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Thread: how powerful would a light need to be to knock your hand back when turned on?

  1. #31
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Install the battery backwards...

    Ok, seriously, I don't think it will be possible in the next 10 millenniums. If we happen to make a light that strong, it would burn our earth into plasma just by turning it on

  2. #32
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by MichaelW View Post
    Forget radiation pressure.
    Enough power will vaporize your skin, and provide a jet thrust. (cough lasers cough)
    Not to mention your peripheral nervous system would probably jerk your hand away from the injury source.
    LEDs do about 100 lumens per electrical watt, estimate white light as about 300 lumens per luminous watt.

    Suffice to say that if my Quark suddenly started emitting 300 MW of light, it would generate about half the thrust of a focused garden hose in hand. This would require it to emit something like ninety billion (Thousand million) lumens, which with today's technology would require the emission of about thirty billion watts of heat, the consumption of two hundred seventy billion watts, and produce three hundred sixty billion joules of heat per second (2/3 at the flashlight, 1/3 at the target). At close range this would certainly cut steel of reasonable thickness, but the beam would quickly spread.
    My biggest light-hog is my camera.

  3. #33
    *Flashaholic* fyrstormer's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by AnAppleSnail View Post
    What we call 'reflection' is a very fast absorb/emit process. In fact, light passing through air is absorbed and re-emitted...in the same direction it was traveling.
    The fact that a photon can be re-emitted at the exact opposite angle of its original impact is damn near magical to me. I'd love to know how that works if the original photon is destroyed via absorption.

    In any event, because photon absorption and re-emission obeys classical physics, it is still fair to say photons "bounce" on a macroscopic scale, even though the re-emitted photons are identifiably different than the absorbed photons.

  4. #34
    *Flashaholic* fyrstormer's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by AnAppleSnail View Post
    LEDs do about 100 lumens per electrical watt, estimate white light as about 300 lumens per luminous watt.

    Suffice to say that if my Quark suddenly started emitting 300 MW of light, it would generate about half the thrust of a focused garden hose in hand. This would require it to emit something like ninety billion (Thousand million) lumens, which with today's technology would require the emission of about thirty billion watts of heat, the consumption of two hundred seventy billion watts, and produce three hundred sixty billion joules of heat per second (2/3 at the flashlight, 1/3 at the target). At close range this would certainly cut steel of reasonable thickness, but the beam would quickly spread.
    You reminded me of this what-if scenario regarding relativistic baseball: http://what-if.xkcd.com/1/

  5. #35

    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    hmmm , the big bang theory certainly moved a lot of photons about, along with a fair few paper backs, but i'd have to point out that a fair few heads have met accelerasion 'status' when met by a CREE in the face.. which in turn, certainly moved hands lol...

  6. #36

    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by fyrstormer View Post
    [...]The statuette of Wile E. Coyote using a fan to push a sail is not even a remotely accurate comparison to this. If you actually want an accurate comparison: a reflectored flashlight is functionally identical to a rocket engine with a parabolic nozzle, except it ejects photons from the nozzle instead of superheated gases. Using that comparison, it's easy to see why a flashlight would experience a slight force caused by the radiation pressure of photons shooting out through the aperture.
    rocket => chemical reaction ejects part of its mass in one direction, driving the remaining mass (the payload you are interested in) in the opposite direction

    Wile E. Coyote => electo-mechanical fan generates the wind for the sail, but alas, you can't take it with you

    To a purist, neither of these models a flashlight. To say one is functionally identical and the other is not even remotely accurate is selective ignorance (in my opinion).

  7. #37
    Flashaholic* Cataract's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    I firmly believe it's all an emitter problem only.

    I'm fairly sure a jetfuel flashlight powered by a turboreactor emitter should move and/or break something... And don't give me none of that BC about emitter color or color rendering, it wasn't part of the OP's question.

    On might argue that a TNT flashlight would give much better color rendition, much more light, move, break AND vaporize plenty of things around, including the wielder's arm and itself. The problem is using it to see anything at all, so I discount that as more of a one-time use flashbulb than a flashlight.
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    A TNT flashlight will get Homeland Security all over you, or you all over everything else
    Last edited by M@elstrom; 07-31-2012 at 12:14 PM.
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  9. #39
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    dang i had no idea it was this complex!
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  10. #40

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    200gn bullet moving at 1250 feet per second could knock your hand back if you weren't locking your wrist and arm into your body. Figure that 1/2mass x velocity Squared and that the speed of light is 186,000 miles per second or so..
    Well I'm too relaxed to try and mess up calculating it, (what does a photon weigh again?) but you'd need some significant billionsth to billionsth of a billionstg of a gram of photons or something... The Death Star, that much directef radiation would knock a 24 oz. pistol out of your hand, except it turned the surrounding area into plasma first.

