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7th grader needs help with speed of light and infinite mass

  1. Aug 21, 2012 #1
    In school yesterday my teacher told me that we could not travel at the speed of light because the faster we go the more our mass increases and that we would reach infinite mass before we reached the speed of light so there could never be enough energy to propel us that fast.

    I asked my teacher if we can't go the speed of light because of infinite mass then how come light can? She couldn't tell me.

    I also asked her if we were in a place that had no gravity to pull on us then would mass even matter when it came to moving us? So given enough time of constant pushing couldn't we get to the speed of light then?

    I understand that the photons are really small and have almost no mass but even if it had a mass of 1 with ten billion zeros in front of it wouldn't infinite mass still give the light the same problem?

    Oh also. Someone told me that light speed isn't constant?
  2. jcsd
  3. Aug 21, 2012 #2


    Staff: Mentor

    Light is a zero-rest-mass particle that travels at the speed of light in a pure vacuum. We know from relativity theory that particles of mass can approach but can never travel faster than light speed. This was how Einstein explained the Michelson Morley experiment's failure to discover a preferred direction of movement.

    As a fundamental premise to his theory. Einstein stated that the speed of light is absolute and nothing travels faster and taking that as a fact he derived time dilation, length contraction and the E=mc^2 results.

    We conclude that light is zero-rest-mass particle. You'll never see a particle of light just sitting there like other particles such as electrons, protons...
  4. Aug 21, 2012 #3


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    The only particles that can move at the speed of light are those with 0 mass.
  5. Aug 21, 2012 #4

    Jano L.

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    Hello Timmy,

    you have good questions, and they are not trivial - any scientist would like to know how it works with light and fast motion exactly. Keep asking that way, it is something people easily forget.

    There are different answers to your questions, depending on which theory you base it on.

    I think that so far the most reliable and the least speculative answer, is based on classical electrodynamics and the theory of relativity, which is this:

    Light is a kind of force pulling and pushing the object it falls upon (for example, those in the eye). The light - or the force - is determined by what the source of light did in past time. It turns out that the delay is given by the time it takes the wave of light to go from the source to the object.

    Now the equations describing light (Maxwell's equations) are such that they naturally lead to the idea that this speed of propagation of force is everywhere and always the same.

    This lead Einstein to remake mechanics and to propose a new equation of motion for the objects. This equation predicts that the speed of a body can only approach the speed of light. This is what Einstein discovered in 1905.

    It is probably useless to write down the equation here, but it can be understood in this way: imagine fast moving train and the light from its front lights. The speed of light is always and for everybody the same, 300 000 km/s. Let us assume the train goes faster, say 400 000 km/s.

    Such speed leads us to paradox. From the point of view of the conductor of the train, the light gets in front of the train, while from the point of view of the people near the rail track, the light loses on the train. These two points of view contradict each other, so we have to conclude that the train can go only with speed less than that of light.
    Last edited: Aug 21, 2012
  6. Aug 21, 2012 #5


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    Light is massless, and can travel at the speed of light. In fact, light can travel only at the speed of light.

    Mass is a measurement of the amount of matter. Gravity, such as that on earth, will give you a weight, which is a force. In a location in space with a negligible gravity field, your weight would also become negligable, but your mass would stay the same.

    Perhaps if someone told you that the speed of light is not constant, they meant that the speed depends upon the medium through which light passes, in which case in all mediums it is less that that through a vacuum.

    I am sure you have more good questions. Feel free to ask away any time.
  7. Aug 22, 2012 #6


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    Well the answer is that the "rest mass" of the photos is 0- so when moving at the speed of light (the only speed they will move at) their relativistic mass is very low (and not infinite).

    Gravity isn't the only concern with objects that have mass. Even in an environment with a vacuum and no gravitational pull inertia, momentum, energy etc. are still completely relevant. No, the speed would never be reached. As you've learned, as the object nears the speed of light (let's make up some numbers and say you reach 0.99c) the effective mass, and thus the energy required to increase the speed will become massive- the energy needed to reach the speed of light would be infinite. Not just all the energy on Earth- more energy than the entire universe has. That said, we can get close (with small low mass particles).

    You understand incorrectly- photons have a rest mass of zero- this is why they can move at the speed of light. There's a big difference between 1x10^-(10^9) and 0 when you're talking in terms of the speed of light. It's confusing, but they will have a mass when moving (they will only ever be moving at c)- though still very minuscule.

    Finally, someone told you very wrong. The speed of light is THE constant. Regardless of your reference frame, the speed of light is the speed of light.
  8. Aug 22, 2012 #7


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    Because light has no "rest mass" so its mass can't increase

    No, "F= ma" has nothing to do with gravity. Your acceleration would be a= F/m and as m increases, the same force will give you less and less acceleration.

    Then you understand wrong! photons (light) have NO mass- 0- nada- zilch!

    That depends on exactly what you mean by "light speed". The speed of light in vacuum is constant- if I were flying at .9 the speed of light and throw a ball straight forward to you, at speed, relative to me, of 50 mph, you will see that ball as having speed, relative to you, of slightly more than .9 the speed of light. But if I were to shine a flashlight ahead at you you would see the speed of the light relative to yourself as exactly c. How ever, the (average) speed of light will be less in water than in vacuum. Many people would argue that it is not really the speed of light that has changed- the speed of light between atoms still c and it is the time the light is being intercepted and then re-transmitted by the atoms that accounts for the apparent slower speed.
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