The Theory of Relativity is quite possibly the greatest modern scientific discovery of all time and yet I would venture to guess that most of us have no concept of it or at the very least fail to see how everyday life is absorbed in the major concepts of the theory. We all know it:
But what does that equation really say? It’s simply saying that the mass of a body is a measure of its energy constant, where ‘E’ stands for Energy, ‘m’ stands for Mass, and ‘c2’ represents the speed of light squared. This equation is often in physics referred to as the mass-energy equivalence concept. We wont get into the math behind it, and lets face it, most of us can’t (myself included).
Einstein’s Theory of Relativity consists of two separate theories called special and general relativity. Special relativity is an expansion on Galilean relativity, which expresses how matter moves through time and space. General relativity is an expansion to his own special relativity theory, which essentially adds gravity into the mix.
So many people ask, ‘How is this a factor in my everyday life?’ We’ll let’s take a look at some things that you deal with constantly (and some I hope we never have to deal with) that are directly related to these theories.
So time travel is pretty sweet and one day maybe we can travel forward, far in time, and experience the future. Impossible? Well, the thing is… we already have at a smaller scale. You’re doing it right now in relation to anyone that is at a lower elevation than you on Earth, though it’s a small enough measure that you and I would never be able to tell without extremely sensitive equipment. Basically what time dilation describes is that the stronger the force of gravity the slower time moves for you in relation to someone who is experiencing weaker gravity. That being said, time for you wont seem to be moving slower relative to you, just as time for the other person experiencing weaker gravity wont seem to move any faster. Now only is this mathematically proven, but we have recorded the effect in real life and it is essential to one piece of equipment we all use daily now.
GPS is a staple technology found in almost all phones and every car. We use it to help us get to places we are unfamiliar with, however if GPS satellites didn’t account for time dilation, we would never get where we needed to go. Imagine the Earth with a GPS satellite coasting in motion far in orbit around Earth. The pull of Earth’s gravity (which is quite weak in relation to other objects in space) is weaker for the satellite where as your car on Earth is closer to the Earth’s mass and thus the pull of gravity is stronger. GPS works by essentially firing a radio wave from your phone to a set of satellites that triangulate your position. Easy enough, I suppose.
This is where it gets interesting and where atomic clocks located on the satellites must be exact. Your phone sends requested signals to satellites located above you in orbit, and because they are constantly moving some may be further away from you than others. If a satellite were to be directly above you then the distance the radio waves need to travel is shorter than a satellite that is located north east of you. These radio waves travel at the speed of light (which is about 670,616,629 mph. Not bad.) However, even at this speed, the satellite directly above you has less distance to travel than the one north east of you. Once the distances of at least four of the twenty-four GPS satellites in orbit are estimated by your phone in can then pinpoint your location in three dimensions.
Now time dilation has to be accounted for because the satellites are experiencing time faster than you due to your stronger gravity, so atomic clocks are programmed to account for this small difference and therefore your location will be accurate to around 10 meters or so based on your ability to broadcast and receive signals. This is why GPS tends to have a harder time in wooded areas. If your phone cannot get an accurate idea of how far the satellite is away from you because the atomic clock on board doesn’t account for time dilation (or because your signal is being obstructed), your phone would be completely inaccurate in its guess of your position on a map. Our ability to account for time dilation is precisely why your GPS gets you (most of the time) where you need to go.
Looking Back in Time:
The speed of light is constant. The reason this is so has to do with the fact that mathematically the more mass an object has the more energy it needs to reach faster and faster speeds (remember the mass-energy equivalence?). So an object with mass would need an infinite amount of energy to reach the speed of light. That being said, the smallest mass-less light, energy, or information particles are also limited to this speed. Because light is limited to this speed as well, we know a few things.
Walk outside and look quickly at the sun (don’t stare!). The image burned onto your eyes is what the sun looked like 8 minutes ago. This is because the light that is traveling from the Sun to the Earth is traveling at the speed of light, which even at that great speed takes about eight minutes to reach us. Now do the same thing later in the evening with the moon (feel free to stare all you want). The image you are seeing in the sky is about 1.26 seconds behind. It only takes 1.26 seconds for light to travel the distance between the Earth and Moon. So imagine that the sun exploded. Even after it happened, it wouldn’t be until 8 minutes later that we saw the effects in our sky. The moon would be much quicker but still delayed that 1.26 seconds.
The speed of light expressed in Einstein’s theory allows us to measure great distances in space and so a general rule of thumb is that the further you look into space the older the image you see in the sky really is. So another example is the Helix Nebula (commonly referred to as the “eye of God”), whose image in our skies is about 700 years old. This again means that it takes light 700 Earth years to cover the distance from the nebula to Earth. To see it from Earth is to look back in time 700 years. The furthest galaxies (in our observable Universe) that we can see with our current technology show that they are a distance of about 13.3 billion light years, meaning that the earliest light first generated by that galaxy has been traveling almost the entire life span of the Universe as we know it to be at an age of about 13.7 billion years old. We are literally traveling back in time just by looking in the sky, because not even light photons can travel faster than the speed of light.
Given that light travels at a maximum speed of about 670,616,629 mph, there is a greater beast that even light cannot escape. One of the interesting things about Einstein’s theory was that the math involved ended up predicting an anomaly that was quite perplexing in what it said and meant. Even Einstein thought that despite the prediction his math made, that it more than likely would not actually exist in the cosmos. The math simply predicted that extremely compact mass would deform space-time to form what they labeled as a black hole.
The best way to imagine a black hole is to pretend that you’re floating in a rowboat in an endless body of water. We’ve all seen waterfalls and we all know that the human body has limitations, because we can only row so fast. This limitation would be the speed of light in relation to a black hole. So we paddle towards what looks like a giant whirlpool waterfall where all the water is rushing towards and falling down into. We can paddle fairly well as long as we stay far away from the event horizon. This is the point at which the force of gravity is pulling stronger than the speed at which we can row. Since nothing can travel beyond the speed of light, we cannot simply row our way out of our impending doom. Gravity is stronger and will continue to become even stronger as we fall further and further in.
What this shows is that since light has a maximum speed, even light itself cannot escape the stronger pull of gravity beyond the event horizon. Any light that crosses the event horizon of a black hole cannot escape, hence the name, black hole. The pull of gravity is so strong in fact that it distorts light around the black hole, giving the outside viewer a kind of “lensing” effect around the black hole. Light bends and distorts around the black hole. Till this day we have not physically seen a black hole in the way in which we imagine them, though we are close. Lensing is one way to know a black hole is possibly near by (possibly, because other things in space also cause this), but we have, however, seen objects that are orbiting around a black hole and how they interact and thus can identify them as well.
Our own Milky Way was discovered to contain a super-massive black hole at its center by observing star orbits over a period of 15 years. It was observed that a cluster of stars were traveling in elliptical orbits around an unidentified object at speeds that would require the sort of gravity known only to a black hole. It is now believed that black holes are a prominent feature of most, if not all, galaxies.
Many consider the Theory of Relativity one of the greatest human discoveries, and because it has been rigorously tested and proven to be accurate time and time again in both mathematical and observable experimentation, it has withstood the test of time. It not only explains our Universe and makes sense of our surroundings, but also helps to push us further in understanding even more of the unknown. It makes our everyday lives easier and explains with other collected evidence how much of our Universe operates. Scientists are still looking for the link (via string theory) as to how the theory of relativity connects back to the microscopic actions of gravity, or how to explain the physics of the very small and the very big. Maybe one day Ill try and tackle some of the more easy to understand concepts of string theory or even m-theory, but they are a whole new can of worms that require patience. One day…