Further to Carlo Rovelli’s views on time travel (only a technological problem, not a scientific one) and the order of time in general, views, as set out in The Order of Time, our color commentator Rob Sheldon offers,
If I can speculate about what goes on in physicist’s heads, this issue about time is an attempt to force symmetry on the universe. Sorta like the 2-yr old who wants to regularize irregular verbs. “Mommy not home; she goed to the store.”
Einstein’s Special Relativity (SR) argued that time was a fourth dimension and should not be treated any differently than height, width, and length. To get the units right, one only needed to multiply time by the speed of light–c*t. Only it was a negative dimension, so if distances are measured by r = sqrt(x^2 + y^2 + z^2), then SR distances were s = sqrt[r^2 – (c*t)^2].
Just to add some pizzazz, we can use that remarkable little constant “i” defined to be “sqrt(-1)”, and write this as: s = sqrt[r^2 + (ict)^2], so now it looks like
a normal distance (Eulerian measure).
But what does it mean?
No one has a clue. But it looks neat and tidy. And it makes physicists believe that time is just another coordinate like space once it is multiplied by “ic”. And since space can go from -infinity–>+infinity, then why can’t time do the same thing? So if there isn’t any “flow” to space, why should there be any “flow” to time?
Just for starters, if you multiplied length by i, and talked about “imaginary length” would anybody have a clue what it meant? So why should “imaginary time” have any more relation to length than “imaginary length” has to time?
In addition, if you don’t allow time to flow, if it is static, then it destroys causality. This has to do with how modellers try to put “tetrads” of 4-D spacetime together and derive physical laws from it. It is absolutely essential that time “flow” for laws to exist, for causality to work, for thermodynamics to make sense, for Newton’s laws of motion to work. Therefore it is sheer nonsense to say that “flowing time” is an illusion–because if it is, all of physics is an illusion too. Just crazy talk. Like “Mommy goed to the store.”
Here is the opening paragraph of chapter 6 from G. K. Chesterton’s Orthodoxy:
THE real trouble with this world of ours is not that it is an unreasonable world, nor even that it is a reasonable one. The commonest kind of trouble is that it is nearly reasonable, but not quite. Life is not an illogicality; yet it is a trap for logicians. It looks just a little more mathematical and regular than it is; its exactitude is obvious, but its inexactitude is hidden; its wildness lies in wait. I give one coarse instance of what I mean. Suppose some mathematical creature from the moon were to reckon up the human body; he would at once see that the essential thing about it was that it was duplicate. A man is two men, he on the right exactly resembling him on the left. Having noted that there was an arm on the right and one on the left, a leg on the right and one on the left, he might go further and still find on each side the same number of fingers, the same number of toes, twin eyes, twin ears, twin nostrils, and even twin lobes of the brain. At last he would take it as a law; and then, where he found a heart on one side, would deduce that there was another heart on the other. And just then, where he most felt he was right, he would be wrong.
See also: Carlo Rovelli: Future time travel only a technological problem, not a scientific one. Rovelli: A starship could wait [near a black hole ] for half an hour and then move away from the black hole, and find itself millennia in the future.