[mdturnerphys]

I always want to ask “degrees what?"

I once worked on a system that ran at -40 degrees. I loved leaving off the scale when giving tours and telling about it, because someone would invariably ask if I meant Celsius or Fahrenheit and then everyone got to learn where the crossing point of the two scales is.

[RheingoldRiver]

When I was taking algebra 1 in middle school I learned that all non-parallel lines had to intersect and this made me realize that the temperature scales must intersect, so I calculated their intersection point. I thought it was the single greatest discovery anyone had ever made and told literally every adult in my life about it. -40 C/F has always held a very special place in my heart haha

[j1elo]

Funnily enough, the scale on the linked page doesn't allow me to set -40°. It jumps between -39.9 and -40.1

So seems like -40.0 is really where the singularity lies.

Either that, or a simple rounding issue. I choose to believe it's the former.

[perilunar]

Not a rounding error in this case, but more to do with the precision of your input device and your screen size. The input angle and the converted temperatures are rounded to the nearest 0.1 degrees, but it is possible to hit 40.0° (I should probably add an option to round to whole degrees.)

If you really want to hit an exact number, you can add a query string to the URL.

https://degreeswhat.com/?-40

[joshspankit]

If you’re using a touch display, it’s easier to land on specific numbers by moving your finger away from the centre. The further your finger, the finer the adjustment.

[imglorp]

Remember that time when it was a rounding error?

https://www.theverge.com/tldr/2021/7/13/22575368/apple-ios-1...

[tzs]

For some reason that reminded me of a Futurama joke, although I'm not sure why since it is completely different.

LEELA: Fry, night lasts two weeks on the moon.

MOON FARMER: Yup. Drops down to -173.

FRY: Fahrenheit or Celsius?

MOON FARMER: First one, then the other.

[diego_sandoval]

- Minus 40 degrees

- Celsius or Fahrenheit?

- Yes

[hanche]

That’s how I convert between Celsius and Fahrenheit, if I have to do it in my head: Add 40, multiply by 9/5 or 5/9 depending on what direction, subtract 40. Easily remembered!

[foobarbecue]

Yeah. When I worked in Antarctica this was often the temperature and it was quite convenient.

[lostmsu]

-40K are impossible AFAIK

[Lester_Square]

Kelvin is not measured in degrees however, so is excluded by implication

[arrowsmith]

Wait, so it's just "five Kelvin", not "five degrees Kelvin"? TIL.

Now that I think about, what's the etymological reason we use "degrees" to talk about temperature? We don't measure force in "degrees Newton" or electric current in "degrees Ampère". Why do we do it for Celsius and Fahrenheit?

[ben_w]

> We don't measure force in "degrees Newton"

Not for force, no, but: https://en.wikipedia.org/wiki/Newton_scale

[Y_Y]

Five kelvins. Just like five metres, no capital letters or funny symbols.

The reason you can do it for Celsius or Fahrenheit is that those are scales but not units. The energy in two bodies (with constant heat capacity) at 100° is not the same as in a single body at 200°, but if you use kelvins then it works out.

[evanb]

I don’t know but freezing and boiling of water are separated by 180 degrees F.

[flangola7]

It's not? What is it measured in then?

[unwind]

It is simply measured in Kelvin.

Kelvin is the standard unit for temperature, 0 Kelvin is the absolute zero point. Each step is the same size as a degree Celsius, but there are no "degrees".

Edit: remove assertion that the scale does not go into negative, because physics.

[deelly]

As a non-native reader, I`m not sure that I understand. Could you explain why 100 Kelvin, but 100 degrees Celsius and 100 degrees Fahrenheit? Is there some special meaning in a word "degrees" there?

[forbiddenlake]

Earlier comment - https://news.ycombinator.com/item?id=36584908

[bmicraft]

K, not °K

[onion2k]

You can go below absolute zero, but things get weird. https://www.sciencedaily.com/releases/2013/01/130104143516.h...

[cinntaile]

That seems to be taking a theoretical approach. I don't think there are any experiments that have proven it is possible?

[TeMPOraL]

It's using related but somewhat different meaning of the concept.

It's how these things go. You and me think of "temperature" as something simple, like it all started. You know, you find some material that noticeably expands or contracts when warmed or cooled, you turn it into an indicator along what will be a scale, and you make a mark for e.g. "roughly when water starts freezing over" and "when water starts boiling"[0]. You make a regular gradation in between, let it spill over below and above your "min" and "max" anchors, you put some numbers on this, and "Bob's your uncle".

