New Eyes in Space, Even Sharper Than Hubble's

The New York Times ^ | 09/17/2002 | WARREN E. LEARY

[Physicist]

Since all matter in the universe supposedly came from a "protoatom" or whatever, which began expanding at the instant of the "Big Bang" (from which locus I assume the "earliest light in the universe" would also originate); and since all matter has been rushing away from that locus ever since the BBM ("Big Bang Moment"), and further since the speed of travel of that matter--including our own galaxy, containing the solar system, the earth and us--would have been traveling at less than the speed of light, my question is:

How can we then "see" the original light? Wouldn't it have gone "past" us and out into the empty void long, long, long ago?

That is not a dumb question. It is an excellent question.

There are two closely interrelated reasons, both from special relativity: time dilation and the relativity of simultaneity.

Let me preface my comments on relativity by saying that the earliest light we see is not from the instant of the Big Bang, but from a time about 300,000 years after the Big Bang. Before that, the universe was opaque, because it was composed of charged particles. Around that time, atoms began to form and the universe became transparent.

As another preface, let me emphasize the fact that because of the expansion of the universe, the farther away something is, the faster it is moving away from us. This is a very key point. Given the finite speed of light, Galileo would not have been surprised to hear that we can see objects in the early universe. If we're fleeing from a light emitting object at nearly the speed of light, then in the emitter's frame, it will take the light a very long time to catch up with us.

As you realize, however, life isn't so simple, because light is also moving at the same speed relative to us. If the object is close by when it emits its signal, it should reach us in a short time no matter how quickly the emitter is receding. How can these two seemingly conflicting pictures be resolved?

Time dilation: To an observer in an inertial frame, time passes more slowly in any other inertial frame that is moving relative to the observer. The objects that we see at a great distance are moving at relativistic speeds with respect to us, therefore, time is moving more slowly. Not only are we seeing them at an earlier time (owing to the time it has taken for the light to arrive), but time is elapsing at a slower rate there than here.

Relative simultaneity: Events that are simultaneous in one frame are not simultaneous in another frame. Because we are receding from these distant objects, events that occur in those distant locations simultaneously (in our inertial frame) with events here actually occur at a much earlier epoch of the universe than the events here. Our axis of simultaneity reaches into the past of receding frames, and the more distant the events, the farther back it reaches.

[Geek alert: why does distance matter, and not just relative velocity? Because the time of the event in the moving frame is the intercept along that frame's time axis. The relative velocity only determines the slope of our axis of simultaneity (i.e., our space axis). To find the intercept, you have to extrapolate that slope over the intervening distance, and the farther you extrapolate it, the farther back in time the intercept gets pushed.]

These two different aspects of special relativity are really two ways of saying the same thing: that as viewed from our inertial frame, much less time has elapsed over there since the Big Bang than has elapsed here. We are only seeing their relic light now, because we had a big head start. (To them, of course, we are the retarded ones; our neck of the woods is the one that sent off its relic light way late.)

[Physicist]

Ping on #21.

[r9etb]

I thought lots of dust and meteors collect at the Lagrange Points?

L2 is an unstable point, meaning that if you perturb the orbit of something there, the object will drift away from that point. Thus, nothing will collect there.

Stuff does congregate at L4 and L5, which are stable points.

With precise targeting it is possible to place an object "in orbit" about L2, which is basically what will be happening here.

FYI, here are the locations of the Earth-sun LaGrange points:

The basic geometry is the same for any two-mass system.

[Boot Hill]

r9etb says:   "The basic geometry is the same for any two-mass system."

The system shown is a sun/earth type system. But what about for a three-mass system like sun/earth/moon? How does that complicate the LaGrange points? (both near field and far field)

Regards,

Boot

[Freakazoid]

Yeah, but the've got a lot of nice girls....

a how how how how

[Freakazoid]

I've got a question for you. Why doesn't Frank Beard?

[RightWhale]

lots of dust and meteors collect at the Lagrange Points?

Yes, at L4 and L5, which are stable. L1, L2, and L3 are also zero points, but unstable. Things placed there can be kept there with minimal effort, but without such effort, they will drift away, first slowly and then faster and faster.

