I How Relativistic Speeds Affect Time Perception on a Spaceship

[jj65854]

Assume a space ship is going at relativistic speed in a straight line. Is it correct to say that its own experience of time is slower than that of a stationary person? So in other words the stationary observer perceives it moving at a higher percent of C than does a crew member on the ship measuring their own velocity?

 

[jj65854]

I’m a new member btw. Trying to understand relativity. Thanks for your input!

 

[PeterDonis]

Assume a space ship is going at relativistic speed in a straight line.

Relativistic speed relative to something else. There is no such thing as "speed" in an absolute sense.

In this case you appear to mean "relativistic speed" relative to a person whom you describe as "stationary".

Is it correct to say that its own experience of time is slower than that of a stationary person?

As you state it this question is too vague and ambiguous to have a well-defined answer. What does "experience of time" mean?

in other words the stationary observer perceives it moving at a higher percent of C than does a crew member on the ship measuring their own velocity?

The "stationary" observer measures the ship to be moving at a relativistic speed relative to him. Call this speed $v$.

The crew of the ship itself measures the ship to be moving at zero speed relative to itself. The crew of the ship measures the "stationary" observer to be moving at speed $v$ relative to the ship.

 

[Drakkith]

Assume a space ship is going at relativistic speed in a straight line. Is it correct to say that its own experience of time is slower than that of a stationary person? So in other words the stationary observer perceives it moving at a higher percent of C than does a crew member on the ship measuring their own velocity?

The ship's clock continues to tick at 1 second per second according to anyone on the ship (we say that they are in the same reference frame). To someone that this ship is passing at a high velocity, let's call this person Bob, the ship's clock ticks slower. To someone on the ship, Bob's pocket watch is ticking slower than their clock on the ship.

The exact differences depend on the velocity.

 

[jj65854]

The ship's clock continues to tick at 1 second per second according to anyone on the ship (we say that they are in the same reference frame). To someone that this ship is passing at a high velocity, let's call this person Bob, the ship's clock ticks slower. To someone on the ship, Bob's pocket watch is ticking slower than their clock on the ship.

The exact differences depend on the velocity.

Thank you Drakkith, this is what I was trying to get at. Sorry only high school physics.

But building on this, does someone in the ship’s reference frame perceive their velocity as the same percent of C as Bob would perceive it?

 

[PeterDonis]

does someone in the ship’s reference frame perceive their velocity as the same percent of C as Bob would perceive it?

Someone in the ship perceives the ship's speed to be zero.

Someone in the ship perceives Bob's speed to be the same as Bob perceives the ship's speed to be. (But the motions are in opposite directions, so the two velocities are of equal magnitude, opposite direction.)

 

[Drakkith]

But building on this, does someone in the ship’s reference frame perceive their velocity as the same percent of C as Bob would perceive it?

No, just like when you are in a car on the freeway you don't perceive your car as travelling at 80 mph, you perceive it as being still. Otherwise it would slam into you. What you perceive as moving at 80 mph is all the terrain around you. Similarly, an observer on a spaceship moving past Bob will measure themselves as stationary and Bob moving at, say, 0.5c.

 

[Drakkith]

Note: I've split these posts off into their own thread. Original thread here: https://www.physicsforums.com/threads/battery-life-on-very-fast-moving-object.1061393/

 

[PeroK]

But building on this, does someone in the ship’s reference frame perceive their velocity as the same percent of C as Bob would perceive it?

As you sit at your computer, what do you perceive your speed to be? Recall that the Earth orbits the Sun at a speed of about $30 km/s$. Do you perceive that?

PS define "perceive".

 

[davidjoe]

As you sit at your computer, what do you perceive your speed to be? Recall that the Earth orbits the Sun at a speed of about $30 km/s$. Do you perceive that?

PS define "perceive".

