# What is the difference between SR and GR?

Discussion in 'General Science & Technology' started by Mark Turner, Jul 5, 2019.

1. ### Mark TurnerBannedBanned

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Thank you for the video , I find dynamical process interesting . If I watched that video a dozen times , I'd still probably not understand the kinetic energy involved .
That's physics though ,it is complex when you start to see and hate the complexity of the physics but in a mind boggling good way .

Thanks for sharing and giving me something to think about .

3. ### Mark TurnerBannedBanned

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If nothing changes within the caesium itself , then how can the frequency possibly change ?

Aren't different reference frames accountable for a difference in enthalpic condition ?

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Or explained much more simply here...your question answer at the 2 minute mark......

7. ### James RJust this guy, you know?Staff Member

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Mark Turner,

The frequency doesn't change in the rest frame of the caesium, or whatever. It is only different when you compare it to a second clock in some other frame of reference. In other words, the time dilation effect is fundamentally due to what space and time are like, not due to any physical property of clocks.

8. ### SeattleValued Senior Member

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Time dilation is necessary once you determine that the speed of light is fixed and everything else is relative.

Time has to slow down when speed approaches the speed of light because nothing can go faster. The usual thought experiments involve a stationary observer on a train station platform and another person in the train that is passing by.

The example given is usually a light clock where it takes a second for a beam of light to bounce from the floor to the ceiling of the train. To the person in the train it appears to bounce straight up and down. To the observer it appears to bounce at an angle (due to the speed of the train). The beam of light has to travel further on the moving train but it still has to occur in one second.

The only way to do that is for time to slow down since nothing can go faster than the speed of light. In this thought experiment the train is going at close to the speed of light.

The same concept is involved with someone riding in the back of a truck going 100 miles an hour. This person throws a ball at 10 mph in the direction of travel. The ball therefore is now going 110 mph.

If you do the same experiment but instead of throwing the ball, you shine a flashlight in the direction of travel. The difference is that the light won't be going 10 mph + C (speed of light). It will just be going C no matter how fast the vehicle is going.

It's because of that "math" that we have to have time dilation. Spacetime is one thing. You have more "space", you have less "time", mathematically.

9. ### James RJust this guy, you know?Staff Member

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It takes longer than one second, according to the person watching the train move, because, as you say, it has to travel further, but (and this is the important point) at the same speed.

10. ### Mark TurnerBannedBanned

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Thank you for the videos .

Isn't time dilation based on the caesium and length contraction used in the GPS systems , two separate phenomenons?

11. ### Mark TurnerBannedBanned

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Doesn't the shorter second measured always fit within the longer second measured ?

12. ### SeattleValued Senior Member

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Time dilation has nothing to do with caesium.

13. ### SeattleValued Senior Member

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What does this even mean?

14. ### Mark TurnerBannedBanned

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In the light clock thought experiment the clock at relative rest will measure 1.2s by time the clock in relative motion measures 1.s because of the extra distance travelled of the angular path of light .

To ask my question alternately , if you simultaneous rolled out two balls of string , one slower than the other , the shorter string reference frame will always fit within the longer string reference frame ?

15. ### SeattleValued Senior Member

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So what?

In the thought experiment case there are two different frames of reference, one going close to the speed of light and one at rest.

16. ### exchemistValued Senior Member

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This is Theorist, yet again. I would not waste your time if I were you.

Seattle likes this.
17. ### Mark TurnerBannedBanned

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Yes , I'm aware of that .

The speed of light is constant , the angular path the light travels in the moving reference frame is a greater distance than the light travels in the clock at rest frame . By time the moving reference frame measures 1 tick , the clock at rest measures ~1.2 / 1.3 ticks .

Wouldn't that make 1 tick in the moving reference frame equal to ~1.2/1.3 ticks of the rest frame ?

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19. ### Mark TurnerBannedBanned

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If the clock at rest reference frame was expanded in distance between the mirrors , wouldn't that then be perfectly synchronised with the one in motion ?

Thanks

20. ### Mark TurnerBannedBanned

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Hello , I'm not great at maths but this morning I have attempted to add some values to my question in case it was not understood .

1. Clock at relative rest

H = ~399723277m

Tick rate = 1.3 light seconds per tick

2. Clock in relative motion

H= 299792458m

Angular path distance the light travels

d=~399723277m

Tick rate = 1.3 light seconds per tick

Dilation = 0t

Is that correct because of c constant ?

21. ### DaveC426913Valued Senior Member

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Yes.

Yes. That is all totally correct.

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