Plus Advent Calendar Door #3: Time in a block universe

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On this, the third day of advent, we explore some strange consequences of general relativity on our concept of time.

As much as we would sometimes like to, particularly those of us who would like to return to more normal times before COVID-19, we cannot go back in time. For us, time marches relentlessly on. Which makes it so surprising that, according to the equations that govern fundamental physics, time doesn't only have to move forward, it could move backwards too. "In the mathematics everything is so beautiful and symmetric," says Marina Cortês, a cosmologist from the Royal Observatory, Edinburgh. "The fundamental equations [of physics] are reversible with time."

You can think of this as watching a film playing forwards or in reverse. Watching a video of some events, such as spilling a glass of milk, it's easy to tell when the film is running forwards in time (and the mess ensues) and when it's running backwards (when all the milk leaps back into the glass). But this event is the culmination of many physical processes each governed by different equations: such as those governing the gravity and the force applied to the glass and its contents, and the fluid dynamics governing the behaviour of the milk itself.

For some very simple systems, physics has no preference for time moving forwards. Can you tell which parts of this film are being played forwards, and which in reverse?

But at a fundamental level of physics, for example at the level of Newton's laws of motion that you learnt in high school, things start to look a little different. Suppose you move an object by applying a force. Then by Newton's second law of motion, an equal force applied in the opposite direction will reverse the motion. If you were watching a film of the object's motion, say a ball gently rolling across a floor, you wouldn't be able to tell if the film was playing forwards or backwards. Fundamental physics has no preference for time moving forwards.

Symmetry of time is one of the underlying arguments for the block time. Block time is all of time – the past, present and future – existing in a four-dimensional block of spacetime known as a block universe. If we live in a block universe, our perception of time as only ever moving forward at a steady rate is just an illusion, a "mathematical artefact of the physics". This might sound surprising but this counterintuitive concept of block time is a direct result of one of the most successful theories of physics: Einstein's general theory of relativity, which we explored in previous advent doors and is accepted by the scientific community as the most accurate description of the true nature of the Universe at the cosmological scale. You can read more about block universes and block time, and watch our interview with Marina Cortês, in our package of articles Time in a block universe.

This year's advent calendar was inspired by our work on the documentary series, Universe Unravelled, which explores the work done by researchers at the Stephen Hawking Centre for Theoretical Cosmology and is available on discovery+. Return to the 2020 Plus Advent Calendar.



I love the website, especially articles in which they questioned "Time is just an illusion" ect
But I often hear.. a broken glass with spilled milk as a good example to picture the validity of Time going backwards idea, and it makes me think that some physicists are not going deep enough in thought.
I'd like to see them explain sound coming out of my ear and back into their mouths - then proceed to affect the muscles in their throat, through their nerves and into their brain - then into their *Mind backwards to the genesis of their Thought and the First cause of it.

Let's see that make sense backwards - and about Free Will = "turning left or right at the fork in the road". Again, I see no depth in some of these theorists ideas and I think that's intentional for some, because they need there to be no free will.
When they believe that numbers and variables on paper actually *Make reality -they demonstrate there's something lacking in their reasoning skills. That's why I liked the more insightful ideas on this website.

You know, many people with various gifts in say math, or science or music, have deficiencies in other areas. It's like the strength of the gift in one area subtracts from another area. This is why we see autism or Asperger's spectrum in some of the greats. It's why we sometimes see awkward social disease as well.
The problem is many of these people believe they're geniuses in *everything - when in fact they don't see the whole world correctly.
Free will and Time are properly basic beliefs that are self evident and denying them demonstrates such a lack of insight that the person is essentially pleading that he is *not a person, that he/she does not have a will to even have an opinion, and that there is No actual Truth. How they do not comprehend that is of course the main symptom of their socially awkward dysfunction.

You simply could not *Think without Time. You could not form a coherent sentence without free will. No amount of pseudo philosophy using the lame fallacy that “if we've learned one thing, its don't trust our intuitions” - as if that is a scientific principle we should bank on to actually Reason. Nothing is going to change the fact that tomorrow the sun is going to rise and I'm gonna watch it - something that is utterly incoherent if this is a frozen Block universe.(strict version)

I think their weak idea of what Free Will is (choice at a fork in the road) is a type of strawman in some cases and a deliberate laziness and lack of depth in order to avoid sawing off the branch they're sitting on because the simple act of Speaking cannot be achieved without free will.
Our tone, cadence, melody, percussion, amplitude, emphasis, velocity, and a host of other choices are made on the fly when we talk and that does not even mention the digging into our vocabulary as we select the right words.
We draw on various Sets of Ideas we have stored, calling up conclusions we have reached, piecing together theories and questions we have, and even recognizing plays on words, a turn of a phrase, all while we read the non verbal communication from our associates, and as we back off or charge forward based on these cues we have made literally hundreds of choices that would have resulted in gibberish and outright lunacy if we didn't have free will.

Why don't people as brilliant as Einstein actually take the time to reflect on what it is they are actually asserting? Why do people that are not stupid believe the earth is flat or George Bush bombed the Twin towers or that people who said Joe Biden exhibited clear signs of early dementia were evil liars? Because Bias destroys logic and renders it inert. With these two topics it could be the person is addicted to shock bombs - the Lust to tell people everything they believe is wrong. It could be to sell books or it fits with their Atheism. But if science is going to be about selling books pushing Atheism(which countless modern scientists are doing), well then it's not science, it's propaganda.

Anyway. Again, excellent website. Thanks.