Suppose I'm moving toward the mirror at the speed of v, then relative speed btw me and mirror would be v.

And I thought that we can think like there exist world that is the same as our world inside the mirror, so I guessed we will measure speed of the image as 2v/(1+v^2), according to relativistic velocity addtion.

But if we think that I'm stationary and mirror is moving toward me, then it looks obvious that speed of image should be 2v because distance btw me and mirror and mirror and image must be always same.

So Im very confused about this, and honestly i have no idea about how can we define speed of image because it doesn't actually exist.

(If it existed as normal object, we shoot the light and by measuring reflected light and getting average of two instance, we might be able to specify its position and time)


2 Answers 2


Prepare to have your mind blown: hold up a mirror to the night sky, and point it at a quasar. Flip it 45 degrees. You've now made the mirror image of that quasar travel several billion light years in less than a second.

The motion of mirror images is virtual, just a mathematical construct, and relativity does not apply to it.


The virtual motion of the image does not obey the relativistic velocity addition formula, and thus the relative velocity of the image is $2v$. As another example, if two photons recede from each other, the speed of the increase in the distance that joins the photons is calculated to be $2c$ from the viewpoint of the inertial observers who move and have relative velocities parallel to the said distance. This speed is also a virtual thing.


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