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Questions about E=mc^2

  1. Mar 10, 2013 #1
    Hi.
    I have some questions that I'm not sure about to do with this equation. I may just be missing something obvious but would be nice to get an answer nonetheless..

    First is to do with the LHC. My understanding is that they are colliding particles at such force that they break down into particles that can't be observed as they are without mass. But the energy needed to make this happen is immense, yet the result has no mass. Doesn't this defy the fact that energy=mass?

    The other question is... There is no state of rest, all is relative. So when we measure the speed of light in a vacuum, it's still relative to the speed at which we move... The closer you are to the speed of light, the slower you'd see light as travelling, so if you were to use e=mc2 on your own mass, then the speed of light will have reduced relative to your increase in energy, so the result would come out exactly the same? Or am I missing something really obvious here....
     
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  3. Mar 10, 2013 #2
    Hi,

    I'm not sure what you're asking in the second part, but the speed of light is a universal constant. If you are moving, say "with" a beam of light (side-by-side), it will appear to be moving at a speed of 3.0x10^8 m/s regardless of your relative speed. The constancy of the speed of light is a foundation in special and general relativity, and has profound implications.

    Hope that helps you out!
     
  4. Mar 10, 2013 #3

    phinds

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    No, the speed of light is the same regardless of your motion and/or the motion of whatever emits the light. In the sense in which you are talking, you will always measure the speed of light as c.
     
  5. Mar 10, 2013 #4

    Nugatory

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    This one belongs in the relativity forum, I think.... Likely enough a moderator will move it.

    The speed of light in a vacuum is the same for all observers. Say I'm at rest, and at time zero I send a flash of light off in some direction and at the same time you take off on a spaceship traveling at .25 the speed of light in the same direction.

    Relative to me, you are moving at .25c, the light is moving at c, and the distance between you and the light is increasing at .75c. Relative to you, the light is moving away from you at c, I'm moving on the opposite direction at .25c, and the distance between me and the light is increasing at 1.25c.
     
  6. Mar 10, 2013 #5

    Nugatory

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    Energy always has mass given by Einstein's equation. When someone speaks of a particle "without mass", they mean "without rest mass", meaning that all the energy and hence mass comes from the kinetic energy of the particle's motion. (BTW, these particles are the only ones that can move at the speed of light, and that is the only speed at which they can move).

    There's no problem observing massless particles... Your eyes do it by the trillions every second when they detect light.
     
    Last edited: Mar 10, 2013
  7. Mar 10, 2013 #6
    Hi Dan, and welcome to PF!
    No, this is not correct (except for annihilation). There are many particles produced in colliders which have mass.
    No. As pointed out above, the speed of light is constant according to Special Relativity (see point #2).

    EDIT: I saw a couple of replies about this already being posted, while I was writing my reply...
     
    Last edited: Mar 10, 2013
  8. Mar 10, 2013 #7
    Apologies if these are really basic questions, but it's hard to find answers and information on this sort of thing, so I appreciate the answers...

    In this situation, if you're at rest, how do you observe the light moving away from you at 1.25c? Isn't it then moving away from you faster than the speed of light?
     
  9. Mar 10, 2013 #8

    Nugatory

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    I don't observe it moving away from me at 1.25c, I observe it moving away from me at c while you are moving in the same direction at .25c.

    You, however, see me moving to the left at .25c and the light moving to the right at c.

    No one sees anything moving at a speed greater than c and everyone sees the light moving at c.
     
  10. Mar 11, 2013 #9
    So If I am moving at 80% the speed of light, observing a light beam that has originated from the same location as me, do I see the speed that it is moving away from me as C or do i see it as 20% of C???
     
  11. Mar 11, 2013 #10

    Doc Al

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    You will measure the speed of light as c. Everyone does!
     
  12. Mar 11, 2013 #11
    So isn't Light's speed effectively infinite in that case?
     
  13. Mar 11, 2013 #12

    Doc Al

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    Well, no. Light's speed is effectively c. The tricky part is that it's c with respect to any observer, regardless of that observer's motion.

    Why would you say it was infinite?
     
  14. Mar 11, 2013 #13
    Okay I have a question, E=mc^2 only works with 100% efficiency with anti matter as it's energy source. How do we get anti matter? can we?
     
  15. Mar 11, 2013 #14

    phinds

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    Anti matter is created routinely in particle accelerators, but only in tiny amounts.
     
  16. Mar 11, 2013 #15

    phinds

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    Which part of post #3 did you not understand?
     
  17. Mar 11, 2013 #16
    dude thats not how light works. relativity states that light is special unlike other things so when you move at 80% off the speed of light, you see the light pass you at the full speed of light, about three-hundred million meters per second. unlike when u r in a car and you are going 60 and the guy going 70 passes you at 10 miles per hour. light. is. special.
     
  18. Mar 11, 2013 #17
    yes you are missing alot of things first of all particles are made out of the most fundemental constituants of matter called quarks (forgive my horrible spelling), the quarks cant be seperated so they cant be studied on their own. Hadrons: protons neutrons, and electrons have three quarks. they crash various hadron particles in various different types of experiments at .9c or almost the speed of light. the result is a bunch of other particles from other fields get nocked out of place and into existance ( not necisarilly existance) but into our plane of reference the faster we collide these particles, the more particles we get in result, we collided particles so fast we nocked loose a higgs boson which was really hard because it was so heavy and deeply embeded in the higgs field. you are confussed by the field of quantum electrodynamics, as you know, light is a massless particle called a photon when photons bounce off of the particles from the collision at the lhc they move crazy
     
  19. Mar 11, 2013 #18
    Hey Dan!
    I suggest that you chekcked some introductory relativity textbook (Check the general physics of Serway, Young etc). The law of velocity addition changes when u->c.
    This way, if you approach an object that moves with c, and your velocity is .25c then you will see it moving with c again.
    Same goes if you move away from it.
    All this is a conclusion of the fact that all laws of physics are invariant for observers moving with constant velocity. A.k.a an observer moving with u and another moving with v= u+u' measure the same physical laws.
    One set of these laws are the Maxwell equations. If you go from one reference frame to another moving with different velocity, the electromagnetic wave ecquation will be the same -- (d^2/dx^2)E =(1/c)^2*(d^2/dt^2)E -- the 1/c term remains invariant and this is the 1/speed of light.
    Thus speed of light must be invariant among 2 observers with constant relative velocity and thus they will both measure c no matter what.
     
  20. Mar 12, 2013 #19
    No, this is not correct (you might know this, but wrote to quickly :smile:). Electrons are not hadrons and they are not made of quarks, electrons are elementary particles.

    For Dan and others:

    Elementary particles are particles with no known substructure, that is, the quarks, leptons (e.g. electrons) and the fundamental bosons (including all of their corresponding elementary antiparticles). Hadrons (baryons, mesons) are particles made up of quarks. Baryons (e.g. protons, neutrons) are made up of 3 quarks. Mesons (e.g. pions) are made up of 2 quarks. Welcome to the particle zoo :smile:. For an overview of the zoo, see this link.
     
  21. Mar 12, 2013 #20
    It's an extremely difficult concept to grasp and many people struggle with it including myself. If you and a particle of light were to have a race on a race track. You're running along side the particle of light at 99% the speed of light, that particle will appear to be pulling away from you and gaining distance on you at C.

    The people in the crowd watching see you and the light particle almost tied for the lead.


    If you're sitting in a plane flying at 99% the speed of light and you lean out the window and fire a particle of light, that light will gain distance on you @ about 186,000mph. While the people on the ground watching will see the light barely making any distance on you.
     
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