How can light be both a wave and a particle?

[Mathochist]

@SnakeSpirit

I know you got an exciting discussion going on about parallel universes and all

It’s more about analytical reading ability than anything else. I figured I’d let a proper physicist answer your questions. If there’s none forthcoming, though…

Particles? I don’t believe in them. At least not in the sense of what we conceive of as matter (as in particles of matter). Sub-atomic “particles” are energy (IMNSHO), and like a magnetic field, or electrical field, can act upon other fields, which is why we maybe call them “particles.”

In a certain way, yes. They’re called particles because they have particle-like aspects, just like some quantum phenomena are called “waves” because they have wavelike aspects. As to what they are, the jury’s essentially still out, but “little globs on energy” is as good a way to think about them as any other.

A wave is an expression of energy. Drop a pebble in a pond, and the waves ripple out, bounce off or are diverted around objects and eventually spread out far enough to be imperceptable. The pebble was energy itself, which displaced the energetic expression of a volume of water, etc.

Again, this – while not necessarily the most accurate to the cutting-edge formulas – is as good a loose idea as any other. The idea that energy can be nonlocalized (as in a wave) or localized (as in a particle) depending on how you look at it really goes to the heart of the wave/particle duality.

I was having some problem with resolving this with the speed of light in a vacuum, however, since if space is a true vacuum, how can light create a wave? Can energy exist without some sort of matter in which to express itself? Or is it just that energy needs matter to produce observable effects?

This is actually the line of reasoning that led to the ether hypothesis, which was later disproved. The stunning fact is that light doesn’t need a medium to wave. The electromagnetic potential field is an assignment of a “cotangent vector” (loosely, a collection of four numbers with certain transformation properties as the coordinate system of the observer changes) to each point of spacetime. This satisfies a differential equation very similar to that of waves in a physical medium. In particular, a local disturbance propagates at a certain rate, the speed of light.

I know, I know, simplistic thinking, but I’m a simple person.

Don’t beat yourself up so much. Asking questions, even if they’re based on false premises (say, “what about a Many-Worlds Interpretation of QM where different worlds interact?”), is the best way of learning something. Conversely, the best way of teaching (and in fact being sure that you know it in the first place) is to answer such questions, and to explain the answers (say, why the premises are false rather than just asserting their falseness).

Well, magnets operate in a vacuum, so why not light? I guess gravity (whatever that is) operates in a vacuum, too. Perhaps a vacuum is just another form of energy that we can’t yet measure. maybe it’s that “dark matter” scientists are trying to account for.

There’s a certain sense in which “empty space” is still something under current models, but it’s definitely not dark matter or dark energy. The vacuum is sort of a background on which everything else happens (there are some theories where the vacuum itself is an epiphenomenon: like the force keeping you from falling through the floor is an epiphenomenon of the electromagnetic interactions between the floor and the soles of your shoes). Dark matter and energy are two different “things” that can go on in the vacuum.

[Chronos]

What you mean by a “vacuum” depends on context, but it’s usually considered OK to have fields in a vacuum. And light is made up of electromagnetic fields, qualitatively not unlike the field of a magnet or of a static-charged object. So yes, light works exactly as well in a vacuum as do magnets, because they’re manifestations of the same phenomenon. Gravitational fields do seem to be a bit different, in that they’re made out of the space itself, while it’s not at all clear that the same is true of electromagnetic fields.

Quoth Mathochist:

There’s a certain sense in which “empty space” is still something under current models, but it’s definitely not dark matter or dark energy.

Not so definitely. The dark energy is actually considered by many physicists to be the vacuum itself, or some manifestation of it. What exactly this means, though, is far from clear. The dark matter, however, is not vacuum, and at least some (though not all) of it is perfectly ordinary matter of the sort we’re familiar with.

[SnakeSpirit]

VERY sincere thanks to Mathochist and Chronos for replying cogently to my queries. Although I’m not yet a threat to these high-physics discussions, I have composed my mental picture more completely, and have an increased understanding.

My gratitude to you both.

S

[Mathochist]

@Chronos

Quoth Mathochist:Not so definitely. The dark energy is actually considered by many physicists to be the vacuum itself, or some manifestation of it.

Well, this partially depends on philosophy, as reflected in different ways of writing the Einstein equation. SnakeSpirit, I’ll try to keep this as heuristic as possible.

The Einstein equation involves three things: the “stress-energy tensor” T, which measures how much classical stuff is at a point in space in a way compatible with Special Relativity, the “metric tensor” g, which can be thought of as the gravitational counterpart of the electromagnetic potential, and the “Einstein tensor” G, which is a certain combination of second derivatives of g measuring how the space described by g is curved.

