Register to reply

Does light recalibrate its speed?

by kwestion
Tags: light, recalibrate, speed
Share this thread:
kwestion
#1
Feb14-12, 09:13 PM
P: 63
Hello, I'm looking to understand the concept of the constancy of the speed of light. In particular, I think I need to understand what is meant by the speed being constant for all observers. Even more particular, I'm looking for experimental evidence that I can understand that will show me that light does/does not change speed by the action of being observed.

Suppose I have a system for measuring the speed of light involving beam splitters, mirrors, gears etc. I become satisfied that the speed of light within that system is constant throughout. I realize that all of the devices in the system are moving at the same relative speed as each other, so it doesn't tell me much about the speed of light relative to a moving object. I consider letting light from a nearby planet enter my system for measurement. I realize that since my device is on the moon, say, the light will be traveling in a vacuum all the way up until it interacts with the first atoms of the measuring device.
I become disappointed, because I realize that the device will have tampered with the light that I wished to have measured. For all I know, from the perspective of this device, the speed of light changed from c+v to c upon physical interaction at the boundary of my device--a sort of recalibration. I seem to only be able to measure light that I've interfered with.

What experiments have been done regarding the speed of light where light is closing in on a moving observer and where the speed measurement is done in such a way as to clearly not interfere with the speed? I'm thinking that such a measuring technique must enclose the starting and ending points, but I know there are some pretty clever scientists too.
Phys.Org News Partner Science news on Phys.org
Scientists develop 'electronic nose' for rapid detection of C. diff infection
Why plants in the office make us more productive
Tesla Motors dealing as states play factory poker
ghwellsjr
#2
Feb14-12, 10:00 PM
PF Gold
P: 4,745
Quote Quote by kwestion View Post
Hello, I'm looking to understand the concept of the constancy of the speed of light. In particular, I think I need to understand what is meant by the speed being constant for all observers. Even more particular, I'm looking for experimental evidence that I can understand that will show me that light does/does not change speed by the action of being observed.

Suppose I have a system for measuring the speed of light involving beam splitters, mirrors, gears etc. I become satisfied that the speed of light within that system is constant throughout. I realize that all of the devices in the system are moving at the same relative speed as each other, so it doesn't tell me much about the speed of light relative to a moving object. I consider letting light from a nearby planet enter my system for measurement. I realize that since my device is on the moon, say, the light will be traveling in a vacuum all the way up until it interacts with the first atoms of the measuring device.
I become disappointed, because I realize that the device will have tampered with the light that I wished to have measured. For all I know, from the perspective of this device, the speed of light changed from c+v to c upon physical interaction at the boundary of my device--a sort of recalibration. I seem to only be able to measure light that I've interfered with.
As long as your measuring apparatus is also in vacuum and doesn't interfere with all the light coming from the planet, why would you expect the light to be effected by your measurement? I'm thinking, for example, of a shuttering device that blocks the light and then unblocks it to let some light through at a measured time. Why would you think that as soon as shutter opened, it would have any more effect on the remaining light? Then you let the light progress to a sensor some distance away that measures when the light reaches it. If the difference in the two times is always the same, no matter where the light comes from, wouldn't you have confidence that the light was propagating at the same speed in every case?
Quote Quote by kwestion View Post
What experiments have been done regarding the speed of light where light is closing in on a moving observer and where the speed measurement is done in such a way as to clearly not interfere with the speed? I'm thinking that such a measuring technique must enclose the starting and ending points, but I know there are some pretty clever scientists too.
Those experiments have been done, essentially as I have just described it. And they have been done simultaneously on the light coming from two different stars that are moving with respect to each other (binary stars in orbit around each other). All these experiments show that the speed of light is independent of the motion of the source.
kwestion
#3
Feb15-12, 06:35 PM
P: 63
Thanks for your reply.
Quote Quote by ghwellsjr View Post
As long as your measuring apparatus is also in vacuum and doesn't interfere with all the light coming from the planet, why would you expect the light to be effected by your measurement?
I wouldnít expect a non-interfering apparatus to affect the measurement by definition . My understanding is that vacuums donít contain mirrors. My original device contained a mirror/splitter which interrupted the vacuum.
Quote Quote by ghwellsjr View Post
I'm thinking, for example, of a shuttering device that blocks the light and then unblocks it to let some light through at a measured time. Why would you think that as soon as shutter opened, it would have any more effect on the remaining light?
I wouldnít initially think that it would have an effect, but now that Iím asked, I have two thoughts on the topic: 1) Iíve read of double slit experiments that werenít supposed to interfere with the light in a given slit, but somehow it seemed that the intent of measuring the flow through a slit was sufficient to affect the outcome of the experiment drastically. 2) Iíve opened a shutter (comprised of my hand and a copper pipe) and allowed a magnet to fall vertically through. Even though the magnet did not contact the shutter physically, the magnet's speed was regulated by the fields that existed at the time the magnet was passing through the shutter. It doesn't initially intuitively seem like copper and a magnet would have this relationship, so I'm open to understand boundary conditions of other experiments.
Quote Quote by ghwellsjr View Post
Then you let the light progress to a sensor some distance away that measures when the light reaches it. If the difference in the two times is always the same, no matter where the light comes from, wouldn't you have confidence that the light was propagating at the same speed in every case?
Yes, I would have confidence in the speed being the same in those cases, at least between the front edge and back edge of the device. I would like to understand the boundary conditions of the shutter to see about extending that confidence to the region at and in front of the shutter.
Quote Quote by ghwellsjr View Post
Those experiments have been done, essentially as I have just described it. And they have been done simultaneously on the light coming from two different stars that are moving with respect to each other (binary stars in orbit around each other). All these experiments show that the speed of light is independent of the motion of the source.
I think Iím already satisfied with speed being independent of the source, since even sound and water waves offer that independence, so Iím really more interested in understanding the case where the observer is closing in on the light which seems like a different case. The binary star experiments that I know of were prior to the age of space exploration. Although they speak to the sameness of the speed of the twin lights, those older experiments would have been measuring light that spent the last leg of its journey through the earthís atmosphere which, at least in my imagination, could serve to regulate the twins to a new speed relative to the atmosphere (arghónow that I think about it, even the earthís magnetic field).

