Will the final speed of light will be zero?

[Minato]

So let us all assume that a mirror is going away from us at speed c/2 and we are stationary. Now a ray of light leaves from me reaches to mirror and reflects back. So from our prospective light will be moving at c/2 w.r.t mirror hence when it its reflected it must be the same c/2, but mirror is already moving at c/2 that will only mean light must be stationary and we will never see our reflection. And if we start increasing the velocity of mirror that will mean light will start moving forward but slowly. I know it doesn't take relativity in consideration but since i was stationary observer I don't think I need to. So please point me out where I'm wrong and where can i get solution.

 

Edit1: Corrected to c/2 from c/t in line " light will be moving at c/2 w.r.t mirror"

 

 

Edit2: added assumption that mirror is going away.

Edited June 6, 2017 by Minato

[Janus]

The light will return to you at c, though it will be red-shifted by a quite a bit. It does not matter whether you consider yourself the stationary observer or not, you have to take Relativity into account, including the part that says that you always will measure light as traveling at c with respect to yourself.

[Minato]

But in that case for us the relative velocity between light and mirror will become 1.5c which is physically impossible. as an observer that is.

[Strange]

But in that case for us the relative velocity between light and mirror will become 1.5c which is physically impossible. as an observer that is.

 

 

The relative speed between two things can be up to 2c as seen by another observer.

 

For example, imagine one spaceship flying away from Earth at 0.8c and another flying in the opposite direction at 0.8c. You will see the two spaceships moving apart at 1.6c. This is not a problem, because there is nothing moving at more than c in your frame of reference.

 

But, and this is the important bit, the people on each spaceship will see the other one moving away at 0.98c. So, again, no one sees anything moving at more that c.

[Silvestru]

 

 

The relative speed between two things can be up to 2c as seen by another observer.

 

For example, imagine one spaceship flying away from Earth at 0.8c and another flying in the opposite direction at 0.8c. You will see the two spaceships moving apart at 1.6c. This is not a problem, because there is nothing moving at more than c in your frame of reference.

 

But, and this is the important bit, the people on each spaceship will see the other one moving away at 0.98c. So, again, no one sees anything moving at more that c.

 

Is there any difference between uniform motion and rest? Can you please explain the answer?

[Strange]

 

Is there any difference between uniform motion and rest? Can you please explain the answer?

 

 

There isn't. It is just a point of view or choice of frame of reference that decides who is at rest and who is in motion.

 

This idea goes back to Galileo (at least): http://physics.ucr.edu/~wudka/Physics7/Notes_www/node47.html

[Silvestru]

I see and based on the second law or Relativity

1. The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source.

 

 

And to further elaborate on OP and your example, no matter how fast you chase a light beam, the beam's speed relative to us will be the same as before we started chasing it.

Edited June 6, 2017 by Silvestru

[Strange]

I see and based on the second law or Relativity

 

And to further elaborate on OP and your example, no matter how fast you chase a light beam, the beam's speed relative to us will be the same as before we started chasing it.

 

 

Exactly.

[Janus]

But in that case for us the relative velocity between light and mirror will become 1.5c which is physically impossible. as an observer that is.

The speed of the light with respect to the mirror as measured by us is more properly called the "closing speed", relative velocity is generally velocity with respect to the the object as measured by the object.

Thus the closing speed between the light and the mirror, as measured by us is 1/2c before the reflection and 1.5c after the refection. The relative velocity of the light with respect to us, as measured by us, will always have a magnitude of c.

For someone at rest with respect to the mirror, the relative velocity of the light with respect to the mirror will always have a magnitude of c, while the closing speed between us and the light will be 1.5c before reflection and 1/2c after reflection.

 

This tendency for light(actually, anything that travels at c) to travel at c relative to the frame measuring its speed is fundamental to Relativity and leads to the effects we associate with Relativity (time dilation, length contraction, and the relativity of simultaneity)

[Minato]

Lol if you just think logically its hard to swallow that id we give head start to light we will never be able to catch it no matter how fast we go it will just get away from us at speed c. but c defy all Newtonian logic.

[Janus]

Lol if you just think logically its hard to swallow that id we give head start to light we will never be able to catch it no matter how fast we go it will just get away from us at speed c. but c defy all Newtonian logic.

We don't live in a Newtonian Universe; we live in a Relativistic one. The Newtonian rules are just close approximations to reality at low relative speeds.

Edited June 7, 2017 by Janus