Physicists propose method to measure variations in the speed of light

speed of light
A relation between the angular diameter distance (DA), the Hubble function (H), and the speed of light c at a specific point called the maximum redshift (zM) may allow researchers to detect variations in the speed of light. Credit: Salzano, et al. ©2015 American Physical Society

(—The speed of light, c, is one of the best-known constants, having a value of just under 300,000,000 meters per second in a vacuum. But in some alternative theories of cosmology, the speed of light is not actually constant, but varies throughout time and space. Observational data in support of variations in the speed of light are lacking, but in a new paper, physicists have proposed a way to constrain possible speed-of-light variations and show that future experiments might be able to detect these variations, if large enough.

The , Vincenzo Salzano, Mariusz P. Dąbrowski, and Ruth Lazkoz, at universities in Poland and Spain, have published their paper on measuring variations in the speed of in a recent issue of Physical Review Letters.

"When the data collected by future missions become available, our model will be able to detect a variation of 1% in c in the more pessimistic case, and down to 0.1% in a more optimistic scenario," Salzano, at the University of Szczecin in Poland, told "We want to stress that the main novelty of our method is that it relies on a direct measurement of the speed of light at the cosmological level, with a very minimal number of assumptions about the dynamics of the universe. Other probes, even if giving slightly better constraints, can only test indirect probes and assume c as one of the many cosmological parameters to be determined."

The new method relies on observations of baryon acoustic oscillations, along with a mathematical relationship. Baryonic acoustic oscillations refer to the clustering of baryonic matter in the universe that led to large-scale structures, such as galaxies. To measure how far away a distant object is in space, scientists must account for the object's redshift since the universe is expanding. They do this by using the angular diameter distance, which increases as redshift increases, but only up to a point, which the authors call the maximum redshift, when it starts to diminish. The exact value of the maximum redshift is not known because it depends on the cosmological model used, but it is somewhere between 1.4 and 1.8.

The new method also relies on a mathematical relationship: when evaluated at the maximum redshift, the angular diameter distance (DA) and the Hubble function (H) give the value of the speed of light c through the relation DA(zM)H(zM) = c(zM).

"Here, the distance DA plays the role of a ruler, while the inverse of the Hubble function plays the role of a clock, and their ratio gives the speed of light at the maximum redshift," Salzano explained.

Using the of the angular diameter distance and Hubble function, the physicists were able to calculate a value for the maximum redshift of just under 1.6. This value can then be used to evaluate the above relation and estimate any possible variation in the value of c, should it exist.

The physicists also investigated whether it may be possible to detect variations in the speed of light using future experiments, such as Euclid, a spacecraft to be launched in 2020. They predict that Euclid will be able to detect variations that are 1% or greater with reasonable accuracy, but smaller variations will be more difficult to detect. At this point, observations indicate that any variation in the speed of light would most likely be smaller than 1%. However, Euclid and other surveys may still have a chance of detecting smaller variations if observational errors are sufficiently small.

"Our method is almost perfectly fit for Euclid: it would give its best with an experiment entirely designed on it, but even so, it will be possible to apply it to Euclid data in a very straightforward way," Salzano said. "Therefore, some result will surely be obtained: if positive (detection of a variation of c) or not (constancy of c confirmed), it will be equally important for the understanding of our universe and the validity of Einstein's relativity."

Detecting variations in the speed of light over time could have several interesting consequences.

"If the speed of light was different in the remote past, then this would have some consequences for the future of life in our universe," Salzano said. "Perhaps, it could also prove that some other pieces of the universe or even other universes with different physical properties could exist—ones in which, for example, the radio, television or mobile phone transmission signals would be slower than on the Earth. If the was larger, we would see more of the universe; if it was smaller, we would see less of the ."

Explore further

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More information: Vincenzo Salzano, et al. "Measuring the Speed of Light with Baryon Acoustic Oscillations." Physical Review Letters. DOI: 10.1103/PhysRevLett.114.101304
Journal information: Physical Review Letters

© 2015

Citation: Physicists propose method to measure variations in the speed of light (2015, April 6) retrieved 21 October 2019 from
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Apr 06, 2015
"(—The speed of light, c, is one of the best-known constants, having a value of just under 300,000 meters per second in a vacuum."

I guess this is a typo and You might wanna correct it cause light speed is roughly 100 times faster. :)

Apr 06, 2015
It seems it is a typo day since I mistyped and it is almost 1000 times faster than stated in the article with average value of the speed of light in vacuum 299792458 m/s :)

Apr 06, 2015
If the space-time is expanding, in order to explain the Hubble red shift - why doesn't the speed of light increase proportionally, in order to remain constant? It was one of the original patches of Big Bang theory, to match theory with observation, but it overlooked the fact that if those distant galaxies are actually moving away, such that it will take light longer to cross this distance, that assumes a stable dimension of space, as defined by the speed of light. Some try to argue light is just being carried along by this expansion and is only measured locally, but the basis of the proof for this expansion is the redshift of that very light crossing intergalactic space. So we are supposed to believe space expands, based on the redshift of this light, but have a stable dimension of space against which to compare it, based on the speed of the same light? Go figure.

