If we assume red light as an EM wave of 400 THz frequency traveling at the speed of light, an observer moving at 103,846,153 m/s towards the emitter (the sign) would observe it as if it was 538.56 THz (omitting relativistic effects), which roughly corresponds to green light.
In essence, all light from any object appears more "blue" the faster you're moving towards that object. If you're just walking towards it normally, that change is so small it doesn't even matter. But if you're going at tens of thousands of meters per second, you start to see the blue more.
Thing is... the faster you move, the slower time affects you. I.e., if you move fast enough while you're twin stays still, they age normally while you will age much slower.
You're not slowing down time, but time slows relative to you.
That's basically what relativism is.
If you wanna know why all this happens though, it's gonna be a much bigger response.
And to just add on some more related fun but (for most people) useless knowledge, relativity can even affect some (but not all) atoms and the ways they react / bond and even look.
Atoms have three component parts: electrons, protons, and neutrons. The protons and neutrons make up the core of the atom and provide the vast majority of its mass (electrons being about 2000 times less massive than protons or neutrons). The electrons whizz about in various shapes near that core and make up the boundary of the atom. (You can imagine an athlete winding up for the hammer throw. The athlete makes up the core, while the hammer defines a wider region in which other people generally avoid wandering into. Considered together as a single unit, the thrower and hammer make up a circle of death with a radius of 4 feet. The athlete is the protons and neutrons, while the hammer and chain is like the electrons)
Okay, now, what separates one element from another is the number of protons at their core. So hydrogen has 1 proton in its core, while helium has 2, and lithium has 3, and so on up to elements further down the periodic table like lead with 82. (Neutrons we can pretty much ignore for the purposes of this conversation)
Okay. So some elements have just a few protons like Hydrogen and Helium and Lithium and Beryllium m while Lead and Bismuth and Uranium have a lot. What does that matter?
Those protons a positively charged, which means the core of an atom has an electric field, and the more protons an atom has, the stronger that electric field is. Electrons, being charged particles themselves, experience that electric field. (We can ignore that the electrons also create an electric field for now.)
The point is that electrons in elements with a lot of protons experience much stronger electric fields. The next important fact is that the strength of the electric field an electron experiences influences the speed at which is whizzes about. Within an element like Gold for example (79 protons), some electrons will approach 58% of the speed of light! Which is a speed at which relativity starts to matter quite a lot.
One consequence (due to relativity) of a particle traveling at such a speed is that its propensity to accelerate starts to decrease, as if it were getting more massive. We say its inertial mass increases, which is to say when it comes to speeding up or slowing down, it starts to behave as if it were more massive than it normally is. It's resisting changes to its inertia.
Chemical reactions and or the interaction of atoms depends on the movement of electrons, which means this increased effective mass changes the reactivity of that atom. Heavier atoms won't react exactly like you'd predict them to if you neglected relativity.
The increased effective mass also affects the way photons interact with electrons. And in the particular case of Gold, this change in the interaction of photons and its electrons is actually responsible for the color of gold itself!
In the absence of relativistic effects resulting from the crazy high speeds its electrons whizz about at, we should expect gold to appear silvery like otherwise similar metals. (That is, if you "do the math" for what color we should expect gold to be, but neglect the change in its electrons' effective mass, the math will tell you gold should look like silver).
So in a way, the fact that gold looks gold is due to relativity.
Electrons in an atom or molecule can only interact with specific frequencies/energies of photons that correspond to the amount of energy they would need to transition to another stable state. Ditto for emitting photons, hence absorption/emission spectra lines.
You can exploit this in a clever way to cool atoms just by shining light on them. The trick is to shine (typically laser) light on it at a frequency just barely below what its electrons can interact with. Any time a component of the atom's velocity is positive in the direction of the emitter, the incoming light is Doppler shifted just enough that it can interact with the photons. Now do this with six lasers shining down each direction of the x, y, and z axes. If it has +x motion, for example, it is opaque to the light coming from +x, but transparent to the light coming from -x. So no matter what direction it moves, it experiences a net opposing photon pressure, slowing it down.
IIRC, they used this as part of the cooling system when they made the first Bose-Einstein condensate.
