Today I Learned

[1oldman2]

Not really. Your retina needs (from memory) ~20-30 visible light photons on one cell to trigger a detection event. So to form an image you need 20-30 photons on each light sensitive cell. Anything emitting at a lower rate is not really emitting detectably to an unaided human, although it may be technically emitting.

Proof things aren't as they always seem, Thanks, I had no idea about the sensitivity of the eye to individual photons. (I also learned a bit about black body radiation thanks to Om's post)

 

[mfb]

Not really. Your retina needs (from memory) ~20-30 visible light photons on one cell to trigger a detection event. So to form an image you need 20-30 photons on each light sensitive cell. Anything emitting at a lower rate is not really emitting detectably to an unaided human, although it may be technically emitting.

People can sense single photons

At E/kT > 55, such an emission is incredibly rare, however. Nonthermal bioluminescence is a larger effect, but still tiny.

 

[OmCheeto]

Not really. Your retina needs (from memory) ~20-30 visible light photons on one cell to trigger a detection event. So to form an image you need 20-30 photons on each light sensitive cell. Anything emitting at a lower rate is not really emitting detectably to an unaided human, although it may be technically emitting.

That's pretty good from memory! But according to an article posted by John Baez, someone determined that single cells can be triggered by single photons, but a bunch of cells, 9 in his example, have to be triggered at the same time, or the signal is ignored. [ref: Can a Human See a Single Photon?, Original by Philip Gibbs 1996.]

This whole thing got started yesterday when someone here at the forum asked about a body at room temperature, and I decided to try and answer the question. After 6 or so hours of trying to figure it out, and doing very suspicious maths, even for me, I decided not to engage in the thread. But in the end, I found that wiki listed a number that was very close to mine, so I decided to publish my findings.

Om's very suspicious number: a 700 nm photon is emitted every 24 seconds from a square meter something at 98.6°F (310.15K) [source of original numbers]

Wiki's mathy number: For example, a black body at room temperature (300 K) with one square meter of surface area will emit a photon in the visible range (390–750 nm) at an average rate of one photon every 41 seconds, meaning that for most practical purposes, such a black body does not emit in the visible range. [ref: wiki, Black Body Radiation, referencing: mathy stuff]

 

[Jonathan Scott]

On Thursday, I learned what it's like to ride a Segway PT. On our family vacation, we included a 45 minute "Segway Experience" riding X2s around, first getting to know how to operate it, then a quick obstacle course to make sure we were getting the hang of it, then they turned off the "turtle mode" beginner limiter and let us have half an hour charging along in single file around tracks and forest paths on the Center Parcs Elveden site where we were staying - all in the pouring rain with USAF jets from RAF Lakenheath thundering overhead (if it had been real thunder, they would have canceled most of the outdoor activities).

They insisted that we wear their supplied helmets, knee guards and elbow protectors, but there was nowhere to sit while putting on the knee guards and I managed to fall over trying to put one on, when it got caught on a shoe. In contrast, none of us fell over at all on the Segway, although I had a near miss when the left wheel took off over a tree root and landed at an angle ahead of the other wheel, after which I managed to zig and zag about six times before getting fully back in control.

 

[Jonathan Scott]

On Saturday, I learned a bit of Anglo-Saxon history. We visited Sutton Hoo, where there is an area of Anglo-Saxon burial mounds dating to the 6th and early 7th centuries, where an intact mound was found in 1939 with a ship burial, probably of the local king.

I also visited Tranmer House, where the owner of the Sutton Hoo area lived. Several of the rooms have been set up to illustrate how it would have looked in the 1930s, around the time of the discoveries. In the main room, there was a grand piano by Schiedmayer of Stuttgart, from around 1864, which had been partly refurbished. I hadn't heard of that make before, but it was one of the most beautiful sounding pianos I have ever encountered and had been well looked after. I was invited to play it, and after I played a few bits of classical music from memory they found a book of Scott Joplin ragtime music, so I played The Entertainer and some of the others, which helped to wind back the clock a bit, although the use of it in the film "The Sting" to depict the 1930s is somewhat anachronistic, as it was only really popular until about 1911.

