Today's lecture topic

ExCyber

Staff member
"It is impossible to devise an experiment that determines whether you are at rest or moving uniformly."

Discuss.
 
I remember studying this a long time ago... this demonstrates the problem with newtonian physics depending on who is observing the movement (or lack thereof).


Though, principally speaking, we're always constantly moving if we're sitting still, as the earth is rotating on it's axis and at the same time rotating around the sun. So philosophically, wouldn't it be easy enough to say 'regardless what you're doing, here on earth you're always moving at a uniform speed'?


Quite a cop out answer, I know.
 
Only somewhat a cop-out answer. I would have to agree with you I think.

Considering the question was about devising an experiment, I would have to agree that it is impossible simply because the experiment itself is also moving and that movement cannot be controlled for the reasons MTXBlau mentioned.

I added a cop-out to the cop-out, yay.
 
Hardly what Excyber was looking for, since our cop out answer doesn't lend itself to the theory of relativity but instead 'rolling over and going back to sleep', so to speak.
 
To be fair, ExCyber only presented a quoted question and asked us to discuss it. He didn't actually ask us to discuss what we thought the answer should be.
 
Yeah, the implications are the more interesting part. Anyway, vbt was just lamenting on IRC that hardly anyone posted anymore, and I was doing physics homework, so... :biglaugh:
 
This refers to "frames of reference". You and your experiment and everything you can see is part of a frame of reference, and the entire frame could be at rest or have constant velocity.

But what is at rest? At rest assumes that there is a central frame of reference, against which other frames can be measured, which contracts the theory of relativity. Nothing can be at rest, and nothing has inherent motion. Only relative measurements can be made.

That's the easy part to understand. The part I'm having trouble with seems to contradict it.

If frame B accelerates away from frame A at near the speed of light, and then stops, and accelerates back towards frame A, the calculations say frame B has experienced less time passage than frame A when they meet again.

But my question is, how do you know which frame is accelerating? You do you know that frame B is accelerating away from frame A, and not the other way around? Then you'd have a contradiction.

If a spaceship accelerates off of Earth, how do we know that the solar system isn't accelerating away from the spaceship? What defines that frame?
 
What you're describing is essentially the "twin paradox". I don't fully remember the explanation of it, but there is basically some kind of trick to it. There are several ways to explain it but IIRC the easiest example to understand and calculate is to suppose that each frame is sending light pulses to the other and calculate the relativistic Doppler shift on each leg of the trip.
 
The Twins Paradox is only a paradox if you assume that the earth brother and astronaut sister both see the travel as the same distance, but it isn't.

Since the sister is traveling at 0.6*c, space is compressed, and alpha centuri, 4-light-years away, appears to only be 3.2-light-years-away frm the sister's point of view. But it is still 4 light-years away from the brother's point of view. This is how the sister ages less.

The twins paradox assumes 3 points: a brother and a destination fixed at 4 light years away, and a sister that moves between them. However, here's how I think I create the contradiction, by adding a 3rd point to make the problem symmetrical.

Pretend instead there are two trains that a 4 light-years long, parallel to each other, facing in opposite directions. The sister train's engine starts next to brother train's engine:

CSSSE (sister train)

EBBBC (brother train)

Now, in the traditional twins paradox, Sister's engine (E) begins at brother's engine (E), and moves towards brother's caboose (C) 4 light years away. And then comes back.

From the solution to the twins paradox, sister's engine (E) ages 128 months, while brother's engine (E) ages 160 months. But now I've thrown in sister's caboose (C), which makes this symmetrical.

And this brings out the acceleration question. If sister's train was the one accelerating and deaccelerating, then the sister's train is not always in an inertial frame, while the brother's train always is, and the scenario is not symmetrical, and this is how the normal equation is played out. But how do you know which train is accelerating? Is acceleration symmetrical?

Since acceleration is caused by a force, and forces act in equal and opposite directions, shouldn't it be symmetrical?
 
Movement can only be defined when given a reference point. The object is moving in terms of what location?
 
Kind of like 'when a tree falls in the woods, and no one is there to hear it, does it make a sound?' - no, since there's no observer to 'hear' the sound.
 
MTXBlau said:
Kind of like 'when a tree falls in the woods, and no one is there to hear it, does it make a sound?' - no, since there's no observer to 'hear' the sound.

Except for the squirrel who began to hear the sound, and was squished by the falling tree.
 
Since when does sound need to be heard to exist? I hate that phrase, it is so full of shit.
 
Kuta said:
Since when does sound need to be heard to exist? I hate that phrase, it is so full of shit.

To see what I mean, take a look at:

http://dictionary.reference.com/search?q=sound

You're both right.

It actually just depends on the definition of sound. For example, if you take the first meaning from the definition, MTXBlau is right. The sound doesn't exist because the definition of a "sound" is someone hearing something. If you take the second more technical meaning, it does exist as a sound as it isn't required to be heard.
 
Kuta said:
Since when does sound need to be heard to exist? I hate that phrase, it is so full of shit.


There's the very mechanical, mundane definition that dibz alluded to, and then there's the philosophical question - without someone to observe, does it exist?


Of course, science is based on verifiable, repeatable instances which lend credence to the notion that yes, there should be a sound. But this is based on the premise of what has been observed and a good faith belief while not observed, it will still make the same sound.


Though, much like the question that started all this, it's impossible to test.
 
Considering sound consists of vibrations through matter (eg, air, water) I can't see why it can't be proven it exists where no one can hear it.
 
Recording or transmitting it would indicate there is someone to observe it, even if at a later time.

I believe the question itself alludes to no one _ever_ observing the "sound" in question.
 
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