# Light in Space

*Currently enrolled in University Physics 2*

We have not covered the topic pertaining to light yet, but we are working with sinusoidal waves. I would like to better understand the motion of light in space. It is my understanding that light moves in the form of waves, so what causes the wave motion? The waves we've been working with in class have involved waves within a certain medium and of course we exist within a medium, but how do waves, particularly light waves, for this motion in a vacuum? Is it a result of the electromagnetic fields that exist throughout space that cause this wave-like motion?

Please forgive any "ignorance" in my question--I don't have a complete grasp on either topic yet. Thank you.

Nugatory
Mentor
Is it a result of the electromagnetic fields that exist throughout space that cause this wave-like motion?

Pretty much, yes. Nothing is actually moving sideways, but at every point in space while the wave is traveling through, the electrical and magnetic fields are oriented perpendicularly to one another at any given time and the direction of travel, and their magnitude is varying sinusoidally with time.

(Note that what I said above does not completely specify the direction of the fields - that's what polarization is about).

Does this mean that the light wave is traveling up-and-down and side-to-side at the same time? In other words, what line is light traversing?

I have one other quandary about the subject of waves. How is it possible for a wave, like that in ocean, able to "sense" the boundary of the shore a 1,000 miles away? My professor used the example of a string wave, but does that same principle apply to all waves? I don't understand how the origin of a wave is connected to a boundary point so far away and able to adjust immediately.

Is this why physicists have a hard time believing that quantum mechanics will be able to instantaneously connect to particles across an infinite distance in space?

Thank you.

Nugatory
Mentor
Does this mean that the light wave is traveling up-and-down and side-to-side at the same time? In other words, what line is light traversing?
No. The light is traveling in a straight line. Nothing is moving sidewise, it's just that the strength and direction of the electrical and magnetic field at each point in space is varying with time.

How is it possible for a wave, like that in ocean, able to "sense" the boundary of the shore a 1,000 miles away?
It doesn't. It senses the behavior of the water directly adjacent to it. Of course that water is in turn affected by the water directly adjacent to it, and so on. If you start a disturbance in the middle of the ocean you won't see any shoreline effects until after enough time has passed for the disturbance in the water to reach the shoreline, and then for the effects of the shoreline to be propagated back to you. Although a standing wave will show the effects of a distant boundary, it takes time to set up a standing wave and stabilize it.

Is this why physicists have a hard time believing that quantum mechanics will be able to instantaneously connect to particles across an infinite distance in space?
No. That's based more on an objection to any form of action at a distance , and some conflict with relativity, which prohibits causal influences from traveling faster than light.

1 person
Drakkith
Staff Emeritus
Does this mean that the light wave is traveling up-and-down and side-to-side at the same time? In other words, what line is light traversing?

Imagine a water wave. It oscillates up and down in the water, but the wavefront moves outward and is a 2 dimensional circle since the surface is 2 dimensional.

For light, the wavefront is a 3d sphere. What is oscillating? The electric and magnetic field vectors. These field vectors tell us which way the electric and magnetic force will be exerted on something a test particle or an antenna when the wavefront passes. For example, if the wave's electric field vector is oscillating up and down relative to the earth when it hits an antenna, all the electrons in that antenna will oscillate up and down at the same frequency of the wave.

I have one other quandary about the subject of waves. How is it possible for a wave, like that in ocean, able to "sense" the boundary of the shore a 1,000 miles away? My professor used the example of a string wave, but does that same principle apply to all waves? I don't understand how the origin of a wave is connected to a boundary point so far away and able to adjust immediately.

I'm not sure what you mean here. Why would the wave sense the shore 1,000 miles away? What effect does that have on the wave while it is in deep water?

Is this why physicists have a hard time believing that quantum mechanics will be able to instantaneously connect to particles across an infinite distance in space?

Not really. That has to do with the inability to transmit information faster than the speed of light. Which is still true as far as we know. Entanglement still cannot transmit information faster than c, so no rules have been broken yet even if they are connected across vast distances.

1 person
Response

Thank you both for your responses--this has really helped clarify the points discussed in class.
Do you know where I might be able to read further on this topic in particular? I'd really like to understand this electromagnetic concept better, I find it fascinating.

I'm not sure what you mean here. Why would the wave sense the shore 1,000 miles away? What effect does that have on the wave while it is in deep water?

It was my understanding that when we were discussing waves along a string, the initial wave senses the restricted end (attached to a weight and pulley system in our case) and the wave would adjust accordingly---as opposed to if the string were not attached at the opposite end. Is this correct? I had assumed this same principle would apply to anything. For instance, in space, isn't it true that our planet is effected by the gravitational pull of the other planets, which in turn is affected by other solar systems (creating our galaxy), which in turn is affected by other solar systems--not matter how minutely the force, that affect still exists. I had assumed that "connection" would exist within any medium. Then again, maybe I'm way off base!

I understand the math for waves, but I'm just trying to understand the concept better.

Is this why physicists have a hard time believing that quantum mechanics will be able to instantaneously connect to particles across an infinite distance in space?

So then what is the theory about particles being able to communicate across space...? Please clarify my understanding, I thought that it was believed two particles on opposite ends of the universe would be able to instantaneously communicate with one another...I've heard this elsewhere, but I found this website this morning and will post the particular section along with the link.

"If the boxes are both opened at precisely the same time and the internal states are measured and still identical, then somehow the systems were able to communicate to each other their information instantaneously so they could know which internal states to agree upon. This, according to Einstein and the boys, would be impossible because instantaneous transfer of information is not possible since nothing can travel faster than the speed of light. So how could observation bring these characteristics into being? They thought they had him. But Bohr responded by saying that instead of believing that some faster than light mechanism was at work, he simply said that if subatomic particles don't exist until they're observed, then they could no longer be thought of as independent things. They were part of an indivisible system and could not be thought of as separate entities."
http://www.factmonster.com/cig/theories-universe/action-at-distance.html

Drakkith
Staff Emeritus
It was my understanding that when we were discussing waves along a string, the initial wave senses the restricted end (attached to a weight and pulley system in our case) and the wave would adjust accordingly---as opposed to if the string were not attached at the opposite end. Is this correct? I had assumed this same principle would apply to anything. For instance, in space, isn't it true that our planet is effected by the gravitational pull of the other planets, which in turn is affected by other solar systems (creating our galaxy), which in turn is affected by other solar systems--not matter how minutely the force, that affect still exists. I had assumed that "connection" would exist within any medium. Then again, maybe I'm way off base!

Yeah, you're a little off base. The string has pre-existing tension, which affects the frequency it can vibrate at. Similar to how you tune a stringed instrument such as a guitar. But this only has to do with standing waves on a string, not waves on water. Waves do not "know" anything about the shore.

Also, note that there is a difference between a static field and a wave. The static field exists at all times and does not propagate. It constantly affects whatever is within its range and has no wave-like properties in and of itself. (But the wave within the field does)

So then what is the theory about particles being able to communicate across space...? Please clarify my understanding, I thought that it was believed two particles on opposite ends of the universe would be able to instantaneously communicate with one another...I've heard this elsewhere, but I found this website this morning and will post the particular section along with the link.

It is called entanglement, from theory of Quantum Mechanics. You'll have to hit up the Quantum Theory forum here if you want to know more, as that is a little off topic for this thread.

Be aware that if you don't know much about Quantum Mechanics then entanglement will not make much sense.

It all makes sense now! Thank you! I think I must have let some important information slip that distinguishes between the difference in string waves (tension) and water waves.

If you think there is anything else that might help on the topic, please post! Cheers!