## Time dilation

• 48
Time dilation will cause a lot of problems for mankind when we're able to interstellar travel. Let's say in 2200 we find ourselves advanced enough to plan a visit to a new star system. Those people that leave earth for this trip will never see anyone they know from earth again due to time dilation. They will leave for a 50 year trip (for example) and hundreds and hundreds of years could have past here on earth, because of the time dilation their clock will move slower than ours due to the speed at which they would be traveling.

What this means for us on earth is that we won't learn anything from this mission to a new star system because they won't return before we die of old age, the next generation will gain the knowledge. Scientists will have to invest in these missions so that the people in few hundred years benefit from the exploration of a new star system.

The people who left for this mission will have only lived an extra 20 years and so if they return to earth the trip back would also add the amount of years onto earth, they would have essentially traveled to a future earth. Space missions are about to get very complicated.
• 1.3k
Space missions are about to get very complicated.

• 3.2k
They will leave for a 50 year trip (for example) and hundreds and hundreds of years could have past here on earth, because of the time dilation their clock will move slower than ours due to the speed at which they would be traveling.

Not necessarily. Because of Special Theory of Relativity's Receprocity one can say that Earth is accelerating away from the spaceship, so it is the clocks in the Earth that are slowing down.
• 2.7k
It depends on how fast the ship is travelling. But, say we get all crazy one day and are able to move mass using a warp drive bubble, we could effectively negate time dilation. A nice vacuum for thought.
• 612
Those people that leave earth for this trip will never see anyone they know from earth again due to time dilation. They will leave for a 50 year trip (for example) and hundreds and hundreds of years could have past here on earth,
If the trip takes 50 years (ship time), they're not exactly expecting to see their relatives again anyway. Human life span is not that long. So why is this a problem? A trip like that can only be one way. You kiss your family goodbye.

Not necessarily. Because of Special Theory of Relativity's Receprocity one can say that Earth is accelerating away from the spaceship, so it is the clocks in the Earth that are slowing down.Rich
Earth does not accelerate away. That would require a massive force on Earth, sending it out of the solar system.
• 3.2k
Earth does not accelerate away. That would require a massive force on Earth, sending it out of the solar system.

Reciprocity of Special Relativity says there is no privileged frame of reference. If there is a privileged frame of reference, STR is wrong and Einstein's T is wrong. There is no T in GTR.
• 612
Rich
...because of Special Theory of Relativity's Receprocity one can say that Earth is accelerating away from the spaceship,Rich
Earth does not accelerate away. That would require a massive force on Earth, sending it out of the solar system.
— noAxioms

Reciprocity of Special Relativity says there is no privileged frame of reference. If there is a privileged frame of reference, STR is wrong and Einstein's T is wrong. There is no T in GTR.I didn't mention a frame, privileged or otherwise. I was commenting on your statement about Earth accelerating.

Earth might be moving away from the ship, but it is not ever accelerating away from it.
• 3.2k
Earth might be moving away from the ship, but it is not ever accelerating away from it.

There is no privileged frame of reference under STR? Either viewpoint is coherent according to STR. STR doesn't allow for exceptions when it is convenient for a science fiction story.
• 612
Earth might be moving away from the ship, but it is not ever accelerating away from it.
— noAxioms

There is no privileged frame of reference under STR? Either viewpoint is coherent according to STR. STR doesn't allow for exceptions when it is convenient for a science fiction story.
Rich
All true, but again, I was talking about your use of 'acceleration'. In no frame does Earth accelerate beyond its annual wobble around the sun. It would be quite the science fiction story if it did (and yes, I've read such stories).
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[ In terms of measurement either viewpoint is equivalent. Either body can be accelerating away from the other.
• 612
In terms of measurement either viewpoint is equivalent. Either body can be accelerating away from the other.Rich
Completely false. You seem to not understand the distinction between velocity and acceleration.
• 3.2k
Completely false. You seem to not understand the distinction between velocity and acceleration.

