# Random physics question: Doppler shifting of light

Moderator, ClubPA mod
edited September 2014
So your eye sees things by processing light waves that have bounced off objects and traveled onto your retina. The color we see is based on the wavelength of that light. When an object travels sufficiently fast towards or away from something, the light is blue shifted or red shifted accordingly, affecting the color we see. The human eye is only sensitive to light in a pretty narrow band of wavelengths, with light outside of that band being invisible. Yes to all of this, right?

So let's say you're traveling really fast along the surface of the Earth, sufficiently fast that the light reflected from objects in front of you is blue-shifted by a noticeable amount. (Yes, I'm aware this is somewhere between wildly impractical and literally impossible without dying, ignore that.) The colors of said objects would shift towards the blue end of the spectrum, correct? What would happen if you continued to accelerate beyond that point? Would there be light reflected off these objects that once was of too long a wavelength to be visible, but now is visible? Would it be possible to travel so quickly that all the light reflecting off of objects ahead of you is blue-shifted out of the visible spectrum? If so, would the world in front of you just basically become invisible at that point?

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ElJeffe on

## Posts

• Registered User regular
edited September 2014
ElJeffe wrote: »
So your eye sees things by processing light waves that have bounced off objects and traveled onto your retina. The color we see is based on the wavelength of that light. When an object travels sufficiently fast towards or away from something, the light is blue shifted or red shifted accordingly, affecting the color we see. The human eye is only sensitive to light in a pretty narrow band of wavelengths, with light outside of that band being invisible. Yes to all of this, right?

So let's say you're traveling really fast along the surface of the Earth, sufficiently fast that the light reflected from objects in front of you is blue-shifted by a noticeably amount. (Yes, I'm aware this is somewhere between wildly impractical and literally impossible with dying, ignore that.) The colors of said objects would shifted towards the blue end of the spectrum, correct? What would happen if you continued to accelerate beyond that point? Would there be light reflected off these objects that once was of too long a wavelength to be visible, but now is visible? Would it be possible to travel so quickly that all the light reflecting off of objects ahead of you is blue-shifted out of the visible spectrum? If so, would the world in front of you just basically become invisible at that point?

Basically, yeah.

As you approach the speed of light, the light in front of you would blueshift into x-ray / gamma radiation.

This is like .99999c, in a spherical cow in a vacuum type hypothetical.

zagdrob on
• Crocodylus Pontifex Sinterklasicus Madrid, 3000 ADRegistered User regular
edited September 2014
What you can't see is will turn black, not invisible, until you hit the point where you are blueshifted so far that the only photons visible to you are in the reference frame of the earth gamma radiation. (Which means they don't interact with most matter anymore) A truely absurd amount of energy.

While accelerating, assuming you are only looking forward, everything you see would blueshift. UV would become visible and have colors, just like you see now, until eventually there are just very little photons for you to see, and it slowly grows dark.
This is because photons are distributed on a frequency curve where visible light is near the top, but drops off quickly:

Bees already see more UV as us, and nature has adapted. So there are some hidden treasures on your trip:
turns into

If you have very good eyes that can still see trace amounts photons, you would still see some light, but it wouldn't really be bouncing of matter anymore (that is not plasma), mostly passing straight through. Whenever you face the sun it would be bright, you would see the upper atmosphere because a lot of high energy interactions would be happening, I'm guessing you could probably see the Milky Way, but everything else would be black. You'd see it straight through the earth too. But this happens at extremely high fractions of c.

If you are flying backwards, it'd move the other way, everything would slowly turn more uniform red for a long time (Because everything absorbs light in the Infrared and radiates it). The curve the other way is much more gentle too. Eventually, at incredible amounts of energy in your movement, it would become dimmer and dimmer too.

