the driver noticed a sudden obstacle only 2 meters ahead, then yes the car would get there before the nerve impulse reached the feet!
How does Venus retain such a thick atmosphere despite having no magnetic field and being located so close to the sun?
You are right in observing Venus does not have an intrinsic magnetic field. [However, solar winds interacting in Venus's upper atmosphere ionize particles](_URL_0_) (an ionosphere). This ionosphere induces an external magnetic field around Venus which acts similarly to planets with magnetic fields and excludes solar winds. [Zhang et al.](_URL_1_) also provide evidence that Venus's externally induced magnetic field reconnects, which was previously thought not to occur. Despite this, Venus still does experience some atmospheric loss due to solar wind pressures. Perhaps there is some geological process that replenishes Venus's atmosphere, but this is outside the scope of my study so I will refrain from speculating more on it.
Is there a larger "explosion" than a hypernova?
> If supernovae happen when massive stars reach the end of their life and hypernovae happen when even more massive stars reach the end of their life (as I understand it), what happens when stars like R136a1 reach the end of their lifespan? A really big hypernova. Hypernova is just a catchall term for supernovae that are much brighter than average. A typical supernova puts out something on the order of 10^44 joules of energy. Hypernovae are a few orders of magnitude stronger than this. We don't know enough about the physics of supernovae, or about the upper limits on star masses (especially in metal-poor Population III stars in the very early universe, stars may have gotten up to several hundred solar masses) to put a limit on supernova energies.
If we look billions of light years into the distance, we are actually peering into the past? If so, does this mean we have no idea what distant galaxies actually look like right now?
Yep, exactly that. We will only find out what distant galaxies look like _now_, millions of years in the future (or rather, our descendants, if they exist, will find that out). This is even true on a lesser scale of nearer objects such as the sun. If it suddenly ceased to exist, we would know nothing of that until about 8 minutes had passed.
Why is wine generally made from grapes?
Wine can be made from anything that ferments. There's [Rice Wine](_URL_0_), which is popular in Asia since grapes are not, [here](_URL_1_) is a list of fruits that are used in wine. As for grapes current dominance, that's probably more of an AskCulture thing than AskScience thing, having to do with the relative dominance of Grape wine in Europe, cultural bias and the relative dominance of Europe culturally throughout the world.
What exactly is a hiccup? What causes them?
A hiccup is caused by a muscle spasm of the diaphragm. The diaphragm is a thin muscle layer under your lungs that helps separate the chest space (which holds mainly your heart and lungs) from the abdominal space. The way air is pulled in during a breath is by flexing the diaphragm. The lungs themselves are not made of muscle so they don't actually do any of the pulling. During a hiccup, the diaphragm has a spasm which causes a sudden contraction, just like a spasm in any arm or leg muscle, and this is paired with your throat closing in reflex to this spasm down below.
How carcinogenic are smoked meat products?
There have been some studies which show cooking meat at high temperatures increases the number of heterocyclic amines, which have been linked to cetain types of cancer, namely those of the GI system. _URL_0_
Since all stars are moving with respect to the Earth, will the constellations eventually change over a long period of time?
Yes, they would. A million years ago, the Big Dipper looked more like a spear than what it does now, and it will change shape again in the future. Barnard's Star is a great example of stars moving across our sky.
Why do we get the green/yellow goo in the corner of our eyes when we sleep and why don't we get it in the day when we're awake?
Is there an English word to better describe these "eye boogers"?
What keeps the all the hydrogen in a star from undergoing fusion all at once?
Stars are in hydrostatic equilibrium. There are two forces in play. First, stars are very massive, and thus gravity wants to pull them together. Without fusion, stars would collapse into a much denser, smaller ball. Second, fusion provides pressure outward, pushing the star apart. Without gravity, the fusion would blow the star apart. These two forces are in balance. If the star compresses a little bit, it would make the core get more compressed. The more compressed the core is, the faster fusion takes place. The faster fusion takes place, the more outward pressure there is. This then pushes the star outward, releasing the pressure on the core. Thus, the star burns hydrogen at a rate which keeps the core at its current level of compression.
