**Ask a monkey a physics question thread**

[Eric]

Why is there so much talk about holograms when looking at black holes?


I remember on the science channel or something awhile back they said no information is lost such that there is a hologram that shows info (I think this was in the context of someone entering a black hole). The recent http://www.dailymail.co.uk/sciencete...-surfaces.html article talks about black holes themselves being holograms as opposed to just the info that gets sucked into them.

[OngBonga]

I've been reading about neutron stars, and apparently they can be virtually undetectable if they are not producing pulsars, or are not part of a binary system.

That said, how can we know that any "missing" matter in the universe is dark matter, and not simply neutron stars that we haven't yet found?

[MadMojoMonkey]

Why is there so much talk about holograms when looking at black holes?


I remember on the science channel or something awhile back they said no information is lost such that there is a hologram that shows info (I think this was in the context of someone entering a black hole). The recent http://www.dailymail.co.uk/sciencete...-surfaces.html article talks about black holes themselves being holograms as opposed to just the info that gets sucked into them.

Preface: I may be not 100% aware of all the subtleties of the holographic principle.

The holographic principle is a statement which basically says that if some 3D thing can be wholly described using information in a 2D plane, then there is no rational reason to claim that the 3D description is more real than the 2d description.

If you can't tell whether you're looking at something which is 2D or 3D, that's kinda like looking at a holograph, hence the name "holographic principle."

I'm not aware of any use of the holographic principle outside of string theories. String theories are on the fringe of physics and are not a part of the Standard Model. Some of the most highly trained physicists in the world are trying to find a string theory which is a complete description of reality, but so far, they have not been able to do so.

String theories have shown and hinted at some interesting properties of the universe, but none of those properties was a new addition to the Standard Model, just a corner that hadn't been shown to be a part of the SM.

[MadMojoMonkey]

I've been reading about neutron stars, and apparently they can be virtually undetectable if they are not producing pulsars, or are not part of a binary system.

That said, how can we know that any "missing" matter in the universe is dark matter, and not simply neutron stars that we haven't yet found?

As ever, some people are blaming the WIMPS (weakly interacting massive particles), and ong is blaming the MACHOS (massive astrophysical compact halo objects).

Wimps are more likely than machos to be responsible for this one.

The distribution of dark matter is such that it would take a very high number of machos to account for it, and that would have a discernible effect on the light passing through them. Like, the sky looks blue because of light particles being redirected as they pass quite close to N2 molecules. Blue light is redirected the most (it's another 4th power relation), and the sky looks blue. Or something even more obvious like... darkness where light was blocked.

Keep in mind that a single neutron star is hard to detect, but you're talking about adding millions or billions of them to the solar system. If there were that many, it seems it'd be obvious that there were "particles" in the way of us looking at the galaxy.

EDIT:
This link to the astrophysical society says that there is some evidence for MACHOs. However, their data has ruled out the notion that dark matter is MACHOs. While there may be some MACHOs which contribute to the dark matter effects, they cannot be the whole explanation.

[OngBonga]

Keep in mind that a single neutron star is hard to detect, but you're talking about adding millions or billions of them to the solar system. If there were that many, it seems it'd be obvious that there were "particles" in the way of us looking at the galaxy.

I assume you mean Milky Way where you say Solar System. Obviously there's precisely zero neutron stars in the Solar System, such a presence would be easily detectable due to its gravitational influence on the Earth, moon, sun, et al.

That link just confuses me further. Honestly, I get the distinct impression that dark matter is just matter we know is there but haven't yet directly detected and classified, or profiled.

The fact we have WIMPs and MACHOs shows me that already we have two types of dark matter.

I feel like dark matter ceases to be dark matter when we see it.

[MadMojoMonkey]

I assume you mean Milky Way where you say Solar System.

Ugh. You got me.

That link just confuses me further. Honestly, I get the distinct impression that dark matter is just matter we know is there but haven't yet directly detected and classified, or profiled.

Well, we have detected it, that's why we gave it a name. We know that our current model of gravitation says the galaxies should rotate one way and that they observationally rotate in a different way.

So there's some force acting on/within galaxies which we weren't expecting. Maybe the theories which have so repeatedly NOT been disproved are not so good. Or maybe the data we've plugged into the theory is not so good.

We're pretty sure that gravity is mostly right (Einstein's gravity, that is), and if we adjust data in the equations we have which tell us the expected way the galaxies should rotate... if we fudge those by adding more mass, then it solves the issue quite well.

OK, so we saw a new observation, then made a prediction... and now we're searching for confirmation of that prediction... i.e. that there is a lot more mass in the galaxy than we've directly observed. How can that be? We are looking for this stuff and not finding it.

A) Where are we looking?
B) At the photons.
A) Maybe this stuff doesn't interact with photons.
B) All charged particles interact with photons.
A) Yeah, so maybe this stuff isn't charged and doesn't interact with electromagnetic fields at all.
B) OK... so how do we look for that?
A) Well, if it has mass, it will have gravitational effects.
B) Yeah. That's how this whole mess started.

The fact we have WIMPs and MACHOs shows me that already we have two types of dark matter.

I feel like dark matter ceases to be dark matter when we see it.

All too correct.

Except that dark matter is its name and, historically, it's hard to get physicists to not spout a prepared lecture on the history of physics at the drop of a dime. So like it or not, the phrase dark matter is going to be around for at least as long as luminiferous ether.

[OngBonga]

ugh, I just have more questions now and I feel like they're stupid questions, but hey I'll fire...

How can something not interact with electromagnetic fields while having gravity? Are we not to expect gravity and electromagnetism to one day be unified? Wherever there is electricity, there is magnetism, and vice versa, and the same can be said for time and space. So if gravity and EM are, in essence, one and the same, then why would one exist without the other? Why would dark matter not have an electromagnetic field to compliment its gravity field?

I now feel like dark matter implies gravity and EM are distinctly different forces, that they cannot be unified. But it's much more likely that I'm jumping to some rather extreme conclusions.

