Submitted by The_Dark_Passenger93 t3_zckx8p in askscience
We believe the speed of light, the Boltzmann constant and other basic physical constants to be static over time and space. How are we sure about this? Isn't it possible for speed of light to have varied over time, of under influence of something like dark matter or dark energy? If it is not as constant as we suppose it is, then wouldn't it drastically change our understanding of the cosmos?
How do we account for red shift without knowing distance? AFAIK the shift itself is the only way we can measure distance at cosmological scales.
>How do we account for red shift without knowing distance?
We don't. We know the distance, at least to within some margin of error that is always accounted for.
>AFAIK the shift itself is the only way we can measure distance at cosmological scales.
That isn't accurate. You probably want to do a little bit of reading into the cosmic distance ladder and how it is constructed. Redshift needs to be accounted for at all but the closest distances, and there are close to two dozen different ways of measuring differently-sized distances that cover overlapping distance ranges, and which are all in general agreement with each other within the overlapping ranges, as well as in agreement with the measured redshifts.
Hope that helps!
Tests of the fine structure constant compare different wavelengths from the same source. The ratio depends on the fine structure (for suitable transition choices) while redshift cancels in the ratio. The uncertainty on the redshift is far too large for the extremely precise measurements (~10^(-7) level) we can do with the ratios (/u/forte2718 ).
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We can't know. But we don't have any reason to believe the opposite.
With accurate scientific language we don't say "constants are constant". We say "we had never find any experiment or scenario where the constants are not constant".
It's similar to courts. We say not-guilty instead of innocent. If we suspect a guy killed some other guy but we can't find any clue or evidence that this guy murdered the other we can't accuse him to be the murderer. But we can't say neither that this person is innocent.
We cannot be completely sure that the fundamental constants, such as the speed of light, are fixed over time and space. However, we have collected data over the course of decades that suggests these constants remain constant. If the speed of light and other constants do vary over time and space, it could indeed have a drastic impact on our understanding of the cosmos, particularly when considering phenomena like dark matter and dark energy.
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In short, we can't be certain about anything.
Due to our relatively small window of reference from a basically static position and inability to move throughout deep space, everything we know is based in what we know about how things interact from long distance observation. It is possible, however unlikely, that there are different means and changes to what we know as constants given circumstances that we cannot recreate here or Earth, or even that may change depending on distance throughout space that reaches beyond what we can observe. That is kind of what relativity is about, that we cannot say for certain everything is what we believe it is from a different point of reference. That is why we continue to explore what we can, and learn as much as we can with what we have, in hopes that some day more answers will come, but with them more questions will arise. Discovery is knowing that we can find those answers.
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Great answer. I myself believe that today, with our current understanding and observation of universe these constants seem to work properly; but who knows? Maybe with new observations and breakthroughs in fields of physics we reach some point that these constants become variables. And I think it's the innate beauty of science, to go forward and modify our knowledge for the best.
The constancy of the speed of light and other basic physical constants is supported by a variety of astronomical observations. For example, the light from distant galaxies is redshifted, which indicates that the speed of light has remained constant over time. Additionally, the cosmic microwave background radiation is consistent with the predictions of the Big Bang theory, which relies on the constancy of the speed of light. Finally, the abundance of elements in the universe is consistent with the predictions of nuclear physics, which also relies on the constancy of the speed of light.
In short, the constancy of the speed of light and other basic physical constants is supported by a variety of astronomical observations, and any deviation from this constancy would have drastic implications for our understanding of the cosmos.
Be careful to reformulate your question in terms of dimensionless constants. Changing those has observational consequences. Changing the value of dimensionful constants, like the speed of light, is meaningless.
"speed of light" is the speed in empty space, without any obstructions.
It is already known that any obstruction, reflection or other influence on the light can increase the time it takes to cover a specific distance. But if you would track the exact path it took, you'd get back to the speed of light.
But one of the primary ideas of the scientific method is that only the data from experiments matters. No value is unchangeable.
Since every change to the overall model needs to be consistent with all other data we have gathered and all previous experiments have to be explained as good or better than the previous, every change will only make the model better.
As a principle, if it does not correlate with the data, it is not a valid theory. Theories are "facts" that only are considered facts because all attempts to disprove it have failed.
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not trying to be contrary, here, but....
i was under the impression that the speed of light DOES vary, depending upon the medium its travelling through??
the "speed of light" (c=) constant is actually referring to speed of light in a 'perfect' vacuum only.
right?
and so, i guess, by definition: it must have changed as the density of the universe changed over time with expansion.
right?
i mean, the value for (c) itself wouldn't have changed, but the ACTUAL speed that ACTUAL light was traveling would have changed over BOTH time AND space, as the density of the universe decreased with expansion.
right?
The speed of light depends on the medium, but in vacuum it's always the C. About the question of density, when we are dealing with expansion of universe, we should consider that the fabric of the cosmos itself is expanding, hence all the galaxies and galaxy clusters get drifted apart from each other (expect for gravitationaly bound objects which are attracted to each other more than what expansion of cosmos can drift them apart).
Therefore the density of universe also changes, but it doesn't mean that vacuum has changed or anything, merely it means that there are more vast vacuum space in the cosmos now.
