# How far back into time can we look into? Are we really

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#### andyarok

##### Guest
Sorry if this question is already being asked. I also can't take it that when we look far into space we are looking back into time and that too at the origin of the universe.

I know this could be explained easily by bringing an analogy that what we see from Sun is something that really happened 8 mins ago. So the same way the farther we look, the farther into time we get. Like say Big Bang happened some 15 billion years ago then if we look into space say 15 billion light years away, we see what happened just after or near to Big Bang. But my doubt lies there.

Its just a imagination. Imagine that Earth is at the surface of the Sun and suddenly a bang happens and the Earth moves to where it is right now. Now, let the time at which the Earth start moving be t = 0; now consider after some t = 166 hours we would be where we are now if it traveled at 250 kps . But in the meanwhile the light that emanated from the sun at the same instant that Earth started moving away from the sun would have gone past the place where we are now at t = 8 mins.

Now when we look back from earth at t = 166 hrs, we are just seeing what happened at the sun when the time is at t = 165 hrs, 52 mins.

Now pass this analogy to big bang and where are we now both in space and time. How can we be looking into the mysteries at the origin of the universe.

edited: its 165 hrs 52 mins considering the 8 mins...

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#### MeteorWayne

##### Guest
No, if the earth is where it is now, at 166 hours, when we look back at the sun, we will see the light emitted by the sun 8 minutes before we looked. It makes no difference how long it took the earth to get there, the light we see from the sun took 8 minutes to get to 1 AU.

The light emitted when the earth started to move (in your scenario) will be 40 times as far from the sun as Pluto.

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#### kelvinzero

##### Guest
Hi andyarok

I didn't fully understand your example, but apart from the fact we know we must be seeing the universe as it was billions of year ago, since we know light does not travel instantaneously, we also see different sorts of galaxies when we look at great distances, consistent with what we could expect to see if they were younger. It really is looking back in time and it is showing us very interesting things we would otherwise not know about early universe.

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#### andyarok

##### Guest
@Meteor wayne

I couldnt understand what you are trying to say. Because you emphasized the same thing that I said... So can we ever look back into time just near Big Bang? Because many astrophysicist say that we are looking into the moments that happened just after the Big Bang. How is it possible... explain...

For others who couldn't get the analogy. If the galaxy where we are now dint travel at the speed of light which am sure it dint, then obviously we can never look back into time.

For example. consider the BB itself. now at t =0, Big Bang happened. Everything started expanding and we are now here after traveling some 15 Billion light years. But surely we dint travel at the speed of light. So by the time we reached the distance where we are now the light would traveled far of may be 100 or 1000 times where we are now if space started expanding at really high speeds. So for us to travel the 15 Billion light years it would have taken much beyond 15 billion years.

As you said Meteor Wayne, so if we want to look what happened during the Big Bang, then we should be looking at the light that was emitted at t= 0. So considering the light would have traveled far beyond us by now, we cannot look back into time beyond certain limit. And for me the limit looks really big.

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#### andyarok

##### Guest
No, if the earth is where it is now, at 166 hours, when we look back at the sun, we will see the light emitted by the sun 8 minutes before we looked. It makes no difference how long it took the earth to get there, the light we see from the sun took 8 minutes to get to 1 AU.

Thats what I mean we are seeing what happened at the sun at t= 165 hrs 52 mins. which is 8 mins just before at t = 166 hrs.

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#### nimbus

##### Guest
If you're on the phone with someone on the other side of the planet, the signal takes a while to travel through the network. If you count to 3 in regular tempo, asking the person on the other side to match your tempo and complete your count with "4, 5, 6", you'll hear "4, 5, 6" after a short delay following your "3". If you then take that person and (for argument's sake) make it a phone call to not just the other side of the planet, but to the Moon, the delay will be even longer. With perfectly efficient (for argument's sake again) communications network, you'll have a delay equal to the time it takes from your saying "3" to the signal's traveling time from Earth to Moon, then for the other person's "4" to make its way from Moon to Earth, which is ~1.3 seconds (at light speed). So 1.3 seconds each way adds up to 2.6 seconds: You'll hear "4" 2.6 seconds after your "3".

So when you hear it, the "4" already happened 1.3 seconds ago. It's 1.3 seconds old. Because the Moon is 1.3 light seconds away. Just the same, if you looked into an immensely powerful telescope at a Moon base where an astronaut was waving hello to you, the gesture you see him making is 1.3 seconds old, or 1.3 seconds into the past. The same phenomenon happens at much larger scales between us and objects dozens or thousands or billions of light years away.

Because space is expanding, some objects are receding from each other faster than light travels. That's the limit to how far back into time we can look. There's another practical limit: if you could look back far enough, there would be no light to observe the early universe by. I don't know the exact details, but at some point around the age of the Cosmic Microwave Background Radiation, the universe was opaque. I don't recall if that was the earliest humanly visible light, or if there was another earlier light with a dark period between it and the CMBR.

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