What makes light special?

[Farsight]

I'm 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, it's not mass times velocity. It's Plank's constant, over its wavelength god damn it. And thinking its a wave doesnt get you out of it, this equation still applies. 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.

 

Look at this, swansont. It's not on. And I don't care what "mathematics tells us" because real life tells us about long wave radio. Do not defend this supercilious intellectual arrogance with a skim of scientific fact.

[ParanoiA]

Damn, physicists are confusing. It's one thing to try to understand point particles as opposed to simply particles, but when the physicists are arguing about it, it really makes it difficult to grasp.

 

Farsight - you say photons are not points, so are you saying they are not point particles? Or are you guys just splitting hairs on deeper meaning?

 

I'm asking because, I've just started the section on string theory and I believe Greene refers to them as point particles - as well as electrons and quarks. Which is weird, because I thought a point particle wasn't supposed to have mass or volume.

[Sequence]

Yeah it is confusing.

[D H]

Damn, physicists are confusing. It's one thing to try to understand point particles as opposed to simply particles, but when the physicists are arguing about it, it really makes it difficult to grasp.

 

Farsight - you say photons are not points, so are you saying they are not point particles? Or are you guys just splitting hairs on deeper meaning?

 

I'm asking because, I've just started the section on string theory and I believe Greene refers to them as point particles - as well as electrons and quarks. Which is weird, because I thought a point particle wasn't supposed to have mass or volume.

 

Physicists model the elementary particles as point particles. However, they cannot be viewed as little points zooming around in space. The uncertainty principle comes into play. The point particles must be viewed as wave packets.

 

From http://en.wikipedia.org/wiki/Point_particle

In particle physics, "point particle" is synonymous with "elementary particle", which is defined as a particle without structure or, equivalently, as a particle lacking component parts. According to the Standard Model (of fundamental particles and forces), quarks, electrons and other leptons are point particles in this sense. Nothing in the theoretical framework of particle physics literally refers to the shape of an elementary particle.

 

From http://en.wikipedia.org/wiki/Wave_packet:

In quantum mechanics the wave packet [ ... ] is interpreted to be a "probability wave" describing the probability that a particle or particles in a particular state will have a given position and momentum.

In the coordinate representation of the wave, the position of the wave is given by the position of the packet. Moreover, the narrower the wave packet, and therefore the better defined the position of the wave packet, the larger the uncertainty in the momentum of the wave. This tradeoff is known as the Heisenberg uncertainty principle.

[[Tycho?]]

I can see why whats-his-face got irritated.

 

Light is a particle.

 

Light is a wave.

 

If you say one of these statements is incorrect, you are wrong, period. In his "pissed off today" post, I saw no errors in physics.

[Ragib]

ParanoiA, do you really think that I learn physics to be special, unique, seek attention? Or for a living/career? You could not be further from the truth. I'm sure there are others who can relate when i say I study mathematics and physics for the sheer beauty of it. I believe there is nothing in this world more enlightening and rewarding than the study of these subjects. And no, i dont expect him to know this, but please make an attempt to learn BEFORE you post in such a website.

 

And it interests me how you understand string theory, but not quantum mechanics. I'd imagine the book was The Elegant universe. Let me tell you, You will not believe how much he simplified it so the public would understand it. But he was right to refer to them as point particles. They do not imply mass, but can have it. As to volume, none at all.

 

Farsight: "I don't care what mathematics tells us". Shows what you know. I believe it was Kepler who was the first to believe in a a solar system, with elliptical orbits. Not because of "real life" but because the mathematics demanded it. And it seems real life agrees with him, just as it always does. Mathematics governs our reality, our existence. And, when i said that they are point particles, i was referring to photons rather than waves. How you measure them depends on your observation. But you can not say that light isn't both.

 

swansont: A TAD strong? Thank you for your kindness, but I know i was not pleasant at all. Other than that, I agree with you in every aspect. I didn't give a crap when matt grime completely crushed some of my views in a far more arrogant way. I don't care because i have learned from my errors.

 

Tycho?: Thank you, hopefully you never will.

