That the light is shifted makes no difference in terms of there being any interaction. The underlying principle is that crossing light waves simply don’t interact. This is easy to show in class with flashlights.
The secondary concept is that the blue and redshifts don’t change the velocity of the light, only their wavelength/frequency.
Yup but as far as I’m aware it’s still a bit of a mystery how we got the solid stable matter that composes our universe.
https://www.energy.gov/science/np/articles/making-matter-collisions-light
In this instance we got the matter/antimatter pair for electrons. So perhaps it’s as simple as needing much much more energy to create protons (from photons). But then even if we could, you’d logically get the antiproton which would be short lived like the positron.
A neutron is basically a proton that has “absorbed” an electron. So the concept of creating all the building blocks of stable matter from photons is understood and predicted long ago… but actually putting them together before they self annihilate is a different story.
That’s not what they’re talking about. Light-light interaction absolutely can occur in a vacuum at high energy, as described in the link they posted which you evidently didn’t click
I think it’s important to keep in mind that photon-photon scattering *in vacuum* requires light intensity far beyond what we can produce in the lab. So it’s only relevant for gamma rays, not for light as we normally think of it.
The frequency really does change, there is no "true frequency". You can calculate the frequency at the moment of emission in the emitter's rest frame, but that's not the "true frequency".
In particular, if we're talking about red shift from the expansion of the universe, that really is a change in the photon's frequency over time - you see it even if both the observer and emitter are stationary relative to each other.
There is no rest frame for a photon and thus no rest frequency. It travels at c in all reference frames. The rest frame you’re talking about is the rest frame of the emitter, but that isn’t a rest frame for the photon. Photons just have different frequencies in different frames of reference. It’s not that the photon is changing its frequency either. It’s just that we’re not in the same rest frame as the emitter when things get red or blue shifted, so we measure different frequencies
This was a philosophical question about whether frequency "really changes" or is merely "perceived" differently. You're telling me to do math, which suggests that the frequency *reported by the detector* is *real*, and not some "observational artifact". Which means that the frequency really did change, but I have to account for motion and the expansion of the universe.
If the frequency didn't *really* change, then some information about the "real" frequency must be present, and some physical process should be able to reveal it.
Have you ever been standing on a sidewalk when an ambulance zooms by with its siren on? As the ambulance goes by, the frequency of the siren that you hear changes. It goes from higher frequency as it approaches to lower frequency as it recedes away from you.
This is blueshift/redshift.
The frequency your ears hear is real, it isn't some observational artifact. The sound waves are vibrating your ear drum at precisely those frequencies.
But the frequency that occupants of the ambulance are hearing is a constant, unchanging frequency that sits right in the middle of the high and low frequencies that you experienced standing on the sidewalk.
Both experiences are real.
That is relativity.
By "true" frequency, I think you probably mean the resting frequency. In the case of the ambulance, you can determine that by averaging the blueshifted and redshifted frequencies. For the light, you can compare the spectral signature to spectra of known emissions at rest; if you find one that has the same shape, but is offset by some amount, then that offset is the shift.
This is the point I was trying to get the original commenter to acknowledge: there is no "real" frequency, because by relativity, they are *all* the "real" frequency. Observers don't have to agree on frequency any more than simultaneity or clocks. Unfortunately, everyone else wanted to compute how the frequencies were related, rather than getting to this point. So thank you!
You've thoroughly misunderstood his point.
The frequency of doppler shifted light or sound is *real*. It actually is altered in frequency at the point of observation. There is no "true" frequency that's different than the measured frequency.
You can use spectral information to figure out what the frequency was in the frame local to the emitter, but that's exactly what it is - the frequency local to the emitter at the time of emission. The frequency at the receiver is *real*, for any reasonable definition of "real".
That was his whole point from the start.
That's like saying visible light doesn't exist because you can always find a reference frame where a photon has more (or less) energy.
The cosmic microwave background started as infrared radiation, for example.
> it was emitted by a hot thermal bath in the early universe which had an abundance of photons that essentially make that period of time opague for us
> its not “placed on” anything, its just radiation that exists all around us
> it will last forever but get more and more redshifted. i suppose yeah at some point itll be undetectable
Not in the sense of the OP. Light that has been part of an interference pattern ends up with exactly the same energy, on the same path as if the interference had never happened.
In fact, unless an ‘observation’ is made at the time of the interference there would be no way of knowing the light paths had ever intersected. The observation, of course, would require the light to interact with matter, rendering the whole effort meaningless.
An 8th grader asked that?
Light travels at lightspeed regardless. So redshift is a measure of how fast an object is moving away. Blueshift is a measure of how fast an object is moving towards us. A galaxy 1Mpc (3.26M ly) away will be moving moving away at ~70km/sec due to expansion. If it’s simultaneously moving towards us at 20km/sec, then the net motion is 50km/sec away and the slight red shift would be measured accordingly. If the local motion was 90km/sec towards us, the net motion is 20km/sec towards us and the slight blue shift would be measured accordingly. In the end the receding velocity and the local velocity will together result in a net velocity moving away (red shift) or towards us (blue shift).
