Antony-Jones

hi Alien, I'm not sure what you mean. You say : but then you admit : ie What you call the 'scientific' definition, I found in the dictionary. So it can't be a matter of choosing between the one or the other, can it ? One contains the other. Secondly, I don't agree that only the second definition is 'scientific'. The first concerns sensation and hearing, (as well as sound-waves). It is therefore the subject of scientific enquiry in a number of fields, such as biology, psychology, neuro-biology, anatomy and acoustics. Ant.

It seems to me that the question is highly ambiguous, because the word 'sound' has two very different but common definitions; the definition you use affects the answer you get. from dictionary.com : sound: 1. the sensation produced by stimulation of the organs of hearing by vibrations transmitted through the air or other medium. 2. mechanical vibrations transmitted through an elastic medium... So Sisyphus, you and Klaynos are using the second definition, aren't you, which means that the tree hitting the ground causes vibrations in a medium, and that this happens irrespective of observers ? So the answer is 'yes, it does'. Whereas if one chooses to use the first definition, that sound is an sensation, (requiring the existence of an organism using sense organs to experience the sensation), one can truthfully say that the falling tree does not cause a sound, because the question specifies that there are no auditory sense-organs (I could have said 'ears') within range (earshot) of the physical waves, and therefore no sensations. So yes are and no are both correct, depending on what you mean by 'sound'. Does that make sense ?

I don't understand , can you explain what you mean there, Klaynos ? How would "no it doesn't" contradict the laws of physics ? And thanks for the links, haven't had a chance to look at them yet. And thanks to you and the others for being patient with a non-scientist's stupid questions.... Ant.

cap'n said: Yes indeed, that's the question, extrapolated to include every sense, not just sound. Do you happen to know the answer to it ?

now that's funny, sisyphus. Immediately after I've concluded that light is invisible, your "slightly different way" of putting it is that light is the only thing that is visible. And yet, your comment also makes sense. I'm beginning to conclude that the nature of light is somewhat contradictory.

"But please ask a real physicist, because I'm probably lying to you." You fraud. You will be hearing from my solicitors. But in the meantime, have another question : The thought experiment continues. You're on your spaceship returning to Earth, when you accidentally hit the "Planetary Destruct" button instead of the retro-rockets, thereby wiping out all life on Earth. Assume no life exists elsewhere in the universe. And sooner or later, you die too. (Sorry, this isn't the most fun thought experiment in town, is it ?) At the precise moment of your expiry, what happens ? There are no longer any eyes for photons to go into. So, does the scene change ? Does the universe become invisible ? In an instant ? In what sense could it be visible, if there are no eyes ? What do you reckon, Cap'n ? Ant.

Fair enough. Do you know why it doesn't ? And.. er.. doesn't this mean... If you shone a light on the Invisible Man, you couldn't see him. Which is why he was The Invisible Man. So, iIf you shine a light on light, but you still can't see it.... doesn't that make light The Invisible Light ?

OK, another question.... I seem to remember the thought-experiment in the book continued thus... You've turned on the first searchlight, but you can't tell whether it's on or not because you can't see it. So you do what you normally do when you want to see something - you shine a light on it. You turn on another searchlight below the spacecraft to intersect with the first beam in front of you. What happens, if anything ? Does the scene change ? Can you see the first beam in the light of the second ?

hi guys, thanks, OK, I'm getting there; I switch on the light, which initiates a stream of photons that I can't see because they're travelling away from me and so not going into my eye. So they're invisible to me. But, if I can get the photons to go in my eye, either by bouncing them off an object, or putting my eyes directly into the beam, I will see them. They'll be visible to me. Light is visible if goes in my eye, otherwise it's invisible, is that right ? That seems obvious and weird at the same time.

Help, please. I read somewhere recently (I've forgotten, unfortunately) that light was actually invisible. The reasoning was a thought-experiment. Imagine that you're on a spacecraft in deep space. You're in the cock-pit, looking out. There's a powerful search-light on top of the craft, currently off, pointing in the direction you're looking, You look out at the scene, and then you turn on the light. Does the scene change ? The book suggested that it wouldn't. The beam of light itself would be invisible, you wouldn't be able to see it, and in fact you wouldn't be able tell whether the light was actually operating. Is this correct ? If so, what does it mean ?