You don't understand STR and GTR. There is no T in GTR and STR, which has a T, states all frames of reference are equivalent. Sci Fi adherents love to mix up and intermix the two. Makes Relativity so much more fun.
• 48
yeah I mean this seems like it will pose a big enough problem to at least try and develop time technology and maybe they'll find a way to keep everyone's clocks running at the same speed
• 48
earth doesn't experience any acceleration what so ever. From either viewpoint the perspective is that they are both accelerating away from each other but in reality only the ship is experiencing acceleration.
• 3.2k
earth doesn't experience any acceleration what so ever.

How does someone on the Earth know that they are not accelerating from the spaceship?

Where is the evidence that either equation is ontological and describes biological time/aging?

Where is T in GTR?

As I said, this stuff is great for Sci Fi programming but STR are about measurements and gravity not about biological aging.
• 3k
Reciprocity of Special Relativity says there is no privileged frame of reference. If there is a privileged frame of reference, STR is wrong and Einstein's T is wrong. There is no T in GTR.I didn't mention a frame, privileged or otherwise. I was commenting on your statement about Earth accelerating.

Special relativity only applies to inertial frames of reference - those that are not accelerating relative to each other. Non-inertial frames, those that are accelerating, are covered under general relativity. This is something that has confused me. I've looked for explanations but different people who seem to know what they are talking about seem to have different stories. What follows is my understanding of what happens. I may have it wrong.

If two space ships travelling a significant fraction of the speed of light (c), but not accelerating, pass each other going opposite directions and check each other's clocks, each will observe that the other's clock has slowed down.

If, on the other hand, there are two space ships at rest relative to each other and one accelerates away from the other up to a significant fraction of c then turns around and comes back and then the clocks are checked, both will observe that less time has passed on the accelerating ship.

This is called the "clock problem" or "twin paradox." Look Twin Paradox up on Wikipedia and you'll see the kind of unsatisfying explanation I was talking about. Please don't think I think that "unsatisfying" is the same as "wrong."
• 3.2k
Special relativity only applies to inertial frames of reference - those that are not accelerating relative to each other.

And where does such a condition exist in the universe?

If, on the other hand, there are two space ships at rest relative to each other and one accelerates away from the other up to a significant fraction of c then turns around and comes back and then the clocks are checked, both will observe that less time has passed on the accelerating ship.

Clocks may be affected by acceleration (but which one?), but this has nothing to do with biological aging. In any case, there had to be deceleration somewhere to even check the clocks.

I also want to underscore there is no T in GTR. Only some strange variable that is called space time because the equations entangle the two concepts. However, there is no reason to elevate the equations to an ontologically status since it is about measurement and not biological evolution.
• 3k

How does someone on the Earth know that they are not accelerating from the spaceship?
Rich

Yes, I'm sure the earth is not accelerating. F = ma. If no force is applied, there is no acceleration. The force is applied to the spaceship, not the earth. You know what acceleration feels like. You are pushed back against your car seat. If you don't feel the push, you ain't acceleratin. No force will be applied to the Earth.
• 3.2k
Yes, I'm sure the earth is not accelerating. F = ma. If no force is applied, there is no acceleration.

The Earth is accelerating. It is always accelerating (remember gravity?).

How does an observer who is measuring know what is accelerating. You are using information that is not contained in GTR. In GTR you are taking measurements. That is all. A person in a spaceship could just as easily claim the Earth is accelerating away via use of instruments.
• 3k
And where does such a condition exist in the universe?Rich

Two cars sitting at a stop light next to each other are in an inertial frame of reference. Two cars travelling at constant speed and passing each other in a straight line going opposite directions are in an inertial frame of reference. You are in an inertial frame of reference with anything that is not moving in relation to you.

Clocked may be affected by acceleration (but which one?)Rich

After Ship 1 accelerates away, turns around, and then returns, people on both Ships 1 and 2 will agree that the clock on Ship 1 is behind that on Ship 2.