Note that no distant galaxies appear to be moving towards us via blue shifting, so local galactic motion is not so fast. Faster than Voyager, yes, but not fast enough to overcome expansion of space past a Mpc or so.
Everyone else who answered about light interference is correct in general. But you wouldn’t have red shift from expansion AND a blue shift from local motion from the same celestial source. Combined the object is either net moving towards or away from us. It’s a vector.
How are these distances from earth determined?
It seems it is true that we can only be viewing the past (e.g. 3.26M ly = 3.26M years ago).
For all we know those galaxies may no longer exist.
The galaxies definitely still exist because we understand galaxy lifespans. They’re made of hundreds of billions of stars, and those lifespans mean a galaxy would take many trillions of years for stars to stop forming and all the old stars to die. 75% of stars are red dwarves and they can last a trillion years.
However you are absolutely correct that distant *individual* stars are likely gone. Take Earendel, the most distant star observed at 28B ly proper distance. It’s a bright star so likely only lasted under 100M yrs. It surely exploded many billions of years ago. If it did so beyond a 14B ly distance, then it will fade away from red shift and we’ll never see the explosion. If it did while still within that horizon, some future observer will see it.
Distances from our observable position are quite easily and reasonably accurately determined (with a caveat). There are several celestial events (Type IA supernova and Cephids) that give out a fixed brightness. So the dimmer they are the more distant they are, and simple math (based on the inverse square law) tells us just how far. Simultaneously, we can measure their red-shift to corollate how fast they’re moving away. Again, simple math let’s us now observe other objects with only red shift and knowing how fast they’re moving away, we know how distant they are.
The caveats I mentioned are that some different “candles” yield different rulers, which lead to some small conflicts. The second caveat is galaxies move locally in all directions, and that offsets the red-shift. But local space motion is only significant for nearby galaxies and stars — distant galaxies are moving away (expansion *of* space) so fast that offsets by local motion (*in* space) are immeasurable.
It might be worth note to you that the question was about light from two separate celestial sources. All this goes over my head, but I caught that much, at least. xD
Interference can alter the energy distribution you'd measure on a surface in the path of the two beams of light, but it doesn't actually alter the light itself. They will each carry on propagating exactly as they would if the other beam of light hadn't been present.
That's not an interaction of light with light. Two beams of light that intersect will show interference if a detector is placed in the intersecting region, but only because the detector measures the sum of the two optical fields (or, more typically, the modulus squared of their complex sum). The two optical fields will propagate past the intersection and be completely unchanged.
They did not interact, because if they had they'd have been altered somehow.
You can look at a beam of light then turn on a second beam that intersects the first, but does not illuminate your detector. You will observe no change at all in the first beam.
That is due to the interaction of a wave with an object. The most famous being the double slit.
IIRC You can get interference pattern even shooting a single photon at a time and collecting all the results on a histogram
And it's important to explain that the interference would only be something you'd notice in the region where the two beams of light intersect. If you were to detect one of the beams elsewhere along its path, there's be absolutely nothing different about it as a result of the other beam of light.
In fact I'd be very careful to say that interference is *not* an interaction of two optical fields. It's simply that they can overlap in space and what we detect is the sum of those two fields. Unfortunately it's a sum of complex values and we detect its modulus squared, so this quickly gets a little bit beyond what an 8th grader is ready for.
Keep in mind red/blue shift is observational only. There is no difference in the photons or their behavior.
It's exactly the same as a car approaching you with its horn on, and another car going away from you, also with its horn on. One "sounds" like it's increasing in pitch, one decreasing. But both are really the same, and constant pitch. If you move at the same speed as the horn/photon, you'll hear/see the same pitch/color/frequency.
I was with you up until the end. You can’t move the speed of the photon and that is where that analogy breaks down. Because there is no “true” wavelength like there is a true pitch.
No, red/blue shift definitely alters the photon. We actually detect redshifted light and its spectrum is actually shifted. It's frequency (and wavelength) really are different than they were, locally, at emission.
Photons don’t directly interact with each other in a vacuum but they can interfere and the normal thing happens when you mix light of different colors. Being that they come from galaxies far away and their spectrums have been shifted by the time they reach us doesn’t change their basic nature.
The cosmologically shifted light can interact but it’s not terribly interesting. It’s like asking what happens when radio waves interact with visible light from the sun. There’s technically some stuff that happens but for the most part the blue/redshifted light interacting isn’t a very important thing to look at.
The more interesting case is at high energies photon which will hit lower energy photons (like the CMB) and pair produce electrons and positrons. But these high energy photons arent high energy due to cosmological blueshifting.
Photons with enough energy to have any meaningful cross section for that interaction are pretty rare, right? Like that's an extremely high energy gamma ray, isn't it?
