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

tobrien · Jul 28, 2012

so as the title says, how powerful, if it's even possible, would a light need to be to knock your hand back when turned on? is that even possible?

i sometime fantasize about a light that powerful lol

anyone who's versed in physics or whatever wanna tell us?

wrf · Jul 28, 2012

LED or hotwire?

127.0.0.1 · Jul 28, 2012

this type of problem has already been argued by physicists
the answer is it would not move at all, no matter how many photons

photons do not put pressure on whatever reflects them

DucS2R · Jul 28, 2012

My Surefire shotgun light seems to have some kick to it

ICUDoc · Jul 28, 2012

Electromagnetic radiation does produce a force, but not much. The energy carried by a photon is given (in special relativity) by: E^2 = p^2c^2 + m^2c^4. As they are massless this reduces to E^2 = p^2c^2. And energy = hf (where h is Planck's constant of 6.6x10^-34 Js, and f is frequency). The frequency of visible light is about 10^14 hertz. So we are talking very little energy and therefore extremely little momentum per photon.
To determine pressure, remember pressure = momentum transferred per second per unit area = in this case energy deposited per second per unit area / c = I/c (where I is the intensity of the beam of light (in W⋅m-2)).
For example the pressure exerted by the sun on a square meter of the Earth (at 1373 watts/square meter) is 4.6 micropascals (assuming it's absorbed- if you used a perfect reflector instead you double this)! So when you compare that to a one litre milk carton (weight force of about 9.8N over 1/100th of a square meter= 1000 pascals) you can see that the recoil you might get is pretty small.
Apparently it might be a decent force if you spread it over a long time and a large surface area (eg the "solar sail" propulsion system).....
Bottom line: forget making any palpable force from a hand held device.
If it makes you feel better, a Nichols Radiometer WILL show the effect of the momentum of the photons you shine on it!

127.0.0.1 · Jul 28, 2012

ICUDoc said:
Electromagnetic radiation does produce a force, but not much. The energy carried by a photon is given (in special relativity) by: E^2 = p^2c^2 + m^2c^4. As they are massless this reduces to E^2 = p^2c^2. And energy = hf (where h is Planck's constant of 6.6x10^-34 Js, and f is frequency). The frequency of visible light is about 10^14 hertz. So we are talking very little energy and therefore extremely little momentum per photon.
To determine pressure, remember pressure = momentum transferred per second per unit area = in this case energy deposited per second per unit area / c = I/c (where I is the intensity of the beam of light (in W⋅m-2)).
For example the pressure exerted by the sun on a square meter of the Earth (at 1373 watts/square meter) is 4.6 micropascals (assuming it's absorbed- if you used a perfect reflector instead you double this)! So when you compare that to a one litre milk carton (weight force of about 9.8N over 1/100th of a square meter= 1000 pascals) you can see that the recoil you might get is pretty small.
Apparently it might be a decent force if you spread it over a long time and a large surface area (eg the "solar sail" propulsion system).....
Bottom line: forget making any palpable force from a hand held device.
If it makes you feel better, a Nichols Radiometer WILL show the effect of the momentum of the photons you shine on it!

this is a flashlight problem: no force will affect the reflector...zero nada zip. photons can put some pressure on something
in front of the light but not the reflector. reflectors are not pushed by photons, they reflect alright but exert no force in doing so
photons vs other surfaces and bodies, different story

Retinator · Jul 28, 2012

The only kick you might notice is the fire extinguisher used on your flaming arm?

Seriously now........

I thought photons were mostly immune to gravity, no mass. But then again a black hole can suck them in so what do I know?

127.0.0.1 · Jul 28, 2012

Retinator said:
The only kick you might notice is the fire extinguisher used on your flaming arm?

Seriously now........

I thought photons were mostly immune to gravity, no mass. But then again a black hole can suck them in so what do I know?

from what I know, photons can exert pressure, but not on their own source, and not on reflectors

wrf · Jul 28, 2012

ICUDoc said:
[...]a Nichols Radiometer WILL show the effect of the momentum of the photons you shine on it!

Wow, I'm not too old to learn something new after all. I was thinking along the lines of Crookes radiometer, in believing the interaction of air molecules with the hot window of the light would be many times more significant than photon pressure. But then, I doubt anybody on this forum has a hand sensitive enough to feel it. A window hot enough to "knock your hand back" I figure would be bomb-like, and destroy itself and everything/anyone around it, but that is just a hunch.

Sintro · Jul 28, 2012

Retinator said:
The only kick you might notice is the fire extinguisher used on your flaming arm?

Seriously now........

I thought photons were mostly immune to gravity, no mass. But then again a black hole can suck them in so what do I know?

Because they have no mass, but are affected by black holes, I read that dark matter which has mass, gets pulled an by the black hole, and the space is warped, so the photons follow that normally straight path, which is now a warped path into the black hole.

fyrstormer · Jul 28, 2012

127.0.0.1 said:
this type of problem has already been argued by physicists
the answer is it would not move at all, no matter how many photons

photons do not put pressure on whatever reflects them

*cough* http://en.wikipedia.org/wiki/Radiation_pressure

127.0.0.1 said:
this is a flashlight problem: no force will affect the reflector...zero nada zip. photons can put some pressure on something
in front of the light but not the reflector. reflectors are not pushed by photons, they reflect alright but exert no force in doing so
photons vs other surfaces and bodies, different story

If radiation can exert pressure, as explained by my previous link, then the pressure MUST be exerted in the opposite direction that the radiation is moving. That is simple Newtonian physics. So if the photons exit the flashlight in one direction -- AFTER BOUNCING OFF THE REFLECTOR AND EXERTING PRESSURE ON IT -- then the flashlight must move in the opposite direction.