[Royston]

I don't care because i have learned from my errors.

 

I agree, nobody is here to molly coddle anyone, if that person doesn't understand something. If you're argument or point is completely flawed then it's perfectly acceptable for somebody to be blunt...plus it will probably stop the person asking again. Perhaps people are insulted because you're only 14 Rajib, well age has nothing to do with it, I couldn't careless what age somebody is, if they give me good advice it's good advice, and that's the end of it.

 

I've only been studying seriously for the last 8 or 9 months with a 14 year gap, since studying science and maths. So I'm bound to ask the odd stupid question or two, so it's ridiculous to take it personally, because the person giving you advice has no knowledge of your background or level of understanding. I think people should stop whining, when somebody points out an error in their logic or idea, take the advice on board, and move on...especially when it's obvious the person 'does' have more knowledge in the subject than you, whatever age, and however they come across.

[Janus]

I believe it was Kepler who was the first to believe in a a solar system, with elliptical orbits. Not because of "real life" but because the mathematics demanded it.

 

Kepker's Law actually were purely empirical. They were based on observation of the planets in their orbits. He observed that the planets followed ellipitical orbits and he observed that their periods and distances from the sun were related in a particular way. He then formulated these observations as laws.

 

It wasn't until Newton that these laws were mathemathically derived from his laws of motion and universal gravity.

[Ragib]

Which is why Kepler only believed his theory to be an approximation. He said it was not real, but could be used for calculating the motion of the heavens. Only after the mathematics was provided he believed it reality, because the math demanded it.

[ajb]

Back to the original topic.

 

Light is special because light cones are identical to causal cones. (In the geometric optics approximation within classical GR)

 

Why I do not know.

[Janus]

Which is why Kepler only believed his theory to be an approximation. He said it was not real, but could be used for calculating the motion of the heavens. Only after the mathematics was provided he believed it reality, because the math demanded it.

 

Er, that would have been a little difficult, since Kepler died in in 1630 and Newton wasn't even born until 1642.

[Farsight]

Can we all try not to be a little more pleasant? By all means correct any wrong view and respond to ignorance, but let's not descend into arguments. It happens too much on forums like this.

 

ParanoiA: Yes, I'm saying photons are not point particles. They aren't waves either. They are what they are. Sometimes they look like waves, sometimes they don't. But that's just us. We always try to describe the subatomic world using something we can see in our macroscopic lives.

[Ragib]

My bad, I must have interpreted a certain book wrongly then..Sorry about that Janus. I should have realised....anyway, Farsight beat me to it, but yes we always try to describe things in a way we are familiar with. Whoever said waves and particles aren't the same thing on a microscopic scale? Theres no need to explain why it behaves like both if we realise theres no reason why waves/particles aren't the same on a certain scale.

[Severian]

I disagree with most of what had been said so far. The difficulty is really the use of the word 'particle' which comes with it too much baggage.

 

The word 'wave' is much better - much more accurate. People have a bad habit of only thinking of a wave as a plane wave, spread out over all space. While this is a wave, waves can also be very localised. A wave can look and behave exactly how we percieve a particle would if it is sufficiently localised.

 

Indeed, since our measurements of position are never 100% accurate, we will never know exactly where a 'particle' is, so it is really a superposition of the position eigenstates, i.e. even after measurements, it is still a wave, albeit a localised one.

 

I think we confuse people when first talking about quantum mechanics with this wave/particle duality business. We should be just calling them waves and leave it at that.

[ParanoiA]

ParanoiA, do you really think that I learn physics to be special, unique, seek attention? Or for a living/career? You could not be further from the truth. I'm sure there are others who can relate when i say I study mathematics and physics for the sheer beauty of it. I believe there is nothing in this world more enlightening and rewarding than the study of these subjects. And no, i dont expect him to know this, but please make an attempt to learn BEFORE you post in such a website.

 

And it interests me how you understand string theory, but not quantum mechanics. I'd imagine the book was The Elegant universe. Let me tell you, You will not believe how much he simplified it so the public would understand it. But he was right to refer to them as point particles. They do not imply mass, but can have it. As to volume, none at all.