    ... Reading through more closely, xkcd is a great comic fyrestormer.
    Last edited by eh4; 07-31-2012 at 08:35 PM.
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  11. #41

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    Photons cannot leave a black hole because the gravity is so intense that the escape velocity is more then that of light. No electromagnetic radiation can escape because it is simply not traveling fast enough.

    The event horizon is where the escape velocity falls below that of light.

  12. #42

    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Isn't one of the deep space NASA probes slowing down because heat (light) is radiating from one side or front of the probe?

    I heard that rate of slowing was quite dramatic too, something like 250 miles slower per year.

  13. #43
    Flashaholic* Cataract's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by al93535 View Post
    Photons cannot leave a black hole because the gravity is so intense that the escape velocity is more then that of light. No electromagnetic radiation can escape because it is simply not traveling fast enough.

    The event horizon is where the escape velocity falls below that of light.
    Actually, it has been mathematically proven that black holes lose mass over time in the form of radiation (just ask Dr. Hawkings.) It has also been observed that black holes have radiation jet streams at their poles, so some things do escape from black holes and black holes can eventually disappear or grow and shrink over time.
    Little known fact: just around the event horizon area, photons escape because of all the matter that is overheated by friction, so a black hole is not completely black; the light just comes from above the event horizon.



    Quote Originally Posted by Quiksilver View Post
    Isn't one of the deep space NASA probes slowing down because heat (light) is radiating from one side or front of the probe?

    I heard that rate of slowing was quite dramatic too, something like 250 miles slower per year.
    you mean this?
    http://www.telegraph.co.uk/science/s...eep-space.html

    They're talking about 6mph per century, which is dramatic only in terms of physics since it means we're missing a big piece of the puzzle. These probes are wayyyy too far for any light to have even the fraction of a measurable effect on them. the stars they're headed for are much, much further in front of them than our sun is behind them and even the solar wind does not have any measurable effect on them anymore.
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  14. #44

    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by Cataract View Post
    Actually, it has been mathematically proven that black holes lose mass over time in the form of radiation (just ask Dr. Hawkings.) It has also been observed that black holes have radiation jet streams at their poles, so some things do escape from black holes and black holes can eventually disappear or grow and shrink over time.
    Little known fact: just around the event horizon area, photons escape because of all the matter that is overheated by friction, so a black hole is not completely black; the light just comes from above the event horizon.





    you mean this?
    http://www.telegraph.co.uk/science/s...eep-space.html

    They're talking about 6mph per century, which is dramatic only in terms of physics since it means we're missing a big piece of the puzzle. These probes are wayyyy too far for any light to have even the fraction of a measurable effect on them. the stars they're headed for are much, much further in front of them than our sun is behind them and even the solar wind does not have any measurable effect on them anymore.
    Thank you.

    I was referring to the heat/light being emitted by the probe itself, the internal functional components emitting heat/light radiation.

  15. #45
    Flashaholic* Cataract's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by Quiksilver View Post
    Thank you.

    I was referring to the heat/light being emitted by the probe itself, the internal functional components emitting heat/light radiation.
    According to the article I linked, they ruled out anything coming from the probe itself plus, as stated above in a few of the extensive essays I don't even dare read through myself, no amout of light/heat/radiation would really affect the probes speed in a way we could measure - especially from here. It is definitely something that has to do with physics that no one on this planet understands yet.
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  16. #46
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by Cataract View Post
    According to the article I linked, they ruled out anything coming from the probe itself plus, as stated above in a few of the extensive essays I don't even dare read through myself, no amout of light/heat/radiation would really affect the probes speed in a way we could measure - especially from here. It is definitely something that has to do with physics that no one on this planet understands yet.

    actually they confirmed, the probe component heat is responsible for the speed difference vs predicted speed.

    no mystery when they account for the probes own heat emissions
    posted by jh333233
    Dont cheat me, im expert in using crap light

  17. #47
    Flashaholic* Cataract's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by 127.0.0.1 View Post
    actually they confirmed, the probe component heat is responsible for the speed difference vs predicted speed.

    no mystery when they account for the probes own heat emissions
    Ah... good to know. Not too surprising since they always retract to something they had previously ruled out. But we are talking about a minor speed differential here, nothing that would move your arm. I'm kinda surprised they hadn't put that through the numbers earlier, those over-confident egg-heads.
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  18. #48
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by wrf View Post
    rocket => chemical reaction ejects part of its mass in one direction, driving the remaining mass (the payload you are interested in) in the opposite direction

    Wile E. Coyote => electo-mechanical fan generates the wind for the sail, but alas, you can't take it with you

    To a purist, neither of these models a flashlight. To say one is functionally identical and the other is not even remotely accurate is selective ignorance (in my opinion).
    A rocket works because the ejected mass has momentum, which it gained by physical interaction with the exhaust nozzle. That is exactly the same as what happens in a flashlight, because photons have momentum even if they have no mass. So yes, it is functionally identical. Their nozzles even have the same parabolic shape.
    Last edited by fyrstormer; 08-01-2012 at 03:42 PM.

