Unemployment solved: 20-hour work week

While watching yet another presidential candidate promise how he would stimulate the economy to create more jobs, I found myself wondering why nobody is talking about an obvious solution: restricting the supply of labor by cutting the working hours.
 
Given the current productivity levels and 5 billion people of working age on the planet, it does not seem possible to give everyone a 40-hours/week job doing something useful.  There is a precedent for reducing working hours.  We went from 70 or more to 40 a while ago.  We also eliminated child labor.  Was it a coincidence that real wages went up and the middle class got created since then? 

Why is 20-hour work week a bad idea, or a good one?

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>>28
… it would also reduce people's pay by 20%

It’s highly unlikely that if you significantly reduce working hours across the board, the hourly rates would remain the same. Remember, this is still a market economy.  The price of anything (including labor) is determined by supply and demand.  If the pool of available labor is reduced by 50%, what do you think will happen with the price of labor (your hourly wage)?  It will go up!

And even if the increase in the hourly wages does not completely make up the difference, what do you think will happen with prices?  Let’s say your apartment lease is up for renewal.  Your landlord sees that your (and, importantly, all other potential tenants’) income is down by 30%.  Do you think the landlord will keep the rent the same and risk having an empty apartment or will s/he reduce the rent? Sellers of other goods and services will face the same choice.  Competition is a wonderful thing.