What makes light special?

[Cap'n Refsmmat]

This is something that's bugged me a little bit recently.

 

We all know that according the relativity, the speed of light is constant to all observers. The question is, why light only? There certainly are other particles (most of them theoretical) that travel at the speed of light, like gravitons. Would their speed also be constant to all observers?

[D H]

Anything that travels at the speed of light does so to all observers. What travels at the speed of light? Massless particles. Massive particles cannot move at the speed of light.

 

Electromagnetism, the strong force, and gravity all propagate at the speed of light per current theories. Electromagnetism and the strong force are mediated by the photon and the gluons. That the photon is massless is about as solid as science can get. That gluons are massless is not as strong but the evidence is very good.

 

We don't know what gravity is yet (it is axiomatic right now). The hypothetical graviton should also be massless.

[Ragib]

Yea, what he said...

[Severian]

The residual strong force, that holds together the nucleus of atoms is mediated by pions. These pions are quite massive, so travel considerably slower than light.

[Farsight]

This is something that's bugged me a little bit recently. We all know that according the relativity, the speed of light is constant to all observers. The question is, why light only? There certainly are other particles (most of them theoretical) that travel at the speed of light, like gravitons. Would their speed also be constant to all observers?

 

IMHO: A photon might be hundreds of metres long, as per long wave radio, so I don't like thinking of it as a "particle". I prefer to think of it as a self-propagating electromagnetic variation in space, or a "space effect" for short. Now instead of asking "Why does a photon travel at the speed of light?" you can ask "Why does a space effect propagate at the rate that it does?". The answer is to do with the properties of space. When it comes to gravitons (see DH's note) I'm definitely thinking "space effect" rather than "particle", and a propagation rate that again depends upon the properties of space. Since we hope that all the "forces" can be unified, it seems reasonable to expect the same propagation rate.

[insane_alien]

farsight, a photon is both a wave and a particle depending on the conditions of its observation and the sobriety of its observer.

[Farsight]

I think "particle" is such a bad word though, insane_alien. IMHO it makes people think of some discrete, distinct, "billiard ball" zipping through space, and makes them think about the properties of billiard balls and not the properties of space.

[D H]

I think "particle" is such a bad word though, insane_alien.

 

Go ahead. Invent your own language. Invent your own axioms. Do it all without a bit of advanced mathematics. Do it without studying what the current theories are.

 

The word is 'particle', and this word does not mean a hard little billiard ball zipping around in space.

[Farsight]

Well sadly you're wrong. That's exactly what it does mean to whole generations of schoolchildren.

 

http://www.answers.com/topic/particle

 

What next? Will you be telling all and sundry that anybody who doesn't like the sound of "coloured" quarks clearly knows no mathematics, even less physics, invents language and axioms, and eats children for breakfast?

 

Geddoutofit.

[ParanoiA]

Go ahead. Invent your own language. Invent your own axioms. Do it all without a bit of advanced mathematics. Do it without studying what the current theories are.

 

The word is 'particle', and this word does not mean a hard little billiard ball zipping around in space.

 

Doesn't the word "particle" imply mass? And aren't photons massless? This has been an issue for me dealing with photons, gluons and gravitons. They are particles, yet have no mass, yet behave like waves..with uncertainty

[D H]

Well sadly you're wrong. That's exactly what it does mean to whole generations of schoolchildren.

 

Where, exactly, in that dictionary definition, does it say that a "particle" is a tiny billiard ball? In particular, under physics, I see the classical and quantum mechanical definitions of particle. Not bad for a dictionary definition. You might even see the term 'wave-particle duality' if you look a little deeper than a dictionary.

 

BTW, weren't you the very person who disparaged a dictionary definition in another post?

 

You've got to be kidding Ed. That's a dictionary definition. You know it's not my definition.

 

Hypocrit.

 

What next? Will you be telling all and sundry that anybody who doesn't like the sound of "coloured" quarks clearly knows no mathematics, even less physics, invents language and axioms, and eats children for breakfast?

 

Interesting logic. I see you found some handy Wikipedia entries:

Misleading vividness

Faulty generalization

Red herring

Ad hominem

 

========================================================

 

ParanoiA,

I am answering your question in this same post. Do not take this to mean that I think you belong in the loony bin with Farsight. Just trying to obey site rules against double-posting ...

 

Doesn't the word "particle" imply mass? And aren't photons massless? This has been an issue for me dealing with photons, gluons and gravitons. They are particles, yet have no mass, yet behave like waves..with uncertainty

 

The word particle does not imply mass. A particle is simply the smallest unit of some thing. An atom of gold is a particle in the sense that it cannot be cut into any smaller part that resembles gold. That atom of gold can of course be divided into electrons, protons, and neutrons. Protons and neutrons can be further divided into quarks. Electrons and quarks are elementary particles: they can neither be cut nor divided into some smaller entity.

 

Light, like matter, also exhibits a wave-particle duality. Einstein's Nobel Prize was primarily for his work on the photoelectric effect, where this duality comes to forefront. Photons, the individual units of light are 'particles' in the sense that there is no such thing as half a photon. Photons are uncuttable and indivisible. They are particles in the sense of the word used by particle physicists.

[Royston]

I guess the simplest way to describe light, is that when it interacts with other particles e.g exciting a hydrogen atom, it is a particle a 'photon.' However it propagates as a wave, hence light can be diffracted when passed through a grating.

 

If we did eventually detect gravity waves and they were a result of gravitons, then they would also display wave-particle duality. I'm very tired, and this maybe a stupid question, but wouldn't that mean gravity could be diffracted, somehow ?

 

Apologies to Cap'n for going off topic.

