Travelling back in time

May 20, 2004

Paris, 19 May 2004

Every time you look at the sky, you travel back in time... in a sense.

When you look at a star which is 100 000 light-years away, you see it as it was 100 000 years ago. Its light has needed all that time to get to your eyes.

So what does this star look like 'now'? There is no way to know, unless you wait 100 000 years for when the light emitted at that moment will reach the Earth (but then it won't be 'now' anymore!).

The Sun is a closer example. The Sun is about 150 million kilometres away from the Earth, and the speed of light is about 300 000 kilometres per second. So the light coming from the Sun needs about eight minutes to reach the Earth. The result is that every time you look at the Sun, you see it as it was eight minutes ago.

The same rule has to be applied to all other radiation we can detect and, in particular, the Cosmic Microwave Background (CMB) radiation that ESA's Planck satellite will study.

The CMB is the cooled remnant of the first light that could ever travel freely throughout the Universe. This 'fossil' radiation, the furthest that any telescope can see, was released soon after the 'Big Bang', probably only 300 000 years after.

The Big Bang happened about 15 000 million years ago. This means that the CMB radiation has needed as many thousands of millions of years to reach the Earth, and when we observe it we are seeing the Universe as it was only 300 000 years after the Big Bang.

ESA's Planck mission, due for launch in 2007, will detect this first light, which carries information about our past and future. By observing the 'oldest' detectable radiation, Planck will be seeing the Universe as it was almost at its origin.

European Space Agency ml
Item source: VED_extreme_0.html

You've reached your article limit.

Register to continue

Registration is free and only takes a moment. Once registered you can read a total of 3 articles each month, plus:

  • Sign up for the editor's highlights
  • Receive World University Rankings news first
  • Get job alerts, shortlist jobs and save job searches
  • Participate in reader discussions and post comments

Have your say

Log in or register to post comments

Most Commented

James Fryer illustration (27 July 2017)

It is not Luddism to be cautious about destroying an academic publishing industry that has served us well, says Marilyn Deegan

Jeffrey Beall, associate professor and librarian at the University of Colorado Denver

Creator of controversial predatory journals blacklist says some peers are failing to warn of dangers of disreputable publishers

Hand squeezing stress ball
Working 55 hours per week, the loss of research periods, slashed pensions, increased bureaucracy, tiny budgets and declining standards have finally forced Michael Edwards out
Kayaker and jet skiiers

Nazima Kadir’s social circle reveals a range of alternative careers for would-be scholars, and often with better rewards than academia

hole in ground

‘Drastic action’ required to fix multibillion-pound shortfall in Universities Superannuation Scheme, expert warns