Posted on 04/30/2024 9:11:39 AM PDT by Red Badger
In 1957, the astrophysicist Herman Bondi wrote a paper in which he considered the possible existence of a negative mass in Albert Einstein’s theory of gravity. A negative mass would repel a positive mass away from it. Given that, a pair of positive and negative masses could accelerate together up to the speed of light. The negative mass would push away the positive mass which in turn would pull the negative mass for the ride. The runaway pair would accelerate indefinitely, without any need for fuel or a propulsion system. Energy conservation would not be violated because the sum of the two masses is zero.
However, in a recent discussion I had with my brilliant collaborator, Mark Hertzberg, we agreed that the existence of a negative mass in nature would allow for a time machine in which one can visit the past. The reason is simple. A positive mass delays the arrival of light that passes near it, as recognized in a seminal paper by my distinguished colleague, Irwin Shapiro.
The “Shapiro time delay” near a positive mass reverses sign and turns into a “Shapiro time advance” near a negative mass. If the sender is moving sufficiently close to the speed of light, the Shapiro time advance would mean that a light signal can be reflected off a mirror and arrive back to the sender when the sender’s clock shows a time earlier than the original transmission time of the same signal.
Effectively, the Shapiro time advance is equivalent to communicating with a faster-than-light particle which violates causality. Within Einstein’s gravity, the technical term for a time machine of this type is a “closed timelike curve.”
Within the standard framework of Einstein’s gravity, a negative mass would allow children to prevent their parents from giving birth to them, a logical inconsistency. Time machines can also be created by traversable wormholes which, as Stephen Hawking recognized in a seminal paper about “chronology protection” from 1992, require negative energy as well.
“It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past,” the abstract of Hawking’s paper begins, and thereafter concludes by saying, “These results strongly support the chronology protection conjecture: The laws of physics do not allow the appearance of closed timelike curves.” Hawking’s conclusion was supported in a subsequent paper by Bernard Kay, Marek Radzikowski and Bob Wald, but the final verdict on its validity is awaiting a better understanding of quantum gravity.
If time machines are absent, what are the most extreme distortions of spacetime that our Universe is known to display?
In Einstein’s gravity, the Earth orbits the Sun because the Sun curves space. An analogy is offered by the surface of a trampoline which is curved by a bowling ball, causing a marble with the proper speed to move in a circle around the heavier ball. Removing the bowling ball would release the marble on a straight trajectory on the flat trampoline. Similarly, discarding the Sun would free Earth on a straight path toward interstellar space.
The spacetime curvature created by the Sun at our location is characterized by a length scale that is equal to the inverse of the square root of (GM/r^3c^2), where G is Newton’s constant, M is the mass of the Sun, r is the Earth-Sun separation, and c is the speed of light. Plugging in the relevant numbers yields a curvature length of 10,000 times the Earth-Sun separation. Another way to derive this number is by recognizing that the Earth’s speed around the Sun is 10,000 smaller than the speed of light, c.
Other astrophysical objects display a stronger spacetime curvature over a shorter length scale. The largest distortions of spacetime are expected near a black hole. Limiting our attention to the accessible region outside the event horizon, this curvature length scales in proportion to the black hole mass. The most massive black holes in the Universe have a mass of ten billion solar masses and a curvature scale of 1,000 times the Earth-Sun separation. The curvature length shrinks by billions of times for black holes or neutron stars formed from the collapse of massive stars.
What is the largest spacetime curvature that we observed so far in the Universe? It corresponds to a length scale of about 24 kilometers, the size of a large city on Earth, and is realized on the surface of neutron stars which carry about twice the mass of the Sun atnuclear density.
The same spacetime curvature characterizes an atomic nucleus because it has the same matter density as a neutron star. But a curvature length of 24 kilometers is 19 orders of magnitude larger than the size of an atomic nucleus and therefore has no significance when nuclei are smashed into each other in CERN’s Large Hadron Collider.
In conclusion, based on all we know from observations or theory within the standard model of physics, negative masses are forbidden for the same reason that time machines are not allowed. If aliens were able to create negative masses and use them for travel, then they would have also been able to travel back in time.