Following the posts in the threads and going back pages, supposing we have a voyage to Proxima B in a generation ship, regardless of its speed relative to earth, time passes normally and the voyage time experienced onboard is the conventional function of distance covered, divided by speed during the span travelled, even if traveling at realistic speed?

No compression or affect on ship time such that travelers could perceive by their clocks, disparity in their aging relative to parties conversant with them on earth, that time slowed down, if moving at a significant fraction of C because according to some other object, they have been moving at that significant fraction already, prior to departure, with no such effects.

It seems too straight forward to be the right interpretation of the posts.

 

[Nugatory]

supposing we have a voyage to Proxima B in a generation ship, regardless of its speed relative to earth, time passes normally and the voyage time experienced onboard is the conventional function of distance covered, divided by speed during the span travelled, even if traveling at relativistic speed relative to the earth?

Note the text that I added in italics. With that addition, the answer to the question is as that far as the people onboard are concerned the ship is at rest and proxima centuari is moving towards them - so the time that passes on the ship is the time that it takes proximan centauri to cover the distance between the starting point and the ship.
Without that addition, the answer is "the question is ill-formed and you have not yet internalized the answers you've seen in previous threads".

No compression or affect on ship time such that travelers could perceive by their clocks, disparity in their aging relative to parties conversant with them on earth, that time slowed down, if moving at a significant fraction of C because according to some other object, they have been moving at that significant fraction already, prior to departure, with no such effects.

The travellers certainly can detect the effects of time dilation - all they need to need to do is exchange radio messages back and forth with the earth, each message of the form "when I sent this message my clock read X; when you receive this message reply with a message telling me what time your clock read when you received it". After a few such message exchanges and after allowing for the the time it takes for these messages moving at speed $c$ to make it from sender to receiver, the traveller will correctly calculate that clocks on earth are running slow relative to their own.

But - crucially - the earth person will also be able to use this exchange of messages to come to the equally correct conclusion that the traveller's clock is running slow relative to their own. This is the symmetry of time dilation; it pretty much has to be that way because we can reasonably the ship to be at rest while earth and proxima centauri are moving in one direction relative to the ship, or that earth and proxima centauri are at rest while the ship is moving in the other direction. If the moving clock is slow relative to the stationary one, and we can choose either clock to be the stationary one, then either clock can be slow relative to the other.

It may seem paradoxical that the ship clock is slow relative to the earth clock and the earth clock is slow relative to the ship clock. But that's only because in describing this apparent paradox we have erred by not accounting for the relativity of simultaneity. When we do the paradox disappears and it all makes sense (and the GPS system works properly).

Until you can recognize that every time you mention a speed without saying what it is relative to you have made a mistake, you will find relativity incomprehensible.

 

[Drakkith]

Following the posts in the threads and going back pages, supposing we have a voyage to Proxima B in a generation ship, regardless of its speed relative to earth, time passes normally and the voyage time experienced onboard is the conventional function of distance covered, divided by speed during the span travelled, even if traveling at realistic speed?

Time passes normally on the ship, yes, but voyage time depends highly on the speed, especially as you approach c, via length contraction. The on-ship voyage time could be many hundreds of years if the ship is traveling at a very small fraction of c, or it could be months or even weeks or days if the ship is traveling very very close to c. For example, if the ship is traveling at 0.9999c (99.99% of c) relative to Earth towards Alpha Centauri, which is about 4 light-years away, then the on-ship voyage time takes only 0.056572 years and the distance the ship travels is only 0.056567 light years according to the passengers. Of course, for observers on Earth, the trip took 4.0004 years and the distance traveled was 4 light-years.

 

[davidjoe]

To Nugatory’s post: We are really speaking the same language, now! I was clearly born in the wrong decade. Scorned in the 80’s for saying the same things aloud, hand in the air, all eyes rolling at me, that I’m reading here the last few days as being accepted now. There’s an entire generation of Xers that were basically brow beaten, in order to get a grade, to elaborate on and explain outcomes based on premises (we) I considered to be illogical propositions in the first place.

