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Hi,

I have been told that the 'background temperature' of space is 2-3 *K. How is this measured? How can a vacuum (the absence of matter) have heat? Could knowledgeable astrophysicist explain! :help:

John :happy8:

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Temperature is effectively a measure of the average kinetic energy of all the particles in a volume. A true 'vacuum' can't have a temperature as you suggest, but space has enough particles in it to have a temperature.

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I imagine there is a big difference between space and nothing I do wonder however if the space inside an atom has a temperature?

Alan

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Hi,

I guess it probably can't, because temperature is a result of the energy level of an atom/molecule. If there was nothing there, it couldn't have energy?:homework:

John

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There is also the CMB, which I suspect has the wavelength that correlates to the temperature you give of 2-3K. Even with nothing you would still have that everywhere. According to last nights talk 2.782 kelvin.

The ISS and other orbiting craft have heat shields on them and that is not to protect from heat from a dense mass of energetic particals as in 1 atm pressure, they are for radiation and that does not need matter to occupy a volume of space.

Does make the aguement of what is nothing ? Which was talked about in an Horizon program about what before the big bang.

Edited by ronin

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17 hours ago, Alien 13 said:

I imagine there is a big difference between space and nothing I do wonder however if the space inside an atom has a temperature?

Alan

Temperature is a statistical concept and does not apply in such circumstances as the space inside an atom (if such a concept makes sense). As said above it is related to the (random) kinetic energy of particles but you also have to include radiation. If various forms of matter and or radiation are sufficiently decoupled (i.e. they don't interact enough to reach thermal equilibrium) you can get several different effective temperatures in the same region e.g. electron temperature or hydrogen ion temperature etc.

Regards Andrew

Edited by andrew s
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Surely radiation (as in its energy profile) only tells you about the temperature of where it comes from, not the space it's passing through.

 

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Yes... Always used to vaguely trouble me that the solar disk (T=6000K)
supported a Corona (T=2000000K) or whatever. But the latter is very
tenuous of course. "T" is about constituents and density too, isn't it. :)

Edited by Macavity

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20 hours ago, JohnSadlerAstro said:

I have been told that the 'background temperature' of space is 2-3 *K. How is this measured?

When matter is heated it's mean kinetic energy increases. The change in velocity of the charged particles within the matter emit electromagnetic radiation as a consequence of this acceleration. This is known as thermal radiation. Electromagnetic radiation of say 3K means that the spectral density of the radiation is consistent with a black-body at a temperature of 3K. Try Googling 'Plank's law' for a better explanation.

 

1 hour ago, Stub Mandrel said:

Surely radiation (as in its energy profile) only tells you about the temperature of where it comes from, not the space it's passing through.

Yes. Think what it is like to toast yourself next to an open fire in a cold room.

 

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1 hour ago, Stub Mandrel said:

Surely radiation (as in its energy profile) only tells you about the temperature of where it comes from, not the space it's passing through.

 

Yes that's right but you only have a "temperature" when you have a system in equilibrium or at least close to it. So the CMB was emitted at and equilibrium temperature when the electrons and hydrogen ions combine to form hydrogen. The radiation from the solar surface is in equilibrium with material in the photosphere.

1 hour ago, Macavity said:

Yes... Always used to vaguely trouble me that the solar disk (T=6000K)
supported a Corona (T=2000000K) or whatever

The current idea is that energy is dumped into the  Corona via magnetically driven  waves/interactions so the temperate of the photosphere is not directly relevant.  

1 hour ago, Macavity said:

"T" is about constituents and density too, isn't it.

Not directly, you can have very high or low temperatures at high or low densities in any combination and with any combination of constituents. But, indirectly they are related  as in the ideal gas law.

Regards Andrew

Regards Andrew

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8 minutes ago, andrew s said:

Yes that's right but you only have a "temperature" when you have a system in equilibrium or at least close to it. So the CMB was emitted at and equilibrium temperature when the electrons and hydrogen ions combine to form hydrogen. The radiation from the solar surface is in equilibrium with material in the photosphere.

