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If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough. However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective - that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?

If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough.

Yes, no problem with the gravitational time dilation.

However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective - that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?

No. We'd see the universe progressing normally, because we aren't subject to that gravitational time dilation. (I presume we're at a safe distance). Let's suppose it was a TV camera that took a picture 25 times a second and sent it back to us, adequately catering for redshift. The camera starts off taking 25 pictures a second as measured by us. But after a while we notice we're only getting 24 pictures a second, then 23, and so on. We see events in the wider universe progressing at their normal rate, but eventually the movie starts getting jerky as the frame-rate reduces. In the end the frame-rate reduces to zero, and that's the end of the show.

For simplicity, let's say that the black hole is isolated and non-rotating (and uncharged), so that the situation is described by the comparatively simple Schwarzschild spacetime. Let's also suppose that the camera free-falls radially into the black hole.

What is the camera looking at? Suppose it is looking at some stationary object that does something with a known frequency. Your question is basically how at what frequency it will be observed on the video feed emitted by the camera.

Without loss of generality, we can suppose that the camera is looking at *us*, and that we're shining a laser beam at it: the 'doing something at a known frequency' would be the oscillations in the electromagnetic wave of the laser beam. We can do this because time dilation affects every physical process, so we might as well pick one that is more convenient to think about.

At this point, it is straightforward why the camera feed will not show any time dilation: being equivalent to a reflected laser beam, the gravitational blueshift when going inward will be cancelled by the gravitational redshift going outward.

## A camera and time dilation? - Astronomy

*I am a high school teacher. In books on time dilation I see many examples of completed problems of what would happen in such journeys out into space. An example is the twins paradox - is there a formula where I could calculate how much time has passed on the Earth while the traveler is flying in space ? Other examples I have seen in programs: "It is possible to journey to the center of our galaxy, a flight time of 50 yrs (?) and when you return, 4 million yrs have gone by." OR "12 year journey and the Brother is now 80 years old". I'm told that such a formula exists - what is it and what are the variables that I can calculate ?*

Length contraction and time dilation are both effects of Special Relativity, which take place when an object is travelling close to the speed of light. It's really not considered time travel, except to the extent that we all travel through time inexorably into the future. Nevertheless, these effects are certainly real. You can indeed travel very near the speed of light for a short time and come back to Earth, where some millions of years have passed. The explanation for this is an entire physics course on its own, and can be found in introductory texts on special relativity. Essentially, it is an immediate consequence of the speed of light being a constant for all observers, no matter what their own speed.

Moving clocks run slow and moving sticks are shortened by a factor

So, let's say we're thinking about the "twin paradox," and that our intrepid traveller is moving at speed v equal to 0.9 times the speed of light, c. In this case,

gamma = 1/Sqrt( 1 - 0.9 2 ) = 2.29.

So the twin on Earth sees the spaceship's clock running 2.29 times slower than her own, i.e. every 2.29 years on Earth corresponds to one year by the spaceship's clock. The Earthbound twin also observes that the spaceship travelling at a speed 0.9c is 2.29 times shorter than it was before the launch. The faster the spaceship travels, the more pronounced the effect. Cool, eh?

*This page was last reviewed on January 28, 2019.*

### About the Author

#### Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

## Time dilation

The factual type of time travel is through the concept of time dilation – a difference of elapsed time between two events. It is an extremely hard thing for us mortals to get our head around – that is, people who are not well versed in the fields of physics and astronomy for instance. This is because Einstein’s theory of relativity plays a part.

In short, when astronauts orbit Earth they are noticeably further from the planet than those who reside on the surface. This means their gravitational time dilation is less because gravity is less and that everything they do is faster (even the clocks run slower on space stations). When they return to Earth they are literally travelling back in time. It is by no means a large number because the time dilation is caused by gravity which is a weak force. (As forces of attraction in the Universe goes). It has been calculated that Russian astronaut Sergei Konstantinovich Krikalev has time travelled the most. Here is a brief summary of how he did so.

## Time dilation direction dependant?

It's been a very long day but let me ask a silly question. I was pondering the space travel question on this forum and came up with a something I haven't seen formally addressed.

When thinking about time dilation I always go back to the example of a train passing through the station at speed with one observer on a 100 foot flagpole attached to a flatcar. He drops a ball and he sees the ball drop 100 feet in 1 second. An observer stationary in the station will see the ball move at an angle and during the same one second the ball will travel 133 feet. So one clock would have seen the same event transpire faster.

I never thought about moving the observer before. If the observer in the station immediately jumped on the tracks just as the train passed and the ball dropped by the second observer on the flagpole, both observers would see the ball traverse the same identical distance in the same time frame. So if the train were moving directly away from the observer at the station then would there be a time difference in the stationary observer vs the observer in motion?

(If this is really stupid then I am asking for a friend!)

### #2 LesB

### #3 LesB

That example should not be a ball but a beam of light, and the path of that beam of light as it appears to the observer on train, more specifially, a rail car. The other observer who is watching the rail car pass sees a different path for the light beam and that path is longer due to the motion of the railcar. This is where time dilation occurs.

Central to the understanding of this phenomenom is the acceptance that the speed of light is the same in all directions regardless of the speed or direction of its source. This understanding is furthered by the Pythagorean relationship that derives the formula. So, while the path of the bouncing ball may be analagous to the path of the light beam the analogy breaks down because the path of the ball is parabolic for obvious reasons.

The ultimate path of light is determined by the curvature of space which is warped by gravitational fields. Locally the path of light for this example is treated as a straight line.

### #4 JerryWise

That example should not be a ball but a beam of light, and the path of that beam of light as it appears to the observer on train, more specifially, a rail car. The other observer who is watching the rail car pass sees a different path for the light beam and that path is longer due to the motion of the railcar.

### #5 LesB

### #6 JerryWise

Then why are the GPS satellites adjusted for time dilation when they are not moving at the speed of light?

### #7 JerryWise

That example should not be a ball but a beam of light, and the path of that beam of light as it appears to the observer on train, more specifially, a rail car. The other observer who is watching the rail car pass sees a different path for the light beam and that path is longer due to the motion of the railcar. This is where time dilation occurs.

