
#1
Jun912, 06:43 AM

P: 67

Is there any other object except photon which moves at the speed of light?
Why can't an object moving at the speed of light be taken as reference frame? Can we use the equation m=m(0)/sqrt(1v^2/c^2) for an object moving with speed of light? 



#2
Jun912, 07:11 AM

P: 3,178

And how would you use a photon as reference frame? A reference frame is a system for comparing (measuring) such things as time and distance. If a clock and ruler would be accelerated to light speed (although impossible), they would stop ticking and have zero length. 



#3
Jun912, 07:13 AM

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#4
Jun912, 07:24 AM

P: 67

Object moving at speed of light as Reference frame.
So does that mean it is impossible for an object with nonzero rest mass to move with speed of light? And Why?




#5
Jun912, 07:27 AM

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#6
Jun912, 07:51 AM

P: 848

Notice in the sequence that the moving observer's X4 and X1 axes rotate toward each other, getting closer and closer to each other as the speed of light is approached. In the limit the X4 axis and the X1 axis overlay each other. So, if the observer were actually moving at the speed of light, both his time axis and his spatial axis would be colinear with the photon worldline. How would you define that as a coordinate system? 



#7
Jun912, 08:49 AM

P: 67





#8
Jun912, 09:02 AM

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#9
Jun912, 10:44 AM

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#10
Jun912, 11:02 AM

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#12
Jun912, 11:32 AM

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P: 16,477

As far as why, that is inherently a tricky question. What are you allowing to be assumed when answering? And what kind of answer are you looking for? If I were asking the question I would be looking for a geometric answer and I would allow the Minkowski metric to be assumed. Then the answer is that a massive object has a timelike four momentum by definition, and any timelike four momentum corresponds to a three velocity < c. If that doesn't answer the question then you will need to clarify what you want better. 



#13
Jun1012, 09:30 AM

P: 3,178

If you did not manage to calculate that a division by zero is infinite, the answer is given in section 10 of http://www.fourmilab.ch/etexts/einstein/specrel/www/ : 


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