Relativity (Misconceptions)
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This page deals with Misconceptions and Pseudoscience concerning the Principle of Relativity, and the theories of Special Relativity and General Relativity. For a technical overview of these concepts, see Relativity (Technical Overview)
While the basics of relativity may be understood by anyone, the more fascinating effects of Special and General relativity may require more specialized learning. Accordingly, popular explanations are necessarily oversimplified, and thus, common misconceptions occur.
[edit] All Things are Relative
“All things are relative” is a quote often erroneously attributed to Einstein. It suggests an applicability of “relativity” to morality and other philosophical areas, an application not intended by the physical theory. It dovetails well with the moral relativist maxim of “Everything depends on your point of view”.
The fact is, not everything depends on your point of view. One purpose of the theory of Special Relativity was to demonstrate that certain things are not relative, that there are absolutes. The speed of light in a vacuum is one such absolute. More generally, spacetime interval between two events, defined as
where d is the distance t is the elapsed time and c is the speed of light, is exactly the same for all inertial reference frames and is not subject to any “point of view”.
Furthermore, many commonplace instances in mechanics are not relative. Acceleration, in particular, is not relative. You can tell that you are in an accelerating reference frame by the forces that you feel. Thus, it is not necessary to define acceleration relative to something else. Rotation, which also causes acceleration, is also not relative.
[edit] All Views are Equally Valid
The theories of Relativity say nothing about what reference frames are valid, only which ones are equivalent. “All views are equivalent” is demonstrably false.
For example, in a rotating reference frame, things appear to exhibit “centripetal force” and “Coriolis force”, which are different from one reference frame to another. Another example is that reference frames using different size scales are not equivalent; matter behaves differently at different size scales.
As for “validity”, physicists use non-inertial reference frames all the time. An analysis of “Lagrange Points” for example, is usually done in terms of a rotating reference frame.
[edit] Reference Frames imply Conscious Observers
The term “observation,” also known as “measurement,” is used in a technical sense with respect to Relativity. It means in reference to an idealized inertial frame of reference, where it is imagined that it is possible to make simultaneous measurements all over the frame. In practice, this is impossible, but it can be dealt with by compensating for the distance of far points on the frame.
In no way does this imply that there is a human or even a conscious observer.
The term has given rise to the misconception that actual observers are a necessary component of the Principle of Relativity. Since observers may be distinguished from other things by having consciousness, this creates the additional misconception that consciousness has some relevance to Relativity. From here, the chain of reasoning may reach even sillier conclusions, such as that consciousness creates reality. While this may be debatable, it is in no way related to Relativity.
[edit] Time is the Fourth Dimension
It is convenient to view relativity in terms of a four-dimensional construct called "spacetime," which combines the three ordinary dimensions of space with an additional dimension of time. The idea that time is a dimension, meaning a “degree of freedom” has fascinated the popular imagination since it was suggested in the opening chapter of H.G. Wells’s The Time Machine. By analogy to the spatial dimensions we are familiar with, it seems that it should be nothing more than an engineering problem to move about as freely in time as we do in space.
The theory of Special Relativity made this possibility seem more real, by demonstrating that space and time can in some sense be partially exchanged by the choice of reference frame.
However, this choice of reference frame does not affect in any way the experiences of the physical objects studied, and certainly does not give them any control over their experience of the flow of time.
[edit] Tachyons
In the equations of Special Relativity, there is a term called gamma that asymptotically approaches infinity at the speed of light. This confirms that massive objects cannot be accelerated to the speed of light. It also confirms that massless objects must travel at the speed of light and cannot travel at any other speed. The speed of light is the only speed at which a massless object can have energy and momentum.
It is possible, however, to plug values of speed greater than the speed of light into the equations of Special Relativity. This leads to the idea of hypothetical objects that travel at a speed greater than light. These objects, if they exist, would be called tachyons.
Nobody knows whether tachyons exist. If they did, they would have unusual properties. Their momenta and energy would have to be imaginary. So would their mass, although in the case of tachyons the mass would not even be a meaningful concept, as they could never decelerate to slower than light and therefore could have no rest mass, because they could never be at rest. Due to their imaginary values, tachyons likely could never interact with anything in our everyday world of objects going slower than light, nor us with them. Tachyons have never been observed, and if the theory is correct, they cannot be observed.
[edit] Relativity and the Paranormal
Tachyons have caused much excitement in paranormal circles, where they are supposed to have rather magical qualities not predicted by the physical theories. They are often invoked, for example, to explain the instantaneous transmission of supposed psychic abilities. As it is extremely likely that tachyons cannot, even in principle, interact with ordinary matter, this is an outlandish claim and not supported by Relativity.
Another similar idea exploited is that of time reversal, namely that there is an assumption that the laws of physics would hold regardless of whether time flowed forwards or backwards. This is again used to justify the possibility of precognition, one adherent claiming:
"... laws of nature give no intimation as to the preferred direction of time... In fact, seeing into the past or the future with equal ease is attributing to nature perfectly symmetrical behaviour."[1]
Critics have pointed out that: "The concept of time reversal does not lead to the conclusion that effect precedes cause, or that we flit around in "time" going in one "direction" at one moment and in another at another moment."[2]
Several proponents of parapsychology have attempted to underpin the subject with theory based on relativity. Lawrence LeShan wrote:
- "...we have the normal phenomena of event A occurring before event B from the viewpoint of one observer, the two occurring at the same time from the point of a second observer, and event A occurring after event B from the viewpoint of a third observer. It is literally impossible with mnay events to say whether they occurred simultaneously or in sequence. Fro the "commonsense" everyday theory of reality, this would lead to precognition and retro-cognition - paranormal phenomena."
