IN ALICE LAW’S LIGHT EXPERIMENT

WHY WILL LIGHT CHANGE DIRECTION?

Han Erim

3 November 2010

INTRODUCTION

The speed and behavior of light traveling toward a moving target has never been experimentally measured or examined under laboratory conditions. Alice Law’s light experiment is a setup that offers us this possibility. The experiment is based on reflecting light between two parallel rotating mirrors and projecting it onto a target. Depending on the rotational speed of the mirrors, the point where the light arrives changes.

In this article I will explain to you why light will change its direction. Later in the text, I have prepared a big surprise for you. I leave it to you to judge how important this subject is.

To simplify our reasoning, let us use the following setup. We reflect light several times between two parallel mirrors. The light source is mounted onto the apparatus, and we can move the entire device left and right.

When we examine the situation from the reference frame of the device, we immediately observe the following: moving the device does not change the path of light between the mirrors in any way. This is a natural result, because physical laws do not change in uniformly moving systems.

This information alone is sufficient to understand why light changes direction in Alice Law’s light experiment. The longer we can keep the light between the mirrors, the further we can shift the light’s arrival point. Therefore, it is obvious that in Alice Law’s light experiment, the light will indeed change direction.

However, there is an important detail we must examine. Here the following question arises: How does light know that the mirrors are moving?

Indeed, when we analyze the event from the reference frame of the ground, it is clear that the light behaves in accordance with the reference system of the apparatus.

Classical Mechanics answers this question as follows: Depending on the motion of the apparatus, light carries an additional momentum in the direction of motion besides its normal propagation direction. Because the light source is attached to the apparatus, the light acquires this momentum, and thus its path between the mirrors remains fixed.

Whether the light source is physically mounted on the apparatus or not is not very important. At the first reflection on the mirror, the light will acquire this momentum.

As we can see, discussing whether light will change direction in Alice Law’s light experiment is unnecessary — light will change direction.

But here we must not overlook something very important: Because light does not carry momentum.

From this point onward, you will see how interesting and important this topic actually becomes.

Can light carry momentum?

Consider two bodies in space, A and B, moving relative to each other. There is a light source (a flashlight) on B, and the flashlight is oriented perpendicular to the X-axis.

Now let us assume we are on object A.

Question: Can we know the speed of A?
Answer: No. We can only know our speed relative to B.

Question: Can we assume A’s reference frame is stationary?
Answer: Yes. We do not even know if we are moving. We may say A is stationary and B is moving.

Question: Where must the flashlight turn on for its light to reach A?

This question has two answers:

1) If light carries momentum, the flashlight must turn on before reaching X=0 according to A, and the light must travel along the blue Y-axis. (This is the classical mechanics interpretation.)

2) If light does not carry momentum, the flashlight must turn on exactly at X=0, and the light must travel along A’s red Y-axis. (This is Alice Law’s view. Einstein’s physics has no clear answer for this.)

As we see, there are two possibilities. The strange part is that until now, it has never been clearly determined which one is correct.

How can we measure whether light carries momentum?

We answer this question using the same two-body example. Now let B be our reference frame and assume B is stationary. In this case A is moving relative to B.

Changing the reference frame does not change the event. Therefore the two possibilities remain:

1) If light carries momentum, the flashlight must turn on before A reaches X=0, and the light must move along B’s blue Y-axis.

2) If light does not carry momentum, the flashlight must turn on at X=0, and light must travel along A’s red Y-axis.

From this analysis we conclude: According to B, if light carries momentum, it follows a straight path. If it does not, light changes direction. (Figure 9)

Now we can discuss how to detect this difference experimentally.

How can we experimentally detect whether light carries momentum?

We place a rotatable disk at the center of a square platform. Since no reflection is used in this experiment, no mirror is required. Our goal is to photograph the light falling on the platform.

Let us apply the same logic here. Let B be the light source and A the rotating disk. (Figure 10)

While the disk rotates, a very thin beam of light from a distant source is projected onto the entire platform, and its photograph is taken. This is Alice Law’s Light Experiment.

1) If light carries momentum, the light streak falling on the platform will not be distorted. Both the outer platform and the inner disk will show straight, aligned lines. (Figure 11)

2) If light does not carry momentum, the light streak on the disk will shift in the direction of rotation. (Figure 12)

Conclusion and Discussion

I have not discussed the connection of this experiment to Alice Law here, because this is unnecessary at the moment. Beyond debates such as “Alice Law says this, Einstein says that,” it is evident that this experiment is fundamental enough to redefine the general theory of physics.

I would like to state here that this experiment will result in favor of Alice Law.

And the final question:

Who will perform this experiment?

Han Erim

I prepared the following animation so that you can examine the details. You can download the source code here.