# The Naked Scientists Forum

### Author Topic: Would I measure the speed of light to be different if I was born on Pluto?  (Read 1045 times)

#### Thebox

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##### Would I measure the speed of light to be different if I was born on Pluto?
« on: 07/04/2016 00:47:55 »
If I was born on Pluto and my clock was a Caesium clock, and for thought the clock ran at half the rate of an Earth clock, would I measure the speed of light in a vacuum to be 599585136 m/s?

added- because if we agree on the length of 1 second of light, we have to agree on the length of a second, if I was on Pluto I would disagree on the length of a second if I used a Caesium clock, so either both of our clocks were wrong, or the speed of light is wrong, so which is it?

added - 9 192 631 770 periods of the radiation at ground state  relative to what exactly?

I can't wait to hear you try and slip your ways out of this one....

« Last Edit: 07/04/2016 01:14:58 by Thebox »

#### alancalverd

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #1 on: 07/04/2016 01:21:55 »
If I was born on Pluto* and my clock was a Caesium clock, and for thought the clock ran at half the rate of an Earth clock, would I measure the speed of light in a vacuum to be 599585136 m/s?
No. You would do enough experiments to convince yourself that the speed of light is constant so you would define the second and the meter in terms of the speed of light and the frequenxcy of your clock.

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added- because if we agree on the length of 1 second of light
the phrase is meaningless, so we wouldn't agree on it.

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added - 9 192 631 770 periods of the radiation at ground state  relative to what exactly?
relative to one period. It's a number, obtained by counting.

*this may explain a lot!

#### Thebox

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #2 on: 07/04/2016 01:41:27 »
No. You would do enough experiments to convince yourself that the speed of light is constant so you would define the second and the meter in terms of the speed of light and the frequenxcy of your clock.

You say no then say the same thing as I just said.  If my clock is running twice as slow as your clock we will record a different speed.  In 1 second on your clock light would have travelled 299 792 568 m ,  but my clock runs half the rate of your clock so your second is twice as long, I record 299 792 568m / 0.5s

Look at this way , you are on Earth measuring the speed of light in a vacuum, I am on Pluto observing you doing the experiment, you tell me light travelled 299 792 568 m in 1 second, I tell you according to my clock it only took half a second because my time runs slower than your time.

See the dilemma now?

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the phrase is meaningless, so we wouldn't agree on it.

''The phrase is meaningless'', yet it is in plane English, you know very well  what that means and in typical avoidance reply with meaningless.

All observers have to agree the length travelled of light in 1 second is the same in a vacuum anywhere in the Universe?

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relative to one period. It's a number, obtained by counting.

*this may explain a lot!

A number obtained by counting, relative to one period, please explain what one period suppose to mean?

I  have a feeling you mean a fixed length on a chart or graph,

If 9 192 631 770  relative to one period

and 9 192 631 760 relative to one period

I have a feeling one period = one period   because surely science is not daft enough to measure a shorter period and declare there is a change.

« Last Edit: 07/04/2016 01:58:47 by Thebox »

#### evan_au

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #3 on: 07/04/2016 12:58:36 »
Sorry about the essay, but the slippery explanation is not as short as I hoped....

Quote from: TheBox
A number obtained by counting, relative to one period, please explain what one period suppose to mean?
Imagine a pendulum clock on Earth, with a 0.24m long pendulum, swinging backwards and forwards with a period of 1 cycle per second = 1Hz. If I count the number of periods, 1 period = 1 second; 2 periods = 2 seconds, etc.

Imagine a musician's metronome, set to a moderate 120 beats (clicks) per minute = 2 beats per second = 2Hz.
If I count the number of clicks, 2 clicks= 1 second; 4 clicks= 2 seconds, 1 click= 0.5 seconds, Half a click= 0.25s, etc.

Now imagine a cesium clock, "ticking" at the extremely fast rate of 9.192 631 770 GHz.
If I count the number of periods, 1 period = 1/9192631770 seconds; 2 periods = 2/9192631770 seconds
Similarly, 4596315885 periods = 0.5 seconds; 9192631770 periods = 1 second

To a scientist, telling the time is just like counting sheep; 1 sheep, 2 sheep, 3 sheep....  zzzzz

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I have a feeling one period = one period  because surely science is not daft enough to measure a shorter period and declare there is a change.
Back to the example of a musician's metronome, this time ticking at a very fast 240 beats (clicks) per minute:
• one period of this inverted pendulum is the time for the arm to move from (say) the left side to the right side, and return to it's initial position, which takes 500ms (and produces 2 clicks). This cycle then repeats.
• half a period is the time for the arm to move from one side to the other side. This takes 250ms. This is definitely a change!
• A quarter of a period is the time for the arm to move from one side to the center. It takes 125ms. The musician is not daft!
• It is certainly possible to divide the period of a metronome into smaller and smaller units. Musicians regularly play quavers, semi quavers and semi-demi quavers, which are faster than the clicks from the metronome.
• In practice, we can use electronic circuits like a Phased Locked Loop to divide a period into shorter periods, so we can measure small changes in time.
• Instruments like advanced time-of-flight mass spectrometers have to measure time in intervals that are finer than one period of a cesium atom.

