# Relativity question

Simplex0
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### RE: RE: Ok. one last

Message 48987 in response to message 48986

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Ok. one last question guys. Does frequency or dopler effect have anyting att all to do with how the speed of light will messured or detected by the photo detector? I cant see that thay have but I might be wrong.

Speed is always a constant in free space - no matter who/what/when/where - and independent of anything at all, frequency included.
Frequency will depend on various relative motions etc.

It can be easier to think of frequency as the amount of 'punch' a photon has. While it always travels at a given speed, it doesn't always deliver the same energy or momentum:

for a photon:

Energy = h * frequency

( h = Planck's constant )

Energy = momentum * c

and since speed of light = c = frequency * wavelength

then

Energy = h * c / wavelength

So higher frequency ( shorter wavelength ) gets more punch, with lower frequency ( longer wavelength ) is less punch. Thus Doppler shifting implies a greater punch for light from an approaching source, than light from a receding source. We have an everyday expectation that it would be the velocity that would vary in that manner - as it does for bullets/baseballs/whatever - but not so for light.

If you throw a ball up in the air ( away from the ground ) then it's kinetic energy diminishes with height as it slows down before it falls back. A photon going up will also do something similiar - again, not by varying it's speed - but by lowering it's energy/frequency. This is gravitational redshift.

A black hole is a region where no matter what energy a photon has, it cannot climb out of the gravity well ( if within the event horizon ). The only objects that 'in theory' escape from a black hole are those going faster than light - so the constancy of light speed implies that photons can't go even faster to achieve that! Cuts both ways ...... :-)

Constancy of light speed is one of the hardest, and most counter-intuitive, parts of relativity. You are not alone in this. Many famous physicists are/were similiarly afflicted.

Try and think of 'ordinary' mechanics as a low speed approximation of relativity, rather than relativity as a 'special' extension of Newtonian stuff.

Generally, for any particle:

Energy = 'rest' amount + 'kinetic' amount

for bodies with mass:

'rest' amount = m * c * c ( ie. 'em cee squared' )

kinetic amount = some_function(varies with velocity)

For massless particles:

'rest' amount = zero, or irrelevant because travelling at light speed.

'kinetic' amount = some_function(doesn't vary with velocity)

Cheers, Mike.

Thank you Mike. I will do my best to ask more precise question in the future.

Simplex0
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### Has it ben any attempt to

Has it ben any attempt to measure the speed of light sent out from quasars?

debugas
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### RE: Has it ben any attempt

Message 48989 in response to message 48988

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Has it ben any attempt to measure the speed of light sent out from quasars?

1) The speed of light emitted by any object does not depend on the speed of the object - it has been tested multiple times, in fact if it was not the case then we would have noticed it long ago. After being emitted the light exists independent of the object that emitted it and the light continues to travel at the constant speed of c. Though the frequency of light is dependent on the speed of object that emitted it.

2) Now u may be confused why the speed of the traveling light does not depend on the speed of the detector that catches it. It is because the detectors that have different speeds relative to each other they also have different time intervals estimates(the observers age differently) and also have different estimates of distances. Namely your understanding is that the passing-by detector at some speed v has its time dilated and space contracted in such a way that the speed of light is the same both for you and for that passing-by detector.

One way to prove the 1) is to look at double-star system and compare the period of time when one companion of the star-system is farther away than the other with the time period when it is closer. Both are equal.
The same applies to the sattelites of the Jupiter (which i think could be tested back when Galileo first used his telescope to find them).
One more test was conducted in 1955 - the Sun is rotating slowly so the light emitted by its right and left edges can be tested to see if both beams cover the same distances simultaniously. The beams from both edges where tested and it was found out that they both covered 2 km distance simultaniously ( the possible delta T was proved to be less than 1.4*10^-12 sek which is 60 times smaller than what was expected if light speed was dependent on the speed of emitting object - the edges of the rotating Sun )

as to the testing 2) - i have a question here - what experiments were conducted to prove that the speed of light does not depend on the speed of the detectors ?
i find that round-trip time is not dependant (when you emit the light and recieve it back from the mirror) on the speed but what about one way beam of light ?

Chipper Q
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### RE: as to the testing 2) -

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as to the testing 2) - i have a question here - what experiments were conducted to prove that the speed of light does not depend on the speed of the detectors ?
i find that round-trip time is not dependant (when you emit the light and recieve it back from the mirror) on the speed but what about one way beam of light ?

This page, called Experimental Basis of Special Relativity, may be helpful. (See section 3.2 One-Way Tests of Light Speed Isotropy.)

Simplex0
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### RE: 1) The speed of light

Message 48991 in response to message 48989

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1) The speed of light emitted by any object does not depend on the speed of the object - it has been tested multiple times,

Is there a link where I can read of one of these tests and how it was done?

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in fact if it was not the case then we would have noticed it long ago. After being emitted the light exists independent of the object that emitted it and the light continues to travel at the constant speed of c.

Yes I asume that.

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2) Now u may be confused why the speed of the traveling light does not depend on the speed of the detector that catches it. It is because the detectors that have different speeds relative to each other

If you by that mean that they are traveling at the same speed but in diferent directions rellative to the light beam that is true.

