# Light, Wave or Particle?

1. Nov 27, 2005

### Johnson

Just wondering what everyones view on the properties of Light is. I'm new to these forums, so I don't know if it has been answered previous, and relatively new to physics in general (Grade 12).

Ive read statements that Light is a particle, statements that light is a wave. Is there an intermediate, because light tends to share properties of both, and if so, what it is?

Anything that would help me answer this question would be much appreciated :)

Thank you,
Andrew

2. Nov 27, 2005

### scott_alexsk

Very painfully I have learned that neither theory is adequte. Each theory is only applicable in certain situations and is used accordingly.
-Scott

Last edited: Nov 27, 2005
3. Nov 27, 2005

### ZapperZ

Staff Emeritus
4. Nov 27, 2005

### Johnson

OK, so basically looking at that...

Light is wave-particle duality, meaning that it is both a particle and a wave, but depending on how you look at it, determines its characteristic? So "Light" is a product of when electrons drop from a higher energy level to a lower energy level, creating a photon. In this case light is a particle (photon), but travels in a ?wave? which would thus give its wave characteristics.

Therefore, if photons are particles, and all particles have a mass, how can it travel at the speed of light? I was under the influence that anything with a mass cannot travel at the speed of light? May be wrong, probably am, but anywho.

5. Nov 27, 2005

### simon009988

photons are massless ,they have no rest mass; so it MUST travell at the speed of light

6. Nov 27, 2005

### Johnson

If photons are massless...then what are they? If they are massless, then they cannot be particles, correct? So then if they're not particles...then that would mean light is not a particle thus disproving the particle theory?

So basically to reach the speed of light, there must be no mass? So Light is then the terminal velocity? Nothing can go faster than the speed of light? Would gravity be considered a velocity, since we measure gravity on Earth as 9.8m/s [Centre of Earth], can the velocity of gravity excel the velocity of light? Such as a black hole, once you reach the event horizon of the black hole, the escape velocity of the black hole is faster than the speed of light. But in that case...if photons are massless, how does gravity affect a massless object?

7. Nov 27, 2005

### Staff: Mentor

They are massless particles. Welcome to the wonderful world of relativity, where the general relationship among rest mass, energy and momentum is

$$E^2 = (pc)^2 + (m_0 c^2)^2$$

Setting $m_0 = 0$ gives you $E = pc$ which is the relationship between energy and momentum of a photon. It's also (surprise!) the relationship between the energy and momentum carried by an electromagnetic wave in classical electrodynamics.

In general relativity, gravity isn't a force. It's the effect of the curvature of spacetime on the motion of objects. Massless particles such as photons follow "geodesics" in spacetime, which are curved in the presence of massive objects.

8. Nov 27, 2005

### Gokul43201

Staff Emeritus
Asking if light is a wave or a particle is somewhat like asking if H2O is a solid or a gas. And just as H2O can be either a solid or a gas (or a liquid) depending on conditions, light can appear to behave like a particle or a wave. The point here is not really that light =particle+wave, but that requiring that something be a particle or a wave is looking only at limiting behaviors, and is a restrictive terminology to apply. From the point of it being an archaic terminology, this is like asking whether molten steel is composed of earth, fire or water.

9. Nov 28, 2005

### Johnson

Is there then an equation to determine gravity as the curvature of spacetime?

Like the equation for gravity (as i was told) is Fg=(Gm1m2) / r^2

10. Nov 28, 2005

### FeynmanMH42

Shouldn't this be in the Quantum Physics forum?
But yeah, I've heard one theory that involves extra dimensions.
Just as a 3D cylinder's 2D shadow can seem to be a circle or a rectangle when viewed from different angles, so a 4D "wavicle's" 3D representation in this world can seem to be a wave or a particle, depending on how you're looking.
Wave-particle duality is the cornerstone of modern physics Johnson, it will become very important as you get further into physics.

11. Nov 28, 2005

### tribdog

is light a wave or a particle?
yes, probably

I don't know if this has been asked here before, but Have you ever heard of Einstein? lol, I gotta go back to GD before I get in trouble

12. Nov 28, 2005

### FeynmanMH42

There are many, but as you can probably see from my distress calls in the Relativity forums, they're very, very complicated.
Feel sorry for Einstein though, he found the equations for Special Relativity (i.e. unwarped space) using partly simple diagrams and geometry (the equation that describes the slowing down of time is based on Pythagoras' Theorem!!) but trying to use the same approach in General Relativity is strange.
It's like drawing a diagram on a piece of paper then crumpling it up and then using the diagram to work out the crumpliness of the paper - we're all inside the curved space, so we can't make measurements for how much it curves - since whatever we put into the space will get bent as well!
Einstein had to derive the equations himself, and then see if the predictions fitted the observations.

13. Nov 30, 2005

### Johnson

There are many equations you say? Is there a basic equation though, that would relate to most circumstances? Or depending on the different conditions depends on which variables to use? Also, if you could link me to the relativity forums :) I searched for it, but couldn't find it.

Thank you,
Andrew

14. Dec 1, 2005

### Staff: Mentor

Go to the physicsforums.com home page, the one with the list of forums. The "Special & General Relativity" forum is the twelfth one from the top. It's in the group "Astronomy & Cosmology".

15. Dec 1, 2005

### pmb_phy

Cohen Tannopudji et al state it like this

1) The particle and wave aspects of light are inseperable. Light behaves similtaneosly like a wave and like a flux of particles, the wave enabling us to calculate the probability of the manifestation of a particle.

2) Predictions about the behaviour of a photon can only be probabilistic.

3) The information about a photon at time t is given by the wave E(r, t), which is a solution of Maxwell's equations.

Pete

16. Dec 1, 2005

### Ratzinger

But E(r,t) is not a quantum wave function, or is it? On the other hand I saw in my quantum text that it's interpreted just like that. Then I read here on physicsforums it's all wrong, it's ripples in quantum fields.
Could someone explain the relationship photon-quantum wave-classical EM wave?

thankyouverymuch

17. Dec 1, 2005

### tariq5

Is it correct to compare between particle and wave? They are two different entities. Wave is a moment of particles. The good last century question where there is a wave there should be a medium. Now types of medium should also be redefined to accommodate the ether like theory.
Sorry for my imagination I see photon as a "299 792 458" m long particle!:rofl:

18. Dec 3, 2005

### Patrick_lai

Simple answer, Light is packets of energy.

19. Dec 3, 2005