# The Twin Paradox and Cosmic Inflation **URL:** \ **Category:** Factual Questions\ **Created:** [October 23, 2026, 12:31am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082 "2026-10-23T00:31:03Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [October 23, 2026, 12:31am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/1 "2026-10-23T00:31:03Z")
Einstein had a famous thought experiment: One twin hopped on a space ship that can go at nearly the speed of light, and the other stays behind on earth. For twenty days or so the first twin is flying at near lightspeed for, say, twenty hours. He comes back to earth to find his brother has aged twenty YEARS. The paradox arises from a concept in relativity; from each twin’s point of view, the OTHER twin has…relatively…sped away and moved at near lightspeed, so each would conclude the other should age while time passes slowly for himself. The paradox is resolved when one takes into account that only the twin that got on the space ship was subject to acceleration. Now… Suppose we take this idea and apply to twins in galaxies which are expanding away from each other, as per Hubble. As I understand it, separate galaxies are NOT rushing away from each other through space: space itself is expanding. Is this expansion an acceleration which…theoretically…produces a twin paradox? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 23, 2026, 1:44am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/2 "2026-10-23T01:44:23Z")
That is correct; light from distant galaxies is red-shifted. Like the “twin paradox”, however, it is not really anything to do with acceleration. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 23, 2026, 1:57am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/3 "2026-10-23T01:57:53Z")
The twin paradox is all about changing reference frames. The attendant acceleration is just a feature/symptom, not a cause. If you lived long enough, and you had an equally long lived twin you could each start on one of two nearby galaxies, then wait for the universe to expand, and then travel to your twin’s galaxy to meet your twin again. The reference frame change when you travel to your twin should provide a twin paradox. Just over insane time periods. The two galaxies are each in different reference frames and your travel is in a third, which gets you the paradox. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 23, 2026, 2:01am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/4 "2026-10-23T02:01:01Z")
How do the two twins get back together at the end? If they don’t do that, then no meaningful comparison of their ages is possible. ---
**Author:** ![Stratocaster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stratocaster/32/16393_2.png) [@Stratocaster](https://boards.straightdope.com/u/Stratocaster)\ **Post date:** [October 23, 2026, 9:37am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/5 "2026-10-23T09:37:12Z")
I’m not following how the twin paradox is resolved. Everything in the universe is in motion, correct? Why does space ship guy age more slowly than earthbound guy? Relative to each other, they’re just moving apart…? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 23, 2026, 10:48am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/6 "2026-10-23T10:48:32Z")
It gets resolved when they come back together and it turns out that they disagree on how far the travel distance (relative to each other) was, or, equivalently, how many ticks passed on their clocks. This apparent paradox goes away when you consider 4-dimensional velocity, which they do agree on (it is the speed of light, which is a geometrical constant). ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 23, 2026, 10:53am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/7 "2026-10-23T10:53:05Z")
If nothing else, their experience was much different – one sat restfully on earth while the other experienced many Gs on the way out, got slammed against the other side of the ship as it stopped and reversed, many Gs on the way in, got slammed again as the ship stopped. The acceleration is what moves you from special relativity to general relativity in my simple understanding, and GR is where the twin paradox pops up. If they were just moving at different speeds, they would perceive each other’s clocks moving more slowly or whatever, but both reference frames would be “correct”. Anyway, that’s my simple, mostly incorrect, understanding. I don’t know about the Cosmic Inflation question. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 23, 2026, 11:18am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/8 "2026-10-23T11:18:33Z")
> [@RitterSport](#): > > The acceleration is what moves you from special relativity to general relativity in my simple understanding, and GR is where the twin paradox pops up. No, the twin paradox can be described on ordinary, flat space-time and therefore does not use any General Relativity per se. You can consider accelerated (non-inertial) frames of reference, but there is no actual gravitational field involved. > [@RitterSport](#): > > I don’t know about the Cosmic Inflation question. If you analyse things correctly, you should discover that cosmological redshift and the Doppler effect are the same effect, in some sense. I suppose the expansion of the universe _is_ described by General relativity because now you are describing the universe by putting a metric, involving a scale factor, on it, and the space might not be flat in general (cf. all the other threads about attempts to discover the shape of the universe). ---
