Relativity for newbies (part I)

So, this is a post for little grasshoppers of physics that want to understand the basic ideas of special relativity without going too much into the math…

Let’s start easy, with Bob the scientific fisherman. He’s in his boat, looking at the waves which his fishing line creates as it oscillates in the water. When his boat advances slowly, he realizes that the forward waves move more slowly with respect to him. Kind of, his boat is trying to catch the waves.

Bob the scientific fisherman thinks: “Sure, the waves move with respect to the water with a fixed velocity. I may move, also with respect to the water even faster than that, if I row hard. Then I can overtake the waves”. That’s true, and this is what happens when you break the sound barrier.

Bob thinks even more: “By measuring how fast do the waves escape from me, I may know my velocity with respect to the water… If I am at rest with respect to the water, the velocity of the waves will be homogeneous from my point of view

“While, if I move, it will not be homogeneous, the waves will escape from me with a velocity that will depend on the direction.”

Bob went home happy. He had caught no fishes, but had acquired some wisdom…

In space nobody will hear you cry… because there is no water, nor air. But there are waves out there, anyway. There is light, and we can apply the same idea. Water waves move on water. What does light move on? Just to give it a name, people used to call it aether. Michelson and Morley in 1887 designed a very beautiful experiment to measure the velocity of the Earth with respect to this aether, by measuring the speed of light in different directions, just as Bob wanted to measure the speed of the water-waves in different directions to find out his velocity with respect to the water.

The experiment of Michelson and Morley was a complete failure. The speed of light was the same, no matter which direction you looked at.

So, there were various options: (a) the Earth is really at rest with respect to the aether. That’s a funny option: we know that the Earth moves around the Sun, at approximately 30 km/s!! And the Sun moves with respect to the galactic center, even faster… What’s the other option? It is (b): the Earth drags the aether as it moves. So to speak, the aether which is near us moves with our same speed, but the aether far away from us does not.

This last option is discarded because of stellar aberration. Let us explain it in simple terms. It’s raining, and there is no wind. The rain drops just fall vertically. You’re standing in the street, just holding your umbrella vertically so as not to get wet. OK. Now, you start moving. You should lean the umbrella forward a little bit if you want to remain dry. If you move in the opposite direction, you have to lean the umbrella in the opposite direction.

Now, let us establish the terms of the metaphor. The light from a certain star is the rain, and the telescope is the umbrella. Let us say that, with the Earth at rest, you have to point your telescope in the vertical direction in order to catch the light from your star. Now, the Earth starts moving. Then, you have to “lean” your telescope a little bit, in order not to lose the star. If the Earth moves in the opposite direction, the telescope has to lean in the opposite direction. This is what happens really: when changing from summer to winter, the positions of the stars change a little bit, exactly as predicted by this little story.

So, we know the Earth doesn’t drag the aether as it moves! Our two options are, therefore, invalid. Now, the patent office employee enters the game. But that’s another story that we will tell soon…

The exposition and images are taken from a talk I gave to high school students in 2005, for the centennial of the SR paper at IES Ágora, in Madrid.


19 thoughts on “Relativity for newbies (part I)

  1. Ok, to the nougat!
    This explain the doppler effect, ok, no problem with it. About the Michelson and Morley, i still doubt the experiment were well designed. From my point of view they “measure” light speed in air, since they didn’t did it on vacuum. The experiment was made on open air, with no isolation between the “arms” of the interferometer.
    We can see a variation in the lightspeed in the movement direction i.e “downards”. The gravity affects the light pulling it.
    Whats the difference between saying that speed of light is 300.000km/s and nothing can move faster and saying that speed of light is just infinite? With an infinite speed of light there’s no need to space to “bend”.
    I can concede that light have a weird behavior, but what about other types of waves. why not sound but yes micro or radio waves? How can something travel in vacuum but not other things can? if vacuum was a medium we can explain why ones can and no other due to medium density. but if vacuum is just, you know, vacuum… Theres nothing there to vibrate.

  2. About the effect of air: how does the presence of the atmosphere explain the negative result of Michelson’s experiment? It’s unfair just to say: “the experiment was not careful enough, what about X?” You have to explain how X might affect the result and the absolutely magical cancellation. Speed is the same in all directions. Why?

