Relativity - Oh dear, here we go again!

[Paulhoff]

If a body moves towards me, what will happen to its size relative to me?
If a body moves away from me, what will happen to its size relative to me?

What do you mean by size relative to you?

Paul

 

[BillyJoe]

What do you mean by size relative to you?

Exactly what he said.

 

[Paulhoff]

Exactly what he said.

Sorry BillyJoe, but I want what he means by it and not what you or I think he means.

Paul

 

[BillyJoe]

You mean like:
Relative to you, I'm bigger, but, when I move away from you, I'm relatively even bigger?

 

[Paulhoff]

You mean like:
Relative to you, I'm bigger, but, when I move away from you, I'm relatively even bigger?

Sorry BillyJoe, I want to hear what he has to say, not you.

Paul

 

[Ali]

If there’s time dilation when a body moves could the following be true:
A frame of reference is moving relative to my frame of reference;
Then if I look at the other frame of reference, will the events inside it be happening in slow motion relative to me, as compared if I was in the same frame of reference?

 

[Reality Check]

If there’s time dilation when a body moves could the following be true:
A frame of reference is moving relative to my frame of reference;
Then if I look at the other frame of reference, will the events inside it be happening in slow motion relative to me, as compared if I was in the same frame of reference?

That is correct. Time dilation only depends on the relative velocity between the reference frames. If you have 2 reference frames with a velocity difference then an observer in either reference frame will see the same time dilation in the other reference frame. See the equation for time dilation and note that the Lorentz factor (gamma) has velocity squared in it (it does not depend on the sign of v).

 

[BillyJoe]

Sorry BillyJoe, I want to hear what he has to say, not you.

I was joking. Perhaps I used the wrong smily.

 

[Ali]

That is correct. Time dilation only depends on the relative velocity between the reference frames. If you have 2 reference frames with a velocity difference then an observer in either reference frame will see the same time dilation in the other reference frame. See the equation for time dilation and note that the Lorentz factor (gamma) has velocity squared in it (it does not depend on the sign of v).

Thanks Reality Check,
I now see that velocity sign doesn't count.

 

[ynot]

Relativity = Let’s Pretend?

New thread about same topic merged into this thread.

Replying to this modbox in thread will be off topic  Posted By: Darat

Things can be stationary relative to other things but there is no evidence that anything can ever be absolutely stationary. In other words, things that are stationary in a relative sense are always moving in an absolute sense

A thing isn’t stationary relative to itself as a thing can’t be relative to itself. A thing IS itself. A thing is always constantly moving relative to the previous positions of itself.

No absolute measure of speed can be attributed to anything and it can’t be correctly defined that acceleration represents an absoute increase or decrease in speed.

On what credible scientific basis does Relativity define that one thing is stationary and another is moving; and that one thing is moving either faster or slower than another? Is Relativity based on anything better than “let’s pretend”?

 

[lionking]

Eh, didn't you do this before, and got soundly beaten up?:

http://www.internationalskeptics.com/forums/showthread.php?t=84948

What makes you think it will be any different this time around?

 

[arthwollipot]

Oh, this is going to be good.

 

[sol invictus]

Things can be stationary relative to other things but there is no evidence that anything can ever be absolutely stationary. In other words, things that are stationary in a relative sense are always moving in an absolute sense

Almost right.

A thing isn’t stationary relative to itself as a thing can’t be relative to itself. A thing IS itself. A thing is always constantly moving relative to the previous positions of itself.

Nonsense.

No absolute measure of speed can be attributed to anything and it can’t be correctly defined that acceleration represents an absoute increase or decrease in speed.

Good thing it isn't and it's not.

On what credible scientific basis does Relativity define that one thing is stationary and another is moving;

It doesn't.

and that one thing is moving either faster or slower than another?

It doesn't.

Is Relativity based on anything better than “let’s pretend”?

Yes.

 

[HansMustermann]

Ynot, "relative to" means pretty much: how would it look to someone sitting in that point?

A train is stationary relative to itself, pretty much just means that to somebody on the train it doesn't look like train is moving. It looks like the scenery outside is moving.