Exactly my point, linking back to the OP the CMB doesn't tell us anything about the temperature of the space it is now passing through.

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24 minutes ago, Stub Mandrel said:

Exactly my point, linking back to the OP the CMB doesn't tell us anything about the temperature of the space it is now passing through.

However, if you placed a test black body (say a small cube of carbon soot) in free space it would come into equilibrium with the CMB  at 2-3K and in this sense you could say it is the temperature of space. We notice this indirectly with our telescope radiate into space and cool down causing dew to form.

Regards Andrew 

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2 hours ago, andrew s said:

However, if you placed a test black body (say a small cube of carbon soot) in free space it would come into equilibrium with the CMB  at 2-3K and in this sense you could say it is the temperature of space. We notice this indirectly with our telescope radiate into space and cool down causing dew to form.

Regards Andrew 

 

It would have to be somewhere where no other source of radiation could shine on it, yet open to to the CMB...

Looking here it seems you need to be along way away from anything, anywhere within our galaxy you can only get down to 10-20 degrees above 0K.

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1 minute ago, Stub Mandrel said:

 

It would have to be somewhere where no other source of radiation could shine on it, yet open to to the CMB...

Looking here it seems you need to be along way away from anything, anywhere within our galaxy you can only get down to 10-20 degrees above 0K.

Quite right that's what I intended by free space but was too lazy to clarify. You would need to be well away from galaxies and clusters of galaxies etc.

Regards Andrew

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On 26/10/2016 at 22:10, Alien 13 said:

I imagine there is a big difference between space and nothing 

Alan

There is indeed. John Barrow's The Book of Nothing is a deftly written and unassailably reliable account of that difference. So far as we know, it seems, there is no such thing as empty space. 

Olly

Edited by ollypenrice

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When the universe was still hot enough to be ionized it was opaque and  the temperatures of its matter and its radiation were in equilibrium.

When the expansion of the universe had cooled it down enough, ions and electrons combined to form non-ionized matter and the universe became transparent. The temperature of matter and radiation became decoupled.

In the newly transparent universe the temperature of the radiation dropped, as this radiation became ever more diluted due to the expansion of the universe. This radiation is now only 2.725 degrees above absolute zero. It is the cosmic background radiation (CBR).

It is still thermal radiation. It has a black body spectrum. From the wavelength of the peak of this spectrum its temperature can be determined (Wien's law).

Locally, of course, the heat of a star makes a big difference, but my guess is that the total energy output of the stars is much smaller than the energy contained in the CBR. Strictly speaking though, the radiation of the stars should make at least some contribution to the universe's temperature, even if only a very tiny contribution.

Edited by Ruud

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Open a burning oven, point an infrared thermometer at it from a distance, and you might get a reading of say 170 celsius. This is not the temperature of air around the thermometer, it's the "black body" temperature corresponding to the electromagnetic radiation that the thermometer is detecting. Take the device into the depths of interstellar space and you'd get a reading of a little under 3 kelvin (if it can read that low). This is not the temperature of space around the thermometer, it's the cosmic microwave background temperature, coming from the edge of the observable universe. The glowing gas of the Orion nebula (or any emission nebula) has a temperature of thousands of degrees. You would freeze in it. The sparks of a sparkler firework have a temperature of over a thousand celsius but you don't get burned (unless you touch the hot metal end). Temperature is average molecular kinetic energy, heat is total energy - they're different. A bath of water at 100 celsius has a lot more heat than a thimbleful at the same temperature. The temperature of space depends where you are, how many particles are moving through it, how heavy they are and how fast they're going. It can be anything from about zero to millions (gas near a black hole).

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19 hours ago, acey said:

The sparks of a sparkler firework have a temperature of over a thousand celsius but you don't get burned (unless you touch the hot metal end)

I tried that once...never again!

John

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