# A few questions on time

Hello,

If a human could travel at .9999c would their perception of time (in their own frame) be the same as the perception of time of a stationary observer (in their frame)?

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malawi_glenn
Homework Helper
yes of course

That's what I thought, but how is this known?

Time ticks by at a constant rate. At different speeds, the perceptions of the two times would be different. But in their own frame of reference they would be the same. If this wasn't true, then the equations on relativity would be incorrect. Since the majority of scientists trust the data from all of the experiments done on relativity, I think that we can believe that time would tick by the same at any speed in reference to someone going at that speed.

I know this isn't quite what you're looking for, but unless we send something that can take readings while going 0.999c, then I think we'll just have to trust science.

DaveC426913
Gold Member
...unless we send something that can take readings while going 0.999c...
We have ample data of such things.

Particles with a known decay rate, when accelerated to near light speed, show a time dilation (i.e. live longer) exactly in accordance with SR.

Extremely accurate atomic clocks flown on jetliners high in the atmo (where gravity is weaker) have shown a dilation of time exactly in accordance with SR.

GPS satellite data takes SR into account when measuring distances. If SR were wrong, the GPS data would be wrong.

We have ample data of such things.

Particles with a known decay rate, when accelerated to near light speed, show a time dilation (i.e. live longer) exactly in accordance with SR.

Extremely accurate atomic clocks flown on jetliners high in the atmo (where gravity is weaker) have shown a dilation of time exactly in accordance with SR.

GPS satellite data takes SR into account when measuring distances. If SR were wrong, the GPS data would be wrong.

I'm not questioning SR time dilation, though.

malawi_glenn
Homework Helper
what are you then asking about? you asked how observers would notice time when travelling at high speeds...

How is it known the observers (or travelers) mental perception of time is "in sync" with all other time dilation effects?

malawi_glenn
Homework Helper
the Mind is not a subject of physics, but if one has the view that the mind is just particles and forces described in principle by physical law, then it should be "in sync".

And let us just play here, HOW would he notice things different, if the mind was detached from the physics: if the observer moving was raising his arm, would he thinking "gosh, what a long time it takes to raise my arm" ??

the Mind is not a subject of physics, but if one has the view that the mind is just particles and forces described in principle by physical law, then it should be "in sync".

And let us just play here, HOW would he notice things different, if the mind was detached from the physics: if the observer moving was raising his arm, would he thinking "gosh, what a long time it takes to raise my arm" ??
I have no idea... Maybe their movement would seem restricted?

I was also curious if the travelers size would increase with velocity relative to the observer, and if that could change their perception of time as well.

Depending on what direction the traveler was going and relative to the observer, the traveler would get skinnier if he/she was going directly forward or backward, shorter if he/she were going up or down (relative to him/her), or a combination if in some other direction. However, the traveler wouldn't change in his/her reference point.

Also, the traveler would appear to be moving slowly (from the observer's reference). As to what that traveler would experience would be nothing more than if he/she weren't moving at all.

JesseM
I'm not questioning SR time dilation, though.
But whether you realize it or not, you're questioning the first postulate of relativity which says all laws of physics work the same in all inertial frames, and that along with the second postulate that light moves at c in all inertial frames is what all the predictions of relativity (including time dilation) are derived from. It's also possible to show that any law of physics will respect these postulates as long as its equations when expressed in one inertial frame have a mathematical property called "Lorentz invariance" (meaning the equation is unchanged under the Lorentz transformation which relates different SR frames), and if you look at all the most fundamental laws of physics with good experimental support that are known today, they do all have Lorentz-invariant equations.

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But whether you realize it or not, you're questioning the first postulate of relativity which says all laws of physics work the same in all inertial frames, and that along with the second postulate that light moves at c in all inertial frames is what all the predictions of relativity (including time dilation) are derived from. It's also possible to show that any law of physics will respect these postulates as long as its equations when expressed in one inertial frame have a mathematical property called "Lorentz invariance" (meaning the equation is unchanged under the Lorentz transformation which relates different SR frames), and if you look at all the most fundamental laws of physics with good experimental support that are known today, they do all have Lorentz-invariant equations.
Again, I'm talking about human time perception, and this probably isn't the appropriate place to discuss it unless SR talks about it specifically.