[OngBonga]

Just looking at dark energy to try and get my head around that.

Best I can imagine is that it's essentially tension. If one imagines the bedsheet anaolgy for gravity that we're all familiar with, well if one is to pull the sheet uniformly in each corner, it would lift the ball from its well... antigravity.

Can dark energy be viewed like this? That it's spacetime tension?

[OngBonga]

I also have some issues with the age of the universe.

I mean we're told it's 14 billion years or whatever.

I have to ask...

In whose frame of reference?

It seems to me that relativity utterly shits on the concept of the universe having an "age". As far as the photon that originated from the big bang is concerned, the universe is merely a fraction of a nanosecond old.

It just seems nonsense to say the universe is x amount of years old. Our concept of time is way too limited for it to mean anything.

Where does 14 billion years come from?

[OngBonga]

For the same reason, the concept of a "light year" seems flawed.

How can a light year be a particular value, when two people will have a very slightly different idea of exactly how long a year is?

[Savy]

For the same reason, the concept of a "light year" seems flawed.

How can a light year be a particular value, when two people will have a very slightly different idea of exactly how long a year is?

Because these things are defined. Being unaware of a definition doesn't make it not exist.

I also have some issues with the age of the universe.

I mean we're told it's 14 billion years or whatever.

I have to ask...

In whose frame of reference?

What do you think?

[OngBonga]

Because these things are defined. Being unaware of a definition doesn't make it not exist.

The definition itself is flawed.

A light year is the distance light travels in a year.

Once again, I ask... in whose frame of reference? I get the idea that you're not entirely sure what I mean when I ask that.

Your year is different to my year. We are travelling at very slightly different velocities, primarily due to our (perhaps slightly) different altitudes, and thus distance from the Earth's centre of mass. Time dilation implies that two people travelling at different velocities experience time differently.

Ergo... time is relative.

So when I ask "in whose frame of reference", I'm pointing out that it's a flawed concept in the first place. Time is not a constant, so it's like measuring distance in horse lengths. Which horse?

[OngBonga]

What do you think?

Ours.

But that's a bad frame of reference to compare with the age of the universe, because we are in motion.

[Savy]

Ours.

But that's a bad frame of reference to compare with the age of the universe, because we are in motion.

As you said before ours isn't the same, so that isn't the reference point. Was kind of a trick question to get you to realise things aren't loosely defined like saying "our" reference point.

To answer your other post it's a case of knowing what the definitions are. Just because you read something and think you know what all the words mean when it comes down to science you probably don't. A year is a well defined thing.

https://en.wikipedia.org/wiki/Julian_year_(astronomy)

That being said the word year gets thrown about with loads of different meanings, you only have to look at the wiki page for that. So when people say things like a year is how long it takes the earth to orbit the sun, well no as that changes everytime it happens, same for a day. Some days are > 24 hours, others less*.

http://www.timeanddate.com/time/earth-rotation.html

*By this I clearly mean more and less than the real average time it takes the for a day, I dunno if it would ever be < 24 hours as I don't know how much fluctuation we see.

[OngBonga]

A year is a well defined thing.

I can see...

In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each.

Let's talk about seconds instead of years then, seeing as a year is ambiguous in its own right.

Here's the definition of a second...

It is quantitatively defined in terms of exactly 9,192,631,770 periods of a certain frequency of radiation from the caesium atom: a so-called atomic clock.

Two people will observe the time it takes for such a frequency of radiation differently, based on their differing velocities.

So, a second is ill defined.

Relativity is a beast, and it shits on our concept of time. When we apply time dilation to universal scales, we're not talking about negligible amounts like when we talk of horse lengths. We could be talking infinite amounts. To a photon originating from the big bang that is constantly in motion at c, the universe is exactly zero seconds old. To something that is not in motion at all, maybe it's infinitely old. Motion, of course, is another thing that's relative. Nothing within the universe is motionless, so nothing within the universe is infinitely old.

But maybe the universe itself can be considered motionless, because there is nothing that it can be in motion relative to.

So, I conclude that the universe is somewhere between zero and infinite years old, depending on the frame of reference.

[MadMojoMonkey]

Some particles have mass, but no charge. Ignoring the neutron (which is composed of 3 quarks, each of which has non-0 charge, but whose net charge is 0), there are still the neutrinos, gluon, photon, Z-boson, and Higgs particle.

The unification of forces would happen during the first 10^-35 seconds of the universe, when the energy density was so high that "massive" particles couldn't even condense out of the fields. This would be prior to the annihilation of matter-anti-matter that left only 1 billionth of the original matter in the universe... which is what we observe now. So take everything in the universe multiply it by a billion, then smash it into a radius of 10^-35 light seconds (~3*10^-24 mm) and try to describe physics.

IDK about dark energy. Maybe like tension, probably not exactly like that.

It's always "in an inertial reference frame," unless otherwise stated.
An inertial reference frame is one in which an object at rest remains at rest if and only if the vector sum of forces acting on it is 0 N. Also, a body in motion will continue to move at constant speed in a straight line if and only if the vector sum of forces acting on it is 0 N.

The Earth is not an inertial reference frame, but we know how to account for these effects. We know how to describe rotating reference frames and the "illusion" forces, e.g. centrifugal force, and how to describe motions in even complicated frames which actively twist and distort. The inertial frames are the easiest to work in, though.

~13.7 billion years is the age of the photons from the Cosmic Microwave Background Radiation which correlate to an event in the early universe when the energy density became such that electrons in Hydrogen atoms could spontaneously flip their spin's alignment in relation to their proton's spin. This re-alignment has a very well-defined energy gap between the high and low energy states. So we have a very strong idea of what the event was that created these particular photons, and the wavelength of those photons when they were created.

Give both observers their own Caesium clock. They travel past each other at a significant fraction of the speed of light and manage to measure the frequency of both clocks as they pass. Both people would observe different frequencies, yes... but they see one zipping past at a decent fraction of the speed of light and the other sitting still. Provided both understand GR, they will agree that the frequency in the other's rest frame is the same as their own once they account for the relativistic effects.

