Dear Shruthi,

Superfluidity is a state of matter in which it behaves like a fluid devoid of any viscosity. Discovered in 1937 by Pyotr Leonidovich Kapitsa, simultaneously with, apparently, John F. Allen and A. Don Misener, it was first described as a property of helium (at very low temperatures) enabling it to flow through the capillary channels or narrow slots without viscosity. Subsequently, the phenomenon has found applications not only in the theory of liquid helium, but also astrophysics, particle physics and quantum field theory.

Physicists mentioned above found that below the critical temperature of 2.17 kelvins (or -270.98 ° C), which is called the lambda points (λ), helium 4 was undergoing transition phase. He spent a liquid state to another with significantly different properties. Indeed, experience, confirmed later, showed that the new state of helium driving heat very well, which could be explained by a low viscosity.

More specific experiments to fluid mechanics then showed that the flow of the helium in a pipe was substantially independent of the pressure applied to the pipe walls. It was also shown that this flow was independent of the pipe section in question. This could be explained by a complete absence of viscosity, hence the name of superfluidity.

Finally, a liquid is said superfluid if it offers no resistance to flow. Accordingly, the solids which move in the liquid undergo no viscous friction.

With my best regards

Prof. Bachir ACHOUR

Shruthi,

I don't think that it is possible to distinguish between a liquid with no viscosity, and one which behaves as though it has no viscosity.

The measurement of pressure independence described by Prof. Achour is all that we have.

If I detect a chair, does that mean that I have a chair? Or just an object with chair-like properties?

Is it meaningful to distinguish  between these two things?

I personally think not.

The observation of implied zero viscosity is good enough for me - until another measurement method shows otherwise!

Prof. Achour and Dr. Garry, thank you for your answers.

Dr. Garry,

For the sake of discussion, let's say we have 2 particles- one red, and the other blue. We assume that the red and blue particles are similar in all respects except for color. If I discover what I think is a red particle, I could assume it is a red particle, or a blue particle seen under different conditions, lighting, for example, without my knowledge.

Going by that argument, I do think it would be possible to distinguish between detecting an entity and finding another entity with the same properties. 

Shruthi,

You touch on a central topic of all of physics. We are unable to directly interact with anything, except through their physical properties.

What *is* an electron? I have no idea - but I do know its mass, spin, charge, etc.

Can I *know* the electron independently of these tests? Probably not.

So at best, with your particles, I think that the only statements that can be made with certainty are that you have found a particle that appears to be red (given the limitations of the sensor, such as the lighting, in your example).

Naturally, if another test were devised that allowed one to determine colour without the use of reflected light, well, that changes everything.

A good physicist is always aware that every measurement is a measure of a property, and not a measure of the object itself.

(thank you for a meaningful and important question!)