How do we know that light is actually made of particles, and not waves upon pure energy? Where's the proof?

Question

Has anyone ever witnessed a photon, or seen a photon cause things that only an inertial particle would cause? I read that photons are massless, so how can they have inertia? If photons are massless, why is it that they can't travel through all mediums? Why is it that the scientists seem to think there are an endless supply of photons.

I understand that sound won't travel through a vacuum. Therefore, light shouldn't be able to travel in a vacuum if it is simply a wave, right? I don't believe so. I believe that solid substances (in terms of particles like protons and electrons) are made of intensely grouped bundles of energy. A vacuum, in turn, would be a level, dispersed, sort of flat region of indifferent energy. Stephen Hawkings suggested this with the string theory.

Where is the proof that light is made of photons???

I understand the photoelectric effect, but it's possible that the particular frequency of the light actually affects the metal (like a resonant frequency) .

I guess traveling in a straight line would be one good evidence

Answer 1

Clearly your current belief system conflicts with much of what modern physics believes to be true. That's okay. Everyone is entitled to their own beliefs.

Please understand this: it is impossible to prove any physical theory. The best anyone can do is demonstrate that a theory provides correct predictions for a given set of circumstances. Another, competing theory, may also provide correct predictions for the same set of circumstances. Is one theory correct and the other theory incorrect, or are both correct? Is it possible that both are incorrect?

You state "light shouldn't be able to travel in a vacuum if it is simply a wave, right?" And then answered your own question with "I don't believe so." Science is all about belief, but it is belief based on rational thought, reasoning, and experiment.

The Scientific Method: observe, explain, predict, then verify. These four steps are absolutely the bedrock foundation of modern science. Note that the last step, verify, leads back to the first step, observe. If you can traverse this loop without stumbling or waving your hands or invoking a higher power (like Stephen Hawkings) then you probably have a robust theory that explains something.

Why do I say it is impossible to prove any physical theory? Because it only takes one counter-example, one single fact, one observation that does not agree with the theory, to absolutely prove that theory is wrong. Even the most trusted theory must be considered to be potentially wrong, because some new observation could render the theory incorrect... like light from a distant start bending as it passes through the gravity field near our sun. Who ordered that?

So, we offer NO PROOF that light is made of photons. All we can offer is observations, explanations for those observations, and predictions of what will happen if obeservations are made in the manner our theory says is required. It is then up to the individual to study our evidence, our theory, our predictions and then decide whether or not to believe our theory.

So here is what we observe when we are looking for photons:

There is a device called a photomultiplier that emits electrons when light of an appropriate wavelength (color) shines on its photosensitive cathode. The electrons are emitted with a maximum energy that is inversely proportional to the wavelength of the light. These photoelectrons constitute an electron current that can be collected at a nearby anode, amplified, and measured.

As we reduce the amount of light shining on the photocathode, the number of electrons emitted per unit time interval, which by definition is the electron current, decreases. We can choose to decrease the amount of light in any manner we care to: by adding neutral density filters in the light path, by passing the light through smaller apertures, by moving the light source further away from the photocathode, whatever.

At some point in this light dimming process, we note that the photoelectron current is no longer a smooth and steady current that is a continuous function of the light intensity. The light has a constant intensity but it is very dim. The electron current now appears to come in short random pulses. If we count these pulses over a long period of time we discover that the number of counts per unit time interval is linearly proportional to the light intensity, exactly the same as the continuous electron current we measured at higher intensities. This occurs no matter how much we reduce the amount of light.

From these observations what may we conclude? In other words, what theory do we propose to explain these observations?

It is reasonable to conclude that each pulse of current at low light levels must have originated from a single electron emitted from the photocathode. But what would cause a single electron to be emitted at a specific energy if not a small packet of light, what we now call a photon, of nearly the same energy?

You are free to make up your own theory, but that works okay for me.

We know it takes a certain amount of energy to free the electron from the photocathode, the so-called work function. And we know that the maximum energy of the ejected electron is inversely proportional to the wavelength of the light that produced it. Shorter wavelengths produce more energy in the ejected photoelectron.

The single photoelectron event we detected was observable because the photoelectron was accelerated by an electrical field in the photomultiplier (adding kinetic energy to the photoelectron) until the photoelectron struck an internal structure called a dynode. The dynode is coated with a substance of low work function. It is easy to knock multiple secondary electrons loose from its surface, simply by hitting the surface with a high-energy electron.

When the accelerated photoelectron strikes the first dynode it causes two or more secondary electrons to be ejected from the surface. These ejected electrons are accelerated by another electric field toward a second dynode, where the process repeats. After ten or so stages of this, with more secondary electrons emitted at each stage, the final result is millions or billions of electrons collected for measurement as a pulse of current from a single electron emitted from the photocathode.

