frisch and peierls then considered the speed of a uranium fission chain reaction, exponential in nature, where "τ is the time required for the neutron density to multiply by a factor "e"." the available data was very approximate, but their central point – that a bomb was possible using fast (~2mev) neutrons – remains. jeremy bernstein remarked of this effort: "let me make the same point by asking a somewhat different question but using the correct numbers. how much time does it take to fission a kilogram of u using fast neutrons?" using modern values he found that to be "equal to about a microsecond, which makes the point about the rapidity of fission with fact [sic] neutrons".