>>>> Again, how can you measure a resistance without moving any current
>>>> (charge carriers) through the resistance?
>>>
>>> It doesn't stop being a resistance when there's no current flowing.
>>
>> How do you know? So far, I thought that resistance was being defined by
>> the ratio between a voltage and a current. What would be the value of
>> 0/0 (if there really was a 0)?
>
> No, resistance is intrinsic to the material. Don't confuse the thing,
> with how it's measured.  A gallon is not a container.

I don't think I confused it with how it's measured. I thought resistance
was /defined/ as the quotient of voltage and current. How would you define
resistance (in a way that is independent of current and voltage, so that
the definition works at V=I=0)?


>> I agree that every material has a resistance. But the value of it under
>> certain conditions is not known until it has been measured.
>
> Ok, but it still is resistive.. This makes as much sense as putting
> resistors in a box, and saying that they aren't resistors anymore.

They have a resistance, but of unknown value. The value that's on them is
spec'ed for certain conditions, and literally 0 current is not part of the
specs.


> Superconductors are like quantum mechanics, they don't follow the rules
> that you are used to for other things.

If that's true, then it is at least a possibility that some classic
electric concepts like resistance are not anymore useful in that realm --
in the same way as some classic mechanical concepts are not very useful in
quantum mechanics.

>> Actually, if you take it by face value, no scientist would ever say that
>> there's 0 (or infinity) of anything. In order to be able to say so, you
>> have to measure. You can't measure neither.
>
> You can measure the magnetic field produced by a current flowing. Since
> the field remains constant, you know the magnitude of the current flow,
> and that it is not changing.

Again, "not changing" is something you should be careful with if you really
want to get to the bottom of things. With any experiment, the most you can
say is that the rate of change is smaller than x, with x>0.

>> Back to superconductors: The fact that there is no potential difference
>> in the first place that makes the charge carriers move when applying a
>> magnetic field to a superconductor doesn't necessarily mean there is no
>> potential difference once they started moving.
>
> Apply ohm's law. When R is zero, E must be zero, even when I is
> non-zero.

/When/ R is 0. How do you know that it is? As a thought experiment: how
would you measure 0 Ohm? If it exists, there must be a way to measure it.
(The other question is whether Ohm's law still makes any sense in a realm
that, according to you, doesn't follow normal laws. But your assumption is
that it still makes sense. So let's work with it.)

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