Time is not universal: how speed and gravity bend it

The Short Version

There is no single clock for the universe. Michelle Thaller puts the fact plainly: "Time is variable depending on how fast you're going and also how far off the Earth's surface you are." [1] Moving fast makes less time pass for you, and so does sitting deeper in a gravitational field, and both effects have been measured, down to a clock raised two feet and up to the satellites that run GPS [1][2]. Brian Cox draws the consequence for travel: a fast enough ship could cross millions of light years in minutes of its own time and return to an Earth millions of years older, which is time travel into the future, while travel into the past appears to be forbidden [2]. Sean Carroll gives the reason all of this holds together: space and time are one four-dimensional thing, different paths through it take different amounts of time, and gravity is its curvature [3]. None of the three claims to know what time is [1][2][3].

Table of contents

What They Said

Michelle Thaller: your head and your feet keep different time

Thaller, an astrophysicist who spent years at NASA, starts by taking the subject out of the realm of theory. Time dilation, she says, is everyday engineering [1]. Clocks are now accurate enough to detect time running differently when one is moved just two feet higher [1]. So, she tells Joe Rogan, "your head is in a different time frame than your feet right now." [1]

Two things change the rate. One is speed. The other is how far you are from a mass like the Earth [1]. The Global Positioning System has to deal with both at once, and she explains that they pull in opposite directions. The satellites are moving fast, which slows their clocks. They are also farther from Earth's gravity, which speeds their clocks up, and that second effect is the larger one [1]. Without correcting for both, she says, positions would be off by roughly six miles within a day [1].

The balance comes out the other way for astronauts. The space station is closer to the Earth and moving faster, so speed dominates. A year aboard leaves an astronaut about a hundredth of a second younger than they would have been on the ground [1].

When Rogan asks what a clock is actually measuring, Thaller says that is exactly the question that revolutionized physics [1]. She walks through Einstein's thought experiment: a pulse of light bouncing between two mirrors on a moving spaceship [1]. However time is measured, motion changes it [1]. And the change is mutual in a way that defeats the idea of a correct clock. Sitting still and watching your own watch, you see time pass normally, but "If someone's flying by it close to the speed of light, they won't see you measure time the same way." [1]

She explains why speed has a ceiling. Rogan had heard that a ship at the speed of light would have infinite mass. Thaller's version is that "anything with mass can't travel at the speed of light" [1]. The reason is energy: "As you approach the speed of light, if you have mass, it takes more and more energy to accelerate you even just a little bit more." [1] At the limit the requirement is infinite; in her phrase, "the equations blow up" [1].

On gravity she gives the idea in a sentence, while explaining gravitational waves: "gravity is actually a curvature of space and time itself" [1]. A moving mass sends ripples of that curvature outward at the speed of light, and detectors now pick them up routinely [1]. She offers other observations that show the bending. The first image of a black hole shows a shadow slightly larger than the event horizon because space and time bend around it [1]. An instrument on the space station sees hot spots on neutron stars that should be hidden on the far side, because space and time curve around the star [1]. And stars whipping around the black hole at the center of our galaxy, one at nearly 20 million miles an hour, let scientists test time dilation directly [1].

Thaller also takes up the philosophical end. Rogan asks about the idea that all time exists at once. She explains that because all speed is relative, Einstein concluded the universe may be one whole block of space and time, and she quotes his description of past, present and future as "persistently annoying illusions." [1]

And she is frank about the limit of all this. "I don't think we have an answer to what time is," she says, and suggests that answering it would be the next revolution in physics [1]. What physics has established is negative: of space and time, "We don't know what they are, but we know they're not a simple flow" [1].

Brian Cox: the trip to Andromeda, and the clock you come home to

Cox teaches relativity to first-year students at the University of Manchester, and he opens the subject with distance, not time. "if you had a spacecraft traveling close to the speed of light, then distances shrink from your perspective," he says [2].

