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Tapsnap
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How do we know the speed of light is constant?
Is it possible that we only perceive it as constant?
Is it possible that we only perceive it as constant?
Tapsnap said:How do we know the speed of light is constant?
Is it possible that we only perceive it as constant?
I did say 'at least'!Nugatory said:Mad Scientist said a "half century"; he's understating the case
Mad scientist said:I did say 'at least'!
Just a quick note, nowadays we know the laws of electricity and magnetism are not exactly Maxwell's, they are a good approximation to a quantum field theiory(QED). But of course Einstein had no way to foresee this in 1905.Nugatory said:Strictly speaking, it's not something that we "know" - it's a postulate, and is explicitly described as such in Einstein's 1905 paper introducing special relativity.
However, it is a very plausible postulate.
First, we have over a century of increasingly accurate results (Mad Scientist said a "half century"; he's understating the case) some of which are described in the sticky thread at the top of this forum, suggesting that nature really does behave that way.
Second, the speed of light in a vacuum can be calculated from the laws of electricity and magnetism (Maxwell, 1861) so any non-constancy in that speed would imply a corresponding non-constancy in the laws of E&M (for example, electromagnetism on the surface of the Earth would behave differently at noon and at midnight, and in June and December, as the Earth is moving in different directions at different speeds).
Tapsnap said:How do we know the speed of light is constant?
Is it possible that we only perceive it as constant?
loislane said:nowadays we know the laws of electricity and magnetism are not exactly Maxwell's, they are a good approximation to a quantum field theiory(QED).
The speed of light was first measured in 1676 by Danish astronomer Ole Rømer. He observed the eclipses of Jupiter's moon, Io, and noticed that the time between eclipses was longer when Earth was moving away from Jupiter and shorter when Earth was moving towards Jupiter. This was due to the difference in the speed of light when measured from different points in Earth's orbit.
The speed of light is considered to be constant because it has been measured and confirmed by numerous experiments and observations. These experiments include the Michelson-Morley experiment, which showed that the speed of light is the same in all directions, and the Fizeau experiment, which measured the speed of light in moving water. Additionally, the principles of special relativity, proposed by Albert Einstein, rely on the constant speed of light.
The most common method of measuring the speed of light is using a device called a laser interferometer. This instrument sends a beam of light through a vacuum and measures the time it takes for the light to travel a known distance. Other methods include using mirrors and timing the reflection of light, or using the properties of electromagnetic waves to calculate the speed of light.
The speed of light has been constant throughout human history, as far as we know. However, some theories propose that the speed of light may have been different in the early universe, during the Big Bang. This is still a topic of debate and further research is needed to fully understand the speed of light in the early universe.
If the speed of light was not constant, it would have significant impacts on our understanding of the universe. The principles of special relativity would no longer hold true, and the laws of physics would need to be rewritten. It would also have practical implications, such as affecting our ability to accurately measure distances and time. However, based on current evidence and experiments, it is widely accepted that the speed of light is indeed constant.