On 1/28/2012 3:42 PM, Russell Standish wrote:
On Sat, Jan 28, 2012 at 12:05:57PM +0100, Evgenii Rudnyi wrote:
Let us take a hard disk. Can I save more information on it at higher
or lower temperatures?
This is a strictly ambiguous question. If we take the usual meaning of
hard disk as including a particular apparatus (heads, controller
logic, SATA interface and so on) to read and write the data, then
there will be a limited range of temperatures over which that
apparatus will operate. Outside of that range, (both higher and lower)
the information storage will fall to zero. That is a purely
On the other hand, if you just gave me the metallic platter from the
hard disk, and did not restrict in any way the technology used to read
and write the data, then in principle, the higher the temperature, the
more information is capable of being encoded on the disk.
I don't think this is quite right. A higher temperature means that there are more energy
states available. But the concept of 'temperature' implies that these are occupied in a
random way (according to the micro-canonical ensemble). For us to read and write data
requires that the act of reading or writing a bit moves the distribution of states in
phase space enough that it is distinguishable from the random fluctuations due to
temperature. So if the medium is hotter, you need to use more energy to read and write a
bit. This of course runs into the problems you note below. So in practice it is often
colder systems that allow us to store more data because then we can use small energy
differences to encode bits.
In practice, various phase transitions will make this more difficult
to achieve as temperature is increased. Passing the curie point, for
instance, will mean we can no longer rely on magnetism, although
presumably even below the curie point we can increase the information
storage in some other way (eg moving atoms around by an STM) and
ignoring the ferromagnetic behaviour. By the same token, passing the
freezing and boiling points will make it even harder - but still
doable with sufficiently advanced technology.
> From an engineering viewpoint it looks a bit strange.
If engineers would take the statement "the maximum possible value
for information increases with temperature" literally, they should
operate a hard disk at higher temperatures (the higher the better
according to such a statement). Yet this does not happens. Do you
In general we are surrounded devices that store information (hard
discs, memory sticks, DVD, etc.). The information that these devices
can store, I believe, is known with accuracy to one bit.
Because they're engineered that way. It would be rather inconvenient if
one's information storage varied with temperature.
suggest a thermodynamic state which entropy gives us exactly that
amount of information?
Here would be again a question about temperature. If I operate my
memory stick in some reasonable range of temperatures, the
information it contains does not change. Yet, the entropy in my view
Sure - because they're engineered that way, and they operate a long
way from the theoretical maximum storage capability of that
matter. What's the problem with that?
So these are my doubts for which I do not see an answer.
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