How quickly is stellar fusion depleting the hydrogen in the universe? When will there be more heavy elements than hydrogen?
Not very fast at all. Less than 10% of the mass of a star undergoes fusion during its entire lifetime. A lot of people have that misconception about stars and the sun. It is not a giant ball that is constantly undergoing fusion. Only a very small part of the core is, and the high energy output is only due to the sheer amount of mass there is at the core, but the number of fusions per unit volume even at the core is very low.
Why don't antibiotics work against viral infections?
Bacterial cells are living organisms. Anti-biotics (there are various types) end up destroying bacterial cells one way or another (destroying their cells walls, stopping the bacteria from making proteins needed for survival etc) Bacteria belong to the domain of prokaryotes. Mammals are Eukaryotes. Bacteria have primitive/different ribosomes (make protein) which can be specifically targeted without affecting Eurkaryotic cells. Virus' aren't classified as living. They have a envelope made of phospholipds/proteins derived from their host. Virus' need the host to reproduce. Since viral envelopes have a completely different structure from bacterial cells walls, anti biotics don't target them. Virus' also do not have ribosomes that can be targeted (they use host cell machinery to reproduce) Soruce: Biology major
Have planet sized objects fallen into the sun?
I think almost certainly. A lot of scientist believe that when the solar system was young there were many planets. In time some collided and some likely fell into the sun and a few maybe were ejected from the solar system. Eventually within a hundred million years or so it settled into the 8 or so planets we have today.
Is there anything in the Universe that is truly "random"?
[particle decay](_URL_0_) is widely considered to be [acausal](_URL_1_), and thus random.
Why do some birds bob their heads when they walk?
It's probably an artifact of their highly developed vestibular sense ~~used to cope with flying around in three dimensions~~. If you move a bird around like in [this video](_URL_0_), they will instinctively keep their head stationary and only move it by jerking to a new steady position. This is independent of walking, but if a bird does walk around its head would tend to stay put until it can't stretch its neck any further, at which point it thrusts its head forward and repeats this throughout the walk. I imagine their vision would be slightly impaired during the head movement itself, much like human blindness during saccades, but the rapid motion affords them relatively long periods of stability which would allow normal vision. [This](_URL_1_) is a video from a camera strapped to a rooster's head. Notice how smooth it is between jerks, even with the car driving on a bumpy road. Try holding a camera that still.
Where does a photon come from within an atom?
Where do waves in the ocean come from? Maybe a stone drops in and causes them to ripple. It's kind of the same question. With photons sometimes the stone is an accelerating charge, and other times it's an electronic transition. The electromagnetic field always exists, the photon is just a ripple in it.
Do photons come from molecules or only atoms?
Both. There are [molecular orbitals](_URL_0_) that can undergo transitions and release photons, which are generally of lower energy than those from atomic transitions. There are also transitions involving atoms within molecules.
When I dry off after a shower, why does the water transfer to the towel, rather than just smear around on me?
The basic mechanism is [capillary action](_URL_1_), each fibre of the towel has a hydrophillic surface that attracts water molecules and quickly spread them out along the length of the fibres. This results in the surface energy of the water droplets being lower when in contact with the towel fibres than when attached to the skin. This is called [wetting](_URL_0_). Trying to dry yourself with a hydrophobic material such as nylon it would just smear the water around. New towels can be somewhat hydrophobic as well, due to oils attached to the fibre, and only improve after some washes.
Why do nuclear bombs make mushroom clouds?
This is not unique to nuclear weapons. Any sufficiently large explosion will produce a mushroom cloud. This is because the explosion pushes air out of the blast area, but after the explosion the air rushes back in. This is what causes the dust and smoke brought on by the explosion to form into the mushroom shape. Air rushes in at all angles, and as it collides in the center it rushes up (because the ground prevents down from being an option) this drags the particles in the air with it. Nuclear blasts produce very prominent clouds not just because of their size but also because of the smoke they produce rapidly from the intense light of the blast. If I was not on my phone I would link to a few very good videos demonstrating the "waves" of a nuclear blast (intense light, explosion, rush of wind back into the explosions area).