### #8 LesB

There is one observer on the moving rail car and the other observer is stationary. You have confused this by saying that the stationary observer is walking behind the train.

The observer on the rail car experiences time dilation but doesn't sense it. The stationary observer experiences time in a different way because the light path of what he observes on the rail car takes a longer path from its source to its destinaton. If the rail car were stationary then both observers would have the same experience.

Why? Because c is the same in all directions and in all inertial frames. The analogy of the ball can illustrate that but in fact all is reference to c. The analogy of the ball breaks down when the math of the light path is analyzed.

### #9 JerryWise

I haven't confused this, that is the question. Drop the ball if you like. The light path works as well. I understood the comment was the observer on the platform would "see a longer light path".

Add a third observer if the observer moving behind the train confuses. We have one standing on the platform, one on the train and one behind the train. The trains motion will be the same to the two observers not on the train. The observer behind the train will see a different light path than the one on the platform. We have the same train velocity for both off train observers, we have a longer light path for the observer beside the train than the observer behind the train.

I understand the C and inertial frames. Do we have to use light speed and light paths for reference when experimentation confirms time dilation at less than C? The question on the GPS satellites is still open and very relevant in illustrating this.

### #10 LesB

Add a third observer if the observer moving behind the train confuses. We have one standing on the platform, one on the train and one behind the train. The trains motion will be the same to the two observers not on the train. The observer behind the train will see a different light path than the one on the platform. We have the same train velocity for both off train observers, we have a longer light path for the observer beside the train than the observer behind the train.

I never disputed that time dilation occurs at speeds less than c. However, the addition of a third stationary observer in your example is unnecessary because the *speed of light* is the same in all directions regardless of the source and the observer's position. Both stationary observers will see the same result. The observer, who is in motion on the train, will experience a slowing of time. Adding a another observer in the same stationary frame will produce another observer who experiences that same sensation of time.

If I remember, *time dilation* was the subject of this thread. Consider that the rail car is in motion relative to both observers who are stationary. The rail car is the inertial frame that moves relative to both stationary observers.

### #11 JerryWise

.. I never disputed that time dilation occurs at speeds less than c.

I agree you never disputed it. I assume you are now acknowledging it. Maybe if we got back to the ball now that it is acknowledged?

You mention an observer observing a longer light path. Stay right with that for the meaning of the thread. Would the angular difference of two stationary observers see a different "longer" light path. Or is that no longer relevant? Just a yes or no will work. Say no and I'll be happy with your opinion.

..If I remember, time dilation was the the subject of this thread.

No need to snip and snipe. You win if thats the point.

Please excuse. I don't mean to cut out on this but "Freddie vs Jason" is coming on and. well. .

### #12 LesB

Please excuse. I don't mean to cut out on this but "Freddie vs Jason" is coming on and. well. .

### #13 JerryWise

Please excuse. I don't mean to cut out on this but "Freddie vs Jason" is coming on and. well. .

### #14 Jarad

Okay there a couple of different things going on here.

#1 - Time dilation is usually defined as measured by a stationary observer parallel to the event being measured. In other words, the formula calculates the time dilation effect if the observer is next to the train at the moment of the event he is measuring, so the train is neither approaching nor receeding. This is the "true" time dilation factor.

#2 - If the observer is instead either behind the train (so it is receding from him) or in front of the train (so it is approaching), there is an additional factor due to the red-shift or blue-shift.

A train that is receeding will have the later events occur farther away. Therefore it will take light from those events longer to reach the observer, causing them to seem even slower than the "true" dilation factor. So to an observer behind the train, things seem to slow down even more than the normal dilation factor.

A train that is approaching will have later events occur closer to the observer, therefore light from those events will arrive sooner than they otherwise would, and time will seem to speed up. This effect will oppose the dilation effect, and be larger than it as well, so that an observer in front of the train will see things on the train occuring faster than normal despite the time dilation factor.

For example, the lorentz transform factor for 90% of the speed light is sqrt(1-0.9^2) = 0.141. So time is running at 1/7 on a ship moving at 0.9C relative to earth. But if the ship is moving away from us, the light it emits at T + 0.141 seconds ship time takes 0.9 seconds longer to reach us than the light emitted at T. So that 0.141 on the ship appearss to take 1.9 seconds to us (1 second for the dilation factor, plus 0.9 seconds since it ended 0.9 light seconds further away), for a total apparent time dilation factor of 0.141/1.9= 0.074, or just under 1/13 as fast as it seems to go on earth.

If the same ship were approaching, then light emitted at T + 0.141 seconds ship time would be emitted 1 second later earth time, but from 0.9 light seconds closer. So it would arrive 0.1 second later, for a time dilation factor of 0.141/0.1 = 1.41. So time would appear to go 1.41 times faster on the approaching ship.

## Relativity and the "Triplet Paradox"

Orbits, the actual physical time the ship took to complete the journey, these too are facts. If the clock on the ship said 17 years and the ship took 20 I see a lot of interesting possibilities (is the ship 17 years older or 20 from launch).

Absolutely. I agree 100%. Where I've been going with this is one is a perception brought about by observations in an inertial frame of reference, the other observations in the system of co-ordinates at rest relatively to the moving body. The measurements taken in the inertial frame of reference must be transformed into a system of co-ordinates at rest relatively to the body in motion if you are to determine intercepts and orbits. This to place problems (need for emergency intercept of the ship, etc.) in the optics of moving bodies to a series of problems in the optics of stationary bodies. (1)

Both measurements are very real. One measurement must be transformed for physical intercepts and orbital calculations while the other needs no transformation of the calculation parameters. By nature of the transformation requirement this set of observations is a perception engendered by moving through a light path propagating from Point A (Planet) to Point B (Planet) at significant percentages of C.

And this brings the point of the ponder. Why are all readings taken in an inertial frame of reference transformed back to system of co-ordinates at rest when the ship ceases motion (moves out of an inertial frame of reference) except the age of the triplet aboard the ship? What special frame of inertial reference allows this observation to stand while all other observations must be converted? This in a system operating under the law of no special frame of reference?

