Hi everyone, welcome back to Cody's Lab.
Sorry about the sound quality, I am working on an underground concrete room,
but that is because I have been trying to measure something which is incredibly weak.
And that is the pressure
put on a reflector by bouncing photons of light off of it.
You see: photons, of light, carry energy, therefore they carry mass,
e=mc²
and when they hit something, they impart a little bit of momentum as they bounce off.
If they are absorbed, they also impart momentum,
but half as much.
Just like throwing a ball at something: if the ball sticks, it imparts less momentum than if the ball bounces.
Theoretically, you could take advantage of this and use either high-powered lasers or the Sun
to accelerate spacecrafts up to high velocities by bouncing the light off of them.
The spacecraft, of course, wouldn't have to carry any of its own fuel.
And it does work: it has been proven to work out in space using solar-sail spacecrafts.
But the effect is quite tiny, and measuring it down here on the Earth
is difficult even in the most advanced labs.
So about a week ago, I published a video were I was pushing a piece of aluminum foil around
in a vacuum chamber using my high-power laser.
It seemed to me that I was using the light pressure to do it.
But a lot of my friends pointed out that it was probably mostly actually
the same thing that causes a Crookes radiometer to spin,
and that is the fact that the foil heats up
and the particles of air, since I didn't have a perfect vacuum,
bounced into it, since it was hot,
gained momentum, as they flew off,
imparting force onto the sheet,
which was probably more than the amount of force put on by the photons themselves.
So I've had to redesign everything:
in fact, I have got a device over here,
which is incredibly sensitive.
And so here it is: I built a balance beam,
with an incredibly long bar, as you can see here,
balanced on a knife edge,
those are razor blades there.
I have got a magnet and a lead counterweight over here, along the bar of silver:
this is to dampen any motion so that vibrations even out quickly.
You know, eddy currents
only occur when you got relative motion,
so if there is no relative motion it will just settle out.
And a thin wire pointing at
lines drawn on individual pages of a book.
All to measure the tiny amount of force that this 8 W laser will apply
to this little tiny mirror that I have got inside of this evacuated glass bottle here.
So the idea here is that the glass will actually catch those particles that fly off of the mirror
and run into the glass, thus canceling out any momentum that they would be applying to the mirror.
And since photons can go through the glass,
I should be only detecting the radiation pressure.
Now to make sure that I have actually got that effect, I have got a piece of activated charcoal in there:
that's to absorb photons.
Theoretically I should get half the amount of displacement here with that,
and I also have left a little bit of room so that I can shoot the laser through the bottle without hitting anything.
So by comparing those three measurements I should be able to determine the exact amount of pressure that the light is applying.
Now of course I can put a tiny little piece of foil on here
to calibrate the scale.
In fact, I think I am going to do that right now.
I have cut out a 9 cm² piece of aluminum foil.
There is my 1 mm² piece of aluminum foil.
It should weight only around 50 µg.
Let's see if I can get it onto this scale.
I am going to set this camera down, looking through this telescope.
Okay, the camera is in focus,
and I have got a value there.
Let's see what happens when I set the foil on there.
Okay, I think I have got it, now let's let that settle back up
and see what our value is.
Alright.
I think I can use that information.
So it would appear that I have got this balance set up that is so sensitive
that just putting that tiny piece of foil on there
was enough to bring this down by several pages worth of distance.
Which is good,
because as it happens,
my calculations for the amount of thrust that this laser will produce
is about 1/10 of what this piece of foil weights.
Not exactly a lot…
But I think I can do it.
I have replaced the camera,
in that way I can film in two locations at once.
Let's turn on the laser, and see what it is pointing at.
Okay, I have got it aimed right at the mirror inside there.
I actually want to aim the laser off the mirror for the first test.
That way, I can cancel out any effect that the wires are going to have
electromagnetically on the metal there.
See, now I have just got it aimed at the aluminum foil.
That shouldn't hurt anything.
I reviewed the footage and it looked like when I put the 50 µg weight on the scale
it only moved down by 6 pages worth of distance on the paper ruler.
