# Does Relativity cut us off from Reality?

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##### Guest
Two cars, driving head to head, with a relative closing speed of 60mph are only 24 yards apart but brake safely to a stop bumper to bumper. How is this when the stopping distance at 60mph is 48 yards? It's a simple calculation because we have the absolute means of measuring due to the road.<br />Drifting in space without the equivalent of a 'road' how can we know the true reality when we only have relative motion?<br />

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#### Saiph

##### Guest
You've got a problem. The braking distance is dependant upon the speed relative to the ground, while the 60mph you list is between the cars.<br /><br />In space, the answer would be simple, they don't stop in time, they hit hard. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### bobvanx

##### Guest
You've got a gut-level understanding of the problem Einstein solved with reference frames. Your wacky math aside, the question of how do we know where we are in space, in relation to the rest of space, is exactly what many many philosophers, theoreticians and physicists struggled with for a dozen decades. We got ideas about "ether," the boundaries of the Universe, and all the rest of cosmology from the struggle to understand just that question.<br /><br />Einstein pointed out that it in fact doesn't matter. You can behave, and make measurements which, for the most part, treat your frame of reference as the stationary one.<br /><br />Lots of wacky, counter-intuitive effects proceed from this, such as the twin paradox, but so far it seems to be an excellent description of what's going on.<br /><br />It also means that yes, there are parts of reality that we are forever cut off from.<br /><br />For example, we don't know what's going on, out at Saturn, right this minute. We'll know in about an hour, what was happening now, but we can't know <i>now</i> what's happening now.

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##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>Einstein pointed out that it in fact doesn't matter. You can behave, and make measurements which, for the most part, treat your frame of reference as the stationary one. <p><hr /></p></p></blockquote><br />Thanks. My point is that's untrue as treating one car as stationary would require 48 yards stopping distance not the <i>actual</i> 24 yards. This anomaly is apparently unknown to us floating in space without a common reference frame. Therefore how can we be sure anything we measure is true & not based on a fallacy. It does seem we base everything on gravity which is still a mystery.

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#### bobvanx

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>This anomaly is apparently unknown to us<p><hr /></p></p></blockquote><br />Well, it's unknown to <i>me</i>, that's for sure. If it takes 48yds to stop a car travelling 60mph, then two cars in a head-on will require a seperation of 96yds to keep from colliding.<br /><br />Not at all sure why you write they'll be able to stop in 24yds.

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##### Guest
Hm I always have problems talking to scientists being an engineer.<br />I thought my description, by way of cars, would underline the apparent problem of measurements in space. Not knowing the mass & speed of both objects would lead to the wrong answer.<br />What I don't see is how has the problem been overcome.

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##### Guest
You've seemingly misread my topic, I said a <i>closing</i> speed of 60mph.

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#### bobvanx

##### Guest
Ah, yes, I did misread your word problem. Let me restate it:<br /><br />A car traveling 60mph requires 48yds to stop.<br />A car traveling 30mph requires 12yds to stop.<br />Two cars approaching each other with a closing speed of 60mph will require 24yds to stop.<br /><br />Here's a more precise way to phrase it (which removes the guesstimated stopping distance numbers):<br /><br />Why will a car traveling 60mph require a greater stopping distance than two cars each traveling 30mph?<br /><br />(The thing is, that it's irrelevant whether they are pointing towards each other or not. What we're interested in is the sum of the stopping distance of the two cars, versus the stopping distance of the single car)<br /><br />The momentum of a moving object increases with the square of the velocity. So a car moving 60mph has 4x the momentum of a car at 30mph, so it requires lots more distance to come to a stop. About 4x the distance (reaction time is a factor, as are tires, and so on).<br /><br />As an aside, where this really gets people in trouble is in the slight increases from 60mph to 70mph, and 70mph to 80mph. That extra 10mph results in a huge increase in the distance required to stop. It's not additive. It's not even a straight multiple. It goes up as a geometric progression, and most drivers don't understand that. They are genuinely surprised when they discover (usually as they impact a wall or another car) that they couldn't stop in time.<br /><br />So this isn't a cosmology problem. It's a mass and acceleration problem. The simple answer is that 2x the velocity gives 4x the momentum, so the deceleration distance is 4x longer.

S

##### Guest
Yes I appreciate it's a square law but my topic is about not knowing this when, as we are, drifting in space. We only know the relative velocity, having no reference frame as the road, which I showed by example of cars gives an anomalous result.<br />You've correctly sussed the two cars going at 30 mph.<br />What I don't understand is how is this overcome as I can't be the only one to be confused. Further I can't believe I'm the only one on Earth to have found a drastic problem in space physics so presumably am missing something.

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#### paintwoik

##### Guest
You've created your own problem by saying one has a reference frame while another does not, when in reality there is no such difference.

S

##### Guest
Well that's a nice re-statement of what Einstein said but I can't understand how I find the relative mass of a distant object. Is there a difference between a common reference frame, as my cars & unconnected ones.<br />Maybe my problem has something to do with the Pioneer probe anomaly<br />

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#### paintwoik

##### Guest
It seems you have decided that somehow when in space that you have escaped reality, when in reality no escape is possible.

