A means of effective time travel has been invented. People can freely traverse time, travelling from the present to any point in the past and vice-versa. Access to time travel is pretty universal, and due to this, knowledge of conditions at all points in time is acute.
For those who believe there is a moral obligation on the part of the better-off to help the less well-off, and who believe in redistributive policies to do this, play along and consider the situation.
People in the present are outrageously wealthy compared to people in the past. Even the poorest Americans today have access to abundant clean water, hot and cold water, heated shelter, air conditioning, an overabundance of cheap, calorie-rich food, more clothing than they need, refrigeration, telephony, transport by internal-combustion engine, laundry facilities, bathing facilities, vaccinations, pharmaceuticals, emergency care, and on and on. These present poor are better off by almost any measure than even the wealthy a thousand years ago.
Do people in the present have an obligation to give some of their wealth to those in the past? Is there some minimum standard of living that we need to keep the ancients up to? Do the poor among the rich (present day poor Americans) have an obligation to the rich among the poor (the well-off a millennium ago)?
What kind of redistributive policies should be enacted? Would they work? What might some side-effects be? Is it required to fulfill a moral duty? Is it wrong for someone born in the present to enjoy the relative luxury and wealth they are inheriting from their era, by no merit on their part? Should they pay an inheritance tax to support people in the poorer past?
What about future generations. What if the future is also poorer; does the present owe them a chunk of our wealth? What would be the result of efforts to redistribute from the present to the future? What if the future was wealthier; do they owe the past a portion of their bounty? What would happen if resources flowed to us from the future, in order to ease our relatively lower condition?
Spanning all of human history, would we have a moral obligation to attempt to make all people across all eras more equal? Would we be obligated to narrow the gap between the caveman and the flying-car-owning future woman? How big could we let the gap be? Would narrowing it be possible? Would there be any side-effects of efforts to try?
What is the difference, morally and practically, between redistribution across time and that across space?