Because speed and time are linked, regardless of how fast you are travelling, if you were to switch on a torch, you would always measure light moving away from you at 299,792,498 metres per second. Regardless of whether you are standing still, or travelling at just 1 kilometre per hour slower than the speed of light, if you turned on a torch and measured how quickly the beam of light accelerated away from you, you would always record the same number: 299,792,498 metres per second. It doesn’t matter how fast you are going; you would always see the same thing, light racing away from you extremely quickly. This is because one hour lasts for much longer when you are travelling close to the speed of light than when you are stationary. The speed of light is a constant regardless of how quickly you are moving.
Such behaviour is very different from our experience on Earth. If you are travelling in a car at 95 mph and another vehicle overtakes you at 100 mph, it does not appear to be travelling much faster than you. In contrast, if you are travelling at 100 mph, a car passing you at 200 mph moves away from you at a much faster speed. At these low speeds, we do not notice time running at different rates as a function of how fast we are travelling, but even at these velocities time does run at ever so slightly different speeds.
The link between speed and time has some unusual consequences. If one twin was to head off on a space journey at 99.99 per cent of the speed of light aged twenty years old, returning to Earth after what would seem to her to be one year later, she would be twenty-one years old but her twin sister would be ninety. On Earth, we think of time as a constant, always ticking away at the same rate, but this is not true when we start to move faster. Time is relative.
As I teenager I was fascinated by the thought of travelling at light speed, even though I knew it could never happen. I wondered what the universe would look like out of my spaceship’s window as it hit light speed. Because time had stopped for me, but not objects travelling at less than the speed of light, would I be able to see all the future locations of all the objects in the universe? Would Earth’s orbits of the sun appear like a smear across space? In imagining this, I had made an error of logic, but I learned an important lesson. My mistake was forgetting that space and time are linked. As speed increases towards the speed of light, and time slows, space contracts, and this means the universe would appear to get smaller and smaller until, at light speed, it would appear as a very bright but infinitesimally small dot. The lesson I learned is that science proceeds by trial and error. What might be a good idea may prove to be wrong. Science is about explaining patterns in data by ruling out hypotheses that turn out to be incorrect. In forgetting that space contracts as speed increases, I had arrived at a flawed hypothesis but learned something important: that science proceeds one disproved hypothesis at a time.
Einstein got it all squared away. He showed not only that space and time are changed by speed but also that they are impacted by gravity. His general theory of relativity includes space, time, speed and gravity. The stronger gravity is, the slower time flows. It is even possible to measure this effect on Earth, with time running faster at the top of a mountain than at sea level. The effect we see on Earth is very small but is exactly what Einstein’s equations predict.
Gravity bends space as well as slowing time. As a beam of light travels through space, if it passes by an object with a large mass, its trajectory bends. A widely used analogy to describe the effect of gravity on space, but in two dimensions rather than three, is to imagine a trampoline with nothing on it. Let us assume that the surface of the trampoline is perfectly flat, such that if you were to roll a marble across it, it would continue in a straight line until it fell off the other side. Now repeat the experiment but having placed a weight in the centre of the trampoline. The weight is sufficient to cause the trampoline to sag in the middle. If your marble is travelling fast enough, its trajectory across the trampoline will now be curved. The gravity of an object curves space much in the same way the weight has changed the surface of the trampoline from a flat plane to being depressed in the middle. And if the heavy object is very heavy, your marble’s trajectory will be so sharply curved it’ll go into orbit around the heavy object.
The greater the mass of an object, the stronger gravity becomes. The sun, for example, has a much stronger gravitational pull than the Earth because it has over 333,000 times more mass. The sun bends space to a greater degree than the Earth, and time runs slower the closer to the sun you get. But even the sun’s gravitational pull pales in comparison to that of black holes. Around each black hole is something called an event horizon. It is the point at which gravity becomes so strong that even light cannot escape its attraction. The gravity in black holes folds space in on itself and stops time for any object that crosses the event horizon.
Einstein’s theory of relativity has repeatedly been tested and has always been found to be remarkably accurate. It describes how (and why) the moon orbits the Earth and the Earth orbits the sun, and why we do not float off into space; it has been used to age the universe and to reveal why it continues to expand. It also reveals that there must exist another type of matter. Without this matter, galaxies would not exist.
When scientists discovered galaxies spinning at such a speed that solar systems towards the edge should not be held in their orbits by the gravitational pull of the rest of the galaxy, they had a problem. The speed at which the galaxies spun meant the solar systems towards the galaxy’s edge should be jettisoned into space. Cosmologists resolved this contradiction by devising a hypothesis that particles exist that do not interact with other particles via the strong and weak nuclear forces and electromagnetism. They called them ‘dark matter’. According to the theory, they only interact via gravity. This makes them extraordinarily hard to detect and, so far, scientists have not been able to observe dark matter particles, even with the LHC. Nonetheless, dark matter particles must be very common given Einstein’s theory appears to be correct.
Dark matter is not the only stuff that scientists hypothesize must exist if Einstein’s theory is true. The universe is expanding quickly, and its rate of expansion is increasing. Physicists have calculated that something other than dark and observable matter must exist to explain this expansion. On very large scales, something is pushing galaxies apart. Scientists have termed this mysterious stuff dark energy briefly mentioned earlier in this chapter. Dark matter and dark energy must be extraordinarily abundant given the observations of the universe we have made. Estimates suggest that 5 per cent of the universe is made up of energy and matter we can see, 27 per cent is dark matter and a whopping 68 per cent is dark energy.
Although scientists know little about dark matter particles or where dark energy comes from, there are many measurements that suggest they must exist. The fact that particles and energy exist that we know so little about is not entirely bonkers. Before the discovery of the Higgs boson, you might have argued that the Standard Model must be wrong because a particle fundamental to it had not been measured. The fact we have not yet been able to directly measure dark energy and see dark matter does not mean it does not exist. In addition, as you will have gathered, not all particles in the Standard Model are influenced by all four of the fundamental forces. In fact, it is only the quarks that feel the strong and weak nuclear forces, electromagnetism and gravity. Electrons are not impacted by the strong nuclear force, and neutrinos are so hard to study because they only interact with other particles via the weak nuclear force and gravity. If dark matter, as is hypothesized, does not interact with the electromagnetic or nuclear forces, the particles that constitute it will be hard to directly detect, requiring levels of energy well beyond those generated by the LHC.