[bitsen]

That’s the stupidest argument I’ve ever heard from someone attempting to be a physicist.

The gear experiment is the simplest answer. With a gear whose diameter spans the distance between the light source and receiver, and where the speed of rotation is controlled by an atomic clock, the gear could have a small hole through it. If light sent from one side to the other while the gear is spinning is too slow, it will not make it through the gear.

The size of the bike through the gear will dictate the speed, because the gear itself measures duration and distance in a single direction.

[quiescant_dodo]

I think you missed the point.

Measuring one-way speed of light requires perfectly synchronized clocks. You cannot do this over any distance, as it requires the speed of light to propagate between the clocks. So you must synchronize the clocks when they are together.

But then you must move at least one clock, which subjects the clocks to unknown time dilation. We can't even assume that the time dilation from movement X is the same as from movement -X, even to impossibly-precise measurements and controls. The _premise_ of the question about one-way speed of light is un-divorceable from time dilation. If you _assume_ that the time dilation is the exact same, you are also _assuming_ that the amount of time light takes from A-to-B is exactly half of A-to-B-to-A.

Einstein himself articulated this problem. He's certainly known for more than "attempting to be a physicist".

[bitsen]

A physicist came up with the gear solution which was stated in the video and dismissed because the mock physicist didn’t understand why the gear or wheel solved the problem.

If the gear or wheel is moving at a known rate, it can define the minimum speed of light passing through it, if the gear goes sufficiently fast and is sufficiently large with a sufficiently small hole.

Einstein was not infallible or all-knowing. His insistence on having two clocks when all that is needed is one time-accurate gear just shows he wasn’t thinking clearly. Einstein was clearly not an idiot.

However the mock physicist ignoring the very solution he presented is an idiot.

[justin66]

> Measuring one-way speed of light requires perfectly synchronized clocks.

But disproving the oversimplification provided above, For all we know, light travels infinitely quickly in one direction and at c/2 in the return direction, only requires clocks.

[pxx]

But it doesn't. The experimental procedure seems to involve a spinning disk that either allows light to travel through its entire diameter or not in an instantaneous fashion. I challenge you to try to fabricate such a device.

You'll run into an obvious problem that the disk doesn't have infinite rigidity. Otherwise you could solve the one way speed of light problem by simply poking a button at the receiver's clock with a long stick. Clearly the wave propagation through the long stick happens much slower than the light getting there...

[justin66]

Disproving light travels infinitely quickly in one direction is easier than you think and does not require the apparatus you've described. Perfect synchronization and extremely high resolution are definitely not needed to measure that one...

[pxx]

Can you describe or point to a simpler (or any) apparatus that would be capable of this result?

[ramses0]

That's actually a super interesting point and problem! One of my favorite museum visits was "the museum of time" at Greenwich (of GMT fame). Well worth the visit!

A summary of the difficulty and "technology" of time + clocks.

1) Latitude (up+down) is easy to calculate + agree upon anywhere in the globe (but what does this have to do with time, you say?). To calculate latitude, you (mumble mumble) look at where the sun is at high noon, do some math and basically figure out if you're in a "small circle" (near the poles) or "big circle" (near the equator). Effectively, latitude is "defined" as "how big around is the ring you are currently on". https://commons.wvc.edu/rdawes/ASTR217/Gnomon.pdf

2) Conversely: longitude is nigh impossible to determine, and very few people think about why that might be. (and why it has huge commerce and military implications).

Why is it difficult to determine? ...well... what is "zero"? For the equator, it's well-defined. The biggest ring around, getting smaller as you go "north" or "south" (towards the poles).

For longitude... is it "how far away from mount everest?" "how far away from new york city?" "how far away from greenwich?" (Turns out, we've settled on the last definition).

Once you've gotten farther away from visible land (can't see the tallest tree/mountain any more) you don't know how far _away_ you are from anything (east-west) ... but you can trivially determine how north/south you are (look at the sun, do some math, figure out which "ring" you're on).

...but _wait_! You _CAN_ figure out how far _AWAY_ you are from something. If you "steal" the sun at high noon in greenwich, wait 24 "true clock hours" (in greenwich), and you'd expect the sun at high noon (12:00pm on your watch) to be exactly in the same spot. Go 100 miles west, and the sun would show up a little late (compared to your 100% accurate "true clock watch"). Go 1000 miles west, and it'll be quite a bit later until the sun shows up at exactly solar noon (overhead, casts no shadow). Figure out exactly "how late is the sun?" and bam, you've calculated "how far away you are from greenwich".

Summary: North/South is "easy" (how big around is the globe). East/West is "hard", and the first, best way to measure it was "how many hours east/west are we from greenwich".

Military and Commerce implications? Accurate maps. If you go west for a month, and run into a spot of land... you know your "Y" (north/south), but you may not know your "X (east/west). If you send 10 boats with 10 inaccurate clocks going "west", they may have wildly varying calculations as to what their "west" is, depending on how accurate their _clocks_ were!

So it turns out that if you ever really do get sent back in time, and happen to be wearing a decent watch, you could become godly rich simply by being able to produce maps that are accurate in the east-west direction and not just the north-south direction (since early on, you're competing with boats carrying grandfather clocks, or burning 24 candles that each last an hour) as a mechanism to keep track of when noon is when they left, so they can calculate how far east/west they've gone.

Spoiler alert, eventually somebody figured out some other fancy math involving maybe the moon or something in order to be able to calculate a longitude, but I believe that was after or near co-incidental with more accurate spring-wound clocks which reduced the importance of being able to do it simply via calculation.

Nowadays we take GPS for granted, but it's surprising how important of a problem it's actually solving.

[jlgaddis]

On a somewhat related note, most people aren't aware that you can easily figure out direction (i.e., "which way is North?") using just your wristwatch -- an analog one, not your smart watch!

I learned that one as a kid in Boy Scouts and it somehow stuck in my mind -- and there were a few times throughmy life that it came in handy.

Of course, it's less useful now that we all have a smartphone with a compass GPS in our pockets.

(And if you're unlucky enough to not be wearing your wristwatch when you get lost, you can find your way using nothing more than a stick in the ground -- but this method will take longer as you'll have to wait 20 or 30 minutes.)

(And if it's night -- meaning you can't use either of the two methods above, you'll have to rely on the North Star.)

[chrisshroba]

Really leaving us hanging! How do I find out which way is North using my wristwatch or a stick?

[mavhc]

https://www.techblazer.com/2018/05/find-true-north-using-wat...

[reasonabl_human]

What is the wrist watch trick? Keeping time and looking at the sun / shadows?

[jtbayly]

That inability to figure out longitude also led to thousands of people dying when ship captains had to guess whether they were west or east of the island they needed to hit for water and food resupplies.