Fat people in zero gravity

[Alorael at Large]

Your assumption that this is an engineering problem rather than a simple example physics problem with no real-world use or basis also has its problems, though.

 

—Alorael, who does not think the original problem could be any more clear. The cube is filled with vacuum to make the density simple. The cube is made of generic strong, relatively dense material, called "steel" for convenience. It requires no reinforcement. You can solve other problems, and they're interesting problems, but they all require making explicitly different starting assumptions.

[Harehunter]

My apologies. I do tend to see things through the lens of an engineer, having been trained in that field. When a theoretical situation is presented, whether it involves engineering, computer programming, or home improvement projects, I take it as a challenge to try to figure out a way to make it actually possible. From this perspective, engineering is only applied physics, where niggling little details such as force, vectors, chemical properties and their effect on material characteristics, friction, Standard Temperature and Pressure (STP), and other things well understood from the physics perspective, actually become important.

 

What's the difference between a chemist and a chemical engineer?

The chemist works on solving the theoretical aspects, the engineer works on the practical aspects. The same rule applies to physicists and engineers; theoretical vs practical. Don't get me wrong; without the work of the former, there could be no latter. Basic research does not produce a product, but without it, no product can be developed. Theoretical thought problems such as presented here are extremely valuable. They are what motivates us to seek new knowledge and grow intellectually. I believe that that is the moral of the story told in the first Star Trek movie.

 

Note: From the desk of Isaac Newton.

Gravity... it's not just a theory, it's the Law.

[Actaeon]

As someone who is neither an engineer nor a physicist, I think you chaps both have good points. On one hand, there are too many factors for any model to be perfectly representative of real life. On the other, a sterile and theoretical system that ignores those things entirely isn't terribly useful, except as an exercise.

 

Either way, you've managed to take a couple of frivolous queries and make them of intellectual interest. There is no reason to build a steel balloon when more suitable materials exist, and while there are real issues associated with obesity and space travel, we are not yet in a position to worry about them. While we're at it, we might as well consider how we could go about siphoning Venus's dense atmosphere off to Mars's nearly nonexistant one in an attempt to tereform them both.

[Dintiradan]

For what it's worth, I really don't mind. Originally, it was just some bonus question that came up in our unit on buoyancy, probably during junior high. Of course people here got bored with it and went above and beyond. And hey, now I know how cubes implode, and why hydrogen storage sucks. And the concept of a 'hot vacuum' blows my mind.

 

(Though I'd be grumpy if this was a test and someone talked about all those things instead of answering the question as stated. Dealing with students who go beyond the assignment description is one of my dilemmas as a TA. On the one hand: Youpi! You actually care about the course and are willing to do your own thing! On the other hand: The purpose of the assignment is to teach Technique X, and you're solving the assignment with Technique Y. It doesn't matter how well you use Y, I still have to be convinced that you know X. And of course, most of the time you're only doing Y because you couldn't puzzle out how to do X.)

[Harehunter]

Back on the theoretical side of things...

 

How would you be able to measure the heat of a vacuum? Knowing how to do that could be the key to understanding how a Dewar flask 'knows' the temperature of the liquid within, and keeps hot liquids hot while still being able to keep a cold liquid cold.

[Niemand]

Quote:

How would you be able to measure the heat of a vacuum?

Just measure the spectrum of the radiation. SInce there is a direct, known relationship between the temperature and the spectrum, computing the temperature is then easy. Astronomers do this all the time. For example, this is how a temperature is assigned to the CMB. I was going to to go on to say that this is basically the temperature of the vacuum of space, except that this isn't quite correct, as evidenced by the presence of other radiation in the universe which doesn't fit the single Planck distribution of the CMB. Clearly, the universe's vacuum isn't in thermal equilibrium. (And no surprise, since it's contaminated with a bunch of baryonic matter, and also some charged leptons. We should get rid of those.)

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keeps hot liquids hot while still being able to keep a cold liquid cold.

If this is your goal, a passive dewar with no knowledge of its internal temperature is already sufficient, however.

[Harehunter]

I think I must be losing my mind. I should have thought of that one myself. Astronomy, anyone?

