Frequenctly asked questions
Unlike its use in the everyday world, scientists use the word theory only when there is substantial and verifiable evidence for an observed phenomena. In this sense a scientific theory is quite unlike a conjecture or idea, and more often than not requires a great deal of observational and experimental evidence before attaining the status of theory. In the case of special relativity there have been very many direct experiments carried out and all of them have provided evidence that special relativity is correct. These range from experiments involving sub-atomic particles in high speed accelerators, to the slightly different clock rates of some space probes. Even the GPS satellites, which work by measuring time, have to take relativity into account to stop their clocks quickly becoming unsynchronised with those on Earth. There is an important distinction between a scientific law and a scientific theory. A law states that something happens, and there’s a lot of evidence for it, but a theory says why something happens in the first place. A good example is Newton’s law of gravity. This works very well at low speeds and low masses, such as on the surface of the Earth or Moon, or in the calculation of the orbits of planets and comets. However, at very high speeds and very large masses it increasingly breaks down and becomes more and more inaccurate. Einstein’s general theory of relativity not only accounted for these discrepancies but also explained why they happen -- the bending of space-time due to mass or speed. In this sense Einstein’s theory is superior to Newton’s law, although we still use the latter in most cases because it’s usually accurate enough for most purposes and much easier to calculate.
Isn’t it just a theory?
Here you will find answers to frequently asked questions relating to E = mc 2  and special relativity.
What happens if you travel faster than light?
According to special relativity it’s not possible for anything with rest mass to travel at or beyond the speed of light. The reason for this is because as we go faster our mass (as measured by an external observer) increases in proportion to our speed. As the time dilation equations show, it would require infinite energy to accelerate something with mass to the speed of light, and that’s clearly not possible. There is a great deal of practical, as well as theoretical, evidence for this. For example, physicists routinely send particles around accelerators with energies very far in excess of what would be needed to move them faster than light if relativity wasn’t correct. Instead, the particles get progressively faster as the energies increase but it needs more and more energy just to gain smaller and smaller increases in speed, until a huge amount of energy is needed just to move a tiny fraction closer to the speed of light. There are some exceptions to this, such as quantum tunnelling, but in such cases it has been shown that relativity (and, in particular, causality) has not been violated, but this is a deeply complex subject and outside the scope of these pages.
Fact box… Distances shrink in the direction of motion. For example, at 90% of the speed of light distances contract to only 44% of their usual value. Try it yourself using the dilation calculators here.
Fact box… Nuclear fission involves splitting the nucleus of an atom apart. Nuclear fusion involves squeezing particles together at the nucleus of an atom.
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Frequenctly asked questions
Isn’t it just a theory?
Here you will find answers to frequently asked questions relating to E = mc 2  and special relativity.
Unlike its use in the everyday world, scientists use the word theory only when there is substantial and verifiable evidence for an observed phenomena. In this sense a scientific theory is quite unlike a conjecture or idea, and more often than not requires a great deal of observational and experimental evidence before attaining the status of theory. In the case of special relativity there have been very many direct experiments carried out and all of them have provided evidence that special relativity is correct. These range from experiments involving sub-atomic particles in high speed accelerators, to the slightly different clock rates of some space probes. Even the GPS satellites, which work by measuring time, have to take relativity into account to stop their clocks quickly becoming unsynchronised with those on Earth. There is an important distinction between a scientific law and a scientific theory. A law states that something happens, and there’s a lot of evidence for it, but a theory says why something happens in the first place. A good example is Newton’s law of gravity. This works very well at low speeds and low masses, such as on the surface of the Earth or Moon, or in the calculation of the orbits of planets and comets. However, at very high speeds and very large masses it increasingly breaks down and becomes more and more inaccurate. Einstein’s general  theory of relativity not only accounted for these discrepancies but also explained why they happen -- the bending of space- time due to mass or speed. In this sense Einstein’s theory is superior to Newton’s law, although we still use the latter in most cases because it’s usually accurate enough for most purposes and much easier to calculate.
What happens if you travel faster than light?
According to special relativity it’s not possible for anything with rest mass to travel at or beyond the speed of light. The reason for this is because as we go faster our mass (as measured by an external observer) increases in proportion to our speed. As the time dilation equations show, it would require infinite energy to accelerate something with mass to the speed of light, and that’s clearly not possible. There is a great deal of practical, as well as theoretical, evidence for this. For example, physicists routinely send particles around accelerators with energies very far in excess of what would be needed to move them faster than light if relativity wasn’t correct. Instead, the particles get progressively faster as the energies increase but it needs more and more energy just to gain smaller and smaller increases in speed until a huge amount of energy is needed just to move a tiny fraction closer to the speed of light. There are some exceptions to this, such as quantum tunnelling, but in such cases it has been shown that relativity (and, in particular, causality) has not been violated, but this is a deeply complex subject and outside the scope of these pages.
Fact box… Distances shrink in the direction of motion. For example, at 90% of the speed of light distances contract to only 44% of their usual value. Try it yourself using the dilation calculators here.
Fact box… Nuclear fission involves splitting the nucleus of an atom apart. Nuclear fusion involves squeezing particles together at the nucleus of an atom.
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