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SPEED OF LIGHT
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- Can anything travel faster than light?
- The speed of light in a vacuum is 299,792.458 km per second (roughly 300,000 km/s). The Sun is 150 million km (15 crore km) away from Earth and light takes just eight minutes and 20 seconds to travel that far.
- This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether. It has since been consistently confirmed by many experiments.
- According to special relativity, the energy of an object with rest mass m and speed v is given by ?mc2, where ? is the Lorentz factor. When v is zero, ? is equal to one, giving rise to the famous E = mc2 formula for mass–energy equivalence. The ? factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light.
- It was September 2011 and physicist Antonio Ereditato had just shocked the world. The announcement he had made promised to overturn our understanding of the Universe. If the data gathered by 160 scientists working on the OPERA project were correct, the unthinkable had been observed. Particles – in this case, neutrinos – had travelled faster than light.
- According to Einstein's theories of relativity, this should not have been possible. And the implications for showing it had happened were vast. Many bits of physics might have to be reconsidered. In the end, it turned out the OPERA result was wrong. A timing problem had been caused by a poorly connected cable that should have been transmitting accurate signals from GPS satellites.
- Can any of humanity's creations compete in a race with light? One of the fastest human-made objects ever built, the New Horizons space probe, passed by Pluto and Charon in July 2015. It has reached a speed relative to the Earth of just over 16km/s, well below 300,000km/s.
- However, humans have made tiny particles travel much faster than that. In the early 1960s, William Bertozzi at the Massachusetts Institute of Technology experimented with accelerating electrons at greater and greater velocities.
- Because electrons have a charge that is negative, it is possible to propel – or rather, repel – them by applying the same negative charge to a material. The more energy applied, the faster the electrons will be accelerated.
- So can be just increase the energy applied in order to reach the required speed of 300,000km/s, but it turns out that it just is not possible for electrons to move that fast. Bertozzi's experiments found that using more energy did not simply cause a directly proportional increase in electron speed.
- As objects travel faster and faster, they get heavier and heavier!
- Light is made up of particles called photons. Why can these particles travel at the speed of light when particles like electrons cannot?
- As objects travel faster and faster, they get heavier and heavier – the heavier they get, the harder it is to achieve acceleration, so you never get to the speed of light. A photon actually has no mass. If it had mass, it couldn't travel at the speed of light.
- Photons are pretty special. Not only do they have no mass, which gives them free reign when it comes to zipping about in vacuums like space, they do not have to speed up. The natural energy they possess, travelling as they do in waves, means that the moment they are created, they are already at top speed.
- In fact, in some ways it makes more sense to think of light as energy rather than as a flow of particles, though truthfully it is – a little confusingly – both.
- Still, light sometimes appears to travel more slowly than we might expect. Light actually travels around 40% slower through the glass of optical fibres than it would through a vacuum.
- In reality, the photons are still travelling at 300,000 km/s, but they are encountering a kind of interference caused by other photons being released from the glass atoms as the main light wave travels past.
- Albert Einstein's theory of special relativity explores many of the consequences of these universal speed limits.
- One of the important elements in the theory is the idea that the speed of light is a constant. No matter where you are or how fast you are travelling, light always travels at the same speed.
- But that creates some conceptual problems. Imagine shining light from a torch up to a mirror on the ceiling of a stationary spacecraft. The light will shine upwards, reflect off the mirror, and come down to hit the floor of the spacecraft. Let's say the distance travelled is 10 m.
- Now let's imagine that the spacecraft begins travelling at a hair-raising speed, many thousands of kilometres per second. When you shine the torch again, the light will still seem to behave as before: it will shine upwards, hit the mirror, and bounce back to hit the floor. But in order to do so the light will have to travel diagonally rather than just vertically. After all, the mirror is now moving quickly along with the spacecraft.
- The distance the light travels therefore increases. Let's imagine it has increased overall by 5m. That is 15m in total, instead of 10m.
- And yet, even though the distance has increased, Einstein's theories insist that the light is still travelling at the same speed. Since speed is distance divided by time, for the speed to be the same but the distance to have increased, time must also have increased.
- Yes, time itself must have got stretched. That sounds strange but it has been proved experimentally.
- It is called "time dilation". It means time travels slower for people travelling in fast-moving vehicles, relative to those who are stationary.
- For example, time runs 0.007 seconds slower for astronauts on the International Space Station (ISS), which is moving at 7.66 km/s relative to Earth, compared to people on the planet.
- Things get interesting for particles, like the electrons mentioned above, that can travel close to the speed of light. For these particles, the degree of time dilation can be great.
- When objects move quickly relative to other objects, their length contracts as well. These consequences, time dilation and length contraction, are both examples of how space-time changes based on the motion of things – like you, me or a spacecraft – that have mass.
- There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light
- Crucially, as Einstein said, light does not get affected in the same way – because it has no mass. That is why it is so important that all of these principles go hand-in-hand. If things could travel faster than light, they would disobey these fundamental laws that describe how the Universe works.
- Exceptions: While nothing has ever been observed travelling faster than light, that does not mean it is not theoretically possible to break this speed limit in very special circumstances. Take, for instance, the expansion of the Universe itself. There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light.
- Another interesting situation concerns particles that seem to be expressing the same properties at the same time, no matter how far apart they are. This is called "quantum entanglement". In essence, a photon will flip back and forth between two possible states at random – but the flips will exactly mirror the flipping of another photon somewhere else, if the two are entangled.
- However, in both these examples it is crucial to note that no information is travelling faster than the speed of light between two entities. We can calculate the Universe's expansion, but we cannot observe any faster-than-light objects in it: they have disappeared from view.
- Humans might one day build a faster-than-light spacecraft. One of the ways to do this might be to travel through a wormhole. These are loops in space-time, perfectly consistent with Einstein's theories, which could allow an astronaut to hop from one bit of the Universe to another via an anomaly in space-time, a sort of cosmic shortcut.
- The object travelling through the wormhole would not exceed the speed of light, but it could theoretically reach a certain destination faster than light could if it took a "normal" route.
- But wormholes might not be available for space travel. What if instead you actively distorted space-time in a controlled way, to travel faster than 300,000km/s relative to someone else?
- Light is the Universe's broadcast. That speed – 299,792.458 km/s – remains reassuringly constant. Meanwhile, space-time is malleable and that allows for everyone to experience the same laws of physics no matter their position or motion.
- In science fiction, it may be said that if this Universe is an artificial construct, then the Creator has put a limit on the speed of light, to enable rendering of images beyond that while someone approaches. It’s just like a video game that gets rendered in real time, as field of vision keeps on changing!
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