Apr 06, 2015
Does dark matter (or dark energy) have an index of refraction?

Apr 06, 2015
richard wenzel: Think instead of a varying relationship between units of time and units of space. True, c defines the rate at which information flows, but the definition is functionally "1 unit of space or less for every equivalent unit of time."

The idea (though I doubt it will ever bear fruit, mind you) of a variant c is that as the universe evolved, the conversion factor between how much space equals how much time has changed. Perhaps the time dimension "stretched" a little bit more over its history than the spatial dimensions have. Or vice versa.

To borrow the rubber sheet analogy, suppose you have a rubber sheet with a grid of points. If stretched uniformly, the ratio of horizontal to vertical distances stays the same. But if stretched non-uniformly, that ratio changes slightly as it stretches.

Apr 06, 2015
If the speed of light varies over time and space, that could also have an impact on the estimated age of the universe and the old and new celestial objects.


That's really a very good question. Scientists usually go for "gravitational lensing" (bending light) in detecting Dark Matter as well as other effects it has on ordinary matter.

Apr 06, 2015
Jim: By definition, no, dark matter would not have an index of refraction (in the traditional sense). Dark matter doesn't interact with electromagnetism.

It does indirectly refract light through gravitational lensing, but that isn't the same thing as material indices.

Dark Energy, as presently thought of, would likely not create any index of refraction, since 1) it maybe doesn't interact electromagnetically? (we don't yet know one way or the other) and 2) it likely is uniformly dispersed throughout the universe (so no area varies compared to another, even if it would interact electromagnetically)

Apr 06, 2015
I forgot to mention earlier that since the advent of empirical science, "randomness in nature" and "infinity in Math" have been two of the most difficult challenges for scientists, to this day.

Apr 06, 2015

That's a tough sell since light itself is an electromagnetic wave, and Dark Matter bends light, aka "gravitational lensing."

Apr 06, 2015
I think the equation has a typo, shouldn't it be DA(zM) / H(zM) = c(zM)?

Apr 06, 2015

I believe that relationship is in the frequency domain, hence the inverse of the Hubble function of z.

Apr 06, 2015
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Apr 06, 2015
The speed of light can indeed fluctuate until we admit the existence of dark matter lensing and the possibility, that this dark matter passes through solar system occasionally. Which is one of indicia of cosmic origin of global warming, IMO. Currently both time, both distance units are defined with light speed in lasers, so that the light speed actually cannot change, even if the physicists would want to. But the older meter or kilogram prototypes aren't based on ad hoced light speed and they indicate, that the space-time geometry changed recently (1, 2). The evolution of gravity constant even indicates the similar temporal evolution, like the global warming.

Apr 06, 2015
BTW The detection of slow gravitational waves with pulsar networks are also based on irregular changes in vacuum density and corresponding speed of light between various pulsars pairs. So if we want to find such a wave we should also admit that the speed of light changes during it. Therefore the above observations can be also interpreted like the passage of slow gravitational wave.

Apr 07, 2015
If the space-time is expanding, in order to explain the Hubble red shift - why doesn't the speed of light increase proportionally, in order to remain constant?....... So we are supposed to believe space expands, based on the redshift of this light....

There is an excellent paper that describes this in superb detail, by explaining the observed combination of redshifts from both relative speed and cosmological expansion:
"Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe"
downloadable at Cambridge University Press
or Cornell University Library
The graphics are enlightening.

Another quality read with good illustrations is this Scientific American piece:

Apr 13, 2015
It would be good if we could nail down the nature of time dilation. At this point in the history of physics, nobody is compensating for time dilation in red shift measurements. This is consonant with Special Relativity's definition of reciprocal time dilation: since both moving objects are thought to experience the same time dilation relative to each other, there's no need to compensate in red shift measurement.

Only problem is, that's the one feature of SR that has never been experimentally nailed down.

GR, by contrast with SR, treats time dilation as directional. So does Lorentz's Absolute Lorentz Transformation.

Lots of experiments have validated that time dilation is a real phenomenon; but the only experimental evidence which teases out the distinction between the two versions of time dilation is GPS - which *must* use directional time dilation or geoposition results are erroneous.

If reciprocal time dilation isn't valid, cosmology needs a major rework.

Apr 13, 2015
Testing Lorentz symmetry of special relativity by means of the Virgo or Ligo set-up, through the differential measure of the two orthogonal beams time-of-flight

Apr 13, 2015
gsardin wrote, "Testing Lorentz symmetry of special relativity by means of the Virgo or Ligo set-up, through the differential measure of the two orthogonal beams time-of-flight"

Thanks. I was sure there must be ways to test SR's time dilation reciprocity, but hadn't actually stumbled across any proposed methods. I hope it gets done.

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