Posts Tagged ‘speed of light

Low-cost experiment to measure the speed of light

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Faraz Mehdi, Kiran M. Kolwankar
In this paper, we demonstrate a low-cost method to measure the speed of light. It uses instruments which are readily available in any undergraduate laboratory in a developing country and some components which are inexpensive. The method is direct as it measures the time of flight of the LASER beam and easy to implement. It will allow students to verify the finite value of the speed of light first hand. It can be part of the undergraduate syllabus as a regular experiment or a demonstration experiment.


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August 18, 2021 at 6:11 pm


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Measuring the speed of light and the moon distance with an occultation of Mars by the Moon

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a Citizen Astronomy Campaign

Zuluaga et al
In July 5th 2014 an occultation of Mars by the Moon was visible in South America.
Citizen scientists and professional astronomers in Colombia, Venezuela and Chile performed a set of simple observations of the phenomenon aimed to measure the speed of light and lunar distance.
This initiative is part of the so called “Aristarchus Campaign”, a citizen astronomy project aimed to reproduce observations and measurements made by astronomers of the past.
Participants in the campaign used simple astronomical instruments (binoculars or small telescopes) and other electronic gadgets (cell-phones and digital cameras) to measure occultation times and to take high resolution videos and pictures.
In this paper we describe the results of the Aristarchus Campaign.
We compiled 9 sets of observations from sites separated by distances as large as 2,500 km. We achieve at measuring the speed of light in vacuum and lunar distance with uncertainties of few percent.
The goal of the Aristarchus Campaigns is not to provide improved values of well-known astronomical and physical quantities, but to demonstrate how the public could be engaged in scientific endeavors using simple instrumentation and readily available technological devices.
These initiatives could benefit amateur communities in developing countries increasing their awareness towards their actual capabilities for collaboratively obtaining useful astronomical data.
This kind of exercises would prepare them for facing future and more advanced observational campaigns where their role could be crucial.
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June 4, 2015 at 7:17 am


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Scientists slow the speed of light

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Shapely photons break rules to fly slower than light

by Jacob Aron
Anyone struggling with a New Year’s fitness regime knows that you move slower when you’re out of shape. Now it seems the same is true even for light, which up until now physicists had thought travelled at a constant speed.

It’s well known that light travels more slowly when it passes through different materials. This phenomenon, known as refraction, creates optical illusions like a seemingly broken drinking straw sticking out of a glass of water. But the speed of light in a vacuum, a little under 300,000,000 metres per second, is an unwavering constant that underpins much of modern physics, including Albert Einstein’s theory of relativity. It’s so important that physicists give it a single letter: c.

Now, Miles Padgett at the University of Glasgow, UK, and his colleagues have shown this isn’t quite right. Light travelling in a plane wave – the traditional up and down squiggle you learn about in school – always travels at c, but light with a more complex wave structure travels slightly slower, by about a thousandth of a per cent.

Light on time?

The team revealed this oddity by studying two kinds of shaped light: a Bessel beam, which looks like concentric rings of light, and a Gaussian beam, which spreads out as it travels. They used an ultraviolet laser to produce pairs of photons and passed one photon through a filter to shape it into either a Bessel or Gaussian beam. Both photons travelled one metre before hitting a detector, so they should have arrived at the same time, but the shaped photon was slightly delayed.

Why does this happen? One way of thinking about it is that some of the light in a structured beam is moving in the “wrong” direction – sideways rather than forwards. This isn’t a strictly accurate picture of the energy distribution within the beam, warns Padgett, but it is a way to imagine what might be going on. “Personally I think that’s a useful concept, though the scientific rigour police might not welcome it.”

Don’t rip up your physics textbook just yet though – the implications are likely to be minor, only affecting certain short-range experiments that rely on very precise time-of-flight measurements, for example. “We’re not challenging Einstein,” says Padgett.

Hints of this effect have been seen in other experiments, but no one had quite pinned it down before, says Ulf Leonhardt at the Weizmann Institute of Science in Rehovot, Israel. “[This] is really the first clean and clear experiment where the speed of photons in structured light beams is directly measured,” he says. Now that physicists understand it, they might be able to exploit it. “I do not foresee immediate applications in the short run, but important fundamental physics always has implications and applications in the long run.”

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January 23, 2015 at 7:18 am


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The One-Way Speed of Light

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Speed of light experiments measure the average speed to a destination and back, leaving open the possibility that the speed may differ over each leg. Now Canadian physicists have measured the one-way speed of light to test whether Einstein really was rightOne-Way Speed of LightOn any list of the most important discoveries in 20th century physics, the constancy of the speed of light must come near the top. The idea of a universal, constant ‘c’ has profoundly influenced our ideas about the universe ever since.

When Einstein put forward this idea in the special theory of relativity, it immediately solved an important conundrum. Many physicists believed that light, like other waves, must travel through a medium which they called the luminiferous ether. Since the Earth moves around the Sun, it must move relative to this ether. Therefore, the speed of light in a given direction ought to change during the course of the year and even throughout the course of a day.