    About gravity: the effect is absolutely negligible. Even in the case of the sun it is rather low! Also, the same question remains: how would gravity explain the negative result?

    About the finiteness of the speed of light: it is a huge difference. It amounts for the whole of relativity. But I’ll tell these things in the next post.

    About light being special: when we say light, we mean all electromagnetic waves: radio, microwave, X ray, gamma ray, infrared, everything. Of course, not sound!

    About the medium which is necessary, it’s ok for me. But clarify me one thing: what does “medium” mean for you? If you define medium as “anything that can carry a wave”, then you’re right by definition. Do you mean anything else? What?

  3. Air does mess with the experiment as Earth carries the atmosphere, so there won’t be a diference in the speed. Wind don’t varies the speed of light. Does it means that there’s no air?

    Gravity won’t explain a negative result, but is a “fact” that a ray of light near a masive body would bend, so gravity mess with light even if photons don’t have mass. How does the effect of gravity beign negligible on one side and a “proof” on another?

    I find difficult to think in a particle of sound (soundon, maybe?) and if all waves goes through void, why sound don’t?

    I mean something like that the space itself is a medium hence never can be “empty”. So any vacuum only can be a “aparent vacuum”.
    I have some other issues with light, but these are other tales.

  4. Fact: both the atmosphere and gravity change the trajectory of the beams of light. Question: does this explain why the speed of light is the same in all directions, as measured from my lab? Only if gravity or the atmosphere can “drag” the aether. But they do not, ergo… None of them can explain the negative result.

    The “particles” of sound are called phonons. But that’s just a name. Sound waves, by definition, are oscillations in a material medium. So they need a material medium.

    And Erynus, seriously, I think you should clarify your ideas about what you mean by empty, medium, apparent… before trying to reach any further. Can you think of any experiment which will distinguish whether we need a medium or not for the propagation of light? No matter how difficult. I think your position is more linguistical than physical: you want a medium to be there, but without the properties attached to material media. OK, I call that vacuum. For example: why doesn’t it oppose the movement of the Earth or the Sun? Because it is infinitely thin… OK, I prefer to call that vacuum, and to say that EM waves propagate in vacuum.

  5. I don’t hate relativity, i’m just positioning against it to “looking for its tickles”.
    I think i understand it quite well, but i’m trying to break the “constant speed light” postulate.

  6. Tickles… ok. No single experiment has been able to detect the slightest difference in the velocity of light in vacuum. So, why tickle something that works so well? If something is not broken, do not repair it.

  7. Manu is right. There are no dogmas, but also there is no reason to doubt the postulate of invariance of the speed of light. Some guys have proposed (Moffat) that the speed of light may evolve with time, but at this moment we have absolutely no reason to doubt that all observers measure the same value for the speed of light in vacuum.

    But this is physics, no metaphysics. When the evidence comes, we will think again.

  8. Well, evidence. Relativity says nothing can achieve speed of light aside from photons wich are massless, and thath a masive object acelerated to c would require an infinite ammount of energy.
    But gravity affects things based upon their mass and miraculously gravity affects light, and with a minimun effort we can get a lot of not-so-massless photons at a, theoretically, imposible to achieve speed of light, investing a quantum or two of energy (wich sounds so far from infinite to me).
    So, something don’t work.
    About repairing things… the way to know how something works is to break it down to pieces and see how is it build.That’s why we collide protons to spray their innards around.

  9. Erynus, again, I have to make an important distinction: (a) special relativity may or may not be a coherent physical theory; (b) special relativity may or may not apply to the real physical phenomena. Please, I’d like you to state where is your problem, in a or b.

    SR, according to me, is coherent and applies to the phenomena amazingly well.

    Gravity affects things independently of their mass. According to general relativity, spacetime is curved and all particles move along geodesics, i.e.: paths of “shortest length”. Also photons, of course. No miracle, and still we believe they have no mass. Sorry, Erynus. Things work. Perhaps we will have to go deeper, but they do work.

    And, last thing in this topic: separating an object into parts and then putting them together is called analysis. There are many things in physics which need fixing, but SR is not one of them. No logical reasons, since the theory is coherent, no empirical reasons. That’s why manu said if it is not broken, do not fix it.