**Author:** ![Stratocaster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stratocaster/32/16393_2.png) [@Stratocaster](https://boards.straightdope.com/u/Stratocaster)\ **Post date:** [October 23, 2026, 11:56am UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/9 "2026-10-23T11:56:02Z")
Let me provide how I finally came to understand time dilation. I understand no one here needs this explanation, but it will give you the background for how my confusion continues. I don’t remember what article or online piece provided this, but here’s how it clicked for me. I’m sure I’m butchering this and using terms poorly, but please indulge me. Picture a stationary boxcar. In the boxcar sits a man, let’s call him Fred, who has the ability to measure with precision what occurs outside his car. On a parallel track a train passes traveling at near light speed. It pulls a boxcar with a glass side (so Fred can see into it). In that moving boxcar there’s a fellow (Steve!) who has a device that emits a photon from the floor straight up to the ceiling where it hits a mirror and returns. Ten feet up and ten feet down. That up and down takes a nanosecond (or whatever the right time would be for c). Steve fires away as he passes Fred’s stationary boxcar. From Steve’s perspective, the photon traveled 20 feet in a nanosecond. From Fred’s perspective, it was NOT straight up and down because the train was moving. Let’s say he witnesses 15 feet up (on a slant) and 15 feet down (also on a slant). Well, since c is constant in every frame of reference, the photon’s trip up and down must have taken longer than a nanosecond for Fred. Hence, time moved more slowly for Fred than for Steve. Eureka, I shouted (well, not really). This is how it clicked for me…until it didn’t. Because suppose we switched roles and Fred in the stationary boxcar was the photon shooter-upper and Steve measured it as he sped on by. Would the same effect, only reversed, occur? It would take a nanosecond for Fred, but Steve would measure a greater distance and consequently a longer time occurring for the photons round trip, even though he was traveling at near light speed. Again, thanks for indulging me, but what am I getting wrong? Obviously they can’t both be experiencing a slower passage of time. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 23, 2026, 12:10pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/10 "2026-10-23T12:10:24Z")
> [@Stratocaster](#): > > Again, thanks for indulging me, but what am I getting wrong? Obviously they can’t both be experiencing a slower passage of time. Nothing; they indeed both see each other’s photons red-shifted/time passing slower. As @Chronos says, there is not going to be any difference unless one of them turns around and comes back (and subsequently they will look blue-shifted), in which case they are not going to agree on when that happens, that is, on the duration of the inbound/outbound legs. In the case of the Milky Way and some cosmologically distant galaxy, same thing, we see them red-shifted and they see us redshifted. ---
**Author:** ![Stratocaster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stratocaster/32/16393_2.png) [@Stratocaster](https://boards.straightdope.com/u/Stratocaster)\ **Post date:** [October 23, 2026, 12:12pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/11 "2026-10-23T12:12:42Z")
So would their clocks be in sync so long as Steve’s train never made a return trip? And thanks for your response. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 23, 2026, 12:13pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/12 "2026-10-23T12:13:41Z")
Yes, it is symmetric, but not in “sync”, each one sees the other’s clock ticking _slower_, by the same factor. ---
**Author:** ![Stratocaster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stratocaster/32/16393_2.png) [@Stratocaster](https://boards.straightdope.com/u/Stratocaster)\ **Post date:** [October 23, 2026, 12:14pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/13 "2026-10-23T12:14:38Z")
I think it’s sinking in now. Thanks for providing some “time dilation for dummies.” ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 23, 2026, 12:38pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/14 "2026-10-23T12:38:56Z")
> [@DPRK](#): > > No, the twin paradox can be described on ordinary, flat space-time and therefore does not use any General Relativity per se. You can consider accelerated (non-inertial) frames of reference, but there is no actual gravitational field involved. I don’t understand this. Without the acceleration, both twins see the other aging more slowly and there’s no paradox. I thought that once you get into non-inertial reference frames, you leave SR. Is the OPs question, “is cosmic inflation the same as other forms of acceleration”? I don’t know the answer, but I’m interested in it. I would say that there seems to be something different with cosmic inflation because I understand that objects can move apart faster than c (in a sense?) since space itself is expanding. ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [October 23, 2026, 12:44pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/15 "2026-10-23T12:44:42Z")