[OngBonga]

Ok, I'm cool with intertial frames of reference.

Let's say we observe a photon that originated from the big bang and moves at a constant velocity of exactly c. That's an intertial frame of reference, right? The photon, as far as it is concerned, is motionless, because it is not accelerating. So it does not experience time dilation, rather everything moving relative to it experiences time dilation, at least from its pov.

So how old is the photon, from its pov? How about from our pov? How old does the photon think we are?

So the question begs... exactly how old is the photon? Is the photon right, or are we right, or are we both right?

[MadMojoMonkey]

This may take some time. I may have to explain multiple points separately.

Photons travel at c, which means they travel on paths with 0 proper delta-t (change in time) over all lengths. delta-proper-t? I'm talking about proper time.

A photon looks the same at all times which it exists. There is no way to determine the photon's age by observing some intrinsic property of the photon.

The age of anything is frame-dependent. Reference the twin paradox. A different amount of time passes for each twin, who observes (naively) that both are the same age - i.e. the same amount of time passed for one as the other; each observes their own time passing and knows that the other existed the whole time and none more or less.

Einstein strictly ruled out reference frames moving at c as non-inertial.

Here's a tricky one. You can't watch a photon age. Once you observe it, you destroy it. There is no frame in the universe in which you can see a photon "moving in time," inertial or not.

[MadMojoMonkey]

Ok, I'm cool with intertial frames of reference.

Let's say we observe a photon that originated from the big bang and moves at a constant velocity of exactly c. That's an intertial frame of reference, right?

Unfortunately, no. Newton's laws break down when infinities are involved.

By definition, if Newton's laws don't work, then it's not an inertial reference frame. The two ideas are inseparably linked. Newton's first says, "this is an inertial reference frame," The other laws are, "In an inertial reference frame..."

The photon, as far as it is concerned, is motionless, because it is not accelerating. So it does not experience time dilation, rather everything moving relative to it experiences time dilation, at least from its pov.

*nods*

So how old is the photon, from its pov?

If assigning a number even makes sense, that number is 0 [time units].

How about from our pov?

If we can determine the source position in a distance measure, d, in an inertial reference frame, then we know that light moves at c, so the time traveled in our reference frame is d/c.

If we can determine the time, t, it has traveled in our reference frame, then we can calculate the distance d = ct.

How old does the photon think we are?

Is this a koan?

It is in the location of its creation and its annihilation at the same moment in its reference frame.

So the question begs... exactly how old is the photon? Is the photon right, or are we right, or are we both right?

I'm going with both right. Time is relative.

***
Photons have highly well-defined velocity and wavelength - perhaps infinitely well-defined, even. This means the only room for uncertainty is in position, which is not defined, let alone well-defined. The best we can do to find a solvable (converging) position function is put some limits on space.

Say we stipulate that we have a system which contains a photon in a region of space (particle in a box, so to speak). Still, the photon has very well-defined velocity and wavelength. (Whether or not we observe it; these are intrinsic properties.) Now we can force the position function to tell us something... which is that the position is equally probable everywhere in the box. We gained nothing. The position is not defined any better than our original stipulation that there was a photon in that region of space.


*not a real word

[OngBonga]

I'm going with both right. Time is relative.

Right. So how can we assign an age of the universe? Time is relative. It's a different age depending on the frame of reference one calculates the age from.

If I go for a ride in a really cool spaceship at 0.99c today and arrive back in exactly one year (from your pov), then maybe a few minutes or whatever have passed for me, while an entire year has passed for you.

So as we stand next to each other discussing the age of the universe, practically in the same region of spacetime, we have nearly a whole year discrepancy between our values. I say it's 13.7 billion years, you say actually it's 13.70000001 billion. Obviously I'm being silly here because it's not like the 13.7b is supposed to be a precise value. But this just demonstrates that the concept of the universe having an age seems utterly ridiculous as a direct result of relativity. The discrepancy in universal age between us here on Earth and an object orbiting very close to a black hole would be enormous. But, because time is relative, both are right.

This conversation is particularly enjoyable, I must say, even if I'm failing to get my head around this.

[OngBonga]

Ok so let me see if I can figure out where the discrepancy comes from...

Let's assume Earth is an intertial frame of reference to keep it simple.

Me travelling at 0.99c, well that requires two accelerations... going faster, and slowing down. I'd probably need to steer the ship around a black hole too. So my frame of reference is non inertial.

Once I factor in the relavistic attributes to my journey, we will be in agreement.

So it isn't ridiculous to assign an age to the universe once we factor in the relavistic properties relevant to the FoR that the age was calculated from, because once all these factors are considered properly, all are in agreement.

Am I getting there?

[MadMojoMonkey]

Ok so let me see if I can figure out where the discrepancy comes from...

Let's assume Earth is an intertial frame of reference to keep it simple.

Me travelling at 0.99c, well that requires two accelerations... going faster, and slowing down. I'd probably need to steer the ship around a black hole too. So my frame of reference is non inertial.

Once I factor in the relavistic attributes to my journey, we will be in agreement.

So it isn't ridiculous to assign an age to the universe once we factor in the relavistic properties relevant to the FoR that the age was calculated from, because once all these factors are considered properly, all are in agreement.

Am I getting there?

You're getting there.

[MadMojoMonkey]

The bartender says, "We don't serve your kind, here."

A tachyon walks into a bar.

[OngBonga]

You're getting there.

Time dilation is a piece of cake.

[MadMojoMonkey]

Right up there with the "We are literally made of atoms created in a supernova." level of cool.

Well, close.



We live at a time when we can see the distant universe. This will not always be the case.

[wufwugy]

Legit question incoming.

So in a Captain America movie, Cap jumps out of a plane and lands without any protection. I don't remember anything else. This has me thinking, what would it take for a human to be capable of such a feat? The feat being leaping out of a plane with no protective gear and landing at terminal velocity while taking no damage. How strong would his bones have to be, what sort of squat numbers would his muscles have to be able to put up (human peak is around 3x body weight), what about organs?

