# Invariant upper bound for velocity of a particle

In the book Relativity book by Wolfgang Rindler, he explains that the Galilean and Lorentz transformations exhaust all the possible transformations. And explaining that he says, "Now, either there is, or there is not, an upper bound to the possible speeds of particles. Suppose, first, that there is. Then, mathematically speaking, there must be a least upper bound, which we will call c. This speed c, whether attained or not by actual particles, must be invariant. For suppose some velocity of magnitude c in an inertial frame S corresponds to one of magnitude c' > c in another inertial frame S'. By continuity there will then exist a velocity of magnitude slightly less than c in S (that is, a possible particle velocity) that still corresponds to one of magnitude greater than c in S', a contradiction. Similarly c' < c can be ruled out. So there exists an invariant speed, which is essentially Einstein’s postulate."

Here I do not understand how is he contradicting the assumption by saying that a velocity slightly less than C in S transforms to a velocity greater than C in S', as both are different inertial frames with different speed limits and so one velocity in S can transform to a velocity which is less than c' but may be greater than c.

• I have no idea what an invariant that is not actually attained by particles is supposed to mean. Such a thing may have mathematical sense, it sure doesn't have any physical meaning. – CuriousOne Jun 14 '16 at 6:23
• In the special theory of relativity, speed of light is the limit to the velocity that can be attained by any particle. Here this upper bound is the same in all reference frames. Now is there a transformation where this upper bound is different for different reference frames. The invariant velocity is being referred to this upper bound of velocity. – lattitude Jun 14 '16 at 6:26
• @CuriousOne for massive particles, $c$ is precisely such an invariant: $v < c$, not $v \le c$: it's the supremum of velocity, not the maximum. – tfb Jun 14 '16 at 6:30
• @tfb: Such a thing is not physically measurable and I am simply going by what can be measured, not what works for a mathematician. – CuriousOne Jun 14 '16 at 7:37

I think what he is saying is simply that if there exists a speed limit then the transformation law for velocity must be of such a nature that the speed that corresponds to the speed limit must remain invariant under such transformations. Because otherwise the law of the speed limit itself will be violated.

As he has said, suppose the rule is that nothing can travel faster than $k$ in any frame. Now if the transformation law for velocity is of such a nature that it transforms the velocity $k$ to a velocity $k'>k$ in another frame then because of continuity there will be particles with speed $w<k$ which can transform to some speed $w'<k'$ but $w'>k$ in another frame. This would violate the law of the speed limit.

The speed limit which is not achievable by particles might be thought of as an asymptotic limit of the speed that particles might attain. We can go as close as it pleases us but can't reach it. In such a case, a plain statement of the transformation of $k$ to $k'>k$ would not as such violate the law of the speed limit as nothing actually travels at that speed. But this would immediately imply the violation of the speed limit if one invokes the continuity argument as he has used.

• When you say that a frame has a speed limit k, is it that all the speeds of the particles as measured by that frame should be less than k ?? If so then why is it that w>k in another frame violate the limit of the frame under consideration? – lattitude Jun 14 '16 at 6:55
• The argument is quite physical. It has incorporated the principle of relativity in it. The speed limit it assumes is universal - i.e. for every frame. If the speed limit is $k$ then it doesn't really refer to any particular frame but rather it refers to all the frames. So $w'>k$ in the $O'$ frame violates the speed limit law in $O'$ frame. – Dvij Mankad Jun 14 '16 at 7:05
• But then, is it possible that different frames have their own speed limits which obey the transformation laws? – lattitude Jun 14 '16 at 7:19
• Mathematically, yes. But then it would provide an absolute standard to distinguish among different inertial frames - which is just not in the spirit of the principle of relativity. – Dvij Mankad Jun 14 '16 at 8:57
• It's always gratifying to think about Rindler's statements! Thanks for the post! – Dvij Mankad Jun 14 '16 at 9:32