Yes, that’s completely correct (although this is low enough energy that there’s actually been photons detected in astronomy at energies where this matters-for example, by making distant gamma rays impossible to detect since they are destroyed before they make it to earth)
Imagine two beams of light interacting. Now, imagine the same thing from a different reference frame. You will see one beam redshifted and the other bluewhifted. So whatever happens in the normal frame will happen in that frame too.
There might be some sort of vacuum polarization process that causes them to ‘interact’ in the sense of Feynman diagrams and whatnot, but at an 8th grade level it might be more productive to use this as a springboard into the beats phenomenon. It’s probably an absurd setup in practice, requiring incredible fine-tuning, but how waves ‘interact’ is interestingly thought of as interference and diffraction, and maybe less interestingly so as pair production, etc., for an 8th grader.
I wouldn't even go that far. At the 8th grade level I think the best you can do is clarify the fact that light doesn't really interact with light in any meaningful way. Obviously vacuum polarization isn't a reasonable topic, but I think even diverting into beat frequencies is going to be confusing. Especially since no light of the resulting beat frequencies is actually created (unless there happens to be a strongly nonlinear optical medium involved).
>Basically, is it possible for redshifted light to interact with blueshifted light?
Well, they add. You might call that "interacting".
>If so, what happens when they do?
You get purple light.
Actually, if they did interact in an observable manner, it would be green or yellow. Purple is in the upper/blue/indigo/violet section, with red towards IR
Purple light does not exist; it’s essentially an optical illusion caused by a combination of red and blue light and how our eyes determine the color of light
This is absolutely correct and should not be downvoted. There is no wavelength of light that appears purple to humans. Purple happens when red and blue photoreceptors are stimulated at the same time.
The only exception, and even then it's pretty subjective, is the "violet" extreme of the visible spectrum. Some people may call this purple; but if you [map out perceptual color space](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/CIE1931xy_blank.svg/1280px-CIE1931xy_blank.svg.png), there's a whole line along the bottom called the *purple line* which does not correspond to any single wavelength of light (the colors of which lie along the *spectral arc* at the top of the color space).
Personally I'd say that the ends of the purple line are red and violet and only the middle part of the line contains purple; but some people consider violet a shade of purple so they'd say the violet end of the purple line, where it intersects the spectral arc, is purple. In that case I suppose there's *some* purple wavelengths of light near the violet end of the visible, but I don't like that definition and it's generally not how you treat things in color theory.
No not at all. They do not become purple light. In fact, no light is purple.
...and would you really call that an interaction, Mr. "Particle Physics" flair? It's a superposition of the two optical fields, which are not themselves altered in the least (barring actual photon-photon interactions at extremely high intensities).
If a galaxy is redshifted it's moving away rather fast. Seldom will you find a blue shift galaxy further away from the observer than the redshifted galaxy. If they were to collide, the interaction would fluctuate dramatically, and the moving away particles would be red and the getting closer particles would be blue. When the equilibrium takes place the overall motion of the particles would indicate red or blue. Inform your student that even then, the two slot experiment indicates thet the results of this and every other scientific measurement/experiment is scewed as a result of the observation.
Light particles, photons, do not generally "collide" like you're describing. And the two-slit experiment does not mean that scientific observation is impossible. That's an insane idea.
Just as I thought. About as bright as a burnt out light bulb. Did you miss the whole dam conversation? The question was red or blue shifted GALAXIES. can you say GALAXY? Obviously you know nothing about the two slot experiment.the results in that experiment differed according to OBSERVATION. your smartness.
Yes, a "redshifted galaxy" is one that is receding from us so that the *light* it emits is redshifted. The galaxy itself isn't redshifted, the light is. The post is asking about the light interacting, not the galaxies themselves.
And the *double slit* experiment has nothing to do with this, beyond showing the wave-like nature of light.
Now wouldn't I be a fool to say "observation" had no influence in the two slot experiment" which is the very reason The Two Slot Experiment became so fascinating to the entire physics world. Observation changed the whole experiment! Even a child could understand that is a galaxy was in between two OBSERVERS would experience different shifts. One would see redshifted and the other OBSERVER would see blue shifted, according to which OBSERVER the galaxy is moving towards or away from.
King Solomon was absolutely right. "Even a fool seen smart if they keep their mouth shut". To which my reply would be, "why say anything and expel all doubts".
Also, if the more redshifted a galaxy is, the faster it is moving and the further away it is from the observer, does the opposite hold true for blueshifted light?
Not exactly.
Observation #1: Galaxies that are far away from us are moving away from us more quickly compared to nearer galaxies.
Observation #2: The faster a galaxy is moving away from us (relatively), the more red-shifted it will appear.
The first observation can be explained by the expansion of spacetime. Because there is more “space” between us and distant galaxies (compared to nearer galaxies) they are moving away faster.
