# Measuring the distance to the moon with laser reflections and the speed of light

#### Trailblazer

##### Moderator
Staff member
Good work.. but like Mick stated before.. we first need to know the distance to the moon for these numbers to check out.. if you put the distance to be.. hmm lets say 3000 miles.. you get "slightly" different numbers..
Well we do know the distance to the moon. Unless you also want to throw out the most fundamental constant in the universe, ie the speed of light, then it's easy to measure. You fire a beam of light at the moon, and time how long it takes for the light to bounce back.

(Photo by M3long - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3512980)

You find that it takes approximately two and a half seconds. A little less when the moon is at its closest, and a little more when the moon as at its furthest.

That's the time taken for a beam of light to travel there and back, so we multiply the time by the speed of light and divide by two:

#### Abishua

##### Member
Well we do know the distance to the moon. Unless you also want to throw out the most fundamental constant in the universe, ie the speed of light, then it's easy to measure. You fire a beam of light at the moon, and time how long it takes for the light to bounce back.

(Photo by M3long - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3512980)

You find that it takes approximately two and a half seconds. A little less when the moon is at its closest, and a little more when the moon as at its furthest.

That's the time taken for a beam of light to travel there and back, so we multiply the time by the speed of light and divide by two:

speed of light is not a constant.. it depends on the medium.. and can be slowed even in vacuum if light shape is changed.. as shown here..

"Everyone knows that the speed of light in a vacuum is constant, but now it appears that there is a way to indirectly alter its speed—by running it through a special mask. Doing so apparently causes a change to the shape of the photon, making it move through a vacuum slower than an unaltered photon.

The researchers built what they called a racetrack—setting up a dual course for firing photons and detecting when they struck a detector a meter away. The first group of photons was fired in the normal way, while the second group was fired through a filter to shape it into either a Gaussian or Bessel beam. The photons from both groups were launched at the same time but the unshaped photons beat the shape-altered photons to the finish line by approximately 0.001 percent. There were two reasons for that. The first was that moving through the filter slowed the photons, much as would happen were they to pass through other mediums such as water or glass. The second reason was more complex, because it demonstrated that the speed of the photons was slower than normal after passing through the filter—light is supposed to speed back up to its normal constant after passing through a medium. The experiment showed that light can be caused to travel slower than c, by changing its shape.

The researchers explain this result by noting that they were using group velocity to measure the light's speed—a measurement of the group's envelope speed. The mask, they explain, caused some of the photons in the group to move at a slight angle to the other's causing a slowdown for the group as a whole. Thus, their results are not going to upend one of the basic tenets of modern physics, it is more likely that future researchers will have to make sure lab or astronomical observations are not being impacted by shape changes that occur naturally."

Read more at: http://phys.org/news/2015-01-physicists-air.html#jCp

#### Abishua

##### Member
That changed the speed by .001%, or one part in 100,000. So maybe 4km?

yeah.. the filter they used changed it that much.. but isn't there a "van alans belt" out there? 60 000km thick if I'm not remembering thing wrong? Shouldn't the light pass through it 2 times.. going to the moon and back? How does this radiation belt impact light, if it impacts it at all... They certenly did not use a filter 120 000km thick.. so unless we want to dable in guesswork I cannot use that as evidence.. since speed of light clearly is proven not to be the most fundamental constant in the universe as Trailblazer stated..

#### Spectrar Ghost

##### Senior Member.
yeah.. the filter they used changed it that much.. but isn't there a "van alans belt" out there? 60 000km thick if I'm not remembering thing wrong? Shouldn't the light pass through it 2 times.. going to the moon and back? How does this radiation belt impact light, if it impacts it at all... They certenly did not use a filter 120 000km thick.. so unless we want to dable in guesswork I cannot use that as evidence.. since speed of light clearly is proven not to be the most fundamental constant in the universe as Trailblazer stated..

The researchers explain this result by noting that they were using group velocity to measure the light's speed—a measurement of the group's envelope speed. The mask, they explain, caused some of the photons in the group to move at a slight angle to the other's causing a slowdown for the group as a whole. Thus, their results are not going to upend one of the basic tenets of modern physics, it is more likely that future researchers will have to make sure lab or astronomical observations are not being impacted by shape changes that occur naturally."

Did you read this part? You posted it. Or did you just look for the first result that seemed to confirm your beliefs?

#### Abishua

##### Member
Did you read this part? You posted it. Or did you just look for the first result that seemed to confirm your beliefs?

did you read this part?

"it is more likely that future researchers will have to make sure lab or astronomical observations are not being impacted by shape changes that occur naturally."

#### Trailblazer

##### Moderator
Staff member
@Abishua , that experiment didn't alter the speed of light. That is just a simplistic explanation for laymen. What it altered was the group velocity, in other words the overall speed of a bunch of light waves.

And the experiment you describe was about making group velocities FASTER than c, which is totally irrelevant here. The speed of light does change depending on the refractive index of the medium it is travelling in. In air, it travels fractionally slower than it does in a vacuum. In glass, it travels about one third slower.

The lunar ranging experiments take into account the refractive index of the earth's atmosphere. It is equivalent to about an extra six metres of distance to the moon. http://adsabs.harvard.edu/full/1968IAUS...32...86A

The Van Allen belts have an incredibly low density of particles compared to the Earth's atmosphere, trillions and trillions of times lower, so their effect on the speed of light will be immeasurably small. In fact the entire Van Allen belts weigh about 10 grams, or less than half an ounce. How do you imagine that much matter will refract the light?

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