but how would you experience time, if it wasn't by measuring physical activity?

For a guy in a rocket getting faster and faster, the speed of a clocks hands and its ticking doesn't change, it only changes for an observer of the rocket looking though the window of the rocket....the observer will see eg it take a clock's second hand maybe 10minutes to make its click to the next second.

If he could observe the guy/s in the rocket, he would see them move in slow motion, but for the people in the rocket, nothing would have changed, and everything would appear as usual.

but how would you experience time, if it wasn't by measuring physical activity?

My premise is that time is constant; it does not speed up, nor slow down. So, if we are using measuring devices to indicate time, those devices have to be constant. So, when someone travels at the speed of light away from a clock, and sees that the hand on the clock has stopped moving, they know that time hasn't really slowed down; it appears that the clock hand has slowed down but, in reality, the clock hand is still moving at a constant rate (assume for the sake of the thought experiment that it's a perfect clock that is not prone to mechanical failure), it's just that the light from the clock hand will never reach you.

Yes, well you can argue that time is some kind of objective thing, but it is clocks, and balls bouncing around etc that we actually experience and scientists can measure.

Scientists like Einstein set up thought experiments, like a scientist on a rocket, with a clock, and a scientist as an observer of the rocket with his own clock.

The observer see that the hands on the clock on the rocket moving more slowly, and the observer on the rocket sees the clock the observer has as moving more quickly....but both observers see their own clock as working as usual....according to the special relativity theory.
Which is one of the reason for the 'relativity' part in the name of the relativity theories.

The observer's experience depends upon his situation, and isn't necessarily the same to another observer's..

but how would you experience time, if it wasn't by measuring physical activity? — wax

Through an internal intuitive experience. In same way that motion of the body (raising your arm for example) is experienced from within. It does not require an outside observer. You experience time tacitly by existing.

So, if we are using measuring devices to indicate time, — philosophy

What are you really measuring with those devices? For example, doesn't a traditional clock really measure the motion of its gears? Doesn't a sundial really measure the motion of the sun relative to a shape that produces a shadow? Etc.

yes, I know what you mean; your own experience is a perception of your arms moving as an example....but for the outside observe, who is not travelling close to the speed of light, relative to the rocket, his experience of you moving your arms would be that you would be moving them slowly.

Say you are on the rocket and bouncing a ball off the wall....the outside observer will see you slowly throwing the ball, and then the ball moves slowly towards the wall, bounces off slowly, and returns to your hands, as you slowly reach out to catch the ball.....for you on the rocket, all seems as usual, and you are just bouncing the ball off a wall, like you might have done a thousand times before you got off the rocket.

The conclusion of relativity is that if you were in a room in a rocket, without a window, you wouldn't be able to tell if you were stationary to an outside observer, or travelling at close to the speed of light, relative to the observer.

But isn't the key word in all this appearance? The ball appears to travel slowly for the observer on Earth, but it doesn't follow from this that the ball is really travelling slowly.

You experience time tacitly by existing. — emancipate

This is interesting. So, if a person were locked in a room with nothing in that room, would that person still retain a sense of the passage of time?

But isn't the key word in all this appearance? The ball appears to travel slowly for the observer on Earth, but it doesn't follow from this that the ball is really travelling slowly. — philosophy

but what do you mean when you talk about the 'real' speed of the ball?

Is there somehow an objective speed of the ball?

This is interesting. So, if a person were locked in a room with nothing in that room, would that person still retain a sense of the passage of time? — philosophy

the person might still be able to see their own body as they move it...if they are blindfold, then all they may have is their own thought processes....but you might have experienced that time seems to pass very quickly sometimes, and very slowly sometimes...if say you are bored..

Is there somehow an objective speed of the ball? — wax

I get your point here. But what if we had a universe with no observers? If motion is relative to an observer, would it not follow that there would be no motion in said universe?

I get your point here. But what if we had a universe with no observers? If motion is relative to an observer, would it not follow that there would be no motion in said universe? — philosophy

Maybe one could argue that there would be no motion, on the other way of looking at it, everything could happen at once..!

Is there somehow an objective speed of the ball? — wax

I think so, yes. Wouldn't the objective speed of the ball be the speed of the ball as it is, independent of observation? The moment we observe the ball we are doing so from a perspective. If the light rays travelling from the ball take longer/shorter to reach me than they do you, then we reach different perspectives regarding the motion of the ball. But why should we tie the motion of the ball to the distance that light rays travel? This seems arbitrary. There is an objective motion to the ball but it cannot itself be observed.