And since I have calculated that the laser is at most going to thrust the equivalent of 5 µg,
just air currents of any kind are going to move that needle more than the half of page I should expect.
So I have moved outside of the room,
I have let everything calm down in there,
and I think that the only thing that should be happening now is seismic vibrations moving the needle,
so I will be very still while I plug this in.
I am going to be turning the laser on remotely.
Okay, I am going to hold still for a little while,
I think I am going to try turning the laser on for 10 seconds at a time,
because [in other tests that I have done in the same way, I have seen the maximum displacement of the needle after 10 seconds].
Here we go!
The laser is on.
... 4,
... 4, 5,
... 4, 5, 6,
... 4, 5, 6, 7,
... 4, 5, 6, 7, 8,
... 4, 5, 6, 7, 8, 9,
... 4, 5, 6, 7, 8, 9, 10
Okay.
That may not have been exactly 10 seconds but,
close enough, yeah
Hopefully with the laser aimed at the floor, it shouldn't have moved the scale at all.
We'll find out.
[Background, quietly] Laser's on.
[Background, quietly] *counting*
I looked at the footage and it appeared that the needle moved hardly at all,
just a few vibrations due to seismic activity.
So now let's move this laser so that it is pointing at the bottle.
So I will have the laser go through the glass, without actually hitting the mirror inside.
That way we can see what the laser does to the glass itself.
I have got my glasses over the camera so that you can actually see what is going on here.
See, if I turn the laser on,
you can see that the beam is off to the side of the bottle.
Okay, so now we should be able to cancel out any effect that is just from the glass.
Shooting through the bottle in 3, 2, 1,
1, 2, 3, 4, 5,
6, 7, 8, 9, 10.
So now I have got the laser pointed at the charcoal,
so the photons should be being absorbed.
Let's see if we can actually see any movement on the scale now.
1, 2, 3, 4, 5,
6, 7, 8, 9, 10.
It's time for the final test,
where I have actually got the light bouncing off of a mirror inside the bottle.
And with any luck, this should produce twice the effect
that the light on the carbon produced.
Here it goes.
1, 2, 3, 4, 5,
6, 7, 8, 9, 10.
There it is, I hope it moved more than it did last time.
Let's actually open the door,
see if I can show you guys what is going on in there without, going in.
Now you are seeing there,
you are seeing that I am just turning on the laser
by touching these wires.
Let me turn the camera in
You guys should get to see
what I am doing.
Here we go.
Let's go ahead and look at it.
I have just been reviewing the footage and it appears
that my experiment was a total success:
I saw exactly what I expected.
The light bouncing off the mirror
moved the needle by about half a page,
whereas the light being absorbed by the charcoal moved it by about a quarter of a page.
I did notice that the light going through the bottle itself did move the needle very slightly,
but I guess I expected that, because it is heating the glass a little bit.
I am pretty stoked though, because this took me so long
to be able to actually measure.
I have had to tinker with this thing for days.
Here is the equation for figuring out how much force
photons of light apply.
If you take the amount of power, in Joules per second or in Watts,
and divide that by the speed of light,
then you get the amount of force in Newtons that that light will be applying if it is hitting a target and being absorbed.
If it is being reflected, then you double that number.
That works out being .05 µN for an 8 W laser
bouncing light off of a mirror.
We are about at 5 µg,
the same amount of force that would be applied by reflecting 16 in² of sunlight off of a mirror.
If you would like to make a similar balance by your own I will put a link on the description,
to where you can see the plans for it.
Keep in mind that I have made quite a few modifications,
like the dampener and stuff.
But anyway, about the first video that I was talking about.
I ended up taking the video down and making it unlisted because
if I am going to have one of the most popular videos on the subject on YouTube,
I figured it may as well have correct information.
So it is unlisted, I will put a link in the description if you would like to see it,
but that's why I took it down.
Hopefully, I didn't do anything majorly wrong here,
if I did, please let me know quickly.
But here we go!
I hope you all enjoyed,
I'll see you next time!