(1) "On the Electrodynamics of Moving Bodies" By A. Einstein, June 30, 1905. Section 8, final paragraph.

### #27 Jarad

Because it wasn't just an optical effect.

The distance travelled doesn't go back when the ship stops. After the ship stops, if they look backwards, they will see the distance as 20 light years, and they will see the clocks all moving at the same rate. But elapsed time doesn't change, and if the ship had an odometer, it would still show 17.3 light years, not 20.

### #28 JerryWise

Because it wasn't just an optical effect.

The distance travelled doesn't go back when the ship stops. After the ship stops, if they look backwards, they will see the distance as 20 light years, and they will see the clocks all moving at the same rate. But elapsed time doesn't change, and if the ship had an odometer, it would still show 17.3 light years, not 20.

### #29 Jarad

The rods aren't so rigid. If they measured the rods, they would measure them as shorter than the people on the planets would (in other words, the on-board instruments would still have to convert to calculate intercepts).

### #30 JerryWise

### #31 Jarad

It's necessary when they are in a different reference frame. So while the ship is moving relative to the planets, they measure things differently. When they are stationary relative to the planets, they measure things the same.

### #32 JerryWise

### #33 HiggsBoson

I'd like to post a thought experiment that seems to question time dilation as a permanent effect. If a voyage is looked at from this perspective I can't see how time dilation can be other than an observation within a frame of reference rather than a permanent effect on the occupants of the ship in motion. (Time dilation proofs from Muon experiments and the GPS compensations are considered fact but for sake of this one example I'd like to reference the following set of circumstances.) For this example three girls are born at exactly the same instant, 2 on Earth and 1 on a distant planet.

What you have done here is posed a postulate as follows:

Time dilations as a result of relative motion are a result of light travel time and do not persist when the traveler and reference observer are returned to the same reference frame.

The twin paradox is not a â€˜thoughtâ€™ experiment. It has been done using real clocks. In 1971 Hafele and Keating flew matched cesium atomic clocks around the earth. The reference clocks were at the US Naval Observatory. When the clocks were reunited the clocks that flew no longer agreed with the reference clock by an amount that could not be attributed to measurement error.

The outcome of the experiment is sufficient to show that the postulate on which your example is based is false. One need not reference Relativity in any way. Nature does not support the postulate. The experiment was repeated using better clocks with similar results.

It is important to remember that it is the outcome of experiments that determine which theories are more useful. Einstein did not embrace Quantum but he never suggested that experiments that supported Quantum were not valid results. He simply wanted more from the theory.

In this case we do have a theory which predicted the results accurately accounting for both the slow down in clock rate due to motion and the acceleration due to being at a greater distance from the center of the earth. While one can suggest that some new currently unknown theory may be more correct there is really no room to suggest that the dilations are not happening.

If the muon data, and the clock test are not convincing the GPS data is overwhelming. There are

2-dozen satellites with 2 dozen cesium clocks. There are hundreds of millions of receivers each producing a navigation solution per second for as long as they are powered and receiving signals. There are few applications of technology that present such a large body of verifying data. If these dilations were not real the whole system would not be as accurate as it is observed to be by 10s of millions of examples. As I understand it the clocks in the original system had to be corrected for Relativistic effects twice a day to keep the clocks sufficiently synchronized to meet itâ€™s accuracy requirements. It appears that in this case the dilations were a very persistent effect.

### #34 Joad

[snip] "And this brings the point of the ponder. Why are all readings taken in an inertial frame of reference transformed back to system of co-ordinates at rest when the ship ceases motion (moves out of an inertial frame of reference) except the age of the triplet aboard the ship?" [snip]

Jarad and HiggsBoson have answered the technical side of the question, and llanitedave has covered what might be called the psychological side. But I am glad that you asked it, because by asking it you have enabled our well-informed members to drive home once again what a strange thing relativity is.

### #35 JerryWise

I'd like to post a thought experiment that seems to question time dilation as a permanent effect. If a voyage is looked at from this perspective I can't see how time dilation can be other than an observation within a frame of reference rather than a permanent effect on the occupants of the ship in motion. (Time dilation proofs from Muon experiments and the GPS compensations are considered fact but for sake of this one example I'd like to reference the following set of circumstances.) For this example three girls are born at exactly the same instant, 2 on Earth and 1 on a distant planet.

What you have done here is posed a postulate as follows:

Time dilations as a result of relative motion are a result of light travel time and do not persist when the traveler and reference observer are returned to the same reference frame.

The twin paradox is not a â€˜thoughtâ€™ experiment. It has been done using real clocks. In 1971 Hafele and Keating flew matched cesium atomic clocks around the earth. The reference clocks were at the US Naval Observatory. When the clocks were reunited the clocks that flew no longer agreed with the reference clock by an amount that could not be attributed to measurement error.

The outcome of the experiment is sufficient to show that the postulate on which your example is based is false. One need not reference Relativity in any way. Nature does not support the postulate. The experiment was repeated using better clocks with similar results.

It is important to remember that it is the outcome of experiments that determine which theories are more useful. Einstein did not embrace Quantum but he never suggested that experiments that supported Quantum were not valid results. He simply wanted more from the theory.

In this case we do have a theory which predicted the results accurately accounting for both the slow down in clock rate due to motion and the acceleration due to being at a greater distance from the center of the earth. While one can suggest that some new currently unknown theory may be more correct there is really no room to suggest that the dilations are not happening.

If the muon data, and the clock test are not convincing the GPS data is overwhelming. There are

2-dozen satellites with 2 dozen cesium clocks. There are hundreds of millions of receivers each producing a navigation solution per second for as long as they are powered and receiving signals. There are few applications of technology that present such a large body of verifying data. If these dilations were not real the whole system would not be as accurate as it is observed to be by 10s of millions of examples. As I understand it the clocks in the original system had to be corrected for Relativistic effects twice a day to keep the clocks sufficiently synchronized to meet itâ€™s accuracy requirements. It appears that in this case the dilations were a very persistent effect.

