# Return to Answer

 2 added 653 characters in body edited Dec 3 '18 at 10:47 Emilio Pisanty 88.9k2323 gold badges226226 silver badges462462 bronze badges The existing answer by flippiefanus is on the right track, but there is no "perhaps" to the thing. The second is definitely on track to be redefined, at some point in the future; it's not so much a question of "if" but of "when" and "to what". This is because optical frequency standards are already better than microwave caesium-based standards, and they are improving at a much faster rate, with no real obstacles foreseen to substantial progress at a similar pace for quite some time. The BIPM has an excellent graphical depiction of the situation: Image source As you can see, there are already multiple optical-clock frequency standards whose precision exceeds that allowed by the fundamental limitations of the SI second as currently defined (i.e. the blue dots). Thus, those experiments are able to e.g. measure frequency ratios to a higher precision than allowed by the SI second, but they cannot quote absolute frequencies because the caesium standard they'd need to compare to just isn't precise enough. (This is analogous to the situation with the meter just before it was redefined to set $$c$$ to a fixed value: we had extremely precise measurements of e.g. the distance to the Moon, but the dominating uncertainty on those was the uncertainty in $$c$$ itself.) So, this will need to be fixed at some point. However, the optical-clock technology is still developing, so we don't quite know just yet what the optimal techniques, platforms and media will be, and it isn't clear which atom or ion is best for the role of standard reference. Thus, it will take a fair few years of scientific and technological exploration to have a better understanding of that landscape, followed by some time to build up a consensus about which system is best, before a redefinition can be put in place. As for the broader question for the rest of the SI units: who knows! It's entirely possible that improvements in technology will lead to a similar replacement of a mise en pratique or a constant, though it's hard to see what could change given the general nature of the other six constants (versus the particular nature of $$\Delta\nu_\mathrm{Cs}$$). Similarly, it's entirely possible that at some point we'll need to do an overhaul of this magnitude, given a sufficient advance in both our technical and conceptual standpoints. However, beyond the change to optical clocks, no further changes to the SI are foreseeable at present. The existing answer by flippiefanus is on the right track, but there is no "perhaps" to the thing. The second is definitely on track to be redefined, at some point in the future; it's not so much a question of "if" but of "when" and "to what". This is because optical frequency standards are already better than microwave caesium-based standards, and they are improving at a much faster rate, with no real obstacles foreseen to substantial progress at a similar pace for quite some time. The BIPM has an excellent graphical depiction of the situation: Image source As you can see, there are already multiple optical-clock frequency standards whose precision exceeds that allowed by the fundamental limitations of the SI second as currently defined (i.e. the blue dots). Thus, those experiments are able to e.g. measure frequency ratios to a higher precision than allowed by the SI second, but they cannot quote absolute frequencies because the caesium standard they'd need to compare to just isn't precise enough. (This is analogous to the situation with the meter just before it was redefined to set $$c$$ to a fixed value: we had extremely precise measurements of e.g. the distance to the Moon, but the dominating uncertainty on those was the uncertainty in $$c$$ itself.) So, this will need to be fixed at some point. However, the optical-clock technology is still developing, so we don't quite know just yet what the optimal techniques, platforms and media will be, and it isn't clear which atom or ion is best for the role of standard reference. Thus, it will take a fair few years of scientific and technological exploration to have a better understanding of that landscape, followed by some time to build up a consensus about which system is best, before a redefinition can be put in place. The existing answer by flippiefanus is on the right track, but there is no "perhaps" to the thing. The second is definitely on track to be redefined, at some point in the future; it's not so much a question of "if" but of "when" and "to what". This is because optical frequency standards are already better than microwave caesium-based standards, and they are improving at a much faster rate, with no real obstacles foreseen to substantial progress at a similar pace for quite some time. The BIPM has an excellent graphical depiction of the situation: Image source As you can see, there are already multiple optical-clock frequency standards whose precision exceeds that allowed by the fundamental limitations of the SI second as currently defined (i.e. the blue dots). Thus, those experiments are able to e.g. measure frequency ratios to a higher precision than allowed by the SI second, but they cannot quote absolute frequencies because the caesium standard they'd need to compare to just isn't precise enough. (This is analogous to the situation with the meter just before it was redefined to set $$c$$ to a fixed value: we had extremely precise measurements of e.g. the distance to the Moon, but the dominating uncertainty on those was the uncertainty in $$c$$ itself.) So, this will need to be fixed at some point. However, the optical-clock technology is still developing, so we don't quite know just yet what the optimal techniques, platforms and media will be, and it isn't clear which atom or ion is best for the role of standard reference. Thus, it will take a fair few years of scientific and technological exploration to have a better understanding of that landscape, followed by some time to build up a consensus about which system is best, before a redefinition can be put in place. As for the broader question for the rest of the SI units: who knows! It's entirely possible that improvements in technology will lead to a similar replacement of a mise en pratique or a constant, though it's hard to see what could change given the general nature of the other six constants (versus the particular nature of $$\Delta\nu_\mathrm{Cs}$$). Similarly, it's entirely possible that at some point we'll need to do an overhaul of this magnitude, given a sufficient advance in both our technical and conceptual standpoints. However, beyond the change to optical clocks, no further changes to the SI are foreseeable at present. 1 answered Dec 3 '18 at 10:26 Emilio Pisanty 88.9k2323 gold badges226226 silver badges462462 bronze badges The existing answer by flippiefanus is on the right track, but there is no "perhaps" to the thing. The second is definitely on track to be redefined, at some point in the future; it's not so much a question of "if" but of "when" and "to what". This is because optical frequency standards are already better than microwave caesium-based standards, and they are improving at a much faster rate, with no real obstacles foreseen to substantial progress at a similar pace for quite some time. The BIPM has an excellent graphical depiction of the situation: Image source As you can see, there are already multiple optical-clock frequency standards whose precision exceeds that allowed by the fundamental limitations of the SI second as currently defined (i.e. the blue dots). Thus, those experiments are able to e.g. measure frequency ratios to a higher precision than allowed by the SI second, but they cannot quote absolute frequencies because the caesium standard they'd need to compare to just isn't precise enough. (This is analogous to the situation with the meter just before it was redefined to set $$c$$ to a fixed value: we had extremely precise measurements of e.g. the distance to the Moon, but the dominating uncertainty on those was the uncertainty in $$c$$ itself.) So, this will need to be fixed at some point. However, the optical-clock technology is still developing, so we don't quite know just yet what the optimal techniques, platforms and media will be, and it isn't clear which atom or ion is best for the role of standard reference. Thus, it will take a fair few years of scientific and technological exploration to have a better understanding of that landscape, followed by some time to build up a consensus about which system is best, before a redefinition can be put in place.