 

In other words, if I interpret you correctly, space is not truly a vacuum, since there is matter, albeit sub-atomic matter, that pollutes the otherwise rarified volume. And since you mentioned 'spectrum', that reflects (no pun intended this time) on the presence of light, a form of energy which can behave like a particle.

 

And while my question regarding the nature of a Dewar flask was facetious, I am being completely serious when I ask,

"Does light have mass?" I suppose it can be argued that according to Einsteins' formula, E=MC2, light has the *equivalence* of matter. That being said, it might be surmised that space is not truly a vacuum.

 

The thing I understood about the vacuum contained in the walls of the Dewar flask is this;

A true vacuum can be neither hot nor cold. It does not contain anything that is capable of transmitting any sort of energy. A perfect vacuum is therefore the perfect insulator, insulating the energy level of the material contained within from the energy being carried by the matter without.

The trick, of course, is trying to achieve a perfect vacuum.

 

Like I said, you guys challenge me.

[Student of Trinity]

On the large scale there is the very curious fact that thermal equilibrium is actually unstable. The specific heat of self-gravitating systems is negative; their temperature goes up when they lose energy (because they shrink under their own gravity enough to overcompensate for the energy loss, and store the extra energy as heat). So your average random cloud of cold dust and gas can spontaneously condense around any random clump of higher density, and the more it condenses the hotter it gets, even though it is also steadily losing energy by radiating into space. Eventually, if the initial clump is big enough, a star is born. And so out of what was once nearly uniform low temperature, a stellar core at millions of Kelvin has formed, quite spontaneously. The result is sunlight, with all that means in the way of further interesting processes. Also geothermal heat from the Earth's own core, which got its heat in the same way the sun did, from gravitational contraction of matter that was originally much less dense.

 

Gravity doesn't actually contradict the Second Law of Thermodynamics, of course. But the way the Second Law works when gravitational change is involved is substantially different from the way it works otherwise. Heat localizes instead of dispersing, and temperature gradients form spontaneously.

 

We owe everything interesting to gravity. I find this amazing. Even more amazing is the fact that I didn't learn it until a couple of years after I got my PhD. It's a fact that deserves a lot more publicity, in my opinion.

[Randomizer]

Originally Posted By: Harehunter

"Does light have mass?" I suppose it can be argued that according to Einsteins' formula, E=MC2, light has the *equivalence* of matter. That being said, it might be surmised that space is not truly a vacuum.

Photons are assumed to be massless particles. However they are given an equivalent mass since they have energy equal to Planck's constant times the photon's frequency.

Neutrino's were thought to be massless a few decades back until it was determined that they had to have mass in order to oscillate between neutrino types.

[Niemand]

Quote:

In other words, if I interpret you correctly, space is not truly a vacuum, since there is matter, albeit sub-atomic matter, that pollutes the otherwise rarified volume. And since you mentioned 'spectrum', that reflects (no pun intended this time) on the presence of light, a form of energy which can behave like a particle.

The first part, yes. There's a bunch of matter at different temperatures, emitting various black-body spectra. (There's also non-thermal emission from processes like synchrotron radiation and inverse Compton scattering.) Clearly, then, there's no single temperature that describes it all. I'm not sure what the significance of the quantization of light is, here, though (besides the fact, which I hadn't previously mentioned, that Planck's law is derived from the quantization of radiation). Somewhat amusingly, though, it's worth noting that without some matter present, it is difficult or impossible for the radiation present in a vacuum to come to thermal equilibrium by itself.

Quote:

I am being completely serious when I ask,
"Does light have mass?" I suppose it can be argued that according to Einsteins' formula, E=MC2, light has the *equivalence* of matter. That being said, it might be surmised that space is not truly a vacuum.

No, light doesn't have a mass, and doesn't need to, as seen by looking at the entire formula: E^2 = p^2c^2 + m^2c^4 . Light has energy, so it must have either mass or momentum, and in fact it always has just the right amount of momentum to require no mass at all. Your point about a vacuum being imperfect if it contains any mass or energy is a perfectly good one, however. One could then take the view that a perfect vacuum is a very fragile thing, since as soon as you wrap it around your hot liquid to keep it from leaking heat to the cold surroundings (or wrap the vacuum around the hot surroundings to keep them form leaking heat to your cold liquid) whichever side is hot will start radiating and contaminate your vacuum, and once your vacuum is contaminated it's no wonder that heat starts leaking!

