# Lorenz contraction - physical phenomenon or "optical illusion"?

I’ve been reading Greene’s “The Elegant Universe.” There’s a part where he mentions that no object is totally rigid - fair enough, have heard that many times on this very board, when people think they’ve discovered a way to get around Special Relativity using light-year long pieces of wood.

He adds that the fact that no object is totally rigid is what allows objects to contract when moving relative to the observer’s reference frame.

Huh?! I thought the Lorenz contraction was a sort of optical illusion, caused by the relative motion of one reference frame to another. I didn’t realize the object was actually, physically being compressed! If so, if Albert, holding a ruler, whizzes by me at 0.9c in a space ship, are the molecules of wood [can one have a molecule of wood?! probably not, but you get my point] compressed, just as if you’d put the ruler in a mechanical press? He alludes to a paper about “Einstein and the rigid spinning disk” that deals with this, apparently.

Because of the finite travel time of light, what you would see is difficult to say, but you would definitely measure a contraction. It is a physical phenomenon, and I do not think it’s an optical illusion in the sense you mean.

Right. There is no such thing as a truly rigid body: the part you push on has to push on the part in front of it to get it to move, this doesn’t happen instantly. Because there is a slight delay, the push moves up the body (wood in this case) in a wave, or ripple. to see this in sction, get a slinky and stretch it out (have a hooker hold the other end, it’s more fun that way!) and push your end sharply toward the other end. You’ll see a ripple move down the slinky: in more rigid objects the ripple moves mush faster, so you don’t notice it. the ripple could never move faster than light, because to do so, the individual parts would have to move (transmit the ripple) faster than light–and as we all know: as objects move faster, they weigh more, and thus it takes more energy to make them go faster. So, to get to the speed of light, you would have to apply an infinite ammount of energy, needless to say, this is very difficult to do with the electrical prices being what they are these days.

It is not an optical illusion. If it were merely an illusion, there would be no reason the have a whole Theory of Relativity. The Albert’s ruler is only compressed as you observe it. To him it appears normal and yours appears compressed.

It depends on what you define as illusion and what you define as reality. First I will have to define two concepts, stress and strain, in engineering terms. Stress is the application of a force per unit area, like psi. It is stress that compresses gases, but the same principle applies to solids - they just stay in more or less the same shape. Strain is a deformation of an object. If I have a 100 cm bar and I pull on it so hard it lengthens to 101 cm, it is under 1% strain. It doesn’t matter if the bar is rubber, steel, wood, or titanium - the force is different, but if it’s stretched 1%, that’s 1% strain. The stress (force) required to stretch it is different, though.

If Albert measured everything in the spaceship, assuming its engines were shut down, as it zips through space at 0.9c, he would find that there wasn’t anything distorted inside it. There would not be any extra strains from Lorenz contraction or anything of the sort measured inside the space ship. There is no difference to an observer on the spaceship whether it is traveling at 0.9c or at rest with respect to an outside observer.

On the other hand, to you, as an observer who is not moving with the spaceship, it will appear to be compressed. You can measure the deformation, perhaps by taking a picture of it and measuring the picture. You could even measure the strains from the Lorenz contraction, but only if your device for measuring strain did not move with the spaceship. Strain is defined as a change the shape of an object, so the strain would be real in your frame of reference. It would not be real to a strain gauge mounted on the spaceship, though.

Confused? It gets worse. Normally, the rule is that strain on a material is produced by applying a stress. But in the case of Lorenz contraction, the strain is not produced by any applied stress at all. There is no force changing the length of the spaceship. What is happening is that while the speed of light is a constant, all other measurements - length, time, and even mass - must change to accommodate the constant speed of light. This happens automatically, without applying any forces.

It’s hard to explain what is happening, but it is not an optical illusion. It isn’t that the material is squeezed to contract it as much as the length that the material occupies is contracted. I know this is not making any sense. It is late and relativity doesn’t even make sense to me when I am wide awake.

It’s not an optical illusion…but it’s not really happening, in that Albert (being in the same frame of reference) would witness nothing unusual about his ruler.

OK, if instead of a ruler, Albert were holding something brittle and fragile, like an egg - would the Lorenz contraction cause it to crack? I have to think not - since the contraction is a relative effect, and relative to some other frame of reference there would be a different contraction. IOW, I have to think that the symmetry of the situation (if I had a ruler, Albert would see it as contracted by the same amount as he whizzes by).

(oops) means that the object itself (ruler or egg) “feels” no real contraction, and that this is a purely observational phenomenon. I have a feeling we’re teetering on the brink of philosophy, here.

I’m suprised that no one has brought up the Barn and Pole Paradox yet. Send an 80’ pole through a 40’ barn at close to the speed of light so that the 80’ pole is contracted and fits in the 40’ barn. Definitely not an illusion.

On the other hand, maybe the definition of illusion is what’s really in question. When your perception of reality is misleading, that would be an illusion. So if you see a 40’ pole when it’s actually 80’ long, that would be an illusion, I suppose. But the pole really is 40’ long in your frame. Does that count as an illusion? Call it as you see fit. But the corresponding time dilation sure doesn’t seem to fit into the category of “illusion” by my standards. YMMV.

And there’s the other half of the pole in the barn paradox. If you were riding on the pole as it went flying through the barn, the 40’ barn would appear to be considerably shorter, and the pole would never be completely inside the barn at any one time. The key there is “at any one time,” because many of the relativity effects depend on time. What is happening “at the same time” in one frame of reference is not happening at the same time in another. So to an observer sitting outside the barn, the time the back end of the pole dissappears into the barn is before the time the front of the pole comes flying out the other side. However, if you are riding on the pole holding on for dear life, these events take place at different times.

