# If a massless observer is going at the speed of light, what speed will it measure of a light beam going parallel to it? [duplicate]

If a massless observer is going at the speed of light, what speed will it measure of a light beam going parallel to it?

## marked as duplicate by WillO, Alfred Centauri, Qmechanic♦Sep 15 '18 at 3:51

what speed will it measure of a light beam going parallel to it ?

Suppose an entity $A$ has speed $c$ in an inertial frame of reference (IFR) then, according to SR, $A$ has speed $c$ in all IFRs, i.e., there is no IFR in which $A$ is at rest.

That is, there are no rulers and there are no clocks at rest with respect to $A$ with which to measure distance and time or change of distance with time.

The speed of light is always constant so it should be considered as an inertial frame of reference

There is no IRF with relative speed $c$, there is no IRF in which a photon is at rest, there is no inertial frame of reference for a photon.

It is not possible to attach a reference frame to anything moving at the speed of light. Anything traveling at that speed will not experience any time and will not be able to perform any observations. Thus, it is meaningless to ask about what one would see if he/she traveled at the speed of light.

However, it the observer were traveling at just below the speed of light, as always, he/she will also observe light traveling at the speed of light.

• An inertial frame of reference is a reference frame in which an object stays either at rest or at a constant velocity unless another force acts upon it. The speed of light is always constant so it should be considered as an inertial frame of reference . – Dhruva Sep 15 '18 at 3:20
• @dhruva sakhare: It is not possible to make any observations when traveling at the speed of light. The Lorentz factor is infinite and it experiences no time. The concept of time doesn't even apply. The lifetime of an object traveling at light speed happens instantaneously. The speed of light is not an inertial frame of reference. – user7777777 Sep 15 '18 at 3:28
• @dhruvasakhare: Have you tried explicitly writing down a frame that includes a vector tangent to the worldline of a light beam? You will learn a lot if you give this a try. – WillO Sep 15 '18 at 3:29
• i got a different answer for this on quora : quora.com/… – Dhruva Sep 16 '18 at 11:34