Today I Learned

[Orodruin]

TIL that and how you can measure c with a microwave and a slice of cheese.

No you can't.

 

[fresh_42]

No you can't.

You can. It only depends on the information your microwave provides.

 

[Orodruin]

You can. It only depends on the information your microwave provides.

No, you can’t. The speed of light is a defined quantity and as such cannot be measured.

At best (assuming the information provided is accurate) you can determine the size of the cheese.

Edit: Or, as I told one of my experimental professors when I was an undergrad and he asked what we had just done (expecting the answer “measured the speed of light”):
- We have checked the calibration of your ruler.

 

[Jonathan Scott]

No, you can’t. The speed of light is a defined quantity and as such cannot be measured.

At best (assuming the information provided is accurate) you can determine the size of the cheese.

Edit: Or, as I told one of my experimental professors when I was an undergrad and he asked what we had just done (expecting the answer “measured the speed of light”):
- We have checked the calibration of your ruler.

I agree that you can't measure c, which is defined, but you can measure the speed of light in a given context, and you would expect to get a value similar to c.

 

[collinsmark]

[...] The speed of light is a defined quantity and as such cannot be measured.

At best (assuming the information provided is accurate) you can determine the size of the cheese.

Edit: Or, as I told one of my experimental professors when I was an undergrad and he asked what we had just done (expecting the answer “measured the speed of light”):
- We have checked the calibration of your ruler.

Technically, this is correct.

[...] It only depends on the information your microwave provides.

This is also correct, in-so-far as the final answer depends on the "frequency" figure typically specified by the microwave oven manufacturer.

 

[Orodruin]

This is also correct, in-so-far as the final answer depends on the "frequency" figure typically specified by the microwave oven manufacturer.

In order to convert that frequency into a speed, you need a distance. A distance that is calibrated by ...

 

[collinsmark]

In order to convert that frequency into a speed, you need a distance. A distance that is calibrated by ...

Correct.

(If you want to "measure" the speed of light, you'll have to pretend the year is before 1983).

 

[Orodruin]

Correct.

(If you want to "measure" the speed of light, you'll have to pretend the year is before 1983).

I mean, you can measure the speed of light if you use units that are not SI units where time and length units are defined in a different manner. However, any such system of units will be less precise than SI units.

 

[fresh_42]

In order to convert that frequency into a speed, you need a distance. A distance that is calibrated by ...

As far as I am concerned, by the length of a piece of metal in Paris.

 

[collinsmark]

As far as I am concerned, by the length of a piece of metal in Paris.

It once was!

But now (assuming SI units), since 1983, the length of a meter is specified by measuring the length that light travels in a vacuum within the timespan of \frac{1}{299 \ 792 \ 458} of a sec.

So today, our system of atomic clocks not only keep official track of time, they also keep track of length too.

 

[fresh_42]

It once was!

But now (assuming SI units), since 1983, the length of a meter is specified by measuring the length that light travels in a vacuum within the timespan of \frac{1}{299 \ 792 \ 458} of a sec.

So today, our system of atomic clocks not only keep official track of time, they also keep track of length too.

I know, but this entire discussion is absurd anyway so referring to the norm meter is as justified as the discussion itself. Nobody would ever actually determine c by a microwave, but it's funny that you can do it with a microwave, a slice of cheese, and a ruler in your kitchen.

 

[OmCheeto]

I know, but this entire discussion is absurd anyway so referring to the norm meter is as justified as the discussion itself. Nobody would ever actually determine c by a microwave, but it's funny that you can do it with a microwave, a slice of cheese, and a ruler in your kitchen.

So when I did this experiment, was I measuring my measuring tape?
Good lord I'm confused.
Btw, I've always heard measuring the speed of light in your microwave involved marshmallows. When did they switch to cheese?

 

[fresh_42]

Btw, I've always heard measuring the speed of light in your microwave involved marshmallows. When did they switch to cheese?

Cheese doesn't explode.

 

[Ibix]

So when I did this experiment, was I measuring my measuring tape?

No. To expand slightly on Orodruin's comment, the meter is defined to be the distance light travels in 1/299,792,458s. So measuring a speed in m/s turns out to be measuring speed in multiples of $c/299792458$. "Measuring $c$" is therefore tautological, give or take your measurement accuracy, and technically sny experiment that does it actually calibrates some combination of the ruler you use and the accuracy of your microwave frequency.

Measuring other speeds (such as the speed of light in a medium) and things like acceleration is not tautological. It's a specific issue with yhe speed $c$.

 

[docnet]

So when I did this experiment, was I measuring my measuring tape?
Good lord I'm confused.
Btw, I've always heard measuring the speed of light in your microwave involved marshmallows. When did they switch to cheese?

 

[jack action]

https://en.wikipedia.org/wiki/Pistol_dueling_at_the_Summer_Olympics​

 

[pinball1970]

TIL that 1 in 12 men are colorblind.

I had the genetics of this wrong for years.

 

[pinball1970]

TIL Limelight actually means light from lime, soda lime that they used to use on the stage.

 

[Hornbein]

TIL Limelight actually means light from lime, soda lime that they used to use on the stage.

Before batteries became a thing bicycle lamps were chemical as well. Carbide I recall.

 

[Ibix]

Before batteries became a thing bicycle lamps were chemical as well. Carbide I recall.

Calcium carbide reacts with water to produce acetylene, which burns, yes.

What happened to the cat who ate calcium carbide? She had a set o' lean kittens. (According to my dad, who had such lamps on his bicycle as a boy.)

 

[symbolipoint]

Calcium carbide reacts with water to produce acetylene, which burns, yes.

What happened to the cat who ate calcium carbide? She had a set o' lean kittens. (According to my dad, who had such lamps on his bicycle as a boy.)

Time taken to figure out what you said took a few seconds. Then I had to reread a certain part.

 

[jtbell]

Today I learned that dental floss expires.

While packing for a road trip starting tomorrow, I grabbed a small unused floss dispenser from a kit that my dentist gave me after a routine cleaning a year or two ago, and noticed that it's stamped "EXP2025-07-30".

Maybe the mint flavoring goes stale?

 

[fresh_42]

... Maybe the mint flavoring goes stale?

Or they just want to sell more of them.

 

[Swamp Thing]

Today I learned that dental floss expires.

It dies of boredom. Use it or lose it.

 

[symbolipoint]

Today I learned that dental floss expires.

While packing for a road trip starting tomorrow, I grabbed a small unused floss dispenser from a kit that my dentist gave me after a routine cleaning a year or two ago, and noticed that it's stamped "EXP2025-07-30".

Maybe the mint flavoring goes stale?

My guess is that the labeled expiration date is listed only for regulatory requirements. If the floss works, then it is not really expired. If it shreds or breaks too quickly, then it could be "expired".

 

[fresh_42]

And it is "best before" and not "suddenly fatal on".

 

[gmax137]

Raisin' it up, waxin' it down...

 

[fresh_42]

They sell Himalaya salt and advertise it as millions of years old. The fine print says best before 2025. Good that they found it in time!

 

[Orodruin]

And it is "best before" and not "suddenly fatal on".

And here I was expecting spontaneous self-combustion …

I once bought a shampoo bottle that was reduced by 30% in price because its best before date was coming up …

 

[Ibix]

I once bought a shampoo bottle that was reduced by 30% in price because its best before date was coming up …

I once bought a shampoo bottle that contained 500ML of shampoo. I can only assume that 1L=1nl.