Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 24/05/2018 08:22:40

Title: If nothing is faster than light, how can we look back in time with a telescope?
Post by: chris on 24/05/2018 08:22:40: This, from Stephen Whiston - @whizzer666 - on Twitter today:

"I have always not been able to reconcile the fact - nothing is faster then the speed of light - yet we look back to the big bang through our telescopes - honesty how does that work? Can you explain?"

Who can help with this?
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: jeffreyH on 24/05/2018 11:31:12: Photons do not travel from one place to another instantly. If they did we would see everything as it is 'now'. Since the light, that contains visual information on objects, can take millions or billions of years to reach us, we see how those objects were at the time the light left them. The maximum speed that information is said to travel is the speed of light.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: evan_au on 24/05/2018 12:39:03: Quote from: Stephen
If nothing is faster than light, how can we look back in time with a telescope?
An example: Chinese observers saw a "guest star" in the sky in the year 1054.
- It was bright enough to see in the daytime.
- We now know that this was caused by a supernova about 6,500 light years away.
- The expanding fragments are now known as the Crab Nebula.
- The supernova actually took place around 5,500BC (roughly 1054 minus 6500 years).
- So when the astronomers saw it in 1054 AD, they were looking back in time about 6500 years.

But supernovae also emit some other particles that travel at well below the speed of light. This probably includes radioactive iron nuclei, ⁶⁰Fe.
- If these are traveling at (say) 2/3 the speed of light, they will take about 10,000 years to arrive at Earth (compared to 6,500 years for light)
- We can expect these to start arriving around the year 4,500 AD.
- So assuming that scientists then have radiation detectors in space, they would see this increase in subatomic particles.
- They would be effectively looking backwards in time at the same event - only now they will be looking back almost 10,000 years
- We can see a sprinkling of ⁶⁰Fe on the sea floor, hinting at nearby supernovae in our galaxy. This isotope has a half-life of about 2 million years, so we know that this did not occur naturally on Earth.

So far as we know, there is nothing that travels faster than light, so no high-technology astronomers on Earth could have seen the Crab Nebula explosion before 1054.
- There is one subtle exception: Neutrinos. It is estimated that a supernova in the core of a giant star may take from a few hours to a day to break through the surface of a star, emitting an intense flash of light, X-Rays and gamma rays. However, neutrinos (which travel imperceptibly less than the speed of light) can make the same trip from core to surface in a few seconds. So neutrinos might arrive at the Earth from the core of the supernova a few hours before the flash of light from the surface of a supernova. With today's neutrino detectors and X-Ray/Gamma ray detectors, I expect we would be able to measure this time difference if another supernova exploded to us as close as 6500 LY.

See: https://en.wikipedia.org/wiki/Crab_Nebula

edit: Correct formatting of the iron isotope, as highlighted by PmbPhy.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: PmbPhy on 25/05/2018 10:11:14: Quote from: chris on 24/05/2018 08:22:40
This, from Stephen Whiston - @whizzer666 - on Twitter today:

"I have always not been able to reconcile the fact - nothing is faster then the speed of light - yet we look back to the big bang through our telescopes - honesty how does that work? Can you explain?"

Who can help with this?
We don't "look back" to the big bang as if it happened a long distance away. In a sense it happened everywhere.

As far as looking into the past we do it all the time. It takes light and sound a finite time to travel the two or three feet between two people having a conversation. In that sense we're communicating with others in the past. In fact most of what we actually experience occurred in the past.

Evan - The atomic mass becomes before the chemical symbol, not after it. I.e. What you wrote., i.e. Fe⁶⁰ should be written as ⁶⁰Fe.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: evan_au on 25/05/2018 11:36:20: PS: Crab Nebula Supernova
If the ⁶⁰Fe nuclei from the Crab Nebula Supernova are travelling at the same speed as the visible expanding edge of the Crab Nebula, they will have a speed of "only" 0.5% of the speed of light.

This means they will take around 6500/0.005 = 1.3 million years to reach Earth.
Since the half life of ⁶⁰Fe is over 2 million years, less than half of the ⁶⁰Fe would have decayed en route, forming another scattered layer of ⁶⁰Fe on the bottom of the ocean basins, and hinting at yet another "nearby" supernova.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: Whizzer 666 on 25/05/2018 21:14:57: Thanks folks for the answer to my original question.

