Special Relativity I

topsquark

Forum Staff
Apr 2008
3,007
635
On the dance floor, baby!
This is a post from
GatheringKnowledge


I have some issues with the Special Relativity and the general understanding of light in mainstream science. For now, I would like to hear an opinion about 2 of my thought experiments regarding the nature of light and 1 scenario, which deals with the problem of Lorentz Transformation in relative motion.

1. Let's imagine a light source, placed inside a hollow sphere with a perfect mirror, as it's inner surface. My question is: is it possible, to explode the sphere, due to increasing pressure of light, which is being constantly emitted by the source inside it?

- from what I understand, according to mainsteream science, the answer is: YES - if light will be emitted long enough and all photons will be reflected by the inner surface of sphere, energy density within a limited space, will grow to the point, where the sphere will explode, similar to a baloon. Of course, the greater will be the intensity of emitted light, the faster the sphere will explode, but even at low intensity, at some point in time, the inner pressure of energy will eventually reach the level, which is required to make the sphere explode - it will just take a longer time...

- in my opinion, if we won't change the intensity of emitted light, energy density in a finite space will remain constant in time. There's no such thing, as density of photons - energy of light is expressed by amplitude and by frequency of electromagnetic waves. Photons don't interact with other photons, so light emitted from 2 different sources won't affect eachother, while passing through the same space. The same goes for light emitted by 1 source, which is being reflected back by a mirror - 2 photons can pass through a single Planck's lenght and it won't affect any of their properties. Of course, it's been proven, that since photons have their own momentum, they will induce a force on matter, while being reflected, pushing back the reflecting surface in direction opposite to their new motion path (this is how the "light sail" works), so it's theoretically possible, that the sphere will explode, if emitted light will have proper intensity - but it won't be possible to cause the explosion using light at lower intensity...
[/QUOTE]
 
Last edited:

topsquark

Forum Staff
Apr 2008
3,007
635
On the dance floor, baby!
Since mirrors have to be made from real materials there will always be a point where they will heat up and explode since they don't reflect the light rays perfectly.

Theoretically, if we have a perfect mirror the energy density will continue to climb without any effect on the sphere.

-Dan
 
Aug 2018
54
2
Neural Network of the Universe
Theoretically, if we have a perfect mirror the energy density will continue to climb without any effect on the sphere.

-Dan
My issue is with the idea, that energy density will grow in a finite volume of space, due to continuous emission of light. Your explanation is in 100% valid, but only if that finite volume of space is filled with somekind of medium (like gas) - in such case photons will heat up gas particles, resulting in the increase of temperature over the entire volume. My mistake, that I didn't tell about the space inside hollow sphere being a perfect vacuum.

Second thing is about the process which could theoretically cause the explosion. You are telling about explosion, due to increasing temperature of the mirror surface - while I was thinking about the mechanism, which allows us to use a light sail, to accelerate a spacecraft (light pushing back the mirror's surface), as it will work even if we would somehow get a 100% perfect mirror
 
Oct 2017
598
306
Glasgow
1. Let's imagine a light source, placed inside a hollow sphere with a perfect mirror, as it's inner surface. My question is: is it possible, to explode the sphere, due to increasing pressure of light, which is being constantly emitted by the source inside it?

- from what I understand, according to mainsteream science, the answer is: YES - if light will be emitted long enough and all photons will be reflected by the inner surface of sphere, energy density within a limited space, will grow to the point, where the sphere will explode, similar to a baloon. Of course, the greater will be the intensity of emitted light, the faster the sphere will explode, but even at low intensity, at some point in time, the inner pressure of energy will eventually reach the level, which is required to make the sphere explode - it will just take a longer time...
Yep, although explode is perhaps too dramatic... it will just yield to increasing radiation pressure forces until it breaks.

- in my opinion, if we won't change the intensity of emitted light, energy density in a finite space will remain constant in time. There's no such thing, as density of photons - energy of light is expressed by amplitude and by frequency of electromagnetic waves.
This is classical thinking. Photons are discrete quanta where light is interpreted as particulate and it makes total sense to ask what the number density of photons is. Photons are bosons and will presumably follow Bose-Einstein statistics.

Photons don't interact with other photons, so light emitted from 2 different sources won't affect each other, while passing through the same space.
Photons cannot interact directly, but they can interact indirectly due to photon shadowing (intermediate pair-production).

Here's my hypothesis:

If the number of photons is increased steadily over time within the interior of a sphere surrounded by perfect mirrors, the expectation value for the number of events involving things like pair production will increase. The photons will interact indirectly with each other, changing their energies. The result will be some distribution (probably a Bose-Einstein distribution, but I'm not certain). Eventually, some photons will be able to create real particles in pair production rather than virtual particles because their energies will be high enough and then you'll start heading towards a matter-radiation coupled system...
then a plasma... you might even get some fusion reactions going
then a degenerate plasma,
then a nucleon degenerate plasma (i.e. like the interior of a neutron star),
then a quark-gluon plasma,
then a singularity.

