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From The Elseworlds episode 3 (Supergirl season 4 episode 9),

When Evil Superman (Dr. Deegan) regained The Book of Destiny and started to rewrite reality (@26:13 min), Flash (Barry Allen) suggested slowing down time by running around the globe in opposite direction @just over Mach 7 speed to create centrifugal force that'll slow down earth and everything else.

So my question is: is it really possible to do such thing in reality in the given circumstance or is it just another science fiction element for entertainment purposes? Is it scientifically possible or is it just a theory?

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  • To be fair, it worked in Superman. If anything, it should have worked doubly well.
    – zero298
    Commented Dec 13, 2018 at 2:58
  • No, my point was if something on earth reaches at such speed, will time slow down in reality? Reference of Superman and this episode belongs to fiction.
    – Jeel Vankhede
    Commented Dec 13, 2018 at 3:11
  • Well, subjective time will slow down. Astronauts orbiting earth in the ISS age slower than us - by some milliseconds per year. Reversing time would - on paper - require to go faster than light. But that has relies on the same logic as a negativ meter or the square root of minus one.
    – Zsolt Szilagyi
    Commented Jul 8, 2020 at 15:31

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If Supergirl and I travel around the globe in opposite directions at just over mach 7, we should be able to create enough centrifugal force to slow the Earth's rotation.

Is it really possible to do such thing in reality

No. It's completely ridiculous.

Physics doesn't work that way, nor does space-time.

Indeed, the speed they mention "Mach 7" is ridiculously slow. Escape velocity (say for Apollo or the Space Shuttle) is Mach 33!

While this is a rather nice tribute to Superman: The Movie and the scene where Superman reverses time by spinning the planet backwards, the physics of what Barry suggests are completely nonsensical within the stated limits. While mass does increase with velocity (per Einstein's theory of special relativity) Mach 7 is nowhere near fast enough for two human bodies to influence a mass the size of the Earth. While gravity has been shown to alter the flow of time (read up on the "Hafele–Keating experiment" for the details) Kara and Barry can't come close to generating that level of power at that low a speed.

Source

Basically, this is a call-back to the Superman where he did exactly the same impossible thing.

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    If they actually managed to slow down the rotation of the Earth, that would affect solar time (the sun would move across the sky slower, and a full rotation of the earth would take more than the usual ~24 hours). However, the inhabitants of Earth would still be experiencing time at the same rate as usual (so, lots of comments about how this day is "really seeming to drag on forever", and wondering why the sun is still up at 7PM, when it was supposed to set at 5:30). However, if this is a magical thing, it's at least possible that this change would have an effect on a spell.
    – RDFozz
    Commented Dec 12, 2018 at 19:17
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    @RDFozz It's much worse than that. Earth bulges in the middle. The surface is 21km "higher" at the equator than at the poles. Its rotation keeps the Earth's oceans and air "uphill". If the rotation were to slow all that water and air would settle at the poles. We'd have a strip of dry land at the middle latitudes between a polar ocean and a high desert with air too thin to breathe. Loss of the Coriolis effect and the longer days and nights would make weather go crazy. youtube.com/watch?v=tx_pawMRPAY Don't let super heroes geoengineer.
    – Schwern
    Commented Dec 12, 2018 at 23:10
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    @RDFozz We have willing suspension of disbelief regarding the basic premise of any SF/fantasy. The question is where the fiction ends. Car engines in the Arrowverse seem to obey the normal laws of physics and chemistry (actually, the TV laws -- they still explode in unrealistic ways).
    – Barmar
    Commented Dec 13, 2018 at 0:16
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    @RDFozz For an example of what Barmar said, the "Superman flies around the Earth making it rotate backwards to turn back time" breaks a lot of reality. "Superman flies around the Earth so far he goes back in time and perceives the Earth rotating backwards" only breaks one thing; flying really fast makes you go back in time. An inconsistent world invites writing characters out of jams with overpowered magic like time travel. Used sparingly it can be an awesome surprise like the scene in Superman. Used too much and you get Superman's eye beams fixing the Great Wall in Superman IV.
    – Schwern
    Commented Dec 13, 2018 at 0:51
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    @Paulie_D: No, superman didn't do the same thing. It's a common misunderstanding of that scene. He is not rotating earth backwards to turn back time. It just looks like it's rotating backwards while superman is travelling back in time. But probably the writers of Flash also misunderstood it.
    – Chris
    Commented Dec 13, 2018 at 6:15
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Well, something the size of a human moving at Mach 7 (or 700) would have approximately zero effect on the rotation of the Earth.

Stopping the rotation of the Earth would again have approximately zero effect on how fast time was passing on Earth in relationship to the rest of the universe. Some effect, yes, enough to matter? No.

Different relative speeds result in time passing at different rates , but any speed difference that isn’t best described as a significant fraction of the speed of light, is only of academic interest.

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What the fictional stories ignores is that the rotation of the earth has more to do with how we measure time (1 rotation = 1 day), and not how space-time is involved.

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