The speed of light must remain constant for observers in all inertial reference frames. Speed is distance divided by time so if time slows then the distance must contract in order for the speed of light to remain a constant for all observers.

What’s really happens is the geometry of Minkowski space-time changes.

Hmm… What surprises me is that no one has brought up particle physics yet. There are certain fundamental particles produced in the upper atmosphere by cosmic radiation and the like which, were Lorentz contraction/time dilation not a real phenomenon, we wouldn’t be able to observe here at sea level because they’d decay before they got here.

The fact that we do see them is explained by Lorentz contraction and time dilation. From the particle’s perspective, it doesn’t have to travel as far (and hence doesn’t have to travel for long enough to decay), while from our perspective, the particle has a time dilation (its clock runs slow and it doesn’t decay as fast as it ought to).

It’s pretty clearly not an optical illusion. But what explanation you accept for what actually is happening depends on from where you look.

So how about that egg Albert is carrying? Will it crack due to the contraction?

No, the egg will not crack. That’s because the “ruler” (aka the “metric”) for the egg is different than the “ruler” you use. Therefore everything, eggshell, atoms, and rulers is contracted when you look at the egg. Of course, to the egg, you’re the one with the contracted stuff.

This is why the barn paradox is so nifty because these effects are REAL. They are physical. You can fit an 80 foot pole in a 50 foot barn and close the doors! This is because the way time and space are connected means that the only consistent way to measure a “length” or a unit of “time” is with respect to each other or with respect to themselves. This is what allows special relativity to be totally consistent.

The egg shell won’t crack, but there is a corollary to the barn paradox you might like, DarrenS. Consider the pole inside the barn. Okay, you can close the doors and the pole is inside the barn and open them again and the pole goes whizzing through. Now, this time don’t open the barn doors. The pole was physically inside the barn. We observed it to be there. Now you have this barn door there and what happens to the pole? Say the barn door is indestructible… how did this pole get inside the barn? After all, as soon as it stops moving at the significant fraction of the speed of light it will again be an 80 foot long pole. Something’s gotta give.

Well, the resolution to this is that there is no such thing as indestructibility in nature. Either the pole will bend/break or the door will break.

Try to think about what happens in the pole’s frame of reference. The front of the pole is going to hit the barn door even before the back door closes… but we know we closed the back door of the barn on the pole so we know that the back part of the pole has to get into the barn. If you calculate how far the back of the pole travels when you have a light-speed signal (call it a sound wave) that transfers the information to the back of the pole that “we cannot go any farther” you will find that it gets inside the barn door. In effect, you cannot get around the finite speed of light. Since the back part of the pole doesn’t know the door ahead of it is closed it can only travel forward happily until it does. Now, what breaks is a whole other ball of wax.

Here’s another way to think about Lorentz contraction: the egg, the ruler, the pole, and so on are not compressed–the space they occupy is what is compressed.

We all know about time dilation, and many of us hear the phrase “space and time are mixed together”. But this goes both ways–if time itself is dilated, then space itself is what is contracted. We hear a lot about time dilation, because it allows to think about the twin paradox; further more, the effects of time dilation can readily be perceived as ‘permanent’. Lorentz contraction, however, seems ephemeral–the space seems to ‘spring back’ to normal when the relative motion ceases.

But that’s only becauses we’re thinking about it in the wrong way. The moving object (ruler, egg, pole) is not the important thing in the problem–it is only a measurement device. **It is the fabric of space itself that is undergoing contraction **, or at least appears to, relative to our own reference frame. The ruler itself is just how we measure the amount of that contraction. And our ruler actually experiences that contraction of space–hence a cosmic ray muon (thanks g8rguy) can actually reach earth because it actually experiences a much thinner atmosphere than we do–so it thinks it has travelled a shorter distance than we think it did.

Here’s another way to think about Lorentz contraction: the egg, the ruler, the pole, and so on are not compressed–the space they occupy is what is compressed.

We all know about time dilation, and many of us hear the phrase “space and time are mixed together”. But this goes both ways–if time itself is dilated, then space itself is what is contracted. We hear a lot about time dilation, because it allows us to think about fun things like the twin paradox; further more, the effects of time dilation can readily be perceived as ‘permanent’. Two atomic clocks which moved relative to each will stay out of synch once the motion stops. Lorentz contraction, however, seems ephemeral–the space seems to ‘spring back’ to normal when the relative motion ceases.

But that’s only becauses we’re thinking about it in the wrong way. The moving object (ruler, egg, pole) is not the important thing in the problem–it is only a measurement device. **It is the fabric of space itself that is undergoing contraction **, or at least appears to, relative to our own reference frame. The ruler itself is just how we measure the amount of that contraction. And Albert’s ruler actually experiences that contraction of space–hence a cosmic ray muon (thanks g8rguy) can actually reach earth because it actually experiences a much thinner atmosphere than we do–so it thinks it has travelled a shorter distance than we think it did.

Oops!! sorry about the double post. Darn dial-up. And “our ruler” should read “Albert’s ruler” as it does in the second post.

So, not to flagellate a defunct equine, but if this is only an effect of perception or measurement due to the relative movement of our reference frames, what has the perfect rigidity (or lack thereof) of the pole/egg/wheel in question got to do with it?

Ew, what an ugly sentence.

Rigidity, AFAIK, only has to do with communication of movement. If this communication were faster than the speed of light, that would violate special relativity. Length contraction, per se, is not the ensuring reason unless the context is constructed correctly (ala the closing barn door).

I agree; rigidity is also only an effect of our perception or measurement.