It was curriculum and they had no choice in the matter, taught it, and I don’t blame them. But suffice it to say that our little scientific souls were split in the formative years and we inculcated everything you guys call outdated now, and revert to it tentatively and unconsciously sometimes, reflexively. We are lost irretrievably, as a product of that decade, as far much of physics is concerned, unless we are either active community members in scientific fields or happen to have a gene that wants to explore for fun what others consider stressful work. I don’t know when all this changed and became more grounded, and
I guess it really doesn’t matter, but it’s night and day, present to past, based on what I’m reading.

I think we did speak at cross-purposes initially. I do appreciate how you guys here have conservatively described any differences you have with your predecessors’ quantifications, conventions or approaches to a subject. Those don’t have to be extremely numerous to make a huge difference to some people. Yes, I appreciate that messages are delayed by distance. I had assumed your italicized insertion.

I’m a step behind you presently on the subject of the clock appearing to have, mutually, slowed. Part of me wonders if that slowing of a clock in appearance that is mutually observed as it passes is the Doppler or red shifting effect first discussed before the thread split, that would be inherent in EM transmission of information, or if this something more than that.

 

[Nugatory]

then the on-ship voyage time takes only 0.056572 years and the distance the ship travels is only 0.056567 light years according to the passengers.

Careful... You meant to say that Proxima Centauri started 0.056567 light-years away from the ship and moved that distance in 0.056572 years to reach the ship (which according to the passengers is not moving).

(Of course Drakkith already knows this, but it really is surprisingly difficult to get this stuff right when using natural language).

 

[Drakkith]

I’m a step behind you presently on the subject of the clock appearing to have mutually slowed. Part of me wonders if that slowing of a clock in appearance that is mutually observed as it passes is the Doppler or red shifting effect first discussed before the split, that would be inherent in EM transmission of information, or if this something more than that.

The doppler effect is something that must be taken into account as well. That is, if you observe signals from a spacecraft receding from you, both the doppler effect and time dilation will act to make the spacecraft's clock tick slower than your own. Or, if the spacecraft is coming towards you, the doppler effect will act to increase the speed of the spacecraft's clock as observed by you, while time dilation still acts to slow the clock down.

Edit: Scratch most of what I just said. See the following 2 posts.

 

[PeterDonis]

That is, if you observe signals from a spacecraft receding from you, both the doppler effect and time dilation will act to make the spacecraft's clock tick slower than your own. Or, if the spacecraft is coming towards you, the doppler effect will act to increase the speed of the spacecraft's clock as observed by you, while time dilation still acts to slow the clock down.

This might be a misleading way to put it.

The relativistic Doppler formula, all by itself, tells you the observed redshift or blueshift of light signals reaching you, which will also be the observed rate that the observed clock runs faster or slower than yours.

The time dilation formula, all by itself, tells you the calculated rate of the other clock in the inertial frame in which you are at rest. You can also calculate this clock rate by starting with the observed clock rate predicted by the relativistic Doppler formula, as described above, and then correcting for the (changing) travel time of the light from the object to you. The result of this calculation is the same as what you calculate using just the time dilation formula. But they are two different calculations.

In other words, there is no calculation in which you use both the relativistic Doppler formula and the time dilation formula.

 

[Drakkith]

In other words, there is no calculation in which you use both the relativistic Doppler formula and the time dilation formula.

Ah, my mistake then. I was under the impression that there was an additional calculation needed.

 

[Ibix]

Ah, my mistake then. I was under the impression that there was an additional calculation needed.

If you correct for the naive (Newtonian) Doppler shift you are left with time dilation, which may be what you are thinking of. But the full relativistic Doppler already includes time dilation.

 

[Drakkith]

If you correct for the naive (Newtonian) Doppler shift you are left with time dilation, which may be what you are thinking of.

Yes, I think that's what I was getting at.