# Will an object decrease in temperature in a perfect vacuum?

1. Nov 16, 2014

### k9b4

It is my understanding that on earth, a hotter object surrounded by colder air will decrease in temperature because the faster moving atoms in the hotter object are repelled by the slower moving atoms in the colder air. This isn't worded very well, I hope you know what I mean.

By what mechanism does an object decrease in temperature in a vacuum, if it does so at all?

Last edited: Nov 16, 2014
2. Nov 16, 2014

### NTW

By emitting electromagnetic radiation to colder surroundings, or to a 'sink' such as interstellar space...

3. Nov 16, 2014

### k9b4

Why would that decrease the temperature of the object?

Does emission of EM radiation reduce the temperature of the object emitting it? If so, why?

Last edited: Nov 16, 2014
4. Nov 16, 2014

### NTW

Because the emitted radiation has an energy that the object loses. Thus, its temperature goes down...

5. Nov 16, 2014

### k9b4

Why? What mechanism causes its temperature to go down? Energy is a concept. Energy does not exist the same way that an electron exists.

6. Nov 16, 2014

### rtsswmdktbmhw

Temperature is dependent on the kinetic energy of the particles in the object. So if the object loses energy, temperature decreases accordingly.

7. Nov 16, 2014

### k9b4

I know. You're repeating what the other guy said and not answering my question.

What physical mechanism causes loss of temperature by emission of EM radiation?

8. Nov 16, 2014

### davenn

Again ... Its the loss of energy ... primarily in the Infra-red spectrum

9. Nov 16, 2014

### k9b4

Energy does not exist in the same way that an electron exists. Energy is a concept, not a thing.

10. Nov 16, 2014

### davenn

put your hand above a flame till it gets burnt, then tell me its a concept not a thing

11. Nov 16, 2014

### k9b4

While I do not understand how a flame works, I can say with confidence that your hand gets burnt because the air molecules between your hand and the flame moves faster. You have sensors in your hand which detect this faster movement of air molecules (which we perceive as heat).

12. Nov 16, 2014

### davenn

That is only part of the heat transfer, the rest is IR EM radiation

13. Nov 16, 2014

### davenn

if the heat source is in a vacuum then your statement wont hold, there are NO air or other gas molecules to transfer any heat (energy)
the ONLY transfer will be by IR radiation

edit
example ....
think of a spaceship or an astronaut on a spacewalk at the ISS the heat he feels from the Sun is purely by IR EM radiation. There is nothing else ( no other medium) by which heat energy can be transferred to him

14. Nov 16, 2014

### k9b4

I know that EM radiation can heat things up, and I understand the mechanism for this, but my question is why does an object emitting EM radiation cool down?

15. Nov 16, 2014

### NTW

Imagine a block of solid metal placed in outer space, away from any stars, and with a given, initial temperature. The block has a thermal energy because of the vibrational status of its atoms. Now, the block, as everything with a temperature above absolute zero, emits EM radiation and will gradually cool down, because the energy lost by that radiation results in a lower vibration of the atoms, i.e. a lower thermal energy of the block.

The block won't receive any EM radiation to compensate the lost energy, because that 'outer space' can be considered as an 'absolute sink', and its temperature will reach absolute zero when the last trace of vibrational energy is emitted as EM radiation.

16. Nov 16, 2014

### Bandersnatch

It's the same process, but in reverse. If you understand one you should understand the other.

One of the problems we have with answering is that we don't know exactly you level of understanding.

Anyway, let me try and take it from the bottom. (all links to wikipedia)

Any object made of atoms that is not at absolute zero (which means every object) emits thermal radiation.

The emission can be understood as follows:
Atoms are made of charged protons and electrons. Since temperature has the meaning of average kinetic energy, all atoms are constantly moving and/or oscilating. Even the hardest, coldest steel has got its atoms vigorously vibrating in its crystal lattice. It is important to note here that while from far away atoms are often electrically neutral, in the lowest of scales they can be no longer treated as such, as the distribution of charges between the nucleus and the orbitals becomes significant. Same thing with molecules, only on a bit larger scale.

Any acceleration of a charge means that the electric field surrounding it is not static - it changes with time. This creates a coupled magnetic field which in turn affects the electric field. The two oscillate in a characteristic fashion and the oscillations propagate through the electromagnetic field as electromagnetic radiation - light.

As anyone who has ever tried to spin an elecric generator (e.g. a bicycle dynamo) can tell, charges resist motion in magnetic field. It takes energy to make electrons move through the wires of the generator as a current.
Similarly, moving in the magnetic field created by their own motion as charged particles slows down atoms.
This slowing down translates to lowering of temperature.

So, to summarize:
-temperature means motion of particles (fas=hot)
-particles are charged (protons and electrons)
-moving charges create disturbances in the EM field (radiation)
-this slows down their motion (->cold)

Note, that there can be other processes that emit EM radiation. Fusion in stars is one of them.

Additionally, to explain why you got the "it's energy!" answers.
At some point trying to visualise how things work, especially in the micro-scale, by using macro-scale analogies stops making sense. When you get down to it, everything you see in the world of quantum interactions and elementary particles is just a set of properties that you've ascertained the system to have.
For example, you say electron "exists" in a more definite way than energy. But the electron is just a set of properties, like its charge, mass, typical behaviour in various circumstances, etc. This includes the property called energy. It's no less real nor more of an abstract concept than mass is.
One can try and visualise particles bouncing off each other like tiny billiards balls, or light waves being akin to waves in water, but the real understanding comes not from how you imagine things to behave using crude analogies from our everyday lives, but from knowing the properties and interaction (this is especially a big issue when learning quantum mechanics).
So saying that it takes energy to emit radiation and it has to come from somewhere is describing the situation in more precise and useful terms than painting a vivid picture.

17. Nov 16, 2014

### k9b4

Hmm, so a single charge in a perfect vacuum which is oscillating will eventually stop oscillating because it is acted upon by its own EM field?
An electron does exist in a more definite way than energy does. An electron has an effect on reality. Energy does not have an effect on reality. Energy is only a concept which is helpful in explaining other concepts. Same with force. Force does not exist in reality. It is a concept only. You could argue that EM field exists and it is a force therefore force exists, but this is wrong, because EM field is not force, it is EM field, it causes 'force' on charged particles.

18. Nov 16, 2014

### rtsswmdktbmhw

How would you explain the mechanism for heating up via radiation (in a vacuum)?

19. Nov 16, 2014

### k9b4

Two charges in a vacuum. One is completely still (no kinetic energy), the other is oscillating (it has some kinetic energy). The EM field created by the oscillating charge pushes the non-oscillating charge around, thus causing the previously non-oscillating charge to oscillate, thus it now has a 'temperature'.

20. Nov 16, 2014

### davenn

Yes

My question to you is ....
What are the 3 methods of heat transfer ?