# Single ray of light

• B
IMAGINE an infinitely large space where there exists no particles or energy fields at all. If we introduce a single photon or a single ray of light, will it be visible from all angles?

mfb
Mentor
To be "visible", a photon has to interact with some sort of detector. Only at the place of this interaction it will be visible. For a ray of light you get some volume where you can detect the light - namely this ray.

Drakkith
Staff Emeritus
If we introduce a single photon or a single ray of light, will it be visible from all angles?
No. The light would only be visible to whatever it struck.

So light will be invisible unless one stands in its path (of in the above environment)?

mfb
Mentor
Yes. That is true for everything. You don’t see objects just because they are there, you only see objects if light from them reaches your eye.

sophiecentaur
Gold Member
2020 Award
To be "visible", a photon has to interact with some sort of detector. Only at the place of this interaction it will be visible. For a ray of light you get some volume where you can detect the light - namely this ray.
You could put it stronger than that, I think. It can only be said to 'exist' when it interacts with something. On the way from the source to the target, the light is best regarded as a wave (or probability density variation, if you want to do it the hard way) and where the energy will turn up (your eye, for example) is never predictable until it actually happens. Diffraction always limits the precision with which you can predict where an individual photon will turn up.

Allen_Wolf
tech99
Gold Member
IMAGINE an infinitely large space where there exists no particles or energy fields at all. If we introduce a single photon or a single ray of light, will it be visible from all angles?
How can we have a detector when there are no particles to make it out of?

sophiecentaur
IMAGINE an infinitely large space where there exists no particles or energy fields at all. If we introduce a single photon or a single ray of light, will it be visible from all angles?
In the real world, you may see the path of a ray of light (consisting of many photons) because the light is scattered from dust and air molecules. Generally this scattering will redirect the light in all directions, including your eye, and what you see is the volume of space the light is scattered from. It would be hard to see a single photon in the real world as that would be scattered in one particular direction that would likely miss your eye, and besides it would just be a single blip of light. In your hypothetical empty space, there would be nothing to scatter or redirect the light to your eye so it would not be visible.

sophiecentaur