If you are familiar with Einstein's special theory of relativity, your answer would immediately be that the mass of the body approaches infinity as you get closer to the speed of light. Though this answer comes directly from the following famous equation derived by Einstein to prove the equivalence of mass and energy, E=mc^2 it is not very intuitive. Last evening, thanks to MinutePhysics, I learnt a much more fun and intuitive answer to this question.
When we look at the above equation, we often fail to realize that the equation is applicable only to bodies that are at rest relative to an observer. Moving bodies have velocity and in turn have momentum, p=mass * velocity This momentum term has to be added to mass-energy equivalence equation which gives rise to the more complete equation, E^2=(mc^2)^2+(pc)^2 Do you see that this equation looks very similar to the pythogoras theorem? a^2=b^2+c^2 where a is the hypotenuse of a right angled triangle, b and c are the sides.
Now watch the following video to understand how this new equation demands that a body with mass cannot travel at the speed of light.
When you watched the video, you might have been confused by a point made by the narrator that if the energy of a body is equal to pc, then it would travel at the speed of light. This implies that the body does not have mass. But from my earlier description of momentum, one can see that it is the product of velocity of the body and its mass. Hence if a body is massless, it should have no momentum and hence no energy. This doesn't sound right. Is Einstein wrong?
Well, he is not. There are two kinds of mass in that equation. When we say that a body is massless, like a photon, its rest mass is zero. Rest mass is the mass of a body measured when it is at rest relative to an observer. As the body starts moving, its mass increases. This is called the inertial mass and it is this mass which is a part of the momentum equation. The rest mass of a body always remains the same while the inertial mass varies with relative velocity. Now if you watch the video again you will see clearly why bodies with mass cannot travel at the speed of light.
When we look at the above equation, we often fail to realize that the equation is applicable only to bodies that are at rest relative to an observer. Moving bodies have velocity and in turn have momentum, p=mass * velocity This momentum term has to be added to mass-energy equivalence equation which gives rise to the more complete equation, E^2=(mc^2)^2+(pc)^2 Do you see that this equation looks very similar to the pythogoras theorem? a^2=b^2+c^2 where a is the hypotenuse of a right angled triangle, b and c are the sides.
Now watch the following video to understand how this new equation demands that a body with mass cannot travel at the speed of light.
Well, he is not. There are two kinds of mass in that equation. When we say that a body is massless, like a photon, its rest mass is zero. Rest mass is the mass of a body measured when it is at rest relative to an observer. As the body starts moving, its mass increases. This is called the inertial mass and it is this mass which is a part of the momentum equation. The rest mass of a body always remains the same while the inertial mass varies with relative velocity. Now if you watch the video again you will see clearly why bodies with mass cannot travel at the speed of light.
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