I think so, yes. Wouldn't the objective speed of the ball be the speed of the ball as it is, independent of observation? The moment we observe the ball we are doing so from a perspective. If the light rays travelling from the ball take longer/shorter to reach me than they do you, then we reach different perspectives regarding the motion of the ball. But why should we tie the motion of the ball to the distance that light rays travel? This seems arbitrary. There is an objective motion to the ball but it cannot itself be observed. — philosophy

I think part of the problem here is the idea that the ball is some kind of object that isn't affected by things like the speed of light....maybe I have put that badly, but if you think about what a ball actually is, it is bunch of atoms, made of fields, and these fields transmit the information and fields relevant to the ball at the speed of light....so I suppose you could say the ball itself doesn't exist in an objective way either.

Indeed in special relativity the ball would be seen by the outside observe as squashed in the direction of travel....which is called 'length contraction', in SR.

if you think about what a ball actually is, it is bunch of atoms, made of fields, and these fields transmit the information — wax

But this still means that there is something that transmits this information. We can distinguish between that which transmits and that which is transmitted. By definition, we can only measure that which is transmitted. But wouldn't that which transmits be the underling, objective basis for what we call the ''ball''?

But this still means that there is something that transmits this information. We can distinguish between that which transmits and that which is transmitted. By definition, we can only measure that which is transmitted. But wouldn't that which transmits be the underling, objective basis for what we call the ''ball''? — philosophy

I don't think science really is at a place where it can say what actually transmits information, but it seems that however this transmission process works, it is measured as being at the speed of light...

Reference frames.

The thought experiment grounding Special Relativity as given in “The Electrodynamics of Moving Bodies” and demonstrated with mathematics, presupposes a third party observer of both the clock on the train and the clock on the platform. This observers sees both the reference frame of the moving passenger and the reference frame of the stationary perspective simultaneously. Or, which is the same thing, the third party observer witnesses the simultaneity of lightening bolts the two other references frames cannot distinguish. When technology advanced far enough to put the mathematics to the test, the clocks in the two reference frames showed the mathematical predictions to be correct, insofar as the respective clocks showed different elapsed times.

Nevertheless, there can be no third party observer for any physical experiment whereby velocities are sufficient for relative time dilation. In other words, no one is going to see the relative speeds of clock movements; all they can do is compare two clocks after viewing them in one reference frame or the other.

personally, I don’t think this changes much of anything. Time dilation is of course a natural phenomenon, but as far as we’re concerned right now, we cannot move at speeds fast enough for the differences in relative times to be noticeable.

My intuition here, of course, is that time is a constant which is independent of any measuring devices and the conditions in which said devices operate.

Or have I missed something? — philosophy

Take a look at the twin paradox thought experiment. To magnify the example, in principle, a twin could depart in a spaceship, travel at close to the speed light for a few years, return to Earth and find that a million years had passed on Earth, with human civilization having died out.

So the time elapsed according to a clock on the spaceship and another clock on Earth would be very different. What they are measuring is the time elapsed in their respective reference frames.

Time dilation is not just an appearance as was demonstrated by the Hafele-Keating experiment

It seems that few posters know their theory very well.

Einstein reasoned that if he were to travel at the speed of light then clocks would appear to stop moving (since the light from the clocks would never reach him). Einstein concluded that time slows down the faster to the speed of light you travel. — philosophy

Einstein did not reason thus, nor did he conclude that time slows for any observer. Anybody will observe their own clock (one in their presence) to run at the normal rate, regardless of speed relative to other objects.

In other words, time is not the same as a measuring device (e.g. a clock).

On the contrary. Time, to Einstein at least, is exactly what clocks measure. If one twin is younger than another, it is because it has been less time since birth for that twin than the other. But if those twins are moving relative to each other, then each twin ages slower than the other one in his own frame, which means the twin that stayed home ages at a reduced pace both in the inertial frame of the departing twin and in the inertial frame of the returning twin.

Could it be that Einstein's theory is just another ''approximation'' of reality like Newton's theory? I mean we've reached some kind of observational barrier in that the fastest thing we can hope to observe is EM waves. Therefore, our theories must be limited by it. What if there's ''other'' stuff that exist that would require a new theory which I guess would also be ''just'' another approximation.

Time doesn't slow down for the one traveling. Time is constant for the observing traveler, but for anyone looking at the traveler, they would see his time as slower. The traveler would see others speeding up. If the traveler reaches the speed of light, he would end up at the location traveled to instantly, meaning, the traveler would end up at a point outside of normal spacetime, since the traveler cannot observe anything if the destination and time to get there is instant and infinite at the same time. Look into how a black hole would work theoretically based on Einstein and Hawkings.

Could it be that Einstein's theory is just another ''approximation'' of reality like Newton's theory? — TheMadFool

yes and every other theory in physics you have heard is an approximation of reality by definition. At the core all physics is is a mathematical model of some reality or observation. By definition this is an approximation. Some of them are very very very good approximations, some not quite as good.

Therefore, our theories must be limited by it. What if there's ''other'' stuff that exist that would require a new theory which I guess would also be ''just'' another approximation. — TheMadFool

could quite possibly be the best single quote about science i have read on the board.