What is time when we live in the past, present and future? Nathan Nelson/Flickr

The past, present and future exist all at once because we are simply living in another part of spacetime, an MIT professor has said.

Brad Skow, in his book Objective Becoming, has analysed several different theories about time and said the most convincing is that of the block universe theory.

Time does not pass, the theory suggests, but instead what we think of as being our past exists in another part of spacetime.

In physics, spacetime is a three-dimensional space that is fused in a four-dimensional continuum. Space and time exist in a single interwoven field.

Skow says we exist in a "temporally scattered" condition: "The block universe theory says you're spread out in time, something like the way you're spread out in space. We're not located at a single time."

Discussing other ideas about time in the book, he said he was interested in looking at what world view you would have if you took metaphors about its passing "very seriously".

Presentism, which says only events and objects in the present can exist, is lacking because it defies the physics of spacetime, Skow said.

Another, the moving spotlight, is more intriguing, however. In this theory, the past and future exist on a par with the present, but only one moment can be absolutely present, and this moment keeps on changing – like a spotlight moving over it.

"I think the theory is fantastic," Skow said. "That is, I think it is a fantasy. But I also have a tremendous amount of sympathy for it. The best argument for the moving spotlight theory focuses on the seemingly incredible nature of what the block universe theory is saying about our experience in time."

He said the moving spotlight is consistent with relativity, but ultimately treats the present as being too distinct. Instead, block universe says the experiences you had 10 years ago are still just as real but are simply "inaccessible" because you are in a different part of spacetime.

"[Moving spotlight] rests on a big confusion about what the block universe theory is saying. Even the block universe theory agrees that ... the only experiences I'm having are the ones I'm having now in this room."

Speaking previously on the subject, he said: "If you could look down on the universe, you would see things spread out in time as you would see the universe spread out in space. You could see that things are one way at earlier times and different at later times, but you wouldn't say the universe as a whole is changing."