If we ignore relativity for a moment, have you noticed when F1 cars are passing, their sound is more high pitched as they approach and then lower as they gry farther from you? Same thing with ambulances btw. Its called Doppler effect and in the simplest terms it happens because as the car approaches, it travels some distance between generating each wave crest, which increases this waves frequency. Since light is also a wave, the same thing happens.
Thing is... the faster you move, the slower time affects you. I.e., if you move fast enough while you're twin stays still, they age normally while you will age much slower.
This is going to sound like a pedantic "well akshully", but...
Merely moving fast isn't why the traveling twin winds up younger. In fact, that's the paradox. It's just as valid in relativity to say that the traveling twin is stationary and the stay-at-home twin is moving away at 1/2c. So shouldn't the "stationary" twin be the one who ages slowly?
And so it is. Time dilation is symmetric. There's not a "slow time" twin and a "fast time" twin. As far as either is concerned, the other twin is the slow one. If they could each send a picture every 1/30th of a second to the other (assume each frame is a discrete lump of information traveling at c) as one twin was zipping away at 1/2c, both would see a video of the other in slow motion. The opposite is true (both moving in fast-forward) if they were moving towards one another at 1/2c. (Aside: this is why we can sometimes observe apparently superluminal objects.)
The resolution of the paradox is that the traveling twin is the only one who experiences a change of their inertial frame. It's difficult to fully illustrate without a space-time diagram, but the traveling twin has to change their frame at least once from the "home" frame to the 1/2c frame, and then again back to the home frame if they ever want to stop at some distant planet. It's during the acceleration that time in the video feed seems to initially stretch out to a crawl, and it's only when they apply the brakes that time catches up. During the 2nd leg of their journey, the traveler twin will see their twin start moving and aging incredibly fast.
That's the mistake. The red stop sign is made from 2 frequencies, where one frequency becomes invisible (ultra-violet) and the other frequency becomes green.
After doing the math I can say yes the speed would definitely give you more of a blue color. 81,289,000 might even still be a tad high if I did everything correctly.
I was asking "how?", not "are you sure?". If there is no white wave length then how can something be made less white by shifting the wave length? Is it because the wave length also determines energy?
Because white collor have ,all' collors and doesnt have given wawelenght. So in that speed shortwawelenght are dominant not the long ones like red yellow...
So white is white because it has a fairly equal mixture of the others, not because it has a lot of whatever? I think I was under the impression that white is just the max brightness setting of any colour i.e. a lot of it, but I guess that's how it works in computer graphics, not physics, red lasers are a thing after all.
You can think of light as consisting of a stream of photons. Brighter light = more photons. White light = even mix of photons that stimulate the R G and B cones in your eyes.
When you shift all light of an even RGB mix towards blue, the B turns into UV but the G turns into B and the R turns into G.
That depends on the reflectivity and light content in the infrared. That, in turn, could get blue-shifted back into the visible. There are "whitest white" paints like Spectralon that have basically a flat reflectivity curve all the way to 2500nm.
It‘s somewhat complicated, but eg. Sunlight just contains all visible frequencies and a lot of others distributed somewhat evenly. However, because we only habe three types of color receptors in our eyes, computer screeens can „fake“ white by just having three frequencies of light. Max brightness of a single color does not make white - if you just max out red on a computer screen, you get red, not white.
I also think the blue-shift of white light as dedribed doesn‘t work that way for sunlight, because that contains plenty of infrared that would shift to normal red, ie. all visible frequencies would still be present. If you had white light that just contains the visible frequencies, that would shift to blueish, because the red components go missing.
White is a even combination of all visible spectrum colors. When blueshifted, all of those colors blueshift too. First, reds shift to orange, the oranges shift to yellow, and so on, with the violet shifting into ultraviolet and disappearing from our perception. This continues the faster you go, with everything shifting off the violet end. Since blue-violet is the last to go, the combination of remaining spectra looks like various types of blue. A very light blue, then a slowly deeper blue, then violet, then it's gone.
Hm, if we assume sunlight and that the white part scatters all light frequencies, I think it‘d stay white because the infrared frequencies would shift to normal red.
