page_title: Natural time analysis
section_title: Time reversal
hierarchical_section_title: Natural time analysis / Time reversal
caption_attribution_description: English: (a) ECG in which the RR distances are marked; (b) the same ECG plotted in (a) but read in Natural Time Analysis (NTA); (c) ECG at conventional time upon time reversal; (d) ECG upon time reversal in NTA.
context_page_description: Natural time analysis is a statistical method applied to analyze complex time series and critical phenomena, based on event counts as a measure of "time" rather than the clock time. Natural time concept was introduced by P. Varotsos, N. Sarlis and E. Skordas in 2001. Natural time analysis has been primarily applied to earthquake prediction / nowcasting and secondarily to sudden cardiac death / heart failure and financial markets. Natural time characteristics are considered to be unique.
context_section_description: Time reversal, in contrast to clock time, is applicable when studying the approach of a system to criticality with natural time analysis. Living systems for example are considered to operate far from equilibrium as there is flow of energy crossing their boundaries, in contrast to deceased organisms where inner driving forces are absent. While time irreversibility is a fundamental property of a living system, the state of death is more time reversible by means of energy flow across the system's boundaries. Thus a critical state of a system can be estimated by applying natural time analysis upon calculating the entropy upon both normal time flow and time reversal and studying the difference of the two results.
caption: (a) ECG in which the RR distances are marked
   (b) the same ECG plotted in (a) but read in natural time analysis
   (c) ECG at conventional time upon time reversal
   (d) ECG upon time reversal in natural time analysis
   The length between RR distances in conventional time is approximately considered as the energy of each pulse (event) in natural time analysis.
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