Physics In the Playground

[Asta Kask]

You can have negative absolute temperature in highly artificial systems, but the energy is still higher than at absolute zero. This has to do with the definition of temperature.

[JCarter426]

Sorry, no. It's like saying the freezing point of water is zero and anything colder has negative temperature. In reality, temperature has an absolute zero it can't go below (or even get as cold as). Everything else is positive temperature. Same with the energy of space; there always some there. You can reduce it but you can't make it negative. You can't even make it zero.

I assure you that you can have negative temperatures, and also that the concept of temperature is not what you are describing; the concept you are describing is no different to the concept of zero point energy - the lowest state of energy you can possibly reach - also known as vacuum energy or the Casimir effect. All of these are considered to be negative energy - which is predicted in relativity - due to the localized energy density, which is negative. Some visual examples here and here.

(If you want to get technical, vacuum energy is the zero point energy of a vacuum, and the Casimir effect is the process of this energy decreasing, making it, paradoxially, negative energy).

[teratorn]

No, what I meant was that the only refutation claimed it broke some hallowed law of science, but rationalism means little in comparison to empiricism, the foundation of science. Empiricism would have actually disproved it.

I don't get your reasoning. The guy isn't going from an experiment towards
a theory, he starts from the current theory and accepts it. The author states that in framework of existing relativistic theory one can generate trust in a closed cavity. He doesn't advance a new theory, he claims thrust is a consequence of existing theory. In this regard breaking a law of science derived in the framework of the theory he uses is enough to show his derivations are flawed. Why would one need empiricism to «disprove» bad math?

[shawnhcorey]

I assure you that you can have negative temperatures, and also that the concept of temperature is not what you are describing; the concept you are describing is no different to the concept of zero point energy - the lowest state of energy you can possibly reach - also known as vacuum energy or the Casimir effect. All of these are considered to be negative energy - which is predicted in relativity - due to the localized energy density, which is negative. Some visual examples here and here.

(If you want to get technical, vacuum energy is the zero point energy of a vacuum, and the Casimir effect is the process of this energy decreasing, making it, paradoxially, negative energy).

Yes, you can have temperature lower than 0C. You can't have them lower than absolute zero. You can have negative energy with respect to some reference but there is only so much energy in a volume of space and you can't take out more than what's there. You can't even get all of it out; some is always left behind. Yes, there is reduced energy but there is no such thing as negative energy. It doesn't exist in our universe.

[JCarter426]

You can't even get all of it out; some is always left behind. Yes, there is reduced energy but there is no such thing as negative energy. It doesn't exist in our universe.

This is precisely what the Casimir effect is thought to do. So don't argue with me, argue with Casimir.

[Grinner]

I don't get your reasoning. The guy isn't going from an experiment towards
a theory, he starts from the current theory and accepts it. The author states that in framework of existing relativistic theory one can generate trust in a closed cavity. He doesn't advance a new theory, he claims thrust is a consequence of existing theory. In this regard breaking a law of science derived in the framework of the theory he uses is enough to show his derivations are flawed. Why would one need empiricism to «disprove» bad math?

We're talking about two different people now. You're referring to Roger Shawyer, the inventor in question. That quote refers to one John Costella, who wrote a paper arguing against Shawyer's reasoning.

Regarding rationalism versus empiricism, I had meant that Costella had been arguing against Shawyer via rationalism. However, rationalism is, in the end, educated speculation.

What irks me about the affair is that no one attempted to recreate the device. If it works, then who cares? It seems like his peers cared more about their precious, admittedly well-founded preconceptions than the results.

But! We've already been through this....Check page 2 of the thread.

[shawnhcorey]

This is precisely what the Casimir effect is thought to do. So don't argue with me, argue with Casimir.

You keep insisting that the mathematics are the physics. They're not. With the mathematics, it doesn't matter if you remove positive energy or add negative energy; the result is the same. But the Casimir effect only removes positive energy. And there is a limit of how much energy it can remove. There's a point that the Casimir effect can't go below; something it could do if negative energy existed.

[JCarter426]

I don't think you're getting exactly what it does. Zero point energy is the point where you can't get any lower. That seems to be the natural energy level of the fabric of the universe. The Casimir effect is a process by which that is still lowered, somehow. Whatever energy causes this is, therefore, negative. The energy density is negative - it is less than the natural level.

This is not the same thing as the temperature dropping below zero. It's not just mathematical semantics.

No, it's not a case of some sci-fi anti-particles, or whatever. It's caused by physical stuff. But it's still a case of an energy that is negative in value - something that is predicted by relativity, and is required for certain things like wormholes to be possible in reality rather than just on paper.

