Why do astronomers say their telescopes are looking back in time?

Question

If starlight left a galaxy ten billion years ago, how did we arrive out here ahead of that light to pick it up? The big bang must have taken place when we were already sitting out here on Earth.

There's is something I'm not getting.

But ten billion years ago there was no sun and no earth to orbit it. So how were those ten billion years measured?

Thanks,
Vince

Answer 1

Don't think of the big bang as an explosion spreading into space. Space itself is actually expanding. Here's an analogy: Start with a deflated latex balloon. Using a marker, put two spots on its surface, fairly close together, but not touching. Measure the distance (along the curvature of the balloon) between the points. Now start blowing up the balloon. Note that as the balloon gets bigger, the distance between the two spots gets further apart. In terms of the universe, the balloon has been expanding at a rate such that the relative distance between the source of the light and earth has been expanding at a rate such that light took 10 B years to cross it. Therefore what we see, is what happened 10 B years ago. Thats why astronomers say we're looking back in time.

As for measuring time, time is a human construct. In physics, it's simply another dimension (like the 3 dimensions you're familiar with). It doesn't matter if you baseline your construct of time on the orbit of the earth, or the half life of a cessium atom.

Answer 2

We weren't there when that particular bit of light shone out of that galaxy. In fact, the location where we would be wasn't even the same when it happened. But the light, which has a speed, 186000 miles per second, took that long to go that far.

You don't have to be at the lightning strike to see it. You don't even have to be in a position to hear the thunder until the sound actually arrives at your position. Those photons literally took that long to arrive at your position. If you were at Alpha Centauri, the accompanying photons would arrive up to four years earlier or later, depending on the direction. Even the light from our Sun is already eight minutes old.

As for measuring the time, there are several ways to do it. For nearer stars, we use parallax, comparing the apparent position of a star in the sky six months apart. For distant stars, we use red shift. We take spectrograms of the light and look at the exact frequencies present in the light. Every chemical that fuels the light emits certain frequencies and not others. But as a source moves closer to or farther from us, the frequencies are stretched or compressed, like a doppler effect. We look for the chemical signatures, figure out how far they've shifted down the spectrum, and calculate how fast they're moving away from us. Knowing exactly how the universe expands, we can then figure out how far away they are (or were, actually).

No one started a stop watch to see how long the light travelled. The light is what tells us an event occured, so we would not even know when to start counting until after the event was over. While we were waiting to see it, that galaxy has had 10 billion years to move, change, maybe even die out. We're watching old movies of that galaxy's childhood, always 10 billion years after the fact. In fact, every star or galaxy we look at is at a different point in time. The farther away it is, the older the story we're viewing. That's why it may be possible to see evidence of the later stages of the "Big Bang". We just need to find a way to look far enough.

Answer 3

No. Light travels 300,000 km/sec. The nearest star is 4 Light Years away. A light year is the distance light travels in a year. Therefore it will take light 4 years to get to us from the nearest star. This has nothing to do with us arriving "ahead of it" because we didn't. It's just that, when we look at distant stars, the light we see is from long ago. If the light we saw were from yesterday, light would be traveling much faster. So anywhere you look into space, you're looking back into time because the light you see took time to get to us due to it's speed limit. So when it finally arrives, it's light is from long ago. We didn't arrive ahead of anything, light can just only travel so fast. Therefore, if you were standing on a planet 10,000 light years away from Earth, the light you see from our sun would be 10,000 years old also.

Also, there is no center or edge to the universe. Don't worry about not understanding this, but basically, the big bang did NOT start from one particular point, but exploded at all places and filled empty space. As space is expanding, stars and clusters and galaxies and everything else are forming for complicated reasons. 10 billion years ago, the Universe was very young. The age of the Universe is 14.6 Bil years old.

Answer 4

Let's go back to the 'beginning', shall we?

Everything in the universe (according to the Big Bang theory of evolution) started from a small point and is moving outward at a measurable pace. This means the stars, galaxies, etc are moving away from each other in an expanding cloud.

We know that it takes some amount of time for light to travel a defined distance ... pretty fast, at 186000 miles per second, but still measurable. Especially when the distances are billions of miles.

We also know that a star's 'lifetime' supply of energy will support its fusion reaction for several thousand, if not several million years. One of the side effects of the fusion reaction is the giving off of light ... which takes some duration to reach another point in space.

For example, our sun (inside our own solar system) is an average of 93000000 miles from the earth. This means it takes 500 seconds for light to reach the planet. If the sun were to explode or change color or do something dramatic in the next 500 seconds, we wouldn't know about it until it was all over ... because it would TAKE 500 seconds for the information to arrive at the planet where we could see it.

The same holds true for distant galaxies and other suns ... the distances involved are greater, so the time lag between something happening and ourt seeing it is greater. For example, the closest star system is Tau Ceti, at around 4.5 light years. What this means is that something happening to the sun of Tau Ceti wouldn't be seen here on Earth for 4.5 YEARS.

That's why telescopes and so on are said to be 'looking back in time'.