Why do nuclear or atomic bombs create mushroom clouds?
Very basically, the initial blast radiates in a sphere around ground zero, however, the ground reflects some of that energy upward, and the cloud is lifted up. The heat generated by the blast rises as well, carrying the cloud higher. Any sufficiently large blast will create a mushroom cloud, they aren't exclusive to thermonuclear or atomic bombs. There's a picture I saw on another sub of a volcanic eruption and it also created a mushroom cloud.
How can a black hole have a spin?
Strictly speaking, only a non-rotating black hole has a point singularity. A [rotating black hole](_URL_0_) has a [ring singularity](_URL_1_). Realistically, all black holes are rotating, but it's a lot easier to understand ones that aren't so we often assume they don't.
How long would we live without the sun?
[Here](_URL_0_) is a video from Vsauce on this subject
Since mirrors reflect light, is there a small delay between something happening and it occuring on the mirror?
Yes. There is a time delay between the image in a mirror and the original that is equal to twice the distance to the mirror divided by the speed of light. For a mirror one mile away the delay would be just under 11 microseconds.
Why is carbon 14 radioactive and carbon 13/12 not?
Carbon-14 is unstable because it has an isobar with a lower mass (nitrogen-14), and a way to decay into it without violating any relevant conservation law (beta decay). The stability of the nucleus depends very much on both the number of protons and the number of neutrons. Different isotopes of the same element will in general have very different half-lives from each other.
The expansion of the Universe is accelerating, will the speed of acceleration asymptotically approach the speed of light, surpass it, or neither?
Gahhh "the speed of acceleration!" Alright, fine, you edited it out :) The rate of expansion isn't a speed. It actually has units of speed divided by distance. You can see this from Hubble's law, v=Hd. v is the apparent recession velocity between two galaxies, d is the distance between them, and H - the Hubble parameter - is the rate of expansion of the Universe. As it turns out, whether the expansion is accelerating or not, there's usually *some* distance past which the recession velocity is greater than the speed of light, just pick d > c/H (often called the Hubble horizon). All that means is that, very roughly speaking, if things continue as they are (for example if H stays the same, which is the hallmark of an accelerating universe), light from regions beyond that won't be able to communicate with us because the light will in effect be unable to "outrun" the expansion. But I'd emphasize that this is a pretty heuristic way of looking at things.
How much computing power would be necessary to simulate our universe?
Wouldn't this be a paradox since such a computer would have to simulate itself?
Why do we see meteor showers at roughly the same times each year?
Debris fields orbit the Sun on highly elliptical orbits because they are generated by comets, which also have highly elliptical orbits. There is debris strewn all along the elliptical path, so whenever we cross said path, we run into some of that debris. [This image](_URL_0_) shows the debris field as the diagonal line stretching across the image, with the comet in the middle.
Why do we see meteor showers some specific days only?
There are various asteroid "clouds" in our neighbourhood, but their orbit around the sun only intersects with our orbit around the sun every now and then. When the orbit of an asteroid cloud intersects that of the Earth, we can see a meteor shower. On other days, there are simply little to no meteors of sufficient size entering and burning up in our atmosphere.
How can spider legs keep moving after they're detached from the body?
maybe you are thinking of daddy long-legs (Opiliones)? spiders do not have extensor muscles through their legs - they extend their legs by using their body to push fluid into the leg - so if the leg is detached it can only curl up. opiliones have actual extensor muscles controlled by neurons in the legs; pull an opilione leg off, and it still has the machinery needed to keep moving, for a while at least.
Why do we tend to sneeze in multiples of 2 or 3 [or more]?
Multiple sneezes may occur when a single sneeze is too weak to remove whatever has irritated the nasal cavity.
Why does fire have the shape it does?
*laymen warning* if you compare how fire looks on earth vs in microgravity [as can be seen here](_URL_0_) it can be seen, that gravity has major effect. In gravity, hot gases/plasma goes upwards as they have lower density then surrounding air. Surrounding air comes from side. I would guess that is the reason why "gravity" fire looks more strong as new oxygen is brought in more readily compared to that (expanding?) ball
Do photons compress, like bouncy-balls, when reflecting off of mirrors?
