DarkRange55
Let them eat cake! 🍰
- Oct 15, 2023
- 2,254
The universe does not merely allow forward time travel, it forces it upon us. We are all time travelers. No matter what we do, we will always inch forward in time, moment by moment. Only under very special circumstances can time appear to stop, and none of those circumstances can really be something normal matter could ever experience, if there is anything to experience at all. What we can do instead is sit back and watch time pass.
Einstein's theories of relativity allow us to control the rate at which time ticks. This is not hypothetical; we actually can and do apply it, most notably in the GPS system. But the cost of doing so is unavoidable. The faster you move forward in time—whether through strong gravitational fields or relativistic speeds—the further you move away from the moment you started from. No matter what you do, you cannot return to that original point.
There does not appear to be any mechanism that allows travel backward in time. You cannot ever get back to where you started, whether you have a flux capacitor and a DeLorean or not. This is not merely a matter of practicality, but one of outright prohibition. And yet, there is a catch. A loophole, if you will. A very obscure one.
One of the major reasons scientists are deeply skeptical of backward time travel is that, unlike traveling to the future, it creates severe paradoxes. In principle, traveling backward in time would require infinite energy. Nothing we know of possesses infinite energy—except perhaps the universe itself in some models—and it appears wholly uninterested in traveling backward in time.
There are tricks, however. Consider a simple system, such as a game of billiards. If every collision were perfectly reversed, the game would play out exactly backward. You might say that represents traveling back in time, and in a limited sense it does. But that sense is confined to returning a system to a previous state. It is simply reversing motion. This is fundamentally distinct from time dilation and from physical time itself, because true backward time travel would require going back and actually visiting the past. In some abstract sense you might be doing that, but relative to the rest of the universe, you are merely recreating the past rather than returning to it.
Time itself is not particularly well defined among physicists. It is one of the concepts few agree on, largely because it behaves in deeply unintuitive ways depending on the scale at which it is examined. That said, there does appear to be a loophole here—actually, several.
One of the primary candidates is the infamous tachyons. The laws of physics do not strictly prohibit particles from moving faster than light and, in doing so, heading backward in time toward the Big Bang. One might reasonably wonder what that would even mean: particles racing backward through time at faster-than-light speeds and colliding with the Big Bang itself. This is not well-trodden theoretical territory, but in principle such particles could exist. The problem is that they could never slow down to the speed of light, because doing so would require infinite energy. As a result, they would be permanently stuck moving backward in time, just as ordinary matter is permanently stuck moving forward.
Another loophole is more literal: that of quantum loop time. Hidden within Einstein's physics has always been the possibility of time loops. These loops appear in General Relativity and are known as closed timelike curves, or CTCs. In principle, if spacetime could be bent back onto itself, you would have a pathway to the past. The issue is that generating such a structure would require an enormous amount of mass rotating at extreme speeds.
Nature does appear to contain such objects in the form of black holes. However, this pathway is useless for macroscopic travel, because the black hole itself prevents anything from surviving the journey. For the quantum world, however, such restrictions may not apply in the same way.
When the idea of backward time travel from General Relativity is shrunk down to the quantum level, these structures are referred to as quantum closed timelike curves. Physicists are deeply uncomfortable with them. It is no secret that there is a massive disconnect between quantum mechanics and General Relativity. That disconnect lies largely with General Relativity itself, which breaks down and stops producing meaningful answers at quantum scales.
On the quantum scale, time behaves more like a ticking clock. On the scale of General Relativity, time can bend and warp just like space in the presence of gravity. How these two descriptions reconcile remains one of the greatest unsolved problems in physics.
Recently, the idea of retrocausality has gained renewed attention. This concept suggests that events in the future can influence events in the past—not by transmitting signals through space, but by establishing connections through time itself. This idea often arises in discussions of quantum entanglement.
Quantum entanglement is a real, experimentally verified phenomenon. Two particles can interact in such a way that their states are correlated regardless of the distance between them. This does not allow meaningful information to be transmitted faster than light, despite appearances. Every experiment confirms this limitation.
Retrocausality reframes the problem. Instead of information traveling through space, the connection occurs through time. When an entangled particle changes state, that change may propagate backward to the moment the particles were originally entangled. The other particle reflects that change, not because the outcome was predetermined, but because the past itself is updated with the information.
This raises profound questions. If a quantum state has never been measured—if its history has never been written—then altering it may not constitute changing the past in a paradoxical sense. Instead, it may simply alter probabilities. In that sense, you are changing the past, but only the unwritten past.
Some interpretations invoke branching outcomes or multiple realities to resolve this, though these ideas remain contentious. Others argue that retrocausality could avoid paradoxes without invoking multiple worlds at all.
Several thought experiments explore this possibility. Some involve quantum computing techniques that simulate particles sending information backward in time. Others propose physical experiments using individual photons sent through quantum time loops. While many of these remain mathematical simulations rather than direct demonstrations, they are testable in principle.
If such experiments were realized, the implications would be extraordinary. A scientist today could establish an experiment whose outcome is determined by a measurement made by scientists in the future. Within minutes of activating the experiment, present-day researchers might receive a result influenced by future observers. In effect, it would function as a kind of time capsule.
Such a result might even represent communication from the future. And if that communication altered the past, the only answer the universe might allow is "don't ask."
These experiments could also provide crucial clues about the relationship between quantum mechanics and General Relativity, and perhaps finally offer insight into how gravity operates on quantum scales, a problem that has resisted solution for over a century.
