# Ask an Astronomer - Scales in the Universe

## Seeing back in time

Published on 08/11/2013
Question:

I was wondering. Is it possible to see back into the past?
I was debating with a friend, about this. And I believe that, it is possible, if you, eg. Travel 3.5 billion light years away from earth in an instant, and if you are able to locate the earth, from where you are, that if you take a REALLY powerful pair of binoculars, and look back at the earth, then you would be able see back into time by 3.5 billion years.
Or, do you just see the present? I would be able to see my house and school, and so on?

I mean, the light reflected, does not come any closer/faster to you, just because you zoom in, right?  It would take 3.5 billion years for you to be able to see, the present?
Or am I completely wrong?

It is indeed possible to 'see into the past' because of the finite speed of light.  In fact it is impossible to see distant objects as they appear in the present.  As an example the Andromeda galaxy, the nearest large galaxy to the Milky Way, is about 2 million light years away, and so when we look at Andromeda (which you can see with the naked eye or a pair of binoculars) you are actually seeing it as it was 2 million years ago.  Now galaxies don't change much in 2 million years so that doesn't make too much difference, but Andromeda is very nearby, if we look at more distant galaxies we see further back in time and then we do see differences with nearby, older, galaxies.

If you imagined an alien 3.5 billion light years away with a powerful enough telescope to see Earth, then they would indeed see Earth as it was 3.5 billion years ago, when it was only a billion years old and the first life was just appearing.

## Light years and Red giants

Published on 24/06/2013
Question:

I was wondering how astronomers know that a star is for example  50 light years away, how can they know when light travels from there and reaches here and even if they did, they would not wait for 50 years. I have asked my physics teacher at school and a student in Glasgow university but they did not have a clear answer. I also checked online but it is too hard for me to understand.
One more question what determines if a main sequence star will be a red giant or a super red giant?.

We know what the speed of light is from experiments conducted here on Earth, and there are various methods of working out how far away a star is.  For example if you take two images six months apart, so Earth is on opposite sides of it's orbit, then relatively nearby stars will seem to have moved slightly compared to distant galaxies since we are looking at them from a slightly different angle, an effect called parallax.  From the amount the star seems to move we can then work out how far away it is.  Once we know how far away the star is can translate that into how long it would take light from that star to reach us, which is where the measurement of a distance in light-years comes from.  Light-years are used just because measuring things in trillions of kilometres even for small distances gets a bit tiresome.

Regarding red giants vs red supergiants the main determining factor is the mass of the star.  Stars that are around half the mass of our Sun, up to ones about 8 times the mass of the Sun will become red giants, stars more than about 10 times the mass of the Sun will become red supergiants.

## Looking back in time

Published on 22/01/2013
Question:

When a picture is taken of deep space and it is said that it is from when the universe was 500,000,000 years old.  Mainly saying that you're looking into the past.  That doesn't make sense to me for the fact that you're able to capture a picture.  Distance and time can coincide but in this case i dont get how this theory works with space?  I understand at such a distance it takes time for light to reach us, the point I'm trying to make is that how can it be said that what we view from deep space is the past not the present?

The effects of large distances and time in astronomy can be a little confusing.  Take as an example Proxima Centauri, the nearest star to our Solar System.  This is 4.2 light years away, which means that it takes light 4.2 years to get from Proxima Centauri to us.  Now since the only way we can see something that has happened at Proxima Centauri is through light, this means anything we see at Proxima Centauri actually happened 4.2 years ago.  If there were a person on Proxima Centauri and they had an exceptionally powerful torch, which the flashed at Earth, it would take 4.2 years for the torch flash to reach us, so by the time we saw it the person would actually have flashed the torch 4.2 years ago.  Now as I said Proxima Centauri is very nearby, when we look at objects in the distant universe they are much farther away, billions of light years, so when we see them we are seeing light that left them billions of years ago, when the universe was much younger.  As a result we can in a way think of looking at objects that are very far away in the distant universe as looking back in time, because the light has taken so long to reach us that the universe has changed a lot in the time it has taken the light to get here.

## Cosmic Expansion Beating the Speed of Light?

Published on 02/04/2011
Question:

When the Universe starts to expand from a single point (singularity) and expands to a size of lightyears in just 10 minutes, then it seems to me that there must have been speeds exceeding the speed of light. Surely that cannot happen?!

Einstein's theory of Special Relativity states that information cannot travel from one place to another faster than the speed of light. When the Universe expands, it is the entire of space and time that is expanding. If you imagine two points in the Universe, as the Universe expands, it is the space between them that expands and everything moves away from everything else, so you will not be able to transmit anything (whether radiation or matter) between them faster than the speed of light (in fact the Universe may be expanding so fast that light travelling from one place to the second may not ever be able to catch up with it, so one place will not be visible to the other!)