...this has nothing to do with biological aging.Rich

That's not correct. The physical and chemical processes in our bodies are affected the same way as the processes in the clock.

...In any case, there had to be deceleration somewhere to even check the clocks.Rich

Why is that? We can just get on the radio and ask what time it is.
• 612
Reciprocity of Special Relativity says there is no privileged frame of reference. If there is a privileged frame of reference, STR is wrong and Einstein's T is wrong. There is no T in GTR
-- Rich
I didn't mention a frame, privileged or otherwise. I was commenting on your statement about Earth accelerating.
— noAxioms

Fixing the quote. I didn't say the 'Rich' part above.

If two space ships travelling a significant fraction of the speed of light (c), but not accelerating, pass each other going opposite directions and check each other's clocks, each will observe that the other's clock has slowed down.

If, on the other hand, there are two space ships at rest relative to each other and one accelerates away from the other up to a significant fraction of c then turns around and comes back and then the clocks are checked, both will observe that less time has passed on the accelerating ship.

This is called the "clock problem" or "twin paradox." Look Twin Paradox up on Wikipedia and you'll see the kind of unsatisfying explanation I was talking about. Please don't think I think that "unsatisfying" is the same as "wrong."
More or less, yes.
I don't think relativity goes into explaining why our universe has this geometry. It just does, and the theory predicts what will be measured.

How does someone on the Earth know that they are not accelerating from the spaceship?Rich
The guy in the ship is plastered into his seat when doing the massive acceleration. The guy on Earth is not. OK, a black-hole sort of gravitational field could do that to Earth, but there is none in the scenario discussed.

The Earth is accelerating. It is always accelerating (remember gravity?).Rich
Trival acceleration to non-relativistic speeds that cancel out over a year. See the part about the wobble around the sun I posted above.

The guy in the ship needs special technology to not die from the massive acceleration needed to get him up to enough speed to notice his age discrepancy, and he needs to do it 4 times (out, stop, start back, stop at end) before he dies of old age. The experiment is not practical even if we had a ship that could do that.

In any case, there had to be deceleration somewhere to even check the clocks.Rich
Clocks can be unambigously compared when in each other's presence, and need not be stationary relative to each other. In short, you can look at each other as you pass by at speed if you like.

Why is that? We can just get on the radio and ask what time it is.
See above. Comparisons of spatially separated clocks are ambiguous and yield different answers depending on the reference frame chosen. The radio doesn't help. This ambiguous ordering is the best explanation of the twins experiment.

I had put together an illustration of the twins experiment which kept things quite simple, involves no acceleration (using instead a tag team), and thus no GR complications. I can post it again if you like.
• 3k
The Earth is accelerating. It is always accelerating (remember gravity?).Rich

Well, yes, the Earth is always accelerating - it is revolving around the sun, which involves acceleration. The whole solar system is revolving around the center of the galaxy, which involves acceleration. I don't see how that is relevant. The space ship moves away from the Earth at a faster and faster speed but no force has been applied to the Earth to make that happen.

Thought experiment. Two cars are next to each other at a stop light, not moving. The light changes and one of them accelerates while the other stays still. They begin to move in relation to each other. Let's say I put a small camera in each car to record what is happening while this is going on. If, later, I showed you the two films, would you be able to tell who was accelerating? Answer - yes. You would see the people in the accelerating car pushed back into their car seats. You would see the cup of coffee spill. You would see the little pine tree hanging from the mirror swing.
• 3k
More or less, yes.

What's the "less" part?
• 612
What's the "less" part?
I didn't like the wording of this part. Each clock is dilated slower in the frame of the other, but that cannot be directly observed.
If two space ships travelling a significant fraction of the speed of light (c), but not accelerating, pass each other going opposite directions and check each other's clocks, each will observe that the other's clock has slowed down.
They only get one peek at each other's clocks as they pass. You can't observe the dilation. If you're watching a moving clock, it appears to run faster if it is approaching. The Doppler effect is far more significant than the dilation.

