Twin paradox: the real explanation (no math)

The Twin Paradox is the most famous of all of the seeming-inconsistencies of special relativity. In this video, Fermilab’s Dr. Don Lincoln explains it without using mathematics. This is a companion video for his earlier one in which the same question was handled mathematically.

Related videos:

[LiveMoksh]

My 55 Yr old wife claims she is 25 as she's been moving faster than me all over the house and around me. I can't disagree.

[colmbracken]

I am a PhD astrophysicist, and I have tried for 10 years as an undergrad and during my PhD to obtain an explanation for this.
Nice video (as always with Dr Lincoln), but I STILL can't see why C can't be assumed to be stationary, and that A and B (and the Earth and Sun) are moving toward C. It still looks like it can be completely symmetric. There is no reason here why we can't assume one frame for C and two frames for A and B. You start be saying we can treat A as stationary, but that is true for C too. In the scenario with C in the rest frame, the clocks on A and B would move slower (when we compare all clocks after we bring them all into contact at a later time). Unless we choose some absolute rest frame (maybe the CMB, which does kinda offer an absolute frame) I simply do not see ANY scenario that can not be flipped through symmetry in a purely special relativistic case.

[laurent]

I think there is one thing that can help understand the paradox, that I have never seen being evoked:

If we "freeze" the world midway into the paradox, when Ron reach the star, as if he never intended to return earth, then by no mean we can distinguish the travel of Don and Ron. It is perfectly symmetrical from the start.

Thus at this moment of the paradox, what is preventing us from saying that Ron is younger than Don from the earth reference, but also the reverse, that is Don is younger that Ron from the spaceship reference?

Well actually nothing. We CAN say that. Ron can "see" (from far away) that Don is younger, and Don can "see" that Ron is younger. One have to remember that simultaneity is relative to the reference frame (look "Relativity of simultaneity" on wikipedia). So Ron and Don are really older than each other on their own reference frame.

Hope this piece of information can help anyone understand better the paradox.

[dhoffheimerj]

Please keep doing sessions on this topic. I don't get it yet, but I think it is a threshold concept to understanding everything else you teach. Thank you. Fabulous lessons.

[matthewtalbot-paine]

I think I'm going to slowly back away from relativity now and accept my simple understanding of it.

[EugeneKhutoryansky]

It is unfortunate that a great channel like FermiLab would give such a highly misleading explanation (twice), and then label it the “Real” explanation. You are only talking about Special Relativity. Relative acceleration does play a role in General Relativity, where a single non-inertial reference frame is allowed for the spaceship’s entire journey. The person in the spaceship can believe that he is standing still by believing that there is a gravitational field present throughout all of space, and that gravitational time dilation is what causes him to be older than his twin.

[valepu]

We can say these videos shows the same topic from two different reference frames

[TacticusPrime]

But of course "acceleration" is just changing reference frames. Acceleration allows one object to experience 2+ reference frames. In the other case, two different objects experience the reference frames and compare with each other.

I think what really trips people up is when you have just two objects that speed away from each other and NEVER come back together. Each experiences just one reference frame. They fundamentally disagree over who experiences more time.

[mikehuber]

I laughed way too hard at the "pair o' docs" joke.

[rentzepopoulos]

For subjects that are counterintuitive (such as time dilation) I consider that a top-bottom approach fits best; first, viewers must understand what is important (time frames) and then they can easily access the mathematical approach as now the reasoning for each step of the methematical approach is easier to understand.

[mnamanm]

1:16 Every single time my teacher makes a joke about physics.

[RickDekkard]

Don't understand the debunking for the acceleration resolution of the paradox. I don't think that acceleration causes time dilation, it is because speed as you stated. But acceleration breaks the symmetry and therefore you can tell which one is older, the one that is not experiencing acceleration, as the original paradox point out that they meet again at the end and compare they ages. So in my opinion, acceleration still resolves the paradox (in the way it is originally presented) although do not explain the time dilation.

