# Questions tagged [observers]

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### If “time is relative”, what does “the universe is 13 billion years old” mean? [duplicate]

Physcists say that the universe is approx. 13 billion years old. However, the amount of time that has elapsed since the big bang differs per object, dwpending on the trajectory that that object has ...
2answers
448 views

### Derivative of proper time w.r.t. time?

On page 86 of Theoretical Minimum (part 3 on special relativity) by Susskind, he writes: $$\frac{d\tau}{dt}=\sqrt{1-v^2}$$ Where $v$ is the velocity of a moving reference frame relative to the ...
1answer
77 views

### Observer problem in Relativity [closed]

Which clock or observer conclude that time dilation has occurred? Is it the stationary clock/observer or the moving one? (Please bear in mind that each observer/clock independently concludes that one ...
4answers
170 views

### In the twin paradox, are some events lost to the observer turning around?

From what I understand of the twin paradox, the time difference in time can be accounted for when the twin on a rocket ship turns around/switches reference frames. Does that imply that, events that ...
4answers
211 views

### Clarification of proper time and inertial frame of reference

“Standing beside railroad tracks, we are suddenly startled by a relativistic boxcar traveling past us as shown in the figure. Inside, a well-equipped hobo fires a laser pulse from the front of the ...
1answer
154 views

### At what moment will matter falling into a black hole affect its size?

Does the mass of matter falling into a black hole affect the size of an event horizon the moment it passes through it, or when it has been incorporated into the singularity?
3answers
326 views

### Einstein's train experiment with clocks

Referring to Einstein's train hit by lightning bolts thought experiment. What if, in the middle of the train, together with the moving observer there is a stopwatch, with 2 displays. One display ...
2answers
183 views

### Young and Freedman Proof: No Observer Can Travel at the Speed of Light

I have been reading the section on Relativity in the ninth edition of University Physics by Young and Freedman. They include the following proof that no observer can move at the speed of light. ...
1answer
204 views

### What would happen if we put Schrödinger inside the box and the cat opened the box door? [closed]

Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen ...
2answers
219 views

### Solving the ladder paradox: Minkowski distance leads to a contradiction

Consider the ladder paradox; Let the length of the barn in its fixed frame be $80m$, and consider the ladder's reference frame which is moving toward the barn with a speed $v$, and in ladder's frame ...
1answer
173 views

### Question on Painlevé Gullstrand coordinates

A known set of coordinates used for the Schwarzschild metric is the Painlevé-Gullstrand coordinates. They consist in performing a change from coordinate time $t$ to the proper time $T$ of radially ...
3answers
134 views

### Special Relativity-Simultaneity

If two events are simultaneous in one reference frame, are they simultaneous in all reference frames? Please provide thorough qualitative explanation If blue light is going at speed $c$, with red ...
1answer
313 views

### Length contraction in GR

In general relativity, $\sqrt{1-v_{e}^2}$ (with $v_e$ the escape velocity as a fraction of the speed of light) is the factor by which time is dilated in a gravitational field. Is it also the factor by ...
2answers
733 views

### Determining the rest frame

Sorry if this has been asked before in some way, I did try looking through the special relativity tag and didn't really see a good answer to my question. I am having trouble determining which frame ...
1answer
92 views

### What if you pointed a laser away from a black hole while falling into it?

Often this is asked if it was pointed towards a black hole. so I thought, if a scientist decided to sacrifice himself and jumped in a black hole, released some dry-ice mist and pointed a laser pointer ...
6answers
623 views

### What does the accelerated twin see in the twin paradox?

I am referring to the usual twin "paradox" where one twin remains on Earth while the other one takes a round trip journey on a rocket. I understand how the experience of the twins is not symmetrical, ...
1answer
68 views

### Observer independent quantities in special relativity

I have been thinking about what things about a point particle do all observers agree about? And I thought trajectories of particles must be the same for all observers, right? But, clearly it is not. ...
2answers
136 views

### Length contraction of a train

Suppose a train of length $3x~\rm m$ and a tunnel of length $x~\rm m$. When the velocity of train is $282842712.5~\rm m/s$ (from length contraction equation), then its length contracts to $x~\rm m$. ...
0answers
88 views

3answers
213 views

### What does “same location” mean in special relativity (definition of proper time), when true length does not exist?

The following statement combined with the application of time dilation principle confused me on how proper length and time are measured and what they represent. "The speed at which electrons traverse ...
2answers
206 views

### Is relativity of simultaneity just a convention?

Lorentz transformations are well known to imply time dilation, length contraction, and relativity of simultaneity. This is prominently featured in any course on Special Relativity (SR), e.g. in ...
3answers
340 views

### Special Relativity Paradox based on Einstein's explaination [closed]

Einstein explained the special relativity (roughly) like this: He first assumed the speed of light would not change regardless the speed of its source. Then he described a person on a moving train was ...