Light can be closed in a mirror's box?

[Darko Dark Shadow]

Hello people, as the title says, I want to know if light can be closed in a mirror's box. My idea is, build a sealed mirror's box (The mirrors must reflect the 100% of the light), in that any light particle can escape, later turn on a light inside for 5 seconds and then turn it off (Let's suppose that the light source is "invisible" to make that the light can't be abosrbed for anything). So if I do that... Will the light be closed forever, bouncing against the walls? Or it will gradually start losing momentum and/or luminescence?

[Strange]

It will be absorbed by the mirrors because they are not perfect reflectors.

If you could create a perfect reflector then, yes, the light would stay in there forever bouncing around. But that is not possible.

[swansont]

One of the reasons 100% reflection is not possible is that momentum must be conserved. When the photon turns around, there must be some recoil by the mirror, so some energy must be transferred.

[Strange]

1 minute ago, swansont said:

One of the reasons 100% reflection is not possible is that momentum must be conserved. When the photon turns around, there must be some recoil by the mirror, so some energy must be transferred.

That occurred me afterwards, as well. Does that mean that with ideal mirrors, the light would get progressively red shifted?

[John Cuthber]

1 hour ago, swansont said:

One of the reasons 100% reflection is not possible is that momentum must be conserved. When the photon turns around, there must be some recoil by the mirror, so some energy must be transferred.

I'm not sure that's a problem.

A photon hits the right hand side of the box, and bounces off with sightly less momentum because some is transferred to the box.
The box is now moving very slightly to the right.
But when the photon hits the left hand side of the box it's hitting something that is moving towards it, and it gains momentum from that interaction.

If anything, I think the conservation of momentum shows that the light does (ideally) bounce forever- if it stopped, where would the momentum go, and which way?

[Strange]

But the energy transfer could warm the material of the box, rather than just moving it back and forth. Maybe.

[John Cuthber]

39 minutes ago, Strange said:

But the energy transfer could warm the material of the box, rather than just moving it back and forth. Maybe.

That would be absorption and we have said that's not happening- the mirrors are 100% perfect

[dimreepr]

1 hour ago, John Cuthber said:

I'm not sure that's a problem.

A photon hits the right hand side of the box, and bounces off with sightly less momentum because some is transferred to the box.
The box is now moving very slightly to the right.
But when the photon hits the left hand side of the box it's hitting something that is moving towards it, and it gains momentum from that interaction.

If anything, I think the conservation of momentum shows that the light does (ideally) bounce forever- if it stopped, where would the momentum go, and which way?

wouldn't that mean that any angle other than 180 would eventually see a loss of energy with respect to the box?

[John Cuthber]

Eventually, the effect of diffraction is that all the photons hit all the walls

[dimreepr]

7 minutes ago, John Cuthber said:

Eventually, the effect of diffraction is that all the photons hit all the walls

TBH I didn't read the OP, just the title and assumed we're talking about 'a' photon, having said that, wouldn't interference create an imbalance that dissipates the energy? 

[John Cuthber]

14 minutes ago, dimreepr said:

...dissipates the energy? 

Where to?

[dimreepr]

1 minute ago, John Cuthber said:

Where to?

I'm guessing the box, but in QI terms I hear a klaxon 

[swansont]

8 hours ago, Strange said:

That occurred me afterwards, as well. Does that mean that with ideal mirrors, the light would get progressively red shifted?

I think an ideal mirror would have to be infinitely massive. Any real mirror would have this tiny redshift.

6 hours ago, John Cuthber said:

I'm not sure that's a problem.

A photon hits the right hand side of the box, and bounces off with sightly less momentum because some is transferred to the box.
The box is now moving very slightly to the right.
But when the photon hits the left hand side of the box it's hitting something that is moving towards it, and it gains momentum from that interaction.

If anything, I think the conservation of momentum shows that the light does (ideally) bounce forever- if it stopped, where would the momentum go, and which way?

Only if you have an infinitely rigid box, another impossible ideal. 

[John Cuthber]

I think it needs to be perfectly elastic, rather than perfectly rigid. So it's impossible for a different reason.

 

The punch-line to all this discussion of why it's impossible is ...

people actually do it.

https://en.wikipedia.org/wiki/Cavity_ring-down_spectroscopy

http://www.chem.ualberta.ca/~xu/research/crds.htm

[Endy0816]

Would traveling infinite distance be the equivalent? Keep thinking frequency should change regardless. Not sure on math though.

[Darko Dark Shadow]

So, we can assume that if the mirror have 100% reflection it will be possible? And I am not reffering to a single photon, I reffer to waves of light bouncing against the walls, as I write "...turn on a light for 5 seconds..."

[dimreepr]

14 minutes ago, Darko Dark Shadow said:

So, we can assume that if the mirror have 100% reflection it will be possible? And I am not reffering to a single photon, I reffer to waves of light bouncing against the walls, as I write "...turn on a light for 5 seconds..."

Well no, as explained above 100% reflection is not possible.

[Darko Dark Shadow]

On 28/10/2017 at 5:37 PM, Strange said:

It will be absorbed by the mirrors because they are not perfect reflectors.

