• Question: What is spacetime? I read the book 'Relativity' by Sir Albert Einstein himself but, it was a little tough for me to understand. I could understand Lorentz Transformation but couldn't get an intuition on spacetime. I don't even know what is time actually. I could understand that time and space are the same. They get converted from one to another like potential energy gets converted to kinetic energy during the fall of an apple from a tree and nothing more. Why is it very difficult to understand relativity? Is it natural for a boy in high school who has not studied physics in depth? What does it mean by saying that space and time are the same?

    Asked by rajathjackson to Chris, Dave, David, Fiona, Jack on 22 Jun 2013.
    • Photo: David Freeborn

      David Freeborn answered on 22 Jun 2013:

      Hi rajathjackson,

      You’re right, relativity is very difficult to understand. The reason is because our brains are used to handling Newtonian concepts of space and time. Pretty much every object we see on a day-to-day basis is between 1mm and 1km in size, and travels no faster than about 500m/s as a speed. We’re so used to seeing things on the Newtonian scale, it’s very difficult to realise that all these concepts we’re so used to in the Newtonian sense, space, time, speed, are just approximations of a deeper theory, Einstein’s relativity.

      Nobody finds Relativity to understand at first! I certainly didn’t. It takes a long time to get your head around it. The further you go in physics, the harder and harder it becomes to view things intuitively. As you advance in physics, you learn to get a whole new set of intuitions, mostly based on mathematics, which helps you to visualise these things.

      In relativity, we treat space and time mathematically in the same way. Basically, in stead of of saying there are 3 dimensions of space that change in time, we treat the system as having 4 dimensions of space-time.

      For example, when a clock travels close to the speed of light, it experiences time dilation and length contraction. An stationary observer will see the length of the clock as being shorter, and see the clock running slowly. These two things are closely mathematically related by a single property called a “Lorentz Contraction”.

    • Photo: Chris Mansell

      Chris Mansell answered on 23 Jun 2013:

      Relativity is hard to understand. I think the reason why it is so hard to understand is that humans don’t travel at relativistic speeds with respect to one another very often during our everyday lives. Consequently, we don’t have much experience to draw on when we are studying relativity.

      I studied special relativity when I was in my first year of university. To be reading about it whilst you are still in high school is very commendable.

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