Is Light a Particle or a Wave?

[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

 

[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

 

[ZapperZ]

Please look at:

https://www.physicsforums.com/showthread.php?t=63802
https://www.physicsforums.com/showthread.php?t=57911
https://www.physicsforums.com/showthread.php?t=56561

etc...

Zz.

 

[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.

 

[simon009988]

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.

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

 

[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?

 

[jtbell]

If photons are massless...then what are they?

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.

if photons are massless, how does gravity affect a massless object?

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.

 

[Gokul43201]

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.

 

[Johnson]

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.

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

 

[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.

 

[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 got to go back to GD before I get in trouble

 

[FeynmanMH42]

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

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.

 

[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

 

[jtbell]

Also, if you could link me to the relativity forums :) I searched for it, but couldn't find it.

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".

 

[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

 

[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

 

[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!

 

[Patrick_lai]

Simple answer, Light is packets of energy.

 

[McQueen]

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 $$E^2 = (pc)^2 + (m_0 c^2)^2$$
gives you $$E^2 = (pc)^2 + (m_0 c^2)^2$$
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.

What about phonons ?

 

[daniel_i_l]

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?

Just because something has "size" doesn't mean that it has mass. In fact, the mass of a particle doesn't have anything to do with it's size. Even though particles are 0-D anyway, just because something exists doesn't mean that it has mass - quite the opposite, it is actually quite puzzling that the fact that a body is big should make it any harder to move, it's almost as if something is "pulling" it back! As a matter of fact, modern theory is looking into something called a Higgs field that interacts with matter and makes it harder to accelerate thus giveing it mass. So anything (photon) that doesn't interact with the Higgs field would be massless!

 

[breezels]

light has two properties:particle and wave.
In diffrent circumstances, one of the properties would behave in priority.

 

[Boffin]

Hans C. Ohanian's in his 1400 page textbook Physics First Edition 2nd volume pg 912 coins what photons are the best.

"Thus photons are neither classical particles or classical waves. They are some new kind of object, unknown to classical physics, with a subtle combination of both wave and particle properties. B. Hoffman has coined the name wavicle for this new kind of object. It is difficult to achieve a clear understanding of the character of a wavicle because these objects are very remote from our everday experience."

There has been some resistance among scientists in using the word wavicle. I like it. What you you think? In my opinion continuing to use words like wave and particle or wave-particle makes people confused as to what light is, as they keep thinking of light as either a marble like object or water wave.

 

[FeynmanMH42]

Hans C. Ohanian's in his 1400 page textbook Physics First Edition 2nd volume pg 912 coins what photons are the best.
"Thus photons are neither classical particles or classical waves. They are some new kind of object, unknown to classical physics, with a subtle combination of both wave and particle properties. B. Hoffman has coined the name wavicle for this new kind of object. It is difficult to achieve a clear understanding of the character of a wavicle because these objects are very remote from our everday experience."
There has been some resistance among scientists in using the word wavicle. I like it. What you you think? In my opinion continuing to use words like wave and particle or wave-particle makes people confused as to what light is, as they keep thinking of light as either a marble like object or water wave.

Yes but we can visualise a wave or a particle, these are everyday concepts and the words conjure up the images you've described.
But what image does the word "wavicle" conjure up?
People need to think of photons as particles or waves if they have a visual imagination, as no-one can visualise a wavicle.
I think that's one reason Einstein disliked Quantum Mechanics, because he had a very visual imagination and couldn't get a grip on photons.
They reckon that the reason Einstein developed this visual imagination is because he was a slow speaker early on, yet very intelligent... was he thinking great thoughts in his head but all in pictures with no math or language?

 

[Boffin]

Yes but we can visualise a wave or a particle, these are everyday concepts and the words conjure up the images you've described. But what image does the word conjure up? People need to think of photons as particles or waves if they have a visual imagination, as no-one can visualise a wavicle."

But isn't it true that the same argument could always be made for not using new words. Before the computer, the closest thing that business and ordinary people used was either an adding machine or typewriter. Would it make sense if someone mentioned that they had just bought an adding machine/typewriter and were having fun playing the latest video games and editing digital video footage on it.

 

[da_willem]

'Wavicles' propagate like waves but exchange momentum and energy like particles. I.e. they are waves until they bump into something and the wave(function) collapses to a point and the wavicle is localised as a particle is.

 

[pmb_phy]

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

The probability density is given by the square of that field. In that sense you can think of it as the wave function.

Pete

 

[Boffin]

'Wavicles' propagate like waves but exchange momentum and energy like particles.

I beg to differ . In my opinion wavicles do not propagate like waves. According to the Free Dictionary a wave in Physics is: "A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself." Don't alll waves spread out in at least 2 dimensions for water waves and 3 dimensions for sound waves. But wavicles do not appear to spread out at all. Also a wavicle does not tranvel in a medium really. The space continuum is physical but not really a medium. That is why I don't like to call light a wave, because it confuses people to say it is a wave when it doesn't act at all like waves in a medium. Light is more like a vibration or occilation than it is a wave.

 

[da_willem]

I beg to differ . In my opinion wavicles do not propagate like waves. According to the Free Dictionary a wave in Physics is: "A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself." Don't alll waves spread out in at least 2 dimensions for water waves and 3 dimensions for sound waves. But wavicles do not appear to spread out at all. Also a wavicle does not tranvel in a medium really. The space continuum is physical but not really a medium. That is why I don't like to call light a wave, because it confuses people to say it is a wave when it doesn't act at all like waves in a medium. Light is more like a vibration or occilation than it is a wave.

It is a very special wave indeed, there is no disturbance of a medium as we are acquainted with from our experience with waves. But it's behaviour is described by a wave-equation just like any other wave. In this case it is not some displacement of a medium that satisfies the wave equation but the particles wavefunction [itex]\psi (\vec{x},t)[/tex] satisfying [itex]|\psi (\vec{x},t)|^2= p(\vec{x},t)[/tex] with p the probablility density of finding the particle.

The wave equation is of course the Scrödinger equation; for a free particle:

$$i\hbar \frac{\partial \psi}{\partial t} = \frac{\hbar ^2}{2m} \nabla ^2 \psi$$

The time-dependence of a particle with definite energy E is

$$\psi \propto e^{-i\frac{E}{\hbar} t}$$

so $$\frac{\partial \psi}{\partial t} = -i\frac{E}{\hbar} \psi$$

substituting in the Schrödinger equation leaves us with

$$\frac{2m}{E} \frac{\partial^2 \psi}{\partial t^2} = \nabla ^2 \psi$$

Unmistakably a wave-equation for a particle with phase velocity [itex]v=\sqrt{\frac{E}{2m}}[/tex]

 

[Boffin]

It is a very special wave indeed...with p the probablility density of finding the particle.

I agree, if a photon can be called a wave at all, it is a very special type of wave as you say. By the way how big is a photon, or "special wave"? By big I mean how far away from the center of the "particle" or "wave" is it able to cause an effect. How far to each side and forward and backwards? Does this depend on the frequency or energy of the photon?

 

[nazgjunk]

I haven't read the whole thread, but I've had the same views upon this for a long time already. My idea basically is that Light (and of course all em ray thingeys) is neither particle nor ray (oh, how poetic), but something other having properties from both.