# Why is light travelling in straight line? (see restrictions)

1. Sep 7, 2009

### sten

Why is light travelling in "straight" line? (see restrictions)

Here's a question: Why is light travelling in "straight" line?

Of course there will be those of you to jump on the question and say it's not a well defined one and they'd be right. For that purpose I'll restrict the question a bit. Consider the question limited over "small" distances and far from "extreme" gravitational fields and allow for some "small" margin of error to compensate for the rest of the causes of bias.

If that restriction is not quite enough, then consider the question as a fifth-grader would understand it, or picture the line drawn by a green laser in a dark room.

So... is the line straight? I'd assume so. So are photons traveling in the same manner? I'd assume so...

But then - there are phenomena such as interference patterns, which don't quite work if everything was plain straight, photons must be able to take multiple paths simultaneously in order to interfere with themselves. Then why doesn't that happen to the laser beam?

2. Sep 12, 2009

### sten

Re: Why is light travelling in "straight" line? (see restrictions)

I wouldn't have expected this question is a hard one, plenty of smart and educated people around here

3. Sep 12, 2009

### JesseM

Re: Why is light travelling in "straight" line? (see restrictions)

You'd probably get more answers in the classical physics section since this is more just an issue of optics than relativity, but in fact laser beams do spread out over large distances, it's impossible for light to be perfectly collimated. You could also approach it from a quantum perspective: because of the uncertainty principle the momentum of a photon can never be perfectly well-defined, so although the probability will be highest for it to be detected at a spot that's in a straight line from where the emitter was aimed, there will be some finite probability of detecting it at different angles.

4. Sep 12, 2009

### HallsofIvy

Re: Why is light travelling in "straight" line? (see restrictions)

The answer is pretty much "because that's how we define a straight line"! Our basic notion of a "straight line" is based on seeing things straight ahead of us.

5. Sep 12, 2009

### JesseM

Re: Why is light travelling in "straight" line? (see restrictions)

I don't think it's correct to say that straight lines are defined only (or even primarily) in terms of the path of light. For example, if you're standing on a flat surface a straight line can also be defined in terms of the shortest distance between points. Also, we don't always define the path of light to be "straight"--just think of refraction.

6. Sep 12, 2009

### atyy

Re: Why is light travelling in "straight" line? (see restrictions)

As JesseM says, light does not travel in straight lines - it can bend around corners. The smaller the corner is compared to the wavelength of light, the more light will bend around it. So in a given situation, the shorter the wavelength of light, the straighter it will go. In many situations in everyday life, the wavelength of light is short enough that it is an excellent approximation to say that light travels in straight lines.

7. Sep 13, 2009

### Staff: Mentor

Re: Why is light travelling in "straight" line? (see restrictions)

The ray approximation of light is just that, an approximation. Massive particles have much smaller wavelengths so the approximation is better with them than with light, provided they are moving inertially.

8. Sep 13, 2009

### sten

Re: Why is light travelling in "straight" line? (see restrictions)

I don't think "definition" is enough - it would be hard to redefine the math of space-time geometry by just changing the definition. It's a nice coincidence that we can, but you can't just pick anything laying around and say "hey, this will be my straight line".

The reason I asked this question here is because I'm wondering about how the particle-like and wave-like properties come together from (I think) a single underlying phenomena (it's a question, not a statement)

9. Nov 3, 2009

### SHAMSAEL

Re: Why is light travelling in "straight" line? (see restrictions)

First of all, over large distances, interference does become apparent. In my experience with lower quality pen type laser pointers, just shining the beam to the other end of a large room there's a noticible difference between the size of the dot on the wall and the width of the beam at its origin.

I know very little of the field of laser science, which is a branch of optics as I understand it, but as far as I do know, with higher energy lasers, there's less interference because they emit a narrower beam consisting of fewer photons but projected at a higher wave frequency, therefor a shorter wavelength.

The fact that the green laser is visible, I believe, has to do with atoms in the air being impacted by photons from the beam, absorbing energy, then emitting it.

10. Nov 3, 2009

### fhisicsstudnt

Re: Why is light travelling in "straight" line? (see restrictions)

There are a few issues here. Firstly, "straight" has no meaning without "not straight". You have to define two conceptual opposites and explicitly define how they are different. In a world where everything is straight, there is no concept of straight, and vice versa.

The second issue is that you're asking a question that is inherently about visualization (imagining light traveling straight). The problem is, nobody has any idea what light is. No visualization that I know of is capable of simulating all of what light does. So you don't really even know what you mean by "does light travel straight?" because you don't even know what it means for light to travel, because you don't know what light is.

The third issue is that nobody can watch light travel. Ever. The only thing a person ever knows about a particular photon is "it's right here (at my eye)" or "it's not here". This is the data we have to work with, light is at the detector or it's not. We can measure two-way transit times over distances etc. but we cannot actually "catch it in the act" of traveling. You can't see light, it's what you use to see.