 

 

Ok, you need to go back and re-read what got us here. I don't know where in the world you got the idea that I know a damn thing about string theory. I said I'm about to start the section on it. And referred to Greene. You seem to know the guy and the book so I'm not sure where that came from.

 

Also, I'm 35 with a full time job and part time investment business, a musician and a writer - I'm not learning about physics for a career either dude, just for the shear beauty of it. I wish I had the time to take some college courses on it and actually learn it - rather than extremely watered down layman literature. But I don't. So, I'm trying to learn the qualitative summary through Greene, at the moment anyway. So, I get where you're coming from. I was just trying to get you to realize, that being jerk and getting pissed at people for not knowing something that you know, is short sighted and obvious - rather embrace that you know something others don't, and have fun and enjoy the reward of teaching and correcting.

 

I don't agree with Snail about molly coddling and so forth. This is just an excuse to carry on the shallow ego infested tradition of scientists brutalizing one another intellectually. This is ridiculous. It serves no useful purpose, and actually holds people back because they get gun shy and run around with incorrect or unclear knowledge just because scientists feel the need to act an ass to each other.

 

Anyway, digs at your age are because while you have the knowledge of an adult, your disposition was childish. You're pissed today. Well good for you. When has that been an excuse to take it out on anyone else? We were all taught that in grade school.

 

I cannot believe a single person in this thread hasn't seen that.

[J.C.MacSwell]

I disagree with most of what had been said so far. The difficulty is really the use of the word 'particle' which comes with it too much baggage.

 

The word 'wave' is much better - much more accurate. People have a bad habit of only thinking of a wave as a plane wave, spread out over all space. While this is a wave, waves can also be very localised. A wave can look and behave exactly how we percieve a particle would if it is sufficiently localised.

 

Indeed, since our measurements of position are never 100% accurate, we will never know exactly where a 'particle' is, so it is really a superposition of the position eigenstates, i.e. even after measurements, it is still a wave, albeit a localised one.

 

I think we confuse people when first talking about quantum mechanics with this wave/particle duality business. We should be just calling them waves and leave it at that.

 

Should we just call an electron a wave also, and leave it at that? I know there is a difference, to a degree, but reduced far enough there is no particle aspect left (I think?), yet both have some particle like properties that emerge from the "wave".

[Sequence]

I saw it, thats why I stopped posting here.

[Severian]

Should we just call an electron a wave also, and leave it at that? I know there is a difference, to a degree, but reduced far enough there is no particle aspect left (I think?), yet both have some particle like properties that emerge from the "wave".

 

Yes, that was partly my point. The electron is also a wave. It only behaves like a particle because its wave is highly localised. In fact, in Quantum Field Theory everything is a wave (or more precisely a field). We only continue to use the word 'particle' to distinguish between different types of field because the fields are quantised.

[Farsight]

Field quantum sounds good to me, Severian. It sheds that baggage. I like entity as well, it's nicely insubstantial in a "ghostly" kind of way. Are there any words or phrases the pros use in lieu of "particle"?

[Cap'n Refsmmat]

Okay, I think the question is pretty much answered now.

 

We can stop the attacks if you'd like.

[ParanoiA]

Well I have a follow up...

 

Light has no mass yet carries energy. Does this energy have weight? Quarks or anything?

 

I don't really understand energy.

[swansont]

Photons have momentum as well as energy. Photons have no rest mass (invariant mass). No quarks or other particles comprise them.

 

Photons will bend in a gravitational field, because the space itself is not flat.

[Farsight]

Light has no mass yet carries energy. Does this energy have weight?

 

No, Paranoia. It's like a wave down at the beach. It has no mass of its own. But it's got enough momentum to knock you flat on your back.

 

I don't really understand energy.

 

Maybe you will after you read this:

 

http://www.scienceforums.net/forum/showthread.php?t=23744

[swansont]

No, Paranoia. It's like a wave down at the beach. It has no mass of its own. But it's got enough momentum to knock you flat on your back.

 

Water has mass, and weight.

[Farsight]

It certainly does.