 

[OmCheeto]

Today I learned, that if you leave a black rock out in the sun, it won't get hotter than 196°F. (sans local reflective sources)
(strictly from theoretical maths, and stuff from wiki, of course.)

But this brought to mind, "It's so hot, you can fry an egg on the sidewalk!"

Which from my calculations, and semi-infinite knowledge of properly cooking turkeys, a 196°F sidewalk should very properly cook an egg.

Which of course led to me googling, and finding: Death Valley employees beg visitors to stop frying eggs on the sidewalk

 

[Ibix]

People can sense single photons

My memory is faulty. According to section 5 of http://webvision.med.utah.edu/book/...nd-cones/phototransduction-in-rods-and-cones/, that rods could react to single photons has been known experimentally as far back as 1979, and was inferred by Hecht et al as far back as 1942. My number is more consistent with Hecht's "seeing curves" (figure 4B in the link) which are a measure of how many photons you need to be able to see a flash, rather than how many you need for a photoreceptor to respond. Although mfb's article suggests even that has been superceded.

At E/kT > 55, such an emission is incredibly rare, however.

I suspected as much but had not done the maths.

 

[OmCheeto]

...even that has been superceded.
...

I had my head in my oven yesterday, with my laser thermometer pointed at the heating element, trying to capture the temperature that my eyes could see it start glowing. It was around 600°F.
Though, I knew that the heating element was some non-perfect, non-homogenous creation, so I thought to myself; "hmmmm... Who routinely looks at glowing hot stuff, and would know the temperatures?"
I ended up at a ceramics website, where they claimed that pottery starts visibly glowing at around 700K.
From that, I determined that our eyesight was about 10 quadrillion times too weak to see our "human" glow.

ps. But as I mentioned above, I find all of my calculations very suspicous, as this is all very new to me.

 

[mfb]

You probably watched it in daylight or at least with other light sources nearby (and with a visible laser pointer?). You'll get better results if you switch off all light sources and wait a few minutes. The single-photon tests had the test persons sit in absolute darkness for 40 minutes.

 

[ProfuselyQuarky]

Today I learned how to properly temper chocolate. Watch out, Benoit Blin.

 

[zoobyshoe]

Today I learned that, in the state of Texas, anyway, there is a crime called "tresspassing by projectile." It consists of shooting from one property ( where you may or may not have permission to shoot) onto another property where you don't have permission to shoot.

 

[mfb]

Texas, of course.

Does an apple count as projectile?

 

[fresh_42]

Texas, of course.

Does an apple count as projectile?

I guess not. But neighbor boy's football might.

 

[RonL]

I had my head in my oven yesterday, with my laser thermometer pointed at the heating element, trying to capture the temperature that my eyes could see it start glowing. It was around 600°F.
Though, I knew that the heating element was some non-perfect, non-homogenous creation, so I thought to myself; "hmmmm... Who routinely looks at glowing hot stuff, and would know the temperatures?"
I ended up at a ceramics website, where they claimed that pottery starts visibly glowing at around 700K.
From that, I determined that our eyesight was about 10 quadrillion times too weak to see our "human" glow.

ps. But as I mentioned above, I find all of my calculations very suspicous, as this is all very new to me.

You might check out the saying that one might make to another, "you are absolutely glowing"

 

[zoobyshoe]

Does an apple count as projectile?

It would be interesting to read the actual law and see if it can be applied to anything other than bullets.

The reason this came up is because a group of people were shooting shot guns into the air from one property and the pellets were falling onto a neighbors cars. They pretty much destroyed the windshields with pits. (The show was called, "Southern Justice." A reality show like "Cops," but set in rural Texas.)

That being the case, if the thrown apples were obviously causing some kind of damage or if they were deliberately being thrown at a neighbor, I'm thinking this law could be invoked.

