Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Why does special relativity exist?

Via studying the outcome(s) of such relativity, one can end up with all the equations involved in special relativity. Thus one runs into things such as contraction, dilation, transformations, velocity addition, etc.

But these are the outcomes.

My question is what is the cause of special relativity, and is the knowledge of this cause currently being shared in the world of physics?

Or, to put it another way, is it currently accepted despite there being no cause that is known of, thus an acceptance of an effect without a cause?

If you give this question a negative vote, please present your reasoning and or evidence to support such a vote.

share|improve this question

closed as unclear what you're asking by BMS, Brandon Enright, V. Moretti, Kyle Kanos, Jim May 7 at 13:29

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

Essentially, the causes are related to the "things" that are not relative: youtu.be/VS46DZe5usY –  cinico May 7 at 0:47
Can you give a few examples from some other area of science that you feel "causes" something else? It's a bit difficult to understand from your text precisely what you think constitutes a cause. –  BMS May 7 at 1:20
The geometric structure of space-time is my candidate for the fundamental "cause" of relativity, but I am acutely aware that there are other ways to structure a minimum assumption scheme. –  dmckee May 7 at 2:53
Physics isn't concerned with "why", only "how!" How these effects take place is due to special relativity, the lorentz invariant universe! –  Benjamin Horowitz May 7 at 4:25
@BMS I would argue the speed of light is an outcome of electromagnetism, not relativity. From Maxwell's equations we can get $c^2 = \frac{1}{\epsilon_0\mu_0}$. If you accept that $\epsilon_0$ and $\mu_0$ are constants they are independent of reference frame so $c$ is constant. –  nivag May 7 at 12:22

5 Answers 5


Your question (at least in the way that you have phrased it), is more of a philosophy question than a Physics question.

Let me put it this way: Why does space exist? Why do particles exist? Why does energy exist? Why does temperature exist?

Physics is trying to observe what does exist, and how it behaves. This can be done by observation, and fitting models to the observed. Answering questions about why something exists is a metaphysical question, and not amenable to the scientific method. This way lies religion: "Beause God made it that way", or "Because the back of the turtle that we stand on is curved".

Why accept Relativity?

Now as to why we accept such a theory "without a cause", the cause is that we observe the constancy of the speed of light in any inertial frame of reference. This result is simply unexplainable in our usual assumptions about velocity.

To come up with a model of the universe where we mess around with times, distances, velocities, mass etc. was not easy, and required an enormous amount of incontrovertible evidence. But, given that evidence, physics was forced into coming up with the simplest (Occam's Razor) theory that could explain the observed, and since nothing very simple fit, we had to keep getting more and more wierd, changing time and space, until the first possible fit was found (special relativity), and soon afterwards general relativity.

Even after having this model, not everyone was comfortable with it, until many more confirmations started to appear. Now there are so many confirmations (and they are shared not only in the world of Physicists, but in many mass paperbacks) that even flight plans of spacecraft include relativistic corrections.

Finally, we realize that even this complex theory is not the "Truth." It is a closer approximation than the first model, but still needs Quantum Mechanics, and who knows how much more tweaking before it is "True," or even if "Truth" can be written in a finite set of equations.

share|improve this answer

There is a fundamental confusion here about what the discipline of Physics is.

It starts because, like small children , the instinctive reaction when encountering an unknown is to ask "why":

"Why is it dark " ? my less than three year old daughter asked when during a summer evening walk.

"Because the sun goes around the earth and now it is the time that it is below us" the physicist mother replied.

She walked for a bit and then said. " You must be right, that is why the asphalt is hot under our feet"

Both asking "why" and looking for a cause are practically in our genes.

But in reality physics does not ultimately answer Why statements . It can only answer How from certain postulates and mathematics ( axioms and proofs and all) one observation is described/predicted. Because we use mathematics for our modeling the behavior of Νature ( from greek Φύσις) we tend to assume that the rigor of mathematics holds all the way through, and forget the postulates we have started with in order to formulate our model. Postulates are a la cart, and chosen so that the mathematical model describes existing data and predicts future ones.

Causes and effects within our modeling of nature are explained by how one goes from cause to effect, the validation being that data are fitted and new data predicted with accuracy.

The question

Why does special relativity exist?

gets the answer because Maxwell's equations and relativistic quantum mechanics equations rigorously contain the transformations. Going within the theoretical model, one can see how it appears naturally in Maxwell's equations.

Then the question becomes why Maxwell's equations describe the electromagnetic observations, and the only answer is : because these elegant equations have been validated by innumerable data and predictions.

Physics is a continuous modelling of observations and the theoretical models are validated by continuous successful predictions. When the predictions fail then the theoretical model is invalidated and is either tweaked to agree again with the data, changing postulates or mathematical axioms, or new theoretical models with new postulates which at the limit merge with the previously successful model are proposed and accepted if validated by data, until the next stage of invalidation.

