Question: Prof. Stephen Hawking and Roger Penrose proved that time began at the moment of singularity. But if there are many universes then, there will be many singularities. Is it that the time of each universe started at its moment of singularity? What is actually time?(I have spend a lot of time thinking about time, so what is it???)

David Freeborn answered on 19 Jun 2013:

Hi rajathjackson,

This is another really interesting question.

You’re right, Hawking and Penrose proved time began at a singularity. If we live in a multiverse of many Universes, then yes, each one begins at a singularity. If the Universe is going to end with a “big crunch”, all of matter collapsing back in on itself in a black hole (this is one possible fate of the Universe), then it will end with a singularity too. Time and space will both have an end at that point. It could be that we live in a multiverse of Universes, each with their space and time starting and ending at points of singularity like this.

Some physicists think the Universe keeps going through cycles of big bang and big crunch. The Universe collapses in on itself in a singularity, and then expands outwards. But at that one moment of the singularity itself, time and space would lose meaning.

Physicists are still unsure about what time actually is. Einstein’s equations of special relativity suggest that time is perfectly symmetrical with space, so the best way to treat time is as another dimension, just like space. When I’m working on a day-to-day basis, I treat time and space exactly the same way in all my equations. Clearly time and space and very closely linked.

We often think of the space as the Universe as curved. Some physicists in the past that time was curved too, so eventually we would arrive back at the beginning of the Universe. But we now know time begins at a singularity, as you said. So we can never reach that original point.

There’s a lot of debate about “why” the dimension of time is different to the dimension of space. A lot of physicists think that the early Universe was for some reason the early Universe was in a very, very improbable, highly ordered state, and ever since then it has slowly been becoming more disordered, moving to a more probable state. They think this is why it feels like “time” has a direction and space doesn’t, and this is the difference between time and space.