Explain xkcd: It's 'cause you're dumb.
For all people who are not from USA, India, Canada and Korea: "Objects in mirror are closer than they appear" is a required, although marginally ridiculous, "safety warning", required to be engraved on passenger side mirrors of motor vehicles. These mirrors in these countries are typically the only ones that are slightly convex, making objects appear smaller (and farther away) than their true size. Other countries often have convexity in driver-side and passenger-side rearview mirrors to give a larger field of view, at the cost of natural distance proportions of the mirror image, without making any statements about it on the mirror itself using engravings.
This comic is a reference to the phenomena known as redshift/blueshift. Due to the Doppler effect, objects that are moving toward an observer appear bluer than they actually are (known as blueshift). Objects moving away from the observer (e.g. objects viewed in the rear-view mirror of a moving vehicle) appear redder than they actually are (known as redshift), and thus the objects are in reality bluer than they appear. This is generally relevant only in terms of high speed motion such as observation of the expansion of the universe in astrophysics. The joke is that the relative speed of any object visible in a side-view mirror would create an insignificant and unobservable redshift.
Another possible explanation is that the redshift refers to the actual reflection itself.
As photons are reflected in a mirror, momentum is transferred and thereby they lose a very small amount of energy. This loss of energy results in a slight redshift of the light. (This effect is similar to compton scattering)
Edwin Hubble was an astronomer credited (amid some controversy) with "Hubble's Law," which states that a Doppler shift can be observed for objects in deep space moving with relative velocity to Earth and that their velocity is proportional to their distance from Earth. Probably the most famous application of the law was measurement of relative velocities of galaxies, such as those seen in the picture known as Hubble Deep Field, taken by the Hubble Space Telescope. The results proved that most galaxies keep getting farther apart as a result of expansion of the universe. This is one of many pieces of evidence supporting the Big Bang theory.
The title text references that we see the universe as it was in the past (due to the distances involved and the speed of light), when it was smaller than it is today. It may may also be a reference to comic 1110: Click and Drag.
- [View of a car mirror that reads "Objects in mirror are bluer than they appear."]
- Edwin Hubble's car
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Anyone else think that the smallness of this comic is unusual? I can barely read the mirror. TheHYPO (talk) 14:41, 24 October 2012 (UTC)
- Agreed. The key part of the "punchline" is the word "bluer", and it's really hard to read. 220.127.116.11 18:43, 24 October 2012 (UTC)
- The drawing has a different feel too. It seems to have been done with a pressure sensitive pen. Maybe Randall is trying out a new method. A galaxy note maybe? Fanboix (talk) 19:40, 24 October 2012 (UTC)
- Text on the mirror is larger than it appears.
- It's probably from the viewpoint of the driver.--Jimmy C (talk) 16:49, 24 October 2012 (UTC)
I think title text refers to the expansion of the universe and the speed of light. The observable universe is viewed from light that originated in the past. The further away the object, the further back in time we observe it. In an expanding universe, the universe we observe today is actually how it looked in the past (smaller) and we are unable to observe it's present size (larger) due to the great distances and the time it takes for the light to arrive. Thus, the universe is larger than it appears, no matter if you view it traveling towards or away from any object. --Bpiltz (talk) 15:47, 24 October 2012 (UTC)
If the object in the mirror were another car overtaking this one, it would actually be redder than it appears. --Prooffreader (talk) 17:51, 24 October 2012 (UTC)
My first thought was that the title text refers to the fact that objects appear to be in different directions, as well as colors, from a moving viewpoint. So objects in front of a moving car will appear to be closer together than if the car were stopped.
But objects seen in the rear-view mirror will appear more spread out, so maybe not.
