# Why Does Time Slow Down or Speed up?

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Posted (edited)

Hi everyone,

According to special relativity, time slows down or speeds up according to how fast you are moving relative to something else. As I understand it, the light traveling from something that tells time reaches you slowly when you are traveling near the speed of light and that is why time slows down. I do not understand why the light from time reaching you more slowly means that time has slowed down?

The time it takes for a second to pass is called a second, any slower time or faster time is not a second. A second passing slowly means that a second will pass slower than the time we call a second, which would mean that time is not a second because it is slower than the time we call a second so it is longer than a second.

Edited by Farid

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2 hours ago, Farid said:

As I understand it, the light traveling from something that tells time reaches you slowly when you are traveling near the speed of light and that is why time slows down. I do not understand why the light from time reaching you more slowly means that time has slowed down?

You understand it wrong.  It has nothing to do with light reaching you more slowly.

While we do use light in examples dealing with time dilation, this is just a convenience as using light makes the examples clearer.  But light itself has nothing to do with the reason for time time dilation.  These examples are just highlighting something much more fundamental about the very nature of time and space in our universe.

Basically,  If we are in motion with respect to each other, we simply do not measure time and space the same.  1 sec for you is not 1sec for me, and one meter for you might not be 1 meter for me.   We will not even agree as to whether given events are simultaneous to each other or not.

Relativity forces us to accept that the idea of universal time, where one sec is the same for everyone, just doesn't apply in our universe.

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Can't agree.
The constant independent speed of light is the reason why EM processes run slower
when in relative motion to an observer. A clock is just a periodic process,
serving as a measuring tool for human activity. The light clock is simplicity
and precision. When the clock is in motion, the light has to chase moving mirrors,
for all modes of oscillation. Why doesn't the observer moving with his clock notice,
his biological clock is also running slower.
A simple analogy is the hunter who aims ahead of the target, to compensate for the target motion.

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1 hour ago, phyti said:

Can't agree.
The constant independent speed of light is the reason why EM processes run slower
when in relative motion to an observer. A clock is just a periodic process,
serving as a measuring tool for human activity. The light clock is simplicity
and precision. When the clock is in motion, the light has to chase moving mirrors,
for all modes of oscillation. Why doesn't the observer moving with his clock notice,
his biological clock is also running slower.
A simple analogy is the hunter who aims ahead of the target, to compensate for the target motion.

It doesn’t have to “chase” anything in its own frame, and this works with other clocks, too.

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Time of course always passes at one second per second within one's own frame of reference.

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...when in relative motion to an observer. I.e. slow clocks are perceived by others.

Local time always appears normal, since the clock owner is affected by time dilation to the same extent as his clock.

It's physics, not relativity enforcement by decree.

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10 hours ago, phyti said:

Local time always appears normal, since the clock owner is affected by time dilation to the same extent as his clock.

That's not a useful way to put it. Only time relative to another observer is affected (the coordinate time of the clock according to the observer). The clock itself is not affected by time dilation and measures "proper time", at 1 second per second.

Your statement can lead to thinking like, if a neutron star (as an observer) rotates rapidly around another that it's colliding with 10 billion light years from here, then my clock slows down, but I don't notice because all clocks on Earth slow down. The real reason I don't notice any time dilation is that another observer's motion and its measure of my clock in its reference frame, doesn't affect time as measured in my own frame of reference.

Edited by md65536

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On 9/1/2019 at 1:02 AM, md65536 said:

That's not a useful way to put it. Only time relative to another observer is affected (the coordinate time of the clock according to the observer). The clock itself is not affected by time dilation and measures "proper time", at 1 second per second.

Your statement can lead to thinking like, if a neutron star (as an observer) rotates rapidly around another that it's colliding with 10 billion light years from here, then my clock slows down, but I don't notice because all clocks on Earth slow down. The real reason I don't notice any time dilation is that another observer's motion and its measure of my clock in its reference frame, doesn't affect time as measured in my own frame of reference.

There is nothing in my response that implies a moving object causes a distant clock to run slower. An observer A perceives a moving clock B run slower, which results from the B clock's motion.

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53 minutes ago, phyti said:

There is nothing in my response that implies a moving object causes a distant clock to run slower. An observer A perceives a moving clock B run slower, which results from the B clock's motion.

And B's "owner" doesn't see its clock slow down, because its clock doesn't slow down in its frame of reference. Not because "the clock owner is affected by time dilation to the same extent as his clock", because what the owner measures is in its own frame of reference where it is not affected by the time dilation A measures.

