People have been measuring the speed of light for a few hundred years, but the universe is nearly 14 billion years old. If the speed of light changes very slowly 鈥� say a small percentage every 10,000 years 鈥� how could we tell?
鈥� If the speed of light varied over time, we would see supposedly constant astronomical processes taking place at the wrong rate, or even changing their rate.
Imagine a small star orbiting a larger one, and that someone close by measures the orbital period as being exactly one Earth year. What would astronomers 100 light years away observe?
Advertisement
If the speed of light is constant, the observed orbital period would remain at one year. But if the speed of light were increasing, light leaving the stars at the end of each orbit would be travelling faster than light leaving one year earlier and would close the gap with it. Light pulses emitted a year apart would then arrive at Earth less than a year apart, meaning we would see each orbit as taking less than one year 鈥� a mismatch between physics and observation. If the rate of change in the speed of light was accelerating, each orbit would be observed to be shorter than the one before, and the physicists would be even more unhappy.
鈥淚f the speed of light varied, constant astronomical processes would seem to proceed at the wrong rate鈥�
Conversely, if the speed of light was decreasing over time, light leaving the stars at the end of each orbit would be travelling slower than light leaving one year earlier, and would lag further behind. Now we would observe the orbital period as exceeding one year. Each orbit might even be observed to take longer than the one before.
However, stellar orbits remain constant to the limits of our measurement technology, so we can say that if the speed of light is changing, the change is too small to observe.
Richard Whybray, Omagh, Tyrone, UK
鈥� Perhaps the issue is not the speed of light, but the speed of time. As the universe expands, the matter within it becomes more spread out. Gravitational time dilation, as predicted by Einstein, causes time to speed up or slow down depending on proximity to a source of gravity: the greater the gravity, the slower the rate at which time flows.
This could create the illusion that the expansion of the universe is accelerating 鈥� something often attributed to the existence of so-called dark energy. A simpler explanation is that as the universe expands and becomes less dense, time itself speeds up. Conversely, at the time of the big bang, when all matter was in a very dense state, time would have been passing very slowly compared with where we are now.
Cosmologists tell us that the initial big bang expansion was very fast. In fact, depending how you look at it, the opposite may be true. Furthermore, the estimate for the age of the universe (nearly 14 billion years) assumes time runs at a constant speed. This may also need to be re-evaluated.
鈥淟ight鈥檚 speed is defined to be 299,792,458 metres per second, so there鈥檚 no way it can change with time鈥�
Mycal Miller, London, UK
鈥� We need to talk about a more basic question: what do we mean by the speed of light or its numerical value? We say that it is 299,000-and-something kilometres per second, but what is a kilometre and what is a second?
Originally, a kilometre was one-10,000th of the distance from the equator to the pole, and a second was one-86,400th of a day. So the metre was based on the size of a physical object 鈥� Earth 鈥� and the second on Earth鈥檚 rotation period. Later we defined the metre in terms of wavelengths of a certain kind of light, and the second in terms of the period of another kind of light. Now we define the metre as the distance that light travels in a second divided by 299,792,458. So by our present definitions, the speed of light is 299,792,458 metres per second, and there is no way it can change with time.
If we went back to defining the metre in terms of the size of some physical object made of atoms, then it could conceivably change with time. This comes down to questions such as whether the 鈥渇ine structure constant鈥� (relating the charge of an electron, Planck鈥檚 constant and the speed of light) or the ratios of masses of the electron, proton and neutron are changing with time. These are meaningful questions; the question of whether 鈥渢he speed of light鈥� has changed is perhaps less so.
Eric Kvaalen, Les Essarts-le-Roi, France
We have covered the possibility that the speed of light might vary over time (4 March 2017, p 28) 鈥� Ed
We pay 拢25 for every answer published in 快猫短视频. To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
快猫短视频 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, 快猫短视频, 25 Bedford Street, London WC2E 9ES.