# How does light travel Through a vacuum

1. Jul 26, 2011

### Darrenmackenz

Hi Im new, Ive read the FAQ's and none of them seem to help.
Its a classic question I know but I cant seem to find a full answer.

I understand that light is part of the electromagnetic spectrum
I also understand that light acts as both a particle and a wave.
I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Thanks

2. Jul 26, 2011

### LostConjugate

A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.

3. Jul 26, 2011

### Drakkith

Staff Emeritus
I think its an electromagnetic force, not an electrostatic. A photon has both electric and magnetic fields.

4. Jul 27, 2011

### Darrenmackenz

I'm going to need more information than that, how exactly does a photon or wave both move in a vacuum?

5. Jul 27, 2011

### mikeph

I don't know if it's a misunderstanding or a grammatical error, light doesn't travel between a magnetic field and electric field. Light is itself an oscillation of electric and magnetic fields. The oscillation propagates according to Maxwell's equations. And a vacuum means devoid of mass, not necessarily electromagnetic fields.

How do you define an ether and what gap in our understanding requires it?

6. Jul 27, 2011

### Darrenmackenz

It's a grammatical error sorry, I get that light itself oscillates between magnetic and electric. Maybe you could explain to me how the oscillation propergates according to maxwell.
I refered to an ether in the context of a medium in which light could travel through. Sound for example travels through air as it's medium. I understand that light does not need a medium in which to travel, I just don't fully understand why.
You say vacuum means devoid of mass which makes sense but not necessary electromagnetic fields, what would cause such a field in vacuum?
Thanks for helping me understand this.

7. Jul 27, 2011

### mikeph

If you combine Maxwell's equations for the case of no charge, the result is a wave equation, and one solution of that equation is an electromagnetic wave. It's difficult to convince a sceptical reader using only words and no equations, but it should be demonstrated in some popular textbooks (Griffiths for example should be in every physical sciences library).

Sound travels through air as it's medium- by this you mean, sound itself is a vibration in the air molecules, correct? A sound wave is fully characterised by the displacement of molecules from their equilibrium point. In the same way, a light wave is fully characterised by the displacement of the electric and magnetic fields from their equilibrium points.

Anything that couples to the electromagnetic field will create a field. Put a charge in empty space and it automatically comes with an electromagnetic field, not just where the charge is, but everywhere. It's the same as the gravitational field, and me asking you "how does the Earth orbit the sun? Why can the gravitational field cause the mutual interaction between a star and planet when there is a huge vacuum between them?" The vacuum simply means no matter is there. The fact that no matter is between two objects does not stop them from interacting.

8. Jul 27, 2011

### Staff: Mentor

Do you understand Maxwells equations, in particular Faraday's law of induction? Do you understand that Faraday's law works even in vacuum? I.e. Do you understand that vacuum does not act as a "shield" for electric and magnetic fields?

If you understand those then light going through vacuum is an unavoidable logical consequence.

9. Jul 27, 2011

### Darrenmackenz

Ok, air molecules who's equilibrium is being displaced is already present before a sound is even made.
Is there already an electromagnetic field present before light is even introduced?
If there isn't does light provide it's own field with which to travel?

10. Jul 27, 2011

### harrylin

Hi Darren, welcome to Physicsforums.

There is no positive evidence that "there is no ether", and a photon in transit can be regarded as a kind of wave packet. Thus it's a concept that you can use. As a matter of fact, many famous physicists (incl. Einstein) concluded that vacuum cannot be nothingness, which implies some kind of ether.

However, there is no evidence of something like "ether particles", and as far as we can tell, no linear velocity wrt an ether can be established by measurements (it would even invalidate relativity theory). That may explain why many people conclude that there is no ether.

Best,
Harald

11. Jul 27, 2011

### saim_

The "medium" that your intuition might require is the electromagnetic field. Oscillation of air particles produces sound. Oscillation of EM field produces EM waves like light. Electromagnetic field is just as physical as the air around you.

12. Jul 27, 2011

### HallsofIvy

Why would a particle (photon) need to travel "within" anything? It is only when you think of light as a wave that you need something to "wave". "Vacuum" means there is no matter. There are still fields such the gravitational field and electromagnetic field at every point in space. A light wave is a "ripple" in the electromagnetic field.

No to both of these. A light wave is a ripple in the electromagnetic field. It does not "oscillate between a magnetic field and an electric field". At every point there is both a magnetic force vector and an electric force vector- at right angle to one another.

13. Jul 27, 2011

### Darrenmackenz

I think I understand it, an electric field can be induced by a magnetic one moving at 90 degrees to it. The basics of a dynamo, and the opposite is the basics of a motor.
Right?
So your saying light induces it's own electric and magnetic fields by oscillating them, this then becomes self propergating.

Last edited by a moderator: May 19, 2013
14. Jul 27, 2011

### mikeph

Yes.

No. Light is just an oscillation in a pre-existing electromagnetic field.

Do sound waves create air particles? Do water waves generate water?

15. Jul 27, 2011

### Darrenmackenz

Ok that all works in my head.
Here's another question, how did the pre existing field get where it is, i.e. the void in space? left over from the big bang?

16. Jul 27, 2011

### jambaugh

It is built into the structure of space. One can speculate about what lies deeper but for now it is sufficient to describe how the "vacuum" behaves and w.r.t. classical moving charges, that is what Maxwell's Equations describe.

17. Jul 27, 2011

### Darrenmackenz

Humour me, what's your speculation? I'm interested in what people think.

18. Jul 27, 2011

### mikeph

19. Jul 27, 2011

### Darrenmackenz

So mikey, you think its an after effect of the cooling of the universe after the big bang? I didn't know that there were theories that the electromagnetic force and the weak nuclear force were once unified or could be unified if the universe heated up.

20. Jul 27, 2011

### Drakkith

Staff Emeritus
Look up Timeline of the Big Bang on wikipedia.