[wufwugy]

I'm guessing it would have to be something ridiculous, like Cap weighing 200 pounds yet squatting 20k pounds or something. Or maybe not that ridiculous. With those stats, Cap would be able to jump over tall buildings.

[MadMojoMonkey]

Legit question incoming.

So in a Captain America movie, Cap jumps out of a plane and lands without any protection. I don't remember anything else. This has me thinking, what would it take for a human to be capable of such a feat? The feat being leaping out of a plane with no protective gear and landing at terminal velocity while taking no damage. How strong would his bones have to be, what sort of squat numbers would his muscles have to be able to put up (human peak is around 3x body weight), what about organs?

Be this guy?

Granted, that chute wasn't open, but definitely made a significant change to his aerodynamics and reduction to his terminal velocity. So not exactly the same as surviving with no chute at all. Still... you stipulated no damage at all. This guy broke his ankle.


This is biology... so not really my field... but I'm curious, so I'll look into it a bit. No promises on my usual level of rigorous answer.

Good grief, this is going to be hard to answer. The way you hit the ground is going to make a huge difference in how the load of catching your weight is distributed throughout your body. The whole, "roll as you hit the ground," thing definitely makes a huge difference in injuries.

After minimal google searching, the most effective way to survive a failed parachute deployment is to deploy the backup chute. lol. Not much info on what to do when that chute also fails. It's actually a rare occurrence for the primary to fail, based on this preliminary search.

[Keith]

The singularity is pretty well defined, as far as I'm aware. It's a region of mass with zero volume in space. If it has volume, it ceases to be a singularity. And if it has no mass, then it has zero density, not infinity.

Can it have density if it has no volume?

[wufwugy]

One reason I'm thinking physics is because there are forces involved. Like if trying to stop yourself using your feet/legs at terminal velocity would amount to needing to over come 2k pounds of pressure, then it makes sense to me that if you could squat in the 1800 range, you would have that one element covered. Or like with bones, is it about density, would your bones need to be as dense as, say, bronze? I don't really know how any of this works, I just thought that Cap jumping out of a plane and tanking the landing is some crazy huge feat that should make him a much more indestructible and agile superhero than he is.

[wufwugy]

I do know that cats can fall very long distances without injury, but I think that's due to ability to slow from terminal velocity.

[wufwugy]

man that video is craze

[OngBonga]

Can it have density if it has no volume?

idk, ask mojo, I can only answer questions directed at mojo.

As for the falling thing, well I remember as a kid watching a tv programme where they threw eggs out of a plane in an effort to not break one, I think it was probably Record Breakers with that dude who died of lung cancer thanks to passive smoking. Roy Castle, that's his name. I digress. They eventually succeeded from what one would consider a ridiculous height considering it's an egg, the egg that made it hit a hill at exactly the right angle to basically continue its fall coming to a gradual halt, rather than hitting the ground perpendicular. So yeah I reckon it's doable if all factors are precisely correct, based on eggs.

edit - I just remembered they were putting a lot spin on the egg as they threw it, so if you ever find yourself falling out of a plane with no parachute, aim for a hill and attempt to get yourself spinning in the air in the direction you'll go down the hill.

[MadMojoMonkey]

Can it have density if it has no volume?

No and yes. This question is a bit trickier than it seems at first glance.


No because you're talking about mass density and implicitly talking about mass per unit volume. I'm not entirely sure if there are any substances which would allow us to talk about mass per unit area as such. Maybe the electrons in graphene?


Yes because mass density isn't the only kind of density worth discussing.

For that matter, electrons have measurable mass and charge, but no measurable volume. We have put limits on the biggest an electron can be, given our subtle observations, but we have yet to prove either that they have small size or that they have 0 size. Turns out that proving a measurement is exactly 0 is easier said than done.

If you're talking about a conductor with a net charge, then talking about the charge density of the conductor still doesn't really make sense, because all the net charge is located at the surface of the conductor. So when we talk about the charge density in that case, we're really talking about charge per unit area.

The same could be said for charge on a line, but in that case, we're making an idealization since there is no truly 1D conductor.

[MadMojoMonkey]

idk, ask mojo, I can only answer questions directed at mojo.

lol

So yeah I reckon it's doable if all factors are precisely correct, based on eggs.

-.-
Not sure if it's physics or if it just sounds like physics.

Anyway, this throws a kink in wuf's question, because I thought it was implied that the jumper hit the ground perpendicular to their velocity... I.e. a square hit. If they're allowed to land on a curved slope, then that changes everything. The only thing to protect against is friction burns, then.

edit - I just remembered they were putting a lot spin on the egg as they threw it, so if you ever find yourself falling out of a plane with no parachute, aim for a hill and attempt to get yourself spinning in the air in the direction you'll go down the hill.

This is what a baseball pitcher does to throw a curve ball. The pitcher puts forward spin on the ball, causing the Magnus effect to pull the ball downward by deflecting the air flowing under the ball such that the air goes upward. Newton's 3rd says that if the ball pushes the air upward, then the air pushes the ball downward, making it curve toward the ground faster than gravity would normally pull it down.

You'd want to spin the other way to exploit the Magnus effect. This would send the air going above you toward the ground. Which would exchange your forward velocity for lift, reducing your rate of falling.

Unfortunately, the Magnus Effect tends to "spin up" the object. Once you start the rotation, it gets amplified. You might have more trouble keeping your limbs in close and not flailing wildly. The flailing would probably not be best for an injury free landing.

[wufwugy]

Square hit. No hitting the branches at the right angles tomfoolery.

[MadMojoMonkey]

I do know that cats can fall very long distances without injury, but I think that's due to ability to slow from terminal velocity.

Umm... this is tricky useage of terminal velocity. If the only 2 forces acting on you are your weight and resistance caused by moving through a viscous fluid (even air) and you're falling at constant speed, then you're at terminal velocity.