Even if you want to talk about yourself relative to yourself... sit still on the bus, and, well, look at yourself. Does it look like your body is getting any farther or closer? No. Although to someone outside it looks like you're zipping past them. when _you_ look at yourself, you're just sitting there. That's really the layman version of "relative to itself."

Now let's take two guys in cars, playing the ancient game of chicken. They're moving towards each other at 100 miles per hour each. Now to each of them, it looks like he's sitting still, and the other one is coming at him at 200 mph.

The gist of it is that the maths is the same. If they start at a mile from each other, they'll collide in the same 0.005 hours ( = 0.3 minutes = 18 seconds) either way. You could calculate relative to the point in the middle, and each does half the distance at 100 km/h. Or you can calculate relative to one of them, and the other is coming at 200 mph and doing the full distance. It's still 18 seconds either way.

Now you could argue that only the former is the real thing, and the latter is "let's pretend". Actually both are "let's pretend". The former pretends that that point in the middle of the road is stationary, but in reality it's moving too, because it rotates around the centre of the Earth, which in turn revolves around the sun, which in turn orbits the centre of the galaxy, etc.

That's really all there is to it. It's doing it all from the point of view of someone sitting in the point you chose at reference.

Also note that so far we've not actually done any relativity. All this is actually just classic mechanics. There too you can put your reference point wherever you want it.

 

[Dorfl]

Things can be stationary relative to other things but there is no evidence that anything can ever be absolutely stationary. In other words, things that are stationary in a relative sense are always moving in an absolute sense

A thing isn’t stationary relative to itself as a thing can’t be relative to itself. A thing IS itself. A thing is always constantly moving relative to the previous positions of itself.

No absolute measure of speed can be attributed to anything and it can’t be correctly defined that acceleration represents an absoute increase or decrease in speed.

On what credible scientific basis does Relativity define that one thing is stationary and another is moving; and that one thing is moving either faster or slower than another? Is Relativity based on anything better than “let’s pretend”?

It makes testable predictions, which turn out to be correct to greater accuracy than any simpler theory (which would have to be very simple indeed) is capable of. So yes, it is based on the best basis any scientific theory can have.

 

[Dorfl]

Wait, what just happened? I could swear that today's posts were the only ones on the list when I wrote my reply, but now there's 14 pages! Am I just imagining things, or did two threads get merged, or what?

 

[paximperium]

Wait, what just happened? I could swear that today's posts were the only ones on the list when I wrote my reply, but now there's 14 pages! Am I just imagining things, or did two threads get merged, or what?

1 page to 14pages? Its all relative man. The same BS is relatively the same now as months ago.

 

[Cuddles]

On what credible scientific basis does Relativity define that one thing is stationary and another is moving; and that one thing is moving either faster or slower than another?

That's pretty much the exact opposite of what relativity does.

 

[Dorfl]

1 page to 14pages? Its all relative man. The same BS is relatively the same now as months ago.

Whatever man.

 

[Fredrik]

Things can be stationary relative to other things but there is no evidence that anything can ever be absolutely stationary. In other words, things that are stationary in a relative sense are always moving in an absolute sense

You seem to think that "X is not stationary in an absolute sense" means the same thing as "X is moving in an absolute sense". Those two statements are very different.

A thing isn’t stationary relative to itself as a thing can’t be relative to itself. A thing IS itself. A thing is always constantly moving relative to the previous positions of itself.

Huh!? Yes, a thing "is itself", but that doesn't make it wrong to say that its velocity relative to itself is zero.

Is Relativity based on anything better than “let’s pretend”?

The answer is definitely yes, as others have pointed out. Relativity does however contain a "let's pretend" part*. All theories do. So if you consider that a problem for relativity, you're really rejecting all of science.

*) In this case, we "pretend" that spacetime can be represented by a particular mathematical model, and that we can interpret the mathematics as predictions about the results of experiments according to a specific set of rules. If the predictions agree with the experiments to a high degree of accuracy, then we feel that we can understand something about the real world by understanding the mathematical model.