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ProximaCentauri

Alive and dead in the cosmos

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We know that the further we look into space, the further back in time we are looking. So for example, when we look thousands of light years into the distance from planet Earth, we see things like incredible giant gas clouds (nebula), stars, galaxies. But we know that some of those stars we are looking at may not actually even be there anymore, as they may have since collapsed and died, maybe even become black holes which we cannot see and wouldn't be able to see for billions of years. Some of the galaxies may look completely different right now to how they appear and some of the nebula may have long disappeared. We cannot know because we can't see these things as they are now but only as they were millions or billions of years ago (however far away they happen to be). Due to light and how long it takes from certain points in the universe to reach us here on Earth. 

Now let's say I wanted to go and visit a nebula gas cloud. In this hypothetical situation I create the fastest spacecraft ever made. I choose to go to the nebula while a friend watches my whole journey from an observatory. My friend watches through a monstrous telescope (which we will say is the largest and most powerful one ever built) as I make my journey to the nebula. I arrive at the nebula and just as I do so, my spacecraft is cooked by the heat of a nearby star and I die. Considering what we know about the speed of light:

 I know my friend will be able to see my spacecraft still on its way to the nebula long, long after the spacecraft has actually been destroyed and I am dead, due to the time delay in light travel. But he was watching my entire journey to the nebula, so as what point does he not see things live in Earth real-time? 

Another spacecraft from a distant galaxy whizzes past the nebula and has a device which picks up my destroyed spacecraft containing my dead body inside it just seconds after this horrific event occurs. It travels faster than the speed of light (nothing can, but in this hypothetical situation, it does) so it is able to instantly get to Earth. The alien life form who was flying the spacecraft delivers my body to my friend.

My friend looks through the massive telescope and sees my spacecraft still on its way to the nebula - and because this telescope is the most powerful ever constructed, it can pick up the tiniest detail, and he sees me looking out of the window of the spacecraft, very much alive. He turns away from the telescope and sees my dead body lying next to him.

If we had the spacecraft which could get me to the nebula so fast in time that I would reach it during the lifetime of my friend back on Earth - and if the alien ship from the other galaxy could travel so fast so as to instantly get to Earth with my wrecked spacecraft and the deceased me - would this situation actually happen? Would my friend be able to see, at the same time, a dead me and an alive me? 

 

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Posted (edited)
59 minutes ago, ProximaCentauri said:

If we had the spacecraft which could get me to the nebula so fast in time that I would reach it during the lifetime of my friend back on Earth - and if the alien ship from the other galaxy could travel so fast so as to instantly get to Earth with my wrecked spacecraft and the deceased me - would this situation actually happen? Would my friend be able to see, at the same time, a dead me and an alive me? 

Short answer: No.

Nothing can travel faster than light* in vacuum. Trying to model the outcome of the chain of events above leads to paradoxes that I guess are not possible to resolve in the context of currently known physics. 
That have of course not prevented many writers from exploring such though experiments in fiction books and movies. I guess in the hypothetical/fiction case described above you could choose an outcome that suits you.

 

59 minutes ago, ProximaCentauri said:

I know my friend will be able to see my spacecraft still on its way to the nebula long, long after the spacecraft has actually been destroyed and I am dead, due to the time delay in light travel. But he was watching my entire journey to the nebula, so as what point does he not see things live in Earth real-time? 

He will watch the entire journey and since you get further and further away the light takes longer and longer to travel the distance from you to him. The light from you will be red-shifted. The person at earth will measure the time from your start to your death to be longer than it takes for you to do the journey. 
I think the following analogy** holds: An ambulance passes you and you hear the siren at a slightly lower pitch when the ambulance travels away from you. Let's assume you can hear the siren over a long distance. At the time the ambulance is 1 km away it shuts off the siren. You will hear the siren for another (approx) 3 seconds.

 

*) according to all available mainstream data

**) At large speeds and when incorporating light this analogy is not true, it is just an attempt to illustrate a delay

Edited by Ghideon
clarified some bad sentences

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1 hour ago, ProximaCentauri said:

 I know my friend will be able to see my spacecraft still on its way to the nebula long, long after the spacecraft has actually been destroyed and I am dead, due to the time delay in light travel. But he was watching my entire journey to the nebula, so as what point does he not see things live in Earth real-time? 

When your spacecraft launches and your fried is watching from a 500 metres away, there will already be a delay of about 2 microseconds. As you fly away that delay will increase. So he will never see things in (your) real time.

