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What If The Speed Of Light Slowed Down?

VO: Noah Baum WRITTEN BY: Caitlin Johnson
The speed of light is the fundamental rule that almost all of physics and science is based on. Ever since the works of Albert Einstein, light speed has dictated what we can and can't do, and what we do and don't know about the universe. But, what if it changed? What if the speed of light suddenly slowed down? Would it really be so bad? In this video, we take the fastest thing ever known, and stick it in slow-mo. Enjoy!
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What if the Speed of Light Slowed Down?


Einstein was only in his 20s when he came up with the ground-breaking theory of relativity, which put a figure to the speed of light and denoted it from that point onwards as “C”, for “constant”. The speed of light, 186,000 miles per second, is the unchanging foundation of modern physics – but now some scientists are thinking that Einstein could have been wrong, or at least not totally right. Light may not be a constant after all… it may have once been much faster and we might be able to artificially slow it down.

Sending light through certain transparent objects has already proven to slow it down significantly from that magic 186,000 miles per second figure – which only applies when light is travelling through a vacuum – and in fact, through diamond, the speed of light more than halves, to just 77,000 miles per second. But what if the universal constant of “C” actually did slow down, not just through other bodies but also in the vacuum of space? What would that mean for mankind, if anything at all?

While light naturally slows down, through natural bodies like diamond and water, there are also ways to artificially slow it. One physicist working at Harvard University, Lene Hau, has even successfully managed to stop light dead in its tracks - almost. She accomplished this by creating what’s known as a “Bose-Einstein condensate,” a cloud of gaseous atoms chilled to absolute zero. The gas is made of elemental sodium first vaporised in an oven, then when the atoms are released they’re blasted with a laser beam. It’s these lasers which force the atoms to slow, stopping them from vibrating. In doing so, this condensate becomes the coldest substance on planet Earth. When a beam of light is shone through the sodium condensate, the speed of light decreases to just 15 miles per hour. This means that even the most run-down, old car has the potential to travel faster than light. Theoretically.

Other experiments have similarly succeeded in reducing the speed of individual photons by changing their atomic structure. At the University of Glasgow, a pair of photons in free space were sent towards a detector – one with its structure temporarily altered, the other without. Researchers discovered that the altered photon arrived at the detector slightly later, meaning that there is an intrinsic link between the structure of a photon and how fast it travels, otherwise they would both have arrived at the same time. While this is just one solitary photon, it’s still a tremendous milestone in physics to be able to slow down light.

But no human experiments are really capable of slowing down light throughout the entire universe, and it seems unlikely that technology will ever reach that point. However, there are plenty of things in the universe that mankind can’t artificially recreate, and there are those physicists who believe that light is indeed slowing down – all on its own. In the grand scheme of things, light doesn’t actually travel all that quickly. 186,000 miles per second in a universe 93 billion light years across – bearing in mind one light year is 5.88 trillion miles – just isn’t that speedy. In one analogy to prove this, if the universe was the same size as planet Earth, walking down your own street at the equivalent speed of light would take you a huge 100,000 years.

The fact is, that the universe is simply too vast for light to have reached all the points in it travelling at this speed. This is called the “horizon problem” and it was one of the largest problems in physics after Einstein determined the value of light-speed. The core issue is that when the edge of the universe is observed, the furthest points in any direction are “uniform”, i.e., they have exactly the same cosmic microwave background radiation. The furthest points we can measure are all 13.8 billion light years away, and are all uniformly -270 degrees Celsius, meaning that light had to have reached all these points at exactly the same time; which is impossible if light moves at 186,000 miles per second. The horizon problem baffled scientists until the application of the theory of inflation. Inflation says that when the universe was created it experienced a rapid period of growth, accounting for the uniformity of the cosmic background radiation.

But other scientists have come up with a controversial counter-theory to the widely-accepted theory of inflation: variable light speed. Physicists Joāo Magueijo and Niayesh Afshordi have been researching the idea of variable light speed for decades, but finally published their findings in a 2016 paper. They suggest that the speed of light has been slowing down in the last 14 billion years since the Big Bang, and that once light actually travelled “infinitely” quickly. They also say that this was possible because at the dawn of the universe, the temperature of outer space was “ten thousand trillion trillion” degrees Celsius. That’s much hotter than anything we can possibly imagine, bearing in mind that the temperature at the centre of the sun is a comparably meagre 15 million degrees Celsius. While this theory makes sense, it’s controversial because of how it disagrees with Einstein’s theory of relativity. If light is not, in fact, constant, then our entire understanding of physics is dismantled – and it stands to reason that the speed of light is still slowing down, albeit much too slowly for human beings to perceive.

But how much of a threat to us is a slowing down of light speed, really? Well, the speed of light slowing down wouldn’t necessarily make it easy for us to “jump to warp speed” or travel through time. We’re still a long way off from even being able to travel close to the speed of light – even if it is slow compared to the rest of the universe. While we potentially could travel through time, if it slowed enough to allow us to reach those speeds, it wouldn’t make travelling through the universe any easier. Light slowing down wouldn’t bring planets and galaxies any closer. If, for example, light slowed to only 100 miles per hour, going faster than light would mean going as slow as 101 miles per hour, which most cars can easily do. But to travel through the stars, despite breaking the light speed barrier and creating a “photonic boom”, you’d still only be travelling at a tedious 101 miles per hour, meaning this wouldn’t be a way to fast-track our journeys to Mars and beyond.

It would also make it far trickier to observe the universe. Already, everything we see in the sky is delayed by light speed; if light slowed, we’d be at even more of a loss when it comes to studying faraway places, knowing less and less about the universe with each passing day. But there’s also a far graver implication, in that light and temperature are directly linked. If the speed of light increases with the temperature of the universe, as Afshordi and Magueijo suggest, then it only makes sense that if the speed of light decreases so will universal temperatures – which could have devastating results; for example, during the last ice age the Earth’s average, global temperature was only 4 degrees Celsius lower than it is now – while the global temperature only has to increase by 2 degrees for the effects of climate change to become irreversible. We’re walking a very fine balance as it is!

So, if the speed of light slowed down, the temperature of the sun could decrease and the goldilocks zone of the Milky Way could be brought much closer in. Perhaps Mercury would become a new, habitable planet, while Earth transforms into an icy wasteland. Essentially, if light slowed down significantly, we’d risk freezing to death, all while finding it impossible to actually observe the wider changes to the universe around us. The really scary part? If scientists are right about variable light speed as opposed to inflation, then this outcome is inevitable – but it wouldn’t reach that point for at least a few million years.

Ultimately though, this theory is still just that – a theory. The theory of inflation and the Big Bang, as well as Einstein’s theory of relativity, are also all technically tentative. They make sense, which is why they’re widely accepted, but we currently don’t have the means to definitively prove them one way or another. There is every possibility that when Einstein said that light is the constant “C”, he was absolutely correct, and light isn’t slowing down at all. But until the planet turns into a big snowball or otherwise, we might not know for sure how reliable the speed of light truly is.
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