Such observations are misleading and show a poor understanding of relativity and simultaneity.
[edit] Light Slows Down in Materials
It is often said that light slows down in materials such as water and glass. This can be used, along with a naive yet largely effective model of light as a wave, to explain the index of refraction, such as in the bending of light in lenses.
Yet this is not quite true. Light does take longer to go through materials. In some recent experiments, light takes so long to go through some materials that it can be outpaced with a bicycle.
It all depends on how one defines speed. Consider an idealized toll road. The speed limit is 55 mph. Every car has a really good engine and brakes. However, every mile, every car has to stop to pay a toll, and this takes some time. If you take a length of road and time a car, what you calculate as the average speed will be much less than 55 mph.
Light going through a material works much like this. While it travels, it always travels at c. At each atom, it must stop to pay a toll, which involves interacting with the electrons in the atom. This is usually described as being absorbed by an electron and then being re-emitted some time later.
Quantum Electrodynamics provides a subtler way of viewing this. The maximal probability path of a photon's amplitude describes a kind of slalom around the atoms it encounters. It is a slalom, however, of all possible paths around the atom. Charles Addams once did a cartoon of a skier on an alp. He was looking at a path of ski tracks that went both ways around a tree. The maximal amplitude is much like this.
Yet the quasi-classical notion of a photon as being absorbed and re-emitted by an electron is good enough to understand this.
While c is usually described as the speed of light in a vacuum, it is important to remember that even the best vacuum is not exactly the same as empty space. Even intergalactic vacuum has some dust and hydrogen atoms in it. Even without these, it is still boiling with short-lived particles that spring out of and are reabsorbed into the vacuum. This has been demonstrated with the Casimir experiments. The speed of light in a vacuum is thus slightly smaller than c.
In a way, it is backward to describe c as the speed of light in a vacuum. Rather, c should be considered a fundamental constant of the universe that naturally binds time to space and energy to momentum. Light must travel at c, because it is the only speed at which it can have energy and momentum.
[edit] Speed of Light vs. Speed of Sound
The prohibition on accelerating past the speed of light places serious restrictions on the future exploration of our universe, an idea offensive to human ambition and disdain for boundaries. If science fiction is representative of our collective imaginations, it is notable that the problem of faster-than-light travel is assumed to have been solved in spacefaring societies.
It is natural to compare the impossibility of traveling faster than light with the former supposed impossibility of traveling faster than the speed of sound. The history of engineering is replete with examples of claimed “impossibilities” which were accomplished after a short time, with much persistence and ingenuity. It is suggested then, that we not be so hasty to declare the speed of light unreachable.
But faster-than-light travel is a different class of “impossibility” than faster-than-sound travel. Even in the 19th century, there were obvious examples of objects that did travel faster than sound. The problem of traveling safely at such a speed has always been an engineering problem, not a law of nature. Faster-than-light travel is impossible in the same way that adding 2+2 and getting 3 is impossible.
[edit] Nothing can Move Faster than Light
Relativity predicts that
- No massive object can be accelerated to the speed of light or greater
- No massless object can travel at a speed other than the speed of light
It also strongly suggests that
- No information or energy can be transmitted faster than light
Our human concept of the “thing,” however, is not limited to these well defined cases. We can imagine and even cause to happen things that travel faster than light. For example, if one points a laser at the Moon, a simple flick of the wrist will cause the “spot” to move across the Moon’s surface faster than the speed of light. Some carefully constructed scissors could be made where the point of contact between the blades moves faster than the speed of light.
However, these are human abstractions. We can talk about a spot as moving, or a point of contact moving, but this is very different from saying that a massive object is moving. The flick of the wrist can still not propagate to the moon in a time shorter than given by the speed of light.
[edit] Einstein was a Plagiarist
This idea has taken hold of the public imagination and has been embraced by many paranormalists. It is mostly due to an extended pamphlet and associated websites by Christopher Jon Bjerknes.
Of course, science largely consists of building on the work of other people. Newton is reported to have said, "If I have been able to see further, it was only because I stood on the shoulders of giants." Credit is given to contributors in the form of citations. Nevertheless, it can sometimes be confusing why one person and not another is given credit for a particular discovery or invention.
Countless people contributed to the study of Relativity. In addition to those mentioned before, there are Henri Poincare, Emmy Noether, David Hilbert, Hermann Minkowsky, and many others. Even the basic transformation of coordinates that occurs in the equations of Special Relativity is still named for Hendrik A. Lorentz.
Yet the name usually associated with Relativity is Albert Einstein. Why is this so?
First, Einstein gave due credit to everyone who had contributed to Relativity. This credit may not always percolate to the public level, but all those who study physics seriously know about it.
Second, Einstein was the first to create a theory of relativity that made sense and was not in any way dependent on fantastic suppositions as the luminiferous aether.
Third, Einstein was the foremost and almost the only person historically associated both with Special and General relativity.
[edit] Relativity Doesn’t Apply to Acceleration
There is a common misconception that relativity does not apply to objects that are rotating or accelerating. The confusion may arise from the so-called “Twin Paradox”, in which an accelerating reference frame is considered. Sure enough, things appear quite different in this non-inertial reference frame.
Accelerating reference frames are not generally equivalent to inertial reference frames. However, the laws governing accelerating objects are identical in all inertial reference frames.
[edit] Further Reading
- Skeptic’s dictionary on Tachyons
- Discussion of faster-than-light motion
- Example of pseudoscientific use of Relativity in support of Mysticism
- Relativity Tutorial
[edit] References
[1]Charles Panati, 'Precognition and time', Parapsychological Review, 6(4),p1-4
[2]James E. Alcock, Parapsychology: Science or Magic? 1981, Pergamon Press, p114