SanderJK on
Steam: SanderJK Origin: SanderJK
• Registered User regular
edited September 2014
Red/blue shift is used to describe bodies on the order of stars to galaxies. I don't think this terminology works with respect to the thought experiment you're talking about. In practice it's likely going to be hard to focus well on something that is moving at relativistic speeds in relation to you unless you are travelling directly towards/away from it.

Djeet on
• Moderator, ClubPA mod
Thanks, SanderJK, for the pretty thorough explanation.

Stuff randomly pops into my head when I'm stuck in traffic.

I submitted an entry to Lego Ideas, and if 10,000 people support me, it'll be turned into an actual Lego set!If you'd like to see and support my submission, follow this link.
• Registered User regular
edited September 2014
SanderJK wrote: »
What you can't see is will turn black, not invisible, until you hit the point where you are blueshifted so far that the only photons visible to you are in the reference frame of the earth gamma radiation. (Which means they don't interact with most matter anymore) A truely absurd amount of energy.

While accelerating, assuming you are only looking forward, everything you see would blueshift. UV would become visible and have colors, just like you see now, until eventually there are just very little photons for you to see, and it slowly grows dark.
This is because photons are distributed on a frequency curve where visible light is near the top, but drops off quickly:

Bees already see more UV as us, and nature has adapted. So there are some hidden treasures on your trip:

If you have very good eyes that can still see trace amounts photons, you would still see some light, but it wouldn't really be bouncing of matter anymore (that is not plasma), mostly passing straight through. Whenever you face the sun it would be bright, you would see the upper atmosphere because a lot of high energy interactions would be happening, I'm guessing you could probably see the Milky Way, but everything else would be black. You'd see it straight through the earth too. But this happens at extremely high fractions of c.

If you are flying backwards, it'd move the other way, everything would slowly turn more uniform red* for a long time (Because everything absorbs light in the Infrared and radiates it). The curve the other way is much more gentle too. Eventually, at incredible amounts of energy in your movement, it would become dimmer and dimmer too.

You have it backwards. Blue shift would move the IR into visible range.

Below is a graph of red shift, which is what you are describing, and what Flash-Jeffe Savior of the Universe would see if running backwards.

e:
*Stuff also wouldn't turn more red under blueshift. A red barn would just look yellow, a yellow barn would look blue and a blue barn would look black. And hot piece of black metal would look redish.
When you are shifting you basically just taking the visable band on the spectrum picture you linked and moving it left(blue) or right(red)

So enough red-shift you'd be able to look into the sky and see pulsars and other such stuff, as x-rays and gamma rays would shift into visual.

tinwhiskers on

• Adventure! Candy IslandRegistered User regular
Sounds like a question for XKCD's what if? section

and I wonder about my neighbors even though I don't have them
but they're listening to every word I say
• Moderator, ClubPA mod
I actually sent it in to xkcd, then grew impatient and decided I'd post the question here.

It would be rad if he chose to answer it, though.

I submitted an entry to Lego Ideas, and if 10,000 people support me, it'll be turned into an actual Lego set!If you'd like to see and support my submission, follow this link.
• Registered User regular
ElJeffe wrote: »
I actually sent it in to xkcd, then grew impatient and decided I'd post the question here.

It would be rad if he chose to answer it, though.

Maybe it made it into the book. I just got it, so I'll check when I get home.

• Moderator, ClubPA mod
I sent it, like, six hours ago.

I suspect the lead time on getting the final draft to the publisher is slightly longer.

I submitted an entry to Lego Ideas, and if 10,000 people support me, it'll be turned into an actual Lego set!If you'd like to see and support my submission, follow this link.
• Unaware of her barrel shifter privilege Western coastal temptressRegistered User, Moderator mod
ElJeffe wrote: »
I sent it, like, six hours ago.

I suspect the lead time on getting the final draft to the publisher is slightly longer.

Maybe he tested it out, and got going faster than c and ended up last year and so it's in there anyway.

You never know.

