On Time Travel

Time travels raise a lot of questions (like the grandfather paradox) - political regimes banned time-travels-in-culture, because they were "irreverent" -  science says it can only go forward.

Portfolios can travel back in time. Providing historic market data are available portfolios could be valued due to "real" market data scenarios of the past, assuming that they are valued with the present models and schemes. You might be interested in return-risk profiles that your current portfolio would have had in certain periods of the history. Maybe your interest is focussed on the most turbulent periods, where markets showed unexpected anomalies. You could do a broad variety of portfolio across scenario time ride.

You know that the historical behavior and risk does not tell you anything about future risk, in the sense of forecast, but it creates insight, what might happen. And especially when driving through stress periods you might get information on the sanity of your portfolio with respect to certain risk factors.

The historical VaR, as an example, is calculated due to market date scenarios of the past. Due to the time series of given risk factors the historical change of these risk factors are calculated. By applying these historical changes to today's risk factors, the historical scenarios are generated. Then each individual instrument is valued under each historical scenario - the difference between these scenario values and today's value are the scenario deltas that are sorted and so on.

The output is a cube spanning VaRs across risk factors and instruments in the portfolio. In the UnRisk VaR Universe also the deltas are accessible for further testing and analysis. So the traveling in the past is in a hyper space of information for a better understanding of sources and impact of risk factors. The portfolio behaves like a chameleon with changing the contribution of instruments to the portfolio VaR - it is not always the same who behave badly. 

Hopefully really we are not traveling back in time, because the future is just to horrifying.