I was talking over the design of such an experiment at lunch with a few fellow physicists, and we came up with a few more:
1: Put an object out in deep space, and let it radiate. If it radiates heat, it’ll get colder, but if it radiates cold, it’ll get hotter. The problem with this is that truly empty space is experimentally unachievable: If nothing else, you’ve always got the cosmic microwave background radiation, and one could argue that the CMB is adding cold to our “isolated” object.
2: As the temperature of an object increases, it’s presumably emitting less and less coldons. Eventually, you would reach some finite maximum temperature, where an object would emit zero coldons. Problem with this is that there’s no a priori way to know what this maximum temperature would be, and it might well be far outside the range of practical experimentation.
3: The one we finally settled on. If you put a test object in radiative contact with a themal resevoir, the temperature of the test object will change in discreet steps, corresponding to the absorbtion and emission of individual photons (or coldons). Suppose, for instance, that you have a cold object in a hot room. Occasionally, the cold object will make a small step downwards as it emits a photon (or absorbs a coldon), and more often, it’ll make a big step upwards as it absorbs a photon (or emits a coldon). By experiment, the size of those upward steps depends on the temperature of the walls of the room, but not on the temperature of the test object. This is consistent with the photon explanation (the hotter the walls are, the “bigger” the photons they prooduce), but not with the coldon explanation, since it would require the object radiating coldons to know its surroundings, to know how “big” the coldons it radiates should be.
It appears we are having problems with semantics.
Cold is a measurable propety called temperature.
Heat is also a measurable propety called temperature.
Heat energy is an entity which is measured in BTU’s or Calories or ?
Heat energy or thermal energy flows from a high temperature domain to a lower temperature domain.
Heat can not flow (radiate) from a low temperature domain to a higher temperature domain. Such would be a violation of the second law of thermodynamics and you could achieve perpetual motion.
How would you measure the radiation (energy flow) from the colder domain to the hotter one? Or are you making the assertion that it just does and the net effect is what matters in the long run?
And here is the problem - flow and** radiate** are not the same thing. All objects above absolute zero radiate heat. If you have a “cold domain” and a “hot domain” they both radiate heat, but the hot domain radiates more heat onto the cold domain than vice versa, so there is a net flow of heat from hot to cold. The Second Law is not broken.
Take a black 50 gallon drum of hot water and plot its cooling curve. Then do it again with a second 50 gallon drum of warm water beside it but not touching, and show that it takes longer for the hot drum to cool down. This is because the cooler object (the drum of warm water) was nevertheless radiating heat onto the warmer object (the drum of hot water). The difference in the cooling curves gives a measure of the intercepted energy radiated by the cooler object.
How does a rock at 100 deg. C “know” to radiate its heat in room at 20 deg C, but stops radiating it if you put it in a furnace pre-heated to 500 deg. C? To make this a decent thought experiment, we’ll say there’s no atmosphere in both cases and have the rock levitating without touching any walls, to eliminate conduction.
I think a lot of people here are using cold or heat in their common senses, rather than their scientific senses… heat being the tendency of something to have a relatively high temperature, cold being the tendency to have a relatively low temperature. They aren’t measurable as such, when used in the common sense, though I’ll agree that they are both underpinned by the measurable property of temperature.
matt has brought up a few good points here. I’ll say that I’m not entirely sure how to reconcile thermal radiation versus second law, except to guess that the second law is more general, as opposed to the specificity of thermal radiation, and that the second law generally seems of the type that “the net effect is what matters in the long run.”
I admit I haven’t measured the temperature increase in the sun, say, from the light of a cold emission nebula shining onto it. I’m working based on theoretical abstracts here, coupled with the certainty, as matt said, that the cold emission nebula has no way of knowing whether the sun is hotter than it is. It just radiates in all directions.
The second law states that the entropy of a closed system can’t decrease. The rule about heat not flowing from a cold to a hot object follows from that.
In fact, heat can be transferred from a cold object to a hot object via a heat pump. That’s how heat can be moved from the inside of a refrigerator to the warmer room outside. It’s just that the entropy decrease from this heat transfer is balanced by an entropy increase from the heat pumping operation - the energy degraded by your refrigerator motor-compressor.
If the Sun is able to intercept the heat radiated from a cold nebula, then the cold nebula is also able to intercept the heat radiated from the Sun - it’s a two-way path. The net energy flow is from the Sun to the cold nebula, the total entropy increases, and there is no 2nd law violation.
Consider a block of ice at 0 degrees Celsius in which has been embedded a thermometer. Put said block of ice in a copper saucepan, and place it on your stove over a high flame.
You are now dumping much heat into the ice, but until it has melted, you will notice that the temperature does not change.
Similarly, take a quantity of water at 100 degrees celsius over a hot flame. Even though you are adding much heat to the boiling water, its temperature does not increase.
This will be clear if you derive the equation for radiative heat transfer from first priciples. If you look at the final equation typically used, the temperatures of two bodies appear like (T[sub]1[/sub])[sup]4[/sup]-(T[sub]2[/sub])[sup]4[/sup] but this comes after combining two independent terms. If you want to know the radiative heat transfer to a colder body, say the one at T[sub]2[/sub], you have two terms each representing the radiation from one of the bodies. One of these terms becomes negative if you’re looking at the heat transfer in a particular direction and you get the combined (T[sub]1[/sub])[sup]4[/sup]-(T[sub]2[/sub])[sup]4[/sup]. But it’s clear that this is a net effect; the colder body is radiating energy but it’s absorbing more than it emits.