A skydiver has a range of terminal velocities at which they can fall, depending on how they hold their body. With arms and legs splayed and the spine bent backward, the body will fall belly-down at it's slowest terminal velocity. (Note the maximizing of surface area to the direction of travel.) When a skydiver pulls their arms and legs together and holds their spine straight, then they tend to fall face-down like an arrow at their highest terminal velocity. (Note the minimizing of surface area to the direction of travel.)

My guess as to why cats can survive falls is due to their relatively low mass to sproinginess ratio. That and they have something like 3x more vertebrae than humans, which contributes to their extreme flexibility. Flexibility seems like a good property to have in these cases. They have 2x as many legs, too, and those legs almost always point downward while the cat is falling. Also, cats strongly arch their backs during the fall, allowing them to use more of their muscles to catch that falling mass and allowing it a greater distance of travel over which that catching force can be applied. Kinda like what an airbag does.

Also, cats are among the animals which give no outward sign of their pain. They could be quite hurt but wont show it unless the pain has physically maimed them.

[MadMojoMonkey]

Square hit. No hitting the branches at the right angles tomfoolery.

Totes, bro.

[OngBonga]

Square hit. No hitting the branches at the right angles tomfoolery.

Yeah, no. I'm gonna say serious injury, if not death, is inevitable with a square hit at terminal velocity (of a human of x weight with maximum surface area relative to direction of travel).

Call it a hunch.

[wufwugy]

I assumed terminal velocity depended on mass alone, no air resistance. So basically I'm thinking of a guy just bolting straight down from the sky like any good superhero should.

Then it would be that what I meant about cats is that they decrease their terminal velocity by doing something like splaying out. People report seeing cats fall from >40 feet distances and running off after landing with no noticeable injuries, but when falling from 20-30 feet they often sustain injury (due to not having ample time to maneuver body and slow down).

[wufwugy]

Yeah, no. I'm gonna say serious injury, if not death, is inevitable with a square hit at terminal velocity (of a human of x weight with maximum surface area relative to direction of travel).

Call it a hunch.

Well, that's why you have to make him super human. How much would his muscular strength have to increase to keep from crumbling to the ground as his legs try to break the fall? How dense/strong would his bones have to be to not snap under such force? Apparently since some people have survived, maybe organs would do fine as is.

[OngBonga]

(Note the maximizing of surface area to the direction of travel.)

I'm gonna be nitpicky and ask if what you actually mean is perpendicular to direction of travel, not just the direction of travel. I mean imagine a pyramid falling upside down at terminal velocity. Well, there's actually a rather large amount of surface area in the direction of travel, more so than if it were falling the right way up with a flat square breaking the fall. Yet upside down it will fall faster, not slower.

[MadMojoMonkey]

I said the thing about biology because I don't think you can treat bone like a homogeneous (same everywhere) crystalline solid. Bone density is important, but I'm not sure if the strength of the bone is a function of density like that. More importantly, I don't know the range of values which would represent a good guess for a person in top physical condition nor do I know what I would have to do to those bones to make them strong enough. Double their diameter? Triple? More? At some point I just have to replace them with Wolverine's whateverium skeleton.

UNLESS the action of the muscles reduces the stress applied to the bones, in which case, I'd have a balancing act of adding more bone or more muscle to absorb the impact. That sounds like a pretty easy equation to work out, though.

Of course, I have to choose the way in which the person hits the ground. That means everything as far as what parts of the body need to absorb/dissipate how much kinetic energy.

Argh... then there are further restraints like the skull has the lowest allowed impulse (force times duration of that force's application). The skull also has the lowest amount of instantaneous force that can be applied without causing a concussion.

[OngBonga]

Well, that's why you have to make him super human. How much would his muscular strength have to increase to keep from crumbling to the ground as his legs try to break the fall? How dense/strong would his bones have to be to not snap under such force? Apparently since some people have survived, maybe organs would do fine as is.

I calculate his muscular strength would have to be at least 68, and his bone strength would need to be 675.4.

I have no idea what units I'm using though.

[OngBonga]

Perpendicular.

That's four times this evening now we've used that word. Good work.

[MadMojoMonkey]

I'm gonna be nitpicky and ask if what you actually mean is perpendicular to direction of travel, not just the direction of travel. I mean imagine a pyramid falling upside down at terminal velocity. Well, there's actually a rather large amount of surface area in the direction of travel, more so than if it were falling the right way up with a flat square breaking the fall. Yet upside down it will fall faster, not slower.

You got it right. I was trying to use simple language avoid saying "normal" to mean perpendicular and I forgot how much you love perpendicular.

I mean when looking up from the ground, the big surface area will fall slower than the small surface area, all else equal.

[OngBonga]

This is what a baseball pitcher does to throw a curve ball. The pitcher puts forward spin on the ball, causing the Magnus effect to pull the ball downward by deflecting the air flowing under the ball such that the air goes upward. Newton's 3rd says that if the ball pushes the air upward, then the air pushes the ball downward, making it curve toward the ground faster than gravity would normally pull it down.

You'd want to spin the other way to exploit the Magnus effect. This would send the air going above you toward the ground. Which would exchange your forward velocity for lift, reducing your rate of falling.

Unfortunately, the Magnus Effect tends to "spin up" the object. Once you start the rotation, it gets amplified. You might have more trouble keeping your limbs in close and not flailing wildly. The flailing would probably not be best for an injury free landing.

I think it was back spin, so when it hit the ground, the spin attempts to cause it to roll uphill. Of course, the spin is utterly overwhelmed by gravity, but the tiny effect it has could be the difference between it breaking and not.

Or maybe it was top spin so it hit the ground and immediately rolled quickly down the hill.

I'm pretty sure the spin is intended to help with the roll though, and not reduce its velocity.

[OngBonga]

You got it right. I was trying to use simple language avoid saying "normal" to mean perpendicular and I forgot how much you love perpendicular.