I get the super nova example but not the- We don't "look back" to the big bang as if it happened a long distance away. In a sense it happened everywhere..... :) HELP!!!! :)
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: PmbPhy on 25/05/2018 21:27:27: Quote from: Whizzer 666 on 25/05/2018 21:14:57
Thanks folks for the answer to my original question.

I get the super nova example but not the- We don't "look back" to the big bang as if it happened a long distance away. In a sense it happened everywhere..... :) HELP!!!! :)
The term "big bang" is misleading. It makes it appear as if within the theory itself there is an event which started the universe expanding. There may have been such an event but we have no way to adduce it as of now.

About the big bang being everywhere: Space is not as simple a thing as you may have thought. The big bang is not a theory about something which exploded and sent material whooshing out from a specific point in space. Its about the notion that its space itself which is expanding. A commonly used analogy is to imagine a bunch of two dimensional beings who are sitting on the surface of a balloon and as he balloon expands and gets larger, each being on the surface sees the everyone else moving away from them. They might even conclude that where they sit the expansion started at their position because everything is moving a way from their location.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: yor_on on 26/05/2018 20:57:21: You need to explain more there.
I'm sure you are thinking of light in a certain way.

So please explain
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: Bill S on 27/05/2018 00:37:53: When you look at any object in the Universe, in any direction, the further away it is, the further back in time you are looking; so the nearest we can come, even to thinking we are looking towards the BB, is to look as far as we can in any direction.

It took me a while to realise that, but it's worth the effort.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: yor_on on 27/05/2018 12:19:30: Hmm, yes, but I was looking at Stephens question Bill, and it didn't make sense to me.

"I have always not been able to reconcile the fact - nothing is faster then the speed of light - yet we look back to the big bang through our telescopes - honesty how does that work? "

What has '- nothing is faster then the speed of light - ' to do with the rest? It's not necessary unless Stephen has some notion that I miss?
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: Bill S on 27/05/2018 14:23:25: Quote
...... unless Stephen has some notion that I miss?

I suspect that may be the case. Hopefully he'll tell us. In the meantime, a few related comments may help him to clarify his thoughts.
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: yor_on on 30/05/2018 12:29:29: You're right Bill.

So here goes. The speed of light is a two mirror experiment in where we can verify that light has a certain 'speed'. That speed is called 'c' and it is the same no matter how 'fast' you define yourself to move relative something else. It doesn't matter if you change your speed relative whatever you first defined it from, 'c' will give you the same exact 'speed'.

The other thing 'c' means is a history. It's a telltale of years gone by. The furthest we can see, as far as I remember, is about 13. 7 or 13.8 billions year old? Better look here https://map.gsfc.nasa.gov/universe/uni_age.html

So wherever you are the 'light sphere' you see around you will be that age, with the earliest light coming the furthest away. Do notice that this is true wherever you are. It doesn't matter if you can 'teleport' yourself somewhere else to look out. The 'age' will be the same, and so a Big Bang is not a localized 'source' or maybe you are the one, wherever you are :)
Title: Re: If nothing is faster than light, how can we look back in time with a telescope?
Post by: yor_on on 30/05/2018 12:58:15: Maybe the question is, if this is true that 'c' is a 'constant' no matter how fast you move relative something else. How can we trust it to give us a age? That's a good question :)

It goes back to the way we define 'locality', and 'repeatable experiments' actually. Facts that so far we know will hold for you everywhere, and at whatever time, you repeat those experiments. You can only measure and experiment locally, any other way must become a abstract exercise. We do that too, but when we do we hypothesize. To test those further 'abstractions' you will need some leeway, and we find that in so called 'weak experiments', in where you presume your 'looking' to not 'disturb' the outcome of the experiment.

==

What I say by this is that when you measure the earliest light, that 'furthest away' light, you only can do it locally. Then again, if all observers, at different places and at different times, agree on the light they measure having a same ' historic age', then that is a fact. The exact same sort of fact as any repeatable experiment becomes.

It's also called axioms, things that seem to fit and create a logic that you can understand.