His standing example is the Large Hadron Collider. Protons circulate in a ring 27 kilometers around at 99.999999 percent of the speed of light [2]. At that speed, distances shrink by a factor of about 7,000, "And so that ring is something like four meters in diameter to the protons." [2]

Then he scales it up. The Andromeda galaxy is two million light years away. Build a ship that goes close enough to light speed and you can shrink that distance, and so the time it takes, by an arbitrary amount; the closer to the speed of light, the more it shrinks [2]. "You could traverse across that distance in principle, in a minute, according to physics." [2]

The catch is what happens to everyone else. If you came back to tell people what you had found, "at least four million years would have passed on the Earth" [2]. Exploration is permitted, he says, and reporting home is not: "getting to chat to everybody about what you found is forbidden by the structure of the universe" [2].

Rogan says that sounds like a time machine, and Cox agrees with a qualification. "Well, it's a time machine in the sense that we could go arbitrarily far into the future by flying around in a rocket very close to speed of light." [2] You could come back a million years on and see what had become of the Earth [2]. The other direction is different. "You can't go back as far as we can tell." [2]

He explains the principle the way he teaches it. In Einstein's theory there are events, things that happen in spacetime, and people can take different paths between the same two events [2]. The amount of time that passes is a property of the path, so two people who part and meet again need not have aged the same amount [2]. In ordinary life the difference is tiny. It becomes large when someone travels close to the speed of light or goes near a black hole, where spacetime is strongly distorted [2].

Tiny does not mean undetectable. Cox gives the GPS case with numbers. The clocks on the satellites tick at a different rate from the clocks on the ground, by something like 30,000 nanoseconds a day, he says from memory, because of the weaker gravitational field and the motion [2]. Then the conversion: "light travels one foot per nanosecond" [2]. A clock that has drifted 30,000 nanoseconds gives a position wrong by 30,000 feet [2]. "So the clocks have to be corrected for that effect." [2] It is easily measured with atomic clocks, he adds, "But it doesn't make much difference to us as humans." [2]

Gravity's effect on time runs through the rest of what he says. Gravitational waves from colliding black holes are passing through the room, he notes, very slightly changing the rate at which time passes; he cites Kip Thorne's phrase for them, "a storm in time" [2]. And at the extreme, inside a black hole, the singularity is best understood in Einstein's theory as the end of time, something you can no more avoid than you can avoid tomorrow [2].

Cox then sorts the science fiction. Travel into the future is allowed by the laws of physics [2]. Travel into the past appears to be forbidden, an idea Stephen Hawking called the chronology protection conjecture [2]. Cox is careful about its status: it has not been proven beyond doubt that the exotic conditions needed, such as stable wormholes you could pass through, cannot be made, so it is suspected and not established [2]. Warp drives work mathematically but seem to require kinds of matter or energy that do not appear to exist [2]. Wormholes do show up in current research in a different role, in an idea that links them to quantum entanglement [2].

Sean Carroll: one spacetime, and the path that takes longest

Carroll supplies the structure underneath the other two accounts. The best way into the subject, he says, is special relativity, and he gives much of the credit to someone other than Einstein [3].

In 1905 Einstein worked out that you could get rid of the ether, the idea of a rest frame for the universe, and that the equations of physics would make sense with "the speed of light being a maximum" [3]. Two years later Hermann Minkowski, who had been Einstein's professor, saw the most elegant way to think about it: "to blend space and time together into space time" [3]. The idea is that "there is no hard and fast division of the four-dimensional world in which we live into space and time separately" [3]. Carroll calls it a profound insight and thinks Minkowski deserves more credit than he gets [3].

Einstein at first dismissed this as mathematicians over-formalizing [3]. Then he saw what followed from it. If spacetime is a thing, it can have a geometry, and a geometry can be curved [3]. The final step is the sentence Carroll gives as the heart of general relativity: "gravity is the curvature of space time." [3]

He describes how Einstein got there over ten years. It began with noticing that inside a quiet rocket you cannot tell acceleration from gravity [3]. That led to the conclusion that gravity cannot be an ordinary force, and then to Einstein teaching himself the mathematics of curved spaces [3].