> [@EnolaStraight](#): > > Is this expansion an acceleration which…theoretically…produces a twin paradox? From Star Trek as I understand it, when a ship was in warp drive, there was no twin paradox because space itself was being affected. It was not a situation where one entity was accelerating through space and the other entity was not accelerating through the same space. So it would seem that the answer to your question would be no if that theory is correct. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [October 23, 2026, 3:00pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/16 "2026-10-23T15:00:32Z")
> [@RitterSport](#): > > Is the OPs question, “is cosmic inflation the same as other forms of acceleration”? I don’t know the answer, but I’m interested in it. I would say that there seems to be something different with cosmic inflation because I understand that objects can move apart faster than c (in a sense?) since space itself is expanding. My understanding, at least how I think about it, isn’t that space is stretching out like the ever popular analogy of a rubber sheet being stretched. It’s more like more space is constantly being created, pushing existing space further apart as new space is created. I think of it as a bunch of grains of sand, with new grains of sand constantly popping up into existence between the existing grains of sand.. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 23, 2026, 10:12pm UTC](https://boards.straightdope.com/t/the-twin-paradox-and-cosmic-inflation/1034082/17 "2026-10-23T22:12:56Z")
Paradoxes in special relativity almost always stem from forgetting about the [relativity of simultaneity](https://en.wikipedia.org/wiki/Relativity_of_simultaneity). > [@Stratocaster](#): > > So would their clocks be in sync so long as Steve’s train never made a return trip? No, the clocks of Fred and Steve are progressing at different rates no matter what. Fred sees\* Steve’s clock as slow. Steve sees Fred’s clock as slow. This is true on both the outbound and the return journey. Thus… a paradox is born. While outbound Steve and returning Steve agree about Fred’s clock speed, these two Steves (or two different reference frames for Steve) _disagree_ about which events in the story are simultaneous. **That’s the critical piece.** _Example_: Steve leaves Fred at speed 0.6_c_, travels 3 light-years (as measured by Fred), flips around nearly instantly 5 years after departure (as measured by Fred), and then travels back at the same speed. During the whole trip, Fred says, “Steve’s clock is slower than mine. He will age 8 years before returning here rather than my 10 years.” Right before the turnaround, Steve says: - “Fred’s clock is slower than mine.” - “My clock shows 4 years since departure since I only traveled 2.4 light-years (as measured by me) to this spot, but Fred’s clock shows 3.2 years since departure.” - “That matches with the fact that his clock is going slower.” Right _after_ the turnaround, Steve says: - “Fred’s clock remains slower than mine.” - “My clock shows 4 years since departure still, since turnaround was instant and my clock stayed with me, but Fred’s clock shows 6.8 years since departure.” - “Given the 3.2 Fred-years it will take for my return trip, Fred will show 10 years of aging (6.8 yr + 3.2 yr) when I get there.” That apparent time jump is because simultaneity assessments differ in the old and new reference frames. In the old frame, the following are seen as simultaneous: Steve is at the turnaround point; Fred sees “3.2 years” on his clock. But those events happen far apart from one another, so a different reference frame will judge them as _not_ simultaneous. In the new frame, the following two events are simultaneous: Steve is at the turnaround point; Fred sees “6.8 years” on his clock. * * * \*“See” is a hazardous verb to use here since there is also a mundane Doppler effect on anything literally “seen”, as through a telescope. So, read “see” here as “would see if he had co-moving clones of himself at all points in the universe so that he can read all clocks locally next to the clock in question” or more simply “would later ascertain through a comparing of notes and calculations”. Or even better: just don’t worry about this detail and happily ignore what “see” means in practice. It doesn’t actually matter for understanding this paradox. * * * **More details** The bouncing photon “light clock” you described can be used to work out various consequences of the constancy of _c_. Time dilation is one, of course. Length contraction is another. But relativity of simultaneity is a critical third, yet it is often skipped over in discussions. You need all three. These can all be encapsulated in (or derived from) a single mathematical framework for spacetime, so you don’t have to think about them piecemeal in practice. But if you do think about them piecemeal, and you don’t consider all three carefully, you stumble into apparent paradoxes. Quantitatively, the relative timing shift T between two events separated by a distance d along a direction of relative motion of speed v is given by T=vd/c^2. In this case, we switch from +v to -v, so there is a double-sized shift, leaving T\_{\mathrm{shift}}=2(0.6c)(3\; c\mathord{\cdot}\mathrm{yr})/c^2=3.6\;\mathrm{yr}, as above. Ultimately this seemingly out-of-the-blue time shift goes back to a deeper question: how are clocks in different reference frames synchronized with each other in the first place? You can also eliminate the “instant turnaround” conceit by having Steve’s brother Schmeve handle the return trip, where Schmeve was traveling toward Fred from far away this whole time. It’s a different analysis of the same situation, but it shows that the details of the turnaround aren’t actually important.