# Do we have any idea what would happen at theoretical absolute zero?

• psuedoben
In summary, the conversation discusses the concept of absolute zero and its relation to an object's energy. It is mentioned that quantum effects prevent an object from having exactly zero thermal energy, and that absolute zero is an idealized limiting case that cannot be reached in practice. The conversation also clarifies that when an object has no thermal energy, it may still have other forms of energy such as mass, and that it can still provide an opposing force when struck due to the energy involved in the collision.
psuedoben
i have a simple question, but I'm not sure it can be answered with our knowledge of absolute zero: if an object were to exist at absolute zero, that means it has no energy, correct? if that is true, then if you struck something at absolute zero with another object, would the object with no energy be able to provide an equal but opposite force?

No we don't - that is a theoretical limiting value which can never be reached.
An object cannot have zero thermal energy - quantum effects stop that from happening.

Note: even objects at reasonable temperatures do not always produce and equal and opposite force on being struck.

could you elaborate on that last point you made? about there not always being an equal and opposite force? thanks!

psuedoben said:
if an object were to exist at absolute zero, that means it has no energy, correct?

Not when quantum mechanics is taken into account, no. As Simon Bridge said, quantum effects mean that no object can ever have exactly zero thermal energy. However, for many purposes the quantum effects can be ignored; that's why you will often find sources saying (incorrectly, strictly speaking) that objects at absolute zero have zero thermal energy (instead of saying that they have the minimum possible thermal energy allowed by their quantum states).

Also, as Simon said, there is no way to cool an object down to exactly absolute zero; so if we consider the properties of an object at exactly absolute zero, we are considering an idealized limiting case that can never be realized in practice. (That doesn't mean it's not a useful idealization; for many purposes it simplifies things greatly to assume an idealized object with zero temperature.)

There is also another key thing to bear in mind: when we say that an object at absolute zero has "zero energy" (subject to the corrections related to quantum mechanics, as above), we mean zero thermal energy (note that both Simon and I were careful to include that qualifier). The object can still have other kinds of energy that are not thermal: for example, the energy contained in its mass.

psuedoben said:
if you struck something at absolute zero with another object, would the object with no energy be able to provide an equal but opposite force?

The object would still be able to provide an opposing force, because the energy involved doesn't come from the object; it comes from the work being done by striking it. For example, if you hit the object with a hammer, the hammer has kinetic energy, and that kinetic energy can do work against the repulsive force between the atoms in the hammer and the atoms in the object, even if the object itself has zero thermal energy.

Last edited:
PeterDonis said:
Not when quantum mechanics is taken into account, no. As Simon Bridge said, quantum effects mean that no object can ever have exactly zero thermal energy. However, for many purposes the quantum effects can be ignored; that's why you will often find sources saying (incorrectly, strictly speaking) that objects at absolute zero have zero thermal energy (instead of saying that they have the minimum possible thermal energy allowed by their quantum states).

Also, as Simon said, there is no way to cool an object down to exactly absolute zero; so if we consider the properties of an object at exactly absolute zero, we are considered an idealized limiting case that can never be realized in practice. (That doesn't mean it's not a useful idealization; for many purposes it simplifies things greatly to assume an idealized object with zero temperature.)

There is also another key thing to bear in mind: when we say that an object at absolute zero has "zero energy" (subject to the corrections related to quantum mechanics, as above), we mean zero thermal energy (note that both Simon and I were careful to include that qualifier). The object can still have other kinds of energy that are not thermal: for example, the energy contained in its mass.
The object would still be able to provide an opposing force, because the energy involved doesn't come from the object; it comes from the work being done by striking it. For example, if you hit the object with a hammer, the hammer has kinetic energy, and that kinetic energy can do work against the repulsive force between the atoms in the hammer and the atoms in the object, even if the object itself has zero thermal energy.
Thank you! that clears a lot of things up.

## 1. What is absolute zero?

Absolute zero is the lowest possible temperature on the Kelvin scale, at which point all molecular motion stops. It is equivalent to -273.15 degrees Celsius.

## 2. What would happen at absolute zero?

If we were able to reach absolute zero, all molecular motion would stop and the entropy of a system would be at its minimum. This means that there would be no heat, no energy, and no motion in the system.

## 3. Can we ever reach absolute zero?

According to the Third Law of Thermodynamics, it is impossible to reach absolute zero. This is because it would require an infinite amount of energy to completely stop all molecular motion.

## 4. What are the potential applications of absolute zero?

At this point, the concept of absolute zero is purely theoretical and has no practical applications. However, by studying systems at extremely low temperatures, scientists can gain a better understanding of the behavior of matter and develop new technologies, such as superconductors, that can operate at low temperatures.

## 5. Could reaching absolute zero have any negative consequences?

Reaching absolute zero is currently impossible, so there are no potential negative consequences. However, if it were to ever be achieved, it could have unforeseen effects on the behavior of matter and the laws of physics as we know them.

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