The NIST Reference on Constants, Units and Uncertainty

Fundamental Physical Constants
Constants
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Constants,
Units &
Uncertainty
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natural unit of time
 $\hbar/m_{\rm e}c^2$
 Value  1.288 088 667 12 x 10-21 s
 Standard uncertainty  0.000 000 000 58 x 10-21 s
  Relative standard uncertainty   4.5 x 10-10
 Concise form  1.288 088 667 12(58) x 10-21 s    
 
Click here for correlation coefficient of this constant with other constants
Source: 2014 CODATA     
recommended values
 
Definition of
uncertainty
 
     Correlation coefficient with
any other constant

 
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