Also consider if a beam of light, for example has spent a million years to travel from one galaxy to another, it would have covered one million light years, but with the expansion of universe during that time, the distance between the two galaxies are about for example 1.1 million light years (numbers are arbitrary) during this time the speed of light hasn't changed at all, it is always C, but the meters that the light has traversed are expanded (or squeezed if the expansion of cosmos is reversed), the speed hasn't changed, but the meters got inflation (kinda like my salary being constant and expenses getting inflation 🙄🙄) this notion is called comoving and proper distance. I hope I could have covered your question.
ahhh, yeah that's a great way to think about it.
space-time "inflation".
hey, now i understand part of why they call it the "inflationary period", so thanks for helping me to make a connection of understanding, there!
cheers
Your welcome my friend. During the inflationary period the rate of universe expansion was so much greater than other times. An object as wide as one nanometer expanded to 10.6 light years wide! In less than a nanosecond. It was indeed a great inflation:/
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I like your thought process on this. A couple of things.
The speed of light will actually vary in different mediums. I can't be bothered looking to find what they are, and how much light's speed is reduced.
Veritasium does a great video about how proving the speed of light isn't really measurable, and that we've only measured light speed in two directions. I'd highly suggest watching it.
I know about speed of light in other mediums, but the speed of light in vacuum is C and always constant, isn't it? Thanks for the insight on the speed of light, but what about other constants? How we assure that they are immutable?
It's constant to the observer, perceived changes in the speed/energy of the light they are observing are actually changes in time itself. Measuring the actual speed of light is impossible.
Theoretically.
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as a naive assessment, enstein formula's describe a constant equilibrium between energy and mass, including C constant (velocity in vaccum of ANY mass-less particle, not only photons).
consequently, a variation in C whenever you want during the life of the universe would have provoked a variation in energy itself.
as a less naive argument, physical constants were discovered step by step but every fundamental values seem to be linked to each other, building up few but solid equations describing the universe, in past, present, and future time. Science itself walks forward by successive discoveries. today, relativity and quantic physic are our best models of the universe and rely on such constant (and make it far more easy with math!). but there is a chance a breakthrou happens and shows variations in what we consider (very strongly) today as constants.
Energy is not a conserved quantity in General Relativity because the theory is not time-symmetric. Hence the whole "dark energy is increasing" thing.
I see your point there, excellent answer. The idea of a great breakthrough in physics always gives the goosebumps.
thanks. but honestly, this is mostly an intellectual effort. what actual physics can tell us about the universe is REALLY stunning and seems to answer anything better than any other approach (personnal opinion). but this knoweldge was gained by the ability to fight what we believed and only keep what is strongly demonstrated. the only counter argument I have in mind is the Hawkin beam from black hole wich freely assume those object obey to entropy law. I will personnaly blindly follow the theoric demonstration and "praise" to see evidence while I am still alive.
I agree with you, our current rules of physics explains so many phenomenons with such accuracy that it's an approval for our physics in general. And the approach of correcting ourselves when we find a better explanation is really necessary if we are about to go forward.
About the Hawking radiation, it really fascinates me as well, sadly I don't have enough technical savvy on the subject so I will follow you, accept and "praise".
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forte2718 t1_iyxr6p8 wrote
>How are we sure [that constants don't vary over space and time]?
Well, we test that hypothesis by looking at measurable quantities which would be different if the constant were different. This is easier to test for some constants than others.
For example, if the fine structure constant were to vary across space, that would have a major impact on chemistry — chemical bonds would have different characteristic energies, light emitted when breaking those bonds would have different wavelengths, and different kinds of bonds would be possible in general. For example, the Lyman-Alpha hydrogen line would have a wavelength different from 121.567 nm. But when we look out into the cosmos, and do spectroscopy of distant stars and galaxies, we see that they all have on average the same composition of frequencies, and the Lyman-Alpha line at 121.567 nm is strongly seen (after accounting for known effects such as redshift of course). So, that's one way we know that the fine structure constant is actually constant throughout the entire observable universe.
Likewise, for the speed of light in particular, one possible test (of many) of the speed of light in distant galaxies comes from type 1a supernovae measurements and something called the cosmic distance duality relation (CDDR), which according to the linked source is model-independent and can only be violated by three conditions (non-Riemannian geometry, which would mean general relativity itself is entirely inapplicable and which seems incredibly implausible given the successes of general relativity at modelling the cosmos as a whole, and similarities in distant measurements of other constants such as the fine-structure constant mentioned above; a source of opaqueness in the cosmos, i.e. some kind of foreground dust blocking our view of distant objects, which obviously isn't the case; and, variation in fundamental physical constants such as the speed of light). By comparing these measurements, they determine that the CDDR is respected even in distant galaxies, indicating that none of those three conditions apply.
And there are a variety of other possible tests as well; off the top of my head I vaguely recall hearing about one involving comparing the delay times of light from a certain supernova to neutrinos that were detected from the same supernova, and I think there was also one involving the delay time of light emitted directly by a supernova versus light emitted by a cloud of gas surrounding the supernova as a consequence of a shockwave, or something along those lines ... though I wasn't able to find references for these in a cursory Google search. I'm sure if you searched around you could find these and/or other methods. (Edit: Also I remembered another detail — since the speed of light is related through Maxwell's equations to the electric permittivity and magnetic permeability of the medium it's travelling in, tests of these two quantities for the vacuum or perhaps even for known kinds of systems like gas clouds surrounding a quasar or supernova in distant systems could also help confirm or refute differences in the speed of light within those systems.)
But suffice to say, the way we know it's the same is by looking at distant systems and seeing that they behave the same as nearby systems, specifically in situations where a different speed of light or other different physical constants should cause them to behave differently. To date, there is no convincing evidence of any discrepancies between the speed of light on Earth and the speed of light in distant galaxies.
Hope that makes sense!