Photons are, to the best of our knowledge, points particles, with no internal structure or size.
What causes the weird sensation inside the nose when eating things like mustard, wasabi or horseradish?
I can really only answer one half of this question, which is the botanical part. Horseradish and wasabi both product a compound called *allyl isothiocyanate*, or "mustard oil". It is also found in mustard seeds, and is a defensive plant compound. It *should* taste bad to animals, making them uncomfortable and forcing them to stop eating the plant. I am assuming the physical sensation might have to do with the super pungent and toxic taste associated with the oil, making your smell and taste nerves freak out a bit (very scientific, I know!). Hopefully some physiology person can come by with the other half of that answer!
Why does water not burn?
Because it is already burned. *Free* molecular hydrogen reacts very readily with *free* molecular oxygen, forming [covalent bonds](_URL_1_) and releasing quite a bit of energy in the process. Because each hydrogen atom has one valence electron, and each oxygen atom has six, it is energetically favorable for an oxygen atom to bond with two hydrogen atoms, gaining a full [valence shell](_URL_0_) of eight electrons. So, what is this reaction product of two hydrogens for one oxygen? 2 H + O... H*_2_*O? Yes indeed. Water (in gaseous form) is what happens when hydrogen burns with oxygen. 2H*_2_* + O*_2_* - > 2H*_2_*O Because combining hydrogen and oxygen releases energy (it is *exothermic*; it "burns"), trying to separate water back into its constituents *consumes* energy (it is *endothermic*).
Why doesn't water burn?
If you define 'burn' as react with oxygen in the presence of heat, then hydrogen burns, but oxygen doesn't burn. When you burn hydrogen, it forms water. So water can be thought of as literally the waste product of burning hydrogen. You can't burn something that's already burned.
How dark/bright is our full moon compared to other moons in the solar system?
Compared to other moons in our solar system, our Luna is much larger than average. [Here](_URL_0_) is a pretty good reference for moon data. Using size as the metric, Titan would be the brightest if it were transplanted to Earth orbit. As to reflectivity of each moon, that could change the results of your query. But, you're on your own there.
If alcohol makes you dehydrated, why is urine clear in colour (while drinking a lot), which indicates that you're hydrated?
Alcohol reduces the production of a hormone called vasopressin, which tells your kidneys to reabsorb water rather than flush it out through the bladder. So once you have drank enough alcohol you stop absorbing the water from what you are drinking and it just goes right through you, which is why your urine is colourless but you are also dehydrated.
If there are red, yellow, blue, white, and "brown" stars, why arent there any other colors?
Stars emit radiation roughly like a [blackbody radiation curve](_URL_0_), so if their emission spectra is centered near visible light, their spectral width is typically about as large as the full visible spectrum. You can see red and yellow stars because the majority of their emission curve is in the infrared spectrum, so much more red light gets radiated than blue light. Similarly, blue stars emit primarily in the ultraviolet wavelengths, and of the light that is in the visible spectrum, much more blue light gets emitted than red. If the emission spectrum is centered about green light, then approximately the same amount of light gets emitted throughout the visible spectrum, which we see as white.
Why some stars in the night sky look like they are changing colours or flashing light rapidly?
Small fluctuations in our atmosphere cause the light to refract and can appear as focusing, diffusion, or slight colour changes. Observatories are built as high as possible on mountains to try to reduce the amount of atmosphere they have to look through, and the Hubble gets a very clear view because it is in space. Modern observatories have adaptive optics which counter the effects of the atmosphere and they use a reference 'star' created by a laser shining high in the atmosphere and calculating how the light is being affected.
Do stars have craters?
No. Stars can be quite reasonably approximated as fluids. They're not *liquid,* obviously, in the sense that water is a liquid. But they're *fluids.* That means they have no rigid structure, and thus can support nothing like craters, or any kind of topography. (Incidentally, defining the surface of a star is no easy task. If you have a pot of water on your stove, it's easy to say where the surface of the water is. Heat it until it boils furiously, and it's no longer so simple to identify a continuous boundary between air and water.)