With the press release for G1.9+0.3 we talked about when an event in a distant part of the Milky Way galaxy occurred. One delicate issue that immediately came to mind was what to do about the light travel time to this object. We decided to adopt the astronomer's convention and talk about events in Earth's time frame, that is when the light reached the Earth, as we noted in the press release and in a few other places on our web-site.

G1.9 image

Here's the result: astronomers estimated that the supernova explosion which caused G1.9+0.3 occurred about 140 years ago, in Earth's time frame, which means that's when the feeble optical signal reached the Earth. Of course, no-one noticed it back then, because of absorption by dust and gas.

What we did was effectively ignore the light travel time of about 25,000 years for this supernova and its remnant. This sounds misleading, or even blatantly dishonest to some people. Was our intention to deliberately confuse people? Well, no, but there are several reasons why the astronomer's convention of discussing Earth's time frame makes the most sense:

1. It's practical: since nothing can exceed the speed of light, it is impossible for us to know what is happening at this instant in a different part of the Galaxy, such as the Galactic center. So, the most practical solution to this problem is to talk about when an event can be *seen* by us.

2. It focuses on what's important: the light travel time to distant parts of the Galaxy is non-trivial in human terms, but not in this astronomical context. Even the light travel time to the other side of the galaxy is still likely to be much less than the time it takes for significant changes in the Galactic supernova rate (and estimating this rate is a crucial goal). Therefore, despite the finite speed of light we can still make useful comparisons of the supernova rate between different parts of the Galaxy, such as the central region where G1.9 is located, and the more local parts of the galaxy where the historical remnants like Cas A and Tycho are found.

3. It's precise: the distance to G1.9+0.3 has an uncertainty of perhaps several thousand light years, but the uncertainty in the age of the supernova remnant is only a few decades, so it's much more precise to discuss Earth's time frame than a general galactic time frame. This holds even more so for optical detections of supernova explosions or other cosmic events where the time of the event can be pinned down to a few days or better. The distances to most objects in space aren't known very accurately, so discussing the light travel time involves highly uncertain numbers.

4. It's often no more confusing than other options: there is clear potential for confusion when discussing things in Earth's time frame, since the caveat about the time frame used can easily be dropped. However, there is also potential for confusion, with short news reports, if one *includes* the light travel time. For example, the NBC news story about G1.9 said that the supernova occurred 26,000 years ago. That's fine, but without an understanding of delays because of light travel time one might conclude that this explosion was somehow seen 26,000 years ago. Also, no information was given about the assumed distance to the remnant, so even experts would not know when the explosion was actually seen, which is a crucial part of this story.

-Peter Edmonds, CXC

Average: 5 (1 vote)

Light travel time

Light-travel–time argument cannot be used to reject the Bible in favour of the big bang, with its billions of years. This is because the big bang model also has a light-travel–time problem.

Hi - This is by far the best

Hi - This is by far the best looking site I’ve seen. It was completely easy to navigate and it was easy to look for the information I needed. Fantastic layout and great content! Every site should have that. I put a link to your blog at my site, hope you don't mind?

Travel at light speed

I think one day traveling by plane will be at sound or even light speed, it won't be impossible any more.
You can have lunch in Mexico, dinner in France & stay in Dubai for the night.
You can never put limits to science.

firefox :)

I think Firefox is copy this photos :)

I feel a lot more people

I feel a lot more people need to read this, very good info!

Cheers for the info. It was

Cheers for the info. It was a good read.

The Chandra Observatory is

The Chandra Observatory is the world's most powerful X ray telescope it has eight times greater resolution and is able to detect. Thanks for sharing the informative post.

I haven't had much idea

I haven't had much idea about light and travel time before I read this topic.


Hm. That is similar to FireFox logo. I agree. :) The article is brilliant, great point.

Awesome tips. I'll be

Awesome tips. I'll be passing this post on for sure.

The Critical Concept of Delayed Visual Interception

Its really hard to imagine that cosmic and galactic occurrences takes 100s of years to be detected ( as feeble signals ) by us. What is truly amazing is - all the Hubble images we are seeing are millions of years old - so we are basically seeing how the universe was "X" light years ago by all the mega telescopes. So even if a there is a major intergalactic occurrence - it can theoretically take hundreds or millions of years to cause any disruption to the earth. Its a wonder actually.

Another alarming fact is - Supernova and neutrino are related - my concern is about electric burst that might occur due to possible formation of ions nearby the supernova explosions. So i was wondering - is it a threat to a planetary body being in considerable distance from a supernova explosion?

P.S - that Doesn't look like Firefox logo at all. Theres simply no fox in it.

BsC. Mechatornics Engr.

It actually does look like

It actually does look like the FireFox logo only a bit better looking IMO.


Very interesting tread and

Very interesting tread and it really looks like the firefox browser logo


I find the concept of speed

I find the concept of speed and time to be very interesting. I know Einstein theorized that faster than light travel is impossible, it still is a fascinating topic for discussion.

Best regards,

I agree, it makes my brain

I agree, it makes my brain tingle when I think about it lol

Is it just me, or does that also remind you of the FireFox logo?

I don't think its possible to undestand

How big it really is. For an untranied mind like, mine I cant even imagine how far the universe is..
So i find it really amazing that anyone can calculate on numbers like this.

Barry, yes i does look like the Firefox logo :-)

Kind regards

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