# What is light, and how can it travel in a vacuum forever in all directions at once without a medium?

I know there are many questions that are similar (maybe identical?). I am not a physicist nor a student - I am just interested in physics and have been watching many physics channels on youtube recently in my spare time.

Here is a (nearly) identical question: Why don't electromagnetic waves require a medium?

Here is one response to this question :

Well, I would say the electromagnetic field is the medium.

I don't understand this - How can something be the medium for itself?

The answers to these questions I don't really think address specific points of confusion I am having - so I am going to try to ask the question a little differently

How can light (or electromagnetic radiation) travel through a vacuum when there is nothing there to act as a medium, and do so forever in all directions? For example the light coming from a star millions of light years away.

With water, for example, it is easy for me to see how a wave can disperse over the surface in all directions and travel for as long as the energy in the wave doesn't dissipate. The molecules in the water act as the medium, and are oscillating and pushing all neighboring molecules causing a domino effect outwards.

But what is the light doing?

One thought I had is, if light is made of something then it just moves outwards from the point of origin in whatever direction the light was headed.

Here is the first paragraph from Wikipedia : http://en.wikipedia.org/wiki/Photon

A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force, even when static via virtual photons. The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon has no rest mass; this allows for interactions at long distances. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of both waves and particles. For example, a single photon may be refracted by a lens or exhibit wave interference with itself, but also act as a particle giving a definite result when its position is measured.

So... maybe photons make up light, and they are just shooting through space like a wave, but it really isn't a physical wave (like a sound or water wave), it's just "acting like one"?

Here is a video from minutephysics : https://www.youtube.com/watch?v=Q_h4IoPJXZw

The video talks about the particle-wave duality of an electron. So a single electron can act as a wave, but when it hits something it hits only one place. Is that the same for photons?

Thought Experiment : Out in the vacuum of space there is a huge hollow sphere 1 light year in radius. Inside is just vaccuum, and at the exact center a flash of light is emitted in all directions from a point source. 1 year later the light hits the sphere at all places at once.

That's 12.6 (or 4 times pi) light years squared of surface area to cover! How can there be enough photons to hit every spot inside the surface area of this huge sphere? I understand that very little light would hit this far away, but even if non-zero at least some light is hitting.

Or maybe does this depend on how big the flash was? Maybe it doesn't hit every spot?

If light really is made of photons and acts like a wave but each photon hits only one place, then it seems to me there must be an infinite amount of photons in that flash of light to hit so many places inside the sphere.

But that must be impossible, so there must be a flaw in my reasoning.

• "Act like a wave"? Well, if it looks like a duck and swims like a duck and quacks like a duck, then... Feb 26, 2013 at 1:09

Well, I would say the electromagnetic field is the medium.

I don't understand this - How can something be the medium for itself?

Light isn't just an electromagnetic field. Light (i.e. photons) is a wave---a disturbance or perturbation---in an electromagnetic field. Just like for gravity, fields apparently permeate space. Even massive particles (e.g. quarks or electrons) can be described (or viewed) as waves in underlying fields.

So... maybe photons make up light, and they are just shooting through space like a wave, but it really isn't a physical wave (like a sound or water wave), it's just "acting like one"?

What you're calling a 'physical wave' is an emergent property of a underlying medium (e.g. air, water). For the most part, such an emergent wave is basically the same as a wave in a field. 'Physical waves' are often referred to as 'quasiparticles', because of this similarity. What we think of as 'particles' (e.g. electrons) don't just 'behave like waves', they are also waves, hence the 'wave-particle duality'.

• So in my thought experiment, do photons hit every point in the sphere? If so how can there be that many photons? Are photons like a particle in that when it "hits" the sphere it will make contact in one place? Feb 26, 2013 at 1:33
• You said that light is a disturbance "in an electromagnetic field" - so in the vacuum of space there is an electromagnetic field already there before the light passes through it? Feb 26, 2013 at 1:36
• @codefactor A field is basically defined as something that exists at every point in space and time, you can find this on Wikipedia. Feb 26, 2013 at 1:56
• @MarkMitchison Thanks for that Wikipedia link - so the field is everywhere and light is a wave in which the field is the medium. I thought that the light WAS the field - so it didn't make sense that it would be the medium for itself. But looks like light is just the wave in this already existing field. Feb 26, 2013 at 2:02
• Ok now this brings up another thing - if light is a wave in "an electricmagnetic field" that implies there can be more than one field... is that right? Or is it a wave in "the electicmagnetic field"... i'm confusing myself. Feb 26, 2013 at 2:04

How can light (or electromagnetic radiation) travel through a vacuum when there is nothing there to act as a medium, and do so forever in all directions? For example the light coming from a star millions of light years away.

Light is observed as traveling at velocity v=c, according to the second postulate of special relativity. But according to the principles of special relativity, light is not traveling, and this is why no medium is needed:

The worldline of light is lightlike, and lightlike spacetime intervals are zero (empty). That means that the momentum which is transmitted by electromagnetic interaction from the event of emission to the event of absorption must just cross a zero interval, both events are adjacent in spacetime. But crossing a zero interval is obviously no travel.

However, spacetime intervals cannot be observed. What we observe in our observer's spacetime diagram are photons which are traveling along the light cone. A zero spacetime interval may correspond in our diagram to a distance of billions of light years, and this traveled worldline which is a mere product of our observation is "filled" by electromagnetic waves. Electromagnetic waves are the observable witnesses of the unvisible zero spacetime interval.