Convention says that today is my 41st birthday.

It depends on how you look at it though.

Which part of “what-I-am” is really 41 years old?

My body?

My body is made of cells which are constantly replaced. Between 50 and 70 billion cells die each day due to apoptosis in the average human adult. The whole body is comprised of totally new cells from 7 years ago. So my body isn’t 41 years old.

And by the way, most of the cells in this body aren’t even human.

According to researchers at the 108th General Meeting of the American Society for Microbiology in Boston, the number of bacteria living within the body of the average healthy adult human are estimated to outnumber human cells 10 to 1. You might think “I am my body” but when only 10% of the cells in the body are actually even human cells…. what does that mean for your identity?

The cells themselves are made of atoms which themselves come and go from my body very rapidly. In fact, studies have shown that 98 percent of the atoms in the body are replaced every year.

The human body is roughly made up of:

• 63% hydrogen;

• 24% oxygen;

• 12% carbon.

How old are these atoms?

Most of the hydrogen in the universe was created between 3 minutes and 20 minutes after the Big Bang, which makes it about 13.7 billion years old.

The oxygen and carbon are around 4.6 or 4.7 billion years old, being the remnants of one or more supernovae that occurred just before our Sun itself formed.

So… how old am I?

If I date my age from when my body was made, I’m about a year old.

But if I date myself from when my components were themselves made, then the atoms that make up my body are somewhere between 4.6 and 13.7 billion years old. Let’s take an average of 9 billion years.

All of a sudden, 41 doesn’t sound that old.

I could argue that I’m the age of my DNA. I was formed about 41 years and 9 months ago. The pattern of my DNA has been passed on from cell to cell for 41 years and 9 months, providing a recipe for creating this particular body. So my “pattern” is 41 years and 9 months old.

But we also need to think about our concept of “time” itself.

We tend to think of time as something that flows or unfolds, moment by moment. It seems to us that the future hasn’t happened yet and that the past is long gone.

Physics, however, tells us something completely different.

Einstein was the first to realise that time and space are actually the same construct – therefore we now refer to it as “spacetime”.  Time is really just another dimension that we add to the three dimensions of space. Spacetime therefore has four dimensions: height, width, length – and time. Essentially Einstein demonstrated (and subsequent experiments have extensively confirmed) that time exists as a dimension of space. As Einstein once wrote to the wife of a recently departed friend, “For we convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”

Just as all of space exists “now”, all of time also exists “now”. How we experience time is merely a matter of our relative perspective (or “relativity”, as Einstein pointed out).

Here’s how theoretical physicist Brian Greene explains it in his excellent book “The Fabric Of The Cosmos“:

In this way of thinking, events, regardless of when they happen from any particular perspective, just are. They all exist. They eternally occupy their particular point in spacetime. There is no flow. If you were having a great time at the stroke of midnight on New Year’s Eve, 1999, you still are, since that is just one immutable location in spacetime. It is tough to accept this description, since our worldview so forcefully distinguishes between past, present, and future. But if we stare intently at this familiar temporal scheme and confront it with the cold hard facts of modern physics, its only place of refuge seems to lie within the human mind.

So let’s summarise.

The cells in my body come and go constantly, totally replaced every seven years.

So too the atoms in my body. And the ones in my body right now are an average of 9 billion years old.

But time doesn’t really exist. All of time exists NOW. Our “path” through time is simply a persistent illusion.

My DNA is about 41 years and 9 months old, but it’s just a pattern, a recipe.

So how old am I?