• Registered User regular
I don’t have an answer. You need someone way more knowledgeable than me. But I have though of a few issues. Some may be relevant

I though the effect with distant bits of the universe was not to do with speed. It was to do with the expanding of the universe. The farthest visible space is red. And I was under the impression that that was to do with space stretching and expanding causing the light wavelength into the “red”

Also im sure the speed of light some how throws up a problem. If you have light coming from and object at the speed of light. You going towards it at massive speed. Does not then make that faster than the speed of light. So you are not affecting the wavelength of light. You are effectively changing the frequency in relation to yourself.

Anyone have any idea how to calculate what velocity you would need to be travelling to push UV into the visible?

Also vision with the retina is a chemical reaction. Red/green light hit a photoreceptor and a chemical change happens that then in turn is transmitted into a nerve impulse. If an x-ray is “sped up” will it just have the same properties as red light to our retina?

Please note I cannot be held responsible for any mental, physical, emotional, spiritual, karma, dharma, metaphysical, religious, philosophical, Logical , Ethical, Aesthetical, or financial damage caused by this post
• Registered User regular
I don’t have an answer. You need someone way more knowledgeable than me. But I have though of a few issues. Some may be relevant

I though the effect with distant bits of the universe was not to do with speed. It was to do with the expanding of the universe. The farthest visible space is red. And I was under the impression that that was to do with space stretching and expanding causing the light wavelength into the “red”

Also im sure the speed of light some how throws up a problem. If you have light coming from and object at the speed of light. You going towards it at massive speed. Does not then make that faster than the speed of light. So you are not affecting the wavelength of light. You are effectively changing the frequency in relation to yourself.

Anyone have any idea how to calculate what velocity you would need to be travelling to push UV into the visible?

Also vision with the retina is a chemical reaction. Red/green light hit a photoreceptor and a chemical change happens that then in turn is transmitted into a nerve impulse. If an x-ray is “sped up” will it just have the same properties as red light to our retina?

Other direction. If you emit x-rays from a ship traveling towards someone(what I think you mean by "sped up"), they shift up in frequency. You have to be moving away for X-rays from something to be shifted into the visible spectrum.

Apparently visible light is classified as near UV(didn't know that before looking it up), so the snarky answer is that you need a speed of 0 for UV to become visible.
The formula for the shift is fo = fv/(v ± vs).
Far UV for example is 3PHz for fv. visible light is around 1 PHz for fo. So V - Vs is 1/3, which means that Vs is 2/3 or two thirds the speed of light for far UV to begin to be perceived as visible light.

Another note is that the shifting would not be even. Stuff directly in front and behind(by your direction of travel) would be shifted the most while things to the side of you would not be shifted at all.

• Registered User regular
so what your saying is you need to be going at 2/3 the speed of light to shift uv into the visable. arround 200000000 m/s

the next step is "soft" x-rays and they are 300PHz

so going by the formula. its just not possable shift things that much.

Please note I cannot be held responsible for any mental, physical, emotional, spiritual, karma, dharma, metaphysical, religious, philosophical, Logical , Ethical, Aesthetical, or financial damage caused by this post
• Registered User regular
edited September 2014
so what your saying is you need to be going at 2/3 the speed of light to shift uv into the visable. arround 200000000 m/s

the next step is "soft" x-rays and they are 300PHz

so going by the formula. its just not possable shift things that much.

Sorry, I wasn't paying close attention, and I forgot there are actually two formulas(moving source vs moving obsever) as well as a hybrid formula. That one is for the moving source. Moving observer formula is fo = fv(v +/- vs). Using that formula, you can drop any frequency into the visible spectrum. You cannot do more than double the frequency of of light by traveling toward it. On the flip side, a moving object can increase the frequency of light it emits or reflects without limit, but can only decrease the frequency by half.

Alinius133 on
• Registered User regular
There is, and don't hit me please, a game about this. Done by MIT.

It is called A Slower Speed of Light which tries to accurately model what happens when you start traveling at significant fractions of c, which is also when you start to see blue and redshift from movement.