Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 02/08/2021 12:44:37
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Dearest Lightologists,
As a sheepy I am of course extremely fast and I can propel myself in a variety of ways. I can EWEs my legs, I can back myself into a corner and do a "bottom burp" ! so, this got me thinking....what makes light or any other type or radiation type stuff move ? What propels it and how does it know what direction to go ?
Like, how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ? what actually makes it move and one final kweschun, does it start off slow and then accelerate to C ?
ewe see, I don't know, but ewe telling me will help me know and I will then know !!
yayyyy...i will know stuff soon !!
hugs and shmishes
neil
xxxxxxxxx
Oh light oh light
I want to know what's right
I need to know your groove
About what makes ewe move
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What propels it
It isn’t really propelled, it propagates. Can help to think about the way a wave travels down a rope, the rope doesn’t move forward, just the rope moving up and down.
In the case of light it’s the electric and magnetic fields going up and down.
how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ?
It doesn’t. From a candle flame it goes out in all directions, unlike sheep.
does it start off slow and then accelerate to C
Not as far as we know. Being massless it isn’t a problem to go straight to c.
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Hi.
I hope everyone is well. It's also quite a nice question.
I like most of Colin2B's post.
It's easier to assume light (or similar radiation) is a wave and therefore it has to propagate. This is Colin2B's approach. Minor details like "how does it know which way to go" could be answered just by saying all possibilities come from what we know about wave mechanics and especially Maxwell's equation for electromagnetism. Any solution will show a valid propagation, sometimes initial conditions allow us to single out some directions of travel and eliminate others.
It would also be possible to consider light as a particle. Then your question makes more intuitive sense and will be a little harder to answer. Let's consider the annihilation of an electron with it's anti-particle as a simple example. It's simple because the photon (particle of light) was not there to begin with and it suddenly appears when the two particles come together and annihilate. Technically there are two photons created but let's not worry too much about the details. There is at least one where there was previously none. How does that photon "know" to travel at the speed of light and why doesn't it start slow and accelerate? The usual answer is that there just isn't any choice. We know that a photon should carry momentum and energy but it has no rest mass. The only solution for this in Relativity is that the massless photon must travel at the speed of light. It was created travelling at that speed, it never started slow and accelerated and it has no choice but to continue with this speed. That's it, that's enough for an end of school or early university level answer. However, it is almost a "get out" clause - we don't have to explain why or how it travels at c we just show that there is no other choice. If you really want to dig deeper into this, you will probably return to wave mechanics, since more advanced theories like Quantum Field Theory abandon the notion of particles entirely. Neither the original particles (including anti-particles) nor the photon were thought to behave like solid particles of matter - they were always waves.
Best Wishes.
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Thank ewe Colin2B and Eternal Student for your wonderful answers.
It is hard to grasp that Light does not accelerate to reach its terminal velocity.
It can be slowed down though can't it ? through water or another medium ? so, if it starts in a vacuum and then enters something then returns to a vacuum it has had to change speed twice ?
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It can be slowed down though can't it ? through water or another medium ? so, if it starts in a vacuum and then enters something then returns to a vacuum it has had to change speed twice ?
Yes, it is (locally) only c in a vacuum, and slower in other mediums. It changes speed (and possibly direction) every time the refraction index of its current medium changes.
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Light propagates as an electromagnetic wave, which are synchronized oscillations of electric and magnetic fields. Electromagnetic radiation or electromagnetic waves are created due to periodic change of electric or magnetic field.
Waves of electricity create waves of magnetism, which create waves of electricity and back and forth, making them capable of traveling through space.
https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/Light_animation_2.gif
Calculating the speed of these electromagnetic waves, Maxwell got the number of c (speed of light).
With the theory of relativity, Einstein realized the connection between time and space, a unified it known as space-time. The connection that allowed to translate between movement in space and movement in time. In other words, how much one meter of space is worth in time. Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa. The same velocity value: c.
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how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ?
A sensible sheep will not try to walk through a fire.
But photons are fearless, and will travel in every possible direction.
Some photons do make it through a candle flame from the far side .