[Farsight]

There you go again, DH. You leap on the lack of a billiard ball and ignore an issue that affects generations of schoolchildren whilst skating over the first line of the dictionary definition:

 

1. A very small piece or part; a tiny portion or speck.

 

I point out an issue and get a pile of insults like "looney" and "hypocrite". You know full well Edtharan tried to use a dictionary definition I quoted on a side issue to make out I was contradicting my own essay. This tendency you've got for dishonesty and offense is not how physics ought to be.

 

ParanoiA: The word particle does imply mass to many younger people, which is the problem that earned me a kicking. However I'd say most people on these boards are happy with the idea of massless particles, and don't associate particles with mass. But I think some do still associate a particle with a speck. Ask DH to explain how long a long-wave photon can be, why it cannot be cut in half, and why David Deutsch thinks a photon in two places at once proves the existence of a Multiverse. The way to grasp the problem here is to yell OI! and then try to make any sense of the SHOUT particle.

[Cap'n Refsmmat]

I opened this thread to ask a question, not to start a fight.

[Sequence]

I try to not think about these things, to me, the word particele implies volume and mass, however tiny. BUt, if a photon is a particle, then it sould have mass. The simple equation Momentum= Mass X Velocity would imply that even though it's mass is miniscule, the speed would give it great momentum. Turning on the lights = hit by a sledge hammer. Since we can be in light with out being effected in that way, that is what makes me think of it as a wave.

 

 

This is just my opinion and I have no evidence to back it up. It doesn't mean I don't think massles particles excist, I just don't see how.

[Ragib]

Im pissed off today, so i'm going to do a thrashing. Particle may imply volume and mass to you, but be reminded you don't know jack. Just because all we see in day to day life, in our macroscopic scale, has mass and volume, does not mean its the same everywhere. Now if you want to momentum of light, its not mass times velocity. Its Planks constant, over its wavelength god damn it. And thinking its a wave doesnt get you out of it, this equation still applys.

 

The mathematics tells us to think of fundamental particles as point particles. No Billiard Balls. In Fact, not volume, takes up so space. They are not intervals in geometry, just points.

 

To snail, yes the gravity could superimposed, or be diffracted etc. This has been examined. In fact Eiensteins equations of general relativity incorporate this, but are commonly interpreted rather as combined distortions of spacetime, which are eqivalent.

 

Please, physics is not hard. Im 14. And no, im not a nerd, i have a life, most people here do.

[Royston]

To snail, yes the gravity could superimposed, or be diffracted etc. This has been examined. In fact Eiensteins equations of general relativity incorporate this, but are commonly interpreted rather as combined distortions of spacetime, which are eqivalent.

 

Right, thanks, I'm yet to study Einsteins equations in 'full' so I just needed some clarification...I was very tired when I asked, so apologies if it seemed a bit dumb.

[Sequence]

Is it that time of month man? I said that I had no idea what I was talking about and that physics is not my thing. So, perhaps you should stop being so critical. I don't know or care. Just stating what I think. Not saying I'm right or even close to being right. You don't need to be a dick to anyone who has not read the same things as you.

[swansont]

Doesn't the word "particle" imply mass? And aren't photons massless? This has been an issue for me dealing with photons, gluons and gravitons. They are particles, yet have no mass, yet behave like waves..with uncertainty

 

This isn't the only case where the scientific definitions and descriptions differ from common use, which is one reason why a lot of scientific description is done with equations. Prose can only get you so far along.

[Sequence]

Which is why I have trouble grasping the concept. I'm not into physics. There was no one reason for Ragib with his overinflated head to come in and compensate for his probable lack of pubic hair by flaming someone who obviously doesn't know much about physics. But, if that makes you feel like a big man, by all means go ahead.

 

Ontopic: I don't know anything about physics.

[CanadaAotS]

Don't think of a photon as a particle then simple as that.

 

Think of it as a 'packet' of energy, since thats what it is pretty much. When light isn't interacting with anything, the energy acts like a wave. When it does interact with something it acts like a 'packet'.

 

Hope that helps in your visualization.

[Ragib]

That time of month? Overinflated head? Lack Of Pubic Hair? At least what I say is proven. I'm not even going to bother trying to talk to you.

[Klaynos]

Don't think of a photon as a particle then simple as that.

 

Think of it as a 'packet' of energy, since thats what it is pretty much. When light isn't interacting with anything, the energy acts like a wave. When it does interact with something it acts like a 'packet'.

 

Hope that helps in your visualization.

 

How do you know it acts like a wave unless it interacts with something?

[ParanoiA]

That time of month? Overinflated head? Lack Of Pubic Hair? At least what I say is proven. I'm not even going to bother trying to talk to you.

 

From what I've read, you haven't bothered talking to Sequence at all - unless you call berating a self confessed layman, talking. I'm sorry but I have to completely agree with Sequence that you seem to expect an unrealistic level of knowledge by non-physics folk.

 

What if we all had that knowledge? Then you wouldn't be so special and no one would pay any attention to you at all. If everyone's an expert, then no one's an expert.

 

It's good that you understand something that the masses don't. This gives you a living - a career. Learn to embrace the ignorance of others and enjoy the capacity of enlightening them. It's a far more noble and grown up attitude.

[swansont]

From what I've read, you haven't bothered talking to Sequence at all - unless you call berating a self confessed layman, talking. I'm sorry but I have to completely agree with Sequence that you seem to expect an unrealistic level of knowledge by non-physics folk.

 

Part of the issue of whether some behavior is called for depends upon the conviction of the statement that elicited the response. If you state something, and are wrong, don't be surprised that you get corrected.

 

Ragib's response was a tad strong, but it was stated at the outset that that would be the case. Restraining one's self from posting while angry should be occurring. But telling somebody they don't understand physics is not inherently a personal attack, and people should realize the difference between opinion and fact, and the appropriateness (or lack thereof) of the former in a factual/scientific discussion.