The now famous Michelson-Morley experiment knocked this idea on the head when it failed to find any variation in the speed of light relative to the Earth’s motion through space. Physicists have repeated this experiment with increasing accuracy many times since with the same result. The speed of light really is constant.

But there’s an interesting loophole in these experiments. They all rely on measuring the round trip speed of light between two points, for example, by bouncing light back and forth between a pair of mirrors. In other words, these experiments measure the average speed of light over two legs, there and back again.

That leaves open the possibility that the speed of light over each leg could be different. So various physicists have attempted to close this loophole by measuring the one-way speed of light.

This is no easy task. The history of these tests is filled with controversy, disputed results and more than one sensational claim that, yes, the one-way speed of light is different from the conventional two-way result.

Today, Farid Ahmed at York University in Toronto and a few pals reveal the results of their own measurement of the one way speed of light. And their results offer some comfort to fans of the status quo.

First, how do you measure the one way speed of light? There is no shortage of experiments which claim to have done this but have been later shown to have measured the two-way speed because of some overlooked factor.

So Ahmed and co’s method is important. These guys create two identical pulses of light and send them in opposite directions along the same length. If there is any difference in the speed of these pulses, that ought to be detectable by photodiodes at each end of the experiment.

Of course, the devil is in the detail. To measure any difference, the experiment has to be run over at least 12 hours, to allow the Earth’s rotation to reorient the experiment. Ahmed and co performed their experiment on 14-15 November and 28-29 November 2009.

And the effects of any environmental changes need to be carefully accounted for. The most significant of these is temperature which can play havoc with experimental equipment and therefore the results. But there are also other factors such as humidity and so on.

Ahmed and co say they’ve carefully accounted for all these factors and that their results are conclusive. “Our results do not report any significant diurnal variations,” they say. “This is consistent with Einstein’s Special Relativity.”

Ahmed and co’s results will need to be carefully scrutinised by the community. Nevertheless, the result will be the one that most physicists expect.

But the result will also be a disappointment for the small band of theorists who say that string theory predicts small variations in the speed of light in these kinds of experiments.

Of course, Ahmed and co don’t rule out the possibility that these variations might exist on a scale too small for this experiment to pick up.

But a test of that will have to wait for another day. Which in some ways is the beauty of science—there is always new physics to be found somewhere over the horizon (we hope).

Ref: Results of a One-Way Experiment to Test the Isotropy of the Speed of Light


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October 14, 2013 at 11:53 am


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Error Undoes Faster-Than-Light Neutrino Results

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OPERA detector

by Edwin Cartlidge
It appears that the faster-than-light neutrino results, announced last September by the OPERA collaboration in Italy, was due to a mistake after all. A bad connection between a GPS unit and a computer may be to blame.

Physicists had detected neutrinos travelling from the CERN laboratory in Geneva to the Gran Sasso laboratory near L’Aquila that appeared to make the trip in about 60 nanoseconds less than light speed. Many other physicists suspected that the result was due to some kind of error, given that it seems at odds with Einstein’s special theory of relativity, which says nothing can travel faster than the speed of light. That theory has been vindicated by many experiments over the decades.

According to sources familiar with the experiment, the 60 nanoseconds discrepancy appears to come from a bad connection between a fiber optic cable that connects to the GPS receiver used to correct the timing of the neutrinos’ flight and an electronic card in a computer. After tightening the connection and then measuring the time it takes data to travel the length of the fiber, researchers found that the data arrive 60 nanoseconds earlier than assumed. Since this time is subtracted from the overall time of flight, it appears to explain the early arrival of the neutrinos. New data, however, will be needed to confirm this hypothesis.
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(updated) Read also:

1. ‘Faster than light’ measurement blamed on loose cable

2. OPERA in Question

3. Opera Result Affected By Instrumental Error !

4. Faster-than-light neutrinos could be down to bad wiring

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February 22, 2012 at 8:49 pm

Faster Than Light Neutrinos (maybe): Field Trip!

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December 30, 2011 at 4:02 pm

Capturing video at the speed of light

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…. One trillion frames per second
MIT Media Lab researchers have created a new imaging system that can acquire visual data at a rate of one trillion frames per second. That’s fast enough to produce a slow-motion video of light traveling through objects. Video: Melanie Gonick.

Read also:  How to make the Slowest Slow-Motion Video Ever!

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December 18, 2011 at 3:08 pm


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On the invariance of the speed of light

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Harihar Behera, Gautam Mukhopadhyay
The invariance of the speed of light in all inertial frames – the second postulate of special theory of relativity (STR) – is shown to be an inevitable consequence of the relativity principle of special theory of relativity taken in conjunction with the homogeneity of space and time in all inertial frames, i.e., the 1st postulate of STR. The new approach presented here renders the learning of special theory of relativity logically simpler, as it makes use of only one postulate….
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November 30, 2011 at 7:23 pm


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