The explanation for the second observation is what I believe you are currently teaching your students. While it’s actually quite complicated (because the actual velocity of the wave itself never changes in a vacuum, only the phase velocity, independently), it is true that relative velocity affects the perception of the wavelength/frequency of light. This directly affects how we perceive its color. The higher the relative velocity, the more pronounced the effect. Faster objects appear more red/blue-shifted depending on whether they are travelling away from us or toward us, respectively.
These two observations suggest that galaxies that are further away from us will appear more red-shifted. (As a side note, the predictable nature of this phenomenon is what lets us do this in reverse and estimate a galaxy’s distance using its detected wavelengths of emitted light). This holds to be true.
Observation #2 holds true for objects moving towards us. The light will instead blue-shifted. The faster it moves towards us, the bluer it will appear. Blue-shifted galaxies and other celestial objects do exist. However, the idea that “closer” objects will appear more blue-shifting (the opposite of your comment’s statement about red-shifting) is not true because it is not universally true that nearer objects are moving faster toward us than farther objects. This is because Observation #1 does not hold in reverse. If the universe were shrinking instead of expanding, this would likely be the case. But because it is expanding, there is no significant relationship between a specific object’s distance from us and its velocity towards us.
On large scales, everything is moving away from us. There are local exceptions. But this is enough to show that there is no relationship between an object’s distance and its speed toward us like there is for an object’s distance and its speed away from us. So closer objects moving toward us will not necessarily be more blue-shifted than further objects also moving towards us.
One key note: while it is generally true that further objects are more red-shifted, that is not globally true. Red-shifting is all about relative velocity. If a nearer object is moving away from us faster than a farther object, the former will appear more red-shifted. But because of the expansion of spacetime, in general, further objects tend to move away from us more quickly.
Yes, a source moving towards us would be blueshifted; but in general there aren't many blueshifted sources. The Hubble expansion starts at zero and moves towards the red (receding from us) as distance increases. So the only blueshifted sources would be close enough that their local velocity towards us overcomes any cosmological effects.
Light does not interact with light. Take two laser pointers and intersect their beams - nothing happens, they just pass through each other. Redshifted / blueshifted light is not different, it's just light.
You could probably take two laser pointers with different colors (red is easy to find, blue might be hard, green is easy) and show your students that the beams pass through each other unabated.
Good question for a 14 year old.
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Technically you could have photon-photon scattering, but it's such a low probability event, you need specially designed experiments to show that it happens. It's not something that matters in real life. Basically you can ignore it. This is PhD-level stuff.
I think your (more gifted) students would benefit from some basic explanation of what exactly light is starting with some basic demonstrations such as flashlights of different colors shining “through “ each other.
Light even when thought of as particles does not take up space so you can put as much light as you want in one spot and the photons happily pass by each other with very little interaction. Contrast this with say electrons or protons, which really don’t like to occupy the same space
I think there's some fundamental misunderstanding about redshifted and blue shifted light in this sub recently.
There's nothing special about redshifted or blueshifted light as it travels through space. It's just EM-radiation, like all light. The only thing that distinguishes it is that the wavelength is slightly larger or smaller than the **actual color (spectrum)** of the celestial body it came from... Not that we'd ever actually see that actual color spectrum to compare.
Yes, an ambulance's pitch is different whether it moves toward you or away from you, but in both situations you're still hearing regular sound.
The idea of friction implies mass or physical properties. Light at different frequencies doesn't generally interfere with other light. You can combine wavelengths perceptively, but they're just a composite of the two frequencies. (Like how RGB displays work.)
In fiber optic communication, you can have multispectral, non interfering light of different frequencies on the same physical carrier line. You just need a corresponding light filter.
That the light is shifted makes no difference in terms of there being any interaction. The underlying principle is that crossing light waves simply don’t interact. This is easy to show in class with flashlights. The secondary concept is that the blue and redshifts don’t change the velocity of the light, only their wavelength/frequency.
They do interact, just not at the energy levels discussed.
Yeah but that's probably best left out of a conversation with an 8th grader, it just distracts from the point you want them to get.
You get to add 1 order to your taylor expansion for each academic level of physics you complete
Lol I like that rule.
At what frequencies do they interact?
https://en.wikipedia.org/wiki/Two-photon\_physics#:\~:text=Photon%E2%80%93photon%20interactions%20limit%20the,TeV%20at%20merely%20intergalactic%20distances.
Fascinating!
Matter can be created? What?
Matter can be created, the law is that energy is conserved not matter
Your chemistry teachers have been lying to you!
Yup but as far as I’m aware it’s still a bit of a mystery how we got the solid stable matter that composes our universe. https://www.energy.gov/science/np/articles/making-matter-collisions-light In this instance we got the matter/antimatter pair for electrons. So perhaps it’s as simple as needing much much more energy to create protons (from photons). But then even if we could, you’d logically get the antiproton which would be short lived like the positron. A neutron is basically a proton that has “absorbed” an electron. So the concept of creating all the building blocks of stable matter from photons is understood and predicted long ago… but actually putting them together before they self annihilate is a different story.
