-1
$\begingroup$

Let us suppose I have an laser.I'm swinging my hand so that the spot of light moves in screen. Screen is at arbitrary distance so when I swing my hands the spot go up and down on screen, Now It is clear that speed of spot will be greater than that of my hands. Now I'm swinging my hands with speed of light (almost) then spot must go with speed greater than speed of light. which is a contradiction? Please Correct me.

$\endgroup$
3

3 Answers 3

1
$\begingroup$

Take a slice in time of your beam spot. It hits the screen. The next in time slice hits the screen, but it is not the same light energy. At the level of photons , it is not the same photons. The relativistic limit of the speed of light in vacuum is about the same light slice ( or photon, at the quantum level) . The energy cannot reach the screen faster than the speed of light.

$\endgroup$
1
$\begingroup$

This is called Nemiroff experiment

Einstein's theory of special relativity says it is impossible for an object (like a single photon of light) to look as though it is traveling faster than the speed of light. Nemiroff experiment doesn't break this physical law because the laser pointer produces a stream of photons, not a single photon. No single photon in the stream ever moves faster than the speed of light, but collectively, they can create the illusion of faster-than-light travel.

Another way to understand this illusion is to imagine an electronic ticker tape, which creates the illusion of a word moving horizontally across the screen. In reality, the word is created by a series of flashes from many individual, stationary lights. In theory, it would be possible to have a word move across the screen faster than the speed of light- because nothing has actually moved. Each flash of light is an isolated event, and they only create the illusion of movement collectively.

You can also watch the "veritasium" video, in which he has explained this phenomenon.

https://en.m.wikipedia.org/wiki/Faster-than-light

$\endgroup$
0
0
$\begingroup$

Nothing in special relativity forbids a spot of light from moving faster than c. The restriction is that objects with mass cannot move at c in an inertial frame. The spot of light isn’t an object with mass (it isn’t even an object).

There is also a restriction that SR together with causality forbid information from traveling faster than c. A spot of light carries neither information nor energy along its path (information and energy are carried from your hand to the screen, but not from one side of the screen to the other).

$\endgroup$
2
  • $\begingroup$ I’m not convinced about the second part of the answer. It’s hard to see how the spot does not carry information. I think, too, that “a spot of light does not carry information” but only because I think that SR is true, thus it impossible that it carries information. But in the first place it’s is only a claim and I don’t see how to prove that it is correct. $\endgroup$ Jan 31, 2020 at 7:55
  • 1
    $\begingroup$ @HartmutBraun a spot of light certainly does carry information, but it carries the information from the hand to the screen, not across the screen. That is the key. A person on one side of the screen cannot change the spot in a way to send information with the spot across the screen. $\endgroup$
    – Dale
    Jan 31, 2020 at 11:47

Not the answer you're looking for? Browse other questions tagged or ask your own question.