# Has anything in the universe ever reached 0 Kelvin?

1. Oct 21, 2013

### Remon

I was just wondering, has anything in the universe ever reached 0 Kelvin? Is it even possible for particles and sub atomic particles to completely stop moving within an object? If so, what happens then? I heard things start to get "weird"
Also, would reaching the famous "absolute zero" result in the object emitting absolutely no energy at all (so we can't see it, hear it, or even feel it)? therefore resulting in us never even knowing about it and never realizing it actually occurred? I know it's a lot of questions but the simple mind sometimes has to wonder lol

Last edited: Oct 21, 2013
2. Oct 21, 2013

### dauto

3. Oct 21, 2013

### Khashishi

Short answer is no, but a longer answer is that temperature only has meaning at a macroscopic scale, and there is no precise boundary between macroscopic and microscopic. You can have a group of particles in their ground state of energy without violating thermodynamics. But this is not the same as absolute zero. Temperature is defined in terms of a derivative of the density of states, and the derivative only exists in a continuum approximation.

4. Oct 21, 2013

### Drakkith

Staff Emeritus
First, absolute zero does not mean sub atomic particles stop moving. Absolute zero is a minimum energy state. The particles still have energy, they just can't give it up because there is no lower energy state to fall into. So in a macroscopic object that is hypothetically at absolute zero there is still plenty of internal motion.

Second, as far as I know, nothing has ever reached absolute zero nor will anything.

An object at absolute zero would cease emitting thermal radiation, but we could still feel it, touch it, and see it if light were reflecting off of it.

5. Oct 21, 2013

### Remon

How could we see it though? I mean if it gave up almost all of its energy and entropy to reach 0 Kelvin, then it still has very small amount of light energy/radiation absorbed which would result in it being almost completely invisible to an eye, infrared camera, and even telescopes that detect different forms of light, since it emits very little light. And it would like an almost perfect mirror because most of the light surrounding it is being reflected off of it

6. Oct 21, 2013

### Drakkith

Staff Emeritus
If it was at 0 kelvin it would not be emitting ANY light of ANY wavelength.

Which would mean that it would be visible similar to a mirror. Unless you mean "visible" in another context.

Also, we are bordering on breaking forum rules, as an object cannot reach absolute zero anyways, so discussion as to what it would be like it is pretty pointless.

Last edited: Oct 22, 2013
7. Oct 22, 2013

### Staff: Mentor

What?

8. Oct 22, 2013

### Drakkith

Staff Emeritus
Because if it absorbed it, then it wouldn't be at zero k anymore.
Hmm... now that I read it again, that's really confusing the way I wrote it.

9. Oct 22, 2013

### Crazymechanic

kinda confusing indeed , but we all get that time to time.
Indeed to see something means something needs to reflect atleast a bit of light or em wavelength at any other spectrum depending on what you use for " seeing" like a infrared sensor or whatnot.
But since em waves interact with matter that would mean that if they were to be shined onto the material that would have 0 kelvin after that it wouldn't have 0 kelvin anymore so there you go.

10. Oct 22, 2013

### Baluncore

If it was at absolute zero, then would it not be superconducting? That would reflect 100% of any EM radiation, and so prevent it absorbing energy from the EM radiation. Would you have to hit it with a hammer to warm it up?

11. Oct 22, 2013

### ZapperZ

Staff Emeritus
1. One can have zero resistivity but still not be a superconductor. Not all material, no matter how low of a temperature, can be a superconductor. Superconductivity is a special case, whereby one of its characteristics is having zero resistivity. There are other criteria.

2. Even for a superconductor, there is a penetration depth for EM fields. So you will have finite, non-zero absorption. This is because the field can also interact with the conduction electrons that did not form the superconducting condensate (the supercurrent).

3. Note that for a superconductor, the DC resistivity may be zero, but the AC resistivity may not! Since EM radiation has oscillating E and B fields, there can be energy being observed due to such resistivity.

Zz.

12. Jan 6, 2017

### pebbleanrock

Would light be constrained to wave energy at zero kelvin abandoning its particle nature?

13. Jan 6, 2017

### weirdoguy

There is no partice-wave duality, so your question does not make sense.

14. Jan 6, 2017

### hilbert2

Nothing in the universe has ever exactly even had a temperature at all, because it is defined only for a system that's at thermodynamical equilibrium.

15. Jan 7, 2017

### Staff: Mentor

Light is light, whatever the temperature. Also, what @weirdoguy said.

In any case, if there is light then the temperature cannot be 0K. Look up black body radiation.

16. Jan 7, 2017

### Staff: Mentor

That doesn't make sense to me. Any isolated system in internal equilibrium has a defined temperature.

17. Jan 7, 2017

### hilbert2

The idea I was after was that if you start asking whether some system has been at exactly 0 K temperature, you can also ask whether things like systems that are absolutely isolated, or are exactly at thermodynamical equilibrium have existed. In the strictest sense, a system has to contain an infinite number of microscopic mechanical degrees of freedom and be at exact equilibrium to have a well defined temperature and pressure, while some other functions of state like volume and internal energy are defined for any system regardless of its size or physical state.