# Frames of Reference

1. Jul 29, 2008

### H0T_S0UP

I'm not going lie I don't know too much about relativity, though I do understand it. I haven't even gone to college yet and from what I've read so far I'm assuming there's much more to know. Anyway, in most of Einsteins thought experiments he speaks about observations in terms of 1 frame of reference. They lead to paradoxes such as time dilation and the increasing/decreasing 'rate' of time. This leads to the conclusion that since each perception is different in its respective frame of reference, all frames of reference are valid. But why is human perception taken into account in the first place? I mean our perception distorts everything. We cant even be sure if everything we are thinking is 100% wrong.

Wouldn't it be more accurate to look at events in terms of how the events are happening rather than how the events are happening with respect to something else? What i mean is wouldn't using only one frame of reference be invalid, there by invalidating all frames of reference, and instead using two equal frames of reference simultaneously under similar conditions? This would create a frame of reference that that uses two different FoR's at the same time to cancel out individual perception and prove that is it possible to do something such as (like Einstein said) synchronize all clocks in a town equally.

Damn I think I just confused myself stating all that. I should probably read more on relativity. Also sometime I find it hard to put my thoughts into words, its a type of 3 dimensional thinking I believe, or so I was told.

2. Jul 29, 2008

### JesseM

What specific thought experiments are you talking about? Most of the thought experiments I have seen involve a comparison of the same situation viewed from two different frames of reference.

3. Jul 29, 2008

### H0T_S0UP

I recall reading about a thought experiment where an observer was placed between two bolts of lightning. At first the observer was not moving confirming that the bolts hit the ground simultaneously. Now the observer is moving toward one bolt, according to Einstein the lighting bolts would no longer have hit the ground the same time (with respect to the observer) because of the Doppler effect. I understand that the bolts did not hit the ground at the same time according to the moving observer but I don't understand why the point would even be valid when in reality the bolts hit the ground at the same time. Get what I'm saying now. Wouldn't it make more sense to distinguish perception from reality rather than integrate them as relativity does?

Let me put it this way: What would happen if were to to make a frame of reference that look at everything that is occurring in the universe all at once? Wouldn't we see some type of synchronization or pattern if everything was look at with respect to everything else?

Last edited: Jul 29, 2008
4. Jul 29, 2008

### JesseM

Actually the whole idea of this thought experiment is to consider the same lightning strikes from the perspective of two observers, one who's on a train moving with respect to the ground, and one who's at rest relative to the ground. And it has nothing to do with the Doppler effect, rather it's based on the idea that each observer assumes that light moves at the same speed c in their own rest frame, which means that if the observer on the ground says the lightning bolts hit either end of the train car simultaneously, then the observer on the train car must say that one bolt hit before the other. Here's a summary of this thought-experiment from the textbook https://www.amazon.com/Spacetime-Physics-Edwin-F-Taylor/dp/0716723271/:
Why do you think the "reality" is that the bolts hit the ground at the same time? The thought experiment simply says that if each observer assumes that light moves at c in their own rest frame, then the observer on the ground concludes they hit at the same time (because the light from each strike reaches him at the same moment, and both strikes occurred an equal distance from his position according to his ruler) while the observer on the train concludes they hit at different times (because the light from each strike reaches him at different moments, and both strikes occurred an equal distance from his position according to his ruler). There is no physical basis for thinking one perspective is more true than the other.

Last edited by a moderator: Apr 23, 2017
5. Jul 29, 2008

### H0T_S0UP

Its tough to explain, if I were to use a frame of reference with an observer 1 mile from the train relativity would still hold true regarding which lightning bolt hit first. What I'm trying to find is a way to take human perception out of the equation to determine that the lightning bolts did factually hit at the same time.

6. Jul 29, 2008

### matheinste

Hello HOT SOUP.

Don't take the word observer too literally. It does not have to be a human observer, it might just as well be a light detector or any relevant apparatus.

Matheinste

7. Jul 29, 2008

### Antenna Guy

Well... JesseM has already claimed that Doppler effects have nothing to do with this thought experiment, but I have yet to see anyone prove that the light coming from either end of the train is the same color in both frames.

Regards,

Bill

8. Jul 29, 2008

### H0T_S0UP

I was unaware of that, looks like I gotta do some more reading. This is very intriguing if these laws hold true for inanimate objects as well. I really do love this kind of stuff but have to be in the mood to look into it most of the time. I've got a few ideas that Im saving for my first year at college. Then again I'm probably not the only one.

9. Jul 29, 2008

### matheinste

Hello HOT SOUP

Physics, and so relativity, is all about measurement and inanimate appparatus is usually more trustworthy and more accurate than human senses at measuring things.

Matheinste.

10. Jul 29, 2008

### JesseM

I didn't mean that the light would be the same color in both frames, I just meant that the Doppler effect has nothing to do with the fact that different observers disagree about simultaneity, which is the point of that particular thought experiment.

11. Jul 29, 2008

### MeJennifer

Note that actual measurements in relativity scenarios are not what many talk about when they talk about what is observed. What is observed is often not what is actually measured! Bondi k-calculus is what should be used to determine and understand what is actually measured instead of imagining some plane of simultaneity that has a zero physical relevance. Probably 80% of all the misunderstandings in relativity is due to the fact that measured and observed data is not identical.

