Andy Little wrote:
I'm unclear what you mean by velocity being unitless. Do you mean that velocity is treated as a dimensionless or numeric type in the relativistic system?
It means that for the sake of computation, it carries no units and no dimensions. So, if I wanted to talk about a velocity of a particle that is moving at 70% of the speed of light, and I make the scale choice that the speed of light is 1, then the velocity is v = 0.7. There are no units to include. I agree with your point below that we need to be a little more careful about the meanings of some of the words we are using. I apologize for my tendancy to use words the way I would with other physicists without defining them for what is largely a non-physicist audience.
BTW I think I detect 2 definitions of unitless at various points in the PQS review. I think one definition that has been used is that a unitless type is a dimensionless or numeric type, while the other sees a unitless type as a dimensioned type which has the *default* units for that system. For example in the SI system a meter is 'unitless' according to this 2nd definition, whereas pi is unitless according to the first definition.
...(snip)
OK . This seems to confirm your use of the term 'unitless' as equivalent to dimensionless or numeric. If the speed of light is a numeric type in the relativistic system it leads me to wonder what math constants such as pi would mean (if anything) in this system?
Pi, and e and all the various others mean exactly the same things they have always meant, and have exactly the same values. All I'm doing is defining relationships between dimensions and selecting scales to measure those dimensions. No matter what scale I choose for the measurement, the ratio of the circumference of a circle to the diameter doesn't change. The values of things like pi and e are in some sense that I think we should avoid trying to describe in detail on this list more fundamental than any unit system.
...(snip)
Essentially the C++ representation of the dimension part of the quantity consists in an array ( such as mpl vector) of numbers representing a sequence of powers of the base dimensions. (Assume for example a system which only has the base dimension length; Then the dimension length itself would have power of 1, area power of 2 and so on ).
So, a refinement of the system described in a few TMP sources. OK, that's what I thought it would be.
The representation is acceptable if I only need to deal with one system, but if I am required to convert between systems, I need to add to the type representing a quantity, some mechanism to tell me which system my quantity is a member of . As it happens I already have the so-called abstract-quantity, which encapsulates the dimension of a quantity together with a tag so that dimensionally-equivalent quantities can be distinguished for output purposes. The abstract-quantity could be extended to include information regarding the unit system, or the abstract-quantity could be an SI-abstract-quantity (IOW in C++ concept terms the si_abstract_quantity would be a model of AbstractQuantity. I could look into how this would work. There are always costs involved though as I said in a previous post. The main problem is that it increases the size of the ball-park quite considerably to put it mildly. I would be wary of adding the functionality without adding some implementation because if I do that I can (almost) guarantee it wont fit when someone tries to implement it, and then it will tend to hang around as a sort of useless tail which no one can get rid of.
I'm going to have to think more to make any useful comments about how it could be implemented, but thank you for the description.
Given that choice of dimensional quantities, it supports scaling between different unit systems. At the moment, I think it does that scaling automagically when the values are compiled and does actual computations in SI units (Please correct me if this understanding is wrong.).
It must be wrong currently, simply because I make the assumption in PQS that there is only one unit system (the SI system), so there is no mechanism in the type to distinguish between unit-systems. (This assumes we are using the term unit-system to refer to a very similar entity of course, which we may not be).
Actually I think I am dealing with another unit system.. the natural unit system. I have assumed that it uses the same dimensions as the SI system. I have therefore expressed quantities in natural units in terms of their scaling to SI units. An energy quantity expressed in electron volts for example would be expressed as an energy unit which if it had the numeric value 1 would be equivalent to 1.602177e-19 joules. The conversion factor is encoded in a compile time entity representing the unit.
So, if I divide 1 eV by 1 s, will the answer of that computation automagically be in Watts (Energy / time = power)? Put a different way, are all calculations on that 1 eV carried out by first converting it to Joules, and then acting on the value in Joules? I again need to apologize for the sloppy language, but this is what I mean by converting to SI units. I am not concerned about your handling of SI prefixes (which I think is fine) nor do I wish to force people to explicitly select the prefix when two quantities that have prefixes but are already in SI units are used in a computation (though some people would probably like to be able to force a certain prefix under some conditions). I'm more worried about things like conversions between pounds and Newtons, or between rods and fathoms, where there is more to do than keep track of a power of 10.
I would prefer for unit conversions to only happen when explicitly requested, and for mixed unit expressions without requested conversions that match the units to produce errors.
First. the definition of what is a unit conversion is problematic. It probably needs further explanation. However some unit conversions in PQS t1_quantity are entirely lossless due to the use of logarithmic notation to express powers of 10. For example the result type of 1mm / 1 millisecond technically involves a conversion, however the conversion is entirely lossless in both runtime speed and accuracy because the calculation is actual a compile time subtraction of -3 from -3 leaving the result in units equivalent to meters per second. This process is very similar to the way one would proceed in a manual calculation. Requiring the programmer to convert millimeters to meters and milliseconds to seconds before the calculation is cruel and any good programmer will feel the ugliness of it!
So, there are a few differences between more generic systems and what you have done, but if I understand what you have done so far, they are not as big as you seem to think they are.
OK. My concern is that I can add the theoretical ability for PQS to encompass unit-systems other than the SI, but the only way to see if it actually works is to write a reasonable sized implementation for a reasonable number of systems. What you are asking for is I know very well a non trivial expansion of the scope of the project! My solution to this is not even to attempt to go there. My goal is simply to cover the SI system and let others extend the project to more generic systems.
I agree that what I'm asking for is non-trivial. And I can't promise that the increased use that would come from this expansion will justify your added time. I'm simply looking at it from the point of view of "what should it do to be the solution for my problems in my work" and adding in some of what I know is done by others in the physical sciences.
BTW Thanks for all your comments and input, and I'm sorry that I keep sounding so negative but I have good reasons for being sceptical, some of which I hope I have explained .
regards Andy Little
And thanks for your willingness to discuss them. I know that you put hours and sweat into this library, and everyone prefers to be told how well they did instead of get into long discussions of what could or should be changed, so I'm sure there are times when this discussion gets tiresome for you. Thanks for continuing to be an active part of the conversation and giving your insight as someone who has actually done the work to implement some of these ideas. John Phillips
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