The Naked Scientists

The Naked Scientists Forum

Author Topic: How do we measure the speed of light?  (Read 1788 times)

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
How do we measure the speed of light?
« on: 01/03/2016 11:19:13 »
I have heard somewhere we measure the speed of light using 30cm, the speed is constant, the speed of light anywhere in the universe can be measured to be constant by  using 30cm. 30 cm is constant, the speed is constant, a light clock with a 30cm length will remain constant.


So what method using 30cm?

added- would an observer in the dark measure the speed of light to be zero?
« Last Edit: 01/03/2016 11:23:17 by Thebox »


 

Offline Colin2B

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1917
  • Thanked: 123 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #1 on: 01/03/2016 12:41:31 »
Haven't heard of the 30cm method.
This is one of the things Wiki is good at, explaining basics so have a look there.
Also look at http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html

added- would an observer in the dark measure the speed of light to be zero?
Of course not.
Either you are being silly and suggesting a situation where there is no light (that would be like asking what is the speed of a car if there is no car), or you are thinking of someone in a dark room with a beam of light to measure. Obviously you need light to be able to measure it.
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #2 on: 01/03/2016 12:57:30 »
Haven't heard of the 30cm method.
This is one of the things Wiki is good at, explaining basics so have a look there.
Also look at http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html

added- would an observer in the dark measure the speed of light to be zero?
Of course not.
Either you are being silly and suggesting a situation where there is no light (that would be like asking what is the speed of a car if there is no car), or you are thinking of someone in a dark room with a beam of light to measure. Obviously you need light to be able to measure it.

I was being a bit silly, but just considering the strict definition of ALL observers in an inertial reference frame.   Not quite all observers.
 

Offline Colin2B

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1917
  • Thanked: 123 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #3 on: 01/03/2016 13:03:11 »
I was being a bit silly, but just considering the strict definition of ALL observers in an inertial reference frame.   Not quite all observers.
Yes, all observers. Think what observer means.
If a car didn't pass you, you can't measure its speed because you didn't observe one!
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #4 on: 01/03/2016 13:11:54 »
I was being a bit silly, but just considering the strict definition of ALL observers in an inertial reference frame.   Not quite all observers.
Yes, all observers. Think what observer means.
If a car didn't pass you, you can't measure its speed because you didn't observe one!

Of course, to observe you need light, sorry a dim thought for a moment. 
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #5 on: 01/03/2016 13:21:13 »
Haven't heard of the 30cm method.
This is one of the things Wiki is good at, explaining basics so have a look there.
Also look at http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html



Thank you I have read the link, the link seems to be based on measuring the speed of the ''tip'' of the light between two points, in the speed of light vacuum test of the speed, is a single flash of light emitted or a ''stream'' of light measuring the tip speed?
 

Offline Colin2B

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1917
  • Thanked: 123 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #6 on: 01/03/2016 17:30:00 »
One method is by chopping the light with a toothed wheel, look up Armand Fizeau. Other methods have used rotating mirrors to cause breaks in the light, try Foucault or Michelson (of Morley fame).
There will be a lot of articles on line that can describe how it was done in far greater detail than we can here.
Come back with (sensible) questions if you have any.
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #7 on: 01/03/2016 21:40:16 »
One method is by chopping the light with a toothed wheel, look up Armand Fizeau. Other methods have used rotating mirrors to cause breaks in the light, try Foucault or Michelson (of Morley fame).
There will be a lot of articles on line that can describe how it was done in far greater detail than we can here.
Come back with (sensible) questions if you have any.

Is the speed of light infinite and the finite speed an illusion by relativistic affects??
 

Online alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4713
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: How do we measure the speed of light?
« Reply #8 on: 01/03/2016 23:51:16 »
Is the speed of light infinite

No. It's quite easy to measure and is about 300,000,000 m/s in vacuo.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3820
  • Thanked: 19 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #9 on: 02/03/2016 00:14:33 »
The length of the meter is defined in terms of the speed of light so you can only confirm the length of your measuring rod!
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #10 on: 02/03/2016 08:46:19 »
Is the speed of light infinite

No. It's quite easy to measure and is about 300,000,000 m/s in vacuo.