If you could create a perfect reflector then, yes, the light would stay in there forever bouncing around. But that is not possible.

 

On 29/10/2017 at 9:00 AM, swansont said:

One of the reasons 100% reflection is not possible is that momentum must be conserved. When the photon turns around, there must be some recoil by the mirror, so some energy must be transferred.

 

On 29/10/2017 at 9:03 AM, Strange said:

That occurred me afterwards, as well. Does that mean that with ideal mirrors, the light would get progressively red shifted?

 

On 29/10/2017 at 0:17 PM, dimreepr said:

wouldn't that mean that any angle other than 180 would eventually see a loss of energy with respect to the box?

 

22 hours ago, swansont said:

I think an ideal mirror would have to be infinitely massive. Any real mirror would have this tiny redshift.

Only if you have an infinitely rigid box, another impossible ideal. 

I have readed your answers and reached to the conclusion that is impossible a perfect boucing without loss energy even with "perfect mirrors", because the same bouncing means a enegy transfer beteween the photon and the wall. I'm wrong?

22 hours ago, swansont said:

I think an ideal mirror would have to be infinitely massive. Any real mirror would have this tiny redshift.

Only if you have an infinitely rigid box, another impossible ideal. 

 

22 hours ago, John Cuthber said:

I think it needs to be perfectly elastic, rather than perfectly rigid. So it's impossible for a different reason.

 

The punch-line to all this discussion of why it's impossible is ...

people actually do it.

https://en.wikipedia.org/wiki/Cavity_ring-down_spectroscopy

http://www.chem.ualberta.ca/~xu/research/crds.htm

The mirrors rigidness can influence in the energy conservation? Can you explain it better.

13 hours ago, Endy0816 said:

Would traveling infinite distance be the equivalent? Keep thinking frequency should change regardless. Not sure on math though.

I think not, because traveling an infinite distance will make them to conserve it momentum and energy as far they encounter a obstacle.

[swansont]

55 minutes ago, Darko Dark Shadow said:

 I have readed your answers and reached to the conclusion that is impossible a perfect boucing without loss energy even with "perfect mirrors", because the same bouncing means a enegy transfer beteween the photon and the wall. I'm wrong?

Perfect mirrors do not exist in reality. You can use them to analyze the ideal case, which is often done in thought experiments. But your wording in the OP implied you wanted a real-world analysis.

[John Cuthber]

1 hour ago, Darko Dark Shadow said:

The mirrors rigidness can influence in the energy conservation? Can you explain it better.

I can try.

If you drop a tennis ball it bounces, but not as high as you dropped it from. Energy is lost as heat.

Other balls bounce better or worse, but none gets back to the height it started from.
In the same way, when you make a box out of mirrors and set a photon bouncing round in it you will lose energy because the box gets stretched (very slightly) when a  photon hits it.

[Darko Dark Shadow]

On 30/10/2017 at 5:05 PM, John Cuthber said:

I can try.

If you drop a tennis ball it bounces, but not as high as you dropped it from. Energy is lost as heat.

Other balls bounce better or worse, but none gets back to the height it started from.
In the same way, when you make a box out of mirrors and set a photon bouncing round in it you will lose energy because the box gets stretched (very slightly) when a  photon hits it.

Now I understand... But, if the shock beteween the mirror and the photon would  a 100% elastic shock?

On 30/10/2017 at 4:55 PM, swansont said:

Perfect mirrors do not exist in reality. You can use them to analyze the ideal case, which is often done in thought experiments. But your wording in the OP implied you wanted a real-world analysis.

Maybe the "perfect mirrors" will be discovered in a future, but for the moment it's only a ideal case as you write. Thanks for your answer

[John Cuthber]

4 minutes ago, Darko Dark Shadow said:

Now I understand... But, if the shock beteween the mirror and the photon would  a 100% elastic shock?

That's a requirement for a perfect mirror.

But the impact of a photon on one side of the box pulls the rest of the box along  by stretching it out a bit.

Unless that's perfectly elastic too, there's a loss of energy.

[Darko Dark Shadow]

2 minutes ago, John Cuthber said:

That's a requirement for a perfect mirror.

But the impact of a photon on one side of the box pulls the rest of the box along  by stretching it out a bit.

Unless that's perfectly elastic too, there's a loss of energy.

Yes, you're right, even under "perfect conditions" the energy loss is present. Thanks for your time and replies

[swansont]

1 hour ago, John Cuthber said:

That's a requirement for a perfect mirror.

But the impact of a photon on one side of the box pulls the rest of the box along  by stretching it out a bit.

Unless that's perfectly elastic too, there's a loss of energy.

Problem: the mirror can't respond by moving at c in its oscillation. The next photons to hit will impact a mirror that is not at the same position, i.e. not at a node of the oscillation. Destructive interference, therefore losses, therefore not perfect.

[John Cuthber]

Interference isn't a loss mechanism.
The dark fringes are dark, but the bright fringes are brighter, to make up for it.

Also, nobody said the box was an integer number of half wavelengths or whatever.

It's not a requirement that the wavelengths "fit"