From this picture Carroll draws the cleanest statement of why moving clocks disagree. In ordinary space the shortest distance between two points is a straight line. In spacetime the rule is reversed: the straight path gives "the longest time between two points," [3]. Someone who stays put between two events takes the straight path and experiences the most time. Someone who travels out and back takes a bent path and experiences less [3]. That is why the traveling twin comes home younger [3].

He follows the idea to the place where gravity bends time most. A black hole is best thought of as a region of spacetime that cannot be left once entered [3]. You would not notice crossing its horizon [3]. The singularity is not at the center of space; it lies in your future, like a moment of time [3]. For a black hole with the mass of the Sun, the trip from horizon to singularity takes about a millionth of a second [3]. Seen from outside, though, someone falling in appears to slow down, redden and fade, so that you never actually see them cross [3]. The two descriptions are of the same fall, timed along two different paths [3].

Carroll adds a point about how seriously to take all this. If a theory fits the data, is simple and makes sense in its own terms, he says, we should attribute reality to the things that play an important role in it, at least until a better theory comes along [3]. He regards general relativity as so successful that "the equation is much smarter than Albert Einstein": it predicted the Big Bang, gravitational waves and black holes, which Einstein did not foresee [3].

Where They Agree, and Where They Don't

They agree on the fact. All three say that elapsed time depends on motion and on gravity, and all three treat it as established [1][2][3].

They agree on the evidence. Thaller and Cox both use GPS, and their numbers match. Thaller says uncorrected satellites would put positions off by roughly six miles in a day [1]. Cox says the clocks differ by about 30,000 nanoseconds a day, which at a foot per nanosecond is 30,000 feet [2]. Thirty thousand feet is a little under six miles, so the two are describing the same error in different units [1][2].

They agree on the speed limit. Carroll states it as the premise of special relativity; Thaller explains it by the energy needed to accelerate anything with mass [1][3]. Cox's examples go close to the speed of light and never reach it [2].

They agree that gravity is geometry. Thaller and Carroll use nearly the same words for it [1][3]. Cox speaks of spacetime being distorted near a black hole [2].

They explain it at different levels. Thaller shows that it is real: clocks, satellites, astronauts [1]. Cox shows what it permits: the trip, the return, the one-way door into the future [2]. Carroll shows why: one spacetime, many paths, each with its own length in time [3]. Cox's account of events and paths is the same idea as Carroll's, in the form he gives to undergraduates [2][3].

They differ in emphasis on what shrinks. Cox describes the traveler's experience as distances contracting, so the journey is short because there is less far to go [2]. Thaller and Carroll describe it as clocks disagreeing [1][3]. These are two views of one effect: from the ship, the galaxy is close; from the Earth, the ship's clock runs slow; both agree on how much the traveler ages [1][2].

They differ in how far they go on the nature of time. Thaller reports Einstein's block universe, in which past, present and future are equally real, and says she does not think we have an answer to what time is [1]. Carroll calls himself a believer in objective reality and treats spacetime as real because the theory works [3]. Cox elsewhere says that the study of black holes is leading physicists to suspect space and time are not fundamental and emerge from something deeper, and that we do not know whether the universe had a beginning in time [2]. Carroll is likewise uncertain whether there was a first moment [3]. None of these contradicts the others; they are statements about different depths of the question.

Only Cox addresses time travel in detail. His sorting of future travel, past travel, warp drives and wormholes is not disputed by the other two, who do not take it up [1][2][3].

Why It Works

Light has one speed for everyone. Einstein's 1905 step was that physics makes sense with the speed of light as a maximum and with no preferred rest frame [3]. Thaller's bouncing-light clock shows what follows: if the light in a moving clock must cover more ground and cannot go faster, the clock's ticks stretch out as seen by someone watching it pass [1].

Space and time trade off. Because the four-dimensional world has no fixed division into space and time, observers in relative motion slice it differently [3]. What one calls purely an interval of time, another sees as part time and part distance. That is why Cox's protons see a 27-kilometer ring as about four meters across [2].

