There's no theoretical problem if we define 86400 seconds = 1000000 newseconds.
One big practical problem is that because SI is a coherent system, the second is embedded in many units. For example, 1 N = 1 kg m/s^2. 1 J = 1 N m. 1 V = 1 J/C. 1 Pa = 1 N/m^2. And so on and so forth. So all of those units will have to be replaced with units derived from the newsecond. This is kind of like how SI has the unit tesla but the CGS electromagnetic unit is gauss.
A long-term problem is that no matter what, the length of the day on Earth will drift from scientifically accurate atomic time. Sometime in the near future, a day will be 86401 seconds. Then 86402, and so on. So the old second or the new second will not solve this problem.
> A long-term problem is that no matter what, the length of the day on Earth will drift from scientifically accurate atomic time. Sometime in the near future, a day will be 86401 seconds. Then 86402, and so on. So the old second or the new second will not solve this problem.
How long term is this problem? For instance, we habitually insert "leap seconds" in order to keep things lined up, but if we didn't insert leap seconds, we might accumulate an error of maybe half an hour between the solar meridian and "noon" in the next 500 years, which is less error than we introduce by putting Madrid and Belgrade in the same time zone. And in 500 years, most of humanity will not even be living on the Earth anyway.
One big practical problem is that because SI is a coherent system, the second is embedded in many units. For example, 1 N = 1 kg m/s^2. 1 J = 1 N m. 1 V = 1 J/C. 1 Pa = 1 N/m^2. And so on and so forth. So all of those units will have to be replaced with units derived from the newsecond. This is kind of like how SI has the unit tesla but the CGS electromagnetic unit is gauss.
A long-term problem is that no matter what, the length of the day on Earth will drift from scientifically accurate atomic time. Sometime in the near future, a day will be 86401 seconds. Then 86402, and so on. So the old second or the new second will not solve this problem.