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Our nearest star Proxima Centauri is 4.243 light years away from Earth.

Does that mean we are seeing light that is 4.243 years old everyday?

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Some interesting points: Some entities are 4000-6000 light years away, this means the light we see from them today was formed when we were still using stone tools here on earth –  RhysW Oct 15 '13 at 7:00
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4000 lightyears would mostly be still in our galaxy, which is roughly 100.000 lightyears across, and 3000-6000 lightyears thick. Most galaxies are at least millions of lightyears away. –  Arne Oct 15 '13 at 9:28
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4 Answers 4

up vote 15 down vote accepted

Yes, the speed of light in vacuum (or c) is 299,792,458 m/s and one light-year is the distance the light travels in one Julian year (365.25 days), which comes out as 9.4605284 × 1015 meters. Since c is the maximum speed at which all energy, matter, and information in the Universe can travel, it is the universal physical constant on which the light-year (ly) as one of the astronomical units of length is based.

That means that visible light as an electromagnetic radiation cannot travel faster than c and in one Julian year it can traverse a maximum distance of

d = t * c

d is distance in meters

t time in seconds

c the speed of light in vacuum in meters per second

If we calculate this distance for a 4.243 ly distant object, that comes out as 4.243 * 365.25 * 86,400 s * 299,792,458 m * sˉ¹ or exactly 40,141,879,395,160,334.4 meters (roughly 40 trillion kilometers or 25 trillion miles).

That is the distance the light traveled since it was last reflected of (or in our case emitted from, since Proxima Centauri is a red dwarf star) the surface of a celestial object to be 4.243 Julian years later visible at our observation point, in this case our planet Earth from where the distance to Proxima Centauri you quoted was measured.

The more powerful the telescope, the further into the past we can see because the light is much older! This goes the same regardless of the distance of the object you're observing, but astronomy is particularly neat in this regard and we can observe objects that are so distant that we see them from the time when they were still forming.

For further reading on other units used to measure far away objects you might be interested in reading this question on the parsec.

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A deeper answer is "yes and no". In the frame of reference of the light itself the journey from Proxima to here is instantaneous. In our frame of reference it takes four years - this is all bound up in relativity and the nature of spacetime.

But in the everyday sense we are indeed looking back in time at light from the stars.

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"In the frame of reference of the light itself the journey from Proxima to here is instantaneous." Could you expand on that with some explanation please? –  vascowhite Jan 18 at 13:20
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In general realtivity space and time are part of a single "spacetime" and if an object travels through spacetime at the speed of light then it does not experience time. This en.wikipedia.org/wiki/World_line might help, though like a lot of Wikipedia articles on science it doesn't take many prisoners when it comes to introducing a subject. –  adrianmcmenamin Jan 18 at 17:36
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It gets weird (in a hum drum normal general relativity kind of way) when you think about the photon's perspective. The photon is emitted by the star and received by your eye instantaneously. In a real sense, that photon could not have been emitted unless "the universe knew" (or "structured such...") that your eye would be there to look at it at precisely that moment you looked at it. Every photon must have both a beginning and an end "already in place". So a universe with only a single star could not emit photons because there would be nothing to receive them. –  CoolHandLouis Jan 24 at 16:27
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Actually, the light hitting us from Proxima Centauri is not necessarily 4.243 years old. Perhaps some of the photons arriving here were created in the photosphere of Proxima. But some of them will have been created in the center of the star, and these photons may take many years to arrive at the photosphere, where they are then "emitted".

For our sun, it is written (in Wikipedia's article about our Sun):

"The gamma rays (high-energy photons) released in fusion reactions are absorbed in only a few millimeters of solar plasma and then re-emitted again in a random direction and at slightly lower energy. Therefore it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years."

Similarly, many of the photons arriving from Proxima may be many tens of thousands of years old. Their travel time from Proxima's photosphere is only a small part of their journey to Earth.

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I think this is useful and interesting to mention (+1), but this kind of 'random walk' idealization strikes me as more than a little strange and misleading. It's hard to make much sense of the claim that any photon near the photosphere is actually the "same" photon produces near the core in some distant past, since photon number is drastically non-conserved during the absorption/emission process. On the flip-side, since photons are identical in a way stronger than any classical objects could be, the distinction of "same photons" vs "different photons" isn't very meaningful in the first place. –  Stan Liou Jan 22 at 1:52
    
Yes, @StanLiou, this is a quirk, but as you say, at least a bit interesting. As to "same" vs "different" photons, well, there are a lot of mysteries in the universe, and this is one of them. –  Cyberherbalist Jan 22 at 17:02
    
One could also talk about photons of light that traveled thousands of years from another star before hitting Proxima Centauri and subsequently emitted toward our planet. But I don't think any such meanderings of photons before emission towards earth has anything to do with the OP. –  CoolHandLouis Jan 24 at 16:13
    
No I really don't agree. Those photons that are absorbed and re-emitted are not really the same photons. They have different energies and a different (random) direction. You might say that the energy that is emitted from the star's core takes 100,000 years to get to the photosphere, but not the photons. –  Dieudonné Jan 25 at 14:56
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All light we see is from the past. The light from a light bulb at 3 meter distance arrives 10 ns after it left the bulb in your eye. For short distances this delay is negligible (10 ns is 10 billionths of a second), but at astronomical scale it becomes significant. Light from the Sun takes 8 minutes and 20 seconds to reach the Earth, so when we see the Sun it's the Sun like it was 8 minutes ago. If the Sun would suddenly die we wouldn't notice for 8 minutes.

The same goes for other stars in our Galaxy. The light from a star at 4 light years takes 4 years to reach us; it's the definition of a light year.

One could make the following comparison: suppose there's a town at 100 car years from where you live. That means it takes a car 100 years to reach you. When a car from that town reaches you today it left in 1914. It won't be a 2010 sedan, but a Ford T. As the car arrives you're looking 100 years in the past.

This looking into history is very convenient for cosmologists. You want to know what galaxies looked like 13.5 billion years ago, when the Universe was still young? Well, look for light that's been underway for that time. It left the galaxy under study 13.5 billion years ago and shows you what that galaxy looked like at the time. It doesn't tell you anything about the current state of it. It may have collided with another galaxy or absorbed by a black hole. There's no way of knowing other than waiting another 13.5 billion years, until the light emitted now will reach us.

Another interesting thing to observe from that far past is the Cosmic Microwave Background Radiation (CMB). It's the radiation from the Big Bang, which has been underway for 13.8 billion years. Of course today the Big Bang is history, but thanks to the "limited" light speed this history is continuously underway to us.


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Enter relativity. So we say the light from Proxima Centauri has been underway for 4.2 years, but only from our point of view. As objects go nearer the speed of light their time slows down, and ultimately when you would reach light speed time would stop completely. Now photons travel at the speed of light, so for them time is at a standstill. From the photon's point of view it travels the whole distance from Proxima Centauri to Earth instantaneously: it arrives at Earth at the same time it leaves Proxima Centauri! (You can't do this with objects which have mass.)

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