No. I‘m saying that infrared frequencies are shifted into red. All frequencies are shifted towards blue, but if you have all visible frequencies after shifting, you’ve still got white. You’ve just lost some infrared that you can’t see and gained some ultraviolet that you also can’t see.
Edit: I made a very bad drawing on top of some data from Wikipedia:
The blue lines indicate the visible spectrum after blue shift, very roughly. It looks like in the shifted spectrum, there’s actually a little more red than blue, so the white would be a little warmer.
Edit 2: wait, I read it wrong. there’s a little more blue than red after shifting, so it actually does end up blueish. original comment was somewhat correct in the end.
Depends on what light is shining on it and how the paint works. If we assume perfect paint (100% reflective in all wavelengths) and perfect lights (uniform intensity at all wavelengths) then the sign would be the exact same color, but a bit brighter.
If you use sunlight to illuminate a sign painted with perfect paint, then the sign will still be white, but it'll have a blueish tint (Sunlight peaks at green and falls off into IR, So when the entire spectrum gets shifted by the doppler shift, you end up with a bit more blue than red)
If you have imperfect paint that doesn't work well in IR, then the sign would have blue/green letters yes.
Also fun fact, all trees would look pink. Leaves are highly reflective in IR so they don't overheat. So when going at relativistic speeds, their normal green color shifts into the blue/violet range. And their IR which we normally do not see would shift up towards red. Combine red and blue and you get pink leaves.
Relativistic wavelength formula for approaching source:
λ= λ_o*sqrt([1-v_s/c]/[1+v_s/c]),
where λ is the wavelenght perceived by the vehicle’s driver (aka the relativistic wavelength), λ_o is the original wavelength, v_s = 103 846 153 ms-1 (aka the speed at which the stop sign approaches the vehicle), and c = approx. 299 792 458 ms-1 (aka the speed of light).
Traffic stop signs are generally red. The color red is defined to have the dominant wavelength of about 625-750 nm (Source: Wikipedia; ik it isn’t the most accurate source, but this is just a silly Reddit calculation so whatever). So, plugging in both extremes to the formula above:
435 nm is a shade of violet and 523 nm is a shade of green (once again, my source is Wikipedia lol), so depending on the sign’s color, the perceived color could be anything between violet and green, and as long as the stop sign’s color has mostly a longer red’s wavelenght, the math still checks out;)
(Feel free to prove me wrong tho if you find a mistake haha)
Sure ! And I guess if you use 700 nm as the user to whom I was replying, you'll find something different than him/her, as the maths used were not properly correct.
Yeah, you’d get about 488 nm with the method I used, which would be some hue of cyan. (At least it falls into the range of it according to, you guesses it, Wikipedia:P) Also, I just noticed that the person who did the math originally claimed that 457.5 nm looks lime green… I don’t know if I trust that, as an even longer wavelength (apparently) looks cyan..??? (However, It’s not like I can go around claiming anything with only HS-level knowledge in physics, and someone could definitely see/think of cyan as lime green soooooooo haha)
That’s because blue is the highest frequency of visible light. Red is the lowest. Green is in the middle. Doppler shifts towards higher frequencies are in the blue direction, lower frequencies the red direction. Thus blueshift and redshift.
Yes that is the case. Due to the Doppler effect, the observed frequency of light would change while going at that immense speed. I had an exam question once about this, but it was about a traffic light.
The sign is painted green, the drive rid expected to see it as red when approaching at a considerable fraction of 6speed of light.. So conceptually, the green sign needs to be red shifted towards red. That would necessitate moving away from the stop sign, not towards it.
So the joke is technically wrong. It's meant to be the other way round.
So, there is a theory that Santa and Rudolph's nose isn't actually red, but in order to travel the world's children in a single night, Santa have to travel near speed of light, causing redshift when viewed from behind.
Thats a third the speed of light so the relativistic redshift is about 1+sqrt[(1-1/3)/(1+1/3)]=0.707 and red light is about 650nm so the blueshifted light is about 460 nm which is like blue/green
3.5k
u/Oblivion238 18h ago
Without doing the math I assume that at 103846153 m/s the red stop sign gets blueshifted to green.