So Micheal, you quote me that I said within the first three sentences of this post that I believe in time dilation and want to post up a thought exercise. (Time dilation proofs from Muon experiments and the GPS compensations are considered fact but for sake of this one example I'd like to reference the following set of circumstances.) I've said in a number of previous post I believe in the time dilation of the GPS system and use it often during night navigation exercise in marine navigation. But yet you want to tell me that what I said is true is true. I agree with you and me on that. I highly respect your previous post on CN and would hope for some effective insight on a simple thought exercise?

This was brought about by thinking over a statement made by Einstein

All problems in the optics of moving bodies can be solved by the method here employed. What is essential is, that the electric and magnetic force of the light which is influenced by a moving body, be transformed into a system of co-ordinates at rest relatively to the body. By this means all problems in the optics of moving bodies will be reduced to a series of problems in the optics of stationary bodies.

in his paper "On the Electrodynamics of Moving Bodies" section 8, last paragraph.

I don't fully understand what Einstein means by that. Maybe your expertise could help out.

### #36 JerryWise

[snip] "And this brings the point of the ponder. Why are all readings taken in an inertial frame of reference transformed back to system of co-ordinates at rest when the ship ceases motion (moves out of an inertial frame of reference) except the age of the triplet aboard the ship?" [snip]

Jarad and HiggsBoson have answered the technical side of the question, and llanitedave has covered what might be called the psychological side. But I am glad that you asked it, because by asking it you have enabled our well-informed members to drive home once again what a strange thing relativity is.

### #37 JerryWise

I pondered over this again and would like to put it in a little different perspective. As Joad mentioned above, the Science has been explained and it is obvious. I suspect myself and many others here are students seeking knowledge (seems many are following this from the view count). I understand the formulas and the logic behind Relativistic examples. Examples like the ball being dropped from a flagpole on a moving train with a platform observer and the poor experimenter sitting on the flagpole at speed. We have a simple example proposed above in the OP. We understand the figures and to some extent the Science. As students we should be allowed to ask â€œwhy is that?â€. I am not refuting the Science (I would never dare) only asking â€œwhy is that?â€.

Since Higgsbosen mentioned the word â€œpostulateâ€ let me formulate (or extrapolate) one.

â€œThe speed of light is the same in all frames of referenceâ€. Given the speed of light is the same in all frames of reference the proportional speed of light is the same in all frames of reference. .5C (1/2 the speed of light) is the same in all frames of reference just as the speed of light is the same.â€

If this is not fully versed in the Science say â€œthat is wrongâ€. But please, if it is wrong, explain the â€œwhyâ€ not just the â€œwrongâ€.

So letâ€™s take our example down to some basics. Ignore the optical contradictions in the first post. We are now only addressing the ship on the voyage under â€œSpecial Relativityâ€ reflecting the conditions Jarad made the calculations on above.

1. The ship travels from Planet A to Planet B above at .5C. Planet A and Planet B above are 10 light years apart. The ship is traveling at Â½ the speed of light and will take 20 years (10 x 2) to make the journey. To make the journey in any less or more time will mean it is not traveling at .5C.

2. The ship is subject to Relativistic Time Dilation going from Planet A to Planet B at .5C. The calculations performed above prove the ship will take 17.25 years to go from Planet A to Planet B at .5C subject to time dilation.

Does any this look out of line?

Looking at these statements, is there a contradiction? If so, what do you think the reason would be? I think I see some.

First, the ship took 20 years to make the journey (2). But the clocks on the ship, the age of the occupants and the shipâ€™s age read/are 17.25 years so the ship took 17.25 years to make the journey. So it looks like the ship took both 17.25 and 20 years to make the journey. We can use Lorentz transforms to transform the time dilated 17.25 years to 20 years. (We could go into how the transform would work both ways but need to understand which observation is the â€œat restâ€ frame and which an inertial or â€œin motionâ€ frame subject to time dilation. But canâ€™t we assume the 20 year figure is the at rest frame since it is stated and it also is determined by the speed of light (invariant)). I think both time frames work just fine since we have a conversion factor (Lorentz Transform) and the science is sound. I donâ€™t misunderstand the Science or definition, only one aspect of the result. We have to use a Lorentz transform for precise location intercepts on the voyage for the physical location of the ship so this converts the time dilated readings to the underlying at rest frame. At the end of the voyage we must use a Lorentz transform on the ships clock to make it read the same as the 20 year time span of the voyage. Why do we not do the same transform on the occupantâ€™s age. Seems a simple enough question. As a student, I ask why that is.

Second, if the ship moves from planet A to planet B at .5C and .5C is an invariant (C/2) then the ship will take 20 years to make the voyage. If the shipâ€™s occupants make the voyage in anything other than 20 years then they will have physically traversed the distance at some figure other than .5C. Seems they will have varied within an invariant. If we use the Lorentz Transform on the occupantâ€™s ages at the end of the voyage to make them inline with the invariant .5C their ages are no longer time dilated. Anybody see a contradiction here? As a student I ask for help on an explanation, not the calculations (those are perfectly correct as given above by Jarad).

Any help with the why of this would be fully appreciated.

### #38 Mister T

Dave, I have been trying to wrap my head around this.

But I was thinking isn't the time dilation effect due to acceleration not velocity??

maybe it is is not traveling at c that distorts space-time, but the acceleration needed to get there.

acceleration is a result of a force

which is why gravity also distorts space-time.

### #39 Jarad

Because space is dilated as well. To the ship, it was moving at 0.5 C for 17.33 years,and only travelled 8.66 light years. It's odometer would show 8.66 light years, and it's clocks will show 17.33 real years (and the ages of the passengers are just more clocks). When it stops, none of those previous measurements change, even though the distance and clock rates reset.

### #40 JerryWise

.

But I was thinking isn't the time dilation effect due to acceleration not velocity??

.

### #41 Joad

I can think of two things, Jerry (which is no guarantee that the thoughts are any good ), but here goes:

1. Planet A and Planet B are not 10 light years apart, as such. Their distance is always relative to the situation of some measuring point. Mathematical transforms can bring the measurements together but do not establish an "actual" distance between the planets because there is no actual distance: there is only distance as measured, and that is always relative. Thus, to the moving ship, the distance is less than it is to the underlying frame measurement. Both measurements are right, just as I weigh 135 pounds on earth but would weigh a lot less on the moon: both measurements are my "real" weight.

