# Why one cannot push light but just in the normal direction to the ray?

1. Mar 1, 2009

### arithmetic

¿Why one cannot push light but just in the normal direction to the ray?

NAIVE QUESTION:

I understand I cannot push a light ray (I mean, to sum speed vectors) in its propagation path,
but ¿why I can certainly push it in the normal direction to the light ray?

Is there some info on this subject anywhere in the web?

Please, I am not looking for opinions nor comments, but just academic sites where the answer to this question have been scientifically developed.

I have not found any info on this matter in any text on special relativity theory and I do not understand how anyone could talk about special relativity without previously explaning such anisotropy of light.

many thanks for your help

I am overwhelmed by this extremely simple question and by the fact that I have not found any info on this matter.

Many thanks for any help, indeed.

2. Mar 1, 2009

### Bob S

Re: ¿Why one cannot push light but just in the normal direction to the ray?

The speed of light is always the same in a vacuum, about 3 x 10^8 meters per second. Einstein conceived this idea to solve a paradox that led to special relativity. This is true in any reference frame. So you cannot "push" it forward or backward in a vacuum to make it go faster or slower. If you look at a light ray in a moving reference frame, the wavelength gets longer or shorter. You can push it sideways, by bending it in a prism. You can also diffract it by using a Fresnel diffraction grating, or by using crystal diffraction (using atomic crystal lattice as a grating).

3. Mar 1, 2009

### matheinste

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Hello arithmetic.

Quote:-
----I can certainly push it in the normal direction to the light ray?----

That is not really true. If you are at the point of emission of a light signal/flash, the expanding light sphere/light rays, behave as if you were at rest with respect to them. This is true for everyone present at the emission no matter what their relative motion. This is a consequence of the speed of light being the same for all observers. So the ray normal to you is in fact, as are all the rays, moving outwards with respect to you.

So, to sum up, the light behaves as you would expect it to behave if you considered yourself at rest.

Matheinste

4. Mar 1, 2009

### lightarrow

Re: ¿Why one cannot push light but just in the normal direction to the ray?

But actually you can: light *has* momentum and you can change it; the fact that its speed doesn't change doesn't mean that you can't "push" it; even if you have a massive body travelling very fast, at speed v1 near c, in a frame of reference travelling very fast at speed v2 near c, they don't sum up in the conventional way; the composite speed V is:
V = (v1 + v2)/(1 + v1*v2/c^2)
so, if v1 and v2 are very near to c, their composite speed V is still near c, not near 2c (make some trials yourself with that formula). This is due to relativistic transformations.

A photon's momentum varies proportionally to its energy, so you can "push" light; the result is a different energy of the photons (quantum description) or a change in frequency and amplitude of the light wave (classical description).

5. Mar 1, 2009

### arithmetic

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Excuse me, but I know the literature on the subject.

Just get a torch and put its axe parallel to your body, turn it on, you will see a light ray (parallel to your body) coming out from that torch, now start walking forward, the light ray will go forward along with you and parallel to your body.
YOU PUSHED THE LIGHT RAY. YOU PUSHED THE PHOTONS.
I do not understand why you do not like the word "push" because one certainly pushed the light ray to make it mosves forward.
You pushed the light ray forward, of course, normal to its propagation path, but you certainly pushed it, so you made vectors sum with photons.

Ask yourself how that happens.

Now, that's the main question which arises when reading any book on special relativity, let me explain with more details:

Two mirrors facing each other separated a distance D

----------Mirror 1

----------Mirror 2

(D vertical), a light ray is moving vertically between them, and the WHOLE DEVICE(two mirros) is moving perpendicular to D at a constant speed v, suppose it is MOVING horizontally to the right ------->.

(Also look at michelson-morley device)

They explain special relativity by arguing that because the whole device is moving then the path of the light ray is as follows (asterisks represent photons):

...*
..*..*
.*....*
*......*

thus, when comparing this "triangular" path to the rectilinear path D between the two mirros, and the constan velocity of light c, the relativity theory comes out.

To follow such triangular path each light photon has to be pushed perpendicular to D.

AGAIN, EXACTLY THE SAME CASE THAN MY PREVIOUS EXAMPLE ON THE TORCH, THE PHOTONS HAVE BEEN PUSHED.

¿What is the physics of such photon-pushing?

None of those books on special relativity explains how those photons get pushed.

I cannot understand who can someone dare to read such books on relativity theory without asking himself about such photon-pushing.

And I have not found any book explaing in detail the physics of that.

That's why I am asking here for some sites or references on such very particualr point.

I hope I have clarified my point.

6. Mar 1, 2009

### arithmetic

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Yes, that is the fairy tale that I have read all through the literature on special relativity, but what i have not read (and I bet you also don't) are the following inquires:

(According to that fairy tale: If I push a photon it dows not increment its velocity, well,
fine, PUSH THIS PHOTONS AND ASK THE QUESTIONS WHICH ARISE FROM SUCH SIMPLE PUSHING:

Just get a torch and put its axe parallel to your body, turn it on, you will see a light ray (parallel to your body) coming out from that torch, now start walking forward, the light ray will go forward along with you and parallel to your body.
YOU PUSHED THE LIGHT RAY. YOU PUSHED THE PHOTONs and they incremented their velocity.

You pushed the light ray forward, of course, normal to its propagation path, but you certainly pushed it, so you made vectors sum with photons speed.

Ask yourself how that happens.

Now, that's the main question which arises when reading any book on special relativity, let me explain with more details:

Two mirrors facing each other separated a distance D

----------Mirror 1

----------Mirror 2

(D vertical), a light ray is moving vertically between them, and the WHOLE DEVICE(two mirros) is moving perpendicular to D at a constant speed v, suppose it is MOVING horizontally to the right ------->.