If you want to get picky, the Earth revolves about its axis, not the sun. It orbits the sun.
• 3k
They only get one peek at each other's clocks as they pass. You can't observe the dilation. If you're watching a moving clock, it appears to run faster if it is approaching.

I think this is not correct. Why do I only get one peek? Let's say the clock on the ship is constructed to flash a light at an established frequency. I can just measure the times between flashes as it passes.

If you want to get picky, the Earth revolves about its axis, not the sun. It orbits the sun.

I love being picky. It is appropriate to say that he Earth revolves around the sun. In the definition of revolve I looked up, it was one of the examples used. It would also be appropriate to say the Earth revolves around it's axis, but I would probably use "rotate."
• 612
I think this is not correct. Why do I only get one peek? Let's say the clock on the ship is constructed to flash a light at an established frequency. I can just measure the times between flashes as it passes.
Right. It will flash faster as it approaches and slower after it goes by. This is why Andromeda is blue shifted when we look at it. Relativity says it should be a little red shifted since it's processes are slower in our frame. Point is, you're not getting accurate timings when you're not in the presence of the source of the signal. You can compute the delay if you know the distance, but the distance to the source is frame dependent, so still ambiguous.

If you want to get picky, the Earth revolves about its axis, not the sun. It orbits the sun.
— noAxioms

I love being picky. It is appropriate to say that he Earth revolves around the sun. In the definition of revolve I looked up, it was one of the examples used. It would also be appropriate to say the Earth revolves around it's axis, but I would probably use "rotate."
Fair enough. I got this from http://wikidiff.com/revolve/orbit:
As verbs the difference between revolve and orbit is that revolve is (label) to orbit a central point while orbit is to circle or revolve around another object. — wikidiff
• 307
Because of Special Theory of Relativity's Receprocity one can say that Earth is accelerating away from the spaceship, so it is the clocks in the Earth that are slowing down.Rich

I'm far from an expert when it comes to time and relativity, but I don't see how this makes any sense. The Earth and the spaceship aren't two isolated objects in a void, this is the real world we're talking about. When one thing moves, it moves relative to everything else in the universe. So when the spaceship accelerates, you could not say instead that the Earth is accelerating because the Earth isn't accelerating relative to the universe--only the spaceship is.
• 3k
Right. It will flash faster as it approaches and slower after it goes by.

I'm thinking about this and I'm not sure. Why would the rate of flashing be different when it is approaching vs. moving away from me? The only difference I can see is that, as it gets closer to me, the angle between my line of sight and the direction of travel increases.
• 612
I'm thinking about this and I'm not sure. Why would the rate of flashing be different when it is approaching vs. moving away from me? The only difference I can see is that, as it gets closer to me, the angle between my line of sight and the direction of travel increases.
For simplicity, assume it is coming directly at you/directly away. No angles to complicate it.
This is Doppler effect, the same reason a train horn pitch is higher when approaching. The effect is far greater at a given point of observation than that of time dilation.

Suppose we flash once a minute. Sun is 9 minutes away, and the flash source is coming at us at half lightspeed. The trip takes 18 minutes, but only 9 minutes to the Earth observer since the first flash is delayed by the 9 light-minute distance.
Time dilation might take one of those flashes off (I'm guessing), so you count 17 flashes in 9 minutes.
• 3k

Still don't understand. This should have nothing to do with Doppler. That changes the wavelength of the light but won't change the frequency of the flash.
• 612
Still don't understand. This should have nothing to do with Doppler. That changes the wavelength of the light but won't change the frequency of the flash.
Same thing. See the example above with the sun. Let's say it flashes 10 times a minute (every 6 seconds).
Without relativity, the trip takes 18 minutes, so the Earth observer sees 180 flashes in 9 minutes, twice the actual rate. Relativity says the fast-moving clock is dilated and only flashes 156 times (my guess was off) in those 18 Earth minutes. Still appears faster to the Earth observer. At 0.5c, the Doppler effect doubles the pace, and relativity removes only about a sixth of the pace.
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