1 hour ago, ProximaCentauri said:

Another spacecraft from a distant galaxy whizzes past the nebula and has a device which picks up my destroyed spacecraft containing my dead body inside it just seconds after this horrific event occurs. It travels faster than the speed of light (nothing can, but in this hypothetical situation, it does) so it is able to instantly get to Earth. The alien life form who was flying the spacecraft delivers my body to my friend.

My friend looks through the massive telescope and sees my spacecraft still on its way to the nebula - and because this telescope is the most powerful ever constructed, it can pick up the tiniest detail, and he sees me looking out of the window of the spacecraft, very much alive. He turns away from the telescope and sees my dead body lying next to him.

When you invent impossible scenarios, then science cannot tell you the answer. You can make up whatever you want. My best guess is that in this scenario, if you were to sneeze, dragons would fly out of your nose.

 

1 hour ago, ProximaCentauri said:

Now let's say I wanted to go and visit a nebula gas cloud. In this hypothetical situation I create the fastest spacecraft ever made. I choose to go to the nebula while a friend watches my whole journey from an observatory. My friend watches through a monstrous telescope (which we will say is the largest and most powerful one ever built) as I make my journey to the nebula. I arrive at the nebula and just as I do so, my spacecraft is cooked by the heat of a nearby star and I die. Considering what we know about the speed of light:

Well, one of the things we know about the speed of light is that it is "invariant"; everyone sees light travel at the same speed.

So, lets say that your spaceship travels at 87% of the speed of light. If you measured the speed that the light from the star passed you, you would see it go past at the speed of light. But when that light reaches Earth your friend will also measure it as travelling at the speed of light.

As a consequence of that, your friend will see your clock running slower than his. And you will see his clock running slower than yours. Also, you will see the distance to that distant star being half what it is measured by your friend.

 

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As Ghideon and Strange have explained, what you see in the distance/past, is what is actually out there.
The speed of light is also the speed of ANY information transfer, and causality depends on it.

There is no universal now, so what you 'see' is what can actually affect you.
The fact that a star 100 LY away has gone nova, will not affect you for 100 years ( if you live that long )

Trying to consider what is happening 'now' to that star 100 LY away, is like trying to consider what will happen to you 100 years from now.

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16 minutes ago, MigL said:

As Ghideon and Strange have explained, what you see in the distance/past, is what is actually out there.
The speed of light is also the speed of ANY information transfer, and causality depends on it.

There is no universal now, so what you 'see' is what can actually affect you.
The fact that a star 100 LY away has gone nova, will not affect you for 100 years ( if you live that long )

Trying to consider what is happening 'now' to that star 100 LY away, is like trying to consider what will happen to you 100 years from now.

So when we look through the Hubble telescope and see nebula billions of light years away, or at galaxies in the Hubble Deep Field, they are all still there today? And all the stars we see in the night sky are all really there right now in our Earth time? 

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20 minutes ago, ProximaCentauri said:

So when we look through the Hubble telescope and see nebula billions of light years away, or at galaxies in the Hubble Deep Field, they are all still there today? And all the stars we see in the night sky are all really there right now in our Earth time? 

Not all. Some might have died or exploded or merged or fallen into black holes or ... 

For example, Betelgeuse is due to go supernova sometime in the next few thousand year. It might have already happened but we won't know until the light reaches us.

You probably want to find the thread(s) by Michael123456 on this forum. He also struggles with the concept of light taking a finite time to reach us and what "now" means. His argument might make sense to you. (It makes no sense at all to me.)

 

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7 minutes ago, Strange said:

Not all. Some might have died or exploded or merged or fallen into black holes or ... 

For example, Betelgeuse is due to go supernova sometime in the next few thousand year. It might have already happened but we won't know until the light reaches us.

You probably want to find the thread(s) by Michael123456 on this forum. He also struggles with the concept of light taking a finite time to reach us and what "now" means. His argument might make sense to you. (It makes no sense at all to me.)

 

Hi no I understand fully about the speed of light being finite - and I was 100% certain that things in space could not exist at all but due to light's finite speed, we could still see them looking the billions of light years away through our telescopes. I only became puzzled when MigL said that everything we see in the universe still exists/is still out there. I knew that wasn't right but didn't know if that's really what he meant to say.

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I would say that if you 'see' them, they are there.
The light coming from them and gravitational effects both travel at c .
Those two long range forces are the only things that make them 'real'.

What 'is actually happening right now' at that far-away star, will not be real until information ( light and gravity ) reaches us in the future.

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I guess you can describe that as causal now. Lol let the metaphysics argue that expression.

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