11 Two-Way Radio Communication
For two-way communication between civilizations in different star systems, the spacing in light years should not be greater than half of the lifetime of the civilization, and should preferably be much less. In addition, exchanges with round-trip times greater than 1,000 years appear unlikely, on the basis that it is difficult to keep up a two-way conversation with such large gaps. Only with lifetimes substantially greater than ten million years is there likely to be an attempt at two-way communication.
Table 1: Likelihood of Two-way Communication
| Number of Civilizations | 33,170 | 3,317 | 332 | 33 |
| Spacing, light-years | 619 | 1,539 | 4,864 | 6,187 |
| Communication Likelihood | Low | Zero | Zero | Zero |
| Number of Civilizations | 1,000,000 | 100,000 | 10,000 | 1,000 |
| Spacing, light-years | 199 | 428 | 923 | 2,802 |
| Communication Likelihood | Fair | Low | Zero | Zero |