# What Exerts a Force on a beam of Light?

1. Aug 22, 2008

### LukeJD

I've been thinking about some really elementary ideas of light and I just can't get a handle on this. Everything on the electromagnetic spectrum travels at 3.0 x 10^8 m/s, but what force is exerted on these waves to travel at this speed and wouldn't newton's third law mean that there would need to be an equal reactionary force?

Last edited: Aug 22, 2008
2. Aug 22, 2008

### ZapperZ

Staff Emeritus
Why would there be a force?

Applying Newton's first law, for example would say that there isn't a force since it is already moving at c and no force is needed for it to maintain that.

Now, if you are asking of there's any recoil, even minuscule, when light is being emitted from an atom, let's say, then yes, but this is due entirely on the fact that light has a momentum.

Zz.

3. Aug 22, 2008

### Staff: Mentor

I don't quite see a simple connection between Newton's laws of motion and EM wave propagation, but even so: Newton's 2nd law says that a force is required to produce an acceleration; Newton's 1st law tells us that no force is needed to maintain a constant velocity.

4. Aug 22, 2008

### LukeJD

btw, I want to preface this with "I don't know" I'm just trying to figure this out.

That makes sense that it would not need a continual force to maintain it's velocity, but what initially forced the wave to that velocity?

Because light shows signs of particle-wave duality, wouldn't the particle be bound to the same newtonian laws?

EDIT: I saw where I messed up in the original post, for some reason I wrote Newton's "Second" law, when I meant the third. Excuse the error!

5. Aug 22, 2008

### ZapperZ

Staff Emeritus
Nothing. That's is the mistake most people make, and why Einstein was so smart. People want to apply the familiar rules on light, when they clearly do not work there. There is no F=ma. Instead, there is F=dp/dt. The only thing you can measure is that a body recoils or moves to preserve the conservation of momentum it interacts with light.

Zz.

6. Aug 22, 2008

### LukeJD

So EM waves have their own set of laws that have nothing to do with classical physics right?

7. Aug 22, 2008

### ZapperZ

Staff Emeritus
No, I was invoking Special Relativity. EM wave can certainly have classical physics description. After all, that's what Maxwell Equations are. However, there is no "force" in here that would apply to light, at least not in the sense that it needs one to be at c. Can light exert a "force"? Sure! That's why we know it has a momentum. But does it require one to be at c in vacuum? Nope.

Zz.

8. Aug 23, 2008

### granpa

the 'springiness' of space is the force you are looking for.

9. Aug 23, 2008

### Staff: Mentor

It seems that you are thinking of a photon as a small pellet that must be accelerated from some initial speed to c. That is not the case. A photon is "born" travelling at c, it never travels at any slower speed, and it never accelerates to c.

10. Aug 23, 2008

### LukeJD

That's interesting, I did not know that. What book/text would you recommend for me to understand photon behavior more completely?

Yes! Problem solved!

11. May 3, 2009

### Bob S

The forces on a photon are:
A) When the longitudinal momentum is changed; like when it is doppler-shifted, either up or down. Its momentum is p = E/c = hv/c, so when the photon energy changes, its longitudinal momentum also changes. Where is the recoil force?
B) When it is deflected transvrsely, like in a prism or mirror (recall that p is a vector along direction of propagation).

12. May 3, 2009

### GetPhysical

I am currently reading Einstein's Special and General Theories of Relativity. There is one part where he talks about light bending by gravity in space. When the light from a distant star passes by a large object in space the beam bends around the object due to its gravity. So essentially you are looking through that object. So forces can act on light. But when it is propagated it is already at maximum speed.