From that point on, all of the new photons would get immediately nabbed by the singularity and it will steadily increase its mass over time, for infinity.
 
Aug 2018
54
2
Neural Network of the Universe
Photons cannot interact directly, but they can interact indirectly due to photon shadowing (intermediate pair-production).

Here's my hypothesis:

If the number of photons is increased steadily over time within the interior of a sphere surrounded by perfect mirrors, the expectation value for the number of events involving things like pair production will increase. The photons will interact indirectly with each other, changing their energies. The result will be some distribution (probably a Bose-Einstein distribution, but I'm not certain). Eventually, some photons will be able to create real particles in pair production rather than virtual particles because their energies will be high enough and then you'll start heading towards a matter-radiation coupled system...
then a plasma... you might even get some fusion reactions going
then a degenerate plasma,
then a nucleon degenerate plasma (i.e. like the interior of a neutron star),
then a quark-gluon plasma,
then a singularity.

From that point on, all of the new photons would get immediately nabbed by the singularity and it will steadily increase its mass over time, for infinity.
Thanks! I think, that this is what leads to the idea of so called "kugelblitz"
.

However, when it comes to this particular scenario, I see a serious problem with this part:
"The photons will interact indirectly with each other, changing their energies".

I know, that matter creation through photon-photon collision is a scientifically proven fact - but it is still impossible for 2 photons to directly collide with eachother.

However, according to Steinberg, it’s not as simple as two solid particles bouncing off each other. Light particles are both chargeless and massless, and must go through a quantum mechanical loophole (literally called a quantum loop) to interact with one another.“That’s why this process is so rare,” he says. “They have no way to bounce off of each other without help.”

And since in the discussed scenario, interior of the hypothetical spherical mirror is a perfect vacuum, there's no way, that emitted photons would collide with eachother - at least, until the energy of emitted light won't exceed a gamma-ray burst...
 
Last edited:

topsquark

Forum Staff
Apr 2008
3,007
635
On the dance floor, baby!
There is no skeletal photon - photon interaction. (Skeletal = a two point diagram by exchange of a virtual particle.) However it can be done. See here. The Feynman diagram is on the right. The wavy lines are photons and the straight lines are any charged particle. (Generically the charged particles are all electrons.) However it is a four point diagram and is very difficult to detect.

-Dan
 

Pmb

PHF Hall of Fame
Apr 2009
1,579
333
Boston's North Shore
Since mirrors have to be made from real materials there will always be a point where they will heat up and explode since they don't reflect the light rays perfectly.

Theoretically, if we have a perfect mirror the energy density will continue to climb without any effect on the sphere.

-Dan
A real mirror also radiates thermal energy so it won't explode.
Anyway, it'd melt, not explode.
 
  • Like
Reactions: topsquark
Jun 2016
1,239
588
England
There have been suggestions of lightsails, that are being postulated for interplanetary (and even interstellar) propulsion
via the reaction of the lightsail to the photons provided by a powerful laser.
Small scale proof of concept experiments have even been performed.

I suppose that a spherical "balloon", made of the ultra-light highly reflective materials used for these lightsails,
could be made to inflate by photon pressure.

If we inore all practical considerations (melting, exploding, etc..) then I guess that I would agree with Benit
eventually, as more and more energy were pumped in, you would create a black-hole.
 

Pmb

PHF Hall of Fame
Apr 2009
1,579
333
Boston's North Shore
There have been suggestions of lightsails, that are being postulated for interplanetary (and even interstellar) propulsion
via the reaction of the lightsail to the photons provided by a powerful laser.
Small scale proof of concept experiments have even been performed.

I suppose that a spherical "balloon", made of the ultra-light highly reflective materials used for these lightsails,
could be made to inflate by photon pressure.

If we inore all practical considerations (melting, exploding, etc..) then I guess that I would agree with Benit
eventually, as more and more energy were pumped in, you would create a black-hole.
Those light sails will never work. The acceleration is way too small.
 
Aug 2018
54
2
Neural Network of the Universe
The general idea, that light is capable to cause a push on a reflective surface, is experimentally proven - although I don't know, if it can be actually used in space travel.
My main issue is with the idea, that the pressure of light will continue to grow inside the empty sphere, even if the source will keep emitting light at a constant energy level...
I don't think, that photons are capable of creating matter without the involvement of other particles - at least I didn't find nothing, that would suggest so.