If time travel had been feasible, then future generations of quantum-gravity physicists could have been able to visit Earth and rewrite our history as they wish. For example, they could have killed Adolf Hitler and allowed six million Jews to thrive in Europe between 1939 and 1945. The fact that this never happened suggests that either time machines will never be constructed in our future or that their creators have no interest in saving human lives.
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Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s – Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011-2020).
He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies.
He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
It’s about Time.....................
It suggests the author has no imagination. The past-present-future we exist in may be the optimum universe, with all other universe possibilities being much worse.
” For example, they could have killed Adolf Hitler and allowed six million Jews to thrive in Europe between 1939 and 1945. The fact that this never happened suggests that either time machines will never be constructed in our future or that their creators have no interest in saving human lives.”
That’s a rather odd conclusion. If they have the technical skills to create a time machine, maybe they have the wisdom not to use it.
We exist on a timeline.
There is only one timeline.
You travel along that timeline in one direction.
You travel along that timeline at a constant speed.
You cannot leave that timeline......................
a time machine is impossible because if it was EVER possible even 1 thousand years from now, we would have time travelers visiting constantly.
What has happened, has happened and nothing can change that.
What has happened, has happened and nothing can change that.
How would we ever know? Once something was changed, everything would be changed. All memories would be changed as well...
Some things are beyond human comprehension.
Go back in time to abort yourself? Sounds like a good idea. Avi Loeb goes first, I say. Harvard needs to change its name to Hamasvard.
Loebs cogitations may go the way of Schiaparellis “canali” on Mars.
You cannot leave that timeline......................
As far as we know.....
Have there ever been “plausible” incidents of time travel-like events happening in our times? A visitor of some sort who blatantly presents as out of place, with artifacts or knowledge that they should not have? I’m thinking that if in the future, time travel becomes possible and somewhat frequent, then either there is little interest in visiting “us” in our time or the time-travellers are very good at remaining in low profile mode?
Einstein’s Theory of Relativity does not preclude the development of a Time Machine.
A Time Machine may be possible, but it would be impractical.
The Universe is not static: everything in it is in constant motion.
Even if you were to create a ‘Time Machine’ it would kill you if you ever used it.
Even if you travelled to just one second in the past or future, you would find yourself in inter-galactic space with no air to breathe and no way to get back................
Most Freepers will say this woman is lying about Einstein and Roswell—but she is persuasive:
Both the author - and Prof. Pangloss.
Regards,
It suggests the author has no imagination.
______________________________________________________
I could not agree with you more. That fact that Hitler was allowed to do what he did could have been much better than what someone he eliminated would have been allowed to do. There are so many possibilities that it is impossible to even think of them.
Experiments with atomic clocks on jet airplanes prove that time is relative so that at least perceived time is different depending on where you are and how fast you are moving. There is a lot we don’t know, yet.
I believe in God, a God that can exist in the past, present and future. I don’t know that He does exist or travel in time or occupy all at once but believe He knows the future from the past which is more than my feeble self can do. So I have to believe that some sort of time manipulation exists. A good friend of mine who passed away a few weeks ago believed that time is a construct that God made especially for us humans. I don’t see that but maybe my imagination is not advanced enough to see that.
Butterfly effects, unintended consequences and IF time travellers exist because time travel is possible, who is to say the travellers have the ability to actually influence what is happening in the times they visit? Perhaps they can only observe events but only negligibly affect them?
The Hitler and Holocaust scenario frequently proposed is interesting because how does anyone know that it happened and was eventually crushed because doing so thus prevented an even worse Holocaust from happening later? Endless possibilities
Thanks Red Badger.
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Tyrone Davis "Turn Back The Hands Of Time" | My Extended Version! | mosogotam
All The Time In The World
Arthur C. Clarke
1952 short story read by Nicholas Boulton
https://archive.org/details/arthur-c.-clarke-all-the-time-in-the-world
Perhaps humanity does not survive long enough to develop the techniques required for Time Travel.
There is only the Present, this moment, the Now... the Past is memory, the Future is Imagination...that’s it.
https://www.youtube.com/watch?v=rWHo0Iw-bvU
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