12. Jul 29, 2008

### Antenna Guy

I assume you agree that color (frequency) can be measured.

Regards,

Bill

13. Jul 29, 2008

### JesseM

The coordinates an observer assigns to events in his frame of reference has nothing at all to do with where he is located, only how he is moving relative to other objects. If you had one observer at the position of the middle of the train when the lightning struck, another 1 mile away, another 100 light-years away, then as long as all these observers are at rest with respect to one another they will all agree about whether or not the strikes were simultaneous.

As I said, one way of dealing with light signal delays is just to factor out the time the light would take to get to you based on the distance of the event and the assumption that light moves at c in your frame. Another would be to use a network of rulers and clocks at rest relative to you, with the clocks "synchronized" using the assumption light moves at c in your frame (the Einstein clock synchronization convention) and only assign times to events based on clocks that were right next to the events as they happened. I discussed this with another poster on an older thread you might want to take a look at.

Last edited: Jul 29, 2008
14. Jul 29, 2008

### H0T_S0UP

Here's another relativity question. Suppose a 2 foot long rod is placed inside a gun when it is launched it will fire near the speed of light, c. Before the rod is launched it is measured in a motionless state. When the rod is launched it travels down a tunnel which has two high speed cameras watching the projectile. The first one moves with the rod and the other remains motionless. As the rod travels past the motionless camera, would the picture it takes of the rod be smaller than its actual size and if so by how much? for the moving camera, would the shot be distored because of both speed and time dilation (say there is counter imposed on the camera's screen as it films the rod fly past/with it).

15. Jul 29, 2008

### JesseM

16. Jul 29, 2008

### H0T_S0UP

So the objects under relativity go under apparent changes as they approach the speed of light. The rod moving at near C wouldn't actually contract, but it would appear to since light takes time to reach an observer. If what I'm saying is correct then this is much more clear to me now. If not then I'm a very lost soul searching for relativity in a lonely world....ha!

17. Jul 29, 2008

### JesseM

No, you've got it backwards, it wouldn't appear to contract because light takes time to reach an observer, even though it "really" would have contracted in your frame. Like I said, if its rest length was 2 ft, then if it's moving at 0.6c in your frame, then its "real" length in your frame would be 1.6 ft because of Lorentz contraction, but visually it would still appear to be 2 ft long because of the Penrose-Terrell effect.

18. Jul 29, 2008

### H0T_S0UP

Ok I think I understand now. I only took a physics course in highschool and I'm just starting college this year so most of this is new to me. Let me ask this: Why would an object contract as approaches the speed of light. What force is it undergoing that causes it to change its shape?

19. Jul 30, 2008

### JesseM

There aren't any forces changing its shape, and the shape-change isn't really objective--in my frame your ruler is shrunk relative to mine, but in your frame my ruler is shrunk relative to yours, there's no physical way to determine a truth about whose ruler is "really" shorter.

20. Jul 30, 2008

### H0T_S0UP

Ok lets verify this one last time. If this 2 foot rod was to launch past me at .75c and I was just standing still, it would decrease in volume but because of the speed that light travels it would appear to retain its motionless volume. Time for the rod would slow down since it is moving so fast. All of this is only relative to my frame so if I was .375c all these properties would change.

21. Jul 30, 2008

### yuiop

Hi Soup,

Please don't let the Penrose-Terell effect confuse you! It should never be introduced to beginners for that reason. Length contraction is nothing to do with light travel times and is more real than a mere optical illusion. What should be made very clear to all beginners in SR is that light travel times are always allowed for and deducted and we are only looking at the residual effects when looking at time dilation and length contraction. A better way to measure the length of the moving rod would be to have a row of ink jets simultaneously spray a short blast of ink at the rod as it passes and observe the silhouette of the rod left by the ink on a wall behind the rod.

22. Jul 31, 2008

### JesseM

It would decrease in length in your frame, although not in its own frame--length contraction is always frame-dependent. And yes, despite the fact that its length is shorter in your frame, its length will look unchanged to you visually because of the Penrose-Terrell effect.
It's only moving fast relative to you, there is no objective notion of speed in relativity. In the rod's own rest frame, its speed is zero while you are the one moving fast, so in its frame your clock would be the one running slow.
0.375c relative to what? Again, you can't talk about an object's speed in any objective sense, only relative to some other object.

23. Jul 31, 2008

### H0T_S0UP

sorry I should have been more specific. By that last sentence I meant that everything I was speaking about is strictly relative between me and the rod.

24. Jul 31, 2008

### JesseM

OK, then in your frame the rod is measured to be shorter, and a clock attached to the rod is measured to be running slow relative to your own, but visually things are a little different thanks to light delays--because of the Penrose-Terrell effect the length of the rod will look unchanged, and because of the Doppler effect, if the rod is moving towards you its clock will look like its sped up rather than slowed down (and if it's moving away from you the clock will look like it's ticking even slower than the amount you measure it to be ticking in your frame).

25. Jul 31, 2008

### MeJennifer

So it is measured to be shorter but visually it is not? That does not make any sense.
You probably meant to write observed to be shorter.