# “We don’t really know the speed of light”

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40 minutes ago, EarthLife said:

As light is affected/bent by gravity, then the speed of a photon moving away from the center of gravity (say the Earth) will be different to the speed of a photon moving directly towards the center of gravity - one way that the speed in each direction can be asymmetric.

If you measure the speed of light locally in a gravitational potential well you will get c. What changes is the frequency the light will be red shifted climbing out of the gravitational potential well and blue shifted falling in.

Regards Andrew

Edited by andrew s

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43 minutes ago, EarthLife said:

As light is affected/bent by gravity, then the speed of a photon moving away from the center of gravity (say the Earth) will be different to the speed of a photon moving directly towards the center of gravity - one way that the speed in each direction can be asymmetric.

Gravity bends spacetime. Light follows the "shortest" path which are straight lines when there is flat spacetime and curved lines (geodesics) when spacetime is curved as in GR by mass/energy.

Regards Andrew

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8 minutes ago, andrew s said:

If you measure the speed of light locally you will get c. What changes is the frequency the light will be red shifted climbing out of the gravitational potential well and blue shifted falling in.

Taking it to the extreme, ie to the edge of an event horizon of a black hole, any photons that cross the threshold towards the center of the well are doomed to move towards the center, any photons just outside of the event horizon (by the tiniest amount) have a chance at moving directly away.

Is the event horizon threshold have zero thickness (towards or away from the center with no if's or buts, a logic 0 or 1) or does it have some thickness, ie could a photon be right on the threshold of being able to move away but with an equal but opposite pull (if you like) towards the center ? .. ie could it end up 'suspended' there (assuming nothing else is around to effect the well).

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Have a look at this posted earlier

Regards Andrew

Edited by andrew s
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1 hour ago, andrew s said:

Have a look at this posted earlier

Regards Andrew

Chuckle. I think there might be some technical issues we’d have to address to actually make this measurement. https://phys.org/news/2014-03-orbit-black-hole.html

Edited by Ouroboros
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Applying the same logic and standards of measurements we do when attempting to measure the speed of light it appears to me, fool that I am, that we cannot be sure of the speed of anything, we just accept a reasonable tolerance depending on the object being measured.
We can say for example that a car is travelling at 30mph, but by doing so we accept the distance measured is accurate and the time taken is also accurate, it’s just a question of degree as to how accurate that speed is or needs to be.  How can we be sure that all clocks used are all reading precisely the same passage of time and start and stop at the same time?
Nothing is certain, quantum theory has taught us that.

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31 minutes ago, Moonshed said:

How can we be sure that all clocks used are all reading precisely the same passage of time and start and stop at the same time?

Time flows the same for all observers in the same frame of reference.

If none of clocks move with respect to each other and spacetime is bent the same for them (for example - they are on the surface of the earth) - then time runs the same for them.

We can use clock synchronization https://en.wikipedia.org/wiki/Einstein_synchronisation

to ensure that all clocks in our frame of reference have the same t0.

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Posted (edited)
2 hours ago, vlaiv said:

Time flows the same for all observers in the same frame of reference.

If none of clocks move with respect to each other and spacetime is bent the same for them (for example - they are on the surface of the earth) - then time runs the same for them.

We can use clock synchronization https://en.wikipedia.org/wiki/Einstein_synchronisation

to ensure that all clocks in our frame of reference have the same t0.

Thank you for that explanation that clears up my synchronisation problem only to replace with an even greater one trying to make sense of all the info in that wiki link. It’s doing my head in!  😂

Edited by Moonshed
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Posted (edited)
2 hours ago, Moonshed said:

Applying the same logic and standards of measurements we do when attempting to measure the speed of light it appears to me, fool that I am, that we cannot be sure of the speed of anything, we just accept a reasonable tolerance depending on the object being measured.
We can say for example that a car is travelling at 30mph, but by doing so we accept the distance measured is accurate and the time taken is also accurate, it’s just a question of degree as to how accurate that speed is or needs to be.  How can we be sure that all clocks used are all reading precisely the same passage of time and start and stop at the same time?
Nothing is certain, quantum theory has taught us that.

The first thing any scientist or engineer will ask when presented with a measured value is "what is the uncertainty".  It was I think Lord Kelvin who said (paraphrased)  "unless you understand the value of the uncertainties you do not understand the value of your measurements"

What we do in practice is quote with each measurement the "uncertainty" in the measurement.  The uncertainty or error in a measurement arises for a number of reasons, most common and perhaps easily understood is the "reading" uncertainty (sometimes called the scale uncertainty).  For example, with an analogue instrument, it would not be unreasonable to take a reading to 1/2 the smallest scale division. This means any reading would have a quoted uncertainty of +/-  0.5 x the smallest division.  As a simple example, let's say you measure the width of disc of metal using a set of vernier calipers at 30.150 mm.  The minimum division on the vernier may be 0.05 mm giving a scale error of +/- 0.025 mm.  So we we would express our measurement as   30.150 +/- 0.025 mm. To get a better understanding of the influence of this error or to compare with other uncertainties to determine the dominant uncertainty we could express this absolute uncertainty as a percentage uncertainty  so becoming 30.150 +/-  0.08 %  (absolute uncertainty divided by the value).  In this case this is a relatively trivial uncertainty for the given measurement. Pending the nature of your investigation/task (tolerable uncertainty levels) you would take a view on the size of the % uncertainties perhaps rejecting (redoing) measurements greater than say 5 %.  Systematic uncertainties and Random uncertainties, Calibration uncertainties are additional sources or error which would be treated in a similar way. In short it is physically impossible to measure and quote anything to an exact value, every measurement has an associated uncertainty level - the lower the better.

Jim

Edited by saac
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17 minutes ago, saac said:

In short it is physically impossible to measure and quote anything to an exact value, every measurement has an associated uncertainty level - the lower the better.

Thank you for your explanation. It was my understanding that every measurement was only accurate to a certain level. I remember being taught in school that when, for example, surveyors were reading from their theodolites they would repeat their measurements numerous times and take the average reading, something like that anyway, it was a long time ago. 😄

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Posted (edited)
34 minutes ago, Moonshed said:

Thank you for your explanation. It was my understanding that every measurement was only accurate to a certain level. I remember being taught in school that when, for example, surveyors were reading from their theodolites they would repeat their measurements numerous times and take the average reading, something like that anyway, it was a long time ago. 😄

Taking repeat readings is a standard laboratory practice as a way to reduce "random variations" in the measurements - in effect you are trying to improve the reliability of your data. To get a feel for the level of these random fluctuations we use yet another uncertainty type called Random Uncertainty.  The magnitude of the Random Uncertainty would be expressed against the mean value of the reading to be compared against other uncertainties to identify the dominant.  Over this, there is a range of statistical algorithms/analysis which can be brought to bear to assess the nature of the errors (uncertainties) to gauge the reliability, accuracy and precision of the measurement.  For example, in the field of particle physics confidence levels are quoted against a standard known as Sigma (a form of Standard deviation). Commonly a measurement for say the energy of the Higgs Boson would be quoted with a 6 Sigma confidence level - what they are saying is that the measurement is unlikely to be influenced by a random error to a chance of one in half a billion (if I remember correctly).  For common engineering purposes (aircraft design, bridges, engines general manufacturing) we would refer to tolerances as opposed to uncertainties typically these being expressed as low as micrometers (one thousandth of a mm).

Jim

Edited by saac

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