# Position measurement

Hello PF members!

First of all, let me start off my post by admitting that I have created a duplicate account. My original account was banned for a good reason.
Now, I am studying physics. More specifically, I'm studying relativity. I trust that you all will help me understand what I am studying.

Anyways, I have a question. Einstein said that the speed of light is the same, no matter where you are or how quickly you are moving. However, due to the effect that velocity has on time, an observer traveling near the speed of light will think that another object is further away than it actually is. Because of this, he will disagree with a stationary observer on how far he actually is from the object. So how can the traveling observer calculate his actual distance correctly? Or is space-time set up such that the both calculations are correct, if you know what I mean?

## Answers and Replies

ghwellsjr
Science Advisor
Gold Member
Hello PF members!

First of all, let me start off my post by admitting that I have created a duplicate account. My original account was banned for a good reason.
Now, I am studying physics. More specifically, I'm studying relativity. I trust that you all will help me understand what I am studying.

Anyways, I have a question. Einstein said that the speed of light is the same, no matter where you are or how quickly you are moving. However, due to the effect that velocity has on time, an observer traveling near the speed of light will think that another object is further away than it actually is. Because of this, he will disagree with a stationary observer on how far he actually is from the object. So how can the traveling observer calculate his actual distance correctly? Or is space-time set up such that the both calculations are correct, if you know what I mean?
An observer traveling in a frame at near the speed of light will think that another object is closer than the frame says it is, not further.

Basically, according to SR, each observer uses radar methods to establish distances and assigns the time of the measurement to the average of when the radar pulse was sent and received. This results in a consistent method of measurement and allows each observer to construct a frame of everything out there, although each frame will be different but there is no way to say that one of them is actually correct and the others actually not correct, so yes, both are correct.

Nugatory
Mentor
Einstein said that the speed of light is the same, no matter where you are or how quickly you are moving. However, due to the effect that velocity has on time, an observer traveling near the speed of light will think that another object is further away than it actually is. Because of this, he will disagree with a stationary observer on how far he actually is from the object. So how can the traveling observer calculate his actual distance correctly? Or is space-time set up such that the both calculations are correct, if you know what I mean?

You will get nowhere in studying relativity if you do not clearly understand that the bolded text above is wrong. One way to see this is to consider that right now, even as we speak, you are moving at .99c relative to some observer somewhere - but does this mean that you think your desk is farther away from you "than it actually is"? Of course not.

You need to back up, try to forget everything that think you know about time dilation and length contract contraction. Study and understand Einstein's train experiment about the relativity of simultaneity, then the Lorentz transformations; only after that should you think about time dilation and length contraction, and you should do so by deriving them from the lorentz transforms.

berkeman
Mentor
Thread locked.