[shawnhcorey]

I don't think you're getting exactly what it does. Zero point energy is the point where you can't get any lower.

No, that is not the zero point. Quantum mechanics says vacuum always has positive energy. It can never be zero.

[JCarter426]

That's what zero point energy is. It's like absolute zero. It's not a literal value of zero; it's the closest you can get to zero. You really could spend your time more wisely by reading on these subjects than arguing with me over the semantics.

[grimbold]

That's what zero point energy is. It's like absolute zero. It's not a literal value of zero; it's the closest you can get to zero. You really could spend your time more wisely by reading on these subjects than arguing with me over the semantics.

i beg to differ (sort of

theoretically absolute zero and zero point energy exist as values of 0

they're just unobtainable

slight semantic difference,, sorry..

[Asta Kask]

Yes, you can have temperature lower than 0C. You can't have them lower than absolute zero.

Yes, we can!

[Yora]

Which is true for the concept of temperature that physicists use, but not for the concept that's used in every day appliances. But under -200° C, every day applications no longer exist anyway.

[teratorn]

In accepted general relativity the equations work equally well if one includes negative mass, and that would imply negative energy. It was never observed, anti-matter seems to have positive mass. I'm not aware of any evidence for negative mass or energy. The closest one gets is indeed with the Casimir effect.

Zero point energy discussion requires people to be familiar with quantum field theory. The formalism is quite hard to grasp because it involves infinite quantities (like the zero point energy) that are dealt with a thing called field renormalization. Basically one subtracts infinite quantities from each other to obtain finites. It's sort of hocus-pocus, and always made me feel uncomfortable with quantum electrodynamics. Technically, zero point energy is the minimum energy the system can achieve, but the energy density is also technically infinite.

Just to finish my exchange with Scotchland. I'm not sure you are aware of the large number of people having proposed things to harness zero point energy in what would effectively be a perpetual motion device, violating the laws of thermodynamics. They were all shown to be crackpots, some of them quite sophisticated. There are a few hints identifying most of them. Not acknowledging obvious errors in their math is the most important, and in itself an indication one should not waste time with them.

What irks me about the affair is that no one attempted to recreate the device. If it works, then who cares? It seems like his peers cared more about their precious, admittedly well-founded preconceptions than the results.

But why should one waste time with an experiment that is based on a guy not knowing how to solve equations? If one of my students fails to solve a simple set of equations I won't give him money to run an experiment, I'll flunk him.

It would have been quite different if he arrived first with a conclusive experiment and said, «I got this funny result and we need to change our theories.» Getting conservation of energy but non-conservation of momentum would mean one needed to adjust current knowledge. The problem is that the guy doesn't get it, he keeps saying he isn't violating anything, that he can derive everything from accepted theories. This talks of ignorance or, worst, fraud. The guy can't be trusted.

He said «as a result of well known physics if I put microwaves inside this apparatus it'll start moving, now give me funding.» He was lying, theory predicts no such thing. He got his funding, made an inconclusive experiment (as expected) and said «wow, give me more funding to put this in lower temperatures, then it will work.» There are many examples of «establishment» not accepting revolutionary results, but this is something different.

[JCarter426]

i beg to differ (sort of

theoretically absolute zero and zero point energy exist as values of 0

they're just unobtainable

slight semantic difference,, sorry..

As soon as I finished saying that, I knew someone would point that out.

It's sort of hocus-pocus, and always made me feel uncomfortable with quantum electrodynamics. Technically, zero point energy is the minimum energy the system can achieve, but the energy density is also technically infinite.

Just to finish my exchange with Scotchland. I'm not sure you are aware of the large number of people having proposed things to harness zero point energy in what would effectively be a perpetual motion device, violating the laws of thermodynamics. They were all shown to be crackpots, some of them quite sophisticated. There are a few hints identifying most of them. Not acknowledging obvious errors in their math is the most important, and in itself an indication one should not waste time with them.

Agreed, but this doesn't change the validity of the phenomenon; there are devices that utilize the Casimir effect, and couldn't possibly work if it were hocus-pocus. It requires more study, certainly. There are still people who feel this way about relativity.

[Rockphed]

Sorry, no. It's like saying the freezing point of water is zero and anything colder has negative temperature. In reality, temperature has an absolute zero it can't go below (or even get as cold as). Everything else is positive temperature. Same with the energy of space; there always some there. You can reduce it but you can't make it negative. You can't even make it zero.

