Superluminal events which do not transmit information or energy

[touqra]

What are the examples of superluminal events which do not transmit information or energy, and hence not violating causality but travels faster than light?
My book here gives an example of a shadow of a bug flying across a projector. Any others?

 

[JesseM]

Some other examples are the spot of a laser moving across the moon faster than light (see here), or the contact point between the blades of a giant pair of scissors moving faster than light, and thus cutting a piece of paper faster than light (see here).

 

[masudr]

Surely with the laser on the moon there will be a lag as the laser pointer is moved here on earth, and the time it takes for more EM wave packets to leave the pointer and reach the moon? Therefore the light will look curved from a "top-down" view, if you like, until the first photon from the pointer in its new position reaches the moon. And surely moving the giant scissors will cause the scissors to just get deformed.

For example, say we have a rigid sphere. When it hits the floor the bottom and top of the sphere instantaneously change velocity; but of course the bottom receives its upwards push from the floor; then through EM interactions the push is transmitted to the top of the ball at a speed less than or equal to c, and so the ball gets slightly deformed; therefore no such thing as a rigid body.

Don't know why I added the last bit; it seemed relevant at the time.

 

[russ_watters]

The laser example isn't about the beam, it's about the spot on the moon. But you are right - the beam would appear curved, kinda like when you spray water from a hose and move it back and forth.

 

[JesseM]

Surely with the laser on the moon there will be a lag as the laser pointer is moved here on earth, and the time it takes for more EM wave packets to leave the pointer and reach the moon?

Sure, there'd be a lag between the moment you began to rotate the angle of the laser and the moment the spot began to move across the face of the moon, but the point is that once the spot begins to move, it can move faster than light if you rotated the angle of the laser fast enough. But no actual photon in the laser is moving faster than light here, and the "spot" is not a single physical entity, it's just a series of positions where different photons from the laser are hitting the face of the moon.

And surely moving the giant scissors will cause the scissors to just get deformed.

That depends whether the angle between the blades is changing--you could just have two separate blades with a constant angle between them that move vertically relative to each other at constant velocity, for example. But yes, if you try to rotate a blade from a fixed pivot the blade will get distorted.

 

[dan_r]

We see red shifts all the time in 'excess of c'. Astronomers routinely 'correct' them, and Einstein is slavishly adhered to in this time of 'preserving one's job, reputation, ad nauseum'. My job is not at stake in this debate so can speak honestly. The pure physics of it would seem to mean that every object effects every other object in the universe: e.g. Two objects approach each other and each has an absolute velocity of almost 'c'; therefore the vector sum of the approach velocities must be the Fourier sums of its unit vectors, ^i,^j,^k; and if any of these vectorial sums is >'c', then the giant hand of the 'god einstein' will descend out of the ether and slow them down...preposterous. The emperor has no clothes! And I think the bushy haired one form long ago would have to scratch his head as well. A viscous space caused by the presence of hidden matter giving rise to a 'reynolds number in space' I could accept, but the idea of every object controlling other objects infinitely far away, of every object or energy field being simultaneousely affected by infinite numbers of other objects being simultaneousely controlled and accelerated (negative acceleration as well) in infinite directions by yet an infinite number of other objects, each with their own infinities of simultaneous controls. How many infinite powers of infinity do we want to have to deal with just to satisfy one man's calculations over a hundred years ago. Of course many potential or actual repliers here have jobs to keep from whoever might listen, wherever or whenever. Just seems that to follow GM to its logical end leads to inevitable insanity among any who cannot break out of the circle of the closed mind.

Do they correct them though? I'm just starting out with physics so bear with me!

As I understand it, redshift is permitted to be in excess of c when dealing with cosmological distance/time/velocity because it doesn't cause a violation of special relativity, given that redshift is moving into infinity from our 'static' point of view, as opposed to measuring a quantified distance, between the Sun and Proxima Centauri for instance?

And while the vector sum you give in your example of two objects approaching each other at near c would be in excess of c.. that's only an abstract value composed from combined two empirical values?