Special relativity was extended from the electromagnetic model of Maxwell's equations into particle behavior because it fitted the new observations and data coming from particle physics . It joins smoothly with classical mechanics transformations in certain kinematic regions and describes the new data appearing with the study of elementary particles. It explains how particles behave. The ultimate Why is metaphysics and philosophy.

share|improve this answer
I just realised my geocentric model :), but I guess at the time I thought it was too much to start on the heliocentric system :) . –  anna v May 7 at 14:04

This is a very serious issue, until one realizes that the assumption which is common in every day lives that there is no maximum speed limit and therefore one can use Galilean transformations, is an equally dubious hypothesis.

It is an assertion about nature to say that one can transform coordinates of an event $(x,y,z,t)$ into another one $(x-v_x t,y-v_y t,z-v_z t,t)$, and that this is what events occurring in the frame moving with velocity $(v_x,v_y,v_z)$ will observe. e.g., that if we apply laws and reasoning in the transformed frame, we get the same physical results as in the untransformed frame.

This is an imposition on nature justified only by everyday low-velocity experience. The various tests of the [Galilean] ether theory of light and of special relativity show that this is not the case. We're simply switching to a result which it sounds like you find philosophically displeasing, from a result which is equally philosophically displeasing. (obligatory Feynman link)

You can paint a truly beautiful picture in terms of modern geometry to see how the structure is much more fundamental than Lorentz transformations, in the same way that Euclidean geometry has structure more fundamental (or at least easier on the eyes) than a general rotation matrix, and this might help, but in the end it comes to experimental verification and consistency of the model.

share|improve this answer
Well one reason I asked this question was due to myself having used intuitive thinking to analyze motion and eventually ended up with all of the equations involved in Special Relativity and a full understanding of it. Yet elsewhere, I have seen nothing of the same nature. See goo.gl/fz4R0I ( My 9 youtube videos ) –  Sean May 7 at 3:56

I will try to give you the answer as to the "Why".

A long as the speed light was considered infinite, all movement appeared the same, regardless of the point of view of the observer. It didn't matter if you were close or far away from a given event, or if the object observed was moving fast or slow, it was commonly believed this event was seen exactly the same and at exactly the same time by everybody.

However, when the speed of light turned out finite, and also constant regardless of the relative movement of the measurer and the measured light, there appeared a need to develop a theory to predict the consequences of this notion. So here we are.

You might want to read about the history of Special Relativity in Wikipedia, as a number of interesting things is mentioned there.

share|improve this answer
As I have stated above, I, using intuitive thinking, eventually ended up with all of the equations involved in Special Relativity and a full understanding of it. Yet elsewhere, I have seen nothing of the same nature. See goo.gl/fz4R0I ( My 9 youtube videos ) As mentioned in the last video, the constant speed of light becomes the FINAL last outcome, thus not the first cause. –  Sean May 7 at 11:13
As to your "seeing is believing" and the last part of your video series. Two different observers cannot actually see the same (ray of) light. To see a photon means it must come to your eye directly. So there is no need to explain a non-existing phenomenon of seemingly contradicting perceptions. As to your graph explaining photon "slowing down": time and distance are orthogonal on graphs only. In reality, when you are accelerating in your car, its tires are not drawing a curve, are they? –  bright magus May 7 at 11:39
I couldn't agree more with the concept of understanding first, and then doing maths (unfortunately current physics follows Feynmann's bullyish slogan: "Shut up and count!"). But then how come we have time dilatation and distance (length) contraction, which means they change in inverse proportion (one is growing, the other diminishing), and yet we have c=x/t and c=x'/t' meaning x/t=x'/t' which proves they should be changing in direct proportion? There are more funny things like that ... (I will watch all your videos later, as they certainly seem worthy of it). –  bright magus May 7 at 11:40

You have to distinguish between the theory of special relativity and the universe. The former is a mathematical model developed by Einstein and the latter is, well, no-one really knows.

The point is that we physicists spend our time developing mathematical models to predict what happens when we do experiments. A successful mathematical model is one that makes predictions that match the experiment, and we call these successful mathematical models theories.

So if you are asking why the theory of special relativity has the form that it does, this is a perfectly reasonable question and I think the best answer is that it's due to the invariance of the line element aka Lorentz covariance. This states that if we calculate the line element $ds$ using:

$$ ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2 $$

Then all observers in all frames whether inertial or not - it is true for accelerated motion as well - will calculate the same value for $ds$. All the effects in SR arise from this simple statement. There is an obvious relation to Pythagoras' theorem, and indeed the statement is a statement about the geometry of the spacetime manifold on which we all move.

But if you're asking why the universe is (approximately) described by the theory of relativity then there is no answer to this. If the universe was best described by some other theory we'd be using that other theory instead and nobody would have heard of Albert Einstein.

share|improve this answer
If you accept Einstein's theory, then you will not accept a greater complete theory. This is a problem. If less than complete is accepted then how can completeness ever be accepted ? –  Sean May 7 at 11:19
@Sean: all, that is all, theories are approximations that work only under a limited range of conditions. So we know Newtonian physics breaks down at high velocity and we know SR breaks down at high energy densities (and presumably GR breaks down when quantum effects become important). We don't accept any theory, we choose to use it in a regime where we are confident it applies. Once outside that regime we choose to use a more complete theory instead. –  John Rennie May 7 at 11:39

Not the answer you're looking for? Browse other questions tagged or ask your own question.