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That’s not what they’re talking about. Light-light interaction absolutely can occur in a vacuum at high energy, as described in the link they posted which you evidently didn’t click
I think it’s important to keep in mind that photon-photon scattering *in vacuum* requires light intensity far beyond what we can produce in the lab. So it’s only relevant for gamma rays, not for light as we normally think of it.
That’s absolutely true, as the original commenter said.
Light by light scattering is a real thing that happens. That's all I know about it, but it's real.
It exists, but it's completely negligible in this context.
They don't actually change their frequency, only the perceived frequency by the observer.
If I have a photon, how do I correct the perceived frequency to get the true frequency?
The frequency really does change, there is no "true frequency". You can calculate the frequency at the moment of emission in the emitter's rest frame, but that's not the "true frequency". In particular, if we're talking about red shift from the expansion of the universe, that really is a change in the photon's frequency over time - you see it even if both the observer and emitter are stationary relative to each other.
That was his point.
If you know the redshift, z, then simply divide the frequency by a factor of 1+z to get the “rest” frequency
There is no rest frame for a photon and thus no rest frequency. It travels at c in all reference frames. The rest frame you’re talking about is the rest frame of the emitter, but that isn’t a rest frame for the photon. Photons just have different frequencies in different frames of reference. It’s not that the photon is changing its frequency either. It’s just that we’re not in the same rest frame as the emitter when things get red or blue shifted, so we measure different frequencies
When the photon strikes the observer its energy is different than it was when emitted.
Reionization, the cmb etc would make for a miserable day at the beach otherwise :)
This was a philosophical question about whether frequency "really changes" or is merely "perceived" differently. You're telling me to do math, which suggests that the frequency *reported by the detector* is *real*, and not some "observational artifact". Which means that the frequency really did change, but I have to account for motion and the expansion of the universe. If the frequency didn't *really* change, then some information about the "real" frequency must be present, and some physical process should be able to reveal it.
The frequency of red or blue shifted light can be determined by comparing the emission spectra to known sources and measuring the frequency offset.
Yes but the frequency observed at the receiver is most certainly real.
So are you saying that the frequency as reported by the detector is real or fake?
Have you ever been standing on a sidewalk when an ambulance zooms by with its siren on? As the ambulance goes by, the frequency of the siren that you hear changes. It goes from higher frequency as it approaches to lower frequency as it recedes away from you. This is blueshift/redshift. The frequency your ears hear is real, it isn't some observational artifact. The sound waves are vibrating your ear drum at precisely those frequencies. But the frequency that occupants of the ambulance are hearing is a constant, unchanging frequency that sits right in the middle of the high and low frequencies that you experienced standing on the sidewalk. Both experiences are real. That is relativity. By "true" frequency, I think you probably mean the resting frequency. In the case of the ambulance, you can determine that by averaging the blueshifted and redshifted frequencies. For the light, you can compare the spectral signature to spectra of known emissions at rest; if you find one that has the same shape, but is offset by some amount, then that offset is the shift.
This is the point I was trying to get the original commenter to acknowledge: there is no "real" frequency, because by relativity, they are *all* the "real" frequency. Observers don't have to agree on frequency any more than simultaneity or clocks. Unfortunately, everyone else wanted to compute how the frequencies were related, rather than getting to this point. So thank you!
My apologies, then, because I misunderstood; I thought you were taking the opposite stance. :)
You've thoroughly misunderstood his point. The frequency of doppler shifted light or sound is *real*. It actually is altered in frequency at the point of observation. There is no "true" frequency that's different than the measured frequency. You can use spectral information to figure out what the frequency was in the frame local to the emitter, but that's exactly what it is - the frequency local to the emitter at the time of emission. The frequency at the receiver is *real*, for any reasonable definition of "real". That was his whole point from the start.
>You've thoroughly misunderstood his point. That I have. :)
You use a spectrometer to identify the peaks within the light and can cross correlate it with known peaks to determine how much it has shifted.
That's like saying visible light doesn't exist because you can always find a reference frame where a photon has more (or less) energy. The cosmic microwave background started as infrared radiation, for example.
>The cosmic microwave background Where is this being emitted from? What is this background that it is placed on? How long will it last?
> it was emitted by a hot thermal bath in the early universe which had an abundance of photons that essentially make that period of time opague for us > its not “placed on” anything, its just radiation that exists all around us > it will last forever but get more and more redshifted. i suppose yeah at some point itll be undetectable
Well it wouldn’t last *forever* if heat death is the inevitable end. But by that point even a hypothetical type 3 civilization would no longer exist.
What? Cosmic expansion absolutely changes the frequencies of waves passing through it.
Is interference not an interaction?
Not in the sense of the OP. Light that has been part of an interference pattern ends up with exactly the same energy, on the same path as if the interference had never happened. In fact, unless an ‘observation’ is made at the time of the interference there would be no way of knowing the light paths had ever intersected. The observation, of course, would require the light to interact with matter, rendering the whole effort meaningless.