11. Nov 3, 2009

### pervect

Staff Emeritus
Re: Why is light travelling in "straight" line? (see restrictions)

Well, if you consider a region far away from any mass, as you carefully do, then ALL particles move in a straight line as long as they aren't subject to external forces, whether the particles are photons, protons, or brickbats.

Light has the advantage of not being influenced as much by external forces, being electrical neutral, and moving very fast. So even under less ideal conditions, it moves in a fairly straight line. The biggest problem with the straightness of the line that light takes in surveying on the surface of the earth is the fact that light gets refracted by the atmospheric density changes. This causes some significant deviations from "straightness" that have to be accounted for.

12. Nov 3, 2009

### Staff: Mentor

Re: Why is light travelling in "straight" line? (see restrictions)

I disagree completely with this. Electromagnetism is the most well understood of the fundamental forces. I don't know of a single behavior of EM that is not correctly described by QED. If you know of some EM phenomenon that is not accurately characterized by QED then please post a credible reference.

13. Nov 3, 2009

### fhisicsstudnt

Re: Why is light travelling in "straight" line? (see restrictions)

QED describes how light behaves, it does not tell you what light is. It tells you the quantities you will measure in experiments, it does not let you visualize light propagating in these experiments so that you can answer whether it is propagating "straight" or not.

14. Nov 3, 2009

### Staff: Mentor

Re: Why is light travelling in "straight" line? (see restrictions)

Well here I think is the core of our disagreement. I think your definition of "is" is completely useless. If you can predict every single aspect of something's behavior and characterize its every property and all interactions and still not know what it "is" then of what possible value would be knowing what it "is". It makes no sense to me to divorce how something behaves from the concept of what it is.

15. Nov 4, 2009

### fhisicsstudnt

Re: Why is light travelling in "straight" line? (see restrictions)

A description/characterization is not unique, it doesn't guarantee understanding. I can describe the ball's motion as 9.8 without understanding. I can describe all the motions of the planets without any understanding. In fact, I can do so with a completely wrong understanding.

So yes, a description can be a very useful thing. It can also be a complete dead end. The question, then, is whether Nature is "understandable" to humans or are we relegated to simply identifying and correlating patterns in the data.

The original poster didn't ask about data or for a description. S/He asked "why" light travels straight. Objectively we can ask why X moves Y. We have to define X and Y to answer the question. If X is an equation or a set of data the question is meaningless.

16. Nov 4, 2009

### haushofer

Re: Why is light travelling in "straight" line? (see restrictions)

Can you give an example of a theory in which concepts are explained in terms of "what they are" instead of "how it behaves"?

17. Nov 4, 2009

### fhisicsstudnt

Re: Why is light travelling in "straight" line? (see restrictions)

Concepts are inherently a description. Objects "are what they are". A theory of "what it is" has to present an object (particle, fluid, string, etc.) first before it can describe the behaviors.

In older times, everything was explained in terms of discrete corpuscles. Atoms and light were all particles (what they are). But it couldn't explain diffraction, then the aether wave hypothesis couldn't explain the MM null result or quantization. I don't know of any more qualitatively different "what it is" hypotheses.

18. Nov 5, 2009

### haushofer

Re: Why is light travelling in "straight" line? (see restrictions)

Well, I don't really see the big difference between the description of light in QED and the description of other physical concepts in other physical theories :)

19. Nov 5, 2009

### fhisicsstudnt

Re: Why is light travelling in "straight" line? (see restrictions)

You don't see the difference between that which you can visualize and make a movie of to explain a phenomenon, vs. a set of equations that gives you the result of an experiment?

20. Nov 5, 2009

### Staff: Mentor

Re: Why is light travelling in "straight" line? (see restrictions)

This goes back to my previous question: if you can predict every single aspect of something's behavior and characterize its every property and all interactions and still not "understand" it then of what possible value would be "understanding" it?

However, it appears to be a purely semantic argument. From your previous responses it seems like you and I both agree on the fact that QED accurately predicts and describes all observed EM behavior. We only disagree if that qualifies as "understanding" what light "is" or not. Personally I find semantic arguments rather boring so I am not inclined to pursue the "understanding" disagreement further.
Good point. "Why" questions are often rather non-scientific. A more scientific line of questioning would have been:
Q: What underlying principle results in light traveling in a straight line?
A: Conservation of momentum.
Q: How is momentum conserved in light?
A: Due to the space-translation symmetry of the electromagnetic Lagrangian.
Q: What makes the electromagnetic Lagrangian symmetric?
I disagree here again. I think the equations of a theory are much more important than the interpretations.
I don't see the difference. I can easily visualize an equation or make a movie of it.