I'm not sure where the 6' number comes from, and I'm skeptical it really holds up as something I'd be comfortable maintaining for an extended period of time (If someone with c19 coughed at me from 6' away I would not feel very safe). I'm guessing the 6' is more like a quick rule for people who are only interacting briefly.
How much does it matter whether you're up/downwind? I've heard conflicting things about how airborne it might be.
I'm interested in this largely for "Okay, assuming we need to be careful about this for months at a time, what sort of practices could we use to maintain in-person social ties, indefinitely, without risk?" (i.e. going on long walks, visiting each other's house where 1-2 people hang out in the street or sidewalk and house denizens hang out on the porch, etc)
I'm guessing this has separate answers for "outdoor" and "indoor."
Indoors: Basically it's not safe.
Outdoors: 10 meters (30 feet) seems safe, if nobody is upwind/downwind of each other. But I'm unclear what the falloff range is.
Given various combinations of an individual patient’s physiology and environmental conditions, such as humidity and temperature, the gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet (7-8 m). [...] Eventually the cloud and its droplet payload lose momentum and coherence, and the remaining droplets within the cloud evaporate, producing residues or droplet nuclei that may stay suspended in the air for hours, following airflow patterns imposed by ventilation or climate-control systems.
So indoors basically no distance is safe, outdoors maybe 10 meters is safe if people aren't up/down wind of each other.
Ruth Backlund, a co-president at the Skagit Valley Chorale, said the group was monitoring public health guidelines at the time of the practice and had asked people to stay home if they showed even minor signs of illness. The group gathered in rows facing a piano and a choir director. They were all in individual chairs and had space to keep separated. Ms. Backlund had made sure there were extra soap dispensers in the bathrooms for people to wash their hands.
“Nobody was sick. Nobody touched anybody. Nobody shook hands. Nobody hugged everybody like you might do in a group. There was none of that,” Ms. Backlund said.
Given that they were spaced out and 1 asymptomatic person probably infected all 45 out of 60 in what must be a reasonably large room, it seems just impractical to keep sufficient distance to be safe indoors.
I'm not yet sure how related this is to Wei_Dai's answer, but found a medium article exploring "how far away to be from others who are walking in front of you". It references a couple other non-english articles, and one english... translation (I think?) on urbanphysics.
Some researchers ran computer modeling of what happens to saliva
The researchers came to this conclusion by simulating the occurrence of saliva particles of persons during movement (walking and running) and this from different positions (next to each other, diagonally behind each other and directly behind each other). Normally this type of modelling is used to improve the performance level of athletes as staying in each other air-stream is very effective. But when looking at COVID-19 the recommendation is to stay out of the slipstream according to the research.
The results of the test are made visible in a number of animations and visuals. The cloud of droplets left behind by a person is clearly visible. “People who sneeze or cough spread droplets with a bigger force, but also people who just breathe will leave particles behind”. The red dots on the image represent the biggest particles. These create the highest chance of contamination but also fall down faster. “But when running through that cloud they still can land on your clothing” according to Professor Bert Blocken.
Out of the simulations, it appears that social distancing plays less of a role for 2 people in a low wind environment when running/walking next to each other. The droplets land behind the duo. When you are positioned diagonally behind each other the risk is also smaller to catch the droplets of the lead runner. The risk of contamination is the biggest when people are just behind each other, in each other’s slipstream.
On the basis of these results the scientist advises that for walking the distance of people moving in the same direction in 1 line should be at least 4–5 meter, for running and slow biking it should be 10 meters and for hard biking at least 20 meters. Also, when passing someone it is advised to already be in different lane at a considerable distance e.g. 20 meters for biking.