In the thick wall of the vessel containing gas, a straight channel of length l is made, which connects the vessel to the vacuum space. To form a beam of molecules, the channel is equipped with two gates. Gate 1 is located at the outlet of the channel to the vessel, gate 2 - at the outlet of the channel to the vacuum space. A beam of molecules is formed as follows: first, gate 1 is opened for a time τ, then, after this gate closes, gate 2 is opened for a time t0. Molecules that have flown through the channel during this process form a beam in vacuum space. What is the length of this beam in time t after closing gate 2?