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By Root 238 0

motion is relative, it is perfectly justifiable for your Earth-bound twin to assume that it is Earth that receded from your spacecraft at 99.5 per cent of the speed of light. However, this viewpoint leads to the opposite conclusion than before—that time slows for your twin relative to you. Clearly, time cannot run slowly for each of you, with respect to the other. The resolution of this twin paradox, as it is known, is to realise that your spaceship actually has to slow down and reverse its motion at Alpha Centauri. Because of this deceleration, the two points of view—your spaceship moving or Earth moving—are not really equivalent and interchangeable.

5 The momentum of a body is a measure of how much effort is required to stop it. For instance, an oil tanker, even though it may be moving only a few kilometres an hour, is far harder to stop than a Formula 1 racing car going 200 kilometres per hour. We say the oil tanker has more momentum.

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E = MC2 AND THE WEIGHT OF SUNSHINE

HOW WE DISCOVERED THAT ORDINARY MATTER CONTAINS A MILLION TIMES THE DESTRUCTIVE POWER OF DYNAMITE

Photons have mass?!? I didn’t even know they were Catholic.

Woody Allen

It’s the biggest set of bathroom scales imaginable. And, oh, yes, it’s heat resistant too. It’s so big in fact that it can weigh a whole star. And today it’s weighing the nearest star of all: our Sun. The digital display has just come to rest and it’s registering 2 × 1027 tons. That’s 2 followed by 27 zeroes—2,000 million million million million tons. But wait a minute, something’s wrong. The scales are superaccurate. That’s another remarkable thing about them, in addition to their size and heat resistance! But every second, when the display is refreshed, it reads 4 million tons less than it did the previous second. What’s going on? Surely the Sun isn’t really getting lighter—by the weight of a good-sized supertanker—every single second?

Ah, but it is! The Sun is losing heat-energy, radiating it into space as sunlight. And energy actually weighs something.

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So the more sunlight the Sun gives out, the lighter it gets. Mind you, the Sun is big and we’re only talking about it losing about a 10-million-million-millionth of a per cent of its mass per second. That’s hardly more than 0.1 per cent of its mass since its birth.

The fact that energy does indeed weigh something can be seen vividly from the behaviour of a comet. The tail of a comet always points away from the Sun just like a windsock points away from the gathering storm.

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What do the two have in common? Both are being pushed by a powerful wind. In the case of the windsock, it’s a wind of air; in the case of the comet tail, a wind of light streaming outward from the Sun.

The windsock is being hit by air molecules in their countless trillions. It is this relentless bombardment that is pushing the fabric and causing it to billow outward. The story is pretty much the same out in deep space. The comet tail is being battered by countless tiny particles of light. It is the machine-gun bombardment of these photons that is causing the glowing cometary gases to billow across tens of millions of kilometres of empty space.

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But there is an important difference between the windsock being struck by air molecules and the comet’s tail being hit by photons. The air molecules are solid grains of matter. They thud into the material of the windsock like tiny bullets, and this is why the windsock recoils. But photons are not solid matter. They actually have no mass. How then can they be having a similar effect to air molecules, which do?

Well, one thing photons certainly do have is energy. Think of the heat that sunlight deposits on your skin when you sunbathe on a summer’s day. The inescapable conclusion is that the energy must actually weigh something.

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This turns out to be a direct consequence of the uncatchability of light. Because the speed of light is terminally out of reach, no material body can ever be accelerated to the speed of light, no matter how hard it is pushed. The speed of light, recall, plays the