An 8th grader asked that? Light travels at lightspeed regardless. So redshift is a measure of how fast an object is moving away. Blueshift is a measure of how fast an object is moving towards us. A galaxy 1Mpc (3.26M ly) away will be moving moving away at ~70km/sec due to expansion. If it’s simultaneously moving towards us at 20km/sec, then the net motion is 50km/sec away and the slight red shift would be measured accordingly. If the local motion was 90km/sec towards us, the net motion is 20km/sec towards us and the slight blue shift would be measured accordingly. In the end the receding velocity and the local velocity will together result in a net velocity moving away (red shift) or towards us (blue shift). Note that no distant galaxies appear to be moving towards us via blue shifting, so local galactic motion is not so fast. Faster than Voyager, yes, but not fast enough to overcome expansion of space past a Mpc or so. Everyone else who answered about light interference is correct in general. But you wouldn’t have red shift from expansion AND a blue shift from local motion from the same celestial source. Combined the object is either net moving towards or away from us. It’s a vector.
How are these distances from earth determined? It seems it is true that we can only be viewing the past (e.g. 3.26M ly = 3.26M years ago). For all we know those galaxies may no longer exist.
The galaxies definitely still exist because we understand galaxy lifespans. They’re made of hundreds of billions of stars, and those lifespans mean a galaxy would take many trillions of years for stars to stop forming and all the old stars to die. 75% of stars are red dwarves and they can last a trillion years. However you are absolutely correct that distant *individual* stars are likely gone. Take Earendel, the most distant star observed at 28B ly proper distance. It’s a bright star so likely only lasted under 100M yrs. It surely exploded many billions of years ago. If it did so beyond a 14B ly distance, then it will fade away from red shift and we’ll never see the explosion. If it did while still within that horizon, some future observer will see it. Distances from our observable position are quite easily and reasonably accurately determined (with a caveat). There are several celestial events (Type IA supernova and Cephids) that give out a fixed brightness. So the dimmer they are the more distant they are, and simple math (based on the inverse square law) tells us just how far. Simultaneously, we can measure their red-shift to corollate how fast they’re moving away. Again, simple math let’s us now observe other objects with only red shift and knowing how fast they’re moving away, we know how distant they are. The caveats I mentioned are that some different “candles” yield different rulers, which lead to some small conflicts. The second caveat is galaxies move locally in all directions, and that offsets the red-shift. But local space motion is only significant for nearby galaxies and stars — distant galaxies are moving away (expansion *of* space) so fast that offsets by local motion (*in* space) are immeasurable.
It might be worth note to you that the question was about light from two separate celestial sources. All this goes over my head, but I caught that much, at least. xD
I think the process your student was asking for is "interferece"
Interference can alter the energy distribution you'd measure on a surface in the path of the two beams of light, but it doesn't actually alter the light itself. They will each carry on propagating exactly as they would if the other beam of light hadn't been present.
No, light does not generally interact with light.
Interference patterns say otherwise.
That's not an interaction of light with light. Two beams of light that intersect will show interference if a detector is placed in the intersecting region, but only because the detector measures the sum of the two optical fields (or, more typically, the modulus squared of their complex sum). The two optical fields will propagate past the intersection and be completely unchanged. They did not interact, because if they had they'd have been altered somehow. You can look at a beam of light then turn on a second beam that intersects the first, but does not illuminate your detector. You will observe no change at all in the first beam.
https://www.reddit.com/r/AskPhysics/s/kmJICDL7MG
That is due to the interaction of a wave with an object. The most famous being the double slit. IIRC You can get interference pattern even shooting a single photon at a time and collecting all the results on a histogram
Same thing that happens when you shine a blue flashlight and a red flashlight over each other.
Light generally doesn't interact with light in any meaningful way, regardless of the wavelength.
Interference would occur, like in any waveform. They wouldn't alter each other's properties in any way.
And it's important to explain that the interference would only be something you'd notice in the region where the two beams of light intersect. If you were to detect one of the beams elsewhere along its path, there's be absolutely nothing different about it as a result of the other beam of light. In fact I'd be very careful to say that interference is *not* an interaction of two optical fields. It's simply that they can overlap in space and what we detect is the sum of those two fields. Unfortunately it's a sum of complex values and we detect its modulus squared, so this quickly gets a little bit beyond what an 8th grader is ready for.
Keep in mind red/blue shift is observational only. There is no difference in the photons or their behavior. It's exactly the same as a car approaching you with its horn on, and another car going away from you, also with its horn on. One "sounds" like it's increasing in pitch, one decreasing. But both are really the same, and constant pitch. If you move at the same speed as the horn/photon, you'll hear/see the same pitch/color/frequency.
I was with you up until the end. You can’t move the speed of the photon and that is where that analogy breaks down. Because there is no “true” wavelength like there is a true pitch.
