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Q & A: relativity fundamentals

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Most recent answer: 10/22/2007
Q:
Hi. The special theory of relativity has bothered me for some time, but now I beleive I know why - I think it is almost always over explained and over extrapolated. It has taken me some time to realise that all the theory offers is that light moves at 300000 km/s within all reference frames. So it seems to me, clocks do not run slower when moving. A train speeding past me at 80% C is not actually shorter. It is all simply at matter that the information being sent to us (ie, light to our eyes) is "lying" to us because it always travels at the same speed regardless of reference. There is nothing peculiar happening to the objects (clock or whatever) that the light is providing us information about, it is simply a "limitation" or "feature" of the information transmission itself. Further, if light always travels at 300000km/s in all reference frames, then there MUST be a subjective relationship (or feedback loop)between the observer and light, and therefore I do not know why there is so much confusion about what quantum physics shows us. Your agreement, refutal or anything in between appreciated, as always. Cheers,
- Brian (age 29)
Victoria, Australia
A:

You’re asking some interesting questions, and your ideas overlap to some extent a fully relativistic perspective.

The constancy of the speed of light as seen by different observers is only one of the simplest peculiarities of the relativistic transformations, the rules for translating back and forth between the descriptions in different reference frames. You’re entirely correct that there’s nothing physically weird about the ’moving’ clock etc. The most basic postulate of Special Relativity is this:

Exactly the same laws of physics work for any one of a collection of reference frames moving uniformly with respect to each other.

Therefore there is no physical meaning to the claim that any particular object is moving or standing still. Each object is at rest in its own frame, which is as good as any other frame because it employs the same physical laws. Therefore nothing at all is intrinsically different about a ’moving’ object.

That does not mean that information you obtain via light is in any way defective. It agrees exactly with the information obtained by any other transmitted signal, traveling at any speed with respect to you. It just means that many things, such as distances and time intervals, which you thought were intrinsic to events themselves actually depend both on events and on how you look at them. Think of the width and depth of a box. Which is which depends on what direction you look at the box from. That’s no big deal because you’re very used to looking at boxes from different directions. Imagine you could only move around by a few inches, and you only looked at things hundreds of feet away. You would probably be convinced that ’depth’ and ’width’ were two entirely different properties of objects, and would be shocked if somehow when you could move more you were to see them convert from one to the other.
That lengths and times depend on from what state of motion you look at them only seems like a big deal because we are not used to changing our state of motion enough to notice the effect. As a result we have evolved a very simple, workable intuition that lengths and time intervals don’t depend on how they are observed. That’s a lot simpler to use than the full picture, but it’s not right.

We happen to have light available to use to look at things, and that makes it easy to tell various stories to explain relativity. However, even if there were no actual signal that could travel at the speed of light, the basic rules describing how length and time look different from different reference frames would be exactly as they are in Special Relativity. The special speed, c, that appears in the formulae for transforming between reference frames would still be there, although we couldn’t call it the speed of anything.

So I’ve agreed with one of your points and disagreed with another. As for feedback loops and ’quantum’, I don’t really understand what you’re asking.

Mike W.

I agree with Brian that the propagation time of light signals must be taken into account when interpreting them. For example, when we look in the sky with our telescopes, we are really looking at events that happened in the past -- the farther away the objects are, the longer the light we’re looking at has been traveling. This is easy to take into account.

What’s amazing is that even after this is taken into account, we can still observe the effects of relativistic length contraction and time dilation.

We can even set up scenarios in which propagation delays are irrelevant. If a car passes a signpost (very close by, with no propagation distance for signals), then the time between the front bumper passing the signpost and the back bumper passing the signpost, as measured by a clock on the signpost will decrease as the car goes faster and faster in accordance with both the speed of the car and its shortening length in the frame of the signpost.

Similarly, the time dilation effect is a real one and not manufactured out of propagation delays for light signals. Unstable particles called muons are present in cosmic rays and they decay with a lifetime of 2.2 microseconds when at rest. When we send them zipping around in a ring of magnets at high speeds, we can observe that they last on average a longer time, and can travel greater distances than 2.2 microseconds times the speed of light (on average). This effect was first measured by comparing the number of muons from cosmic rays on a mountaintop with the rate on the Earth’s surface. Muons are produced in collisions of high-energy protons and air molecules high up in the atmosphere. If it weren’t for time dilation, the muons would nearly all decay to electrons and neutrinos before they got down to the surface of the Earth, but many more muons were observed at the Earth’s surface.

Tom


(published on 10/22/2007)

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