# Time and speed and how speed impacts time

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Excuse my ignorance first of all. I'm just a guy who never really paid attention in any science class, but now, a bit later in life, I've become very interested.

So, (and I may even be starting from a poor understanding) but from what I get, speed, especially light speed, if you're travelling at that speed, has an impact upon your experience of time.

Before I go any further, just want to be sure I have that right!

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Kind of.

You can't reach it, but as you(or something else) approaches the speed of light(c), the time dilation/length contraction effect will become increasingly apparent.

Edited by Endy0816
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Light speed is clearly hypothetical Endy0816. What I'm interested in is the time dilation/length contractioneffect

Say you leave Earth at the speed of light. When you leave Earth you'll see their clock in freeze frame, right?

What fascinates me y'all is how this idea of time might not be true

what you think of time isn't true

Edited by Estranged
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3 hours ago, Estranged said:

Say you leave Earth at the speed of light. When you leave Earth you'll see their clock in freeze frame, right?

Certainly, if you leave Earth at near the speed of light, you will see their clocks running slow. And they will see yours running slow.

It doesn't really make sense to ask what will happen at the speed of light because it is non-physical: if you try and apply the theory, you end up dividing by zero and similar meaningless things.

3 hours ago, Estranged said:

What fascinates me y'all is how this idea of time might not be true

Not sure what you mean by "true" but it does tell us that time (and distance) are not fixed measurements, they depend on who does the measuring.

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5 hours ago, Estranged said:

has an impact upon your experience of time.

You have to be careful here (and in Science generally) since this has more than one meaning.

For any observer time passes normally for herself.

She just witnesses it passing differently for others who are travelling at (any) speed relative to her.

If they are travelling at the same speed together, however great, no effect will be witnessed. The relative speed is zero.

The any effect observed will depend on the relative speed and is so small at to be unobservable at low relative speeds.

That is why we say there is an effect at high relative speed.

Does this help?

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Good catch Studiot

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8 hours ago, Strange said:

Certainly, if you leave Earth at near the speed of light, you will see their clocks running slow. And they will see yours running slow.

It doesn't really make sense to ask what will happen at the speed of light because it is non-physical: if you try and apply the theory, you end up dividing by zero and similar meaningless things.

Not sure what you mean by "true" but it does tell us that time (and distance) are not fixed measurements, they depend on who does the measuring.

Time is not a fixed measurement in the sense that weight is not a fixed measurement. Right? I don't weigh the same in space, or on another planet, because there are gravitational differences.

Is it fair to say that time and space and distance are not fixed measurements, but they are measurements that can be fixed under certain conditions, like weight? A minute is always 60 seconds just like a pound is always 16 ounces. Those are relative fixations, no?

But it seems like time is more easily fixed than lots of other measurements. If I'm travelling at light speed and looking back at a clock that stays where I started, my perception of time doesn't necessarily change, just my perception of that particular clock. If I have a clock with me, it still goes at 60 seconds a minute. Is that right?

If I travel at the speed of light away from the Earth for a year, then travel back to Earth at the speed of light for the next year, I will have aged two years, just the same as the people who stayed on Earth all along, right?

That's where I get really confused because it seems that everything that I read and watch says that travelling at the speed of light, or any really high speed approaching that of light, makes time actually pass differently to the extent that the person travelling at high speed would actually age differently. No matter how hard I try to understand that concept it makes no sense. Where am I going wrong? Or am I going wrong?

Isn't asking what happens at the speed of light what Einstein did? The speed of light is just a speed, the fastest speed we can measure, but it's still just a speed. At least that's the way it seems to this layman.

And "true" was probably a bad word to use.

Next of my lay musings is a consideration of travelling faster than the speed of light. In such a situation it seems to me that things behind me would go dark because no light can reach me. If a sound occurs behind me, yet I'm travelling faster than the waves of that sound, then I can't hear it. Why should light behave differently than sound?

Thanks for humoring me y'all.

Edited by Estranged
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18 minutes ago, Estranged said:

Time is not a fixed measurement in the sense that weight is not a fixed measurement. Right? I don't weigh the same in space, or on another planet, because there are gravitational differences.

Interesting analogy.It doesn't quite work because you will feel a different weight on Earth and on the Moon. But you won't feel time run slower because of relative speed (or gravitational time dilation).