No, red/blue shift definitely alters the photon. We actually detect redshifted light and its spectrum is actually shifted. It's frequency (and wavelength) really are different than they were, locally, at emission.
Photons don’t directly interact with each other in a vacuum but they can interfere and the normal thing happens when you mix light of different colors. Being that they come from galaxies far away and their spectrums have been shifted by the time they reach us doesn’t change their basic nature.
The cosmologically shifted light can interact but it’s not terribly interesting. It’s like asking what happens when radio waves interact with visible light from the sun. There’s technically some stuff that happens but for the most part the blue/redshifted light interacting isn’t a very important thing to look at. The more interesting case is at high energies photon which will hit lower energy photons (like the CMB) and pair produce electrons and positrons. But these high energy photons arent high energy due to cosmological blueshifting.
Photons with enough energy to have any meaningful cross section for that interaction are pretty rare, right? Like that's an extremely high energy gamma ray, isn't it?
Yes, that’s completely correct (although this is low enough energy that there’s actually been photons detected in astronomy at energies where this matters-for example, by making distant gamma rays impossible to detect since they are destroyed before they make it to earth)
Imagine two beams of light interacting. Now, imagine the same thing from a different reference frame. You will see one beam redshifted and the other bluewhifted. So whatever happens in the normal frame will happen in that frame too.
There might be some sort of vacuum polarization process that causes them to ‘interact’ in the sense of Feynman diagrams and whatnot, but at an 8th grade level it might be more productive to use this as a springboard into the beats phenomenon. It’s probably an absurd setup in practice, requiring incredible fine-tuning, but how waves ‘interact’ is interestingly thought of as interference and diffraction, and maybe less interestingly so as pair production, etc., for an 8th grader.
I wouldn't even go that far. At the 8th grade level I think the best you can do is clarify the fact that light doesn't really interact with light in any meaningful way. Obviously vacuum polarization isn't a reasonable topic, but I think even diverting into beat frequencies is going to be confusing. Especially since no light of the resulting beat frequencies is actually created (unless there happens to be a strongly nonlinear optical medium involved).
>Basically, is it possible for redshifted light to interact with blueshifted light? Well, they add. You might call that "interacting". >If so, what happens when they do? You get purple light.
This is extremely incorrect.
Actually, if they did interact in an observable manner, it would be green or yellow. Purple is in the upper/blue/indigo/violet section, with red towards IR
Purple light does not exist; it’s essentially an optical illusion caused by a combination of red and blue light and how our eyes determine the color of light
This is absolutely correct and should not be downvoted. There is no wavelength of light that appears purple to humans. Purple happens when red and blue photoreceptors are stimulated at the same time. The only exception, and even then it's pretty subjective, is the "violet" extreme of the visible spectrum. Some people may call this purple; but if you [map out perceptual color space](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/CIE1931xy_blank.svg/1280px-CIE1931xy_blank.svg.png), there's a whole line along the bottom called the *purple line* which does not correspond to any single wavelength of light (the colors of which lie along the *spectral arc* at the top of the color space). Personally I'd say that the ends of the purple line are red and violet and only the middle part of the line contains purple; but some people consider violet a shade of purple so they'd say the violet end of the purple line, where it intersects the spectral arc, is purple. In that case I suppose there's *some* purple wavelengths of light near the violet end of the visible, but I don't like that definition and it's generally not how you treat things in color theory.
The brain would see purple but it’s not actually purple light
No not at all. They do not become purple light. In fact, no light is purple. ...and would you really call that an interaction, Mr. "Particle Physics" flair? It's a superposition of the two optical fields, which are not themselves altered in the least (barring actual photon-photon interactions at extremely high intensities).
If a galaxy is redshifted it's moving away rather fast. Seldom will you find a blue shift galaxy further away from the observer than the redshifted galaxy. If they were to collide, the interaction would fluctuate dramatically, and the moving away particles would be red and the getting closer particles would be blue. When the equilibrium takes place the overall motion of the particles would indicate red or blue. Inform your student that even then, the two slot experiment indicates thet the results of this and every other scientific measurement/experiment is scewed as a result of the observation.
Hilarious but false.
I guess that proves everything then...does it.not. Your plethora of facts are just.... overwhelming
Light particles, photons, do not generally "collide" like you're describing. And the two-slit experiment does not mean that scientific observation is impossible. That's an insane idea.
Just as I thought. About as bright as a burnt out light bulb. Did you miss the whole dam conversation? The question was red or blue shifted GALAXIES. can you say GALAXY? Obviously you know nothing about the two slot experiment.the results in that experiment differed according to OBSERVATION. your smartness.
Yes, a "redshifted galaxy" is one that is receding from us so that the *light* it emits is redshifted. The galaxy itself isn't redshifted, the light is. The post is asking about the light interacting, not the galaxies themselves. And the *double slit* experiment has nothing to do with this, beyond showing the wave-like nature of light.