I wish I knew of a specific canonical experiment that gave me confidence about the boundary conditions. The one atmosphere-free measurement that I have recently read about involved the GPS satellites, but that experiment claimed to have resulted in a measurement of v+c, so Iím now disoriented.

DaleSpam
#4
Feb15-12, 07:05 PM
Mentor
P: 17,322
Does light recalibrate its speed?

Quote Quote by kwestion View Post
What experiments have been done regarding the speed of light where light is closing in on a moving observer and where the speed measurement is done in such a way as to clearly not interfere with the speed?
The phenomenon you are talking about is known as optical extinction and is a well-known criticism of many speed of light measurements. However, because it is a well-known criticism there are also many experiments that are not subject to it. This is a good place to start:
http://www.edu-observatory.org/physi...cal_Extinction
DaleSpam
#5
Feb15-12, 07:06 PM
Mentor
P: 17,322
Quote Quote by kwestion View Post
The one atmosphere-free measurement that I have recently read about involved the GPS satellites, but that experiment claimed to have resulted in a measurement of v+c, so Iím now disoriented.
Please post your source for this. I suspect it is probably an unreliable source.
kwestion
#6
Feb15-12, 09:21 PM
P: 63
Quote Quote by DaleSpam View Post
Please post your source for this. I suspect it is probably an unreliable source.
I'm not sure about the reliability, so thanks for asking about the source. Here's a link to the paper: One-Way Light Speed Measurement Using
the Synchronized Clocks of the GPS by Stephan Gift


Thanks, for the vocabulary and link mentioned in the other post.
harrylin
#7
Feb16-12, 12:54 PM
P: 3,187
Quote Quote by kwestion View Post
I'm not sure about the reliability, so thanks for asking about the source. Here's a link to the paper: [One-Way Light Speed Measurement Using the Synchronized Clocks of the GPS]
That is a very unreliable site, for sure much less reliable than this one. The claim is probably due to a simple - but frequent - misunderstanding that also appears to be yours: the mere equation "v+c" is in itself not in contradiction with SR. As a matter of fact, the similar (c-v) is even part of Einstein's 1905 derivation of the Lorentz transformations. And sure, GPS also uses it.
In jargon it is called "closing velocity". For example, assume that a radio wave moves at c in one direction, and a GPS receiver in opposite direction. The relative speed (or closing speed) is then c+v by definition, because the speed of light is c and that of the receiver is v as measured in that reference system.


Register to reply

Related Discussions
Exceed speed of light: always exceeded speed of light? Special & General Relativity 11
What is the speed of light of a headlamp of a plane travelling at a speed of 1000km/s General Physics 4
How to recalibrate motherboard temperature sensors Computers 7
Idea for light speed or possibly faster than light speed travel? Quantum Physics 1
Recalibrate Rotameters General Engineering 0