Thank you Alan, I asked another question earlier which Colin said was stupid or along those lines.


In this vacuum, do you measure the tip of the light ''stream'' or a flash of light?
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4120
  • Thanked: 245 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #11 on: 02/03/2016 09:26:06 »
Quote from: TheBox
In this vacuum, do you measure the tip of the light ''stream'' or a flash of light?
To get the most accurate result, you need the biggest sample size possible.

So you don't measure the "tip" of a light stream (ie the first photon to arrive).

You try to take a measurement which represents the velocity of all the photons (or as many as possible).

The toothed wheel and the spinning mirror method both send pulses of light, and you wait a tiny amount of time for the pulse to come back. You adjust the rotation rate so the maximum light intensity arrives back at the detector. From the rotation rate and the distance to the far mirror, you can calculate the average velocity of a pulse of light (ie a bunch of photons).

Quote from: TheBox
In this vacuum...
Large vacuum tubes are expensive, so the first experiments were conducted in air.

This gave a cheap (but pretty accurate) measure of the speed of light. The refractive index of air is 1.000277, so the measurement will be low by 0.0277%.

If you are planning to repeat this experiment at home, I suggest that you measure it in air, and then subtract 0.0277%.
 

Offline Thebox

  • Neilep Level Member
  • ******
  • Posts: 3158
  • Thanked: 45 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #12 on: 02/03/2016 09:34:52 »

Large vacuum tubes are expensive, so the first experiments were conducted in air.

This gave a cheap (but pretty accurate) measure of the speed of light. The refractive index of air is 1.000277, so the measurement will be low by 0.0277%.

If you are planning to repeat this experiment at home, I suggest that you measure it in air, and then subtract 0.0277%.

To be honest I think the rational way would be the Galileo method and a flash/flashes of light.   Surely if we had a vacuum of exactly 299 792 458 m in length between emitter and sensor, and we emitted the light flashes at 1 second intervals, the results would either show a conclusive synchronisation or a complete crock of?

Galileo  had the method but did not have the technology.

 299 792 458 m= 299 792 458.s= 299 792 458 beats

and the delay interval of each beat should be the same fraction of time/length apart on the readout.

P.s it may show gravity waves as well, but that's another story.


P.s we could also add some magnetism on a on and off switch, and see how this affects the result.
« Last Edit: 02/03/2016 09:59:29 by Thebox »
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4120
  • Thanked: 245 times
    • View Profile
Re: How do we measure the speed of light?
« Reply #13 on: 02/03/2016 21:07:16 »
Quote from: TheBox
I think the rational way would be the  Galileo method and a flash/flashes of light
The Galileo method could easily measure the difference between the speed of light and the speed of sound from a cannon - this is well within human reaction times.

To measure the speed of light alone, the Galileo method relied on human reaction times, the brightness of hand-held lamps and the sensitivity of human vision - the errors were far larger than the very small delays he was trying to measure.

Today we have instruments that can measure extremely short intervals of time far more precisely than a human, allowing the experiment to be conducted in a much smaller space.

Quote
if we had a vacuum of exactly 299 792 458 m in length between emitter and sensor
There is nowhere on Earth you can create such a vacuum - it is wider than the Earth.

The first measurement of the speed of light in a vacuum was made by Ole RÝmer, measuring the timing of eclipses of the moons of Jupiter.

He used far more than 1 light-second of vacuum; he used a baseline of 1320 light-seconds of vacuum (the distance between the orbits of Earth & Jupiter).

His answer was not amazingly accurate, because in those days they did not have a good measure of the diameter of Earth's orbit.

See: https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light
 

The Naked Scientists Forum

Re: How do we measure the speed of light?
« Reply #13 on: 02/03/2016 21:07:16 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length