Discussions around the twins paradox showcase the way Modern Theoretical Physics discounts actual physics and logical consistency in favor of mathematical convenience. The twins paradox is a thought experiment where one twin remains on Earth while their sibling travels at some significant fraction of the speed of light to a distant star and then returns home at a similar speed. The result is not in dispute, the traveling twin will have aged significantly less than the twin who remained on Earth due to the time dilation effect described by Relativity Theory.
The problem arises when someone attempts to explain the age discrepancy in terms of Special Relativity rather than General Relativity. In SR, which applies only under inertial conditions, that is in the absence of gravity or acceleration, there is an apparent but not physically real time dilation. For example two travelers in interstellar space moving at some constant velocity with respect to each other can each consider themself to be at rest and the other traveler to be in motion and each will observe the others clocks to be running slower than their own. In fact neither clock is running slow and the time dilation is an illusion induced by the relative motion. This apparent time dilation is described by a mathematical equation known as the Lorentz transform.
In cases where gravity or acceleration are involved General Relativity is the only proper framework for understanding the time dilation situation. This is because under GR conditions time dilation is not merely an illusion owing to a constant relative velocity. An observer at the surface of the Earth will see a a clock in orbit running faster than their own while an astronaut in orbit will observe all clocks on Earth to be running slower than their own clock.
Unlike under SR conditions where each observer sees the other’s clock as running slower though neither is actually running slow, in a gravitational field all observers will agree that clocks lower in the field (where gravity is stronger) are running slower than clocks higher up (where gravity is weaker). This is gravitational time dilation. It is an observed effect that is properly accounted for by GR.
There is a similar time dilation effect associated with an accelerated observer that is also described by GR. This is the case of the traveling twin and this is where the “paradox” arises. For some not clearly explicable reason some physicists like to claim that the Lorentz transform is sufficient to analyze the situation and it is not necessary to account for any acceleration (or deceleration).
A reasonable objection to this would be that if the Lorentz transform is relevant then it applies to both the traveler and stay-at-home twin and consequently there should be no age difference when the traveler returns. Each twin should have perceived the other to be aging more slowly but in fact they aged at the same rate throughout the time of the voyage.
The retort to this objection is that the traveler changes reference frames by turning around and that justifies applying the Lorentz transform only to the traveler therefore affecting a net aging differential. It is also claimed that the periods of acceleration are irrelevant to the time dilation effect and can be ignored. The problem with this is that it is an inadequate, you could even say shoddy, analysis of the physics of any such voyage.
The following discussion of a plausible interstellar trip is based on the section of this Wiki account labeled Specific example.
In the specific example considered a 1g acceleration for 9 months is mentioned but quickly dismissed for the sake of mathematical convenience:
“To make the numbers easy…This can also be modelled by assuming that the ship is already in motion at the beginning of the experiment and that the return event is modelled by a Dirac delta distribution acceleration.“
Do not fail to check out the Dirac delta function AKA the Immaculate Acceleration. There is no better illustration of the inanity of the mathematicist approach to doing physics. What you wind up with is an analysis that is completely unrelated to a realistic account of any plausible interstellar voyage. The resulting claim, that the returning twin will have aged only 6 years while their sibling will have aged 10, is simply wrong. The math is just the math but the given result is not possible because the assumptions of mathematical convenience render the analysis physically impossible which means the result is physically meaningless.
To carry out such a trip the traveler must first accelerate away from the Earth with a sufficient period of acceleration so as to achieve a velocity that approaches light speed. A 1g acceleration (equivalent initially to the gravitational effect at the surface of the Earth) would require 9 months to achieve a velocity of 8/10ths the speed of light. An equivalent deceleration would be required to arrive at the destination and another acceleration/deceleration would be required on the return trip. If the distance to the remote star is 4 light years then the overall distance to be travelled is 8 lightyears from the Earth’s reference frame. In 9 months at an average velocity of .4c then the distance covered during acceleration is .4c x .75y = .3ly. There are 4 such acceleration/deceleration events over the entire trip so 1.2 total lightyears are travelled at an average of .4c. That leaves the remaining, 6.8ly, to be travelled at .8c which requires 8.5 years. Again this is the earth bound perspective
So the four acceleration intervals of 9 months sum to 3 years travel time plus 8.5 years for the two .8c intervals yielding a total trip time of 11.5 years from the Earth’s perspective. Using the inverse Lorentz factor .6 for the .8c interval the Earth twin calculates the traveler’s local elapsed time to be 5.1 years for the 8.5 year interval and 2.7 years for the 3 year accelerated interval (using a .9 inverse Lorentz factor for the .4c average velocity during acceleration intervals) The Earth twin then expects their sibling to have aged 5.1 + 2.7 = 7.8 years to their 11.5 years – a differential of 3.7 years. These results are markedly different than those presented in the Wiki example and confirm that it is simply wrong to ignore the acceleration intervals and treat the entire trip as an Immaculate Acceleration event with a constant velocity.
This error stems entirely from the desire “To make the numbers easy“, i.e. the preference for mathematical convenience over physical accuracy. The results speak for themselves. You cannot ignore the intervals of acceleration by pretending they take place instantaneously. It is rather absurd to expect that such an unphysical approach would work. But such is the state of Modern Theoretical Physics.
To this point the conceit that the Lorentz transform is appropriate for calculating the time dilation effect for the travelling twin has been granted. But is it actually appropriate? And if so, can it be deployed in a physically meaningful way? That is a larger topic and will be the subject of the next post.