Now, the equation can be stated either as

(1) G = T + kg

or as

(2) G - kg = T

for a suitable “cosmological constant” k. Version (1) heuristically says that the curvature of space is determined amount of matter and forces and so on at a point plus a certain contribution from space itself (kg). Version (2) says that the amount of matter and forces and so on determines the curvature, but the appropriate measure of curvature isn’t the original G, but rather the combination (G - kg).

The math works out the same either way. It’s just a choice between considering the cosmological term as “more stuff” (version 1) or “more curvature” (version 2). I tend to prefer the latter in my own private thoughts, but I admit I’m a hobbyist at differential geometry, let alone GR and extensions thereof. There may be a compelling physical argument for placing the term on one side or the other that I don’t know offhand.

[Chronos]

There may be a compelling physical argument for placing the term on one side or the other that I don’t know offhand.

No compelling argument, but most physicists nowadays put the cosmological constant on the “stuff” side of the equation. The reasoning is that most models of particle physics suggest that there should be a contribution of that form from the foam of virtual particles. Of course, a back-of-the-envelope estimation of that contribution is off by between 40 and 120 orders of magnitude, so absent some miraculous almost-exact cancellation, that’s not the source of the actual dark energy. But it’s the closest thing we have.

[SlowMindThinking]

One thing that makes GR hard, is the difficulty in making distinctions of the type Chronos and Mathochist are making. If what I say doesn’t help, then ignore me. Mathochist and Chronos have said nothing wrong.

The metric tensor g tells you how far apart any two points are in spacetime. (So, it takes four numbers, 3 “positions” and one “time”. I use quotes, because one thing relativity does is blur the distinction between space and time.) One result of GR, and GR-like theories, is that g and the electromagnetic four potential are analogous. That analogy is the starting point for trying to unify gravity under a quantum theory with the other forces.

Traditionally, the stuff that is in T, the stress-energy tensor, is considered the “source” of spacetime curvature, and everything outside of it is considered the behavior of spacetime itself in response to the “source”. The problem is, everything is pretty much of a piece. For example, the “kg” term that mathocrist refers to, could be stuffed in side of the stress energy tensor. From a purely GR perspective, a term like kg would be considered vacuum energy because it has a form that looks like a simple energy field, with no obvious source. Whether or not it is vacuum energy would be a prediction of some unified theory.

“Real” stress-energy tensors, such as one describing the solar system, describe matter as a function of position. It sounds simple enough. You specify some positions for some masses, and solve some horribly difficult equations for g, and viola, have g, which tells you everything about spacetime. But there is a chicken and an egg problem here. How do you specify where your masses are, if you don’t know the thing that tells you how far apart your masses are? The meaning of your coordinate system that you use to specify your mass positions is specified by your metric tensor, g. Having a term in your stress-energy tensor that explicitly depends on g makes this interdependence obvious.

Likewise the Einstein tensor, G, is a set of differential equations for g, so you could follow mathocrist and lump the kg term inside of G. The reason that you usually don’t pull all terms proportional to g into G, is because you can sometimes attribute the proportionality to a “real” source. If I keep getting interrupted here at work, I’ll never remember if a photon filled universe, or a dust filled universe is linear in g.

So, where you put a term like kg is arbitrary, within GR. Only some quantum theory distinguishes them.

As an aside, you might ask how anyone solves any problem in GR. All the exact solutions that I can think of right now, assume some symmetry. The symmetry constrains g and T. For example, a spherically symmetric solution requires a spherically symmetric stress-energy tensor.

[SnakeSpirit]

Thanks, guys, but you don’t have to continue to “dumb down” the discussions for me any more. I appreciate the thought (and no, I did not get an overnight Ph.D. in quantum physics), but your explanations for me have caught me up about as much as I’m able to be caught up. I can conceive of the principles involved adequately for my uses (mostly curiosity, a desire to know more about my universe), and I’ll follow along now as best I can.

Please don’t let me slow this discussion down.

[schindler]

@1010011010

Light is neither a wave nor a particle. It is light… and light can behave like a wave or a particple, depending on the experiment you conduct with it… but light itself is neither a wave or a particle.

That’s how I choose to think of it, anyway.

No u misunderstand: It IS a wave, but it can only be emitted in certain quanta or packets, known as photons. So it’s often useful to call the photon a particle, but actually it’s just a packet of light, which is a wave. It is also sometimes better to think of it as a wave instead of a particle, since we can’t predict the path of a particle but with a wave equation we can show the probability of it going to a certain place.

