# Cold vs hot on the atomic scale

• B
• Fishpig
In summary: In fact it would be quite inaccurate to say that a photon in a vacuum is the hottest particle because it is in turn moving at the fastest speed possible.
Fishpig
TL;DR Summary
slowest speed vs the fastest speed and temperature
So I've been hunting google for an answer but i cannot find a definitive one.

If 0 Kelvin is the coldest temperature and it is where atoms cease to move does this mean that a photon in a vacuum is the hottest particle because it is in turn moving at the fastest speed possible?

Second question is with cooling down the atoms they use a laser to push away the warmer particles until it's just the cold ones, so is it really "cooling" or are they just slowing them down? yes i do realize that it's both but it should be one or the other.

Thanks smart people.

Fishpig said:
If 0 Kelvin is the coldest temperature and it is where atoms cease to move does this mean that a photon in a vacuum is the hottest particle because it is in turn moving at the fastest speed possible?
Temperature is a collective phenomenon. We cannot talk the temperature of one atom or one photon.

For a gas of photons, it is not their speed that is related to temperature, since they always move at ##c##. What we have is a distribution of the photons' energy (or frequency, or wavelength) that depends on temperature.

Fishpig said:
Second question is with cooling down the atoms they use a laser to push away the warmer particles until it's just the cold ones, so is it really "cooling" or are they just slowing them down? yes i do realize that it's both but it should be one or the other.
In laser cooling the atoms are slowed down. No experiment is perfect, but the cooling results from the slowing down, not from the loss of hot atoms.

However, there is a limit to the temperature that can be achieved by laser cooling (of the order of 1-10 μK), such that in experiments where lower temperatures are needed, for instance to achieve Bose-Einstein condensation, after the laser cooling phase there is an evaporative cooling phase, where cooling is obtained by letting the hottest atoms escape.

PeroK, Dale and Fishpig
That makes sense thank you, the laser is really just 1 part which i did know i just didn't know the order.
This site is brilliant because i have so many questions that i never knew who to ask.

berkeman, Dale and DrClaude
DrClaude said:
Temperature is a collective phenomenon. We cannot talk the temperature of one atom or one photon.
In principle that's right, but it can make well sense to consider the temperature of a single atom when it is coupled to some "heat bath", i.e., as an open quantum system. Then you expect the atomic states being Maxwell-Boltzmann distributed with the temperature given by the temperature of the heat bath.

vanhees71 said:
In principle that's right, but it can make well sense to consider the temperature of a single atom when it is coupled to some "heat bath", i.e., as an open quantum system. Then you expect the atomic states being Maxwell-Boltzmann distributed with the temperature given by the temperature of the heat bath.
I agree. I should have said that "We cannot talk the temperature of one atom or one photon in isolation."

I think however that it is important in the context of the OP to say that when an atom ceases to move, it does not mean that it is a zero temperature.

vanhees71 and Fishpig

## 1. What is the difference between cold and hot on the atomic scale?

The main difference between cold and hot on the atomic scale is the amount of thermal energy present in the atoms. Cold atoms have lower thermal energy, meaning they are moving slower and have less kinetic energy, while hot atoms have higher thermal energy and are moving faster.

## 2. How does temperature affect the behavior of atoms on the atomic scale?

Temperature plays a crucial role in determining the behavior of atoms on the atomic scale. At higher temperatures, atoms have more kinetic energy and are more likely to collide and react with each other. At lower temperatures, atoms have less energy and are less likely to interact with each other.

## 3. Can atoms be both hot and cold on the atomic scale?

Yes, atoms can have varying levels of thermal energy, so they can be both hot and cold on the atomic scale. This is because temperature is a relative measurement and what may be considered hot or cold for one substance may not be the same for another.

## 4. How does the atomic scale differ from the human scale when it comes to hot and cold?

On the atomic scale, hot and cold refer to the amount of thermal energy present in individual atoms. On the human scale, hot and cold refer to the temperature of a larger object or environment. While the concepts are related, they are not the same.

## 5. Can temperature on the atomic scale be measured?

Yes, temperature on the atomic scale can be measured using specialized tools and techniques such as laser cooling or electron microscopy. These methods allow scientists to directly observe and measure the movement and behavior of atoms at different temperatures.

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