Time is the length of a path. In Cox's and Carroll's accounts, the time that passes between two events depends on the route taken through spacetime, and the unaccelerated route is the longest [2][3]. A traveler who goes out and back has taken a shorter one, measured in time [3].

Gravity curves the paths. Einstein's starting clue was that acceleration and gravity cannot be told apart from inside [3]. If gravity is curvature of spacetime, then clocks at different depths in a gravitational field are on different paths and accumulate different times, which is what the two-foot clock experiment and the GPS correction show [1][3].

The limit protects itself. The energy needed to push a mass faster rises without bound as it nears the speed of light, so no ship reaches it [1]. This is also why the trick only runs forward: speed and gravity can make less time pass for you than for others, which carries you into their future, and nothing in these effects runs a clock backwards [2].

Many independent measurements agree. Atomic clocks at different heights, satellite navigation, astronauts' ages, stars near the galactic center, gravitational waves and the image of a black hole's shadow are different kinds of evidence for the same theory [1][2]. Thaller's standard for science is "what is reproducible, consistently reproducible," and these results are [1].

Where It Applies, and Where It Doesn't

The effects are real everywhere and large only in extremes. Cox says the differences are tiny unless something moves close to the speed of light or sits near a black hole [2]. A year in orbit is worth a hundredth of a second [1].

They matter in technology that measures time precisely. Navigation by satellite turns time into distance, so a small clock error becomes a position error of miles [1][2].

Future time travel is physics; the hard part is engineering. Nothing in the laws forbids the Andromeda trip, in Cox's account, but it requires a ship at a speed only subatomic particles in an accelerator reach today, and Thaller's point about energy explains why [1][2].

Past time travel is not established to be impossible, and not expected. Cox says it appears forbidden, that the prohibition is a conjecture, and that the conditions needed for a loophole do not seem to exist [2].

Warp drives and traversable wormholes remain on paper. The mathematics allows a warp drive; the required matter or energy does not appear to exist [2].

Relativity does not say what time is. It says how elapsed time depends on motion and gravity. Thaller says the deeper question is open [1].

The theory has edges. Einstein's theory by itself implies a singularity inside a black hole and another at the start of the universe, Cox says, but once quantum mechanics is added the picture becomes unclear, and we do not know whether the universe had a beginning in time [2]. Carroll says he is maximally uncertain whether there was a first moment [3].

Block-universe conclusions go beyond the measurements. That the past and future exist in the same sense as the present is an interpretation Thaller attributes to Einstein, and she later marks where she is moving from science into conjecture [1].

What To Do With It

1. Drop the idea of a universal now. Each clock measures time along its own path; there is no master clock for them to disagree with [1][3].

2. Remember the two causes and their directions. Speed slows your clock relative to others, and being deeper in gravity slows it too; being higher up speeds it [1].

3. Use GPS as the proof. The satellites are corrected for both effects every day, and without the correction your phone would be wrong by about six miles within a day [1][2].

4. Keep the scale in view. Two feet of height is detectable only with the best clocks, and a year in orbit changes an astronaut's age by a hundredth of a second [1].

5. Use the twin picture. The one who stays takes the straight path and ages most; the one who travels and returns ages less [3].

6. Think of a fast trip in both views. For the traveler the distance shrinks; for those at home the traveler's clock runs slow; they agree on the outcome [2].

7. Sort the fiction. A one-way jump to the future is allowed. A return to the past is, as far as anyone can tell, not. Warp drives need material that does not seem to exist [2].

8. Be honest about the open question. Physicists can measure how time behaves and still say they do not know what it is [1].

Sources

1. Michelle Thaller on Black Holes, Time, Quantum Weirdness and the Limits of Science · The Joe Rogan Experience
2. Brian Cox on Black Holes, the Great Silence and the Meaning of Life in the Universe · The Joe Rogan Experience
3. Sean Carroll: General Relativity, Quantum Mechanics, Black Holes & Aliens · Lex Fridman Podcast