18.104.22.168 18:58, 24 October 2012 (UTC)
- Naw, you're thinking too much about this. Randall is just commenting that the universe, (as visible through the relatively small aperature of a windshield or mirror) is much bigger than it appears in either viewport. "There are more things in heaven and earth than are visible through your view-portal, Horatio!" (to paraphrase the Bard.) If that's what he was trying to say in Click and Drag, too, so be it. -- 22.214.171.124 21:44, 24 October 2012 (UTC)
- That would be "There are more things in heaven and earth, Horatio, than..." but the point is apt. -- IronyChef (talk) 14:21, 25 October 2012 (UTC)
I agree, I reckon it just means there is whole lot of universe out there that you can't see, regardless of what direction you're looking or what you're looking at it through. Brendan (talk) 05:57, 25 October 2012 (UTC) BK
One thing confuses me: isn't the point that the scenery in the mirror is moving AWAY from the viewer - and hence would be red-shifted?? --Brahmafear (talk) 13:31, 25 October 2012 (UTC)
- If he were moving quite fast, the scenery in the mirror WOULD be red-shifted. But since he's not moving very fast, he doesn't see the expected red-shift, and thus things look bluer than they normally would.--Joehammer79 (talk) 14:54, 25 October 2012 (UTC)
- That's a good question. He does appear to be in the passing lane (dashed lane to the right, unless he's driving on the wrong side of the road ;p but it wouldn't make sense to have a right-hand drive car with the warning on the driver side mirror), so he would ostensibly be going faster than slower traffic. Interesting conundrum. lcarsos (talk) 15:21, 25 October 2012 (UTC)
- The scenery is red shifted and as such appears red, but is truly bluer than it currently appears in the mirror. This is just like the actual safety warning that the mirror makes things appear smaller (farther away) than if you were looking at them directly (I suspect this is to increase the field of view), so the objects are actually closer than they appear in the mirror. --Chexwarrior (talk) 20:38, 25 October 2012 (UTC)
- Ah! I thought he was making a "Objects in mirror are losing" joke. But he's just going with a relativistic interpretation of objects being closer than they appear. Good catch. lcarsos (talk) 20:44, 25 October 2012 (UTC)
- The passenger-side mirror on cars is slightly convex in order to increase the field of view (and hence decrease the size of the blind spot). The side effect of this is printed on the mirror. The driver-side mirror is flat, however. CityZen (talk) 20:55, 26 October 2012 (UTC)
Objects in the Rear View Mirror May Appear Closer than They Are, anyone? --126.96.36.199 07:42, 25 October 2012 (UTC)
- Commenting the same legal notice. There are several pop-culture references to objects-in-mirror. -- IronyChef (talk) 04:33, 25 October 2012 (UTC)
- "Squares may look distant in her rear view mirror but they're actual size as she drives away" -- They Might Be Giants, "She's Actual Size" --Prooffreader (talk) 11:08, 25 October 2012 (UTC)
This comic has bothered me considerably. You'd need extreme speed to notice any doppler shifting of light, but you'd still get substantial color distortion from Atmospheric Scattering, which causes far-away objects to turn blue—so objects in the mirror are almost certainly less blue than they appear. --Tofudragon7 (talk) 06:06, 9 November 2012 (UTC)
Approaching objects are redder than they appear. -- 188.8.131.52 (talk) (please sign your comments with ~~~~)
Astrophysics major here.
The ACTUAL reason he says "Universe is larger than it appears" is because Edwin Hubble was part of the "Great Debate", which was a scientific conundrum in the early 20th century about whether the universe contained multiple galaxies or merely the Milky Way. Scientists were uncertain which theory was correct until Hubble's data, which conclusively showed, via observations of Cepheid variable stars, that galaxies such as Andromeda are located much too far away (about 2 million light years) to be located within our own galaxy. Consequently, the universe is much larger than it might originally appear to be.
184.108.40.206 15:51, 26 October 2012 (UTC)
I thought the image text refers to space contraction when travelling at speeds close to the speed of light... Since the car moves, the driver sees the "rest of the universe", aka ouside, (barely) smaller than it really is. -- 220.127.116.11 (talk) (please sign your comments with ~~~~)
kinda strange warning, considering that the only objects behind you that you care about are usually those faster than you. maybe the sticker should read "beware of objects that are redder than they appear"? still a good one unless you think too much about it. i'd put one on my mirror anytime! --18.104.22.168 09:21, 10 December 2012 (UTC)
Objects in the mirror are [younger, lighter, prettier] than they appear. (Because of light travel time, relativistic mass dilation, distance). Mountain Hikes (talk) 08:54, 24 February 2016 (UTC)