[Mathochist]

@schindler

No u misunderstand: It IS a wave, but it can only be emitted in certain quanta or packets, known as photons. So it’s often useful to call the photon a particle, but actually it’s just a packet of light, which is a wave. It is also sometimes better to think of it as a wave instead of a particle, since we can’t predict the path of a particle but with a wave equation we can show the probability of it going to a certain place.

No, it is neither, but is something which behaves in certain ways like one and in certain ways like the other.

[schindler]

[QUOTE=

Now, the equation can be stated either as

(1) G = T + kg

or as

(2) G - kg = T

for a suitable “cosmological constant” k. Version (1) heuristically says that the curvature of space is determined amount of matter and forces and so on at a point plus a certain contribution from space itself (kg). Version (2) says that the amount of matter and forces and so on determines the curvature, but the appropriate measure of curvature isn’t the original G, but rather the combination (G - kg).

The math works out the same either way. It’s just a choice between considering the cosmological term as “more stuff” (version 1) or “more curvature” (version 2). I tend to prefer the latter in my own private thoughts, but I admit I’m a hobbyist at differential geometry, let alone GR and extensions thereof. There may be a compelling physical argument for placing the term on one side or the other that I don’t know offhand.[/QUOTE]

            Don't forget, Einstein later admitted that the idea of a cosmological constant was the biggest mistake of his life. It is generally believed that the universe doesn't, as he originally said, have an inbuilt tendency to expand, rather expansion is the result of the Big Bang. From here, there are three theories, whether the universe will continue to expand, will someday be pulled back by gravity and come to a Big Crunch, or will continue to stay right on the boundary between collapse and expansion, so it'll lean just a little to the side of expansion. 

But that’s going off on a tangent… Don’t forget, Einstein was brilliant and is the foundation of modern physics, but in his time there was a lot htat wasn’t known about hte universe, and a large part of what wasn’t known wasn’t known because he had yet to discover it. So different interpretations of general relativity and special relativity for that matter have to take into account new discoveries, ie the wave particle duality, which einstein fought till he died (it is now generally accepted as true. He felt that scientific determinism was a must, to quote him “God does not play dice with the universe.” It is possible that his whole idea of a cosmological constant was only stated for the purpose of making sure we wouldn’t have to worry about a big bang, because that introduces a certain amount of chaos into it, and Einstein wanted everything to be predictable, whereas at a singularity like the Big Bang the laws of physics break down because of hte problem of infinite density.

[schindler]

[QUOTE=

Now, the equation can be stated either as

(1) G = T + kg

or as

(2) G - kg = T

for a suitable “cosmological constant” k. Version (1) heuristically says that the curvature of space is determined amount of matter and forces and so on at a point plus a certain contribution from space itself (kg). Version (2) says that the amount of matter and forces and so on determines the curvature, but the appropriate measure of curvature isn’t the original G, but rather the combination (G - kg).

The math works out the same either way. It’s just a choice between considering the cosmological term as “more stuff” (version 1) or “more curvature” (version 2). I tend to prefer the latter in my own private thoughts, but I admit I’m a hobbyist at differential geometry, let alone GR and extensions thereof. There may be a compelling physical argument for placing the term on one side or the other that I don’t know offhand.[/QUOTE]

            Don't forget, Einstein later admitted that the idea of a cosmological constant was the biggest mistake of his life. It is generally believed that the universe doesn't, as he originally said, have an inbuilt tendency to expand, rather expansion is the result of the Big Bang. From here, there are three theories, whether the universe will continue to expand, will someday be pulled back by gravity and come to a Big Crunch, or will continue to stay right on the boundary between collapse and expansion, so it'll lean just a little to the side of expansion. 

But that’s going off on a tangent… Don’t forget, Einstein was brilliant and is the foundation of modern physics, but in his time there was a lot htat wasn’t known about hte universe, and a large part of what wasn’t known wasn’t known because he had yet to discover it. So different interpretations of general relativity and special relativity for that matter have to take into account new discoveries, ie the wave particle duality, which einstein fought till he died (it is now generally accepted as true. He felt that scientific determinism was a must, to quote him “God does not play dice with the universe.” It is possible that his whole idea of a cosmological constant was only stated for the purpose of making sure we wouldn’t have to worry about a big bang, because that introduces a certain amount of chaos into it, and Einstein wanted everything to be predictable, whereas at a singularity like the Big Bang the laws of physics break down because of hte problem of infinite density.)

[Mathochist]

@schindler

Einstein later admitted that the idea of a cosmological constant was the biggest mistake of his life.

And that statement itself is true only insofar as his motivation. It’s a mistake to add it for the sake of preserving the universe eternally, but it’s a mistake not to include it without very strong justification of the assumption that empty space is Ricci-flat.

