# Travelling light...

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Hello all.

Distance from earth to moon is 384,000 kilometres.
Time light takes to travel that distance = over 1 second.
1 second of time is discernible.

A space probe travelling to somewhere happens to be at a location with visual of both earth and moon.
If a laser beam is directed from the earth towards the moon during 1 second, an observer at the probe with visual of both the earth and moon should see the 'rod' of light advancing between earth and moon during 1 second, or will appear instantaneous ?

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You can't see laser beam in cosmos as there is no medium.

You can see laser beam in air just because some photons are bouncing from air molecules and diffuse (effect is the strongest in fog or dust i.e. microscopic water molecules ).

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1 hour ago, Externet said:

If a laser beam is directed from the earth towards the moon during 1 second, an observer at the probe with visual of both the earth and moon should see the 'rod' of light advancing between earth and moon during 1 second, or will appear instantaneous ?

Apart from the practical detail that you couldn’t see it, yes it would advance across the distance in 1 second

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This is really dependent on where the probe is relative to the Earth and Moon.

Assuming, you could see a "muzzle flash" caused by scattering of light from the laser, and then a reflection off the Moon when the beam hit:

If you are positioned so that the Earth- Moon line is at a right angle to your line of sight and are an equal distance from Earth and Moon, then 1 sec after the muzzle flash you would detect the reflection off the Moon.

However, if your line of sight is along the Earth- moon line (and neither of these bodies block your view of either flash or refection), Then:

If the Moon is on the far side of the Earth, you will see the reflection 2 sec after the muzzle flash, as the refection has to travel to the Moon and back to the Earth before it gets to you,while the muzzle flash only has to travel from the Earth.

If the Moon is on the side towards you, you will see both at the same time, as light of the muzzle flash will be passing the Moon at the same moment as the reflection is produced at the Moon.

Other viewing angles to the laser path line will produce results between these extremes.

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9 hours ago, Externet said:

If a laser beam is directed from the earth towards the moon during 1 second, an observer at the probe with visual of both the earth and moon should see the 'rod' of light advancing between earth and moon during 1 second, or will appear instantaneous ?

The nearest thing to this is where we can see the light reflected from a supernova as it travels through clouds of gas and dust. For example: https://www.space.com/38731-exploding-star-echo-of-light-hubble-telescope.html

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Witnessing the speed of light visually.  The radius increasing 300,000Km every second.   What is commonly referred a interstellar 'gas' and 'dust' can be 'particles' the size of planets and asteroids reflecting the light, right ?

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On 12/25/2019 at 11:25 AM, Sensei said:

You can't see laser beam in cosmos as there is no medium.

@koti: Did you see3 this? Nobody objected.

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On 1/16/2020 at 7:45 AM, Eise said:

@koti: Did you see3 this? Nobody objected.

You have to forgive me Eise, I lost count at see1.

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No medium in space, that's nonsense😴

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1 hour ago, Dr.Krumpet said:

No medium in space, that's nonsense😴

!

Moderator Note

If you wish to make your case for space being a medium, you may open a thread in speculations where you can present your model and evidence. Otherwise, kindly do not hijack other threads, especially with non-mainstream commentary.

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