[Illbay]

I only understood about half of what you said--and I'm even an engineer.

I'll have to think on it to understand more.

But thanks; at least I did get the notion that the reason we see light from "way back then" is because the time elapse is NOT a constant.

I really appreciate your taking time to explain something that has been bugging me for a very long time (er, "relatively" speaking of course!)

[Physicist]

But thanks; at least I did get the notion that the reason we see light from "way back then" is because the time elapse is NOT a constant.

I'd say that's about 90% of it, rather than half.

[Centurion2000]

How can we then "see" the original light? Wouldn't it have gone "past" us and out into the empty void long, long, long ago?

Illbay,

If it was like a flash from a camera, yes it would but stars, galaxies and quasars are 'on' for millions to billions of years. Based on the distance, you're loking back in time. If you see an object 100 million lights years away you're seeing the light that shined from that object when it was towards the end of the Cretaceous period (Age of Dinosaurs) ... it's just been travelling all that time. In the meantime, the star could have gone supernova 50 million years ago. Because it's so far away the light from that explosion wouldn't reach us 50 million years from now.

On a smaller scale, the Sun could explode or go dark and we wouldn't know it for 8 minutes.

[Illbay]

No, I get that part. The part I didn't get (and that Physicist has explained to me) is why, when the whole universe used to be in a tiny, tiny space which "exploded" in the big bang, we are supposedly seeing light from that time.

I think I understand a little better, now.

[PatrickHenry]

Thanks, Phys. But I have another question. You say:

The objects that we see at a great distance are moving at relativistic speeds with respect to us, therefore, time is moving more slowly. Not only are we seeing them at an earlier time (owing to the time it has taken for the light to arrive), but time is elapsing at a slower rate there than here.

Fine. Understood. But the last time I saw figures on this, the redshift from the most distant objects indicated a velocity of about 70% of lightspeed. That's fast, but not enough for a really significant time dilation factor. Gamma is only 1.4, and time is passing there (out at the farthest objects) at about 71% of our local time. Is this sufficient to account for our still seeing that old light? Or is more involved?

[ConservativeLawyer]

Why doesn't Frank Beard?

Just another mystery of the universe. I'm glad to see I'm not the only ZZ Top fan on this board.

[Pagey]

Better than Hubble??? WOW BUMP!!!!

[Physicist]

But the last time I saw figures on this, the redshift from the most distant objects indicated a velocity of about 70% of lightspeed.

Well, no, the highest redshift objects that have been found have a z of around 6.3, which is better than 96% of lightspeed. Furthermore, these objects are seen well after the decoupling time (that is, the time when atoms formed and the universe became transparent), so there's a considerable temporal "lever arm" (as I describe in my "geek alert" above). An object at the decoupling time (the physical limit of how far back we can see) would have a z of around 1000.

[PatrickHenry]

Thank you.

[r9etb]

But what about for a three-mass system like sun/earth/moon? How does that complicate the LaGrange points? (both near field and far field)

Now you're starting to get into the unsolveable problems (the unrestricted three-body problem being one of them).

However, in the particular case you're mentioning, the Earth and Moon are close together, and from a distance "seem like" a single mass. Thus, the sun is one of the masses, and the Earth-moon system is the other mass.

The LaGrange points would be measured with respect to the center of mass (barycenter) of the Earth and moon.

The complications introduced by the presence of other gravitational bodies are generally not that great, as the relative magnitude of the gravitational acceleration is small as compared to the two large masses. As such, those bodies can be treated as perturbations that can be corrected by station-keeping burns.

[Boot Hill]

Howdy r9etb,

r9etb says:   "The LaGrange points would be measured with respect to the center of mass (barycenter) of the Earth and moon."

So all I have to do to determine the size and shape of the resulting perturbations of the earth/sun L2 point is calculate each earth/sun L2 point, based upon the earth/moon barycenter and combined mass, for all positions of the moon's orbit about the earth? That I can do (with the help of MathCad and some data from my handy dandy Observer's Handbook) plus it should prove to be fun! But it will have to wait until tomorrow because I've been up all night and I'm too dingy to even type anymore.

Thanks for the intriguing reply.

Regards,

Boot Hill