# Special relativity reference frames

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Bit of a philosophical question here but if everything is relative to something else, who's frame of reference is actually correct? For example an observer moving at 99.9%c will use the exact same laws of physics we use and determine without any doubt whatsoever that Earth is flat... flat as pancake!

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true...

Secondly imagine some object of height H at rest and directly facing a slot of height H/2. Obviously at rest the object cannot fit through the slot but at 99.9%c the object can fit through the slot.

So according to the physics of the observer moving at 99.9% it will fit, but clearly to the stationary observer it won't fit... who is right? Does it fit or does it not fit?

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true...

Secondly imagine some object of height H at rest and directly facing a slot of height H/2. Obviously at rest the object cannot fit through the slot but at 99.9%c the object can fit through the slot.

So according to the physics of the observer moving at 99.9% it will fit, but clearly to the stationary observer it won't fit... who is right? Does it fit or does it not fit?

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#2

(Original post by

Bit of a philosophical question here but if everything is relative to something else, who's frame of reference is actually correct? For example an observer moving at 99.9%c will use the exact same laws of physics we use and determine without any doubt whatsoever that Earth is flat... flat as pancake!

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true...

Secondly imagine some object of height H at rest and directly facing a slot of height H/2. Obviously at rest the object cannot fit through the slot but at 99.9%c the object can fit through the slot.

So according to the physics of the observer moving at 99.9% it will fit, but clearly to the stationary observer it won't fit... who is right? Does it fit or does it not fit?

**AishaGirl**)Bit of a philosophical question here but if everything is relative to something else, who's frame of reference is actually correct? For example an observer moving at 99.9%c will use the exact same laws of physics we use and determine without any doubt whatsoever that Earth is flat... flat as pancake!

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true...

Secondly imagine some object of height H at rest and directly facing a slot of height H/2. Obviously at rest the object cannot fit through the slot but at 99.9%c the object can fit through the slot.

So according to the physics of the observer moving at 99.9% it will fit, but clearly to the stationary observer it won't fit... who is right? Does it fit or does it not fit?

*must*be the case that the object fits through the slot in both cases if it can fit in one case; otherwise, the two frames disagree fundamentally, which the 1st postulate of SR does not allow.

Look at the ladder-barn paradox for more on that.

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#3

Space is like a cake.

In the end, we all just end up taking different slices out of it. Time is included with that cake as the icing.

Either way, we can all perceive the cake differently from different frames of reference, and in the end, we are all correct; the only fact that we can all agree on is that we are seeing an aspect of that cake.

You can't look at the cake from the top and see the bottom (at least from how I'm trying to state it), nor can you go from the side and see the other side, you can only see some part of that cake, and from what you see you can ascertain what part of the cake you are seeing, and compare what you see in that cake to what another person may see in that cake.

You get what I mean! :-;

In the end, we all just end up taking different slices out of it. Time is included with that cake as the icing.

Either way, we can all perceive the cake differently from different frames of reference, and in the end, we are all correct; the only fact that we can all agree on is that we are seeing an aspect of that cake.

You can't look at the cake from the top and see the bottom (at least from how I'm trying to state it), nor can you go from the side and see the other side, you can only see some part of that cake, and from what you see you can ascertain what part of the cake you are seeing, and compare what you see in that cake to what another person may see in that cake.

You get what I mean! :-;

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(Original post by

Answering only the last part, it

Look at the ladder-barn paradox for more on that.

**crashMATHS**)Answering only the last part, it

*must*be the case that the object fits through the slot in both cases if it can fit in one case; otherwise, the two frames disagree fundamentally, which the 1st postulate of SR does not allow.Look at the ladder-barn paradox for more on that.

Care to talk about my first question a little? Even if it's just your personal opinion.

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I just watched a video on the ladder barn paradox and I understand now, damn special relativity is pretty insane!

Care to talk about my first question a little? Even if it's just your personal opinion.

**AishaGirl**)I just watched a video on the ladder barn paradox and I understand now, damn special relativity is pretty insane!

Care to talk about my first question a little? Even if it's just your personal opinion.

For a sort of qualitative description of this I'd recommend reading some popular science books like 'Why does E=mc^2' by Brian Cox

For a more mathematical treatment this wiki books article is pretty detailed although you'll need quite a lot of maths to understand it

https://en.wikibooks.org/wiki/Specia...ransformations

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#6

**AishaGirl**)

I just watched a video on the ladder barn paradox and I understand now, damn special relativity is pretty insane!

Care to talk about my first question a little? Even if it's just your personal opinion.

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#7

**crashMATHS**)

I think Callicious has done a fantastic job at explaining and commenting on your first point, so I won't say anything else as I can't do better than that

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(Original post by

**Callicious**)
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#9

(Original post by

I thought it was a pretty elementary explanation but I appreciate you taking the time to reply

**AishaGirl**)I thought it was a pretty elementary explanation but I appreciate you taking the time to reply

If you want to know more, I recommend a book! Or Khan Academy, they help with this sort of thing... or Britannica! Those guys have everything

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(Original post by

The better explanation I could draw up would include Lorentz transformations and all that 4 dimensional hoobley-joobley, and I don't exactly know the math behind it, not in its entirety at the least ;-;

If you want to know more, I recommend a book! Or Khan Academy, they help with this sort of thing... or Britannica! Those guys have everything

**Callicious**)The better explanation I could draw up would include Lorentz transformations and all that 4 dimensional hoobley-joobley, and I don't exactly know the math behind it, not in its entirety at the least ;-;

If you want to know more, I recommend a book! Or Khan Academy, they help with this sort of thing... or Britannica! Those guys have everything

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#11

(Original post by

Bit of a philosophical question here but if everything is relative to something else, who's frame of reference is actually correct? For example an observer moving at 99.9%c will use the exact same laws of physics we use and determine without any doubt whatsoever that Earth is flat... flat as pancake!

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true

**AishaGirl**)Bit of a philosophical question here but if everything is relative to something else, who's frame of reference is actually correct? For example an observer moving at 99.9%c will use the exact same laws of physics we use and determine without any doubt whatsoever that Earth is flat... flat as pancake!

So who's frame of reference is correct? Is the Earth actually flat or is it spherical? I don't quite understand this If it's dependent on the observer then doesn't that ultimately mean both are 100% true

Philosophically and practically speaking, different approaches are used. One method, which is often used is to talk about proper quantities: the quantities of an object as measured in its rest frame. Quantities in special relativity transform because of different speeds; choosing a set value for the speed (0) to define quantities makes them invariant from frame to frame. In this case we would use the proper length, which makes the Earth round in its own frame. You can see how it is used in momentum: momentum was redefined to the be the mass times the derivative of position with respect to proper time, as opposed to just time.

Another approach and a probably nicer one philosophically and physically speaking is to use the space-time interval to define objects. Relativity says there are 4 dimensions: three of space and one of time. Just like how a position vector with three dimensions does not change magnitude from frame to frame in Newtonian physics, a combination of time, height, length and width does not change in relativity under a rotation and translation: it behaves like a vector with 4 components. The magnitude is the space-time interval.

Interestingly, you can show that the proper quantities are invariant for a fundamental reason, not just for a trivial reason like the speed associated with them being constant. In fact, if you considered measuring simultaneously the two ends of an object, like a rod or Earth, in its rest frame, you would see that this is in fact equal to the space-time interval. Likewise, if you measured the time in an object's rest frame, then the time (multiplied by the speed of light) is the spacetime interval. Thus the proper quantities are invariant because they are actually 4-vectors with a component that is zero, so you can measure a piece of space-time that way.

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