Be Excellent To Each Other

Ok,

Imagine I have a torch, an object and a screen.

I point the torch at the screen, turn it on, and place the object between the 2. I then change the angle of the torch so the shadow is touching the far left of the screen. I then make it so the shadow moves from left of the screen to the right of the screen.

Now the Shadow's terminator (and I'm not talking Arnie here) takes the same time to move from the right to the left as the time I spend rotating the torch. So in theory I could speed up my rotating of the torch until it's moving at say 1/10th the speed of the light. Now, if the screen is 20 times as large as the distance I move the torch is not the terminator moving at twice the speed of light?

I'm sure there is a flaw in my argument somewhere, but I don't see what? I mean I guess the shadow has no mass so that's not going to be a factor.

Any of the people that actually completed their physics degrees care to enlighten me?

Malc

Surely the shadow is the absence of light?

yes, but it's (well it's terminator is) moving faster than light

Malc

I can rub my magic torch at the speed of light.

Define 'terminator'.

And no, the flaw in teh logic is that you will be seeing the relection of that light, so it has twice as far to travel.

Isn't it moving at the speed of wrist? Tbh I'm not sure what you're barking at Malc.

maybe, i don't fully understand. The light is travelling the same distance on each occasion, (to teh screen and back) and not travelling across the screen (left to right).

I've solved this exact problem as part of an undergraduate worksheet. You're correct, it's quite easy to get it over the speed of light (slightly more realistically, the problem I did involved a laser on a turntable projecting a dot of light onto distant clouds a few miles away).

This is perfectly fine though. The terminator isn't a physical object, and doesn't actually travel anywhere. It's just a consequence of the other bits of the system and none of those are going faster than c. The more formal way of restating Special Relativity is that information cannot travel faster than c, and in this context, information is carried by any particle or wave but not by your terminator.

More here.

What you're saying is, you're effectively drawing an arc with the torch. If you are rotating the torch at speed x, the leading edge of the light should be moving a lot faster. In fact, the leading edge of the light would be describing an arc which would move at the same speed of the torch movement. Because the screen is there, it's actually describing a chord of shorter distance in the same time, therefore moving slower, not faster.

Possibly.

Also - what's moving here, exactly?

Yeah, silly.

According to this guy - http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3 - yes it is going faster than the speed of light, but it's OK cos it's not a thing. Phew!

The line between light and dark.

It's the line between day/night on Earth and it's the edge of the shadow in my example.

If the screen was made of photosensitive detectors, and the object was shaped in someway so that you could make it's shadow mean something, then the "data" would travel across the detectors faster than the speed of light, information would travel faster than the speed of light, which is impossible.

Malc

Hmm. Should I use my powers of invisibility for evil, or should I fight crime?

last post was in reply to MaliA, I'll read the others in a second.

Malc

No. In this case, data is flowing from the torch to the screen, and that happens at the speed of light. To violate causality you have to get pass information from one bit of screen to another through the terminator.

Invisibility and a jet pack! Excellent, really.

Or impregnate your own mother.

Yeah, sorry. It took me a while to get the link right and then I kept hitting submit, and there kept being other posts, so I kept hitting it again. Ooops. Stupid internet.