[Chronos]

And that statement itself is true only insofar as his motivation. It’s a mistake to add it for the sake of preserving the universe eternally, but it’s a mistake not to include it without very strong justification of the assumption that empty space is Ricci-flat.

In other words, for generality, the term should be included in the equations, with the stipulation that, absent experiments to the contrary, the value of that term might be zero.

Which is moot, now, since we do now have experimental results which indicate with a very high degree of certainty that the cosmological constant is nonzero, and in fact is greater than all of the other “stuff” in the Universe combined. Einstein was a great thinker, but thought alone doesn’t cut it in science, without experimentation. Since Einstein did not have access to the MAP results or the Hubble keystone Sn1a survey, he was not able to determine that the cosmological constant was nonzero and significant.

And 1010011010’s interpretation of wave-particle duality is perfectly valid, and is probably the safest way to approach the question. Light is light, and depending on the context, it can sometimes behave like a wave, and sometimes like a particle.

[Loopydude]

@Chronos

In other words, for generality, the term should be included in the equations, with the stipulation that, absent experiments to the contrary, the value of that term might be zero.

Which is moot, now, since we do now have experimental results which indicate with a very high degree of certainty that the cosmological constant is nonzero, and in fact is greater than all of the other “stuff” in the Universe combined.

It’s safe to say it’s disconcertingly close to zero, though, correct? Hence all the hand-wringing about anthropic cosmologies?

[Chronos]

Whether it’s close to zero depends on how you approach the question. If you approach it as a cosmologist, the natural thing to compare it to is the total energy density of the Universe, and looking at it that way, it’s not close to zero, being in fact the bulk of the energy density. Certainly, one can’t ignore its effects.

If you approach it as a particle physicist, though, it’s very disturbingly close to zero. Particle physics provides a way to estimate the value it “should” have, but it’s the worst estimate in history, being (as I mentioned above) at least 40 orders of magnitude too high. Note, that’s not a factor of 40, that’s 40 orders of magnitude, or a factor of 10[sup]40[/sup]. In the best case. Now, that would probably be OK if the cosmological constant were actually exactly zero: That would just mean that everything in that (very rough) estimate cancelled out with each other. But as it is, it’s not quite zero, meaning that (if the particle physics estimate is at all relevant) the terms cancel out almost exactly, leaving behind a tiny little fragment 40 orders of magnitude smaller than any given term. That’s inconceivably (and I do know what that word means) implausible to fine-tune, as you say, and does indeed lead to a lot of hand-wringing.

[Loopydude]

Considering the fact that it appears to be the majority of “stuff” is an interesting way of putting it, and I hadn’t thought of that before. Thanks for the anwer!

[TLC1]

I am not a scientist in the least, but unless I am mistaken, doesn’t water often present itself as both a wave, ( stored energy ) and a particle ( molecules)? Wouldn’t a group of photons generated into a wave of light react in a likewise manner?

I’m most likely all wet here (no pun), but would appreciate someone of knowledge to briefly explain the basic difference.

[TLC1]

@TLC1

I am not a scientist in the least, but unless I am mistaken, doesn’t water often present itself as both a wave, ( stored energy ) and a particle ( molecules)? Wouldn’t a group of photons generated into a wave of light react in a likewise manner?

I’m most likely all wet here (no pun), but would appreciate someone of knowledge to briefly explain the basic difference.

Never mind. I just realized that the water itself doesn’t move, but transfers the wave energy from molecule to molecule along the route. Perhaps photons themselves do not actually travel, in the general sense, but somekind of similar energy transfer is taking place through some, as yet undiscovered, medium.

Dark energy?

[Stealth_Potato]

Being a student engineer, I like to think of the wave/particle properties of light in practical terms. The photoelectric effect is a good example of light behaving like a particle, and the fact that I can stick my face right up next to the window of my microwave oven without getting my brain fried is a good example of light behaving like a wave.

[tim314]

@TLC1

Never mind. I just realized that the water itself doesn’t move, but transfers the wave energy from molecule to molecule along the route. Perhaps photons themselves do not actually travel, in the general sense, but somekind of similar energy transfer is taking place through some, as yet undiscovered, medium.

Dark energy?

No, photons really move. In fact, they move at the speed of light.

There’s not an “undiscovered medium” at work here. Light is well understood. It’s not a wave of matter, so it doesn’t require a matterial medium. It’s a wave of electric and magnetic fields.

“Dark energy” has nothing to do with it. It comes up in explanations of the acceleration of universal expansion – it’s not related to the nature of light.