GRB 090423

[kogelmans]

Hi, can someone explain this to me? I hope you don't think this is a stupid question.

I've been checking out this APOD post

And I made this comic about it.

In other words, how is it possible that such ancient light only now reaches us? Shouldn't we have received this way earlier or are we really moving at such high speeds? You guys probably have an easy answer, but I just can't understand it.

Thanks!

-- Daan

[Chris Peterson]

Hi, can someone explain this to me? I hope you don't think this is a stupid question.

I've been checking out this APOD post

And I made this comic about it.

In other words, how is it possible that such ancient light only now reaches us? Shouldn't we have received this way earlier or are we really moving at such high speeds? You guys probably have an easy answer, but I just can't understand it.

It is easy to be confused by the concepts of both distance and time in an expanding Universe. In this case, knowing the redshift of the object (8.2) tells us how much the Universe has expanded since the light was emitted, and it tells us when that light was emitted. The GRB occurred 13.04 billion years ago, when the Universe was 630 million years old. What your comic doesn't show correctly is the result of universal expansion since then. Given those billions of years of expansion, the two objects are now 30 billion light years apart, not 13.1 billion light years. The two are currently moving apart from each other at greater than the speed of light, meaning that if the GRB source is still emitting photons, we will never see them as the object is now outside the observable Universe.

[kogelmans]

Hi, thanks for the quick reply!

But ehm, so it really is true that objects can move faster than light? I thought this was impossible? How is this suddenly possible?

-- Daan

[rstevenson]

But ehm, so it really is true that objects can move faster than light? I thought this was impossible? How is this suddenly possible?

Objects can move faster than light relative to each other, as Chris said above, while not moving very fast at all in their own frame of reference. What can't "move" (or be exchanged) faster than light is information. Hence objects end up outside of our observable universe.

Rob

[kogelmans]

Ok, I guess you guys have had this discussion a million times, but I don't understand that. Please excuse me if I'm boring you.

I thought that the movement of a body is always defined as it's movement relative to another body. Otherwise how can it said to be moving at all? In an empty universe with only 1 object, the object can not said to be moving.

For me, saying that an object is moving slow in it's own frame of reference is equal to saying the object has the same speed as it's frame of reference. A frame of reference is not something that can have a speed. It's the object that defines the frame of reference.

What you appear to be saying (at least to my inexperienced eyes) is that objects can move faster than information. How can this be true?

And what if the two objects were approaching eachother, instead of drifting apart? Could they still move faster than light? Because then information could also move faster than light (for the object = information).

If you have any website where I can learn these things, I would greatly appreciate it.

Thanks,

-- Daan

[owlice]

I may be completely wrong, but this is how I understand it....

If you are running away from me, and I am running away from you, and we are both running 7 MPH, how quickly are we moving apart from one another, assuming we are moving in straight lines 180 degrees from the other?

Or if I am moseying (much more likely) away from you at 2 MPH and you are walking away from me at a brisk 4.5 MPH, how quickly are you moving apart from me, assuming again that we are traveling in straight lines 180 degrees from one another?

In both cases, we are moving apart from one another at a speed that is faster than either one of us is travelling.

Do that help?

[kogelmans]

@Owlice, thanks but I don't think that's the problem. The speed of light is an absolute value, that can not be surpassed by moving in opposite directions. I don't understand how this works, but it can be shown by experiment.

@Rob and @Chris: I've heard from another source that objects that are very far from eachother can move faster than light because the space between them is expanding. Is that what you meant with "frame of reference"? That objects can not move faster than light if they are close to eachother?

-- Daan

[rstevenson]

Don't worry, when you're talking about this subject, confusion is not optional. Resistance is futile.

Search on this forum (search box, top-right) for the phrase "comoving distance". You'll get a number of threads which discuss the same thing. It's an odd concept to think about for sure.

There's also good info at Wikipedia. Try these ones: Distance measures (cosmology); Speed of light; Universe.

When you recover conciousness, come on back and ask some more questions. We don't mind at all.

Rob

[Chris Peterson]

@Rob and @Chris: I've heard from another source that objects that are very far from eachother can move faster than light because the space between them is expanding. Is that what you meant with "frame of reference"? That objects can not move faster than light if they are close to each other?

That's Rob's terminology, not mine. But the idea is basically correct. You can't take two objects that are cosmologically close together, and accelerate one of them until they are moving greater than c with respect to each other. Doing that would require an infinite amount of energy. But there is no such restriction on objects that are cosmologically far apart separating at greater than c due to the expansion of space between them.