2. The three sisters aged 17.3 years on the voyage. They would have aged 20 years if they had stayed home, but time, and their bodies, changed (aged) at a different rate on the voyage. When they land, mathematics can restore a measurement equilibrium of sorts, but their bodies are younger relative to what they would have been if they had stayed "home" because the time in which their lives took place got foreshortened for a while.

In other words, there is no absolute time for their bodies to age.

### #42 HiggsBoson

Jerry, it seems that I missed the point of your post. You are asking why the age dialation sticks and other apparent contradictions go away. Some of the confusion here may be due to language.

First, the ship took 20 years to make the journey (2). But the clocks on the ship, the age of the occupants and the shipâ€™s age read/are 17.25 years so the ship took 17.25 years to make the journey. So it looks like the ship took both 17.25 and 20 years to make the journey.

In these types of problems one must always make sure that the meaning is unambiguous. The trip took 20 years as measured in the reference frame of Planet A. This is a calculation. There is no way for anyone on Planet A to know that this is true. It will take at least 10 years for that information to return to Planet A.

In the statement of the problem and the outcome of experiments one must keep in mind the limitations of each frame. No one can actually know some of the things that are â€˜givenâ€™ in the statement of the problem.

One of my hobbies is film making. Film directors have the ability to show the audience what is â€˜reallyâ€™ going on in two or more locations at a time. This is called â€˜parallel actionâ€™. Imagine a house husband having a rough day with the kids and the wife having a hard time at work. A director can cut between activities going on at the home and work location with reckless abandon. One can even include a home to work phone conversation between the two to clearly show that the views are near simultaneous. The camera can cut to any location in the universe just in time to show the audience relevant information that will propel the story developing in the mind of the audience. **Called the â€˜omnipotent cameraâ€˜, it sees all, it knows all and it gets there just in time to see important events happen.** In some cases it will actually show you the same action from more than one point of view â€˜explosions are expensive they tend to get used 2 or 3 timesâ€™.

The challenge with a thought experiment is to avoid observing the experiment from an omnipotent camera. The mind creates an â€˜absoluteâ€™ reference frame ( Point of View to film makers ) to which everything else is compared. The omnipotent camera is allow to have points of view that do not correspond to any real point of view ( no one actually views people talking in a car from the POV of the hood ornament ).

The two reckonings of the trip time are both valid when the ship reaches Planet B. There is no absolute frame from which one can say what really happened. When the ship returns to planet A, there is a difference in time experience by those who traveled vs those who did not. The dilations that stick are those experienced by those who experience acceleration during the interval in question. Because the travelers had to accelerate to change from moving toward planet B to moving toward planet A they will experience a sticky dilation compared to those who remained in the Planet A reference frame.

### #43 Joad

Jerry's thread has helped me grasp more fully (I think) the often-cited observation in this forum that time does not exist for a photon. I think I can understand better, now, desitter's observation that space doesn't really exist for a photon. It makes sense: if space/time compresses as C is approached, at C space/time would compress to, well, nothing. Right?

But that doesn't mean that there is no space/time. Relative to poky old earth that photon from M51 I looked at last night came a long way baby, and took a long time to get here. So that photon both crossed a lot of space/time (relative to me) and is suspended in its own relativistic "now" relative to its own C movement. And both are real.

### #44 HiggsBoson

Joad, I think that the short answer here is â€˜yesâ€˜. Photons do not experience the passage of time as we know it. However, one result of Relativity is we lost the classical concept of time. In the Relativistic view space and time are a part of an inseparable thing called space-time. A singular noun describing that stuff that contains the universe.

In General Relativity we learn that space-time is flexible enough to twist and form interesting shapes. It is very hard to understand the results of Relativity using the classical concept of time. Unfortunately, the concept is so intuitive to us that it is hard to let it go and embrace this new space-time stuff prior to attempting to understand a problem.

To quote Feynman â€˜no one understands quantum, we just get use to it.â€™ Every quantum mechanical wave function I have ever seen for a real object was non-zero everywhere in the universe. What am I to make of this? The wave function for an electron in the ground state of the hydrogen atom is non-zero everywhere in the universe.(1) Yes, I can talk about what this means from a mathematical standpoint. But as a person with a mind I do not understand why such functions provide accurate predictions of the behavior of electrons.

In the case of that M51 photon, from the reference frame of the Earth, the wave function that describes the probability of an observation of that photon has a maxima that took 23 million years to get here. However, in the reference frame of the photon, no time has passed. While I understand the math this does not conform with anything within my personal experience.

Allow me to express my gratitude for this forum. It helps me blow the dust off of the few remaining brain cells that I have.

1) With the exception of a singularity at the center of the nucleus. The function is zero at the origin and has radial symmetry. Because the Proton is centered at this point and has non-zero radius the probability of the electron collapsing, (being observed), into the Proton is non-zero.

### #45 JerryWise

Jerry's thread has helped me grasp more fully (I think) the often-cited observation in this forum that time does not exist for a photon. I think I can understand better, now, desitter's observation that space doesn't really exist for a photon. It makes sense: if space/time compresses as C is approached, at C space/time would compress to, well, nothing. Right?

But that doesn't mean that there is no space/time. Relative to poky old earth that photon from M51 I looked at last night came a long way baby, and took a long time to get here. So that photon both crossed a lot of space/time (relative to me) and is suspended in its own relativistic "now" relative to its own C movement. And both are real.

HiggsBosen absolutely is on track with the concepts. And I think Joad is on track with what I've tried to understand.

What I did in the OP was present a scenario which tended to free the thought exercise from some conventional thoughts on Relativity. This by placing the motion of the three observers in a common "Omnipotent Camera" (thanks for that Micheal) observational mode. This was done by giving all three observers visual access to the other's clock faces. Note I said clock faces and not "time". This is the clock face "information" and it is conveyed by the one absolute we canâ€™t refute by definition. The speed of light ©. The special optical device (large binoculars?) at each location could observe the clock faces at each of the other observers locations. This Omnipotes (made that up) the observers.

