Frequently 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 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 under certain circumstances, 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
, Special Relativity and related subjects:
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.
Do photons have color?
Scientists sometimes refer to photons or light as being red or blue, for example,
but this is simply a shorthand and photons have no color.
Our eyes are adapted to be sensitive to a certain range of frequencies, called the
visible part of the electromagnetic spectrum. Special cells at the back of the eye
detect these frequencies and send signals to the brain, which we then perceive as
different colours. This is useful. For example, seeing different colours means our
ancestors had a better chance of being able to distinguish between safe and ripe
foods and foods that could be harmful.
Similarly, some animal’s eyes are sensitive to other parts of the spectrum. For
example, bees’ eyes are more sensitive to purple, violet and blue, as well as being
able to detect ultraviolet light. This makes flowers look much brighter than their
surroundings, which is highly advantageous to a bee.
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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,
Special Relativity and related subjects.
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
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 under
certain circumstances, 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.
Do photons have color?
Scientists sometimes refer to photons or light as being red or
blue, for example, but this is simply a shorthand and photons
have no color.
Our eyes are adapted to be sensitive to a certain range of
frequencies, called the visible part of the electromagnetic
spectrum. Special cells at the back of the eye detect these
frequencies and send signals to the brain, which we then
perceive as different colours. This is useful. For example, seeing
different colours means our ancestors had a better chance of
being able to distinguish between safe and ripe foods and foods
that could be harmful.
Similarly, some animal’s eyes are sensitive to other parts of the
spectrum. For example, bees’ eyes are more sensitive to purple,
violet and blue, as well as being able to detect ultraviolet light.
This makes flowers look much brighter than their surroundings,
which is highly advantageous to a bee.
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