  19. #49
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by al93535 View Post
    Photons cannot leave a black hole because the gravity is so intense that the escape velocity is more then that of light. No electromagnetic radiation can escape because it is simply not traveling fast enough.

    The event horizon is where the escape velocity falls below that of light.
    Incorrect. Black holes do lose mass through emitted radiation, which would be impossible if the escape velocity were greater than the speed of light. The event horizon is merely the area where space is curved so strongly that the only direction leading away from the black hole is "straight up", and all other paths point toward the singularity. Radiation can escape by traveling "straight up" from the singularity, but the statistical probability of a photon traveling "straight up" instead of some other direction is very small, so the evaporation process happens very slowly.

    http://en.wikipedia.org/wiki/Event_h...f_a_black_hole

    If black holes couldn't lose mass through evaporation, then the proton-sized black holes already created by the Large Hadron Collider would've fallen to the center of the Earth and started sucking up other particles by now. Fortunately, they are so small they evaporate in a puff of X-rays in a few thousandths of a second.
    Last edited by fyrstormer; 08-02-2012 at 10:16 AM.

  20. #50
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    Quote Originally Posted by Cataract View Post
    you mean this?
    http://www.telegraph.co.uk/science/s...eep-space.html

    They're talking about 6mph per century, which is dramatic only in terms of physics since it means we're missing a big piece of the puzzle. These probes are wayyyy too far for any light to have even the fraction of a measurable effect on them. the stars they're headed for are much, much further in front of them than our sun is behind them and even the solar wind does not have any measurable effect on them anymore.
    That article is ten years old. The question has already been answered. The slowdown is in fact caused by radiation being emitted from the front of the spacecraft.

    Here's a link to the article I posted, in case anyone else missed it on the first page of the thread. http://www.nytimes.com/2012/07/24/sc...ence&seid=auto

  21. #51

    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    hmm intresting, theyve been finding things without an equal and oposite reaction for a few years now.. along with actual footge of the old tackion particles, that apear to be changing shape/form.

    now to wait 25-30 years whilste they discredit any scientists dareing to come up with the proof..again.

    i dont recall the program names about the paricles and reactions mind.

    the bit about the particles involed a nobel prize winner who proved that the then current maths was rong for collecting particles, he got discredeted and fubarred off, spent his life proving it and his math by actualy collecting the right amount of them, as aposed to only a small percent. hell of a hobby there lol.

  22. #52
    *Flashaholic* fyrstormer's Avatar
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    I'm not aware of any process in the universe that doesn't produce equal and opposite reactions in some form or another. If there is one law of physics that underlies all the others, it is Conservation of Energy. I would be interested to know what processes you've read/heard about that don't conserve energy.

  23. #53
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    fyrstormer I admire your tenacity, but it seems everyone else want to argue while in possession of very few facts. Your posts have all been very sensible! BTW, whilst I agree that CofE is up there with the most fundamental laws, have you read about virtual particles being allowed to BREAK that law for a very short time? Hence the "quantum vacuum" and some really weird stuff- I think that's where black hole radiation comes from (too tired / hungover to search). In general terms, the fact that Planck's constant is really small seems to be vital to the way things are...
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  24. #54
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    Default Re: how powerful would a light need to be to knock your hand back when turned on?

    My understanding of virtual particles is they aren't actually virtual, they just exist for a statistically-insignificant amount of time before annihilating themselves against an antiparticle.

    I have heard of quantum particles breaking many laws of "classical" physics so long as the deficit is compensated-for elsewhere.

    I think the hypothesis you're referring to is that black holes lose mass by instigating the creation of particle-antiparticle pairs very close to the event horizon, and then one particle escapes while the other falls in, so the black hole only recoups half the energy used to create the particle-antiparticle pair. This is plausible, but it still requires that energy must escape the black hole in the first place in order to power the creation of the particle-antiparticle pair outside of the event horizon. As far as I know, photons are the only viable candidates for energy to escape from a black hole, because they are massless and therefore are only affected by the space-warping effect of gravity, not by its attractive force.

  25. #55

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    They are "virtual" for the current model.
    The brighter the light, the darker the shadow.

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