EXTREME ASTRONOMY Professor Derek Ward-Thompson Cardiff University September 19-21, 2007

Extremes: Astrophysics teaches us about extreme behaviour in the Universe. Two examples: –Time travel Is it possible? Under what circumstances? –Black holes Do they exist? Can you ever escape from one?

Time Travel

Is time travel possible? Do the laws of physics allow time travel? We all travel in time at 60 seconds per minute Can we alter that?

If you could travel in time…. Who would you like to meet? Where would you like to go?

Time paradoxes

What if I travel back in time and kill my grandfather when he’s still a baby?

Time paradoxes What if I travel back in time and kill my grandfather when he’s still a baby? What if I travel back in time and kill myself?(!!)

Time paradoxes What if I travel back in time and kill my grandfather when he’s still a baby? What if I travel back in time and kill myself?(!!) What if I travel back in time and give myself knowledge I didn’t have?

Time paradoxes What if I travel back in time and kill my grandfather when he’s still a baby? What if I travel back in time and kill myself?(!!) What if I travel back in time and give myself knowledge I didn’t have? Causality

Time travel can happen Einstein’s theory of special relativity predicts ‘time dilation’ Moving objects experience time at a different rate

Time dilation Special relativity tells us that if we travel at velocity v, then time passes for us at a slower rate relative to a stationary observer. This effect is known as time dilation, and the dilation factor is given by:

Our trip: T/4 We travel for time T (at 2g)

Years on journey Years into future , ,000 1,000,000 10,000,000

Years on journey Initial mass/final mass , ,000 1,000,000 10,000,000

Years on journey Initial mass/final mass , ,000 1,000,000 10,000,000 Practical limit?

Years on journey Years into future , ,000 1,000,000 10,000,000 Practical limit?

Wouldn’t it be great to see the future?!

But you can never come back!!!

Extreme time dilation What if I travelled at v= c ? (1 part in )

Extreme time dilation What if I travelled at v= c ? (1 part in ) Some particles do – they’re called cosmic rays

Extreme time dilation What if I travelled at v= c ? (1 part in ) Some particles do – they’re called cosmic rays These are free neutrons which have a half- life around 10 minutes (613s)

Extreme time dilation Free neutrons were emitted at the Big Bang and they are still travelling today

Extreme time dilation Free neutrons were emitted at the Big Bang and they are still travelling today They think the Universe is only 10 minutes old!!!

Other extreme places: The most extreme of all: Black Holes

Black holes What are they? Do they exist? If so, where are they?

What is a black hole?

A black hole is an object so dense and so massive that its escape velocity is greater than the speed of light

Where do black holes live?

We believe there is a black hole at the centre of our Galaxy (the Milky Way)

The centre of our Galaxy:

Now we have observed many of them!

In order to explain these motions of the stars we need a very massive compact object at the centre –3.6 million times the mass of the Sun

A picture of a black hole:

Can we ever escape from a black hole?

Under certain conditions, matter can ‘escape’ from a small black hole

Can we ever escape from a black hole? Under certain conditions, matter can ‘escape’ from a small black hole If a particle and anti-particle are created near the event horizon, one could fall in and the other escape

Can we ever escape from a black hole? Under certain conditions, matter can ‘escape’ from a small black hole If a particle and anti-particle are created near the event horizon, one could fall in and the other escape This is known as ‘Hawking radiation’

Summary Time travel into the future is possible under certain circumstances Time travel into the past isn’t – so you can’t come back There is a black hole at the centre of the Milky Way Galaxy You cn now tell your friends you’ve seen a picture of a black hole!

In the year of '39 assembled here the volunteers In the days when the lands were few Here the ship sailed out into the blue and sunny morn Sweetest sign ever seen And the night followed day And the story tellers say That the score brave souls inside For many a lonely day sailed across the milky seas Ne'er looked back, never feared, never cried Don't you hear my call though you're many years away Don't you hear me calling you Write your letters in the sand For the day I take your hand In the land that our grandchildren knew In the year of '39 came a ship in from the blue The volunteers came home that day And they bring good news of a world so newly born Though their hearts so heavily weigh For the earth is old and grey, little darling we'll away But my love this cannot be Oh so many years have gone though I'm older but a year Your mother's eyes from your eyes cry to me Don't you hear my call though you're many years away Don't you hear me calling you Write your letters in the sand for the day I take your hand In the land that our grandchildren knew Don't you hear my call though you're many years away Don't you hear me calling you All your letters in the sand cannot heal me like your hand For my life - Still ahead - Pity me QUEEN: ’39 - from ‘A night at the opera’ - lyrics by Brian May. 1976