[Alorael at Large]

In practice vacuum in space isn't even just full of radiation. It's also full of very, very diffuse hydrogen gas. That's not terribly important for most purposes, because while vacuum is a perfect insulator, being incapable of conducting energy doesn't make it incapable of transmitting energy. Or, to be more accurate with terminology, there's nothing in vacuum that obstructs radiation, including thermal radiation. That's also a major property of vacuum.

 

—Alorael, who thus assumes, although the deep physics might differ, that a drink kept in an ideal Dewar flask would still change in temperature to match the ambient temperature outside the flask because of the passage of light. You could presumably prevent that as well by making both the inner and outer layers of the flask out of a material that fully absorbs light at all points on the spectrum, but there are several problems with coming up with such a material, starting with the need for infinite cooling to maintain it.

[Harehunter]

Every Dewar flask I have seen has always had a silvered surface on the interior of the glass. I remember always seeing that, but never wondered why. Now I see that having a reflective surface would have the effect of not absorbing energy, but reflecting it away.

 

My analogy of the Dewar flask does have two major flaws;

1) The fact that they are made using some material which will have a thermal absorption coefficient. Glass is a very good insulator, but it will still transmit heat, from the outside surface, around the lip of the flask, and into the interior surface.

2) And even blocking out light will not stop other forms of electromagnetic energy. Microwaves would pass through very nicely, and we all know what they do to water.

 

Let's go visit a scientific discipline that we have ignored up to now; biology, or more specifically, bio-chemistry.

(Bear with me folks. I know that most of you following this thread have some understanding of this. I am going into a little more detail so that those people who haven't had that education yet may follow along.)

 

Any one who has had a little knowledge of chemistry knows that when you burn a hydrocarbon, say sugar for example, it gives off heat. This is what is known in physical chemistry as an exothermic reaction, whereby the potential energy stored in the chemical bonds of the glucose molecule is released as oxidation takes place.

 

If oxidation is an exothermic reaction, what is it called when plants produce glucose?

Answer: an endothermic reaction. This means that the plant has to put more energy into the chemical reactions that build the glucose molecule than it gets out. Where does this energy come from? The chlorophyl in the cells of its leaves, which absorbs the energy of the visible light that hits it (except for green, which is reflected). There are other examples of exothermic / endothermic reactions; a lead-acid battery producing a current when in use (exothermic) while chemically it changes from lead and sulfuric acid into lead sulfate (oxidation), and requiring a reversed input current in order to recharge it (endothermic) causing the lead to be re-plated on the surface of the cathode (reduction). Not all endo/exo thermic reactions are chemical in nature. The cycle of water going from state to state ( solid, liquid, gas) also represents endothermic in the direction given and exothermic when going from gas to solid.

 

The reason for this long winded explanation of the concept of energy absorption and release of materials. So back to the original thought problem; a box containing a true vacuum, that has sufficient tensile strength to resist implosion due to atmospheric pressure, that does not present a surface that folds into the interior of the container as the Dewar flask does, and has a reflective coating on its surface to prevent the passage of most electromagnetic radiation. Unfortunately, there are still some forms of radiation, both particulate like cosmic rays, or just plain electromagnetic radiation like microwaves, that will not be reflected by the mirrored surface, but will happily pass right on through to try to impart its energy into that box.

 

Here we run into my next question.

What is the energy absorption spectrum of a vacuum?

Theoretically, since it contains zero mass, it should be zero. Since there is nothing there to absorb the energy, the radiation should pass right through, out the other side and carry its energy with it.

 

And speaking of electromagnetic energy, where does our good friend, Gravity, fit into this scenario?

 

A couple of bumper stickers seen on a university campus...

"Chemists have Solutions"

"Honk if you passed P-Chem"

[Alorael at Large]

A perfect reflector across the spectrum would also block microwaves, and cosmic rays could be kept out by adding (strong) electromagnetic shielding.

 

Unless gravity is actually moving the ideal Dewar flask or its contents, no work is done and there is no change in energy due to gravity.

