3 ms·
Suppose you have two points, X and Y, each distance d from some origin O. At time 0, some light is emitted from O towards both X and Y. The earliest it can arri
by tzs 3y ago
Suppose you have two points, X and Y, each distance d from some origin O. At time 0, some light is emitted from O towards both X and Y. The earliest it can arrive at either of them is time d/c, where c is the speed of light in a vacuum.
If you want the light to arrive at different times you can either do something to make the paths different lengths, or to make the speed of light different on the two paths, or both. You can't make the speed faster than c on either of the paths but you can make it slower by making the light go through something with an index of refraction greater than 1.
Now suppose you've got some thing that you have built to process light, and you care about the differences in arrival time of the light to different parts of the thing. There's probably going to be some part of your thing that it makes the most sense to use as a reference, and then describe light arrival times elsewhere relative to that reference point.
There's usually no particular reason to pick as the reference the point on your thing that light first reaches. If you do indeed pick a reference that is not the earliest place of contact with the light, then you can have light arrive earlier at other parts of your thing.
- dylan604 3y agolight either arrives exactly on time, or arrives late. what ever other convoluted description you want to come up with, light cannot be early based on your equation of d/c
- tzs 3y agoConsider light from a distant source, sufficiently far away that the wavefront at your instrument is planar. You have a planar sensor array parallel to the incoming wavefront, so the wavefront arrives at all sensors at the same time. There are different kinds of sensors sensing different aspects of the incoming light. You need to combine them, making sure that you are combining readings that come from the same plane of the wavefront. But one of your sensors takes a little longer than the others to process and produce a reading. You either need to make that sensor start processing earlier than the other sensors, or make the other sensors start processing later, or add some sort of delay between the other sensor outputs and the thing that combines all the sensor outputs. If you fix this by moving that one sensor a little out of your sensor plane, moving it toward the light source, most people, including most scientists, would say that your fix was to make the light arrive at that sensor earlier, because the light in fact arrives at that sensor now before it arrives at the other sensors. Let's say that the sensors process different wavelengths of light. Then another adjustment you might do to get the outputs synced would be to fill the space between where light enters and the sensor plane with a gas that happens to have a low index of refraction for the wavelengths the slow sensor uses and a high index for the wavelengths the other sensors use. That would make the light arrive later at all the sensors than it would have if the gas had not been added, but it will less later at the slow sensor. But again nearly everyone would say that what you fix did was make the light arrive earlier at the slow sensor, because before the fix light arrived at the slow sensor at the same time it arrived at the other sensors, and after the fix it arrives at the slow sensor before it arrives at the other sensors.
- Dylan16807 3y agoSo if light arrives through a fiber-optic cable, you will insist it's never on time? I don't think that's a great definition. Please stop acting like it's the only reasonable definition.
- pyinstallwoes 3y agoThat’s somewhat akin to negative voltage relative to ground.