I understand the basic idea of viral plaque assay, but I don't understand how the virus spreads so quickly in the culture.
For example, at 1:30 in an educational video about the subject, you can see various viral plaques all growing from single infection points and then multiplying to cover a circular area as the new virions spread outwards.
However, if you think about it, we know the virus cannot move, so when the new virions bud they can only infect adjacent cells (or bacteria). So the spread is limited to whatever cells are immediately adjacent to the infected cell(s).
So, let's say a virus is 100nm in diameter and replicates every 8 hours. In 4 weeks there would be 30 days or 90 replication cycles. Let's imagine that a virus can "reach" things 5 times its body diameter when it buds, so that is things 5 x 100nm = 500nm away. If we have 90 replication cycles then the total radius of the viral plaque could be at most 500nm * 90 = 45,000nm or 45 micrometers, which is a speck invisible to the naked eye.
However, when we look at the plaques in the video we can see they are 5-7mm in diameter and are very visible to the naked eye. So, how could the virus be traveling this far?