# Speed of Light From Moving Object

*Most recent answer: 05/29/2012*

- Ron Oliver (age 62)

Las Vegas, Nevada

That's one of the two obvious guesses one might make.

Your guess is that light travels at a speed fixed with respect to its source. The other one would be at a speed fixed with respect to some medium. It turns out that, weird as it may sound, light travels at a fixed speed with respect to everybody. That seems impossible only because we have incorrect intuition about how space and time coordinates change as you change viewpoints.

Mike W.

*(published on 05/29/2012)*

## Follow-Up #1: Relativistic Measurements with Light

- Isaac (age 21)

Adelaide, SA, Australia

Hello Issac,

Great question. People are often confused by how speed of light is constant relative to different moving object. But in fact, you were right that speed of light is constant relative to any object moving at a constant velocity.

I'm not sure where that 190m comes from, but you're right that at first glance the combination of distances looks peculiar.

It's often said that "time slows down for the moving observer" but that's misleading. Each observer is moving, according to the other one. Whether the time between events is bigger or larger depends on how the events are moving with respect to each observer. Phrases like "at the end" assume that the different observers agree on what events are simultaneous, but actually they don't.

What we usually do in problems like this is just to carefully write down the t and x coordinates in each frame, using the Lorentz transforms.() When "A" sees "B" go by a marker fixed at distance 0.9ct away, the light has gone distance ct, as you say. As "B" goes by that marker, he says the time elapsed since A went by him is about 0.44t. [0.44 ≈ sqrt(1-0.9^{2}]) He sees the light then as 0.44ct away, and the distance from A to the light as ~0.84ct. You find it all comes out consistent.

Hope this helps,

Lingyi and Mike W.

*(published on 03/05/2013)*

## Follow-Up #2: A simpler replacement for relativity?

- Jay Purrington (age 60)

Irving, tx

I couldn't follow all of your idea, but can comment on two aspects.

1. All clocks- atomic, biological, radioactive, Casio, ....- show exactly the same relativistic effects. So nothing whatever that you look at within your own little local box tells you that the progression of time is strange in any way. It's only comparisons with other clocks in a different state of motion and/or at a different height in a gravitational field that show discrepancies.

2. Relativity is the exact opposite of an epicyclic theory. The problem with epicycles is that you can keep adding, adjusting, and fiddling to fit any data whatsoever, because there's no organizing principle. Relativity is derived by rigid logic from some very simple postulates. All those amazing predictions, confirmed in great detail follow directly from the simple basic assumptions.

And thanks for the tip on the editor. What browser were you using?

Mike W.

*(published on 07/08/2014)*