I suppose the "both B and its owner are affected by the same time dilation" could be used to show that A agrees that observers in B's frame of reference would not experience a difference in time among their own frame's clocks, but that's all described in A's frame. However, there are people who are trying to learn relativity who get stuck on ideas like "B really 'actually' does slow down, it's just that B's owner doesn't notice", because they're not thinking about the effects with respect to different specific frames of reference.

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19 hours ago, md65536 said:

And B's "owner" doesn't see its clock slow down, because its clock doesn't slow down in its frame of reference. Not because "the clock owner is affected by time dilation to the same extent as his clock", because what the owner measures is in its own frame of reference where it is not affected by the time dilation A measures.

Both statements can be correct, and they are.

19 hours ago, md65536 said:

I suppose the "both B and its owner are affected by the same time dilation" could be used to show that A agrees that observers in B's frame of reference would not experience a difference in time among their own frame's clocks, but that's all described in A's frame.

Which is what phyti said was the context of the statement.

19 hours ago, md65536 said:

However, there are people who are trying to learn relativity who get stuck on ideas like "B really 'actually' does slow down, it's just that B's owner doesn't notice", because they're not thinking about the effects with respect to different specific frames of reference.

On the contrary, saying that B and their clock re equally affected by time dilation is pointing out the effects from another frame of reference. Further, it's emphasizing that the effect is on time, and not some mechanical effect on the clock, which is a common misconception.

Whether or not a true statement is useful is often a subjective statement. It may not be useful to you, or it may not be the way you would express something. But that might not hold for someone else. (It's not like you are arguing that it's irrelevant to the discussion, like saying that 'sharks live in water' which is true but has nothing to do with the conversation)

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23 hours ago, md65536 said:

And B's "owner" doesn't see its clock slow down, because its clock doesn't slow down in its frame of reference. Not because "the clock owner is affected by time dilation to the same extent as his clock", because what the owner measures is in its own frame of reference where it is not affected by the time dilation A measures.

I suppose the "both B and its owner are affected by the same time dilation" could be used to show that A agrees that observers in B's frame of reference would not experience a difference in time among their own frame's clocks, but that's all described in A's frame. However, there are people who are trying to learn relativity who get stuck on ideas like "B really 'actually' does slow down, it's just that B's owner doesn't notice", because they're not thinking about the effects with respect to different specific frames of reference.

What about length contraction? A similar case.

The anaut measures his 20 m capsule interior, lc to 90%, with his 1 m tape, lc to 90%, and gets 20 m.

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And if a neutron star rotates rapidly around another that it's colliding with 10 billion light years from here, then my clock slows down, but I don't notice because all clocks on Earth slow down. Because, the distant star's reference frame is just as valid as any other.

Okay maybe that's useful, maybe I should have said I don't think it's helpful. I don't think we're helping anyone who is struggling to understand relativity by stating things this way. My clock doesn't slow down in my reference frame and my ruler doesn't length-contract in my inertial reference frame. It is not a case of it happening without me noticing, unless you want to argue that it merely "appears normal" because I don't see the measurements that could be made in another reference frame. There are so many caveats to explain, when it all could have been said so much clearer and intuitively.

Edited by md65536

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On 8/25/2019 at 9:50 PM, Farid said:

As I understand it, the light traveling from something that tells time reaches you slowly when you are traveling near the speed of light and that is why time slows down. I do not understand why the light from time reaching you more slowly means that time has slowed down?

My understanding (SR & GR) is speed of light (c) is constant and universal for all observers in their respective frame of reference. They must always agree that c is constant and max speed (in vacuum, 186k mps). Time speeding up (example, stationary observer measured against moving/accelerating observer or gravity well observer) is not correlated to the speed of light...at least not in a cause & effect manner.

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2 hours ago, The Shadow said:

My understanding (SR & GR) is speed of light (c) is constant and universal for all observers in their respective frame of reference. They must always agree that c is constant and max speed (in vacuum, 186k mps). Time speeding up (example, stationary observer measured against moving/accelerating observer or gravity well observer) is not correlated to the speed of light...at least not in a cause & effect manner.

Fair lets improve this as there is an inaccuracy.

the cause and effect is correlated to the distance a light signal will take to transverse where the observer velocity must be less than c. The speed of light is not a valid reference frame due to separation distance in that frame being $ds^2=0$ which would imply the photon is everywhere at once which is nonsensical.

in a Cartesian coordinates $ds^2= ct^2, dx^2, dy^2+dz^2$ notice we use the interval ct . this gives v=c a unit of normalized unit of length.

