I Relativism & Reality: Finding Length in Space

[ewq]

Do you regard relativistic effects to be physically real or just apparent (kind of like optical illusion caused by distorted information one is receiving)?

To prevent endless entanglement in "define physically real" arguments imagine this: two spaceships moving inertially relative to each other in space. Both see the other one as shortened in direction of movement. But there is an alien from parallel universe who has the ability to shoot his laser measurer with infinite speed in our universe without affecting it in any way.

What would he say about the length of both ships? Would he find them to be the same?

 

[FactChecker]

They are real in every sense of the word. Remember that E=Mc² and the atomic bomb are consequences. If an alien could shoot a laser at infinite speed then he would be in a different physical world.

 

[vanhees71]

Of course the kinematic effects of relativistic spacetime are real. If they were not, nobody would use relativity in physics.

 

[Nugatory]

But there is an alien from parallel universe who has the ability to shoot his laser measurer with infinite speed in our universe without affecting it in any way.

If you start by assuming something that is contrary to the laws of physics, you will be able to draw conclusions that are contrary to the laws of physics... but that tells you nothing about how things work in a world in which the laws of physics must be obeyed.

However, there is another way you can attack that question about whether length contraction is "real": Search for "Bell's spaceship paradox" and work through it for yourself. That will surely raise some other questions about relativity works, and we're here to help you with those - but there's no way a physics forum can help you with a hypothetical involving parallel universes and infinite speed, so this thread is closed.

 

[Nugatory]

To Nugatory: don't close this thread because you don't like the sound of aliens and infinite speeds- it was just meant to illustrate the point.
In my view this question is fundamental for real understanding of physics and one of most neglected and misunderstood too!

So for all of you who answered "yes relativistic effects are real"- yes of course they are real in a sense that we can see them and measure them. However this is not what I am asking. I am asking if what we see or measure really corresponds to reality.

Sticking to original example: how can both ships be shorter than each other?

OK - hang on for a moment and the thread will reopen without the aliens and infinite speed...

 

[Nugatory]

dont close this thread because you don't like the sound of aliens and infinite speeds- it was just meant to illustrate the point.

Right, and the problem is that it doesn't illustrate the point, it confuses it. I guarantee you that if you introduce the concept of infinite velocity you will start to find contradictions, just as it's easy to find contradictions in algebra if you introduce division by zero. And the cure in both cases is the same - the contradiction is nature's way of telling you that you were mistaken when you thought it was a valid thing to do.

I'm asking if what we see or measure really corresponds to reality.

Yes, with the caveat that what we see may have to be corrected for the time it took for the light to get from whatever we're seeing to the retinas of our eyes. If there are two bombs placed one light-second away from me and two light-seconds away from me and I see two flashes of light at exactly 12:00:02 according to my wristwatch, I'll know that the nearer bomb went off at the same time that my wristwatch read 12:00:01 and the far one went off at the same time that my wristwatch read 12:00:00. But as long as we allow for this, yes, what we observe and measure is real, at least for purposes of science.

how can both ships be shorter than each other?

Ask that question in a thread of its own... But before you do, google for "Einstein relativity simultaneity" to find the classic thought experiment that demonstrates relativity of simultaneity (and we have about 83 bazillion threads on the subject as well). You need it to be able to understand and explain time dilation and length contraction and the apparently paradoxical result that both observers find the other ship to be shorter and the other clock to be running slow. Relativity of simultaneity is also what lies behind the impossibility of infinite speeds and instantaneous signalling.

 

[jtbell]

Another good one to Google for is "barn and pole paradox".

 

[vanhees71]

A very recent high-precision test of time dilation can be found here:

https://arxiv.org/abs/1703.04426

 

[FactChecker]

Some comments on reality versus perception regarding how both rockets can think the other is shorter:
1) Once you read and understand how the definition of "simultaneous" causes that effect, you will see that it is not only logical, but also surprisingly simple.
2) It was the reality of always measuring the speed of light as constant that lead to the theory of relativity. In fact nothing real that can be used to measure time conflicts with SR -- not atomic processes, cell growth, or anything.
3) Modern GPS satellites must take both SR and GR into account or the GPS location errors will be very bad. That is a real-world test of the theory that impacts everyone daily.

 

[ewq]

mhh... the usual non-answers, I heard them all already

They broadly fall into on of three different categories:
-i don't know the answer so i will pretend that this is such a basic question whuch is beneeth my knowledge level and will just point you to some high school textbooks
-they are real followed by an explanation why we measure what we measure but does nothing to address the actual question
-the question "what is real" can never be known because of finite speed of information propagation. As such question is pointless and "philosophical"

I am amazed that anyone can find that level of understanding satisfactory

 

[Nugatory]

I am amazed that anyone can find that level of understanding satisfactory

If that in fact reflected the current level of understanding, you'd be quite justified in considering it unsatisfactory. However, the current level of understanding is not what you seem to think it is, so your dissatisfaction may be misplaced. In particular, "the question 'what is real' can never be known because of finite speed of information propagation" is a complete misunderstanding of relativity.

So I will repeat the offer that we've already made twice: If you want to understand how reality can be what relativity says it is, despite counterintuitive things like mutual length contraction and time dilation, feel free to start threads asking those questions.

This thread, however, has reached its end - twice now - so is closed again.