- Outside the candle, air is cool and clear, so light from the near side has an unobstructed path to your eye
- But a photon from the far side has to pass through a hot maelstrom of unstable reaction products, which are more likely to absorb it, and then reradiate the energy from a closer point
- Some photons do make it all the way through the candle flame - but they are hard to see against the brightness of the nearer side
PS: Is there such a thing as a sensible sheep?
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Hi.
It can be slowed down though can't it ? through water or another medium ? so, if it starts in a vacuum and then enters something then returns to a vacuum it has had to change speed twice ?
Sheep don't ask questions and force logical connections like this. This is not an average sheep.
Halc's answer is a good one: Yes it does - BUT let's not explain how or why.
(F.Y.I. I don't have all the answers. I cannot see how a single photon can be fired into a glass block and somehow it either slows down or takes a non-straight path through the block. I'm suspicious that the photon is required to display wave-like behviour to explain this).
Best Wishes.
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Beware of falling into the common trap of confusing the model with reality. Electromagnetic radiation does what it does, and we need two quite different models to predict it, but neither claims to be what light is.
The remarkable facts to my mind are (a) that Maxwell could calculate the speed of light from essentially static experiments and (b) that whilst the propagation speed of a photon can be predicted from continuum electrodynamics, its direction is probabilistic and its interaction with matter is quantised.
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It can be slowed down though can't it ? through water or another medium ? so, if it starts in a vacuum and then enters something then returns to a vacuum it has had to change speed twice ?
Yes, it is (locally) only c in a vacuum, and slower in other mediums. It changes speed (and possibly direction) every time the refraction index of its current medium changes.
Thank ewe Halcy. So what is the speed of light in my room ? I'm not in a vacuum !
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Light propagates as an electromagnetic wave, which are synchronized oscillations of electric and magnetic fields. Electromagnetic radiation or electromagnetic waves are created due to periodic change of electric or magnetic field.
Waves of electricity create waves of magnetism, which create waves of electricity and back and forth, making them capable of traveling through space.
https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/Light_animation_2.gif (https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/Light_animation_2.gif)
Calculating the speed of these electromagnetic waves, Maxwell got the number of c (speed of light).
With the theory of relativity, Einstein realized the connection between time and space, a unified it known as space-time. The connection that allowed to translate between movement in space and movement in time. In other words, how much one meter of space is worth in time. Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa. The same velocity value: c.
Thank you TommyJ .Did Maxwell create a vacuum to measure the speed of light ?
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how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ?
A sensible sheep will not try to walk through a fire.
But photons are fearless, and will travel in every possible direction.
Some photons do make it through a candle flame from the far side .
- Outside the candle, air is cool and clear, so light from the near side has an unobstructed path to your eye
- But a photon from the far side has to pass through a hot maelstrom of unstable reaction products, which are more likely to absorb it, and then reradiate the energy from a closer point
- Some photons do make it all the way through the candle flame - but they are hard to see against the brightness of the nearer side
PS: Is there such a thing as a sensible sheep?
Thank ewe Evan. I guess this is similar to the way that light takes a gazillion years to finally leave the sun, but (confewesed sheep)...I thought as soon as light interacts with something it gets absorbed or converted into something else. So how does it survive all those collisions in the sun before it finally makes a run for it ?
sensible sheep ? guffaw chortle !!
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how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ?
A sensible sheep will not try to walk through a fire.
But photons are fearless, and will travel in every possible direction.
Some photons do make it through a candle flame from the far side .
- Outside the candle, air is cool and clear, so light from the near side has an unobstructed path to your eye
- But a photon from the far side has to pass through a hot maelstrom of unstable reaction products, which are more likely to absorb it, and then reradiate the energy from a closer point
- Some photons do make it all the way through the candle flame - but they are hard to see against the brightness of the nearer side
PS: Is there such a thing as a sensible sheep?
Thank ewe Evan. I guess this is similar to the way that light takes a gazillion years to finally leave the sun, but (confewesed sheep)...I thought as soon as light interacts with something it gets absorbed or converted into something else. So how does it survive all those collisions in the sun before it finally makes a run for it ?
sensible sheep ? guffaw chortle !!
