Graduate Understanding time invariant of EM

[jdou86]

on the conquering the physics gre book it says e.g. for time invariant "if you can see someones eyes in a mirror, they can see yours as well" so what the hell does that mean?

isnt person A sending photons to person B and person B sending different sets of photons to person A? how does that relate to time invariant of EM

<< Mentor Note -- profanity deleted from post >>

 

[dextercioby]

Technically, the type of photons sent to one person's eyes are not sent by other persons, but by light sources, such as the Sun, or a bolt. Time invariance of electromagnetism means that if a solution of the Maxwell's equations is a function of "t", the same functional dependence but with "t" replaced by "-t" is also a solution of the same equations.

 

[PeterDonis]

for time invariant "if you can see someones eyes in a mirror, they can see yours as well" what the **** does that mean?

It means that the time reverse of the path light rays take from them to you, i.e., the path light rays take from you to them, is also allowed by the laws of physics. If the laws governing light were not time reversal invariant, that would not be the case.

 

[jdou86]

It means that the time reverse of the path light rays take from them to you, i.e., the path light rays take from you to them, is also allowed by the laws of physics. If the laws governing light were not time reversal invariant, that would not be the case.

Thanks! that is a very cool answer!

 

[PeroK]

on the conquering the physics gre book it says e.g. for time invariant "if you can see someones eyes in a mirror, they can see yours as well"

It might be fun to think of all the examples where this is not true.

For example, if you remove a blindfold, then you will almost immediately see someone at a distance. But they won't see your eyes for a tiny bit longer.

I'm not sure why the mirror is relevant either.

You could also be hiding in a dark cupboard, looking out through a keyhole. But perhaps that's getting too silly.

 

[jdou86]

Keyhole I think is fine (both can see) but there is a delay since c =1

 

[PeroK]

Keyhole I think is fine (both can see) but there is a delay since c =1

If there is no light source and all the photons entering the cupboard are absorbed.

 

[jdou86]

If there is no light source and all the photons entering the cupboard are absorbed.

Then the person in the darkness wouldn’t be able to see through it because there is no light

 

[PeroK]

Then the person in the darkness wouldn’t be able to see through it because there is no light

You've obviously never played hide and seek! You can see out through a crack or hole, as light enters, but not enough light comes out for anyone to see you.

 

[jdou86]

You've obviously never played hide and seek! You can see out through a crack or hole, as light enters, but not enough light comes out for anyone to see you.

places with more light have more interference you are not looking at the light from the hole but you also looking at light from the world bruh

 

[jdou86]

okay this is the end, i do not want to receive any more information from this post please.