Storing light and using it to propel an object

[J3J33J333]

TL;DR Summary

Using Light as spaceship fuel

Is it possible to store large quantities of light for extended periods of time? If so would it make any sense to use it to propel a spaceship because it doesn't weigh anything? I read that one of the biggest issues with getting to space is the weight of the fuel.

 

[phinds]

Summary: Using Light as spaceship fuel

Is it possible to store large quantities of light for extended periods of time?

no

If so would it make any sense to use it to propel a spaceship because it doesn't weigh anything?

no

I read that one of the biggest issues with getting to space is the weight of the fuel.

true

 

[PeroK]

Summary: Using Light as spaceship fuel

Is it possible to store large quantities of light for extended periods of time? If so would it make any sense to use it to propel a spaceship because it doesn't weigh anything? I read that one of the biggest issues with getting to space is the weight of the fuel.

Theoretically, one of the most efficient means of accelerating in emty space is a photon rocket:

https://en.wikipedia.org/wiki/Photon_rocket

Note that a container full of light would in fact have a mass related to its total energy content.

 

[phinds]

Theoretically, one of the most efficient means of accelerating in emty space is a photon rocket:

OK, guess I was wrong about it being a good means of propulsion. Directly "storing light" still makes no sense. Have a light GENERATOR, as you pointed out, is a different thing and not what I interpreted the OP as meaning.

 

[J3J33J333]

OK, guess I was wrong about it being a good means of propulsion. Directly "storing light" still makes no sense. Have a light GENERATOR, as you pointed out, is a different thing and hot what I interpreted the OP as meaning.

You're right, I was basically wondering if anyone knew about materials that have seriously low photon attenuation with good internal reflection. The photon rocket does help a good chunk with what I'm trying to figure out

 

[J3J33J333]

Theoretically, one of the most efficient means of accelerating in emty space is a photon rocket:

https://en.wikipedia.org/wiki/Photon_rocket

Note that a container full of light would in fact have a mass related to its total energy content.

What can i look up to calculate the mass?

 

[PeroK]

What can i look up to calculate the mass?

The mass, ##M##, of a box of light in the centre of momentum frame of the box is given by:
$$Mc^2 = E$$ where ##E## is the total energy of all the photons in the box.

Note that if you have only one photon, then there is no centre of momentum reference frame. And this invariant mass, ##M##, cannot be defined.

 

[DaveE]

I was basically wondering if anyone knew about materials that have seriously low photon attenuation with good internal reflection.

You might be interested in the work of Kerry Vahala, a former classmate and one of the smartest people I ever met. He can build small optical resonators with Q → 10¹¹, similar to "whispering galleries" IIRC. They're not for bulk energy storage so much as metrology.

BTW, the materials are basically glass and such. Also, it doesn't take too long for light to bounce around a resonator 10¹¹ times.

 

[Filip Larsen]

You can use the rocket equation to figure out the (theoretical) delta-V possible with pure photon exhaust by setting exhaust speed ##v_e## equal to ##c##, the speed of light in vacuum. A PeroK mentions, you can relate the corresponding propellant mass (e.g. the trapped photons or corresponding matter/anti-matter) with the energy "required" to fuel up the rocket for a particular mass ratio using ##E = mc^2##.

For instance, if you would like to know how much "photon energy" ##E_p## is needed for a particular ##\Delta v## for a spaceship with mass ##m_f## after the "burn", then the relationship can be derived to be ##E_p = m_f c^2 (e^{\Delta V / c} -1 )##. Propelling something to, say, 1% of ##c## would thus take around 900 TJ per kg of spaceship. Note that for high delta-V's you need to consider a relativistic rocket to find the actual "physical" speed after such a burn.

 

[sophiecentaur]

no

He's totally right and that "no" makes storing light in a bucket, to use later, a real non-starter, even with a very shiny bucket.

There's an arm waving objection to the whole notion and that is that light is an essentially kinetic form of energy which is being dissipated or absorbed all the time. Using mechanical kinetic energy for transport is pretty uncommon (apart from freewheeling down hill on your bike and your momentum taking you to nearly the top of the next hill). The Gyrobus was one of those rare examples.

Using a photon beam, on the other hand, is one of many contenders for long distance travel with high final speeds, after many years of the drive being switched on. But the photon beam has to be generated at the time it's used.

 

[berkeman]

even with a very shiny bucket