What Happens if You Focus a 5W Laser With a Giant Magnifying Glass? Negative Kelvin Temperature!

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In this video I show you what it means to have negative temperature by focusing a laser beam down to a single point. I show you what happens if you try to focus a light down to a single point, then I show you how a laser is different due to population inversion.

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  • I see a lot of people are having trouble with this video. First, I am very much aware that the reason the laser it getting hotter when it is magnified is due to the reduced area. That isn't the point of this video. The point is to try to explain why it doesn't break the laws of thermodynamics! Now for the negative kelvin explanation, statistical mechanics tells us that at infinite temperature all atomic states will be populated equally. The Kelvin scale was built upon classical mechanics where it would be impossible to achieve a state in which there are more atoms in a higher state than a lower state. However due to quantum mechanical effects, we know that we can stimulate atoms to be in a higher energy state simply by shining light near them that is at the same wavelength as the light it would emit at that state (stimulated emission). So in a laser, the stimulated atoms actually achieve a population inversion where there are more atoms in a higher energy state than a lower one. This is where the negative temperature comes from. In this case we have to define temperature as negative or else we get into problems that break the second law of thermodynamics. It doesn’t matter that my laser has poor optics. What’s important is that lasers can break the conservation of etendue due to the fact that they have light that doesn’t spread, the reason they have light that doesn’t spread is because of population inversion, and this is why we have to say they have negative temperatures (or they behave as if they have negative kelvin). We can never achieve negative temperature in a non-quantum mechanical system thus anything the laser shines on is always at a positive temperate no matter how hot you get.

    Of course the reason the laser gets hotter when it’s focused is due to the reduced surface area of the light. That was not my point though. The point of the video was to explain why it doesn’t break the 2nd law of thermodynamics. Please research “conservation of etendue” to understand why you can’t focus a flashlight down to a point that is hotter/brighter than the flashlight surface. This is a very good example of how the second law of thermodynamics can never be broken no matter how hard you try.

    The Action Lab July 22, 2020 2:40 am Reply
  • please tell the glass you used here

    ROCK Gamer July 22, 2020 2:40 am Reply
  • Suggest the beginning of laser guns

    physics biology July 22, 2020 2:40 am Reply
  • Imagine if you pointed it at the eye

    MR JADEN July 22, 2020 2:40 am Reply
  • There is a small magnifying lens on the end of your laser that you can adjust by simply unscrewing the lens cap or screwing on the lens cap. By doing this you can tweak the point of the laser to a smaller, finer, hotter point. Then you can do away with your second or hand held magnifying lens. Be careful this gets very hot and burns thru even plywood if left long enough.

    Jim Lee July 22, 2020 2:40 am Reply
  • Thought this was a video where he cuts stuff in half with lasers…..
    My disappointment is immeasurable and my day is ruined.

    Yeet Yote July 22, 2020 2:40 am Reply
  • Why didn't you just use a reflective tube?

    Shine the light into a reflective tube, heck… tinfoil and cardboard should work… and place your magnifier lens at the end, that should do the trick for focusing your flashlight at a shorter distance

    David Meigs July 22, 2020 2:40 am Reply
  • this can also work with the sun on a hot day with no clouds. i tried it and i burned a leaf XD

    Mira Beals July 22, 2020 2:40 am Reply
  • So a curious question, what occurs if a laser is passed from one level of magnification to a stronger magnification? Think of eyeglass lenses for example, stepping up the magnification strength from one lense to the next? Can the negative temperature be stronger?

    John Wieger July 22, 2020 2:40 am Reply
  • I don't like Kelvin anymore.

    Saythein July 22, 2020 2:40 am Reply
  • I feel if you take I cylinder tube that has an aluminum covering around the light you could probably keep all that light down to a small point without losing any

    Earl Fuller July 22, 2020 2:40 am Reply
  • *Styropyro wants to know your location *

    Sanguine Satan July 22, 2020 2:40 am Reply
  • This reminded me of "Final Destination 5"

    Quinnlan Purley July 22, 2020 2:40 am Reply
  • 7:44
    stop at the right milisecond and laugh

    Morry July 22, 2020 2:40 am Reply
  • Amazing

    Hermes de Almeida July 22, 2020 2:40 am Reply
  • FRICK THE LAWS OF PHYSICS,let's double jump,if that's even possible.:(.