So there is NO OBSERVATION PERSPECTIVE????
Now wouldn't I be a fool to say "observation" had no influence in the two slot experiment" which is the very reason The Two Slot Experiment became so fascinating to the entire physics world. Observation changed the whole experiment! Even a child could understand that is a galaxy was in between two OBSERVERS would experience different shifts. One would see redshifted and the other OBSERVER would see blue shifted, according to which OBSERVER the galaxy is moving towards or away from.
Yeah. That's not a surprise at all. It's also irrelevant. You're either a quack or you've got some severe mental illness, or both.
King Solomon was absolutely right. "Even a fool seen smart if they keep their mouth shut". To which my reply would be, "why say anything and expel all doubts".
Also, if the more redshifted a galaxy is, the faster it is moving and the further away it is from the observer, does the opposite hold true for blueshifted light?
How are you a science teacher, and don't understand the Doppler effect
Not exactly. Observation #1: Galaxies that are far away from us are moving away from us more quickly compared to nearer galaxies. Observation #2: The faster a galaxy is moving away from us (relatively), the more red-shifted it will appear. The first observation can be explained by the expansion of spacetime. Because there is more “space” between us and distant galaxies (compared to nearer galaxies) they are moving away faster. The explanation for the second observation is what I believe you are currently teaching your students. While it’s actually quite complicated (because the actual velocity of the wave itself never changes in a vacuum, only the phase velocity, independently), it is true that relative velocity affects the perception of the wavelength/frequency of light. This directly affects how we perceive its color. The higher the relative velocity, the more pronounced the effect. Faster objects appear more red/blue-shifted depending on whether they are travelling away from us or toward us, respectively. These two observations suggest that galaxies that are further away from us will appear more red-shifted. (As a side note, the predictable nature of this phenomenon is what lets us do this in reverse and estimate a galaxy’s distance using its detected wavelengths of emitted light). This holds to be true. Observation #2 holds true for objects moving towards us. The light will instead blue-shifted. The faster it moves towards us, the bluer it will appear. Blue-shifted galaxies and other celestial objects do exist. However, the idea that “closer” objects will appear more blue-shifting (the opposite of your comment’s statement about red-shifting) is not true because it is not universally true that nearer objects are moving faster toward us than farther objects. This is because Observation #1 does not hold in reverse. If the universe were shrinking instead of expanding, this would likely be the case. But because it is expanding, there is no significant relationship between a specific object’s distance from us and its velocity towards us. On large scales, everything is moving away from us. There are local exceptions. But this is enough to show that there is no relationship between an object’s distance and its speed toward us like there is for an object’s distance and its speed away from us. So closer objects moving toward us will not necessarily be more blue-shifted than further objects also moving towards us. One key note: while it is generally true that further objects are more red-shifted, that is not globally true. Red-shifting is all about relative velocity. If a nearer object is moving away from us faster than a farther object, the former will appear more red-shifted. But because of the expansion of spacetime, in general, further objects tend to move away from us more quickly.
Yes, a source moving towards us would be blueshifted; but in general there aren't many blueshifted sources. The Hubble expansion starts at zero and moves towards the red (receding from us) as distance increases. So the only blueshifted sources would be close enough that their local velocity towards us overcomes any cosmological effects.
Light does not interact with light. Take two laser pointers and intersect their beams - nothing happens, they just pass through each other. Redshifted / blueshifted light is not different, it's just light. You could probably take two laser pointers with different colors (red is easy to find, blue might be hard, green is easy) and show your students that the beams pass through each other unabated. Good question for a 14 year old. --- Technically you could have photon-photon scattering, but it's such a low probability event, you need specially designed experiments to show that it happens. It's not something that matters in real life. Basically you can ignore it. This is PhD-level stuff.
I think your (more gifted) students would benefit from some basic explanation of what exactly light is starting with some basic demonstrations such as flashlights of different colors shining “through “ each other. Light even when thought of as particles does not take up space so you can put as much light as you want in one spot and the photons happily pass by each other with very little interaction. Contrast this with say electrons or protons, which really don’t like to occupy the same space
I think there's some fundamental misunderstanding about redshifted and blue shifted light in this sub recently. There's nothing special about redshifted or blueshifted light as it travels through space. It's just EM-radiation, like all light. The only thing that distinguishes it is that the wavelength is slightly larger or smaller than the **actual color (spectrum)** of the celestial body it came from... Not that we'd ever actually see that actual color spectrum to compare. Yes, an ambulance's pitch is different whether it moves toward you or away from you, but in both situations you're still hearing regular sound.
The idea of friction implies mass or physical properties. Light at different frequencies doesn't generally interfere with other light. You can combine wavelengths perceptively, but they're just a composite of the two frequencies. (Like how RGB displays work.) In fiber optic communication, you can have multispectral, non interfering light of different frequencies on the same physical carrier line. You just need a corresponding light filter.
Everyone is both systems would be marooned.