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they also have different time intervals estimates(the observers age differently) and also have different estimates of distances. Namely your understanding is that the passing-by detector at some speed v has its time dilated and space contracted in such a way that the speed of light is the same both for you and for that passing-by detector.

Yes, in the way that rellative the observer at 'A' one detection occures after the light beam has traveled 1 lightyear, that is after he has corrected for tha time the light needs to reach him from respective photo detector.
The other detection will occure, according to 'A', after the light beam have travel a distase close to half the distanse between the photo detector in the fron to the photo detector in the back, the same corrections done here fore the time the light needs no reach him.

The physical slowing of time must affect the both pairs of photo detectors equal.

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One way to prove the 1) is to look at double-star system and compare the period of time when one companion of the star-system is farther away than the other with the time period when it is closer. Both are equal.
The same applies to the sattelites of the Jupiter (which i think could be tested back when Galileo first used his telescope to find them).
One more test was conducted in 1955 - the Sun is rotating slowly so the light emitted by its right and left edges can be tested to see if both beams cover the same distances simultaniously. The beams from both edges where tested and it was found out that they both covered 2 km distance simultaniously ( the possible delta T was proved to be less than 1.4*10^-12 sek which is 60 times smaller than what was expected if light speed was dependent on the speed of emitting object - the edges of the rotating Sun )

In my exempel it is vital that it is the detectors that are moving, that is why I wonder if any have try to measure rhe speed of light from a redshifted quasar. According to the common theori of the expanding univers increeses the speed with the time elapsed from big bang so that make me to conclude that the eart is moving much faster than the quasars becaus the quasars wee observ is much closer in time to big bang than wee are.

I realise that I made a to wide and complex question, an I refer to my self in this case :), I will try to make it in shorter steps in the future.

Simplex0
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### RE: RE: as to the testing

Message 48992 in response to message 48990

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as to the testing 2) - i have a question here - what experiments were conducted to prove that the speed of light does not depend on the speed of the detectors ?
i find that round-trip time is not dependant (when you emit the light and recieve it back from the mirror) on the speed but what about one way beam of light ?

This page, called Experimental Basis of Special Relativity, may be helpful. (See section 3.2 One-Way Tests of Light Speed Isotropy.)

That link was a goldmine Chipper Q

debugas
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### RE: 3.2 One-Way Tests of

Message 48993 in response to message 48992

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3.2 One-Way Tests of Light-Speed Isotropy
Note that while these experiments clearly use a one-way light path and find isotropy, they are inherently unable to rule out a large class of theories in which the one-way speed of light is anisotropic. These theories share the property that the round-trip speed of light is isotropic in any inertial frame, but the one-way speed is isotropic only in an ether frame.

Why do not the experiments rule-out these theories ? Is it because we don't know whether Earth is at rest relative to that ether frame ? Do we need to conduct these experiments also in some fast moving relative to earth laboratory to disprove these theories alltogether?
If these experiments can only testify that different light-beams travel at the same speed then it does not mean they travel at speed c, it could be c+kv for all of them. What i'd like to see is an experiment that tests the same light-beam in two inertial labs moving relative to each other at some constant speed v

In Tomas' example it would mean:
observer B emits the light then rocket observer moving at speed v sees the light-beam passing by at some speed c+kv and the observer A sees it at speed c.
The question remains - will k be = 0 for any v?

debugas
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### Tomas, here is this

Message 48994 in response to message 48993

Tomas, here is this discussion i have found that pretty well explains the difficulty in defining the one-way light speed (OWLS), namely you can postulate OWLS to be always =c and then arrive to simultanity definition based on it OR you can define simultanity in some weird way and then get whatever value of OWLS you want.

So we better stick with two-way light speed (TWLS) definition (sending signal forth to the mirror and getting it back and taking time delay at the same location in order to avoid ambiguity of defining what "there-now" means)
In this way the signals from observer B come to the rocket detector packed 3 times more dense than emitted by B but they still travel at speed c between two rocket detectors. No matter how close detectors RA and RB are but they still are at different locations and require clocks sinchronization - namely how do you define dots RA and RB to be at the same time moment here and there "now"?

Simplex0
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### RE: Tomas, here is this

Message 48995 in response to message 48994

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Tomas, here is this discussion i have found that pretty well explains the difficulty in defining the one-way light speed (OWLS), namely you can postulate OWLS to be always =c and then arrive to simultanity definition based on it OR you can define simultanity in some weird way and then get whatever value of OWLS you want.

So we better stick with two-way light speed (TWLS) definition (sending signal forth to the mirror and getting it back and taking time delay at the same location in order to avoid ambiguity of defining what "there-now" means)
In this way the signals from observer B come to the rocket detector packed 3 times more dense than emitted by B but they still travel at speed c between two rocket detectors. No matter how close detectors RA and RB are but they still are at different locations and require clocks sinchronization - namely how do you define dots RA and RB to be at the same time moment here and there "now"?

Regarding synchronization of the the clocks on the rocket and the clock at 'A' it could be done by a lightsignal sent from a Cephei star system that are sent from a long distanse and perpendicular to the rockets path.

Misfit
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### RE: Relativity question

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