Lun 15 Feb 2016
As we can feel the wind in a moving sailboat and figure out which way and how fast we are moving, someone thought that, as we are traveling through space in our own planet Earth, we should be able to somehow feel the wind. If we can perceive the slight differences in the speed of the light of the stars coming towards us from different directions, we might figure out where are we going and how fast and, most intriguingly, where did we come from.
This was not practical until late in the 19th century. The speed of our Earth was minuscule against the speed of light so there was no way to detect such a subtle difference within the margin of error in our measurement of the speed of light.
When this became possible there was a big surprise, even the most sensitive instruments, some built on purpose to measure this, could not detect any difference. Light came at exactly the very same speed from absolutely everywhere. Could it be that we are completely still at the exact center of the Universe and everything moves around us? That sounds more theology than cosmology.
The measurement got so precise that it should have been able to detect the Earth rotation on its axis or its orbit around the sun. Even if we believed in an unmovable Earth at the center of the Universe, the relative movements of other bodies relative to us should be detectable. But, still, nothing.
Many theories were developed that fitted the observed facts. The most elaborate assumed that our rulers shortened in the direction of the movement and since speed is distance over time, if the ruler measuring distance shortened, we weren’t getting the speed reading right. Numerically, it worked wonderfully but explained nothing.
Then came Einstein who more or less thought, what if the speed of light is constant? He started to work the equations based on that assumption and, voilá, we got the Special Theory of Relativity in 1905. Instead of having arbitrary equations that fitted the numbers but had no underlying justification, he started from a single principle, that the speed of light was constant, and came up with the equations that fitted the observations.
Though we talk about the speed of light as a constant, what we actually mean is that there is a maximum speed in the Universe and nothing can go faster than that, light included. Would-be inventors wonder if there could be something other than light that can go faster but it is not like we have a race-car called light that happens to max-out at that speed but some other more powerful race-car could beat it. It is not light that puts the limit, it is the Universe that sets it and light just goes as fast as it could possibly go.
Photons, the particles light is made of, are like ‘flubber’ the magical bouncy thing in The Absent-minded Professor, and I am thinking the Fred McMurray version not the recent one with Robin Williams. Photons have no mass so, it doesn’t require any force at all to get them going. It is basic Newtonian mechanics, F (force) = m (mass) * a (acceleration) or, a = F / m. If m is 0, a turns infinite, no matter what the force might be. Thus, like flubber bouncing all over the lab, the moment photons get just the slightest touch, they accelerate up to the maximum possible speed.
Ten years after Special Relativity, now a century ago, Einstein published the General Theory of Relativity. He thought, if a particle does not speed up beyond the speed of light, it is as if its mass grows larger. The faster it goes, the more massive it becomes, heavier if you wish. By the time it reaches the speed of light, the mass becomes infinite and no matter how much we push, the particle is not going to go any faster.
It might sound as an explanation no better than rulers magically shortening in the direction of movement. Why mass would grow with speed? Well, we actually don’t know why but, and here is the beauty of it, if we assume that mass does increase, we can make predictions on how things should behave and, if those predictions come true, the original assumption should be true. Even in Einstein’s time, some of the predictions took several years until they could be confirmed and one of them, gravitational waves, has just been confirmed a century after it had been predicted. Not that anyone seriously doubted it would be true, the problem was that the effect is so minuscule that we couldn’t make instruments sensitive enough to detect it. It is an achievement of experimental science since the theory, Einstein’s, was not in doubt.
This change in the mass of moving objects is what relates mass to the speed of light. The closer to the speed limit of the Universe an object travels, the heavier it gets. By playing a little bit with the equations, we get to the famous E = mc², an equation which has been thoroughly proved by thousands of nuclear explosions and many nuclear power plants.
The existence of a speed limit in the Universe means that nothing can get instantaneously to anywhere else. So far we had verified that over all the electromagnetic spectrum, of which light is a part of. Both optical and radio telescopes confirm that. Now we have another type of waves, not electromagnetic but gravitational waves, and they also comply. This is further prove that the Universe has a speed limit. We call it the speed of light but it is more fundamental than that. It is not just light, gravitational waves also travel at that top speed.
Galileo first pointed his primitive telescope to the planets and by discovering Jupiter’s moons started modern observational astronomy. Newton’s reflector telescope allowed us to build larger and more capable telescopes. It wasn’t until the 1930’s that engineers working on microwave transmission equipment for a large phone company figured out that the noise they were getting was coming from the stars and, thus, radio-astronomy was born. So far, we had been limited to light, now we could see the whole electromagnetic spectrum of which light is just a small part.
Gravitational waves are a completely new way to see the Universe. It is like a completely new sense, as if we could suddenly grow ears and perceive the Universe beyond what our eyes can see. The instrument that managed to pick those waves are two enormous installations of several kilometers in length each at two different locations. All these was needed just to confirm an effect so subtle that it took a century from its prediction to develop the technology to make it possible.
Now that we have confirmed that we have the technology capable of sensing them scientists might get the go ahead to build even more sensitive instruments capable of looking at the Universe with this new sense.
Dark matter (or dark energy, since mass and energy are interchangeable) is dark in the electromagnetic spectrum but it does have mass, that is why we figure out it must be there. We might finally be able to confirm its existence.
What else might be out there? We are starting to have the means to find out.