I mean when looking up from the ground, the big surface area will fall slower than the small surface area, all else equal.

It's not just my love for that word. It was also my pedantry. I imagined a really thin and long pyramid falling down, with its massive surface area facing the direction of travel.... but only a tiny tip perpendicular to it. Such a falling object would be massively aerodynamic.

[MadMojoMonkey]

I assumed terminal velocity depended on mass alone, no air resistance. So basically I'm thinking of a guy just bolting straight down from the sky like any good superhero should.

No. Terminal velocity is tricky. Stuff like the Magnus effect gets involved. The surface interactions between the object and the fluid make a big difference. Look into golf ball dimple technology for evidence of how much it matters and how little we understand it.

The skydiver's clothing matters. Baggy clothing dissipates energy as it whips back and forth, which slows the rate of descent.

Then it would be that what I meant about cats is that they decrease their terminal velocity by doing something like splaying out. People report seeing cats fall from >40 feet distances and running off after landing with no noticeable injuries, but when falling from 20-30 feet they often sustain injury (due to not having ample time to maneuver body and slow down).

IDK if the science is 100% in on cats, but I'm pretty sure I've seen this explained and debunked about the height. The problem is that the results are not really reproducible, 'cause throwing cats off of buildings is kinda frowned upon, even when it's in the name of science. The results we have are anecdotal and it is hugely likely that confirmation bias is skewing the rates which are reported. If a cat falls off a building and dies... is that worth getting excited about? But if the cat falls off a building and walks away... that's news.

[MadMojoMonkey]

I mean imagine a pyramid falling upside down at terminal velocity. Well, there's actually a rather large amount of surface area in the direction of travel, more so than if it were falling the right way up with a flat square breaking the fall. Yet upside down it will fall faster, not slower.

Would it? That's not clear to me.

Whatever aerodynamics you gain on the front, you lose on the back... I think.

Fluid dynamics is ridiculously complicated when you do away with the assumptions and start to look at real-world scenarios.

[Savy]

IDK if the science is 100% in on cats, but I'm pretty sure I've seen this explained and debunked about the height. The problem is that the results are not really reproducible, 'cause throwing cats off of buildings is kinda frowned upon, even when it's in the name of science. The results we have are anecdotal and it is hugely likely that confirmation bias is skewing the rates which are reported. If a cat falls off a building and dies... is that worth getting excited about? But if the cat falls off a building and walks away... that's news.

This was brought up on QI a really long time ago. IIRC it's split into 4 heights. Starting from the smallest heights they're fine then they get to a point where they start getting injured/dying then slightly higher up they are safe again before reaching another height where they die again.

I can't remember what the mechanism was they use to survive but it's to do with it taking x amount of time to have an effect.

I tried to find the episode/segment but can't.

edit - here's an article about it

http://www.bbc.co.uk/news/magazine-17492802

[OngBonga]

Anyone want to see footage of a cat being dropped upside down in slow motion with incredible music playing? There's also monkeys.

Skip to 56:10

[CoccoBill]

This was interesting:
http://www.popularmechanics.com/adve...a5045/4344036/

[MadMojoMonkey]

Assumptions:
mass of Cap is ~100 kg = 220 lbs @ 1g
Vertical speed at t = 0 s is 110 m/s ~= 250 mph

I found an excellent .pdf on the military math modeling of parachuting. I'm trusting it for this post.

The pdf gives an equation for the "severity index" of brain trauma due to deformations during the deceleration.
I = [2v/(g*dt)]^2.5 * dt

It says we need to keep the index below 400 to avoid loss of consciousness or mild concussion.
Uh oh. Plugging in 110 m/s and g = 9.81 m/s^2, we need ~3.3 s to decelerate. I was hoping for something in the 1/4 of a second range.

Solve for average deceleration over that time:
v_f = v_i + a*dt
0 m/s = 110 m/s + a(3.3 s)
a = (-110 m/s)/(3.3 s) = -33 m/s^2

Solve for distance covered during deceleration:
x_f = x_i + v_i*dt + (1/2) a*dt^2
x_f = (0 m) + (110 m/s)*(3.3 s) + (1/2) (-33 m/s^2)*(3.3 s)^2
x_f = 181.5 m

***
We're in some room for making some changes. Either he needs to be 182 m tall, or a super brain that can not deform under extreme accelerations.

Which do you prefer?

[wufwugy]

How would you find a rough estimate of what his muscle strength needs to be? Something like calculating the force at which he would hit the ground, which would be approximately the same weight he would need to be able to squat? I know I'm not thinking of the units correctly. I really don't know physics. I just wanna know what kind of strength it would take to land from free fall like the superheroes do in the movies. Would they have to be able to squat 2k pounds, 20k pounds, or what?

[MadMojoMonkey]

How would you find a rough estimate of what his muscle strength needs to be? Something like calculating the force at which he would hit the ground, which would be approximately the same weight he would need to be able to squat? I know I'm not thinking of the units correctly. I really don't know physics. I just wanna know what kind of strength it would take to land from free fall like the superheroes do in the movies. Would they have to be able to squat 2k pounds, 20k pounds, or what?

@100 kg and 110 m/s, that's 11,000 kg*m/s of momentum to absorb.

I've stipulated 3.3 seconds to absorb it.
Newton's 2nd says F = ma or - more properly - F = dp/dt. So we know dp and dt, just plug and chug
F = (11,000 kg*m/s) / (3.3 s) = 3,333 N ~= 750 lb (force)

A more realistic time, assuming a super-brain that doesn't require such delicate treatment, a time span of 0.25 seconds makes more sense. Same math, different dt.
F = (11,000 kg*m/s) / (0.25 s) = 44 kN ~= 9,900 lb (force)

[wufwugy]

Awesome, thanks.

Looks like an "ordinary" superhero feat like Cap jumping out of a plane and tanking the landing would require ~50x body weight in squat. Given that peak athletes do around 3x body weight, he'd have to be about 15x more "athletic" than them. Given that the difference between an American football player running a 4.5 second 40 yard dash and a 4.0 would be something like squatting 2x body weight vs 3x, Cap would be able to run the length of a football field in some crazy low number, like a second.