This of course is super useful, so others refine your "thermometer", as they realize just how stable and broadly-applicable this "temperature" is as a measure. Eventually some start asking how it all works, and you hear some grumbling about average kinetic energy of particles, which still kinda makes sense. I mean, lots of tiny things buzzing around, and there's that German guy saying you can't possibly measure each of the tiny things individually, but whatever - the average, low-frequency part is stable.

But then those theorists push and push, and break through the barriers of sanity. They enter R'lyeh and start transcribing the Eldritch tablets found inside. Suddenly, you see temperature redefined using something else, something you recall should also be a statistical phenomenon but suddenly isn't[1]. You see words like entropy and enthalpy thrown around, and then some American mathematicians get involved, and suddenly there's also negtropy and fractional bits, and nothing makes any kind of sense anymore - temperatures going below absolute zero, the "nothing actually moves anymore" point, leading to negative temperatures where heat flows from colder to hotter...

...really that's one of the least weird thing in hard sciences these days.

--

[0] - In the process discovering that this was a really good choice of anchors, as water behaves in strange ways at exactly those two temperatures.

[1] - Or is it? Are you sure what "statistical" means these days? Are you sure what "means" means these days?

[Y_Y]

Negative temperature is a perfectly normal and well-studied phenomenon with lots of common examples. Temperature is defined as the rate of change of entropy wrt internal energy, usually more energy means more entropy, but it's not a given.

https://en.wikipedia.org/wiki/Negative_temperature

[aziaziazi]

> A system with a truly negative temperature on the Kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system

Wait what?!! That feeds my daily “oh oooh” moment.

[Y_Y]

Energy flow wants to maximize entropy. Usually things flow from hot to cold because a cold thing will "make better use" of the energy to create entropy (i.e. permit microstates). Something with negative temperature will increase entropy by losing energy, and the positive temperature thing will increase entropy by gaining it, so it's (thermodynamically) a no-brainer.

[eru]

It makes more sense, if you don't measure in temperature, but in 'coldness'. See https://en.wikipedia.org/wiki/Coldness or β.

Basically, coldness is 1/temperature but makes more theoretical sense:

> Though completely equivalent in conceptual content to temperature, β is generally considered a more fundamental quantity than temperature owing to the phenomenon of negative temperature, in which β is continuous as it crosses zero whereas T has a singularity.[6]

> In addition, β has the advantage of being easier to understand causally: If a small amount of heat is added to a system, β is the increase in entropy divided by the increase in heat. Temperature is difficult to interpret in the same sense, as it is not possible to "Add entropy" to a system except indirectly, by modifying other quantities such as temperature, volume, or number of particles.

[NavinF]

Lasers often have negative temperatures. It's not that weird

[TeMPOraL]

Okay, can we use it for cooling like in Sundiver by David Brin[0], where people (IIRC) used lasers to keep themselves cool while, as per title, diving their ship into the Sun?

--

[0] - https://en.wikipedia.org/wiki/Sundiver

[SAI_Peregrinus]

Lasers are often used to cool atoms & molecules to extremely low temperatures[1]. Bose-Einstein condensates are made via laser cooling, and it's commonly used in quantum entanglement experiments since it can get the involved particles closer to absolute zero than other methods.

[1] https://en.wikipedia.org/wiki/Laser_cooling

[eru]

No, not like that. Sundiver makes a mockery of the second law of thermodynamics.

[xen0]

Population inversion is a thing; many lasers exhibit this behaviour.

But we tend not to talk about the 'temperature' of electron energy levels. If we did, then they would have negative temperature.

[eru]

Compare https://en.wikipedia.org/wiki/Negative_temperature

[cinntaile]

Interesting I wasn't aware of this definition but I would consider that moving the goal posts (not by you to be clear). Since that's using a different definition of temperature, the original poster was referring to the more common definition of temperature where at 0K nothing moves anymore.

[eru]

> [...] the original poster was referring to the more common definition of temperature where at 0K nothing moves anymore.

That's a common simplification, but not really the definition. At the minimum, quantum mechanical effects make this complicated.

But you are right in a sense. From Wikipedia:

> The absolute temperature (Kelvin) scale can be understood loosely as a measure of average kinetic energy. Usually, system temperatures are positive. However, in particular isolated systems, the temperature defined in terms of Boltzmann's entropy can become negative.

[lionkor]

Probably not, that's why it's called "absolute" zero

[a2800276]

That's why they say it's -40 degrees. :P