Dec 022017

I thought I understood this but the last several times I have thought it through I reach the same sticking point. I will paste in a typical layman explanation from a PhD in Physics below and respond with my questions.

Is it true that a clock ticks more slowly when it moves at a higher speed? Why?

5 Answers
Giordon Stark
Giordon Stark, PhD Physics, University of Chicago (2018)
Answered Mar 15 2014
Ugh. so let me see if I can try and settle a lot of misconceptions here. I do want to point out that this is one of the most common misconceptions with special relativity — assuming simultaneity makes sense (ie: relativity of simultaneity). Disclaimer: I’ll attempt to be as correct/precise as possible, while still trying to make it clear from a laymen’s perspective.

Special relativity comes with two postulates – but let’s focus on only one: the speed of light is a constant in all inertial frames. Let’s call this constant… cc.

Mary’s sitting pretty over at the coffee shop when John squeals his tires and rushes past Mary, on his motorcycle, around 0.8c0.8c (80% of the speed of light). Mary knows that she can turn on a flashlight and light travels at cc. John can also turn on his headlights and see the light coming out at cc. Mary can also see his headlights as well. So what gives? This is, clearly, non-intuitive. Light doesn’t behave the same way that physical, massive objects do, so we have a hard time sort of reconciling that sort of idea.

In order to resolve this, both time and distance have to “warp” or “transform” in a rather non-intuitive way as well. That is, for both observers to agree on the speed of light traveling at cc, what they use to measure it must be different in their frames (in a way, although this may not necessarily be true). To Mary, John appears shorter along the direction of motion, but his watch also ticks slower. Likewise, to John — Mary’s table appears shorter and her watch also appears to tick slower. This is relativity — what is observed depends solely on the inertial frame of the observer!

So how can both people observe the other’s watch ticking slower? The answer is because both people are making measurements at different times and you cannot compare the observations. That is, in order for us to compare measurements of something, we must have some notion of “simultaneity” — but this something unique to the inertial frame of the observer. For example — measuring the length of an object implies that we observe both ends of the object at the “same time” — but if I transform into a moving frame, then I can observe the object as shorter because the light traveling from both ends reaches my eye at different times.

For more information — read up on relativistic phase here; Special Relativity/Simultaneity, time dilation and length contraction


So my question here is whether or not this is a mere measurement problem – an illusion if you will. Because I can use language in a similar way to describe the size of a building changing when I move further away from it. Objects APPEAR smaller in the distance. We know this intuitively. Why is the situation described above not the same sort of thing? i.e. objects in motion APPEAR length contracted?

In other words, just like we know that the building did not shrink in size, don’t we also know that the object in motion did not actually contract in length? In what way is Special Relativity saying something more than that we need to adjust our measurements in these “special” situations?

The word special here is perhaps instructive. We don’t experience relativistic motion in our daily lives but what if we did. i.e. what if the special in Special Relativity were not special, but something we encounter every day – would it not be common sense that objects in motion APPEAR a different size than they are? Wouldn’t we just adopt this common sense reality as an illusion in the same way we think about the illusion of objects APPEARING smaller in the distance?

I feel like I understand why objects in the distance APPEAR smaller, and I feel like I understand why objects traveling near the speed of light APPEAR length contracted, but this understanding in no way leads me to accept that objects in the distance are ACTUALLY smaller or that objects in motion ACTUALLY contract.

What am I missing here?

In the past I would have responded to myself by saying, well if this theory were not accurate GPS would not work, yada, yada, yada. But that misses the point of the question. I am not challenging the claim that an adjustment to our measurements must be made since we are in a different intertial reference frame. Rather, what I am suggesting is that the reason this adjustment is needed is NOT because a physical object changed size. It is needed in the same way that I would need to make an adjustment to my measurement of the height of a building were I to stand back from it 100 yards and hold a ruler up to measure it’s APPARENT height.

Now, I am more than willing to accept the veracity of the claim. I trust that scientists are correct. But I am still in want of a better explanation. The explanation above does NOT do it for me. It explains why things APPEAR this way. But the claim is about more than appearances. And this is where I get stuck. I am looking for the explanation that takes me beyond appearances.

 Posted by at 3:45 pm

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