If time and speed is relative, then we could double the speed of every particle in the universe, as this increases the clocks rate we shouldnt be able to tell any difference.

But my question is, in this new scenario, when two cars collide with double speed, why dont they make much more impact? They now have 4 times the kinetic energy, how is that canceled out?

If this doesnt cancel out, then time isnt relative and there is an intrinsic speed of time?


closed as unclear what you're asking by Emilio Pisanty, knzhou, Dale, David Z May 26 at 16:34

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Why the downvote? Perhaps you didnt comprehend the question $\endgroup$ – TROLLHUNTER May 26 at 14:35
  • $\begingroup$ Yes, it is unclear what you are asking. $\endgroup$ – Feynmans Out for Grumpy Cat May 26 at 14:37
  • $\begingroup$ What is unclear to you? Its complicated, not unclear. $\endgroup$ – TROLLHUNTER May 26 at 14:39
  • $\begingroup$ Well, what do you mean by the speed of time? $\endgroup$ – Feynmans Out for Grumpy Cat May 26 at 14:41
  • 1
    $\begingroup$ Imagine two universe besides each other, then one can physically move twice as fast as the other. Can you detect which universe you are in? $\endgroup$ – TROLLHUNTER May 26 at 14:47

You're grasping towards basic dimensional analysis; what you're talking about can be put in more concrete terms by just imagining that we halve the unit of time. (You have to talk about it this way, because if you just say "everything speeds up", you have to specify what this is relative to. If it's relative to some standard of time, then we can use that standard to define our units, so what you are saying is equivalent to a unit change.)

The point is that if you do this, not every physical quantity changes in the same way. For example, you know the length of a stick in meters wouldn't double, because you're not changing the unit of length. Energy has units of $\text{kg} \, \text{m}^2 / \text{s}^2$, so we expect it to quadruple, so there's no issue here.

For a simpler version of this "paradox", you might note that changing your distance unit from yards to feet will only triple your height in the new units, but it will make land areas $9$ times larger. This does not mean that real estate is now magically $9$ times cheaper.

  • $\begingroup$ So in a universe like ours but where everything moves twice as fast, we should be able to detect differences? Doesnt this mean there is an intrinsic speed of time? And is it a dimensionless number? Is it 1? Why? $\endgroup$ – TROLLHUNTER May 26 at 14:52
  • $\begingroup$ @TROLLHUNTER To be honest, that sounds very vague. Can you rephrase what you think in terms of actual observations? For example, how do you propose the "speed of time" be measured? How are you practically defining the speeds of objects in the first place? Sensible answers to these questions should contain words like "apparatus", "measurement", "stopwatch", "ruler", and so on. $\endgroup$ – knzhou May 26 at 14:57
  • $\begingroup$ Yes god observe everything to move twice as fast in one universe then in the other. How hard is that to understand. Now my question is if they evolve differently. I am op $\endgroup$ – user122089 May 26 at 15:17
  • $\begingroup$ @TROLLHUNTER The point is that time is a dimensionful quantity. So you need to choose a unit. The "dimensionless prefactor" you seem to be referring to is just the conversion factor between different units which you are free to choose, i.e., you are free to choose the unit of time. For example, with the dimensionless prefactor of $60$ you can switch between minutes and hours and make things move $60$ times faster or slower in a numerical sense. This is not called the "relativity" of time in an SR or GR sense. This is just a change of units. $\endgroup$ – Feynmans Out for Grumpy Cat May 26 at 15:26

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