What's wrong with this Hypothesis?

[themos]

http://what-if.xkcd.com/1/

[allcart]

It does my head in everytime I try to figure this stuff out. When we look at Andromeda, are we looking 2.5 million years into the past. If it was 2.5 million LY away, 2.5 million LY ago. where is it now? If it dissapeared 1 million years ago, how and when would we know?

[Ags]

Saying it was the same luminosity of the star was just to make it visible from 5LY away.

To accelerate the ship to (nearly) the actual speed of light, the tail blast would be as bright as the sun :-)

[themos]

There's no universal "now", that's the lesson of relativity. Andromeda is closer now than it appears. If it disappeared 1 million years ago, we will know in 1.5 million years.

[JulianO]

It does my head in everytime I try to figure this stuff out. When we look at Andromeda, are we looking 2.5 million years into the past. If it was 2.5 million LY away, 2.5 million LY ago. where is it now? If it dissapeared 1 million years ago, how and when would we know?

Yes - it was 2.5 million LY away and ago when the light left it. It's not travelling the fast, so its more or less where it was then - well a little nearer, it moves about 400 lightyears every million years so hasn't knocked much off the 2.5 million.

it might have disappeared or anything in the meantime, we won't know until we see it. However it's a big thing and it would take a lot to make any changes in it, and there is nothing that big round about, until it collides with us in about 10 billion years or so.

[allcart]

Guess I shouldn't worry too much then. It is kind of intriguing that the photo I took of Andromeda is 2.5 million years old already.

[symesie04]

Haha its older than the technology you used to take it!

[Tiki]

The simple answer is you can't travel at the speed of light, unless you're a photon or similar - so you can't do the thought experiment.

You can still do the thought experiment, its just that the experiment will 'reveal' an infinitely blue-shifted journey. Not too sure what an infinite blue shift is but i wouldn't want it entering my eye or anywhere else.

[DH88]

To add my contribution ...

Correct anything with mass cannot travel at the speed of light. As speed increases so does mass meaning you would need pretty much infinitely powerful rockets.

Also don't be getting confused between Doppler-effect red and blue shift compared to be redshift from the expanding universe - that is because the space itself is stretching between them ... Not because they are 'moving'

Some people have said the light would be blue-shifted into X-rays or gamma rays .... But no-one would be vapourised because earths atmosphere is damn good at blocking x-rays and gamma (hence why such telescopes are in orbit & not on the ground) although I think the shift would be a lot more than what we are able to detect currently.

When a plane approaches mach1 sound waves gather up around the nose as they cannot go faster to get away from the plane, maybe similar effects would occur with light?

[K3ny0n]

In years and years to come, say 9 billion years, 1 billion off Andromeda colliding with the Milky Way, could Andromeda already have collided with "us" before we see it? Or would it get to a point were we see it 1 light year away, 2 light months away then bang, it's collided.

That's the best way I could try and explain what I meant lol.

Rob

[JulianO]

In years and years to come, say 9 billion years, 1 billion off Andromeda colliding with the Milky Way, could Andromeda already have collided with "us" before we see it? Or would it get to a point were we see it 1 light year away, 2 light months away then bang, it's collided.

Its only moving at a few hundred km/s so it will just ease up to us and slowly crunch into us. We will see it all in real time, if we're around