Where is Everybody?

Is space travel too difficult?

There are certainly problems:

distances are huge and travel times long (but perhaps the long trips could be made easier by time dilation and "suspended animation")
the energy requirements are immense (unless a propulsion fuel can be found during the flight)
biological hazards (cosmic rays, meteor impacts)

Frank Drake and others argue that communication by radio is more cost-effective.

On the other hand, Ben Zuckerman (1985) argues that some long-lived civilizations would have to accept these dangers (and costs) and migrate to a new planetary system when their original star expands to become a red giant and then shrinks to become a white dwarf.