27 minutes ago, Estranged said:

If I'm travelling at near light speed and looking back at a clock that stays where I started, my perception of time doesn't necessarily change, just my perception of that particular clock. If I have a clock with me, it still goes at 60 seconds a minute. Is that right?

Exactly. (Apart from the "at light speed" bit!)

28 minutes ago, Estranged said:

If I travel at near the speed of light away from the Earth for a year, then travel back to Earth at near the speed of light for the next year, I will have aged two years, just the same as the people who stayed on Earth all along, right?

Not quite. You will have aged two years, but for the people on Earth a lot more time will have passed.

31 minutes ago, Estranged said:

That's where I get really confused because it seems that everything that I read and watch says that travelling at the speed of light, or any really high speed approaching that of light, makes time actually pass differently to the extent that the person travelling at high speed would actually age differently.

So if we ignore the "returning to Earth bit" let's just say you are in a spaceship passing the earth at high speed. You will see clocks running slower on Earth (and the people aging less than you). They will see your clocks running slow (and, hence, you aging less).

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This is where I get lost in the twin paradox, right at the beginning..."Relativity dictates that when he comes back, he is younger than his identical twin brother."

My problem is that I don't understand relativity to create this kind of aging difference between the brothers. It makes no sense to me. He travels at near the speed of light and returns. For him, the traveler, time has passed just as it passed for his twin brother. I get that time on Earth for the brother in flight APPEARS to be going more slowly, but how does that translate into time for the brother in flight ACTUALLY moving more slowly?

I'm so confused by that. It plagues me and makes me feel stupid that I don't get it, and I like to think of myself as pretty smart guy, and I'm starting to think that it might just be wrong, which in turn seems equally ridiculous to me, because all physicists seem to just assume the theory of relativity and start with the premise that the twin coming back would have aged differently. Not one thing I've ever read seems to really explain why, and I've read a lot about it. It's just very frustrating.

Edited by Estranged
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14 minutes ago, Estranged said:

This is where I get lost in the twin paradox, right at the beginning..."Relativity dictates that when he comes back, he is younger than his identical twin brother."

My problem is that I don't understand relativity to create this kind of aging difference between the brothers. It makes no sense to me. He travels at near the speed of light and returns. For him, the traveler, time has passed just as it passed for his twin brother. I get that time on Earth for the brother in flight APPEARS to be going more slowly, but how does that translate into time for the brother in flight ACTUALLY moving more slowly?

I'm so confused by that. It plagues me and makes me feel stupid that I don't get it, and I like to think of myself as pretty smart guy, and I'm starting to think that it might just be wrong, which in turn seems equally ridiculous to me, because all physicists seem to agree that the twin coming back would have aged differently. Not one thing I've ever read seems to really explain why, and I've read a lot about it. It's just very frustrating.

What you are probably not realising is that they both have clocks and the time on the returning twin's clock will be earlier than the Earthbound twin's when they meet again. This is a real effect and has been measured. Check out the Hafele -Keating experiment. The only difference is that the differences are large in that twin scenario because the speeds are relativistic whereas in that H-K experiment, the difference of the results is micoseconds. using nanosecond-accuracy clocks.

Edited by StringJunky
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14 minutes ago, StringJunky said:

What you are probably not realising is that they both have clocks and the time on the returning twin's clock will be earlier than the Earthbound twin's when they meet again. This is a real effect and has been measured. Check out the Hafele -Keating experiment.

I've heard of that experiment, but how does the clock discrepancy translate to the twin paradox -- the assumption that the brothers age differently? The clocks going differently doesn't automatically mean there'd be a difference in age if you travel around with that clock or not, right?

Edited by Estranged
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1 minute ago, Estranged said:

I've heard of that experiment, but how does the clock discrepancy translate to the twin paradox -- the assumption that the brothers age differently?

In both scenarios they illustrate the difference in elapsed time between different frames. The frame you measure from is always stationary (or inertial), as long as it's a constant rate of motion.

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19 minutes ago, Estranged said:

I've heard of that experiment, but how does the clock discrepancy translate to the twin paradox -- the assumption that the brothers age differently? The clocks going differently doesn't automatically mean there'd be a difference in age if you travel around with that clock or not, right?