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I (on Pluto) tell you (on Earth) according to my clock it only took half a second because my time runs slower than your time
Time on Pluto runs (ever-so-slightly) faster than time on Earth; not slower.

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on Pluto ...the clock ran at half the rate of an Earth clock
This is a gross exaggeration. Human reaction times could not see a difference between the speed of events on Earth and Pluto, caused by their position in the Sun's gravitational well.

But let's keep the factor of 2 for the simplicity of the numbers.

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we have to agree on the length of a second
This is true. In the metric (SI) system, 1 second is defined as "the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom".

This is defined for all scientists, measuring the duration of events occurring in their own laboratory, using a cesium clock which is also located in their own laboratory.

However, when you are measuring the duration of events in someone else's laboratory, which may be traveling at a different velocity than you, or in a different gravitational field than you, or at a large distance from you, then this definition does not apply. You must use the equations of Einstein's relativity to estimate the time in that other frame of reference.

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if I was on Pluto I would disagree on the length of a second
No, you would realise that the test was not being done in your laboratory; you would make allowances for the different frame of reference, or (if that's too hard) you would just agree to disagree.

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measure the speed of light in a vacuum
When scientists say "the speed of light in a vacuum is constant", they are talking about the speed that you measure in your laboratory, using a clock in your laboratory.
• If you are on Pluto, you measure it in your laboratory, and find that it is 299,792,458 m/s.
• A scientist on Earth measures it in her laboratory, and also finds that it is 299,792,458 m/s.
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you are on Earth measuring the speed of light in a vacuum, I am on Pluto observing you doing the experiment
Now you are observing someone else doing the experiment far outside your laboratory. The standard conditions do not apply, so the standard answer does not apply.

You see the Cesium atoms on Earth oscillating, and a counter counting up the periods.
A flash of light is emitted, and the counter starts incrementing.
When the counter hits 30.663318988 = 9192631770/299792458, the position of the flash of light is measured. It has traveled precisely 1 meter.

You, on Pluto, can see that the experiment on Earth has been conducted properly, and so you agree on the length of a meter, and the speed of light in a vacuum on Earth.

However, if you triggered your Cesium counter on Pluto when you saw the counter on Earth start, your counter would reach 61.326637977 by the time the counter on Earth stopped.

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would I measure the speed of light in a vacuum to be 599585136 m/s?
No, because your measurement of the speed of light in your laboratory  on Pluto is done using a clock in your laboratory on Pluto.

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either both of our clocks were wrong, or the speed of light is wrong, so which is it?
Neither.

The fact that your clock on Pluto looks different to a clock on Earth, and the length of a meter on Pluto looks different from a meter on Earth is purely due to the fact that you are in different frames of reference.

If you transported the equipment from Earth to your lab on Pluto, you would now find yourself in violent agreement about the duration of a second, the length of a meter and the speed of light in a vacuum.

Corrected to account for the fact that metronomes produce 2 clicks (beats) per cycle of the pendulum.
« Last Edit: 10/04/2016 18:48:17 by evan_au »

#### Thebox

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #4 on: 07/04/2016 13:25:49 »

The fact that your clock on Pluto looks different to a clock on Earth, and the length of a meter on Pluto looks different from a meter on Earth is purely due to the fact that you are in different frames of reference.

Different frames of reference, I disagree, both our frames of reference is our Sun.

Your slippery explanation of dogma does not really disagree with me , your last statement I have quoted, agrees with me.

Your one dimensional view of only considering things on Earth is a downfall.

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If you transported the equipment from Earth to your lab on Pluto, you would now find yourself in violent agreement about the duration of a second, the length of a meter and the speed of light in a vacuum.

Exactly that, I was born on Pluto ,  I have transported myself and my equipment to your lab, I am disagreeing with you about the speed of light, a meter and time.  You concur my premise.

All observers agree the speed of light is constant, however my speed is different to your speed because my time runs at half the rate of your time.   Now if we both agreed on the length/duration  of a second that equals 1m in travel of light, then we must agree there is no time dilation because my vacuum clock and your vacuum clock would be synchronised.

#### evan_au

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #5 on: 07/04/2016 17:57:05 »
Quote from: TheBox
Different frames of reference, I disagree, both our frames of reference is our Sun.
You may measure the distance from Pluto to the Sun, and from Earth to the Sun.
But when you do an experiment on Pluto or on the Earth, you are not doing an experiment in the frame of reference (FoR) of the Sun.
• When you do an experiment on the surface of the Sun, you are about 0.7 million km out of the Sun's gravitational well, traveling around the Sun at about 2 km/s (at the equator).
• When you do an experiment on Earth, you are about 150 million km out of the Sun's gravitational well, traveling around the Sun at about 30 km/s.
• When you do an experiment on Pluto, you are about 6,000 million km out of the Sun's gravitational well, traveling around the Sun at about 5 km/s.
These different positions in the Sun's gravitational well, and their different  velocities, means that these three locations are in three different FoR, and you expect that viewing results in a different FoR will produce different answers.

