Relativity - Oh dear, here we go again!

ynot said:
It’s possible to measure a thing that has no mass, travels at c and is “destroyed” the instant it contacts matter? :jaw-dropp
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It's not destroyed. And of course it's possible to measure - you're reading this with your eyes, aren't you?

How big are the dimensions of light?
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Visible light has a wavelength of around 500 nm (half a thousandth of a millimeter).

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Is the size and dimension of light actual or theoretical?
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Both. What's the difference, anyway?
 
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jsfisher said:
But what does it mean for the events to occur simultaneously? If the two events are not at exactly the same place, with time being relative and all, it becomes meaningless to ask if events were simultaneous without also specifying the reference frame. In different frames the temporal ordering of the events could well be different (whether or not actually observed).
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I agree that observation is relative but not necessarily time. It‘s not possible to observe which events occur simultaneously, but as there‘s never a time that events aren‘t occurring, it‘s obvious (to me) that they always do. In other words, it‘s not possible to define what events are occurring simultaneously, but undefined events are always occurring simultaneously.

I‘m not considering things from a “relativity is true“ stance. How can you evaluate a thing without bias when you use a thing to evaluate itself?
 
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ynot said:
I agree that observation is relative but not necessarily time. It‘s not possible to observe which events occur simultaneously, but as there‘s never a time that events aren‘t occurring, it‘s obvious (to me) that they always do. In other words, it‘s not possible to define what events are occurring simultaneously, but undefined events are always occurring simultaneously.
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I came in late to this thread, so I am not sure why you reject the notion that time is relative. It is a required consequence of the speed of light being a constant.

Also, you may be taking the concept of "observation" a bit too literally. Observation in the literal sense is not required. An event is an instance in time. It may be that events A and B occur in that temporal order for one reference frame, B then A in another, and the two simultaneously in a third...even if no one observes the either or both events.
 
ynot said:
Is measuring the wavelength of light actually measuring light? Isn’t that a bit like measuring water by measuring waves at the beach?
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No. Light in classical electrodynamics is a wave - there is no analogue of water (or ether as it used to be called), and you can't make something out of waves that's smaller than the wavelength.

Read up on the photoelectric effect. Taking into account quantum mechanics, light is composed of photons, each of which has a size and a certain amount of energy. The size is the wavelength (more or less).
 
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ynot said:
I agree that observation is relative but not necessarily time. It‘s not possible to observe which events occur simultaneously, but as there‘s never a time that events aren‘t occurring, it‘s obvious (to me) that they always do. In other words, it‘s not possible to define what events are occurring simultaneously, but undefined events are always occurring simultaneously.

I‘m not considering things from a “relativity is true“ stance. How can you evaluate a thing without bias when you use a thing to evaluate itself?
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So what are the definitions that you are useing?

What you seem to be likely unknowingly argueing for is a priviliged reference frame. So if the laws of physics vary depending on how you are moving why does this not show up in experiments?
 
ynot said:
I‘m not considering things from a “relativity is true“ stance. How can you evaluate a thing without bias when you use a thing to evaluate itself?
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Depends what you're evaluating it for. If you're evaluating a theory for self-consistency, then the theory is all you need.

The only people I've ever met who objected to relativity didn't understand it. If you don't understand it, it's easy to get the impression that the theory isn't self-consistent (hence terms like the "twin paradox" which isn't a paradox at all). But it is. There's absolutely zero question on that front, because its self-consistency is a mathematical property.

Now, self-consistency is a separate question from whether or not it describes reality. Newtonian mechanics is self-consistent, but it's not correct. But every single test of relativity has produced a confirmation. It's as good a theory as science has ever developed. It's so good that even if it ever gets supplanted, it's accurate enough for so much of physics that it will still get used, just like Newtonian physics has been supplanted by relativity but it still gets used because it's an excellent low-velocity approximation.
 
ynot said:
I‘m not considering things from a “relativity is true“ stance. How can you evaluate a thing without bias when you use a thing to evaluate itself?
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Of course, we don't say
Suppose relativity is right. Well, then, relativity is therefore right. QED​
What we say is this:
Suppose Newtonian mechanics were right. What sorts of phenomena would we expect to see, and do they match what in fact we see? (Some don't.) Now suppose, on the other hand, that relativity were right. What sorts of phenomena would we then expect to see, and do they match what in fact we see? (They all do, as far as we can tell.) Therefore, Newtonian mechanics is wrong; and relativity is right, as far as we can tell.​
You shouldn't use the theory of relativity to evaluate itself. You should evaluate it by noting the close agreement between its predictions of what we should see and what we actually do see.
 