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#### rpmath

##### Guest
<font color="yellow">Hm I always have problems talking to scientists being an engineer. <br />I thought my description, by way of cars, would underline the apparent problem of measurements in space. <br /></font><br />I am an engineer too, and I have some problems too…<br />We as engineers are used to know some fast solutions to common problems. We must take care to see if we are missing some of the conditions required to use them. <br /><br />The solution “the stopping distance at 60mph is 48 yards” is valid if:<br />- The brakes give the "standard" deceleration<br />- The observer (reference frame) was not accelerating, (like standing on the street) <br />- The car was moving at 60mph relative to him <br />- The car ended stopped relative to him. <br /><br />In your example, the second car is accelerating too, so it cannot be a good reference frame. It will see the second car acceleration been 2 times the standard one. If you want to do your calculations with this frame, you must forget the easy solution, and do the full calculations… or choose another reference frame where equations are not so complex.<br /> <br /><font color="yellow">Not knowing the mass & speed of both objects would lead to the wrong answer. <br />What I don't see is how has the problem been overcome.<br /></font><br />You don’t need to know mass for a cinematic problem…<br />You only need distance, speed and acceleration.

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#### bobvanx

##### Guest
RPMath, thanks for working to defeat stereotypes about engineers.<br /><br />I've known lots of 'em, and there have been great communicators and terrible ones, spiritual ones and agnostic ones, techno-fiends and social activists, and yet more and more variation. For someone to use their training as an excuse for an unrelated disconnect rankled me.<br /><br />On the other hand, you used your training to excellent effect to show that if we want to measure acceleration from a given frame of reference (a moving car) we can't hop out part way through and use the frame of reference of the roadbed.<br /><br />Hats off to you!

S

##### Guest
big hurry<br />limitation gravity<br />60mph car pulls 1g 48yards<br />30mph cars <i>each </i>pull 1g sum 2g<br />hence half stopping distance 24yards

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#### bobvanx

##### Guest
RPM,<br /><br />Do you get what siarad's trying to point out? I don't see it.

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#### bobvanx

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>my topic is about not knowing this when, as we are, drifting in space. We only know the relative velocity<p><hr /></p></p></blockquote>I tell you, if you really want to toast your marshmallows, play catch on a merry-go-round. If you throw the ball directly towards your playmate, you'll miss them everytime. And what's really crazy is that with enough practise, your plastic mind will adapt, and you'll learn throw the ball in such a way that to your eye it looks like it curves through space to "catch up" with your playmate.<br /><br />So, are you wondering since the earth is "drifting in space," how there might be some sort of "hidden" velocity that should be changing how quickly we can stop? While traveling in a complementary direction?

S

##### Guest
So my illustration of a nag in my head was not perfect & became the topic.<br />Lets look:<br />no one pointed out that 2g braking is possible, dragsters convert torque into extra weight, not mass, F1 cars do similarly with air pressure.<br />It seems the topic has failed but I leave you with this:<br />two space ships approach each other just like my cars. One escaping from the suns gravity & both break at 1g stopping facing each other.<br />the one escaping the sun breaks at 2g, they end up facing each other but falling into the sun.<br />the opposite one breaks at 2g & they both escape the sun.<br />I leave you to decide regarding your own frame <img src="/images/icons/laugh.gif" /> of mind which reality you choose.<br />

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#### bobvanx

##### Guest
I choose the reality where I maintain clear, consistent trains of thought.<br /><br />RPMath already pointed out that you can't shift reference frames in the middle of your calculation. You've got to crunch the long, hard numbers.<br /><br />The sentences you leave out of your query make it difficult to parse your thought, but one thing is clear: there is no ship decelerating at 2g in your example.

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#### newtonian

##### Guest
siarad - Hi! I'll skip the details and simply respond:<br /><br />Assuming relativity theory to apply to all reference frames in our universe at all times may indeed cut us off from reality.<br /><br />Perhaps, and for a number of reasons:<br /><br />1. The anomaly you mentioned with the Pioneer probes.<br /><br />2. Inflation theories posit faster than light expansion while light speed is considered both constant and an upper limit to travel in relativity theory.<br /><br />Yes, I know the space expansion vs. actual motion of matter explanation - I doubt it.<br /><br />Also, some scientists (including astronomers) postulate a different speed of light for the very early universe - hence not totally constant.<br /><br />3. Reference frames do matter and some reference frames beyond our universe's visibility horizon are apparently receeding from our reference frame faster than light.<br /><br />4. Tachyons.<br /><br />In short: accepting a theory as fact can lead to wrong conclusions.<br /><br />However, the actual discussion of details in your example, such as how wet the road surface is, how good the tires are for braking or avoiding hydroplaning, whether the braking system is anti-lock, whether there is variable slope along the distance between the cars, the density of the air (including temperature), the temperature of the road surface, etc., is far too complex for me to post on now.<br /><br />Our universe is similarly complex and there are many unknowns like dark matter and dark energy which can skew your calculations.

S

##### Guest
Thanks. The car was me trying to engender thought about space & not the topic but seems to have become it.<br />For reference I measured these with my car without anti-lock brakes but merely paced out the distances without any allowances for accuracy as it was just an <i>example</i><br />For interest I measured 30 mph stopping at 12 yards & for every extra 10 mph another 12 yards added up to 70 mph. This followed the expected square law & needed no calculation or change of reference plane <img src="/images/icons/laugh.gif" />

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