JesseM
Again, I'm talking about human time perception, and this probably isn't the appropriate place to discuss it unless SR talks about that specifically.
Unless you think human time perception isn't based on events happening in the physical brain which obey the laws of physics, then the first postulate should apply to human time perception too.

Maybe if you gave an specific problem related to your question we can be more helpful?

DaveC426913
Gold Member
Again, I'm talking about human time perception, and this probably isn't the appropriate place to discuss it unless SR talks about it specifically.
ALL things are affected.

you need to realize that you are right this very moment undergoing time dilation due to relativity. Do you notice anything out of the ordinary?

ALL things are affected.

you need to realize that you are right this very moment undergoing time dilation due to relativity. Do you notice anything out of the ordinary?

Heh.. No, but I could argue that there's nothing to compare the feeling of my frames "time dilation" to.... We've all pretty much evolved in it.

DaveC426913
Gold Member
Heh.. No, but I could argue that there's nothing to compare the feeling of my frames "time dilation" to.... We've all pretty much evolved in it.
If there is no way to detect something then how can one say exists at all?

We're talkin' invisible, intangible, backyard elves here...

If there is no way to detect something then how can one say exists at all?

We're talkin' invisible, intangible, backyard elves here...
Right, I guess that's why someone mentioned this is more of philosophy discussion... How could you measure a personal experience traveling at .9999c?

DaveC426913
Gold Member
Right, I guess that's why someone mentioned this is more of philosophy discussion... How could you measure a personal experience traveling at .9999c?
No. Nonono.

You are travelling at .9999c right now.

The whole point of relativity is that it's relative. There is no experiment you can do that can say that you are not moving at .9999c right now. Your velocity is only relative to a point of reference of your choice. You might choose the Earth as your frame of refence, but there is no way you can claim that the Earth is not moving at relativistic velocities.

And it's not that you "can't tell" if you've moving; it's that there is no such thing as an objective frame of reference. It is all relative.

Well, how about with gravitational time dilation effects instead of velocity? If you could feel the gravity of 100 suns (and the "time" in that frame), would this be considered relative as well?

BWV
hell my mental perception of time is different travelling at 0.0000007C on Continental Airlines

DaveC426913
Gold Member
Well, how about with gravitational time dilation effects instead of velocity? If you could feel the gravity of 100 suns (and the "time" in that frame), would this be considered relative as well?
Yes. The principle of equivalence.

If I put you in an elevator that was free-falling into a black hole, there is no experiment you could do, even in principle, to show that you were undergoing gravitational acceleration.

malawi_glenn
Homework Helper
No Dave, since there are no observers, as we know, with human perception observing us when we move relative to them at very high speeds. It might happen, that the human mind live in another "space-time" (parallel universe) similar to the aether where to mind is not affected by the body moving with respect to the minds space-time :-)

This is not a physics question, it is not a question of time either, it is a question if and how the mind "live" in the physical reality and is hence a subject of philosophy. Everything we way about the mind will be circular in some sense, since the mind is saying something of the mind which is saying something of the mind etc..

to Nuby, the traveller will shrink in its direction of motion according to the observer.

The point of SR is that there are no special reference frame, according to the travaller, it is the observer who shrinks and have time running slower. Time is local.

Another question you could ask is if conservation of momentum would be measured by a very fast rotation observer (we neglected SR here) - you are asking if the effect of a symmetry generation is perceptible when the symmetry action itself is being turned on.

russ_watters
Mentor
This is not a physics question, it is not a question of time either, it is a question if and how the mind "live" in the physical reality and is hence a subject of philosophy.
Philosophy is not just random idle thoughts vomited onto a keyboard. Philosophy is about facts and logic. And one cannot weasel out of a question that is most certainly scientific in nature by saying they are looking for a philosophical answer.

The question, as it is being presented, either has the simple, logical scientific and philosophical answer as already given or it is nothing but gibberish. A meaningless, pointless question with no answer at all because it isn't based in reality.