# Does heat have mass?

1. Jun 7, 2011

### brollysan

Hi guys just something I am a bit curious about, if heat is a form of energy, doesn't it follow from the mass-energy equivalence that heat energy has mass, however small? Then does this mean that heat energy has gravity/can be affected by gravity?

I am curious about energy in general but heat was a bit harder to imagine, do the photons that carry thermal/EM energy from the sun to us have mass then?

2. Jun 7, 2011

### Staff: Mentor

I believe you are correct. Everything I've read says that increasing energy of a system increases its mass. According to my knowledge, all EM radiation has momentum and according to GR will have gravitation.

3. Jun 7, 2011

### chroot

Staff Emeritus
Yes, energy causes exactly the same gravitational effects as an equivalent amount of mass. The equations of general relativity include a mathematical object called the "stress-energy tensor" which encapsulates both mass and energy in the same way. Light does indeed produce its own (very meager) gravitational field.

- Warren

4. Jun 7, 2011

### khemist

"Heat" is a measure of the kinetic energy of a system. Kinetic energy is a function of momentum, which will increase as the velocity increases. "Heat energy" again is just a measure of the kinetic energy of a system, so it is not necessarily carried by photons (rather by the interaction of molecules/atoms). Photons carry energy (They are little packets of energy), and this can be turned into kinetic energy (aka heat) by absorbtion.

When EM radiation is emitted from a hot object, it is doing so because of the atomic transitions that are going on within the object. As the object cools, the atoms go from an excited to ground state, and there is an emission of a photon because of that.

In short, I do not believe that heat is carried by photons, simply because the definition of heat is dependent on the kinetic energy of a particle, and the kinetic energy of a photon is zero (but their momentum is a finite number, dependent on the wavelength/ frequency). However, the energy that is carried in the EM wave CAN be CONVERTED into heat.

I am not 100% of what I just claimed but I hope it makes sense...

5. Jun 7, 2011

### cosmik debris

Heat doesn't have mass but it contributes to the mass of matter by adding energy to it. The photons that carry heat energy gravitate and are affected by gravity, they do not have mass.

6. Jun 7, 2011

### Staff: Mentor

I don't see the distinction.

7. Jun 7, 2011

### Staff: Mentor

The term heat has various meanings, but I think we can all agree that the energy of a system which determines its heat and temperature and all that does have mass. A hotter object has more energy in it and is therefore more massive than a similar system which is at a cooler temperature.

8. Jun 7, 2011

### khemist

Agreed, but the "heat" itself does not have mass. The energy added to increase the temperature increases the mass of the object.

Heat energy as a stand alone product does not exist as far as I understand. The only way energy can be called heat is when discussing temperature, kinetic energy of the molecules, or a transfer of that kinetic energy.

9. Jun 7, 2011

### Staff: Mentor

Isn't that what I just said?

10. Jun 7, 2011

### khemist

Upon rereading it appears so, though the distinction that the energy that is called heat does not have mass doesnt seem explicit.

11. Jun 7, 2011

### WannabeNewton

Energy density effectively curves space - time as per the energy momentum tensor.

12. Jun 7, 2011

### davenn

and all the msgs from others since....

OK Im totally confused. how can increasing energy of a system by heating it, increase its mass ?

take an enclosed container of whatever... liquid , gas... its a fixed amount
you heat it over a flame... that heat gets the atoms in the container material in motion and they (according to Feynman) start bouncing off each other and heat is produced. That heat is then transferred to the contents of the container and the atoms of that liquid/gas get active and start bouncing off each other and the walls of the container and so heat inside the container is produced.

Assuming I havent totally screwed up there (always possible ;) ) you have a fixed amount of gas etc in the container .. HOW can it increase in mass ? its a fixed amount

Dave

Last edited: Jun 7, 2011
13. Jun 7, 2011

### davenn

Now I can accept that in the case of physically bombarding a material with particles... electrons, photons etc then yes you are adding physical mass to the original material.

But in my understanding, that isnt happening with just pure heating ... radiated heat

Dave

14. Jun 7, 2011

### Staff: Mentor

I believe it is shown in E=MC^2

15. Jun 7, 2011

### Staff: Mentor

If you do the calculations you will find that it takes more force to accelerate a box containing a hot gas than it does to accelerate the same box containing the same amount of cold gas. By f=ma since a greater f is required for the same a that means that m is greater.

16. Jun 7, 2011

### davenn

hey Dale,

I cant accept that on face value, I would have to see the working for that. As it doesnt answer my basic question. How can a fixed amount of matter be more massive just becauser its hotter? there have been no physical particles added to increase its mass, where's the extra mass coming from ?

posted in the interests of learning :)

Dave

17. Jun 7, 2011

### hillzagold

E=MC^2, so heating up your gas by the degree you're imagining will increase mass by that ammount, divided by the speed of light squared. It's a ridiculously tiny amount of mass, and I don't think you're used to thinking in such a small scale.

18. Jun 7, 2011

### Staff: Mentor

Sorry that you can't accept that. I don't have the time to work it out this week, so if you really want to see it you will have to work it yourself. It is tedious but not that difficult, simply calculate the weight of a box of ideal gas and see that it depends on the temperature.

I think you misunderstand mass. Mass is a property that material objects have, but that doesn't mean that mass is only associated with matter. Similarly with energy, energy is a property that material objects have, but non-material things such as fields and photons also have energy. You can add energy without adding matter, similarly you can add mass without adding matter.

Did you realize that most of the mass of ordinary matter is not in the matter (fermions) itself but in the forces (bosons) that hold the matter together? That is one reason why nuclear reactions can convert mass into energy.

19. Jun 7, 2011

### davenn

no thats not the problem

Dave

20. Jun 8, 2011

### davenn

dunno where to start on that one ;)

yes probably ;) ... ok I understand photons having more energy than others but are still just a photon

visible light photon has more energy than a microwave one, a gamma ray photon more energy than a visible light photon.

ok, but in any given atom all those things are present within the atomic structure?
what is being added to change the mass of that structure ?

ok a physics lesson... so you are saying its the Bosonic force that holds the atoms of a particular element together ?

Crap ;) we are getting in much deeper than I ever did in 1st year university physics so so long ago :)

cheers
Dave

PS ok taking the E=MC^2 if the amount of energy in a system increases, as the speed of light is a constant, the only thing left means that the mass has increased. thats logical.

still leaves me trying to understand where the additional mass has come from in a closed system

Last edited: Jun 8, 2011