Despite all of this, backward time travel for macroscopic objects appears to remain forbidden. Nature seems to enforce a one-way street for ordinary matter. If time travel exists at all, it may be confined to quantum information, probabilities, or interpretation — not physical motion.
Einstein's theories of relativity allow us to control the rate at which time ticks. This is not hypothetical; we actually can and do apply it, most notably in the GPS system. But the cost of doing so is unavoidable. The faster you move forward in time—whether through strong gravitational fields or relativistic speeds—the further you move away from the moment you started from. No matter what you do, you cannot return to that original point.
There does not appear to be any mechanism that allows travel backward in time. You cannot ever get back to where you started, whether you have a flux capacitor and a DeLorean or not. This is not merely a matter of practicality, but one of outright prohibition. And yet, there is a catch. A loophole, if you will. A very obscure one.
One of the major reasons scientists are deeply skeptical of backward time travel is that, unlike traveling to the future, it creates severe paradoxes. In principle, traveling backward in time would require infinite energy. Nothing we know of possesses infinite energy—except perhaps the universe itself in some models—and it appears wholly uninterested in traveling backward in time.
There are tricks, however. Consider a simple system, such as a game of billiards. If every collision were perfectly reversed, the game would play out exactly backward. You might say that represents traveling back in time, and in a limited sense it does. But that sense is confined to returning a system to a previous state. It is simply reversing motion. This is fundamentally distinct from time dilation and from physical time itself, because true backward time travel would require going back and actually visiting the past. In some abstract sense you might be doing that, but relative to the rest of the universe, you are merely recreating the past rather than returning to it.
Time itself is not particularly well defined among physicists. It is one of the concepts few agree on, largely because it behaves in deeply unintuitive ways depending on the scale at which it is examined. That said, there does appear to be a loophole here—actually, several.
One of the primary candidates is the infamous tachyons. The laws of physics do not strictly prohibit particles from moving faster than light and, in doing so, heading backward in time toward the Big Bang. One might reasonably wonder what that would even mean: particles racing backward through time at faster-than-light speeds and colliding with the Big Bang itself. This is not well-trodden theoretical territory, but in principle such particles could exist. The problem is that they could never slow down to the speed of light, because doing so would require infinite energy. As a result, they would be permanently stuck moving backward in time, just as ordinary matter is permanently stuck moving forward.
Another loophole is more literal: that of quantum loop time. Hidden within Einstein's physics has always been the possibility of time loops. These loops appear in General Relativity and are known as closed timelike curves, or CTCs. In principle, if spacetime could be bent back onto itself, you would have a pathway to the past. The issue is that generating such a structure would require an enormous amount of mass rotating at extreme speeds.
Nature does appear to contain such objects in the form of black holes. However, this pathway is useless for macroscopic travel, because the black hole itself prevents anything from surviving the journey. For the quantum world, however, such restrictions may not apply in the same way.
When the idea of backward time travel from General Relativity is shrunk down to the quantum level, these structures are referred to as quantum closed timelike curves. Physicists are deeply uncomfortable with them. It is no secret that there is a massive disconnect between quantum mechanics and General Relativity. That disconnect lies largely with General Relativity itself, which breaks down and stops producing meaningful answers at quantum scales.
On the quantum scale, time behaves more like a ticking clock. On the scale of General Relativity, time can bend and warp just like space in the presence of gravity. How these two descriptions reconcile remains one of the greatest unsolved problems in physics.
Recently, the idea of retrocausality has gained renewed attention. This concept suggests that events in the future can influence events in the past—not by transmitting signals through space, but by establishing connections through time itself. This idea often arises in discussions of quantum entanglement.
Quantum entanglement is a real, experimentally verified phenomenon. Two particles can interact in such a way that their states are correlated regardless of the distance between them. This does not allow meaningful information to be transmitted faster than light, despite appearances. Every experiment confirms this limitation.
Retrocausality reframes the problem. Instead of information traveling through space, the connection occurs through time. When an entangled particle changes state, that change may propagate backward to the moment the particles were originally entangled. The other particle reflects that change, not because the outcome was predetermined, but because the past itself is updated with the information.
This raises profound questions. If a quantum state has never been measured—if its history has never been written—then altering it may not constitute changing the past in a paradoxical sense. Instead, it may simply alter probabilities. In that sense, you are changing the past, but only the unwritten past.
Some interpretations invoke branching outcomes or multiple realities to resolve this, though these ideas remain contentious. Others argue that retrocausality could avoid paradoxes without invoking multiple worlds at all.
Several thought experiments explore this possibility. Some involve quantum computing techniques that simulate particles sending information backward in time. Others propose physical experiments using individual photons sent through quantum time loops. While many of these remain mathematical simulations rather than direct demonstrations, they are testable in principle.
If such experiments were realized, the implications would be extraordinary. A scientist today could establish an experiment whose outcome is determined by a measurement made by scientists in the future. Within minutes of activating the experiment, present-day researchers might receive a result influenced by future observers. In effect, it would function as a kind of time capsule.
Such a result might even represent communication from the future. And if that communication altered the past, the only answer the universe might allow is "don't ask."
These experiments could also provide crucial clues about the relationship between quantum mechanics and General Relativity, and perhaps finally offer insight into how gravity operates on quantum scales, a problem that has resisted solution for over a century.
Despite all of this, backward time travel for macroscopic objects appears to remain forbidden. Nature seems to enforce a one-way street for ordinary matter. If time travel exists at all, it may be confined to quantum information, probabilities, or interpretation — not physical motion.