The photons that leave the surface are not the same photons that started at the core. If a photon interacts with an atom, it adds energy to that atom. That atom will end up shedding all or part of that energy (depending on the type of interaction) by emitting another photon. That photon won't necessarily be emitted in the same direction as the original photon.
So while "a" photon eventually leaves the surface, it removed by a chain of many, many, of these types of reactions from the photon that started at the core.
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Hi again.
I'm enjoying this discussion. I like the little logical connections from one thing to another. Have you considered becoming a teacher? The thing is, you're using a discovery based style of teaching here. Unless there's been a revival that's not popular anymore. Never mind, I like it, let's roll with it.
I thought as soon as light interacts with something it gets absorbed or converted into something else
In the circumstances you were describing, yes. @Janus has presented a good discussion.
However, in general, we believe that light can also interact through gravity. Photons do seem to be a source of gravity and also the path taken by photons does seem to be influenced by gravity. Additionally the frequency of a photon can be influenced through interactions with gravity. So, for example, a photon could perturb a nearby atom and transfer some energy to it without being totally absorbed by that atom.
There are other interactions that may be possible without requiring total absorption. For example, you can change the polarisation of e-m radiation without without seeming to need to absorb it all.
Light is complicated stuff.
Evan_au also made this comment:
- Some photons do make it all the way through the candle flame - but they are hard to see against the brightness of the nearer side
Not only are they hard to see but they wouldn't be any different to the yellow/orange photons created on the other side of the flame. How would you know which side of a flame that light came from? Due to density fluctuations in the air and the issue of refraction your distance perception isn't really going to discern any difference. Anything I see beyond a flame is just a blurry and wavy image anyway.
Best wishes.
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It's easier to assume light (or similar radiation) is a wave and therefore it has to propagate. This is Colin2B's approach.
My approach was somewhat guided by our favourite sheepy, s/he mentioned a flame. The vapour in, say, a candle flame contains a vast number of atoms, each with changing energy levels and emitting millions of photons in all directions. When we have a mass exodus of photons we generally attribute wave properties to that collective, hence my approach. Sorry for teaching granny, but given the sheepy nature of this topic ........
By the way, there follows more granny stuff:
It would also be possible to consider light as a particle. .... Let's consider the annihilation of an electron with it's anti-particle as a simple example.
A valid approach, although I wasn’t thinking of annihilation in the case of a flame, but it has been suggested in the high energy effects of lighting storms.
theories like Quantum Field Theory abandon the notion of particles entirely.
We don’t abandon completely. When a field is quantized we can find discrete quantum excitations that we can interpret as particles, which is justified by the excellent agreement with experiments.
I'm suspicious that the photon is required to display wave-like behviour to explain this.
As@Janus says, we don’t need to use wave behaviour in that particular example. As you will know from studying QM, QFT often starts with derivations from Maxwell’s equations and photons are given attributes that we would normally associate with a wave.
Did Maxwell create a vacuum to measure the speed of light ?
(sings) Oh no sheep, no sheep, no sheep no.
Maxwell didn’t measure it, it came out of his calculations - not just light but any em radiation. Since the 1700s it had been calculated as around 300,000km/s based on stellar aberration, more accurate experiments followed although most will consider Roemer to be the first to make a measurement using Jupiter’s moons, but it was a bit low at around 214,000km/s.
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Hi again.
Did this one ever get answered?
Thank ewe Halcy. So what is the speed of light in my room ? I'm not in a vacuum !
Light in air is 1.0003 times slower than light in a vacuum, which slows it all the way down from 299,792,458 meters per second to 299,702,547 meters per second.
[taken from forbes.com]
I hate air. I know everyone else likes the stuff but I'm deeply suspicious of it. Air is a mixture of gases and so it is deeply sinister stuff. None the less, it's considered a (single) medium by most people.
On a similar note, light never travels at the speed of light in a vacuum - because there are no vacuums found in the universe. We believe that there is interstellar gas, neutrinos (CNB) and other photons (CMB) flying around in every part of space.
I think most of us accept that matter seems to slow down light but it's also quite likely that any form of energy does. For example, other photons may interact with and hinder the passage of photons. NOTE: I've not seen any hard and reliable references to back that up. It just seems likely. I'd be grateful for any good references that support or refute that.
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Hi again.
