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curiousat61

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- Thread starter curiousat61
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- #1

curiousat61

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- #2

Edit: although you're right in natural units lol

- #3

curiousat61

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- #4

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So ##c^2## isn't a speed. What it actually is here is a scale factor relating our units of mass to our units of energy (which we thought were unrelated concepts pre-Einstein).

- #5

If you have something moving at ##0.8c## with respect to something else, which is itself moving at ##0.8c## with respect to a third thing, you might reasonably ask what speed the first thing is going with respect to the third thing. Relativistically, this is given by the velocity addition formula, and it is

But, that doesn't change the fact that ##0.8c + 0.8c = 1.6c##, that is still clearly true. It's just that the '+' operation is no longer the operation that represents adding two velocities relativistically.

- #6

curiousat61

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Thank you for the replies. I'm beginning to see the "light"...

- #7

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##c^2## isn't a speed, it's a speed squared, and has different units. You cannot compare the two, because ##c## and ##c^2## have different physical units.Thank you for the replies. I'm beginning to see the "light"...

Something like ##c^2 = c, \ c^2 < c## or ##c^2 > c \ ## would all be physically meaningless.

- #8

Sagittarius A-Star

Science Advisor

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Yes.I’m obviously looking at this the wrong way

Maybe, you believe due to bad formatting and wrong interpreting of it, that the formula is ##e= mc *2##, but it means ##e=mc^2=m*c*c##.…..with reference to the formula e = mc2 isn’t “c” (the speed of light) a constant? So if that is true doesn’t c2 (or any other multiple of c) equal c?

- #9

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It depends on your unit system. If you use geometric units where c=1 and c is dimensionless, then you could say c^2=c.

However, if you are using the standard SI units (meters, kilograms, seconds), c neither has a numerical value of 1, nor is it dimensionless.

1 foot does not equal 1 foot squared, they are different things. The same with c, in the usual unit system where it has dimensions.

2 does not equal 2^2, the only case where dimensionless numbers satisfy x^2=x is when x=1 or when x=0.

The square of a dimensionful quantity is never equal to the unsquared value, it is a units error.

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