Thanks for the feedback.

As I understand it, when a thing is in an inertial frame of reference it has a constant velocity and is not undergoing any acceleration. It would seem to follow that a thing can’t be in more than one inertial frame of reference at a time. In other words, a thing can’t be in an inertial frame of reference and be moving in relation to it, because that contrary movement puts the thing in a different inertial frame of reference. Is this correct?
 
That would be accurate, but given what he's been saying all along I don't think he meant that. Let's hope.
 
Ziggurat said:
What he might have been trying to say is that for any non-accelerating object, there's a unique inertial reference frame in which that object is stationary.
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That’s exactly what I meant (and what I thought I had said). :confused:

I’m not talking about whether things can be observed or measured from other inertial frames. Just that they can and do only actually exist in one inertial frame at a particular time (not two or more at once). So is that correct?
 
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ynot said:
That’s exactly what I meant (and what I thought I had said). :confused:

I’m not talking about whether things can be observed or measured from other inertial frames. Just that they can and do only actually exist in one inertial frame at a particular time (not two or more at once). So is that correct?
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Ummm, no. That is not correct.

These inertial frames aren't some form of parallel universes. They are just different frames of references with different points of view.

The Statue of Liberty exists, and it exists in the inertial frame of an observer standing at its base, and in the inertial frame of a passenger on a boat cruising past, and in the inertial frame of a pilot in a plane zooming by.
 
jsfisher said:
Ummm, no. That is not correct.

These inertial frames aren't some form of parallel universes. They are just different frames of references with different points of view.

The Statue of Liberty exists, and it exists in the inertial frame of an observer standing at its base, and in the inertial frame of a passenger on a boat cruising past, and in the inertial frame of a pilot in a plane zooming by.
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So if I’m in one vehicle travelling in one direction, I’m also in another vehicle that passes going in the other direction!?

ETA - Surely to be in the inertial frame of a vehicle, you have to be travelling with it?
 
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ynot said:
So if I’m in one vehicle travelling in one direction, I’m also in another vehicle that passes going in the other direction!?
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A vehicle has limited size. So you can be in it or out of it.

A reference frame is infinite in size. You can't be out of it. You can be stationary relative to it or in motion relative to it.

As a physical example, think of a pilot in a plane on an aircraft carrier. He's in the plane. He's also on the ship. As the plane is taking off, he's motionless relative to the plane and moving relative to the ship.
 
69dodge said:
A vehicle has limited size. So you can be in it or out of it.

A reference frame is infinite in size. You can't be out of it. You can be stationary relative to it or in motion relative to it.

As a physical example, think of a pilot in a plane on an aircraft carrier. He's in the plane. He's also on the ship. As the plane is taking off, he's motionless relative to the plane and moving relative to the ship.
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Are we talking reference frames or inertial frames, and is there a difference?
 
Let me try to explain what I mean in planet X language. A particular thing “at rest“ (non-accelerating) is in a particular “burp”. Anything and everything else in the universe that’s not moving relative to this particular thing is also in the same “burp”. Anything and everything that is moving relative to this particular thing are in different “burps”. A thing “at rest” in a particular “burp” cannot simultaneously exist in any other “burp”. A thing can’t be moving relative to a "burp" and still be in that “burp”, because that different movement puts it in a different “burp”.
 
ynot said:
Are we talking reference frames or inertial frames, and is there a difference?
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An inertial frame is a specific kind of reference frame, namely, one relative to which the law of inertia holds.
 
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