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  1. Subscriber joe shmo
    Strange Egg
    17 Oct '09 15:41
    E = mc^2

    Do we agree that light has energy?

    Here is whats not clear to me.

    In order for light to have energy it must have mass, as shown by the equation, but my physics professor(without going into any great detail)mentioned that anything with mass cannot accelerate to the speed of light....So if light has mass, then how can it travel at "c"

    is this a paradox, or a simple misunderstanding?
  2. Subscriber coquette
    Already mated
    18 Oct '09 05:16
    Originally posted by joe shmo
    E = mc^2

    Do we agree that light has energy?

    Here is whats not clear to me.

    In order for light to have energy it must have mass, as shown by the equation, but my physics professor(without going into any great detail)mentioned that anything with mass cannot accelerate to the speed of light....So if light has mass, then how can it travel at "c"

    is this a paradox, or a simple misunderstanding?
    Neither. It's a complex misunderstanding. Few manage to understand it. The ones who think that they understand it are often deluded or delusional.

    The fact that a formula exists that shows a relationship and equality of balance does not mean that the factors are identical. Light behaves as a wave when you approach it as a wave and as mass when you approach it as mass. There is no contradiction, other than breaking the concrete ideations of which we've been imprinted in order to make the complexities sensible.
  3. 18 Oct '09 06:55
    Originally posted by coquette
    Neither. It's a complex misunderstanding. Few manage to understand it. The ones who think that they understand it are often deluded or delusional.

    The fact that a formula exists that shows a relationship and equality of balance does not mean that the factors are identical. Light behaves as a wave when you approach it as a wave and as mass when you approa ...[text shortened]... e concrete ideations of which we've been imprinted in order to make the complexities sensible.
    I'm surprised that noone has attempted to answer the question. There are many people here who has the capacity to explain it. Where are you?
  4. Standard member sonhouse
    Fast and Curious
    19 Oct '09 03:04
    Originally posted by joe shmo
    E = mc^2

    Do we agree that light has energy?

    Here is whats not clear to me.

    In order for light to have energy it must have mass, as shown by the equation, but my physics professor(without going into any great detail)mentioned that anything with mass cannot accelerate to the speed of light....So if light has mass, then how can it travel at "c"

    is this a paradox, or a simple misunderstanding?
    Mass is the other side of energy, light (photons) impart momentum, if they hit stuff, that stuff gets a little impulse, which is why solar sails work, they can move around the solar system for free, tacking like a boat with sails to change direction. If you hit a high enough energy photon with another one of the same energy, you can actually create matter from seemingly nothing and when you combine anti-matter with matter it converts all of that into gamma rays, which is very high frequency, short wavelength photons way smaller than light or X rays. Some individual gamma rays hitting the top of the atmosphere hits atoms up there with the energy of a well placed tennis slam, all that from one photon smaller than an atom.
  5. 19 Oct '09 09:58
    Originally posted by joe shmo
    E = mc^2

    Do we agree that light has energy?

    Here is whats not clear to me.

    In order for light to have energy it must have mass, as shown by the equation, but my physics professor(without going into any great detail)mentioned that anything with mass cannot accelerate to the speed of light....So if light has mass, then how can it travel at "c"

    is this a paradox, or a simple misunderstanding?
    I don't know how relevent this is, but I was taught that the equation should/could simply read e=m, and leave the speed of light out completely. Since I don't claim to understand relativity, I don't know how true that statement is (energy = mass), but it sure did simplify things for me at the time. Still, I feel pretty certain Albert would not have put SOL squared in there if it were superfluous...
  6. 19 Oct '09 10:17 / 1 edit
    Originally posted by PinkFloyd
    I don't know how relevent this is, but I was taught that the equation should/could simply read e=m, and leave the speed of light out completely. Since I don't claim to understand relativity, I don't know how true that statement is (energy = mass), but it sure did simplify things for me at the time. Still, I feel pretty certain Albert would not have put SOL squared in there if it were superfluous...
    I've seen the same thing E=M, but then it only says that you can convert energy to mass and vice versa. But in real fact you have to bring in c^2 to do math with it.

    I've also seen that time=money, but you cannot do any math with this either. Samo samo.

    Yes, a massless photon contains energy so why can we use E=mc2 here? The question still stands unanswered.
  7. 19 Oct '09 11:07 / 2 edits
    E = m in natural units, but I'm not going to discuss that here.

