# Light's constant speed and frames of reference

Been reading up on physics out of pure interest, just learning really.

Mainly started special relativity and learning about the inertial frames of reference. I think I understand that, but here's what I don't get.

If from one inertial frame of reference moving at a constant 100 MPH, I toss a ball at a speed of 10 MPH, from my frame of reference the ball has moved at a speed of 10 MPH.

A viewer from a different inertial frame of reference moving at 20 MPH in the same direction as the other reference would witness the same ball traveling at a speed of 90 MPH.

But the speed of light is said to be constant.

So, does that mean that if I am in an inertial reference frame traveling 1/2 the speed of light, and an outside viewer is on a "stationary" reference frame (not sure if such a thing exists, but for simplicities sake..) and I shine a flashlight...

From MY reference frame, traveling 1/2 the speed of light, I "witness" the electromagnetic waves emitted from a flashlight traveling at 3 X 10^8 m/s, correct?

But if someone from a stationary reference framed witnessed my frame of reference traveling at 1/2 speed of light AND witnessed the flashlight, would they also "witness" the electromagnetic waves emitted from the flashlight at a speed of 3X10^8 regardless of the speed of the source (flashlight-1/2 the speed of light)?

Is that what it means for C to be constant? Both viewers see it at C, even if one viewer is traveling with the lightsource at 1/2 the speed of light and the other is stationary, they both witness the light from the flashlight at the same speed?

Related Special and General Relativity News on Phys.org
Dale
Mentor
Is that what it means for C to be constant? Both viewers see it at C, even if one viewer is traveling with the lightsource at 1/2 the speed of light and the other is stationary, they both witness the light from the flashlight at the same speed?
Yes, exactly.

yes.....

and so despite what your everyday experience suggests to you, Einstein explains the constant speed of light in all reference frames by the fact that it is space and time that vary by observer....by reference frame.....

and in general relativity he goes even further showing that not only speed but also gravity (gravitational potential) affects space and time....in fact gravity IS the warpage of space and time.

Our world is often NOT as it appears to the casual observer.

ghwellsjr
Gold Member
...
From MY reference frame, traveling 1/2 the speed of light, I "witness" the electromagnetic waves emitted from a flashlight traveling at 3 X 10^8 m/s, correct?

But if someone from a stationary reference framed witnessed my frame of reference traveling at 1/2 speed of light AND witnessed the flashlight, would they also "witness" the electromagnetic waves emitted from the flashlight at a speed of 3X10^8 regardless of the speed of the source (flashlight-1/2 the speed of light)?

Is that what it means for C to be constant? Both viewers see it at C, even if one viewer is traveling with the lightsource at 1/2 the speed of light and the other is stationary, they both witness the light from the flashlight at the same speed?
Yes, that is true, but keep in mind, you cannot directly "witness" the electromagnetic waves. And let's consider a "flash" of light rather than just turning on a flashlight and letting it shine. As soon as you turn the light on and off, the light travels away from you and you don't know where it is. You can't know where it is unless it bounces off something and comes back to you at which point you can now see it. So to measure the round-trip speed of light, you have to put a mirror some measured distance away from you and then measure how long it takes for the flash of light to leave your flashlight, travel up to the mirror, reflect off the mirror, and travel back to you. Then you calculate the speed of light by taking double the distance to the mirror and divide it by the total time. Any one who makes this measurement, no matter what their relationship to the source of the flash of light, as long as they are not accelerating, will get the same measured value for the speed of light. This was established by experiment prior to the Theory of Special Relativity and has nothing to do with relativity or frames of reference. It's just the way nature behaves for us.

Last edited:
yes.....

and so despite what your everyday experience suggests to you, Einstein explains the constant speed of light in all reference frames by the fact that it is space and time that vary by observer....by reference frame.....

and in general relativity he goes even further showing that not only speed but also gravity (gravitational potential) affects space and time....in fact gravity IS the warpage of space and time.

Our world is often NOT as it appears to the casual observer.
So here are you referring to the concept that time slows (and mass increases?) at speeds nearing the speed of light? And does time also slow in stronger gravity (ie a black hole?)

While we're on that subject, if time slows to a stop at lightspeed, why doesnt light reach any destination instantly?

D H
Staff Emeritus