Question on Photons and perception

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    Perception Photons
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Discussion Overview

The discussion centers around the perception of light, particularly whether light that has passed a location can still be detected and the differences between light waves and sound waves in terms of detection. Participants explore concepts related to the nature of light, redshift, and the mechanics of wave propagation.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions if light from a distant galaxy can be perceived after it has passed by a location, suggesting that it might be possible to see the same object at different times from different positions in the sky.
  • Another participant challenges the idea by stating that once light has passed a location, it cannot be seen because it has already moved away.
  • Some participants express curiosity about the fundamental properties of light waves that limit detection to when light is moving towards an observer, contrasting this with sound waves that can be heard from multiple directions.
  • There is a discussion about the nature of light as electromagnetic waves and the conditions under which they can be detected, emphasizing that light must enter the eye to be perceived.
  • One participant uses an analogy involving water waves to illustrate that waves must interact with an observer to be detected, reinforcing the idea that light waves, like other waves, cannot be perceived once they have passed by.

Areas of Agreement / Disagreement

Participants generally disagree on the possibility of detecting light that has already passed by a location. While some seek clarification on the properties of light, others assert that light cannot be seen once it has moved away.

Contextual Notes

There are unresolved questions regarding the fundamental nature of light and its detection, as well as the comparison to sound waves. Participants express varying levels of understanding about these concepts.

Ryanw58
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Its my understanding that a photon will travel and exist in space forever. My question is if light, whether it be from a distant galaxy or star, can be perceived after it has passed by our location in space. To me it seems a confusing subject to think about but i assume you can see a beam of light from both directions, traveling towards your direction and past you? For example, If the light from a galaxy that was billions of years old passed through our location in space before humans were here to see it, it would continue on its path through space. Once humans are here and able to detect light wouldn't we see not only the light from that galaxy as it is currently from its source but also the light that has passed us going in the opposite direction ? This implies that we could see the same object in two different spots in the sky from two different times. I also understand redshift says that the light from a moving object is stretched in wave length to the red end of the spectrum, but is it possible that as light passes your location towards the other direction that it also is stretched to longer wavelengths as it travels away?

I understand i must be missing a fundamental concept of the way light works, so I'm just looking for a answer that will explain why this won't work. Thank You
 
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If the light has passed you, how can you see it?
 
I guess that was my question, once light passes your location is it possible to detect it going away from you? If not why? I realize this sounds like a stupid question but I'm curious.
 
In order to detect light, it must hit you. If it's already passed by you and moving away, you missed your chance.
 
I understand that you have to detect the light, but what is it about the light wave that makes it only detectible from head on as opposed to going away from your direction. For example, and i know light waves are not sound waves, but you hear sound coming towards you and also away. What makes light waves different in that aspect?
 
Ryanw58 said:
For example, and i know light waves are not sound waves, but you hear sound coming towards you and also away.
No you don't. You might hear the sound emitted by something moving away from you, but the sound itself is moving towards you. Else you won't hear it. Same for light.
 
Understood but what's the scientific reasoning of why light cannot be seen unless directly looked at. What are the properties of a light wave that make it detectable from only straight on and not from behind or the sides or any other angle.
 
Ryanw58 said:
Understood but what's the scientific reasoning of why light cannot be seen unless directly looked at. What are the properties of a light wave that make it detectable from only straight on and not from behind or the sides or any other angle.
No, not understood I'm afraid. Any wave - even in photon description - is detectable from all angles as long as it comes towards you. Try again to analyse better the example of sound: you can hear a sound wave coming at you from all angles, but of course you cannot hear a sound wave going away from you. And also the same for water waves. And even the same for bullets: a bullet that is going away from you can't hit you!
 
Ryanw58 said:
Understood but what's the scientific reasoning of why light cannot be seen unless directly looked at. What are the properties of a light wave that make it detectable from only straight on and not from behind or the sides or any other angle.

I think you have a misunderstanding of what light is. An EM wave, or light wave, IS light. You cannot see the wave because you are using it to see in the first place! You cannot "look at the light", you can only look at an object that the light has bounced off of or an emitter that is sending out light.

If the wave isn't able to enter your eye, which is transparent to the wave, then it is absorbed by your skin or whatever is blocking your view.

Imagine you are in a boat on the water. A large wave comes by and raises you in the air before setting you back down. This is analogous to an EM wave passing by. Unless you have something past you to reflect the wave back, neither the water wave nor the EM wave will come back and interact with you again. The wavefront has already passed!
 

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