Did this one ever get answered?
Thank ewe Halcy. So what is the speed of light in my room ? I'm not in a vacuum !
Light in air is 1.0003 times slower than light in a vacuum, which slows it all the way down from 299,792,458 meters per second to 299,702,547 meters per second.
[taken from forbes.com]
I hate air. I know everyone else likes the stuff but I'm deeply suspicious of it. Air is a mixture of gases and so it is deeply sinister stuff. None the less, it's considered a (single) medium by most people.
On a similar note, light never travels at the speed of light in a vacuum - because there are no vacuums found in the universe. We believe that there is interstellar gas, neutrinos (CNB) and other photons (CMB) flying around in every part of space.
I think most of us accept that matter seems to slow down light but it's also quite likely that any form of energy does. For example, other photons may interact with and hinder the passage of photons. NOTE: I've not seen any hard and reliable references to back that up. It just seems likely. I'd be grateful for any good references that support or refute that.
Light does not interact with neutrinos, and no, other photons have no effect on its speed. As far as interstellar gas goes, we are talking about a few atoms per cubic meter. Since the spacing between the particles is so much larger than the wavelength of the photons, they only interact by scattering the light which only occurs if a photon strikes a particle.
But, because the particles are so few and spread out, this occurs rarely. Put another way, the mean free path of a photon( The average distance it travels before interacting with one of those particles) in interstellar space is pretty long.
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Thank you TommyJ .Did Maxwell create a vacuum to measure the speed of light ?
Thank you, neilep.
Maxwell had been working with the phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Waves of electricity create waves of magnetism, which create waves of electricity and back and forth, making them capable of traveling through space.
And when he calculated the speed of these electromagnetic waves, Maxwell got the number of c.
https://en.wikipedia.org/wiki/Electromagnetic_radiation#Maxwell's_equations
https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#The_origin_of_the_electromagnetic_wave_equation
Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell, distilled his theory and found that this conversion rate is exactly same number.
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Hi again.
Thanks for your reply Janus. I like it a lot and will probably continue to think about it and seek more references.
Light does not interact with neutrinos, and no, other photons have no effect on its speed.
I like this bit and it seems reasonable. I may speak more about it later.
Since the spacing between the particles is so much larger than the wavelength of the photons, they only interact by scattering the light which only occurs if a photon strikes a particle.
But, because the particles are so few and spread out, this occurs rarely. Put another way, the mean free path of a photon( The average distance it travels before interacting with one of those particles) in interstellar space is pretty long.
This is also reasonable. However, scattering would cause significant changes in direction of travel for a photon and much of this change would be random. When light is sent into a medium (like a block of glass), it doesn't alter direction while travelling through the medium. There may be some change in direction at the interface between two mediums but even this can be prevented if the angle of incidence is 0 degrees. Now obviously some mediums do block light and this will have atoms in it which light can be scattered off. For example, if we made a glass block with some microscopic particles of silver embedded in the glass. However, glass (mostly Silica atoms) just doesn't absorb or block light. There should be no scattering but somehow the light is slowed down while passing through the glass.
I also appreciate what you're saying about wavelength and the space between atoms - but some of this can be brushed aside if we take a broader definition of light. We can use radio waves of very long wavelength where the wavelength is the same order of magnitude as the space between atoms. Now there is a relationship between wavelength of light and the refractive index of a medium (how much it is slowed down) - which is why a prism will effectively spread out white light into a rainbow. However, the refractive index varies in a continuous way with the wavelength (to the best of my knowledge) so that light of any wavelength should be slowed down a little as it passes through a medium like interstellar gas.
The best explanations of why light slows down as it travels through dense mediums that I have seen involve using a wave description of the light. The electric field in the incident light agitates electrons in atoms which they pass close to - not enough to excite them to a higher energy state in the atom (that would effectively be total absorption) - but enough to cause those electrons to create their own electric and magnetic fields. The apparent slowing of the light is then a result of superposition of the all the electro-magnetic fields in the medium. I have never seen a convincing explanation for why light slows down as it passes through a dense medium using a particle model of light.
