# Gravity and Absolute Zero?

1. Jan 12, 2013

### Bboy Physics

Last edited: Jan 12, 2013
2. Jan 12, 2013

### Staff: Mentor

No, absolute zero is a MINIMUM ENERGY state, not a zero energy state. Even if we could get a material down to zero kelvin it would still have loads and loads of energy locked up as invariant mass, which gravity works on just fine.

3. Jan 12, 2013

### Bboy Physics

Then wouldn't with the mass energy equivalence, if E = 0, then that means either mass has to equal zero or c has to equal zero, but according to Einstein c is constant in all FoR. So that means m has to change to zero?

4. Jan 12, 2013

### VantagePoint72

As Drakkith said, E does not equal zero for a system at absolute zero, it's just at its minimum. Since the rest mass of a body, $m_0$, remains when all kinetic energy is gone, the minimum energy of a system is at least $m_0 c^2$. In a classical system, this is the energy level that would correspond to absolute zero. In a quantum system, something at absolute zero can have higher energy yet than that because of the possibility for zero point energy in some systems.

5. Jan 12, 2013

### Staff: Mentor

Why would you think that? It's the molecular kinetic energy that goes to a minimum value. Mass doesn't disappear.

6. Jan 12, 2013

### VantagePoint72

Incidentally, there are quite a few other incorrect statements in the blog post you linked to. Don't get me wrong, it's great to see a high school student who's excited about advanced physics and wants to share their knowledge—I definitely don't want to discourage that. However, it might be a bit premature to start trying to explain some of these concepts to other people, rather than focusing on your own understanding for now.

7. Jan 12, 2013

### 0xDEADBEEF

I checked your page. It seems that you have read a lot about Physics, and understand a few things well. But you don't have a good feel for how things work and fit together. So you apply formulas in places where they are inadequate. You understand the basics and then jump to the formulas, but you neglect the models that lead to the formulas.

Gravity is caused by the rest mass (and a tiny bit by the kinetic energy of a system) and affects particles with relativistic mass. Neither rest mass nor relativistic mass of matter cease to exist at zero temperature.

8. Jan 12, 2013

### F1225

I am just curiuos to know what exactly happen in absolute zero? Particle still got mass? at temperature near to 0K particle undergo phase change right? Do Bose-Einstein condensate have mass?

9. Jan 12, 2013

### Bboy Physics

Ah, this is probably what I'm looking for.

From what I understand NOW, it is not that they have zero energy(BECAUSE they have mass), but they have 0 kinetic energy.

I'm guessing that what everyone's trying to tell me is that ƩE= mc^2 and not just KE=mc^2. Makes a lot more sense now can't believe myself for being such an idiot into skipping that.

That's true, I am an amateur. Could you provide examples of such models so I could look these up?

There are a LOT of incorrect statements in the blog post that I linked to. I'm not really 'trying' to explain to other people. The abillity for other people to view my work is there, but mainly this is for me to track my own work and try to explain to myself if I really do understand this material or not. I dive into a concept of Physics, try my best, and hopefully somebody criticizes and fixes so I learn.

I thank you both for your criticism it is much appreciated.

10. Jan 12, 2013

### Staff: Mentor

I've never seen the equation written like this. Are you sure this is correct?

11. Jan 12, 2013

### VantagePoint72

Kinetic energy does not equal $mc^2$. The full formula for relativistic energy is what you have on your blog post: $E^2 = mc^2 + p^2c^2$. For massive particles, this can also be written $E = \gamma mc^2$ where $\gamma$ is the Lorentz factor that shows up all over in relativity, $\gamma = 1/\sqrt{1-\frac{v^2}{c^2}}$. The difference between $\gamma mc^2$ and $mc^2$ is what you would normally think of as kinetic energy (though in practice people will often call the whole $\gamma mc^2$ the kinetic energy, even though part of it is due to the invariant mass). If your thermodynamic system as a whole is moving (i.e. its centre of mass is moving) with some velocity according to some reference frame, then the above will just shift the minimum energy of the system in that frame. All this is to say that whether or not a system is at absolute zero is a relativistically invariant statement: all observers will agree on it. So, not only does the rest energy remain at absolute zero but the system as a whole can still have kinetic energy. In other words, if I had a thermally isolated system—some perfectly insulated container that was internally at absolute zero—then it will still be at absolute zero even if the system as a whole is moving relative to me. It's just that I would assign a different value to the minimum energy that corresponds to absolute zero (which I believe is the point 0xDEADBEEF was making, though I don't think relativistic mass is the most pedagogically useful way of discussing it). When we talk about the energy of a system at some temperature, we almost always mean "in a frame reference where the system's centre of mass is at a rest" because anything else is irrelevant. The part of gravity that couples to energy would ultimately be described in some frame by the energy of the system itself (meaning its rest mass plus its kinetic energy if the centre of mass is moving) plus its internal energy (meaning energy due to its temperature, etc.)

Last edited: Jan 12, 2013
12. Jan 12, 2013

### VantagePoint72

This is slightly off topic, but it does relate to the relationship between gravity and energy. There is one very problematic statement in your blog post:
Ignoring the slight misunderstanding about what the Eddington experiment was looking at, you're confusing the two ways in which gravity interacts with energy: energy and momentum create gravity by warping spacetime, and curved spacetime affects how things move. Light is deflected by gravity because, like everything else, it really just goes in straight lines—but curvature changes our notion of what "straight" means. Additionally, electromagnetic radiation also contributes to gravity because it carries energy and momentum. Light does indeed affect gravity, but the fact that its deflected in gravitational fields shows that gravity affects light—which is another thing altogether.

13. Jan 12, 2013

### 0xDEADBEEF

You could always get the Feynman lectures and study those or you can get another book like Tipler or Halliday but the main point is that you should also do some exercises, just to get a feeling of how a physicist or an engineer would describe a hose in the garden, a spinning top, a pendulum, a boiling pot of water. Then maybe you already have a good idea how he would describe a planet. The other theories are an extension of this basic understanding of everything, and this understanding you will get from exercises, never from reading alone.

If you can do the exercises in classical physics, you can try to learn enough to do exercises in quantum physics or relativistic physics. If you cannot do the exercises in these topics you have not understood the topic yet.

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