**How to calculate the speed of anything, when their speed becomes closer to the speed-of-light. **

*This article was originally a comment in the linked article*; Why Nothing Moves Faster Than Light.

— To correct the comment I have made earlier ” unless *something is completely massless in its rest-frame* ” I also add the following. This is a fact which I have realized lately — or rather trapped myself to commit an inconsistent remark, by following the same comment.

But it’s better late than never; **when ***something is massless, it will never have a rest-frame, because *by Einstein’s transformation rules known as Theory of Relativity to be consistent, *a massless particle will always move at the speed of light, no matter which frame we are looking at it from*.

This then leads to the velocity addition formula of Einstein.

According to Newton and Galileo — Galileo Project., known by a name **Galilean Relativity**, the following follows; if **C ***moves *at speed** speed_cb ***wrt*** B**, **B ***moves *at speed** speed_ba ***wrt*** A** then **C **must *move *at speed **speed_ca ***given* by** speed_ca = speed_cb + speed_ba.** The speeds get added up.

Let’s take this simple example,* to illustrate the above*; A bullet moves at speed **200 **km/hr in a train and the train moves at **250 **km/hr** **wrt Jason, who is standing still, on ground, then ground is A, train is B and bullet is C.

So Jason will see that the bullet will move at speed

200+250 = 450km/hr. ThusGalilean Relativity:speed_ca = speed_cb + speed_ba.Note that the b as an index vanishes, if we keep the indices in proper sequence. ca = cb + ba

But Einstein modified this, **to make the rule consistent**, with how it will work out *for light or any relay of information at that speed*. To make *light always move at speed c = 2.99 x 10 ^8 meter/sec* the simple addition becomes **“warped”**.

It’s still known as addition of velocities or addition of speed and given by **speed_ca = [ speed_cb + speed_ba ] / [1 + speed_cb * speed_ba]** where *we will get the results in terms of speed of light being 1* **and** *speed of anything else as a fraction given by*** speed_ca**.

We then simply multiply **c = 2.99 x 10 ^8 **meter/sec to that fraction, to get the exact speed, in terms of meter/second or km/hr** etc**. Not only we have to multiply this value to any result we obtain from this formula, we also have to divide this value 2.99 x 10 ^8 meter/sec before carrying out any calculation, in order to use the formula, given in this form.

— Literature will make it slightly more complicated, but less attractive, for modern Physicists, to get consistent results in meter/sec or km/hr.

So

Einstein’s Relativity:speed_ca = [ speed_cb + speed_ba ] / [1 + speed_cb * speed_ba] .

When this fraction ** [1 + speed_cb * speed_ba] **is divided, its no more a simple addition, but a warped addition and known as Einstein Velocity Addition Formula.

This article has a good deal of similitude of content with the linked: Why Nothing Moves Faster Than Light — so they can be read together.

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