Edited by Mordred

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md65536;

Okay maybe that's useful, maybe I should have said I don't think it's helpful. I don't think we're helping anyone who is struggling to understand relativity by stating things this way. My clock doesn't slow down in my reference frame and my ruler doesn't length-contract in my inertial reference frame. It is not a case of it happening without me noticing, unless you want to argue that it merely "appears normal" because I don't see the measurements that could be made in another reference frame. There are so many caveats to explain, when it all could have been said so much clearer and intuitively.

When high altitude high speed muons show their decay times extended, allowing more to reach the ground, is the decay process really slowing or are the scientists imagining it? In the H-K experiment, precision clocks were flown around the world, in both directions. The SR and GR effects of timekeeping were revealed when compared to ground base clocks, though they weren't aware of them while in the air.

You may not like the idea of 'things happening without your awareness', but that's the essence of relativity. Mental activity is chemistry, which is EM activity.

It was unthinkable before then, that motion can alter perception and measurement.

NIST, who's business is time standards, will tell you all clocks drift. GPS will not work without constant periodic adjustments. The world is not the 'ideal' setting people imagine/want it to be.

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1 hour ago, phyti said:

When high altitude high speed muons show their decay times extended, allowing more to reach the ground, is the decay process really slowing or are the scientists imagining it?

In the scientists frame, the process is really slowing down. In the muon's frame, the process is really NOT slowing down, but the height of the atmosphere is really length contracted, meaning it can pass through it in a shorter proper time.

The unhelpful part is trying to describe what is "really" happening in terms of only one frame of reference (like the scientists'), because many people think that different frames' measurements include one "real" measurement and just how it "falsely appears" in other frames. And once again, what the muon observes, it measures in its own frame, where the process really does not slow down. The muon doesn't have to transform its measurements to the Earth frame (or any other), to say "the process really slowed down but my clock is also slowed down." It can just say, "My clock did not slow down."

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Might help to consider you cannot have a length contraction without simultaneous time dilation. The two go hand in hand and are not seperarable.

If you have a frame described by length contraction you must also have a corresponding time dilation of  equal ratio to t4he same gamma factor.

(A good way to examine this is the invariance of c to all observers) in order to preserve that invariance you require both length contraction and time dilation.

Edited by Mordred

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22 hours ago, md65536 said:

In the scientists frame, the process is really slowing down. In the muon's frame, the process is really NOT slowing down, but the height of the atmosphere is really length contracted, meaning it can pass through it in a shorter proper time.

The unhelpful part is trying to describe what is "really" happening in terms of only one frame of reference (like the scientists'), because many people think that different frames' measurements include one "real" measurement and just how it "falsely appears" in other frames. And once again, what the muon observes, it measures in its own frame, where the process really does not slow down. The muon doesn't have to transform its measurements to the Earth frame (or any other), to say "the process really slowed down but my clock is also slowed down." It can just say, "My clock did not slow down."

When the atomic clock returns to earth and is compared to the ground based clock, there is a real measurable difference in readings. How can that be if the time dilation isn't real? The length contraction of the atmosphere is the human interpretation of the muon time dilation, to explain the early arrival of the ground. It's a form of reciprocity.

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8 hours ago, phyti said:

When the atomic clock returns to earth and is compared to the ground based clock, there is a real measurable difference in readings. How can that be if the time dilation isn't real?

Because time dilation is real. And yet, the atomic clock recorded time at one second per second, never slowing down or "being affected" by time dilation in its own frame. This is all consistent with relativity.

8 hours ago, phyti said:

The length contraction of the atmosphere is the human interpretation of the muon time dilation, to explain the early arrival of the ground. It's a form of reciprocity.

Human interpretation??? What do you mean by that? The length contraction is just as real as time dilation. What does it have to do with humans? Do you need the humans' frame to describe the proper time measured by the muon? Does the humans' frame have some special status?

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Lol sounds like your both arguing the same side. You both agree time dilation is real.

11 minutes ago, md65536 said:

Does the humans' frame have some special status?

My human frame is unique only I have my reference frame 😈😈😈😃😃😃

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12 hours ago, phyti said:

How can that be if the time dilation isn't real?

It depends on what you mean by "real".

When you say things like:

On 9/4/2019 at 5:31 PM, phyti said:

What about length contraction? A similar case.

The anaut measures his 20 m capsule interior, lc to 90%, with his 1 m tape, lc to 90%, and gets 20 m.