  10. Erynus, now I realized what is the problem with your comments. Take, for example, gravity action on the photons, which are massless. You don’t ask “How come this happens? I don’t understand”. No, you assume something is wrong, because you don’t understand it. That’s not a good way to approach problems. Your doubts are fine, and I’m eager to solve them, but the way you pose them is not. Are you trying to say we physicists are fools? Don’t you think that if the problem with SR was so obvious we would have realized in so many years? So, please, I beg you: pose your doubts in a different way.

    Otherwise, you’re in risk of qualifying as a physics crackpot. See

  11. Javi, can you explain a little bit Moffat’s theory? I heard about the speed of light changing with time, but I don’t understand what was that idea good for. In the past it was slower or faster, according to him?

  12. Hypazia, it is to explain the homogeneity in the universe. The expansion, just after the big bang was so fast that different parts of the universe were separating at speeds faster than light. This means that there was no communication between them, so explaining the homogeneity is difficult. The standard model of cosmology uses inflation, a time of even faster growth of the universe. How can it explain the homogeneity? Because *before* inflation the universe was so incredibly small that it became homogeneous at that stage.

    Moffat proposes that the velocity of light was larger near the big bang. This way, you don’t need inflation to explain homogeneity. Different parts of the universe were separating faster than light *today*, yes, but they were not disconnected because light was faster then.

    In fact, he mixes up c and G, so that the quotient remains constant. But, to tell you the truth, I don’t know the reason for that… We should read smth! :)

  13. The term spacetime itself is made up based on constant lightspeed postulate, “if we can’t change the speed of light we need to change the time”. And for me, slowing the physical mechanism to measure the time (i.e acelerating a clock or a cesium 133 atom) don’t slow time itself.
    I know all of this would seem a tantrum but i’m just trying to clarifying my doubts.
    It is logical to think in a way if we follow the postulates, but i always try to get “beyond” (like questioning what was before big bang).
    My doubts…. well… a lot, for example i don’t “understand” even the two slits experiment, but not the experiment itself or the explanation of it. I see what it say (the interference pattern on a light beam proyected on the slits) and “believe” that it happens because they say it happens, but somehow there’s something don’t fit to me.
    Either we make the slits so small a photon can get through both of them at once or we “aim” to the division between them (so the photon bounces). I don’t understand how can a photon pass through two slits at once.

  14. “slowing the physical mechanism to measure the time (i.e acelerating a clock or a cesium 133 atom) don’t slow time itself”

    What is “time itself”? Imagine that all ways to measure time agree on a certain value. All of them. How can you say that the “real” time is different from that? Occam’s razor: it’s simpler to say that time changes than to say that all clocks change, each one in its own way… The second way of thought does not lead to conclusion. The first one does, and the conclusions are verified by experiment, so…

    Within a certain framework, there are questions which do not make sense. For example, if I ask you “what does the Earth stand upon?”, you will tell me that within the “newtonian gravity” framework, the question does not make sense. The same goes for the “time before the big bang”. Time started with the big bang. Otherwise, don’t call it big bang, the concept is different. You can tell me, I don’t believe in the big bang. Time extends back to minus infinity. And I’ll tell you ok, because really we don’t have any proofs against that idea.

    About QM… ok, there will be more posts! :) Anyway, it’s ok if you don’t understand. Feynman used to say that those who considered QM intuitive really didn’t understand it… Somehow, one develops an intuition. But, nonetheless, QM is not the same as special relativity. The foundations of QM are all but clear…

  15. The way to measure time is due a change of anything, but if things don’t change time still flow.
    I’m thinking about neutron half-life problem (well… “problem” to me, as i think it could be alternate explanations to the increment of half-life).
    Occam’s razor says then that it’s simpler to change all the universe than a single particule.
    It still don’t explain to me the paradox of generating a photon without infinite amounts of energy.
    From my point of view, time is a must to everything to move, cause if you supress time theres no movement posible, by definition. If you freeze a particule, its movements will be slower, does that mean that we’re messing with the time?
    Some people think that if we reverse the arrow of time events would go backwards ant time will flow from future to past. But i think that to move things “backwards” energy will be needed. I’m again on the metaphysics field.
    Seems that i should lear maths to “see” whats going on. I can follow the logic but a can’t “see” the proofs.

  16. I don’t see the point of discussing this here in English, nobody else is following. I’ll send an email in Spanish to you… :)

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