<Tr> <Td> Concrete (dry) </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Vacuum </Td> <Td> 0 </Td> <Td> 4π × 10 (μ) </Td> <Td> 1, exactly </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Hydrogen </Td> <Td> 3008780000000000000 ♠ − 2.2 × 10 </Td> <Td> 6994125663710000000 ♠ 1.256 6371 × 10 </Td> <Td> 7000100000000000000 ♠ 1.000 0000 </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Teflon </Td> <Td> </Td> <Td> 6994125670000000000 ♠ 1.2567 × 10 </Td> <Td> 7000100000000000000 ♠ 1.0000 </Td> <Td> </Td> <Td> </Td> </Tr>

What is the unit for mu in physics