If what you were saying about a diffuse gas like interstellar gas is correct, then we should be able to do the following experiment:
Prepare a diffuse (near vacuum) sample of a gas like Hydrogen and identify all the emission lines of the atoms. Do not send in light of these wavelengths (hoping to ensure that absorption or scattering won't happen). Now send in one photon at a time and allow it to cross the gas. Measure the time it takes to cross the gas and calculate the propagation speed through the medium. If scattering or absorption did happen then we should know - because the photon either won't appear at the other end or it will be scattered off and leave the gas chamber at some unusual position and angle. I haven't done this experiment nor do I have any references for such an experiment having been done. However, I suspect that even if you send in one photon at a time they will always be sowed down while travelling through the medium. There may be a more dramatic scatter event sometimes but this will be easily detected - but even those photons that exit the gas chamber exactly where they were expected and in the expected direction should have been slowed.
You (Janus) probably won't need a Pop Sci video but if anyone else is reading and seeks clarification then this YT video by FermiLab takes less than 10 minutes and explains why scattering or absorption by atoms is not a good explanation for why light slows down:
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If I may return to the earlier point about neutrinos and other photons....
Photons aren't just limited to interactions through the electric and magnetic fields. Photons have energy and so they contribute to the stress-energy tensor as a source of gravitation in General Relativity. More generally, photons can interact with any other gravitating source. Neutrinos don't have an electric charge, so I'm sure that a photons interaction with them is limited but I can't be certain that there is no interaction. This is speculation not hard facts and I would not want anyone to think that light propagates more slowly when it passes through a chamber of neutrinos - it's just that I can't eliminate the possibility.
As regards an interaction with other photons - well this does seem to happen. The preferred method of explaining why light propagates more slowly through a dense medium is based on the principle of superposition of waves. I don't seem to need the atoms of that medium to be there, just the right electric and magnetic fields at the right places along the path of travel of the incident light.
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Summary: I don't know. The apparent slowing of light as it travels through a dense medium is complicated. Sorry if I have side-tracked this thread a bit too much.
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Speed of light can be misleading about experiments (explained below).
Let me address two points: slowing speed of light in a transparent media (1) and speed of light in vacuum (2).
Light moves slower through transparent denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.
The c = 299,792,458 m/s was never proved by an experiment. It was derived from round-trip time delay.
Speed of light is rather a convention rather than a fact.
Einstein himself in 1905 put it in his paper ‘On the Electrodynamics of moving bodies’.
There is no way for synchronizing the clocks, measuring light start point time and end point time.
Quote from the paper: ‘‘Stipulation that I can make on my free will to arrive at a definition of simultaneity.’
His ’convention definition’ that time travel is the same toward and backwards (Einstein’s synchronization convention).
This convention is accepted as one of the asymmetries in the universe (for instance, uneven matter distribution).
If the speed of light is not the same in different directions. It may go c/2 in one way and instantaneous in another. So even we might see stars exactly as they are now.
When light arrives at the detector, you know nothing about light. You just see it, and it’s there.
As long as c is correct for round trip delay, none of the physics breaks.
Since 1905 we have used Occam's razor for the exploration (same speed of light in both directions).
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Hi TommyJ,
Thanks for your reply.
Light moves slower through transparent denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement
This is a common explanation and it's good enough for school. Unfortunately, I don't think it fully explains what is observed. Absorption and re-emission by atoms should alter the direction of travel of the light.
I'm concerned that this is hijacking the original post. The OP should feel free to shout stop at any time.
Best wishes to everyone.
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I don't think of it as 'propagating' as much as it follows those laws we define, and the properties we define to it of course. It's very abstract light and nothing I can hold in my hand. One description is the one in where it becomes a emanation in a field. I think Professor Strassler uses that analogue too, although describing the Higgs field and Higgs boson? Thinking of it that way, and then thinking of the reason given to why it has a 'recoil' it moves it into a very abstract sphere. The law of conservation of momentum
https://www.nuclear-power.com/laws-of-conservation/law-of-conservation-of-momentum/
https://profmattstrassler.com/
and https://scienceblogs.com/builtonfacts/2010/04/21/laser-rifle-recoil
and this law of momentum is a observed fact, but why it must exist moves it away from practical experiments into something abstract.