Photons have no mass, but photons don't need mass in order to exert pressure. Photons contain energy, and a certain amount of energy can always be mathematically equated to a certain amount of momentum. Quantum physics is confusing at times, but basically, quantum particles are capable of behaving like all of their mathematical equivalents at once. That means photons can bang into things and move them even though they have no mass, because their momentum is still absorbed by the object they collide with. Older versions of physics didn't predict this because older physicists had no experimental knowledge of quantum particles, and they were unable to discover and measure quantum effects because they didn't have equipment sensitive enough.

Notably, lasers are used in experiments with antimatter to pin the antimatter particles in-place so they can be observed. This works because the photons do exert pressure, and because they are laser photons, they all travel in the same direction with the same energy so the amount of pressure can be very precisely controlled.

fyrstormer · Jul 29, 2012

Retinator said:
I thought photons were mostly immune to gravity, no mass. But then again a black hole can suck them in so what do I know?

Black holes don't "suck in" photons. Photons have no mass. Gravity affects photons by bending the space they travel through, so even though they travel in a straight line, the definition of "straight" near a heavy object is actually curved. Black holes trap photons by bending space so much that all possible travel directions at the event horizon point straight into the black hole.

Chrontius · Jul 29, 2012

Let me put something in perspective - by the time the photon pressure is generating a pound of thrust, the air in front of the light will be superheated into a giant plasma explosion that will be... unhealthy for a normal human operating the device.

Norm · Jul 29, 2012

Pretty sure if you want to see photons move something you need a radiometer.

Norm

Norm · Jul 29, 2012

127.0.0.1 said:
this is a flashlight problem: no force will affect the reflector...zero nada zip. photons can put some pressure on something
in front of the light but not the reflector. reflectors are not pushed by photons, they reflect alright but exert no force in doing so
photons vs other surfaces and bodies, different story

Kinda like

127.0.0.1 · Jul 29, 2012

fyrstormer said:
*cough* http://en.wikipedia.org/wiki/Radiation_pressure

If radiation can exert pressure, as explained by my previous link, then the pressure MUST be exerted in the opposite direction that the radiation is moving. That is simple Newtonian physics. So if the photons exit the flashlight in one direction -- AFTER BOUNCING OFF THE REFLECTOR AND EXERTING PRESSURE ON IT -- then the flashlight must move in the opposite direction.

Photons have no mass, but photons don't need mass in order to exert pressure. Photons contain energy, and a certain amount of energy can always be mathematically equated to a certain amount of momentum. Quantum physics is confusing at times, but basically, quantum particles are capable of behaving like all of their mathematical equivalents at once. That means photons can bang into things and move them even though they have no mass, because their momentum is still absorbed by the object they collide with. Older versions of physics didn't predict this because older physicists had no experimental knowledge of quantum particles, and they were unable to discover and measure quantum effects because they didn't have equipment sensitive enough.

Notably, lasers are used in experiments with antimatter to pin the antimatter particles in-place so they can be observed. This works because the photons do exert pressure, and because they are laser photons, they all travel in the same direction with the same energy so the amount of pressure can be very precisely controlled.

light has different properties than ALL the other particles which are in the example you mention

light is special and does not apply

the math is sound but not when using photons, and the reflector doesn't get pressure

use a ground-based laser to elevate a space vehicle. yup, it goes up. but there is no force
inside the laser assembly pushing back on the earth. it is all pressure on the target after the photons leave

light can push, except it's source
use graphite instead of a reflector and then you get blackbody radiation and then that can push you along

and I could be full of it too but I am not just making this up

The ability of light to apply pressure to objects has been known for some time
but the reflector is the problem here in the example of a flashlight propelling itself

any other type of radiation or ion drive would push it right along fine

fyrstormer · Jul 29, 2012

You must not have read the article you're arguing against. The first particle it discusses is the photon, i.e. light, and they describe how photons exert radiation pressure on objects they impact. You don't know what you're talking about.

Let me make it really simple for you; read this section: http://en.wikipedia.org/wiki/Radiation_pressure#Quantum_theory_argument

I would be fascinated to know why you think the reflector is somehow immune to radiation pressure, but all other objects in the universe are susceptible to it -- especially since the article I linked also explains that reflective surfaces experience twice as much radiation pressure as absorptive surfaces.

fyrstormer · Jul 29, 2012

Norm said:
Pretty sure if you want to see photons move something you need a radiometer.

Norm

That particular radiometer is a bad example, because the motion is actually caused by IR absorption on the black vanes causing the low-pressure gas in the bulb to expand, pushing against the black vanes and making them move. There are other radiometers that can measure the radiation pressure of photons alone, but they aren't as entertaining to watch as the one you linked.

wrf · Jul 29, 2012

Norm said:
Kinda like

I vote for this one. I think it is funny as heck, and at the same time conceptually accurate (Newtonian wise).

ICUDoc · Jul 30, 2012

127.0.0.1 said:
this is a flashlight problem: no force will affect the reflector...zero nada zip. photons can put some pressure on something
in front of the light but not the reflector. reflectors are not pushed by photons, they reflect alright but exert no force in doing so
photons vs other surfaces and bodies, different story

I disagree. See Newton's laws. Momentum is a conserved quantity. You are clearly incorrect.
NORM- you'll need a Nichol's radiometer for the truth. The one you show WILL turn, but only due to heating effects, not momentum. A hotwire at close range will make that one you show really spin!

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

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