## Relativity question

Discussions on classical and modern physics, quantum mechanics, particle physics, thermodynamics, general and special relativity, etc.

### Relativity question

We can approximate the speed of light to 1ft/ns for this experiment.
We set up transparent light sensors every 10 ft which transmit a pulse every time a laser hits them.
We then send a laser through them. The receiver at the source detects pulses every 20 ns indicating the speed of light is 10ft/20ns, half its well known speed. Why?

I don't have the STD but if you draw it yourself, the space and time units are 10ft, 10ns and c is a diagonal line with points 10 ft/10 ns, 20ft/20ns, 30ft/30ns etc. From each of these points is a diagonal light line that goes back to the time axis and intersects it every 20 ns. What is that saying about this way to measure the speed of light? Why does it get the wrong answer?
ralfcis
Member

Posts: 860
Joined: 19 Jun 2013

### Re: Relativity question

Incorrect thinking. You are looking at two-way light travel times, 20ns, 40ns, 60ns, etc. For two-way distances 20, 40 ft, etc...

BurtJordaan
Forum Moderator

Posts: 2592
Joined: 17 Oct 2009
Location: South Africa
Blog: View Blog (9)

### Re: Relativity question

Oh yeah. Thanks
ralfcis
Member

Posts: 860
Joined: 19 Jun 2013

### Re: Relativity question

That also answers my 2nd question where the source is separated from the receiver and the laser is being shown from source to receiver. The receiver will have to wait to get the 1st pulse but subsequent pulses will come in every 5 ns or double the speed of light. However, if you average in the initial delay, the average speed of light will be correct.
ralfcis
Member

Posts: 860
Joined: 19 Jun 2013

### Re: Relativity question

The speed of light is the same in both directions; no averaging required. Since you are asking about relativity, we assume that all clocks are Einstein-synchronized.

BurtJordaan
Forum Moderator

Posts: 2592
Joined: 17 Oct 2009
Location: South Africa
Blog: View Blog (9)

### Re: Relativity question

I see what I've done wrong here. I'm confusing the physical characteristics of the laser light with the information the light pulses are carrying back. The information each individual pulse is carrying is not about the speed or frequency of the laser, it's about light pulse number. What the 1st pulse is telling the receiver is that the laser has hit the first marker 10 ns after the start of the experiment. The delay of that info needs to be filtered out in post processing so it is not involved with measuring the speed of the laser light. The spacing of when the info returns is not important because the guy who analyses the data knows each pulse represents the laser light advancing 10ft in 10ns.
ralfcis
Member

Posts: 860
Joined: 19 Jun 2013

### Re: Relativity question

Ok I'm being really dumb today. Yes if you shine a laser towards you from a distance all the lights and the laser will hit you simultaneously. I was thinking about the STD for .6c. Sorry.
ralfcis
Member

Posts: 860
Joined: 19 Jun 2013