    Coltitive July 22, 2020 2:40 am Reply
  • I believe that's a 1W not a 7W

    Rock Saturn July 22, 2020 2:40 am Reply
  • 2:20
    Is that really a problem?

    Michael Wright July 22, 2020 2:40 am Reply
  • Who got more confused?

    Missing Link July 22, 2020 2:40 am Reply
  • Your fridge actually warms food in Antarctica.

    GoldenzYae July 22, 2020 2:40 am Reply
  • That laser is actually 1.4 watts not 5 because if it was five it would easily blind him. I know this because I have the exact same laser and I also have a watt measure and its 1.4 not 5

    Finny boii July 22, 2020 2:40 am Reply
  • Scientists: 2nd law of thermodynamics
    People: what about laser?
    Scientists: … Make it negative

    calvin lee July 22, 2020 2:40 am Reply
  • Now try it with the last prism.

    Coding Hub July 22, 2020 2:40 am Reply
  • 1:55 I was literally JUST, about to make a mean comment about how u were wrong. But you had to put that caption up didn’t u?!?!

    Po_ l July 22, 2020 2:40 am Reply
  • The highest temperature is not Infinity, it's the Planck temperature. Although it might seem unlimited, because the more Energy you put in, the hotter it should get, calculations break down above this temperature.
    It's roughly 1,416 784(16) · 10^32 Kelvin.

    Dominik Riegler July 22, 2020 2:40 am Reply
  • HOLY !! The Comment Section is ToXic…………………….

    Sarthak Nimbalkar July 22, 2020 2:40 am Reply
  • Quantum mechanics:
    * Violates general relativity *
    * Respects thermodynamics *

    Ben Silva July 22, 2020 2:40 am Reply
  • The only thing that is infinitely hot is hot pockets.

    Spruce July 22, 2020 2:40 am Reply
  • when you say negative temperature, are you exactly saying that as we get colder and colder, temperature becomes hot? because yah know – degrees are usually cold outside. i'm kinda confused.

    FLAMESIS July 22, 2020 2:40 am Reply
  • Use fresnel and magifier

    v12345vtm July 22, 2020 2:40 am Reply
  • You Said In You’re Thumbnail That It’s Hotter Than Infinity. Well That’s Not But A Lightsaber Is.

    JNB217 July 22, 2020 2:41 am Reply
  • Eh.. That explanation as to why the second law of thermodynamics isn't broken by lasers feels like a cop out..😂

    Reegan Layzell July 22, 2020 2:41 am Reply
  • Hey dude I made the flashlight Brighter than the actual lens itself because of a special type of flashlight

    Pixel I.G July 22, 2020 2:41 am Reply
  • Use a concave lense directly in front of the flashlight, then a second convex one a couple of inches away

    Pascal Baryamo July 22, 2020 2:41 am Reply
  • You made a mistake when you introduced second law of thermodynamics. you are focusing the energy, therefore theoretically you can get infinitely big intensivity if your area is infinitely small

    Dachi Okropiridze July 22, 2020 2:41 am Reply
  • Now imagine what the government can do with the newest technology and like unlimited cash……. I kinda want to know if there is a lazer that cuts trough anything like butter

    nocheat July 22, 2020 2:41 am Reply
  • Can't you use 2 glasses for the flashlight? One close to narrow the light in and one far to focus

    Matt O'Dowd July 22, 2020 2:41 am Reply
  • 2:33 i think some youtubers might disagree with that

    Roel atsocialfunstuff July 22, 2020 2:41 am Reply
  • Guys go check out styropyro hopefully you will sub him.

    Lazar The Shinobi July 22, 2020 2:41 am Reply
  • styropyro: "DID I HEAR LASERS?"

    Lazar The Shinobi July 22, 2020 2:41 am Reply
  • The Highest temperature We have on earth is 100 billion degree Celsius and You need -400°~ c to cool the Super magnets around them.
    There is no way a 5 w laser can Reach temperatures hotter than that.(The temperature of the 'artificial sun' is 4 to 5 times greater than our own sun
    So yeah)

    P.s Im saying this at from the intro mark
    So this might change later on

    Jack July 22, 2020 2:41 am Reply
  • I think it's one of the things that's easier to understand on a theoretical model than with the experiment. I mean both laser emitter and flashlight produce additional heat

    Sk0lzkiy July 22, 2020 2:41 am Reply
  • What if you focused the laser through a magnifying glass, and than focused that through another magnifying glass, and so on

    Matthias Manneraak July 22, 2020 2:41 am Reply

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