Cool stuff. If you spend any time reading /r/whowouldwin, you occasionally come across examples of comic book writers totally flubbing feats of superheroes. I recall one example being something along the lines of one comic of Flash having him save tons of people from a sudden catastrophe by transporting them a long distance away one by one. This feat apparently put Flash's "real" speed at something like several times past the speed of light even though the character was supposed to not be nearly that fast.

[MadMojoMonkey]

Cool stuff. If you spend any time reading /r/whowouldwin, you occasionally come across examples of comic book writers totally flubbing feats of superheroes. I recall one example being something along the lines of one comic of Flash having him save tons of people from a sudden catastrophe by transporting them a long distance away one by one. This feat apparently put Flash's "real" speed at something like several times past the speed of light even though the character was supposed to not be nearly that fast.

I wonder if they accounted for time dilation and space contraction in the direction of travel.

Bear in mind that if you were traveling at 99% the speed of light, your relativistic gamma factor would be ~7. The distances in front of and behind you are 1/7 what would be measured by a stationary observer. You can get from here to a star 7 light years away in only 1 year of travel time as observed by you.

[MadMojoMonkey]

Awesome, thanks.

Looks like an "ordinary" superhero feat like Cap jumping out of a plane and tanking the landing would require ~50x body weight in squat. Given that peak athletes do around 3x body weight, he'd have to be about 15x more "athletic" than them. Given that the difference between an American football player running a 4.5 second 40 yard dash and a 4.0 would be something like squatting 2x body weight vs 3x, Cap would be able to run the length of a football field in some crazy low number, like a second.

The problem is that we're facing a "rocket equation" when we extrapolate.

He would need to be ~15x more athletic at the same mass. If he puts on mass to become stronger, then he needs to be stronger to compensate for decelerating that additional mass. It's a feedback loop. It will fall apart if there is no convergence, i.e. the added strength needs to be at least enough to stop the added mass + some extra.

[wufwugy]

Yep. I was looking for a scenario in which the body mass is unchanging.

[MadMojoMonkey]

New video showing an example of conservation of angular momentum using fire.



The idea that "hot air rises" is a pet peeve of mine. It's true, but there is nothing gravity-defying about hot air. It's the fact that colder air is more dense which displaces the hot air upward.

Anyway, with the hot air going upward, it draws in cool air from the sides. As that inrushing air passes through the spinning screen, it gets a little rotational push.

Just like an ice skater spinning slowly with their arms extended, but spinning quickly when their arms are in, the rotation of the cool inrushing air increases as it approaches the rotational axis. The result is that the hot air rushing upward is now somewhat contained in a vertical column.

[wufwugy]

Cold air falls!

[OngBonga]

Hot air is buoyed by cold air, resulting in a "rising" effect, relative to the Earth.

[OngBonga]

Hot air ascends.

[OngBonga]

Why does hot air rise?

I would speculate that the increased energy of warming air would result in more collisions between molecules, thus increasing the average distance between each molecule, and thus the average space a molecule occupies. That decreases density, relative to cooler air.

But collisions are governed by probability.

So hot air probably rises.

[OngBonga]

How much rocket fuel would it take to carry a 2 ton rod of tungsten into space?

Don't forget to factor in how much rocket fuel is needed to carry the rocket fuel needed to carry the tungsten!

Also, how fast would the rod need to be travelling through the atmosphere for it to turn the air it displaces into plasma? What other signatures would such a falling object be likely to have? Could it be detected by radar, or infrared or ultraviolet cameras on satellites?

In case you wonder why I ask, there is a theory that the Tianjin explosions last year were an act of war, an attack by USA in retaliation for the devaluation of the Yuan and threats to dump their dollar reserves.

It seems to me that it would be counterproductive to protect the economy by sending very heavy rods of tungsten into space and then dropping them on a target. Also, there is no evidence of a falling object at high velocity on any of the footage I've seen. I'm skeptical of this one.

[OngBonga]

To clarify the theory...

USA has "kinetic bombardment" weapons on space, tungsten rods that can be dropped onto a target with immense kinetic energy, comparable to a tactical nuke. China and USA are currently engaged in economic warfare, and this was a warning shot.

I do not believe this, but it certainly interests me.

[MadMojoMonkey]

As a broad rule of thumb, the fuel is about 90% the total mass of a space-bound rocket. The fuel tanks, engines, and the structures which hold it all together and control it are all in that remaining 10% of the total mass, and whatever is left is payload.

The Falcon 9 has a listed max payload of 13,150 kg or about 14.5 tons at surface gravity.

***
Another concidental rule of thumb is that the entry speed into the atmosphere in m/s is equal to the peak shock layer temperature in K. E.g. a spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7,800 K. Air can become a plasma at temperatures in a range of 7,000 to 10,000 K - hotter than the "surface" of the sun. So a vehicle entering the atmosphere at ~7 to 10 km/s would ionize the atoms in the air, creating a plasma.

As for other emissions, the only I can think of would be to look at the meteor streak with a spectroscope and see if the spectral lines associated with Tungsten are there.

I'm sure the atmospheric entry and compression heating is producing a blackbody radiation signature which has a bandwidth of all wavelengths greater than 0. I.e. there is a non-0 chance of the thing emitting a photon of any conceivable energy, but the probability is like a bell curve with one short tail and one long tail. I suspect any light sensor pointed at an atmospheric entry will see it, because it will emit radio waves, microwaves, infra red, optical, ultra violet, hopefully not too much gamma.

[MadMojoMonkey]

To clarify the theory...

USA has "kinetic bombardment" weapons on space, tungsten rods that can be dropped onto a target with immense kinetic energy, comparable to a tactical nuke. China and USA are currently engaged in economic warfare, and this was a warning shot.

I do not believe this, but it certainly interests me.

