Can a ratio level variable have negative values? According to Stevens (1946) ratio level variables require an absolute zero. The implication, I believe, is that 0 = an absence of the thing being measured.
As such, can ratio level scales have negative measurements? We can manipulate the measurements mathematically to get negative numbers, but can we have actual negative measurements?
An internet search results in competing arguments, and I cannot find a peer-reviewed academic work that provides an answer.
Against negative numbers:
http://www.statisticshowto.com/ratio-scale/
https://www.dummies.com/education/science/biology/levels-of-measurement-for-biostatistics-data/
For negative numbers (they all seem to use bank balances):
http://web.pdx.edu/~newsomj/pa551/lecture1.htm
https://www.spss-tutorials.com/measurement-levels/
 A: By definition, ratio scales include an absolute zero point. If it's possible to have negative quantities than it's not an absolute zero.
For example, in the physical realm, you can have 0 or 3 apples but you can't physically have -1 apples. Because the concept you are measuring is the existence of apples it's possible to use a ratio scale. 
But if the thing you were measuring is the abstract concept of possession of apples, you can have -1 apples. But this means it's no longer on the ratio scale but is instead on the interval scale. Since it's possible to owe 10 apples or 10M apples, it's no longer possible to define an absolute zero. Any choice of zero becomes arbitrary.
As another example, the Kelvin temperature system is a ratio scale and 0°K is representative of an absolute physical inexistence of kinetic energy. While -273.15°C and -459.67°F are equal to 0°K, °C/°F are considered interval scales. That's because they are actually representing the concept of existence of energy in relationship to some criteria, like the freezing temperature of water. This again makes zero an arbitrary point which means it's impossible to define an absolute zero.
Without an absolute zero, ratios of values have no meaning.
In Stevens, S.S., 1946. On the theory of scales of measurement. he says:

If, in addition, a constant can be added (or a new zero point chosen),
  it is proof positive that we are not concerned with a ratio scale. 

More importantly, though, Steven's framework is a tool for thinking, not an absolute truth. You can read about how it fails in real data in Velleman, P.F. and Wilkinson, L., 1993. Nominal, ordinal, interval, and ratio typologies are misleading. The American Statistician, 47(1), pp.65-72.:

Many of the discussions of scale types, and virtually all of the
  mathematical results, treat them as absolute categories. Data are
  expected to fit into one or another of the categories. A failure to
  attain one level of measurement is taken as a demotion to the next
  level. However, real data do not follow the requirements of many scale
  types.