Is it possible to do this? Yes or much of what we have learned about light and distances to galaxies, stars and the edge of the Universe is flawed (possibly a valid assumption but not one we can take based on present knowledge). Here we are looking at light as a carrier of information (what we discern in the image of the clock face or the time it shows at the distant location and not a photonâ€™s characteristics). This information is moving between these locations at C and in the omnipotent camera's eye (visual perception of all three locations) this would make the three observers frames of reference available to each observer in real time minus the necessary transit time of light (the carrier of the information). Light does propagate at a given rate and we can transit along that path. This is why we see Red and Blue shifts indicating a motion relative to the emission source. This is also why I suggest in the OP the clock face information is valid visually between all observers at each stage of the journey. We would also see the light from the clock face of the approaching planet B as slightly blue tinted (blue shift of the light frequency indicating an approach to the light source) and the clock face of planet A slightly red tinted (indicating a move away from that light source). It is how these readings contradict with how some calculations are applied that is a concern. (I hope I'm making this clear. I have it thought out but putting something this involved to pen is rough.)

As Joad says, "time does not exist for a photon". But that Photon has a given set of physical characteristics that define its behavior. It moves at "C" in all frames of reference and moving from an emitting location to a perceiving location it moves along a physical path at "C" which is a given and fixed speed/time span. So time does not exist for the Photon but time does exist for any journey a photon will make. Photons are worthless to us without bringing a shape in their group presentation (what we see). If they bring the shape of a distant clock face we can read that face just as the observer at that location. So the camera doesn't need to relocate and we become the omnipotent observers at each location.

I am not saying there is a problem with Relativity. Nor am I saying there is a problem with the intuitive visual scenario in the original post. I am proposing a mental exercise that appears to show a contradiction. If we can make a visual observation of an approaching and receding light source traveling between the light sources at the same instant (did not say simultaneously) we can compare the information as presented. If this information does not agree with the calculations then we have a "Triplet Paradox". To say the visual indications of the omnipotent binoculars is flawed disputes the nature of light propagation. To say it is valid disputes the application of some Relativistic calculations. Peculiar.

Iâ€™d like to post up a suggestion of why there might be a misconception using the moving train, dropped ball and flagpole scenario so common in discussing Relativity. But first need to run put out a fire.

## What Will An Action Camera Record If It Falls Into A Black Hole?

In the whole universe, there is only one place where our modern technology cannot survive, and obviously, it is a Black Hole. But just for a minute, suppose that scientists have made a powerful and strong action camera that can survive any threat in the universe and dropped it into a black hole. What will it record for us?

The action camera falling into a black hole may transmit a signal to us up to the intersection of the event horizon. As you approach a black hole, the time for the camera will slow down, which will lead to a shift of its signal to the region of long waves (gravitational redshift), and the shift will noticeably increase every second.

Schematic representation of the gravity wells of the Sun, a neutron star and a black hole

The redshift is a consequence of the time dilation for the action camera in the frame of reference of an observer located far from the black hole. That is, the camera in its frame of reference of 60 frames per second will still shoot and send us its records, but 1 second for the camera can turn into hours, years and ultimately billions of years for us.

That means that huge intervals of time will pass for an observer on the earth between the arrival of frames, and the picture will change very slowly.

Therefore, soon after starting the camera, the usual technique will not be able to catch this signal and restore the video from it, but if necessary, you can create devices that can capture the long-wave signal and turn it into a video.

Another, more significant problem will be the noise of the signal and the huge temperature, because a huge amount of substance usually falls into a black hole, which, when dropped, heats up to hundreds of millions of degrees

Nevertheless, let’s say that we were able to protect the camera from high temperature and filter the signal, so what will we see?

The view that the camera records depends on how massive the black hole will be. In black holes of stellar mass, the gravitational field is very heterogeneous, huge tidal forces appear in it that will tear the camera apart long before it approaches the black hole. All that we will see in this situation is a hot accretion disk and darkness in the middle of horizon events. To see something more interesting, you need to send the camera into a supermassive black hole, where the tidal forces are much weaker.

Near a black hole, space-time is strongly curved, and the closer to the singularity, the greater the curvature of space-time. When the camera begins to “sink” into the gravitational well of the black hole, the field of view of the camera will begin to narrow, in fact, the light on one side (right or left) of the camera will cease to reach it, and eventually all the light of the universe will degenerate into a small blue dot (due to gravitational blue shift). Before passing the event horizon, total darkness will come, however, we will not have time to see this, because for us, by that time, billions of years will pass.

## Time Dilation Examples

The effects of time dilation are used often in science fiction stories, dating back to at least the 1930s. One of the earliest and most well-known thought experiments to feature time dilation is the famous Twin Paradox, which demonstrates the curious effects of time dilation at its most extreme.

Time dilation becomes most apparent when one of the objects is moving at nearly the speed of light, but it manifests at even slower speeds. Here are just a few ways we know time dilation actually takes place:

## Was time dialation understood before 1905?

I seem to recall the concept going back before Einstein's Special Theory of Relativity. In fact, I believe Galileo knew of time dilation in an early form of relativity he proposed.

### #3 jayhall0315

There were a number of physicists playing around with different dimensions and the ether including Poincare and Lorentz but none really made note of variable time (it simply did not occur to them that something so fundamental could be based on the observer reference frame). The only person I am aware of who actually looked at variant time was the polymath Fredrick Carl Gauss. He was the first I am aware to look at the ancient equation d=vt on a curving surface and realize that time might actually be distorted by curvature. (this is really more in the general relativity realm) So, the short answer is no. Einstein really was the first to see the whole picture from special relativity including the variable passage of time depending upon the observer's reference frame.

### #4 astro_NC

Others did in fact hit upon some of the ideas central to relativity theory, but Einstein's way of thinking about it is what revolutionized physics.