By the way, there is a way that an observer on Earth & Pluto can observe an experiment done in the Sun's FoR: The Sun's atmosphere absorbs certain wavelengths of light, creating Fraunhofer lines. These spectral lines can be observed by telescopes on both Earth and Pluto. Both Earth and Pluto will consider the wavelength of the Sun's light to be slightly red-shifted compared to measurements in their own lab. A lab on Pluto will consider it more red-shifted than a lab on Earth.

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Exactly that, I was born on Pluto ,  I have transported myself and my equipment to your lab, I am disagreeing with you about the speed of light, a meter and time.

If you transported your lab equipment to Earth, you would agree with a measurement made on Earth of these fundamental quantities, which are believed to be invariant when measured in all FoR.

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you are on Earth measuring the speed of light in a vacuum, I am on Pluto observing you doing the experiment
This is where a possible disagreement could occur - a naive observer sees a measurement which is done in a different FoR, and is puzzled by the apparent difference in results.

#### Thebox

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #6 on: 08/04/2016 08:29:29 »
Quote from: TheBox
Different frames of reference, I disagree, both our frames of reference is our Sun.
You may measure the distance from Pluto to the Sun, and from Earth to the Sun.
But when you do an experiment on Pluto or on the Earth, you are not doing an experiment in the frame of reference (FoR) of the Sun.
• When you do an experiment on the surface of the Sun, you are about 0.7 million km out of the Sun's gravitational well, traveling around the Sun at about 2 km/s (at the equator).
• When you do an experiment on Earth, you are about 150 million km out of the Sun's gravitational well, traveling around the Sun at about 30 km/s.
• When you do an experiment on Pluto, you are about 6,000 million km out of the Sun's gravitational well, traveling around the Sun at about 5 km/s.
These different positions in the Sun's gravitational well, and their different  velocities, means that these three locations are in three different FoR, and you expect that viewing results in a different FoR will produce different answers.

By the way, there is a way that an observer on Earth & Pluto can observe an experiment done in the Sun's FoR: The Sun's atmosphere absorbs certain wavelengths of light, creating Fraunhofer lines. These spectral lines can be observed by telescopes on both Earth and Pluto. Both Earth and Pluto will consider the wavelength of the Sun's light to be slightly red-shifted compared to measurements in their own lab. A lab on Pluto will consider it more red-shifted than a lab on Earth.

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Exactly that, I was born on Pluto ,  I have transported myself and my equipment to your lab, I am disagreeing with you about the speed of light, a meter and time.

If you transported your lab equipment to Earth, you would agree with a measurement made on Earth of these fundamental quantities, which are believed to be invariant when measured in all FoR.

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you are on Earth measuring the speed of light in a vacuum, I am on Pluto observing you doing the experiment
This is where a possible disagreement could occur - a naive observer sees a measurement which is done in a different FoR, and is puzzled by the apparent difference in results.

Well this Pluto observer happens to be clued up and not naive and he asked the Earthling what gives you the Universal right to define the length of a second and explains that the second is much shorter than a second, in fact my Pluto second is provable to be a different length than yours.  Also the observer understands simultaneity and tells the Earthling it is relative to nothing apart from your say so.

#### Bored chemist

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #7 on: 10/04/2016 10:16:41 »
Do we have to get as far as Pluto?
When I grew up the speed of light was 186000. Now it's 300,000.
But the speed of light hasn't changed. What changes was the units people used (MPS to KM/s).
It's like saying that a car is more efficient in the UK because it gets more miles per gallon in the UK than in the US..
But that's nonsense- the difference is because the gallon isn't the same as the gallon.
(There are about 1.2 gallons per gallon or about 0.83 gallons per gallon, depending on how you look at it.)

If you have a "second" that isn't the same as the second then you will get a different number for the speed of light.
But the actual speed of light is the same.

#### Thebox

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##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #8 on: 10/04/2016 10:22:16 »

If you have a "second" that isn't the same as the second then you will get a different number for the speed of light.
But the actual speed of light is the same.

Yes the speed of light is constant, the speed of time is constant even if the observers disagree on the rate of time and the numbers, if on Pluto I agree that the speed of light is the same for 1 m of space as in your space, then we have to agree that time is an invariant.  We cant disagree on time and have different numbers, that would be a different speed.

#### The Naked Scientists Forum

##### Re: Would I measure the speed of light to be different if I was born on Pluto?
« Reply #8 on: 10/04/2016 10:22:16 »