 

—Alorael, who finally will complain about the use of endothermic/exothermic to describe phase changes. A phase change is not a reaction, and it does not actually require any change in temperature at all. You can instead increase or decrease pressure. And you're playing fast and loose with enthalpy and free energy in general. An oxidative reaction may be exothermic, like combustion, or it can be exergonic (energy-releasing) without necessarily having the energy take the form of heat. Glucose production in plants, in particular, isn't endothermic: the necessary energy input is light, not heat, and the reaction is endergonic (products have higher Gibbs free energy) not endothermic (products include heat).

[Niemand]

Quote:

cosmic rays could be kept out by adding (strong) electromagnetic shielding

Cosmic rays are not electromagnetic radiation (thanks to being named before people knew what they were, and generally sloppy naming); at low energies they are electrons and positrons, at higher energies they are protons and heavier nuclei, and no one knows what the composition is at the highest energies. (The debate is whether the answer is 'protons only' or 'heavy nuclei'. I belong to the protons camp, as I believe in photodisintigration, but there are recent results arguing for heavy nuclei.) What we typically notice at ground level is not the cosmic rays themselves, but the muons they produce when they cause particle showers in the atmosphere. The point is that the only way to stop any of these particles is with kilometers of dense material, usually combined with hundreds of kilometers of atmosphere. (The detector I work on is under 1.5 km of ice as is still constantly bombarded with muons.) It's tough to stop things that come at you with energies of more than a Joule, packed into a single particle.

Sorry harehunter, I know you wanted to discuss bio-chem now, but particle astrophysics grabs my attention.

[A less presumptuous name.]

Whether it's the most accurate description or not, most intro-level classes use endo- and exothermic to describe phase changes. If your professor is good, though, he or she will emphasize that such descriptions are true at constant pressure.

[Alorael at Large]

Cosmic rays are charged particles. The charge means you could, in theory, stop them or deflect them with a strong eletromagnetic field. That's not actually feasible, as I understand it, but neither is the rest of our hermetic Dewar flask experiment.

 

—Alorael, who has looked at the internet and determined that phase transitions are, in fact, labeled exothermic and endothermic. Sua culpa.

[Harehunter]

My puny attempt to drag biology into all this mess was weak indeed. In fact, photosynthesis is more closely related to P-Chem than biology per se. The point I was trying to make is that each form of matter has an energy absorption spectrum, and that by absorbing that energy, the enthalpy of the material increases.

 

Since the crux of this discussion deals mostly with physics, I thoroughly understand your attraction to the characteristics of particles. That thought crossed my mind when I wrote that piece, and that cascaded into the effects of gravity on those particles, and in fact, its effect on matter in general. The thing that most puzzles me about gravity is this: "Is it electromagnetic in nature, or is it something else? How does it fit in with Einsteinian physics?"

 

And for the bonus question:

"How is it that the Speed of light is constant, yet Time is not? Since Velocity is defined as Distance / Time, why is it not that Time is constant, but Distance is not?" This is the crucial point of one of Einstein's theories that Time passes more slowly when in a gravitational field than it does in the spaces between those fields. The theory goes that if you take two identical clocks and synchronize them precisely, then take one of those clocks out of the gravity well for a period of time, then return it to its origin, it will have been faster than the clock that stayed in the gravitational field.

 

I've heard that this experiment has been done, with that very effect observed. My skeptical mind wonders this:"Did we measure that time does actually progress more slowly in a gravity well, or do clocks simply run faster when out of the effects of the gravity field because there is less force in a particular direction causing friction of the moving parts of that clock?" Even an atomic clock has moving parts, i.e. the atoms and sub-atomic particles that transmit energy towards the receptors.

[Alorael at Large]

Gravity is almost certainly not electromagnetic in nature. Gravity fits very nicely with general relativity, or so I am led to believe; GR itself is beyond my understanding.

 

In special relatively, both time dilates and length contracts. But I think you've got it backwards: in any frame, no matter how much time and space differ from another frame, the speed of light in a vacuum is observed to be c.

 

While some experiments have used atomic clocks, others use red-shift. I am not a physicist, so I can't speak to the technical quality of the experiments, but I imagine that confounding factors were kept very much in mind.

 

—Alorael, who now has to turn this over to Randomizer and SoT. Real physics is for real physicists.

[Erebus the Black]

Let's start at the beginning.