I read about it in a Robert Heinlein book, I think. Good pulp fiction, but all the characters are sexist in an out-dated kind of way.

EDIT: I think the book was "The moon is a harsh mistress," but I could be mixing up titles.

[OngBonga]

Ok so at sea level the speed of sound is ~343 m/s, and these rods are theorised to impact at ~MACH 10, which is ~3.5 km/s, well short of ionisation velocity if we're in need of 7 km/s, which is ~MACH 20 at sea level.

Would a tungsten rod shed particles on its descent? Could it be coated to reduce particle loss?

If the only problem with this theory is the cost, well I wouldn't rule out superior propulsion technology than is known to exist. Granted, it's still pseudoscience, but I can at least entertain the idea that John Hutchison was onto something with his bowling ball experiments.

[MadMojoMonkey]

Ok so at sea level the speed of sound is ~343 m/s, and these rods are theorised to impact at ~MACH 10, which is ~3.5 km/s, well short of ionisation velocity if we're in need of 7 km/s, which is ~MACH 20 at sea level.

You're talking about the speed upon impacting the ground. I'm talking about the speed upon impacting the air.

The rule of thumb I quoted is for the entry velocity as object enters the Earth's atmosphere.

Would a tungsten rod shed particles on its descent? Could it be coated to reduce particle loss?

Yes. It will either ablate material from its surface as it falls the way a meteroid does or it will explode entirely.

Yes. We've had this technology since the early years of space travel. The Apollo landing capsules had heat shields.

If the only problem with this theory is the cost, well I wouldn't rule out superior propulsion technology than is known to exist. Granted, it's still pseudoscience, but I can at least entertain the idea that John Hutchison was onto something with his bowling ball experiments.

I am quite skeptical about secret "superior propulsion technology."

In theory, it's fine. Drop a rock from high enough and it's going to make a big mess. Drop bigger rocks for added effect.

[OngBonga]

I'm less skeptical about secret propulsion technology than I am other things. Tesla was doing some really interesting work, and others (with far less credibility) have claimed to have continued this work. I can certainly see why it would be kept secret, if indeed it is for real.

I was talking about this with my friend. he suggested that it's possible to propel something upwards by firing lasers at the underside to cause the air to expand. I kind of like the idea of that, I mean essentially it's turning the air into the fuel, which means it isn't necessary to take any fuel. That isn't what Hutchison was doing, that's a second theory I've heard which might explain how heavy things can get into space cheaply.

And yes, velocity would be much lower at ground level. At what altitude would plasma production cease? Could it look from the ground like a shooting star, or lightning, or something else perceived as innocent?

[MadMojoMonkey]

I'm less skeptical about secret propulsion technology than I am other things. Tesla was doing some really interesting work, and others (with far less credibility) have claimed to have continued this work. I can certainly see why it would be kept secret, if indeed it is for real.

I'm not going to get into the whole "Tesla was an under-appreciated god of science who hid/destroyed all his best work" conspiracy theories.

Whatever he hid and or destroyed was done in an era before modern computers. He was intelligent, but he wasn't the most intelligent person ever to study this stuff.

Your supposed reasons to keep efficient energy sources a secret are lost on me. All I hear is conspiracy theories. Trust me, if my department got word of a practical power source, we're not keeping it a secret. We're testing it, quantifying it and monetizing it. The blind accusation that "big oil" or something similar is somehow incentivized to suppress innovation is lost on me.

If there is a real conspiracy, then show me the evidence. Make it good. Don't make it a series of open-ended questions to which you have no verifiable answers. Don't then attempting to wrap up your ignorance with accusations.

I was talking about this with my friend. he suggested that it's possible to propel something upwards by firing lasers at the underside to cause the air to expand. I kind of like the idea of that, I mean essentially it's turning the air into the fuel, which means it isn't necessary to take any fuel. That isn't what Hutchison was doing, that's a second theory I've heard which might explain how heavy things can get into space cheaply.

It's possible to do so in theory. Control and stability would be ever-present issues, but they usually are with rockets.

I fear that it would cost obscene amounts of energy to attempt to do this. Plus the whole "Don't kill everything behind you when send a rocket into space" thing might come into play... again. (Project Orion is so cool.)

And yes, velocity would be much lower at ground level. At what altitude would plasma production cease? Could it look from the ground like a shooting star, or lightning, or something else perceived as innocent?

IDK about the altitude. It would probably be a function of the initial speed of the falling object. If it starts faster, it'll take longer to slow down.

It would look exactly like a shooting star, 'cause that's what a shooting star is - i.e. a hunk of metal falling from space.

[OngBonga]

I wasn't intending to get into a discussion about what Tesla might or might not have done, I was merely pointing out that he inspired a great many people. Whether ot not they succeeded where he failed (or succeeded in secret) is another topic. I'm just open to the possibility, without intending to impress the concept as truth.

I'm utterly convinced though that the big oil companies would do their utmost to stifle new energy technology. Why wouldn't they? Would they sit idle while their business model is utterly destroyed? Of course not, they're too rich and powerful.

I'm not going to start digging for evidence of conspiracies, because if the evidence was compelling, it wouldn't be a conspiracy, it would be science. That wasn't my intention when I made those comments.

All I'll add is that there are reasons why such technology would be kept secret, and not just oil. We're talking about immense energy here, which has profound implications when it comes to military and global security. Of course scientists would want energy technology to advance, but if such technology is widely available, then we have a very dangerous arms race on our hands.

It would look exactly like a shooting star, 'cause that's what a shooting star is - i.e. a hunk of metal falling from space.

Yes but a shooting star is not coated with anything that is designed to protect it from atmospheric entry.

I'm simply trying to find reasons why such a weapon could go unnoticed until impact.

[OngBonga]

I hope you noticed that so far my language has been deliberately ambiguous. Words like "might" and "if indeed it's for real" were intended to ensure that my comments are not perceived as blindly accepting of conspiracy theories.

I'm just not rejecting such theories either, though.