Clocks measure the elapsed time, so I don't see how you avoid aging at the rate the clock indicates.

The paradox is that the time dilation is symmetric - but the symmetry is broken by the acceleration of the twin who leaves the earth.

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7 minutes ago, swansont said:

Clocks measure the elapsed time, so I don't see how you avoid aging at the rate the clock indicates.

The paradox is that the time dilation is symmetric - but the symmetry is broken by the acceleration of the twin who leaves the earth.

Can we say that the accelerating twin is in a stronger gravitational field, or equivalently?

Edited by StringJunky
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52 minutes ago, StringJunky said:

Can we say that the accelerating twin is in a stronger gravitational field, or equivalently?

We're ignoring gravity in this scenario.

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2 minutes ago, swansont said:

We're ignoring gravity in this scenario.

OK. i was just wondering if that was a possible explanation for the asymmetry.

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1 hour ago, swansont said:

Clocks measure the elapsed time, so I don't see how you avoid aging at the rate the clock indicates.

The paradox is that the time dilation is symmetric - but the symmetry is broken by the acceleration of the twin who leaves the earth.

How much of a difference will it make to the time discrepancies if the accelerations required for departure,turnaround and homecoming are maximized or minimized?

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2 hours ago, Estranged said:

I've heard of that experiment, but how does the clock discrepancy translate to the twin paradox -- the assumption that the brothers age differently? The clocks going differently doesn't automatically mean there'd be a difference in age if you travel around with that clock or not, right?

This was very hard for me to wrap my mind around as well. Its not just the clocks everything is moving at a different rate including ageing processes.

If you look out at earth from your spaceship traveling at near light speed not only will you see clocks moving slower but the people will be moving slowly as well.

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Just now, Outrider said:

This was very hard for me to wrap my mind around as well. Its not just the clocks everything is moving at a different rate including ageing processes.

If you look out at earth from your spaceship traveling at near light speed not only will you see clocks moving slower but the people will be moving slowly as well.

What I found hard to accept was that both parties saw the others' clocks (and general movements) as slower than each others'  .

Also bizarre was the finding that when the mutually moving parties were reunited the time difference accumulated did not disappear to zero.

So no "optical illusion" ,but a stark reality born out by experimental observation.

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2 minutes ago, geordief said:

What I found hard to accept was that both parties saw the others' clocks (and general movements) as slower than each others'  .

Yep that too but the first thing I had to loose was the notion that there was something special about the clocks.

4 minutes ago, geordief said:

So no "optical illusion" ,but a stark reality born out by experimental observation.

Exactly!

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1 minute ago, Outrider said:

Yep that too but the first thing I had to loose was the notion that there was something special about the clocks.

Yes it took a long time to sink in.

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Relativity has a more complicated elephant-in-the-room surprise in store that no one has mentioned.

Say you set a clock on a table and receded at an appreciable fraction of the speed of light.

Or if you prefer when could you read that clock?

You need to invoke the relativity of simultaineity since there would be an increasing time lag.

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11 minutes ago, studiot said:

Relativity has a more complicated elephant-in-the-room surprise in store that no one has mentioned.

Say you set a clock on a table and receded at an appreciable fraction of the speed of light.

Or if you prefer when could you read that clock?

You need to invoke the relativity of simultaineity since there would be an increasing time lag.

Can you fire a continuous beam of light at it and read the  " reflection"?

A laser radar/sonar in other words.

Edited by geordief
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The use of lasers ie null geodesics signaling is part of the Einstein synchronization procedure.

This arxiv is fairly low key on the math working under SR.

Edited by Mordred
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4 hours ago, geordief said:

How much of a difference will it make to the time discrepancies if the accelerations required for departure,turnaround and homecoming are maximized or minimized?

The discrepancies depend on the elapsed time and the speed. If the turnaround takes longer the discrepancy will be larger. It's typically assumed to be negligible in duration, so as not to be a distraction.

4 hours ago, StringJunky said:

OK. i was just wondering if that was a possible explanation for the asymmetry.

It would be an asymmetry, but ultimately a small effect given the numbers usually used. Ballpark is around a part in 10^10 frequency shift, while the speed used has a gamma of 2 or more.

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