3 ms·

Okay, let's do some math. Very rough napkin numbers, but it'll get us close. A 150Hz blink rate means a period of 1/150s, or ~6.7ms. The speed of radio signal

Okay, let's do some math. Very rough napkin numbers, but it'll get us close. A 150Hz blink rate means a period of 1/150s, or ~6.7ms. The speed of radio signals through air near the surface is a bit lower than c, let's call it 299,250 km/s. [0] When you multiply those numbers you get ~1500km. That means each 'on' and 'off' signal is 1500km long, moving near c. More specifically, the signals are concentric rings around the transmitter 1500km in width. Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2. Assume the transmitter is in the center of the perfectly circular city, that means it takes 3.3ms for each 'on' and 'off' signal to cover the entire city. Each on and off pulse is just about exactly long enough to cover the entire city. All that together means that at any given moment 50% of the lights in the city are on, and the other 50% are off. That's about as far from synchronized as you can get, really. Also, light is slightly faster than radio in these conditions[1]. The light reaching the center of the city won't be pulsing in the same phase as the signal you're sending out. It will also be blurred and muddied due to the difference in signal speed. You probably wouldn't see much flicker in the light at all, as 50% of the lights are always on. On average, you receive 50% of the light you would otherwise, but that value will never reach 0. There won't be any periods of total darkness where you can make an observation, as the article suggests. Moral of the story: light (and radio) is much, much slower than most people realize. [0] https://ieeexplore.ieee.org/document/170108 https://ieeexplore.ieee.org/document/170108 [1] I couldn't find a confident number in my 16 seconds of searching. Seems like just a few percent faster, but the real number doesn't matter here. Bonus: you can synchronize the streetlights exactly, but it's much more expensive and complex than a single radio transmitter. You need to have every single light contain a receiver with an accurate clock that synchronizes with a geosynchronous satellite with an atomic reference clock. Search for "GPS disciplined oscillator" if you're curious.

anon_cow1111 3y ago
>Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2 Um am I really really tired and not reading correctly, or is this confusing linear area with square area? London doesn't stretch from New york to Florida. 186000 miles/second * pulse of .003 = 558 miles ~= 900 km
- calamari4065 3y ago
  I knew I fucked something up. Don't do math while smoking, kids. Either way, propagation delay is a huge problem with synchronizing systems. It's not something you usually think about
  - sand500 3y ago
    Dont you have propagation delay in reverse too? Let's say you do turn off all the lights at the exact same time. The light hitting the telescope also travels at the speed of light.
namuol 3y ago
There’s engineering feasibility, and then there’s feasibility. Is it any less feasible to operate fewer or dimmer street lights? How about directing light more efficiently? This is a problem of bureaucratic and societal engineering.
- calamari4065 3y ago
  I was really just exploring a hypothetical. This whole idea of synchronizing lights is pretty dumb. It won't work, and even if it did it's still a bad solution. Better solution is less lights, or turn them off at night or any of the other solutions we've been talking about for 30 years. Remember when street lights were killing sea turtles?
peddling-brink 3y ago
I’m not a physics person, but couldn’t this be easily solved by adding a programmed delay that corresponds to the distance from the transmitter? Street lights aren’t known to migrate.
- calamari4065 3y ago
  In principle, yes. But not easily. The physics of radio wave propagation are surprisingly complex. Buildings and objects in the path between transmitter and receiver can cause various interference effects that basically slow down the signal. I'm glossing over a lot, but it would take significant effort to precisely measure the exact delay between any one receiver and the transmitter. It's much more complicated than just the distance, and each one has to be measured individually. I'm not an expert in RF, but I think this concept of a single synchronizing transmitter just doesn't work in the real world. You'd probably build a cell network with many transmitters covering small areas. Each transmitter has the hardware to synchronize with thr others, and the lights just have a simple circuit slaved to the transmitter clock. There's probably more clever ways to synchronize signals over long distances, but RF is black magic to me.

anon_cow1111 3y ago

>Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2 Um am I really really tired and not reading correctly, or is this confusing linear area with square area? London doesn't stretch from New york to Florida. 186000 miles/second * pulse of .003 = 558 miles ~= 900 km

calamari4065 3y ago
I knew I fucked something up. Don't do math while smoking, kids. Either way, propagation delay is a huge problem with synchronizing systems. It's not something you usually think about
- sand500 3y ago
  Dont you have propagation delay in reverse too? Let's say you do turn off all the lights at the exact same time. The light hitting the telescope also travels at the speed of light.

namuol 3y ago

There’s engineering feasibility, and then there’s feasibility. Is it any less feasible to operate fewer or dimmer street lights? How about directing light more efficiently? This is a problem of bureaucratic and societal engineering.

calamari4065 3y ago
I was really just exploring a hypothetical. This whole idea of synchronizing lights is pretty dumb. It won't work, and even if it did it's still a bad solution. Better solution is less lights, or turn them off at night or any of the other solutions we've been talking about for 30 years. Remember when street lights were killing sea turtles?

peddling-brink 3y ago

I’m not a physics person, but couldn’t this be easily solved by adding a programmed delay that corresponds to the distance from the transmitter? Street lights aren’t known to migrate.

calamari4065 3y ago
In principle, yes. But not easily. The physics of radio wave propagation are surprisingly complex. Buildings and objects in the path between transmitter and receiver can cause various interference effects that basically slow down the signal. I'm glossing over a lot, but it would take significant effort to precisely measure the exact delay between any one receiver and the transmitter. It's much more complicated than just the distance, and each one has to be measured individually. I'm not an expert in RF, but I think this concept of a single synchronizing transmitter just doesn't work in the real world. You'd probably build a cell network with many transmitters covering small areas. Each transmitter has the hardware to synchronize with thr others, and the lights just have a simple circuit slaved to the transmitter clock. There's probably more clever ways to synchronize signals over long distances, but RF is black magic to me.