    E = mc² is the notation used by Einstein where m represents the relativistic mass. In modern notation m usually represents the rest mass and the equation reads:

    E = gamma * mc², where gamma is a velocity-dependant factor that is not defined for v = c. For v = c special relativity gives another value for E, that is, relativity permits massless particles provided they move at the speed of light and obey:

    E = cp, where p now is the momentum of the particle, which by the way also defines the momentum for massless particles.

    And just to get this clear: light consists of massless particles.

    An alternative way of formulating E = mc² which perhaps clarifies this issue a bit is:

    E² = m²c^4 + c²p², where m now represents the rest mass. In the case of a particle with mass m at rest it reduces to E = mc² (now with m rest mass, so not the same as Einstein's E = mc² !) and in the case of a particle with no mass it reduces to E = cp.
  8. Standard member smw6869
    Granny
    19 Oct '09 17:17
    Originally posted by KazetNagorra
    E = m in natural units, but I'm not going to discuss that here.

    E = mc² is the notation used by Einstein where m represents the relativistic mass. In modern notation m usually represents the rest mass and the equation reads:

    E = gamma * mc², where gamma is a velocity-dependant factor [b]that is not defined for v = c
    . For v = c special relativi ...[text shortened]... same as Einstein's E = mc² !) and in the case of a particle with no mass it reduces to E = cp.[/b]
    You're kidding, right !?!

    GRANNY.
  9. Subscriber coquette
    Already mated
    21 Oct '09 05:42
    Originally posted by KazetNagorra
    E = m in natural units, but I'm not going to discuss that here.

    E = mc² is the notation used by Einstein where m represents the relativistic mass. In modern notation m usually represents the rest mass and the equation reads:

    E = gamma * mc², where gamma is a velocity-dependant factor [b]that is not defined for v = c
    . For v = c special relativi ...[text shortened]... same as Einstein's E = mc² !) and in the case of a particle with no mass it reduces to E = cp.[/b]
    Exactly my point.
  10. Subscriber AThousandYoung
    Do ya think?
    22 Oct '09 05:20
    Originally posted by KazetNagorra
    E = m in natural units, but I'm not going to discuss that here.

    E = mc² is the notation used by Einstein where m represents the relativistic mass. In modern notation m usually represents the rest mass and the equation reads:

    E = gamma * mc², where gamma is a velocity-dependant factor [b]that is not defined for v = c
    . For v = c special relativi ...[text shortened]... same as Einstein's E = mc² !) and in the case of a particle with no mass it reduces to E = cp.[/b]
    I didn't know that gamma mc^2 is the same as that last formula. The latter formula is the one I remember.
  11. 22 Oct '09 11:17
    Originally posted by AThousandYoung
    I didn't know that gamma mc^2 is the same as that last formula. The latter formula is the one I remember.
    Not really the same, since E² = m²c^4 + c²p² is more general and also applies to massless particles.
  12. Standard member sonhouse
    Fast and Curious
    31 Oct '09 17:04
    Originally posted by KazetNagorra
    Not really the same, since E² = m²c^4 + c²p² is more general and also applies to massless particles.
    The units of p=newtons in CGS system?
  13. Standard member sven1000
    Astrophysicist
    02 Nov '09 21:19 / 1 edit
    Originally posted by sonhouse
    The units of p=newtons in CGS system?
    Newtons are units of force in the SI system, force in CGS would be dynes. Momentum in CGS would be g*cm/sec, or dyne*sec.

    p is a momentum variable
  14. Standard member sonhouse
    Fast and Curious
    03 Nov '09 03:15
    Originally posted by FabianFnas
    I've seen the same thing E=M, but then it only says that you can convert energy to mass and vice versa. But in real fact you have to bring in c^2 to do math with it.

    I've also seen that time=money, but you cannot do any math with this either. Samo samo.

    Yes, a massless photon contains energy so why can we use E=mc2 here? The question still stands unanswered.
    Because if you slam two photons together with sufficient energy, they will create mass all by themselves, converting their zero mass to actual mass that weighs something. Maybe the stuff won't hang around long till it craps out and becomes pure energy again but the point is the energy in massless photons can convert to actual mass if the two photons possess enough energy. Some cosmic rays, a single pulse of cosmic ray which is EM with zero mass, contains the same amount of energy as a well served tennis ball.
    Think about that, a condensed version of pure EM energy so small you could not see it in the best microscopes has the energy of a well hit tennis ball. That sounds pretty impressive to me!
  15. Subscriber AThousandYoung
    Do ya think?
    05 Nov '09 05:47
    Originally posted by FabianFnas
    I've also seen that time=money, but you cannot do any math with this either.
    http://komplexify.com/math/humor_pure/EvilnessOfGirls.html