As other people have said, negative temperatures totally exists. Not only do they exist, but people have measured them. Also, infinite temperatures. You can go past infinity in temperature, but not zero. This implies that our measurement of temperature is actually measuring the inverse of some quantity.

[Yora]

If negative temperatures do exist, how can you not go past zero?

[Grinner]

If negative temperatures do exist, how can you not go past zero?

I get the feeling this discussion has become muddled with different systems of measurement.

Edit: A distant memory and some brief research have revealed that zero Kelvins is absolute zero, as opposed to zero degrees Celsius, which is the freezing point of water.

[Gwyn chan 'r Gwyll]

Kelvin is the truly scientific measurement of temperature. Celsius is the same, the only difference being Kelvin is measured from Absolute 0, while Celsius is measured from the freezing point of water.

Fahrenheit is totally different and confusing and not worth learning unless you're in America.

[PirateMonk]

Actually, the Planck temperature is the truly scientific measure of temperature*. It's utterly impractical, of course.

I get the feeling this discussion has become muddled with different systems of measurement.

Edit: A distant memory and some brief research have revealed that zero Kelvins is absolute zero, as opposed to zero degrees Celsius, which is the freezing point of water.

No one is talking about Celsius. Read Asta Kask's link. Calling that phenomenon negative absolute temperature seems like something of an abuse of the term, but we should probably just take the scientists' word for it.

*Edit: This sentence is not intended fully seriously.

[JCarter426]

Kelvin is the truly scientific measurement of temperature. Celsius is the same, the only difference being Kelvin is measured from Absolute 0, while Celsius is measured from the freezing point of water.

Fahrenheit is totally different and confusing and not worth learning unless you're in America.

I'd say they're both pretty worthless for different reasons. Fahrenheit is also based on the freezing point of water, incidentally - salt water, though. The freezing point of water isn't all that important in most aspects of science, rendering both Fahrenheit and Celsius problematic. Kelvin suffers from its relationship with Celsius, as well - the system of degrees they use is just not very practical in most applications, because it creates arbitrarily low numbers and requires decimals in many situations. And don't even get me started on the metric system.

[Gwyn chan 'r Gwyll]

My point about Fahrenheit and Celsius isn't that they're both based on water, that's all basically arbitrary anyways. The fact remains, however, that
a) Kelvin is the universally accepted scientific measure of temperature
b) Celsius shares a scale with Kelvin
c) The only places in the world that uses Fahrenheit is the USA, Belize, and the Cayman Isles.

All numbers on any scale is arbitrary. To me, Fahrenheit has arbitrarily high numbers in many situations. I have never seen a Celsius/Kelvin number with a decimal outside of incredibly specific science.
There are no inherent advantages or disadvantages to either scale, other than my 3 points above.

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.
Again, however, in the end, everything there is completely arbitrary. You just use whatever measuring system you grew up with. Hell, in Canada we unofficially measure PEOPLE only in feet and pounds. Why? I have no clue.

The third point for Fahrenheit is just as applicable for non-Metric systems either. Metric is the universally accepted scientific measurement system, and is used in all countries across the world, officially, except the USA, Liberia, and Myanmar. The only reason these countries haven't changed over to Metric is because, for Liberia, they're too poor for it to have a difference, Myanmar, they're too isolationist for it to have a difference, and for the USA, they're so economically huge that all the economic problems that arise from having a complete different system of measuring EVERYTHING can be successfully absorbed.

[JCarter426]

All numbers on any scale is arbitrary. To me, Fahrenheit has arbitrarily high numbers in many situations. I have never seen a Celsius/Kelvin number with a decimal outside of incredibly specific science.

I'm talking more day to day uses, such as weather. Fahrenheit is more precise. Its precision might not make a lick of sense, but it is more precise.

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.

I said don't get me started on the metric system.

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.

The only thing I hate more than the metric system is this. Well, and the number seven.

But my problem with the metric system isn't the logic - it's a fair attempt at a logical system. My problem is 10 is just a horrible, horrible, horrible number to work with. Yeah, it's "easier to remember", but it only has four divisors. It's practically a prime number. You can't divide it into thirds or even quarters without going into fractions or decimals, or rounding. And that makes it annoying to the point of useless in many applications - publishing, carpentry, cooking, just to name a few. It might work in strictly scientific settings, but it's impossible to use (for me at least) day to day.

Anyway, I agree, it's all arbitrary. But I still think both Fahrenheit and Celisus are inherently flawed. They were designed for specific things, not as universal systems of measurement. Kelvin has problems, as I said, due to its connections to Celsius; the degree system could be easily changed to make absolute zero and the freezing point of water (or some Earth temperature) align better. As it is now, Kelvin is still designed to have exactly 100 degrees between the freezing point of water and the boiling point of water, which really doesn't make any sense when both those temperatures aren't even integers on this scale.