(Also look at michelson-morley device)

They explain special relativity by arguing that because the whole device is moving then the path of the light ray is as follows (asterisks represent photons):

...*
..*..*
.*....*
*......*

thus, when comparing this ^ path to the rectilinear path D between the two mirros, and the constan velocity of light c, the relativity theory comes out.

To follow such triangular path each light photon has to be pushed perpendicular to D.

AGAIN, EXACTLY THE SAME CASE THAN MY PREVIOUS EXAMPLE ON THE TORCH, THE PHOTONS HAVE BEEN PUSHED.

¿What is the physics of such photon-pushing?

None of those books on special relativity explains how those photons get pushed.

I cannot understand who can someone dare to read such books on relativity theory without asking himself about such light anysotropy .

And I have not found any book explaing in detail the physics of that.

That's why I am asking here for some sites or references on such very particualr point.

I hope I have clarified my point.

Last edited: Mar 1, 2009
7. Mar 2, 2009

### Saw

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Yours is a question that many people ask themselves and for some reason it is rarely answered . At the end of thread Questions abouut:Special Relativity, Time Dilation, Light Clock, Velocity of light, I contented myself with a certain solution. Maybe someone can comment on it.

8. Mar 2, 2009

### Al68

Re: ¿Why one cannot push light but just in the normal direction to the ray?

No, they didn't. The photons comprising the light ray in your example after walking are not the same photons that comprised the light ray initially.

No photon "changes velocity" in your example. Obviously, I can shine a flashlight and move it around, but no individual photon changes speed or direction because I moved the flashlight.

9. Mar 2, 2009

### Saw

Re: ¿Why one cannot push light but just in the normal direction to the ray?

AI 68 is giving the answer that I received from the beginning, although it took me some time to understand it. In the end, I think I understood it as follows: Light is omnidirectional within the apparatus but the opening of the apparatus only lets out the photon or ray that has the adequate direction to hit the target, the mid-point of the top mirror (otherwise, if the photon didn't have that "successful" direction, it wouldn't be there, going through the opening!). But I am no guru on SR. Is this right?

10. Mar 2, 2009

### Al68

Re: ¿Why one cannot push light but just in the normal direction to the ray?

No, they are not. The triangular path is due to using a different reference frame.

If I throw a ball straight up in the air and catch it, the ball's path is triangular to an observer driving by in a car. This is not due to the ball being "pushed".

The path of the ball to the observer in the car will look like this:

....*
..*..*
.*....*
*......*

The ball is not being pushed to the side, it is just being observed from a different reference frame.

11. Mar 2, 2009

### lightarrow

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Apart from the answers you have already received, please think about this: if you are moving with respect to the air here on Earth, does sound speed vary? No.

12. Mar 2, 2009

### Al68

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Sure, unlike light, the speed of sound is different to different observers.

To an observer at rest with the air, the speed of sound in the air is ~1100 fps.
To an observer in a jet moving at 500 fps, the speed of sound in the air (outside the jet) is between 600 and 1600 fps relative to the jet, depending on direction.

13. Mar 2, 2009

### lightarrow

Re: ¿Why one cannot push light but just in the normal direction to the ray?

Yes, my mistake, I wanted to talk about the movement of the source only. Thank you for pointing out it.

14. Mar 2, 2009

### Fredrik

Staff Emeritus
Re: ¿Why one cannot push light but just in the normal direction to the ray?

All of them do. They all explain that physics is the same in all inertial frames. If you hold the torch stationary, the photons go straight up in the torch's rest frame. If you push the torch forward, the photons still go straight up in the torch's rest frame. That's why they don't go straight up in the frame where the torch was at rest before you pushed it.

Note that you don't push the photons. You just push the torch, and the consequence is that the frame in which the photons go "straight up" is a different frame than before.

The velocities add according to this rule:

$$\vec u\oplus\vec v=\frac{1}{1+\vec u\cdot\vec v}\bigg(\vec u+\vec v+\frac{\gamma_{\vec u}}{1+\gamma_{\vec u}}\vec u\times(\vec u\times\vec v)\bigg)$$

Here $\vec u$ is the velocity of the torch in its original rest frame, and $\vec v$ is the velocity of whatever the torch is emitting (in this case photons), in the torch's rest frame.

Last edited: Mar 2, 2009
15. Mar 2, 2009

### Bob S

Re: ¿Why one cannot push light but just in the normal direction to the ray?

There is an interesting effect called Mossbauer effect spectroscopy that applies to this subject. It involves the emission of a very narrow nuclear gamma ray line (about 14.4 keV) in a Fe^57 source and its consequent absorption in another Fe^57 atom. The absorption is a very narrow resonance absorption line, corresponding to the same long lifetime excited state that emitted the gamma. Both atoms must be bound in crystal lattices to absorb nuclear recoils. If the Fe^57 source ("flashlight") is pushed either toward or away from the Fe^57 absorber ("observer") at velocities of the order of 1 cm per second or more, the Fe^57 absorber (observer) no longer absorbs ("sees") the gammas ("light"). So why does pushing the flashlight toward or away from the observer prevent the observer from seeing the light? The velocity of the light is unchanged because it remains the same in both reference frames. So what is different? The gamma ray energy (wavelength) seen by the observer has changed, and is off-resonance in the Fe^57 absorber held by the observer.

If a Mossbauer source is "pushed" toward the observer at say x cm per sec, then the observer has to move away from the source at about x cm per sec in order to absorb the radiation from the source. So "pushing" the source toward the observer has a significant effect even though the speed of the radiated light has not changed.

Last edited: Mar 3, 2009
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