 

[OmCheeto]

You might check out the saying that one might make to another, "you are absolutely glowing"

I think my comment, and subsequent comments, deserve a new thread, as I took mfb's advice, and did the "stick my head in an oven" experiment again at 5 am this morning.
I saw the strangest of things.
And the maths that followed, were even stranger.
But the new thread will have to wait, as it's time for my nap.

 

[Pepper Mint]

Today I learned how to hit a ball with a stick and there will always be "Next".

 

[demelzareveley]

Today, I learned hundreds of French names from babynology.com and completed my first assignment of my French language course.

 

[Pepper Mint]

I Learned today about brainstorming (why and how etc). I find the professor and her assistants are just so informative in their lecture and only hope the students later graduating from Stanford will be similarly good or probably better . The one thing I like from her lecture is instead of starting my comments in other people's talks or works with YES, BUT pattern, I need to learn to practice doing so with YES, AND one. How thoughtful!

 

[jim hardy]

I need to learn to practice doing so with YES, AND one. How thoughtful!

That's how you make the whole greater than the sum of its parts...
Harry Truman said "It's amazing what you can accomplish when you don't care who gets the credit"

old jim

 

[OmCheeto]

TIL that seabirds take naps while flying out at sea.
They can also nap just half their brain at a time, so the other half can pay attention, and not run into other birds.
[ref]

 

[RonL]

Til, never trash your wife's favorite political candidate 30 minutes before she trims your hair , not my brightest moment

 

[dlgoff]

TIL This material is darker than anything I've ever seen.

 

[DrGreg]

Til, never trash your wife's favorite political candidate 30 minutes before she trims your hair , not my brightest moment

That explains Donald Trump, then.

 

[collinsmark]

Today I learned (or rather just realized) that "Little Bow Peep" can be used as a legitimate archery term.

A "peep" is analogous to the rear sight of a gun (they are functionally equivalent), except peeps are used in archery. They attach to the bowstring. Peeps are standard on compound bows and are sometimes used on recurve bows. (Not all recurve and/or standard bow archers used peeps. Some do, some don't. But a compound bow is almost guaranteed to have a peep.) Peeps come in several different sizes and styles.

After that it's just a matter of punctuation. "Little Bow Peep," or "Little, Bow Peep" -- one describing a peep used on a small bow, and the other describing a small peep for a bow.

 

[Borg]

After that it's just a matter of punctuation. "Little Bow Peep," or "Little, Bow Peep" -- one describing a peep used on a small bow, and the other describing a small peep for a bow.

Reminds me of a panda in a cafe.

 

[dlgoff]

Peeps are standard on compound bows ...

I've hunted with a compound bow for years. No front or peep; who needs them.

 

[Jonathan Scott]

Today (well, yesterday) I learned that it's good to trust my instincts when I spotted a UFO.

The UFO was initially just a slow-moving bright point of light in the daytime sky, too slow to be an aircraft. When I looked at it using binoculars, it was a tiny bright golden-yellow circle, just like a picture of the sun. My wife and I concluded that it must be a balloon, and the direction of motion appear to be consistent with the prevailing light wind, although we couldn't tell the distance.

Several hours later, we spotted a golden-yellow party balloon floating low over the house, again moving slowly downwind. We rapidly realized that it must be identical to the earlier UFO, and sure enough, as it floated into the distance, it looked very similar to the earlier one, except that this time the sun was further round, so it was only half-illuminated directly, and other half was a darker yellow-brown colour.

 

[1oldman2]

TIL, that for the second time since installing Win10 it has reset my computer to boot on only one core after an update, I hate the way it changes other minor settings also. The moral of the story is if your running 10 you might want to check the system configure on your machine.

 

[Pepper Mint]

I learned about epilepsy today.
I don't know how true these people's descriptions of what they've been through actually are but am so happy I haven't had it so far. I did witness several kids around my house experiencing it painfully (like those being possessed in exorcism related movies).