This video contains perhaps the most beautiful visual demonstration of how special relativity works. Darren, your question is also discussed and answered in a most profound way. One must pay close attention to the video, but it is well worth it.

**Edited by astro_NC, 08 January 2017 - 12:49 AM.**

### #5 GJJim

There were a number of physicists playing around with different dimensions and the ether including Poincare and Lorentz but none really made note of variable time (it simply did not occur to them that something so fundamental could be based on the observer reference frame). The only person I am aware of who actually looked at variant time was the polymath Fredrick Carl Gauss. He was the first I am aware to look at the ancient equation d=vt on a curving surface and realize that time might actually be distorted by curvature. (this is really more in the general relativity realm) So, the short answer is no. Einstein really was the first to see the whole picture from special relativity including the variable passage of time depending upon the observer's reference frame.

Leibniz, and later Mach argued that the concept of time could be cast in terms of relations between physical events (order, simultaneity, etc.) and there was no need for the absolute framework championed by Newton. Time and space are more akin to a relational database than a giant aquarium. Einstein's stroke of genius was to condense these largely philosophical arguments into a mathematically consistent model that was acceptible to scientists of his day.

**Edited by GJJim, 08 January 2017 - 11:38 AM.**

### #6 JonNPR

Yes. Fitzgerald in 1889, followed by Lorentz in 1892 devised the contraction equations in order to explain the Michelson-Morley aether experiment. But as the Wiki explains, there's depended upon ad hoc assumptions and only an hypothesis. Einstein was the first to show a better explanation, in 1915. He demonstrated that contraction wasn't necessary due to traveling through a hypothetical aether, in an attempt to save the aether idea. Instead, it was directly due to special relativity, a full fledged (scientific, formal) theory.

## Is Time Dilation Real?

Is time dilation real? Such as when something speeds up or when nearing a black hole, like in

Interstellar?

Some supermassive black holes at the centre of galaxies are devouring the surrounding material. They . [+] also divert part of it away through powerful winds and jets. This artist's impression shows how the black hole accretes the surrounding matter through a disc (orange). Part of the accreted material is pushed away in a wind (blue), which in turn powers a large-scale galactic outflow of molecular gas (red). Image credit: ESA/ATG medialab

Time dilation is real! And anyone who uses a GPS, or the “My Location” option on Google Maps, is making direct use of the fact that time dilation is real.

The idea behind time dilation is just as you describe, where your perception of time is dependent on how fast you’re going, or how close to a very large object you are. *Interstellar* actually did a remarkably good job of discussing time dilation in general, especially given how counter-intuitive ideas to do with relativity can be.

In *Interstellar*, the major plot points surrounding time dilation are the difference between time passing in two different locations, where the astronauts sitting near the black hole felt time passing at a slower rate than their families back home on Earth. Similarly, when some of them head down to the water world, they feel time passing at a slower rate than the astronaut left on the spacecraft. Now, ignoring the exact numbers of this difference, which depends on things like how big the black hole is, how far you are from it, and how much movie magical math you’re willing to go for, this kind of time dilation does happen.

The principle is this: the deeper you find yourself in a strong gravitational field, the slower your clock will run, relative to someone who is not in as strong a gravitational field. This can be translated into meaning the closer you are to something large, the slower your clocks will run. However, it also means that if your two clocks are the same distance from two objects, one which has a much stronger gravitational pull than the other (say, a planet for one clock, and a black hole for the other), the clock around the more massive object (the black hole) will run slower, even though both clocks are the same distance away.

Artist's illustration of a Navstar-2F satellite of the Global Positioning System (GPS). Image use: . [+] public domain.

A GPS satellite, which orbits about 12,500 miles above the surface of the earth, is further away from the Earth than those of us who live on its surface, which means that our clocks on the surface will appear to progress more slowly, relative to the clock on the GPS. Unfortunately, we really need these two clocks to operate in sync with each other, or the location information you get back starts to be increasingly inaccurate, defeating the point of having the GPS in the first place. This effect can be mathematically calculated, so we can correct for this slight difference in clock speed by setting the GPS clock to run a little slower than normal. Just a little slower this effect is only nanoseconds in size near the Earth.

But for a GPS, another form of time dilation is also in effect, because the fact that the satellite is in motion changes the clocks as well. This form of time dilation goes such that the faster you’re moving, the slower your clocks appear to move, relative to someone who is not moving. This effect gets more and more dramatic the closer to the speed of light you travel, so the offset in your clocks would get more severe.

A GPS satellite, which has to travel in a circle 12,500 miles above the Earth’s surface every 12 hours, is therefore going around 8,670 miles per hour. The speed of light is about 670,000,000 miles per hour, so our GPS is only going about 13 millionths of the speed of light. This is *not* a significant fraction of light speed, but it’s enough that we can calculate and measure the impact that it has on the onboard clocks on the GPS.

The speedy motion of a satellite in space slows down its clocks relative to ours on earth, while its . [+] distance out of the earth's gravitational well makes satellite clocks go a bit faster. Thus shuttle pilots age less than a couch potato at the south pole, while geosynchronous orbiters (as well as interstellar dust particles) age more rapidly. This also means that the surface of the earth may be more than a year older than the earth's center, assuming that both were formed at the same time. Although the resulting errors in satellite timing are measured in nanoseconds, lightspeed is 30 centimeters (1 foot) per nanosecond so that the combined effects can result in GPS errors as large as 15 meters if not taken into account. Image credit: P. Fraundorf, CC BY-SA 3.0

As it happens, the gravitational time dilation has a more significant impact on the clock than the speed of the GPS relative to us on the ground. While the speed-up due to gravity is partially canceled out by the slow-down imposed by its 8,670 mph speed, there’s still some residual extra speed on those clocks because they’re so far away from the most concentrated part of the Earth's gravitational field. It’s this slightly diluted speed-up that’s calibrated into the clocks that we send up onto our satellites for the GPS network. (You’ll notice from the chart above that the space shuttle’s orbit was so close to the Earth that the gravitational effect wasn’t a big one, so the only clock-altering effect they were dealing with was due to their orbital speed around our planet.)