Answer 5

This might be easier to get than all the scientific mumbo jumbo. The closest star you can see (besides the sun) is 4 lightyears away. So, when you look at it, what you are seeing is actually 4 years old because it took 4 years for it's light to travel the distance to Earth. The more powerful the telescope you could get, the more closer you would get to the star and you'd be cutting some of the distance, which means cutting some of the time, as well. If you could look through the ultimate powerful telescope, you could see the star in it's present form, as it really exists at this moment, rather than 4 years ago.

But say tou wanted to do the same with the ultimate powerful telescope and see a star that's 200,000,000 lightyears away. Say you could see it in it's present form because the telescope is that strong; then you might see nothing at all if the star hadn't been born yet.

Do you see what I mean?

Answer 6

Since some stars viewed by scientists are so far away, their light takes a long time to reach Earth. Think of it this way: you're running in a race. After they call start, it still takes some time before you get to the end. Therefore, the people at the starting point saw you before the ones in the end did. In the same way, aliens or UFOs have probably seen the event happening, but by the time the light reaches us, it's been a long time already. So that's what they mean by the fact that we see into the past through telescopes... get it?

Answer 7

Telescopes look back in time because it takes light time to get from the star to the telescope. In some cases, for very distant stars, it takes 10 billion years.

How did we arrive out here ahead of that light?

Two points:
1) Don't think of the Big Bang as happening at a location. Think of it as an infinite (or at least very, very large) space at all times. That infinite space is expanding at all times after time zero, yet it is still infinite. Even at a fraction of a second after the Big Bang, it is still effectively infinite.
2) The relative speeds of expansion are not limited by the speed of light. Distant parts of the Universe can and do move apart faster than the speed of light. The speed of light limit in Special Relativity only applies to two things that are near each other. If they are far apart, they can move faster than light compared to each other without creating time paradoxes.

When we look back as far as we can see, we see the Big Bang. All of that matter that we see "banging" is "now" as old as we are, and in 20 billion years from now, we will see that photons that are leaving those stars "now", and those distant stars and galaxies will look like we do now, 13.7 billion years after the Big Bang. Of course by then, they and we will be 13.7 billion years older than we are now. What we see "now" is a dense, hot plasma where the stars and galaxies haven't formed yet. Luckily, it is highly redshifted so we don't get burnt---we see it as only being 2.7 degrees.

There is a whole lot more stuff further away than 20 billion lightyears, but we can't see it yet. That's the furthest we can see, because when we look back, the furthest back we can see is back to the time of the Big Bang. What we can see is limited by the finite amount of time since the Big Bang, not the finite size of the Universe, which is so big that it would take a billion billion years for light to travel across it. Our Universe has been in existence for a much shorter time than the time it would take light to cross it.

Time is measured in terms of physical processes. In General Relativity, the dynamics of the Universe evolve in a definite way with respect to a "timelike" variable. The cumulative amount of that timelike variable since the Big Bang is the age of the Universe.

Answer 8

Light travels at 186,000 miles per second, it actually takes TIME to travel, it's not instant like when you turn your headlights on.

So, a galaxy or star 200 light years away, is as it was 200 years ago, it could be long gone or changed by now.

Sorry, I am editing this, I missed part of your question, no at the time of the big bang there was nothing, no earth, nothing. Ok so the big bang occours let's say 10 billion years ago for the sake of argument, our planet didn't arrive ahead of time, we could have formed many billions of years later, cooled, now have life looking into the heavens at older objects. The same as when you were born, you didn't arrive ahead of your grandfather, he was here before you.

Those galaxys had formed long before we were here in much the same way, due to thier distance however, we are in essence looking back in time. I hope that helps?

The time and distance ARE measurable, even though we werent here when it started, The year 2007 is only relative to Christ, nothing else, so it's arrogant to think (not you personally) that nothing could have happened before we were here to measure it.

Note to SS poster 186,000 miles per second IS 300,000 km/sec!

Answer 9

I'll give a slightly different perspective

At any point of time when you look at our sun, you are actually seeing it as it was 8 min 18 sec ago (warning never look at our sun directly with your eyes, it can be harmfull to your eyesight)

That is because light does not travel instantaneously, but at a speed of 186,282 miles per second

On average the earth is 92,957,000 miles from the sun

Light is a stream of photos. So if a single photon was emitted from the sun it would take 92,957,000 miles / 186,282 miles per second = 8 min 18 sec to reach earth for you to see it.

Now think about how far away the next nearest star is to us. Billions of miles. So when we see the light from that star, we are actually seeing it as it was, and not as it looks right now. Since we are seeing it as it was, we are actually looking back in time.

Answer 10

no one knows where the universe started, since its infinite theres no center.........we were probably created before that star of after doesnt matter, if earth was created before a particular star, (say ten billion light years away), we wont see that star for 20 billion years, cuz if the creation of our universe is that of a ripple in water that in must have been created eqally in all directions......si it would take ten billion to reach its position in space , then ten more billion to reach us...theres no way to tell if we were here or that particular star....