What else do we observe? These photoelectron events appear to occur randomly at low light levels. There is no way to predict exactly when a pulse of amplified photoelectron current will occur. If we place a closed shutter in front of the photomultiplier, to exclude all light from the photocathode, the number of pulses per unit time decreases. If we open the shutter, the number of pulses per unit time increases. This always happens no matter how low the light intensity is made.

What is causing the bursts of current when the shutter is closed? In any device of this nature, there is a small but positive probability that an electron will be ejected from either the photocathode or from one of the dynodes by thermal motion of the electrons comprising those structures. No light is necessary, it just happens. We observe it happens less often if the photomultiplier is cooled, which is why we attribute the random pulses in the absence of light to thermal excitation. But there is another mechanism: cosmic rays, highly energetic particles coming in from outer space to attack us and mutate our precious DNA!

It is fortunate that the cosmic ray flux at the earth's surface is very small. About one every hour or so crosses any given volume of about a cubic meter. The chance that one will hit our photomultiplier, and cause one or more electrons to be ejected from whatever surface it passes through, is small but still greater than zero. So even cooling the photomultiplier to absolute zero will not eliminate the cosmic ray "noise". And we don't have to! The cosmic rays, as well as the thermally ejected electrons, will be present no matter whether the shutter is open or closed. The extra pulses they add will statistically add up to zero if we subtract the number of pulses counted with the shutter closed from the number of pulses counted with the shutter open, assuming the time open is the same as the time closed.

Now here is a spooky thing: We can set up a Young's double-slit experiment between the attenuated light source and our photomultiplier. Turn the light intensity up and characteristic fringes will appear on a white card placed in front of the photomultiplier. Turn the light down until all we get are random pulses from the photomultiplier, the average rate of which is proportional to light intensity.

Now mechanically scan the photomultiplier across the plane where the white card and the fringes were. Record the pulse rate at each position. You will find the pulse rates correspond exactly to the light and dark stripes of the fringes. How is this possible if light is made up of discrete photons? Where did the wave interference come from to produce a fringe pattern from single photons? Somehow each one of those photons that were counted had to make a decision, as it passed through one of the two slits (how could it have passed through both?), that it was going to land on a bright fringe or a dark fringe or somewhere in between. That photon, somewhere in its lifetime from light source to absorption on the photocathode, had to somehow also act as a wave during passage through the two slits.

I hope this clears things up for you.

Answer 2

I think the complementary nature of light or any electromagnetic radiation has been well established. Optical phenomena like Interference, Diffraction and Polarisation can only be interpreted by the wave concept. On the other hand Photoelectric effect or Compton effect can only be interpreted on the basis of the particle or photon concept. Einstein had extended Planck's hypothesis of energy quantisation to the photoelectric effect by postulating that each quanta of light (namely photon) has an energy proportional to the frequency of light and successfully derived the Planck's constant from the photoelectric equation.
A photon is emitted whenever there is a transiton of an electron from a higher energy level to a lower enrgy level. The photon energy being equal to the difference between the two energy levels.

Some of your premises sound strange indeed when there is ample experimental evidence about the true nature of light. Light is an electromagnetic wave unlike sound which is a mechanical disturbance. Hence it does not require a physical medium at all for propagation and travels always with the same speed in vacuum.

Answer 3

Einstein won his Nobel Prize for figuring this one out. Referring again to the photoelectric effect, the energy of an emitted photoelectron increases uniformly with increasing frequency, once the minimum photon energy has been attained. Further experiments varying both frequency and amplitude of the incident radiation show conclusively that it is a photon process. More advanced studies, using things like the Compton effect, also show this. (I did such an experiment in college.) The quantum theory, and its nephew quantum eletrodynamics, are now the most reliable theories in all of physics, capable of predicting some phenomena to an accuracy of 12 decimal digits -- and probably better, but that's as good as we can measure right now.

Answer 4

The proof that light is a wave is that it produces interference patterns. The proof that it is a particle is that it hasnon-continuous magnitudes, transfers momentum to other objects, and is affected by gravity. The first no particle can do, the second no wave can do. Whether it really is an observable particle or not doesn't matter as long as it behaves like one, but in fact the effect of individual photons has been observed.

Answer 5

There is no proof that light is a particle or indeed that its a wave, but depending on the circumstances the best way to think of light is as either a particle or a wave. For example if its radiating from a source or being defracted its best to think of it as waves. If its bouncing off a mirror or triggering a detector to give a single "ping" each time the detector senses light, then the best explanation is as a particle.
In truth light isn't either; its a quantum "entity" and no-one really "knows" what it is.

Answer 6

I have no clue .