You could look at the net radiation to the hot body. This simply reverses the negative signs in the equation and, since the hot body’s temperature is higher, you get a negative result. Negative heat transfer to the hot body means a net loss from the hot body to the cold body.
This is a typical case where using only the derived equations may hamper real understanding. It’s always good to go back and look at the derivations of those equations so you understand what the terms represent and what assumptions are implicit in the result.
Good one. Out of curiosity, can this be done with today’s technology? I don’t know what the state of the art is for infrared bolometers.
Another scheme that comes to mind: Put two bolometers in far ends of a very large cryogenic vacuum chamber with low-emissivity high-reflectivity interior walls. Occasionally one of the bolometers will emit a photon or coldon, which travels to the other bolometer and gets absorbed. So temperature of one jumps down, and the other jumps up by the same amount. In theory, it should be possible to tell which occurs first. If one is emitting a coldon which is absorbed by the other, the temperature rise of one bolometer should happen before the temperature drop of the other. If it’s hotons, temperature drop would happen before the temperature rise.
scr4 said: Point a thermographic camera at a cold object, and then start accelerating towards the object.
Wow, this is pretty good. I can’t see any reason your approach wouldn’t work and let you figure out that the positive sense of the thing radiated correlates with the positive sense of hotness and the negative sense of coldness. I also think that if you figured out it was all electromagnetic radiation, you could do the same experiment with light, which might be easier (in fact astronomers do this with the earth’s orbit redshifting and blueshifting the same source at times a half year apart).
Chronos said: If you put a test object in radiative contact with a themal resevoir, the temperature of the test object will change in discreet steps, corresponding to the absorbtion and emission of individual photons (or coldons).
This should also work in principle, so hats off to your band of physicists. But not very many hats. Coldons would be awfully damn small - that is, the temperature jump associated with individual coldons would be impossibly small to measure. scr4’s approach certainly works with visible light, and to the extent that we can test relatively hotter and colder things that still radiate in the visible, we could say it’s practically useable.
spingears said:It’s fundamental physics/engineering.
Heat radiates.
There is no such thing as “COLD.” i.e. an object may be colder that another, it is a comparative term.
You and the OP need to review the fundamentals.
Wow, spingears, what made you so irritating? Look, I am a physicist and my focus right now is in heat transfer and thermal physics. Next to the bed I have two books on thermal physics, one on statistical mechanics, 3 on heat transfer, and on and on. Last night I was writing out the uncertainties in the Stefan-Boltzmann and Planck and Wien laws for a 4 hour course I am giving tomorrow morning at 8:00. And I have an international reputation for being strong on the fundamentals. I gotta say, I have almost no idea what you are getting at with the above quote. And I also think the OP was a fascinating and open-ended question and many of the replies have been ingenious. Most of the relevant things I can think of to say to you aren’t of the General Questions nature - but the most involved posters here don’t need remedial studies; rather, they could better use exactly what they are getting from each other. What an excellent discussion!
I believe that “cold” can radiate, in the sense of one’s perception of it.
To set up our experimental area, we need a large, round room with rings of couches all facing the center. Label the rings of couches A, B, C, etc., starting from the innermost.
Take an extremely cold object (which to make this observable, you’d likely have to continue to refrigerate over time and pump the heat to some other discussion) and place it in the center of the room.
The couches in ring “A” would cool rapidly, due to the large differential in temperature between themselves and the Cool-O-Matic
Those couches, having been cooled, would create a greater differential in temperature between themselves and Ring B’s couches, which would begin to cool more rapidly than they did when A was still nice and toasty.
The cooling of Ring B would create a temperature differential with Ring C, causing it to begin cooling at an accellerated rate.
The radius of the zone of coolness has increased. The cold has “radiated” out from the center of the chamber and will soon pose a danger to the city, and only a crack team of Navy Seals can stop it!
You know, if you want to feel the cold radiating off of something, you’d have to be pretty sensitive.
The total radiation of heat varies as the fourth power of absolute temperature, if it’s a blackbody or greybody (that is, if its spectral emissivity were constant over wavelength).
You could radiate into a cold object from your body temperature which is about 37 degrees C or 310 K. How much energy would this be? The same as you would receive from a somewhat warmer object, one that radiated twice the energy you do. So it’d have to have a temperature of 310 K times the fourth root of two which is 1.189, or 369 K, or 96 degrees C. That is, if there were a blackbody at absolute zero on your left, and another blackbody at 96 degrees C on your right, you’d feel as many watts per square meter leaving you on the left as you felt entering you on the right.
Bear in mind that this would be the maximum cold radiation you could ever feel, given that absolute zero and your body temperature and an emissivity of 1 are all fixed. It isn’t like radiant heat where the source could be arbitrarily hot.
This isn’t very much, eh? I mean, I don’t sense a lot of radiant heat thrown off by things at that temperature.