You seem to be saying that something physically changes in the length of the tape and spacecraft. And similarly with the clocks that measure time dilation.

That doesn't work because we are talking about relative measurements. How could another spacecraft speeding up or slowing down cause your astronaut's rule to shrink and expand?

And how much should the ruler shrink? I am stationary (in my frame of reference) so my ruler is 100% of its normal length. When a spacecraft flies by at 10% of the speed of light then my ruler is now 0.5% shorter. But if, at the same time another spacecraft flies past at 20% of c, then my ruler is 3% shorter. So which is it? It can't be both.

So, yes, length contraction is "real" but it isn't something that changes in the "moving" object, it is just a change in relative measurements.

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In case anyone reading is confused about whether the effects are "real", there's no argument here from anyone that they're not. They're real.

"It can't be both" (a ruler can't be .5% shorter and 3% shorter) can be understood to mean, "It can't be both in any single frame of reference."

Because length and time are relative, it is perfectly reasonable for example for one observer to say "Clock A is slowed but clock B isn't; ruler X is .5% shorter" and another to say "Both clocks A and B are slowed; ruler X is 3% shorter", and another to say "Clock B is slowed but clock A isn't; ruler X is not length-contracted." All of those are real physically measurable quantities made by individual observers, none of them need to be interpreted by another observer. It is not reasonable to say a rocket is length-contracted to 90% in all frames but the other frames measure it differently because their rulers are also length-contracted differently.

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md65536;

An anaut Al, leaves Earth for a target object Q 12 ly distant by an earth star chart.

Al's flight plan is to orbit Earth at a large distance until his ship reaches .3c. From reading his 'relativity' manual, he knows he has two options.

1. remain an anaut moving at .3c, or

2. assume a pseudo rest frame, with Q approaching him at .3c.

If 1, he arrives at 38 yr, knowing he and his ship have lost 2 yr due to time dilation.

If 2, Q arrives at 38 yr. He knows that a clock at rest does not lose time. His watch and computer both agree with the onboard clock. He can verify the speed of Q using light signals. The only other variable is distance. He concludes the universe has apparently contracted in the direction of motion, to explain the time difference, i.e. a mental reconciliation, without any physical processes.

No.1 is believable since SR predicts on the basis of physical processes, fast moving clocks run slower.

No.2 is absurd, since a fast moving space craft cannot influence distant parts of the universe, but it can influence the perception of an occupant. When this is accepted, there is no need for no.2. When an observer measures lc of objects, he does not conclude lc of the space between objects. This also points to the anaut's conclusion in no.2 as perception, not corresponding to anything physically real.

_________________________

Your puzzle would imply you accept time dilation as a physical phenomenon.

When the anaut twin returns to the earth younger than the other, isn't that proof that the anaut twin biological processes occurred at a reduced rate, even though the anaut was not aware?

That's the implication of postulate 1, 'physics is the same for all inertial frames', thus the observer cannot detect any differences. Being 'aware' contradicts postulate 1.

SR does not say you can't determine the how and why of td and lc.

________________________

Inanimate muons need someone to speak for them.

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8 minutes ago, phyti said:

md65536;

An anaut Al, leaves Earth for a target object Q 12 ly distant by an earth star chart.

Al's flight plan is to orbit Earth at a large distance until his ship reaches .3c. From reading his 'relativity' manual, he knows he has two options.

1. remain an anaut moving at .3c, or

2. assume a pseudo rest frame, with Q approaching him at .3c.

If 1, he arrives at 38 yr, knowing he and his ship have lost 2 yr due to time dilation.

If 2, Q arrives at 38 yr. He knows that a clock at rest does not lose time. His watch and computer both agree with the onboard clock. He can verify the speed of Q using light signals. The only other variable is distance. He concludes the universe has apparently contracted in the direction of motion, to explain the time difference, i.e. a mental reconciliation, without any physical processes.

Did they forget about relativity in #2? Otherwise they would understand what had happened, and one would not fret about “physical processes”

8 minutes ago, phyti said:

No.1 is believable since SR predicts on the basis of physical processes, fast moving clocks run slower.

It’s not an effect caused by “physical processes”. That’s a bad way of describing it.

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42 minutes ago, phyti said:

2. assume a pseudo rest frame

There is no such thing as a “pseudo” rest frame.

The astronaut is at rest in his frame of reference and so there is no local time dilation or length contraction.

56 minutes ago, phyti said:

No.2 is absurd

Not a valid scientific argument

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