Anytime you pull your phone out to get directions to a restaurant, or to check how far you have left to walk, and your phone accurately figures out where you are, the GPS connection that underlies that software is relying on our understanding of relativity, and the time dilation the satellite is undergoing, to do its job properly.

## Answers and Replies

Hi All. I have a little bit of a strange question I'm trying to figure out.

If a human body was undergoing time dilation relative to another object, we believe that their brain, and therefore mind would slow down along with the rest of their body, correct?

If this is so, and light continues to **hit a time-dilated person's eyes at a constant rate** (because c is constant), would light accumulate on their retina and cause their vision to become blurred or brighter than usual because it's **hitting their eyes faster** than their brain can process it?

I suppose the same question applies to sound as well, though the speed of sound is not constant.

I may be off base here and have some incorrect assumptions. Thanks for humoring me!

How can the light be hitting their eyes both at a constant rate and faster?

And why are you concerned that a human body is undergoing time dilation relative to just one other object--there are billions of objects out there, which one is going to make a difference to the human body?

Sorry, I know my question wasn't very clear.

Say there is a light source, and I am far away from it. I begin moving towards it, and as I do so, time dilates for me (though it seems constant to me) and my body slows down relative to the light source. Even though my body has slowed down, the light hitting my eyes remains at a constant speed from my perspective due to the time dilation I'm undergoing. However, my brain is moving more slowly and taking longer to process the light hitting my eyes.

Would there be some sort of "build up" of light caused by my eyes not being able to process the light quickly enough?

Basically what I'm wondering is if things get brighter as you move towards a light source, and therefore more slowly when moving away. I know that they red/blue shift, but I'm wondering about brightness or some blurring due to not being able to process the light as quickly as it's coming in due to your body being slowed down.

I'm just using a single light source/object in this example to make it as simple as possible. I know that at the speeds we travel at in our human experiences that time dilation is largely negative, so I'm guessing this affect would be as well, but I'm curious about the principal of it.

I'm pretty sure the answer is "no," or, "this is an invalid question," but I don't understand why.

Sorry, I know my question wasn't very clear.

Say there is a light source, and I am far away from it. I begin moving towards it, and as I do so, time dilates for me (though it seems constant to me) and my body slows down relative to the light source. Even though my body has slowed down, the light hitting my eyes remains at a constant speed from my perspective due to the time dilation I'm undergoing. However, my brain is moving more slowly and taking longer to process the light hitting my eyes.

Would there be some sort of "build up" of light caused by my eyes not being able to process the light quickly enough?

Basically what I'm wondering is if things get brighter as you move towards a light source, and therefore more slowly when moving away. I know that they red/blue shift, but I'm wondering about brightness or some blurring due to not being able to process the light as quickly as it's coming in due to your body being slowed down.

I'm just using a single light source/object in this example to make it as simple as possible. I know that at the speeds we travel at in our human experiences that time dilation is largely negative, so I'm guessing this affect would be as well, but I'm curious about the principal of it.

I'm pretty sure the answer is "no," or, "this is an invalid question," but I don't understand why.

Your brain doesn't slow down due to time dilation nor do your bodily processes take any longer to process light coming into your eye. Time dilation is something OTHER people see you experiencing. You don't experience it at all. You WILL see light coming into your eyes differently than if you were not traveling a substantial fraction of c, but that's not due to any change in your mental processes.

Although I don't have a link handy, there are "movies" on the internet that show what incoming light would look like as you increase in speed, and similarly as you go into a black hole and "experience" time dilation due to gravity.

It's my understanding that time dilation is an objective phenomenon, but that the subject(s) of the time dilation wouldn't know it because their brain is slowed down along with their body, so they wouldn't see themselves as moving in slow motion, but they would see other people moving quickly, relative to them.

Given this, and that the stimuli hitting their senses (e.g. Light) is NOT slowed down, wouldn't their brains "fall behind" and not be able to process the sensations as quickly as they would be coming in?

Maybe I'm just reasking the same question again.

It's my understanding that time dilation is an objective phenomenon, but that the subject(s) of the time dilation wouldn't know it because their brain is slowed down along with their body, so they wouldn't see themselves as moving in slow motion, but they would see other people moving quickly, relative to them.

Given this, and that the stimuli hitting their senses (e.g. Light) is NOT slowed down, wouldn't their brains "fall behind" and not be able to process the sensations as quickly as they would be coming in?

I think I know what you are getting at and I think you are on the right tracks once we unravel which reference frames you are talking about. It is a good idea with relativity subjects to always be clear about which reference frame you are talking about.

OK, I will use the example of a camera rather than a human brain and eye, because it is simpler to analyse, but the principles are the same.

Consider:

A laser light source that emits 10 photons per second (in its rest frame).

A camera that has a fixed shutter speed of one second (in its rest frame).

The camera and laser source are moving towards each other at 0.8c as measured in the rest frame of either.

__As seen in the camera reference frame:__

Taking only the simple classical Doppler effect into account, about 50 photons would arrive at the camera film in the the one second the shutter is open, due to the light source going towards the camera. On top of this the light source is subject to time dilation, so it actually only emits 60 photons per second in this reference frame and the nett result is that about 30 photons per second arrive at the camera film in the one second the shutter is open. All this produces a blue shift and a brighter image than if the source was stationary with respect to the camera. __As seen in the laser source reference frame:__

In this reference frame the camera appears to be moving towards the laser source. The laser is now not subject to time dilation and emits its regular 10 photons per second. The fact that the camera is going towards the source means that it would receive 18 photons per second if we consider only simple classic Doppler shift. However, due to time dilation the camera shutter is actually open for 1.6666 seconds in this reference frame and the end result is that 30 photons arrive at the film in the time the shutter is open. (Note that this is the same result as for the camera reference frame but the explanation is different.)

Now if you liken your eye and brain to the camera and its control circuits, then yes, in the reference frame of another observer moving relative to you, he explains why you see the blue shift and brighter image, partly in terms of your slowing brain processes just as if your brain was a mechanical device. However, in your own rest frame you do not experience any brain slow down and you explain everything in terms of Doppler shift and the slowing down of the emission rate of the light source.