[PirateMonk]

All numbers on any scale is arbitrary.

Right, but √(ħc⁵/(Gk²))=1 is less arbitrary than the alternatives.

[Drolyt]

I've been staying out of the temperature discussion because I'm not that knowledgable about it but... I'll go ahead and explain how I think it works.
On the Kelvin scale 0 is absolute zero, no temperature at all. In classical physics this was a real but unreachable value, while in modern, quantum physics it is believed to simply be non-physical since 0 temperature would imply 0 energy which is impossible in quantum mechanics. That said, negative temperature is possible. It has to do with what temperature is, which is kind of complicated. I won't go into that since I haven't actually figured it all out, but realize that negative temperature isn't less than 0 regular temperature, it defines an unusual variant of regular temperature. This can be observed in the fact that negative temperature doesn't necessarily mean cold. It is just like negative charge, it doesn't mean less than 0 charge, it means a different kind of charge.

[Yora]

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.
Again, however, in the end, everything there is completely arbitrary. You just use whatever measuring system you grew up with. Hell, in Canada we unofficially measure PEOPLE only in feet and pounds. Why? I have no clue.

With non-metric measurements, you can often still see it in the name, that they were not origiginally meant to be accurate measurements on a specific scale, but estimations for people who had no measuring devices. Obviously the foot, but also the mile comes from lating "thousand paces".
In a world were accurate measurement devices are everywhere, those scales are redundant.

[Form]

I've been staying out of the temperature discussion because I'm not that knowledgable about it but... I'll go ahead and explain how I think it works.
On the Kelvin scale 0 is absolute zero, no temperature at all. In classical physics this was a real but unreachable value, while in modern, quantum physics it is believed to simply be non-physical since 0 temperature would imply 0 energy which is impossible in quantum mechanics. That said, negative temperature is possible. It has to do with what temperature is, which is kind of complicated. I won't go into that since I haven't actually figured it all out, but realize that negative temperature isn't less than 0 regular temperature, it defines an unusual variant of regular temperature. This can be observed in the fact that negative temperature doesn't necessarily mean cold. It is just like negative charge, it doesn't mean less than 0 charge, it means a different kind of charge.

Statistical mechanics and thermodynamics course powers ho! The temperature of a system is defined in terms of how the energy depends on the entropy of the system. It is defined as the derivative of the energy w.r.t. the entropy. The larger the energy increase due to an increase in entropy, the higher the temperature.

When talking about negative temperature a better parameter to use is coldness or the inverse temperature. The coldness of a system is of course, by definition, related to how the entropy depends on the energy. Systems with negative coldness exist and are hotter (lower value of coldness == less cold) than systems with positive coldness. One can easily convert coldness to temperature and vice versa since they're each other's inverse, but the coldness parameter is usually more useful because the temperature can get a bit funky about 0. Anyway, systems with negative temperature are hotter than systems with positive temperature.

I'd also like to stress that 0 temperature does not mean 0 energy. The energy of the ground state of a system can be non-zero. The energy of the ground state of a harmonic oscillator, for instance, is non-zero. A system consisting of particles behaving like harmonic oscillators will have a non-zero energy at T=0. The particles will occupy the ground state, but the ground state energy is non-zero and so the energy of the system will be non-zero.

(Purely for completeness: For a system of bosons all particles will inhabit the ground state at T=0. For a sytem of fermions the situation is different.)

The above should be correct, but if someone more knowledgeable detects an error or feels the need to elaborate on any of my points they should of course feel free to do so.

[Yora]

Why is purple such a rare color in nature? There's lots of reds, browns, yellows, and greens, and even some stuff that is blue. But pink and light purple colors seem to appear only in a couple of flowers. Is there a reason there are so few substances that reflect purple light?

[Grinner]

Why is purple such a rare color in nature? There's lots of reds, browns, yellows, and greens, and even some stuff that is blue. But pink and light purple colors seem to appear only in a couple of flowers. Is there a reason there are so few substances that reflect purple light?

While a can't give a scientific perspective on it, I can tell you that violet paints are exceedingly difficult to mix by hand. So, the particular pigments required for violet must be in precise ratios.

But that's just my guess.

[Yora]

How is that? When working with pigments, it is more difficult than making green or orange? Or is it, that our minds classify shades as either blue or red, with only a small segment that we would call "purple"?

Also, what makes the flames that often come from molten metal? Is it carbon impurities that burn up?