Mankind has observed four different forces of nature, each with it's own unique characteristics:

1. Electromagnetism

2. Gravity

3. Weak

4. Strong

 

To the best of my understanding gravity affects space in a very peculiar way, it bends or (logic, not exclusive) curves it.

The usual example is a reduction to 2 dimensions:

take a taut bed sheet and place a bowling ball on it, as long as the ball remains the sheet will have a dent in it so any ant, for example, walking along the sheet will walk a longer distance with the ball than without the ball.

 

Before the speed of light has been axiomized to be a constant, time (or rather it's flow) was the axiomized constant. During several years of mathematical work, in order to solve some kinks with the now moot luminiferous ether hypothesis, time was shown to noticably change when systems (i.e. the moving bits) where observed from different vantage points that were moving in relative velocities, which differed by at least c/3(i.e. v2-v1>c/3), to one another.

 

It's not a matter of time not being constant but rather that the ratio between time and space is constant for ripples advancing within an electromagnetic field (i.e. light a.k.a. radiation). According to wiki :

Quote:

Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore traveled at the speed c appearing in his theory of electromagnetism.

 

Even when light passes through matter it doesn't slow down, it is just bent by the medium it passes through, this makes it travel a longer path and so it has the appearance of slowing down.

[Student of Trinity]

No, light does slow down as it passes through a medium. Well, kind of. To be really accurate, you have to be more precise about what you mean by 'light'.

 

Suppose you send a pulse of light into a thick wall of glass. You flip your laser pointer on and off really quickly, and that makes a pulse. It zips through vacuum (let's say) at the universal speed c, then hits the glass. A bit of it reflects, and the the rest of the pulse goes into the glass.

 

This reflection and entry process is actually a bit complicated. Light piles up around the surface for a little while, and the molecular vibrations in the glass start getting cranked up. Higher frequencies get involved, so some of the reflected light is actually ultraviolet, having been excited by the high-frequency vibrations of the molecules.

 

But all of that happens in maybe a few picoseconds, and when it's over, there's a reflected flash heading back at you through the vacuum, and the rest of your light pulse is heading onwards inside the glass. The very first, faintest leading edge of your light pulse is racing ahead, diffracting around molecules in the glass, and traveling at the same speed c that it would travel in vacuum. And behind it there is a trail of excited glass molecules, with their electrons all swinging around. This drains energy from the leading edge of your light pulse, so it gets fainter and fainter as it propagates. The main body of your light pulse, following behind the leading edge, then has to move through the wake of molecular excitation.

 

Those vibrating electrons are exciting and absorbing light, and so this wake of molecular excitation behind your light pulse's leading edge has a big effect on the main body of your light pulse as it follows along. What ends up happening is that most of your light pulse effectively becomes a hybridized pulse of synchronized light wave and molecular vibration. And this hybrid pulse of synchronized electromagnetic field oscillation and electronic motion travels at less than the speed of light in vacuum.

 

Do you still call this hybridized pulse 'light'? Well, this is a mere matter of terminology. In practice, yes, people just call it light. It behaves really very much just like pure light, except for its speed. It travels more slowly than pure light. Also, the precise speed of a hybridized light-electron excitation wave travelling through matter depends on wavelength, whereas pure light in vacuum travels at the same speed regardless of wavelength. But otherwise it's just like pure light in pretty much every way, so the accepted terminology is to say that the hybridized wave is just 'light travelling through an optical medium'. Given this language, then, it's correct to say that light travels more slowly through material media.

 

It is nowadays possible to engineer highly artificial (and powered) media, in which the speed of "light" (in this hybridized sense) is extremely low. Like, walking speed, literally. But at this point it really becomes a semantic issue, whether what you have is slow light, or just a slow wave of light absorption and re-emission in matter, in which almost all of the energy at any given time is held in electron motion.

[Harehunter]

Many thanks SoT for shedding light on that topic; it was very illuminating for me, and I do mean that seriously. I always heard the 'constant speed', but I glossed over the phrase 'in a vacuum'. And the differentiation between 'pure light' and 'light within matter' is new to me as well; the last course I had in physics was almost 30 years ago. Intuitively, I 'knew' this, but I don't ever recall it being explained so lucidly before.

 

Special relativity, anyone?