[mheermance]

Assume for a minute that the universe has a closed spherical topology, it is small enough to traverse in a finite amount of time, and one twin is moving close to C. That should allow the traveling twin to return to their origin without frame jumping. How would the paradox be resolved?

[garysimpson]

The two frame explanation is the only explanation that I have ever seen or heard that ALMOST makes sense to me. Thank you. I can not help but think that there is a closed integral hiding in there somewhere to describe the condition that the two frames sum to get back to the starting point. BUT, why not make two moving earth frames from the rocket's perspective? It seems to me that brings back the paradox.

[Davemester]

I just can't escape the feeling that you created the assimetry in the thought experiment. I mean you intended to exclude acceleration of "Ron" by separating him into two frames (B, and C). But you didn't do it with A, so the only reason why Don (A) is only in one frame is because you did not separate him into two frames. (I know you didnt say your thought experiment was "related" to Don and Ron, but the connection seems obious.)

I just cannot grasp what you mean by "A is only in one reference frame" and "B and C are in two". Maybe its a language barrier, but I can't wrap my head around what it means.

Also, you said that the measurements are mada at A's location. So how would the thought experiment looked like if it were measured at B"s or C's location? Would the result be the same (as Don aged more), even measured at different places? Sigh.. :/

[altrag]

I still don't get it. Isn't it equivalent to acceleration to switch between measuring a clock going +v to a clock going -v? That seems like an instantaneous jump of -2v in my mind and thus not constant velocity and thus an acceleration. The fact that the same theoretical person isn't physically holding the clock in both directions seems rather irrelevant. I mean obviously Dr.Don is way smarter than me so I'll trust that he's right, but this explanation doesn't really explain much.. it just introduces some additional variables that seems to mask the paradox more than resolve it. Even in the previous video where he did that math, he was always explaining from A's perspective (he put up the equations for B's and C's perspectives but didn't really get into why B/C think they're younger than A at the end of the trip and I'm neither smart enough nor motivated enough to figure it out for myself.. I learn my physics from soundbite Youtube videos for a reason!)

[jagmarz]

This video did make it a lot clearer. The apparent paradox of the problem comes from missing the fact that the traveling twin has to change reference frames when turning around. I think driving this point home is really the crux of understanding the problem, an the first video, despite having observers B and C really didn't stop to emphasize that fact enough. For those of us who really don't understand the physics fully, it's very easy to just gloss over the importance of this subtlety. In fact, it might have been helpful to show the perceived ages each of the three observers have of each other at the beginning, when B&C cross, and at the end, when A&C cross.

[thenewtalkerguy]

It's not the math, it's the fact that this guy is just a bad explainer.

[Seehart]

Perfect explanation. Another way I like to debunk the bogus acceleration explanation is to note that the twin at home is also experiencing 1G due to earths gravity. And 1G is plenty sufficient propulsion to zip around the galaxy and do twin paradox experiments (indeed relativity is not as much of a buzzkill for interstellar travel as most people think, but that's another topic).

With a ship capable of accelerating at 1G, you can accomplish a round trip visit to alpha centauri in about 10 years ship time, while about 15 years pass on Earth (accelerate to halfway point, decelerate, arrive and repeat to come home). The twin at home also experiences 1G in the form of gravity on the surface of earth, so both twins are experiencing the same acceleration effect (aside from the launch, of course). So it's not the acceleration.

Incidentally, the general relativity dialation effect at 1G is negligible for both twins: it comes to about 3 milliseconds per year, or 0.03 seconds for the traveller or 0.045 seconds for the twin at home, which doesn't make much of a dent in the 5 year difference. So, again, it's not acceleration!

[mrbatweed]

Hmmmm... A bit obsessed with debunking the acceleration argument.
The problematic question is: why is there no symmetry?
If we took the Ron's reference frame as stationary and tried to repeat the argument about Don changing reference frames, where does it all break down?