1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strong light

  1. Nov 22, 2003 #1
    hi i'm new, i was doing some pondering after watching a show about new modes of transportation. one of the modes was for space travel and using solar sails. in the show it was stated that the main force would be from light emitted from the sun. i thought that was cool, as it was explained (and shown) that photons exert force upon the sail itself in a similar fashion to wind and conventional sails. i got to wondering how much does a photon weigh if anything. i say if anything because force = mass*acceleration. well at 186,000mps you'd think that we would be crushed by light. here's my question is it possible for light to actually not exist within known dimensions? that perhaps light is a product from the 5th dimension that warps the 3rd and 4th?

    now for the biggy, do i sound stupid?
  2. jcsd
  3. Nov 22, 2003 #2
    Photons are massless, but they do carry momentum, and so can exert a force (which is change in momentum).

    You'll notice that the equation for force you gave doesn't actually involve speed, so where exactly are you trying to plug in 186,000 mps?

    Depending on latitude, time of year, etc., the Sun radiates about 1360 watts per square meter of light upon the surface of the Earth. That's power per unit area, P/A. If we know power is energy per unit time P = dE/dt, and photon energy is related to momentum by E = pc, then the momentum per unit area per unit time is, (dp/dt)/A = F/A = pressure = (P/A)/c.

    Therefore, sunlight exerts a pressure of 4.6 microPascals, or a force of 4.6 microNewtons upon one square meter, equal to the weight of a 460 microgram mass.

    Note that the large numerical value of the speed of light contributes to the numerical smallness of the light pressure, since you're dividing by c.

    If light doesn't exist within known dimensions, then how can we see it?

    If by "warps" you mean "curves", what does spacetime curvature have to do with anything? That would just induce a gravitational effect, but it wouldn't let us see light.
  4. Nov 22, 2003 #3
    please forgive my lack of knowledge, if photons are massless and do carry momentum and can exert force then your getting something for nothing (this doens't include the source of the light)am i incorrect in assuming that? and about the 5th dimension thing, i guess i'm asking is it possible that photons cannot be measured because they do not originate with in the 1st 2nd 3rd and 4th dimensions? from what i've read and you've stated they shouldn't exist but do. and by warps i mean warps causes an anomoly that eye's pickup as light. but once please forgive my lack of knowledge.
  5. Nov 22, 2003 #4
    What does "something for nothing" mean in this context, and why do you think anyone is getting it?

    But photons can be measured.

    Why shouldn't they exist? Who said something had to have mass to exist? Photons aren't matter, but they certainly exist and have physical properties, including energy, momentum, and angular momentum.

    Why so convoluted? The eye picks up light when a photon is absorbed. What purpose does introducing extra dimensions serve?
  6. Nov 22, 2003 #5
    by something for nothing, if photons have no mass then how can a object with no mass have momentum, and exert force? which actually brings up another thought if photons(light) have no mass then how can they become trapped in a black hole?(ignore the last one)

    and by measured i mean their weight i mean, the actual mass of a photon. and finally i dunno its late for me, and i'm tired.(22 hours and still going sluggishly.) it just doesn't seem right that something can exist without any physical properties. and why shouldn't there be anomolies that cause light instead of the direct assumption that light is photons being absorbed? i'll get back to you on it in say 12-20 hours.

    ps i see where yer coming from but puting what i'm thinking into words is proving to be much harder than i had first anticipated.
    Last edited by a moderator: Nov 22, 2003
  7. Nov 22, 2003 #6
    Maybe if you explained why you think massless objects can't have momentum, we'd get somewhere. You keep making these statements about how things are impossible, but I don't know what your underlying assumptions are.

    For instance, you may be using the non-relativistic equation,

    [tex]p = mv[/tex]

    instead of the relativistic

    [tex]p = \sqrt{E^2-(mc^2)^2}/c[/tex]

    The curvature of spacetime influences the motion of everything within spacetime, mass or not.

    Weight isn't mass; weight is force. Photons exert force, so in that sense you could say they have "weight": I calculated that quantity above.

    What? I just listed several physical properties that photons have.

    Well, you could assume that when we detect a photon in a photon detector, it's really just some convoluted extradimensional illusion that just makes it look like a photon is being detected ... but why, and what evidence supports this idea over the interpretation that you simply detected a photon?
  8. Nov 22, 2003 #7
    Not really that silly a question can see how it is that u got confused, for i myself was also confused by that, couldnt get my head round how a particel with no mass could have momentum but eventually i did, so its not such a stupid question afterall.

    Why did i bother typing that crap?
  9. Nov 23, 2003 #8
    if it has wieght, doesent it have to have mass,


    where f is the weight due to gravity, a accelration, and mass...so if photons have no mass...how can it have a force
  10. Nov 23, 2003 #9
    Newton originally defined force as F=dp/dt, where p is momentum. That definition still holds in relativity, but F=ma doesn't, because p=mv doesn't.

    (And who said anything about weight due to gravity?)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook