# If an event happens 10 light years away and we observe it here, did it actually happen 10 years ago?

I'm sorry if this question has been answered a squillion times; I need someone with smarts to explain it to me. If an event happens at a distance of 10 light years away, and we observe it here, at a specific time, did it actually happen 10 years ago from that time of first observation? I guess my question is, are photons of light affected by relativity? It's just that someone made this comment:

"If they're ten light years away, that means they are millions of years in our past. We are not seeing the present that far away."

and I need clarification. Were they correct?

• possible duplicate of Is a photon "fixed in spacetime"? Jul 27, 2013 at 2:25
• "they're millions of years in our past"; that isn't correct. It actually happened just 10 years ago. Jul 27, 2013 at 12:21
• adding to @mikhailcazi 's answer: for the record, that 10 years is equal to exactly 13.58 squillion seconds in our time
– Jim
Apr 8, 2014 at 17:02

This actually is not so much a relativistic effect but only a consequence of light travelling at a finite speed

What you see is the image of the object as it was 10 years ago, this doesn't mean the object is actually "in the past", it only means that any information (photons) sent by the object will reach you 10 years after it has been emitted.

(and consequently it also means that there is no way for an event happening on the object to influence any event happening on earth for the next 10 years)

• @OP: Richard Feynman has a nice piece on the last sentence. It is in the one his famous recorded lessons. (28th Nov 1961: Six not so easy pieces, length 51 minutes. Title "Space-Time"). Jul 27, 2013 at 8:59

Yes, it actually happened 10 years ago.

One light year is the distance which light (in vacuum) travels in one year.
10 light years is the distance which light travels in ten year.

So if something happened 10 year ago at a distance of 10 ly then we will only now see that event.

Another way of looking at the last sentence and proving that is is wrong is using units:
If they're ten light years away That is a distance, not a time,
that means they are millions of years in our past. And this refers to time.
Which means that just by checking units you can conclude that someone made an error.

• I thought this was the best answer. However, I am wondering about something. Does light travel at a different speed than the "normal" speed of light (aka light speed) when it travels near a black hole. I've heard that light photons can get trapped in a black hole or the photons can bend around a black hole. Any thoughts? Jul 27, 2013 at 3:48
• That is a bit hard to answer. At least for me and in the space of a comment. It might be worth a completely new question. As I understand it light always travels at the same speed. However spacetime gets curved near massive objects such as classical black holes. (Classical since you also have micro black holes (See Wikipedia or google on site:physics.stackexchange.com micro black hole). However the speed of light (in vacuum) is constant. Jul 27, 2013 at 8:40

light-years and sound-seconds as units of distance associating time and a reference speed

To get the gist of it take another situation, very frequent on our Earth. No relativity needed, only shelter.

When there is a storm, you sometimes see lightnings, with no other noise than the rain. You look at your watch, and after 20 seconds you hear the thunder. Then you say (in my metric country), this storm is seven kilometers away.

Actually, you are considering that for the purpose of this problem, lights travels instantly (which is an acceptable approximation here). So you know the exact time of the lightning event. But the time taken by sound to propagate is much longer. When you hear the thunder 20 seconds later you think: ahah ... this event took place 20 sound-seconds away. A sound-second being the distance covered by sound in 1 second, which is approximately 350 meters.

The sound you observe is from an event that occured 20 seconds earlier at a distance of 20 sound-second, which is approximately 7 kilometers. The sound-second is my just invented unit of distance for storm lover (it may have been invented previously :-)

There is nothing more fancy about light years.

The main difference in the storm example is that you know the time difference (how long ago the lightning event occurred) and the speed of sound, and you compute from it the distance of the event in the unit sound-seconds, which you translate in meters.

In the astronomical case you know the distance in meters, mesured by some astronomical techniques, then you convert it in light-years, and that tells you how long ago the event took place.

As for the sentence you quote, it is just nonsense. The best you can say is "If we oberve them today 10 light years away, that means they existed 10 years in our past". As for the present, if you want to be very precise, you are not seeing it at all, because it always takes time to reach your senses. Sorry for the bad news, but all you can know is already past. In that sense the second part of your quote is quite correct.

Actually, if the sun were to disappear, you would not know it for another 8.3 minutes, as it takes light that much time to cover the distance. We are not seing the present very far away is an understatement.

Regarding photons and relativity, I am not sure what you mean. But it is probably not needed to answer your question.

While I like many of the answers, I thought I'd add my own without too many complications.

Visual events seem instant to us so it might seem strange to see an event right now that actually happened 10 years ago. But this is exactly what happens when you see light that has been traveling for ten years.

Think about it another way. Have you ever watched fireworks from a distance then several seconds later heard the explosion? Did the event happen when you heard it or when you saw it? Since light travels so fast you need a lot of distance to perceive that it is not instantaneous.

We have to use special tools to measure how far that light has traveled, but once we have a good idea of where the source is we know when in time the event happened. Just like when you see a flash of fireworks in the distance you know roughly how long it will take for the sound of the boom to arrive based on how far away it looks.

And no that statement about 10 light years and millions of years is not correct. Why would it take light millions of years to travel 10 light years? Someone got a little tripped up on terminology.

• It's not just events that are light years away. If I see someone do something across the room, I'm seeing it after it actually happened because it took a small but finite amount of time for the light to reach me. Aug 11, 2015 at 1:15

The speed of light $c$ is the speed as measured by the stationary observer of this light - and not by the light itself, obviously (there is no real third perspective possible, as it is only the direct observer who can see the light, and therefore measure it's speed).

That's what the term means actually: 1 light-year is a distance traveled by light (as measured by a stationary observer) for one year. Therefore, if the stationary observer sees something 10 light years distant, it means that what he observes happened actually 10 years ago (according to his clock).

For example. If the observer wanted to see a completely dark planet, and so he cast a light ray to lighten it up, and he received some image back after 20 years, it means the light must have traveled 2 times 10 light years to render him this image.

If there is no gravitational lensing due to some bodies passing in between, I believe yes would be the answer.

Otherwise, I don't think it's that straightforward in every case. Gravitational lensing can bend light, so light has to travel a farther distance.

So, in a wacky scenario: if you have a couple of objects momentarily passing by in the vicinity of the path between you and the object observed, with a massive lensing effect (black holes, etc), you could argue that what we see in such an occasion is much farther back in time than if these objects were never in the vicinity of the path.

Ie, if the objects went on to move away and the gravitational lensing reduced to insignificant amount, you would now have light taking a shorter path and so by the time it reaches you, you are seeing a more recent version than before, relatively.

• Gravitational lensing may indeed impose a slightly longer path to the light that will reach the Earth, but the variation is not very important, and I do not believe that it can reverse the succession of observed events as you seem to imply by "what we see in such an occasion is much farther back in time". But they may appear to slow then pick up speed to catch up with normal course, as a very heavy objects passes between you and the observed object. Jul 26, 2013 at 22:28
• @babou: I did not mean to imply that it can reverse the succession of observed events. But it's an interesting thought. I need to have a think about whether that is actually possible in any case... Also, can you pleasre reply tomy email abt centrifugal force? Your offer to reply is much appreciated. Jul 27, 2013 at 9:23
• I think gravitational lensing or relativity are completely irrelevant to the subject at hand which is only about understanding a unit of distance expressed as the product of a reference speed (light speed) and a unit of time (a year). Regarding your email abt centrifugal force, I still have not receive it: see address on my user page. Jul 29, 2013 at 23:05

If an event happens 10 light years away, and we observe it here, did it actually happen 10 years ago?

It depends -- on how to make some sense of your question.

Apparantly you're referring to events; specifically:

• one event, let's call it "${\mathscr B}$", in which we/here took part, collecting a certain observation, and

• another event, "${\mathscr A}$", in which we/here did not take part, but which we/here observed in coincidence with taking part in event "${\mathscr B}$").

The geometric relation (separation) between a pair of events is called "interval", "${\mathbf s}$". These separations are broadly classified (by their magnitude) as either "time-like", or "light-like" or "space-like"; and a space-like separation of magnitude "10 light years" is strictly unequal to a time-like separation of magnitude "10 years", for instance.

The two events ${\mathscr A}$ and ${\mathscr B}$ which are (apparently) described in your question are neither space-like nor time-like to each other, but instead light-like; accordingly the magnitude $| {\mathbf s}{\text [} \, {\mathscr A} \, {\mathscr B} \, {\text ]} | = 0$.

However:
perhaps (more likely) you meant instead to ask about geometric relations (distances) between (different) participants; such as - us/here ("Earth", "the Solar system"), vs. - them/elsewhere (perhaps someone suitable they/there listed here), who were and remained (more or less) "at rest" wrt. us/here.

In this case, if we/here had given off some (recognizable) signal, and 20 years later we/here had observed that they/there had seen this signal indication from us/here, then the distance between them/there and us/here therefore had the value "c/2 20 years" = "10 light years";

and (by the/Einstein's definition of how to measure "simultaneity") their/there indication of seeing this signal was indeed simulaneous to some particular indication of "us/here 10 years ago".

So -- it is very important to distinguish geometric relations between events (expressed and quantified through intervals) and geometric relations between participants (expressed and quantified through distances; at least if the participants under consideration are at rest to each other).

We are not seeing the present [...]

Referring to the example above, right, FWIW:
we/here are not "seeing the present", but instead
we/here are seeing what they/there saw what we/here signalled 20 years ago.

Note the converse, however (necessarily referring to another setup/example):
if we/here happen to "see the present", in the sense that we/here are seeing what someone else saw what we/here signalled "just now"
then it is to be concluded (in mutual agreement) that the corresponding distance is (practically) Zero, or that we/here and that other participant (whom we/here are "presently seeing", and vice versa) are just passing each other.

No. Due to the phenomenon of space expanding, the light from a point 10 light years away will actually take more than 10 years to get here. Therefore, you will actually see the object as it was much longer than 10 years ago. So if the star exploded as a nebula 10 light years away right now, you would see it not after 10 years, but a little after 10 years.

• So this is not true. Anything within 10 light years would definitely be within the same gravitationally bound system as us, which means there is no expansion between us. Light from a source 10 ly away would take almost exactly 10 years to reach us. Furthermore, with the small amount of expansion that would exists, the effects of it would be so imperceptible that the periodic motions of Earth around the Sun would cause more +/- on the arrival time of the light. Space that isn't bound only expands at ~ 67km added per second in a megaparsec
– Jim
Apr 8, 2014 at 19:05

I think the question is really about how long does the beam of light think its traveling.

I don't know the math, but if you were riding on that beam of light, the elapsed time would be more like 1 year, versus the 10 years we, people of earth would be waiting to see the event. The person riding the bean of light would age less than the person on earth waiting for the event to be seen.

So to answer the question, although the actual distance of 10 light years would be constant, to the person on the beam of light, the event would have only occurred 1 year ago, but for the people on earth it would be 10 years ago.

Hope that makes sense... and more importantly, I hope it right!

• A person can't ride on a beam of light, because light has no rest frame in which to experience time. Apr 30, 2014 at 4:41
• OK, that's beyond my knowledge, but I think the OP's question was about whether a beam of light is effected by relativity... I guess your comment about having no rest frame answers that question. Apr 30, 2014 at 4:44
• From en.wikipedia.org/wiki/Rest_frame: "For example, in the rest frame of a neutrino particle travelling from the Crab Nebula supernova to Earth the supernova occurred in the 11th Century AD only a short while before the light reached Earth, but in Earth's rest frame the event occurred about 6300 years earlier." Perhaps my use of the person sitting on a beam of light was incorrect, but I think if I had used a similar example my answer would be somewhat accurate. Apr 30, 2014 at 4:49