Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Effect of Light on Mass

  1. Aug 1, 2004 #1
    I'm looking for the conventional explanation, rather than unsubstatiatied theories.

    Mass is the prerequite property expressed in my physics texts for measuring quantities. In Newtons Laws, I have learned to this point mass is necessary to get a value of displacement. Without a physical form sensed by a human such as mass, displacement quantities may not be aprehended, which has implied to me that concepts such as speed through energy are unmeasureable unless mass is sensed.

    But, I've been told authoritatively that light has no mass. Newton laws have assured me that things with no mass may not interact with mass, and if it has interacted then this is mass. If light has no mass, if it's not substance, if it's not physically hard, how is light's velocity substatiated when no mass exists to have an effect upon another mass (the measuring instrument), which would be used to express a quantity such as displacement, speed, acceleration, force, impulse, work or energy?
  2. jcsd
  3. Aug 1, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It's not really mass that is important at all in classical mechanics... it's momentum. Newton's second law is F=dp/dt; the derivative of momentum with respect to time. F=ma is a convenience in a theory where p=mv and in a situation where m is constant.

    And even before 1900, it was known that mass can interact with fields. For example, a charged, massive particle can interact with an electromagnetic field... something that does not have mass. However, it was already known that the interaction of an electromagnetic wave with charged particles could be described as if the wave has momentum. For example, it was known that an electromagnetic wave that reflects off of a mirror will impart momentum to the mirror.

    Quantum mechanically speaking, the photon is an approximate piece of the electromagnetic field. Since the field does not have mass, but can impart momentum, then those are properties photons have.

    Back to relativity, momentum is no longer described by p=mv; it is instead described by [itex]p=\gamma mv[/itex]. In the prerelativistic case, if m=0, then p=0, period. However, in the relativistic case of a massless light speed particle, this equation becomes indeterminate; while m=0 says that p should be zero, v=c says that p should be infinite. (as v-->c, [itex]\gamma[/itex]-->infinity) Through a manipulation of the total energy equation, [itex]E = \gamma m c^2[/itex], which is the sum of the rest energy [itex]E = mc^2[/itex] and kinetic energy [itex]K = (\gamma - 1)mc^2[/itex], one can derive the formula p = E/c which holds for a light-speed massless particle.

    Since a light-speed, massless particle has momentum (proportional to its energy), it is capable of interacting with massive tardyons (sublight-speed particles), as can be seen via Newton's law F=dp/dt, or by conservation of momentum.
  4. Aug 1, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Now, the other side of the coin, the problems with your conclusions.

    We can, and frequently do, talk about displacement of things that aren't even matter, let alone massless. For example, as I type, I can observe the displacement of the text cursor. However, the cursor is an abstract object, not a real thing.

    The reality is that sometimes one part of my monitor stops glowing, and sometimes another part stops glowing. However, I can still, to some extent speak about this abstraction as if it is a real thing. For example, I can measure displacement, its velocity, and its acceleration.

    There are lots of other common examples; the crest of a wave, a spot on a wall as I wave around a flashlight...

    The wave example is interesting because it obviously does carry momentum and energy. So, we see that even in prerelativistic mechanics, abstractions can carry momentum, do work on objects, et cetera.
  5. Aug 1, 2004 #4
    To Hurkyl
    Whilst I know what your getting at and agree , I do not agree with your last statement , an abstraction is capable of moving at any speed, such as your wall spot or the conjunction of a wave with an oblique shore line , this shows that abstactions carru NO momentum and is the way of distinguishing them. As such whatever a wave decribes , it itself is not abstract if limited to 'c' or less .
  6. Aug 1, 2004 #5
    I don't follow. What does that mean?
    What they meant was the proper mass (aka rest mass) of light is zero. Light still has non-zero inertial mass (m = p/c), passive gravitational mass (responds to gravitational field) and, active gravitational mass (generates a gravitational field).
    Sounds okay to me.
    Let me remind you of something Einstein said in his famous 1916 General relativity paper
    Since light is an EM field then it can be considered to be "matter".

  7. Aug 2, 2004 #6
    Thats very interesting, but what does the gamma symbol represent?
  8. Aug 2, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    \gamma := \frac{1}{\sqrt{1 - (v/c)^2}}
  9. Aug 2, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I disagree with this. A wave is describes the behavior of objects (like water molecules) and is not an object itself; thus, it's an abstraction. Maybe we just disagree on the meaning of abstract.
  10. Aug 3, 2004 #9
    I don't see how that's clear. Momentum may not be represented, unless mass is there. Momentum is p = mv. I don't sense the v, but I can sense the m in this equation. I use the motion of mass against a stationary point to develop the concept velocity. Multiplying the mass(weight or N) by the number of motion frames gives me momentum. I'm assuming momentum is only a productive consequence of mass and velocity.

    Isn't interaction just another word for N III Law? A massive partical interacts with an electromagnetic field.

    Could I say a hunk of gold is just a field, then too call it non-mass?

    Is my assumption that mass is identical to any dimensional entity that exists that may have interactions incorrect?
  11. Aug 3, 2004 #10
  12. Aug 3, 2004 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I have to ask if you bothered to read my post...

    How? I honestly don't know how one would even begin to go about sensing mass.

    As the old riddle goes, if I say a tail is a leg, how many legs does a horse have? (4)

    Yes. Although I don't know what you could mean by "dimensional entity that exists", but it doesn't resemble any definition of mass I've ever seen.
    Last edited: Aug 3, 2004
  13. Aug 4, 2004 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    You'll need to think about where you got these ideas. As the example you yourself have brought up show, they are basically not true in relativity.

    Think of momentum as a fundamental physical quantity, one you can interact with, not a derived quantity. Momentum is not just "mass * velocity", and as many have attempted to point out to you, that formula doesn't work anymore in relativity. (And there are some problems with your definitoin even with classical physics, when calculating what the momentum is in generalized coordinates, for instance).

    Momentum *is* something physical, something you can feel. When you get hit by a baseball, it's not the mass you feel, it's the momentum.
  14. Aug 5, 2004 #13
    What is a good definition of mass?

    I like to think of mass as a set of force fields that occupy some amount of space. These force fields include electric fields, which are always connected to magnetic fields, and potentially other force fields which have not yet been observed.

    I also believe that there is no physical or mechanical mass in the universe. This means that all particles do not have any mechanical mass at the center of the particle. The mass, that we perceive, is simply the interaction of one set of force fields against another set of force fields.

    Does this make it easier to consider "The effect of light on mass" ?
  15. Aug 5, 2004 #14
    They occupy space, therefore something could not be in the space they are in.

    But we wouldn't know they are there unless something attempted to occupy it's space, which makes necessary an interaction.

    Does this means we call it mass until we detect that it is an interaction rather than some expression of mass?

    I'm assuming things that occupy space and interact with other things that occupy space to be mass. If not, then what are the general category names for things that have interactions, which denotes the axiomatic fact it occupies space, so we can move closer to the original questions answer?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook