Recently a reader, having read my post about why the speed of light seems so fast, sent me two questions that highlight important cosmic issues.
- Is there in fact anything within physics as it’s presently understood that indeed prevents […] there existing something other than atoms as some basic “unit”?
- I’ve long wondered why it is that despite the seeming brilliance of humans at building such complex understanding, we are still pushing at such limits as the time it would take to fly a space ship to another galaxy. Is it really true that nothing could ever exceed ‘c’ and thus we are indeed doomed to take lifetimes to travel beyond our solar system? Or is it because we have not yet discovered something much more fundamental about the universe, such as an ‘alternative to’ the atom?
These deep questions are examples of an even broader pair of questions about reality.
- Which aspects of the cosmos are contingent?—in that one could easily imagine a similar universe in which these details are thoroughly altered.
- Which aspects of the cosmos appear rock solid?—in that they are so deeply integrated into the universe that it is difficult to imagine changing them without ruining everything.
Could something other than atoms serve as a basic material unit?
The answer to this question is “absolutely yes.”
If we look at the composite objects that make up ordinary matter, we are looking at specific particles and specific forces. There are four levels of composite objects:
- protons and neutrons made from quarks and gluons via the strong nuclear force
- atomic nuclei made from protons and neutrons via the residual strong nuclear force
- atoms made from nuclei and electrons via the electromagnetic force
- molecules made from atoms via the (often residual) electromagnetic force
But the details are complex and have to do with the precise natures of the particles and the forces. A universe with different particles and/or different forces might make entirely unfamiliar composite objects—or none at all.
Here’s where the power of theoretical physics shows itself. We can in some cases calculate what would happen in an imaginary universe with its own types of particles and forces, and gain some insights into the composite objects that might result. More challenging is to figure out whether some macroscopic material, analogous to the ordinary large-scale solids and fluids we’re familiar with, could exist in that universe. But it’s easy to show that many types of composite objects could potentially exist in other, imaginary universes, and though different from our familiar atoms, they could nevertheless serve as building blocks for complex materials.
How about in our own, real universe? There’s still a lot we don’t know about it. Experiments leave open the possibility that there are types of particles that we haven’t yet discovered, perhaps entire classes of them. There are two reasons we might not have found them.
- Too Much Mass: If a particle’s mass is very large (roughly a thousand times the mass of a typical atom, or more), then we can’t make it directly in our particle accelerators; and if such particles are also rare in the universe, with very few of them surviving from the Big Bang, then we would neither find them on purpose nor by accident. They might form unknown forms of composite objects, through either known or unknown forces.
- Too Hidden: If a type of particle is insensitive to the known types of forces (excepting gravity), then our experiments would neither detect their presence nor be able to produce them directly. Such particles are referred to as “hidden” or “dark”. They might even make up dark matter, giving us a hint of their existence through their gravity, but no other direct information that would help us learn their details. If they interact with each other via their own set of forces, those forces might allow them to make unknown and hidden composite objects.
For some types of particles, both of these reasons could simultaneously be true.
Composite objects formed by these unknown particles, through known or unknown forces, could potentially be as complex and variegated as atoms. As an example, researchers have taken seriously the possibility that dark matter is made from some sort of exotic atom, formed from dark elementary particles and forces, and asked about the particle physics and astrophysics consequences. (Here’s one paper on the subject; here’s another more recent one.)
And so, both theoretical considerations and existing experiments allow for the possibility of an unknown material made from unknown basic building blocks or units. This is true both in the abstract, where we imagine other possible universes, and in the concrete, in that it may even be true in our own universe. It may be that dark matter, or some other substance as yet unknown, has this property.
Can the speed of light be exceeded?
Before answering this, one must state carefully what one means by this question; I have pointed out pitfalls here. The proper form of the question is:
- When two objects pass each other, could an observer traveling with one object find that the speed of the second object is higher than the cosmic speed limit?
(If you ask the question in the wrong way — for instance, if you ask, can I observe two objects whose relative motion is faster than the speed of light from my point of view? — then the answer is “yes, and it happens all the time; just look at two oppositely-directed flashlight beams, or, as viewed from the laboratory, the two proton beams in the Large Hadron Collider.” Clearly that’s not what the reader is asking.)
In any universe in which Einstein’s view of gravity (known as general relativity) is true, for which local processes are described by special relativity, taught in first-year physics classes, the answer would be firmly “no.” In such a universe, there is a unique, unbreakable cosmic speed limit that applies to all objects equally. The very nature of space and time prevent anything from breaking it.
For example, if you tried to overtake a light beam, you’d find that the faster you go, the faster the light would seem to go, too, making it impossible for you to catch up to it. (In my book, I called this the “nightmare property” of the universe, since it sounds uncannily like a certain type of bad dream.) No matter what you do to improve your chances, your experience of time and space will adjust in such a way that your efforts will fail. It’s not a matter of better technology. Even infinitely powerful technology cannot beat the universe’s basic structure.
It is widely believed that, in our universe, Einstein’s general relativity is correct to a very good approximation. It can’t be exactly correct, because it doesn’t meld well with quantum physics, which we know is another feature of our universe. When quantum physics meets space and time, it might not even be meaningful to define “speed”, at least not in a straightforward way. So there might be circumstances in which the cosmic speed limit does not apply in the ways we are used to.
However, it seems to me profoundly unlikely that any violation of the cosmic speed limit, induced perhaps by quantum physics, will permit humans to travel faster than light. We ourselves are creatures of ordinary space and time, and in any situation in which space and time behave in an extraordinary way, or in which we try to move across it an extraordinary way, would probably kill us. (I’ve just finished reminding you how fragile we are and why this means that we must travel slowly relative to our surroundings. As another unrelated but amusing example of this point, see section 3.4 of this paper, a light-hearted yet scientifically rigorous look at just how difficult it would be to make wormholes that humans or spacecraft could safely cross through.)
Even if you just wanted to send a message faster than light, you would presumably still want to be sending it across normally-defined space and time. The structure of the cosmos would likely ensure that you would fail.
This is not to say that we should be closed-minded about this question. Sometimes our understanding of the universe takes a strange twist, and there’s a lot about space and time that we don’t yet understand. But being open-minded is not the same as being empty-headed. Any chance of violating this basic cosmic constraint on space-time, at least in any way that would affect our ability to cross the cosmos, currently seems like a very, very long shot.
One more point: could there be imaginary universes with no cosmic speed limit at all? Maybe. But in such a universe, extremely distant events in the universe could potentially have an instantaneous impact on our lives. Cause and effect might be harder to understand, and it’s not clear (to me, anyway) that such a universe would function well.
Final cosmic thoughts about speed and time
The bottom line:
- No principle or experiment forbids the existence of unknown materials made from unknown basic objects, much as familiar matter is made from atoms.
- The existence of a cosmic speed limit seems to be a basic structural element of the universe; this statement is suggested by theoretical principles and has a solid experimental basis (see for instance this recent paper).
So it turns out, though this would hardly have been obvious a century ago, that it’s much easier to imagine replacing atoms with something else than to evade the cosmic speed limit.
As a last thought, let me add something regarding this part of the reader’s second question:
- Is it really true that nothing could ever exceed ‘c’ and thus we are indeed doomed to take lifetimes to travel beyond our solar system?
“Yes” for the first half of the question; but “no” (in a sense) for the second.
Even though nothing can exceed the cosmic limit under any familiar circumstances, it is still true that time can play tricks, as it behaves unexpectedly in our universe. It is possible in principle (though probably impossible practically, due to the difficulty of building suitably safe rockets) for you to travel to many stars, even all across our entire galaxy, in your lifetime. Unfortunately, for those left behind on Earth, your trip will take far longer than their lifetimes.
This is sometimes called the “twin paradox” (and it underlies the emotional plot of the movie Interstellar) but there’s nothing paradoxical about it. It’s just unfamiliar. It rests on a basic fact: the amount of time that you measure between one event and another depends on the nature of the journey that you took to get from the initial event to the final one.
Said another way: time is something experienced by each object separately, as measured by a clock carried along with that object, and it depends on how the object moves around within the universe. There is no universal clock that exists across the universe, and outside individual observers and objects, that can measure some universal notion of time.
Specifically, the amount of time that elapses for someone traveling far from Earth to distant stars and then returning home can be far less than the amount of time that elapses meanwhile on Earth. This is not an illusion or a trick; it’s just a fact about time that’s not at all obvious from daily life. The consequence is that you yourself could visit many stars, but your friends or family (and multiple generations after them) would be long dead when your rocket landed back on Earthly soil.
(Note: In a perfectly smooth and uniform universe, there would be some reasonable notion of “universal time”; and since our universe is approximately uniform on very large distance scales, there is an approximate notion of universal time, which is quite similar to Earth time, that is useful on very large distance scales. That’s why we can talk about “the time since the Big Bang”, using this approximate universal time, and say that the universe is 13.8 billion years old; it’s approximately true for observers and objects that have not moved rapidly relative to the average objects in the universe, such as typical galaxies and our own planet. But this universal time does not apply to, say, individual observers taking extremely rapid, complex round trips around the galaxy. Such observers may live far longer than 100 years of approximate universal time — though for each of them, life will feel just as long as it does for us, because the rate of their thinking, breathing and metabolism relative to the time they experience is the same as it is for any human. Again, see the movie Interstellar for illustrations of this effect.)
17 Responses
Well, thank you so much for such clear and thoughtful explanations. What a treat!
A few comments that arise from the above, in no particular order:
What does theoretical physics say about entanglement? Assuming I’ve correctly understood the thought experiment a pair of entangled particles is created, with the particles travelling in opposite directions. If the spin of one particle changes, the spin of its entangled pair also ‘instantly’ changes. And that this property holds true. Thus it would be easy to think that before much time has elapsed (to an observer) the amount of time necessary for any ‘information’ about spin state to travel between the two particles would be noticeable. Of course… I may again be misunderstanding an aspect of what relativity says about our universe.
It was very interesting to note that there’s nothing preventing there being alternative types of basic material units in our universe, and furthermore that such “dark matter” can be thought of as a class of such units (or whatever they should properly be called). So perhaps all of the currently unsolved properties for things such as gravity belong to this class and not their currently assigned class of ‘regular’ stuff (meaning quantum or classical forces)? But a lack of knowing how to examine ‘dark matter’ is a barrier to experiments that would enable us to understand it?
Entanglement: “If the spin of one particle changes, the spin of its entangled pair also ‘instantly’ changes.” This isn’t the right way to think about it. Reality is already stored non-locally, and the particles’ spins are correlated: whatever the spin of one, the spin of the other is the opposite. If the spin of one particle is then *measured*, the spin of the other is consequently *determined.* That is not about changes, it is about reality about the spins becoming more definite (by correlating at least one spin with a measuring device.) And this does not involve faster-than-light travel of any object, nor is it possible to send information faster than c using this fact, as someone measuring the second spin cannot recognize that it has been determined without talking to the person who measured the first spin, which takes time.
Gravity: Dark matter (assuming it exists) is a substance that, like all other substances, is acted on and causes gravity; it may also be subjected to other forces of its own. Gravity is just gravity; it affects everything in the universe. These sets of ideas are not in the same class, and can’t be put into the same class as their properties are very different.
Dark Matter: “a lack of knowing how to examine ‘dark matter’ is a barrier to experiments that would enable us to understand it?” Yes, it is a barrier. Experiments that search for dark matter directly or indirectly always have to make assumptions about how it might interact with ordinary matter. If dark matter only interacts with ordinary matter via gravity, then only measurements such as gravitational lensing will be able to tell us about it, and that will be a long, slow road.
Recently, I came across a beautiful video explaining the fundamentals of electromagnetic radiation, what is light? [3Blue1Brown on YouTube]
Fine, EMR is the wave created by a charged particle. But, what is a charge? I mean the fundamentals of a charge, one step below the creation of the electric field that propagates the waves.
What is the physical explanation of a charged particle. What is going on in the electron cloud that creates this property of charge.
Can I assume it’s the fluctuations of a magnetic field with the electron and if so how is the magnetic field created with the boundaries of the electron?
A question about artificial gravity: while it is possible to travel to a different galaxy in a (subjective) human lifetime, it would still take years to accelerate to relativistic speeds with an acceleration that we can withstand.
Is it concievable or compatible with what we know about the universe that it would be possible for a spaceship to accelerate mich faster than that, while the travellers won’t experience strong acceleration?
I think I agree, having done a quick calculation, that for nearby stars the travel is doable but for the Andromeda galaxy it is not, unless one increases human lifetimes by a factor of order 10 by biomedical means.
Not only is there no known way to shield gravity (which is equivalent to acceleration, under general relativity), I personally suspect that the unshieldability of gravity is a central principle of the universe… that the very nature of space in quantum gravity makes it impossible to avoid gravity, and thus (equivalently) the effects of strong acceleration.
(I had not thought about much about this limitation because there’s another practical problem that kicks in first: collisions of the ship with stray atoms and even cosmic microwave photons will produce a high-energy particle bath in the ship that will kill the occupants. Most of these particles could be shielded, unlike gravity, but the ship would require a vast amount of shielding, making the whole idea impossible.)
Clearly this is a job for radiation-hardened, long-lived robots. But it’s not clear anyone from our species’ descendants will be around to receive messages back.
Questions one and two are linked to each other.
We would be narcissist to think we are alone in the Universe. Such as our Theological doctrines, we have always believed that there is no place where there is no world and the same is applicable to laws. Just because we cannot observe the multiplicities of celestial objects because of time or space(distance) and to limit our mindset to extra terrestrials as humanlike also limits our possibilities of what to expect to see. Life in frozen planets of methane and those in extremes like of kelvin, indeed exist. One would say..prove it… This is where the concept of moving beyond the dimensions of the ‘atom’ , including moving faster than light without the inherent barrier of force and fields that impact solid matter such as a mortal or a spaceship.
However in order to achieve such, one would have to experience such beyond the limitations of a physical body or physical spacecraft.
We are made of three components, Mortal (clay), spirit (higher refined matter) and Mind or Intelligence. In order to seek the answers beyond the test tube and telescope one would have to apply the last two. It has always been our belief that there is no such as ‘immaterial matter’, hence as you scientists say ‘The law of the Conservation of ‘Matter and Energy’ and that matter and that energy have always existed and cannot be destroyed only altered and if that be the case, can any of the latter two entities of the soul be altered to achieve phenomenon beyond the our 5 limited senses.
To go down that road is to step into the assumed unknown. But is the unknown achievable? Indeed it is, but to do so is to step out of the lab and into possibilities that presently seems impossible.
Biology is produced by the process evolution, and that means cells and bodies constitute biochemical machines, no more and no less.
I don’t think physics can be said to have personal traits such as narcissism, and AFAIU anthropic multiverses they suggest that language capable organisms may be alone in their universes. (But more likely only very rare, necessarily since evolution of language was late and rare on Earth however habitable our universe is.)
“One more point: could there be imaginary universes with no cosmic speed limit at all? Maybe. But in such a universe, extremely distant events in the universe could potentially have an instantaneous impact on our lives. Cause and effect might be harder to understand, and it’s not clear (to me, anyway) that such a universe would function well.”
It is widely recognized that time exists so not everything happens at once, and that pace exists so not everything happens to ‘me’.
If the cosmic speed limit was zero or infinite one spacetime dimension would effectively collapse, and I doubt structures (say) would exist.
“In any universe in which Einstein’s view of gravity (known as general relativity) is true, for which local processes are described by special relativity, taught in first-year physics classes, the answer would be firmly “no.””
“When quantum physics meets space and time, it might not even be meaningful to define “speed”, at least not in a straightforward way. So there might be circumstances in which the cosmic speed limit does not apply in the ways we are used to.”
Special relativity with its cosmic speed limit underlies quantum field theories, so [apparently being a doubter today] I doubt that they can break it. That it is a space property shared with all fields makes Lorentzian spacetime [reneging my doubter attitude] universal.
If one wants to make gravity work in space and works with a classical as well as geometric model, general relativity is what you get. I note that this is the opposite view from taking general relativity, which isn’t quantized, as enforcing special relativity on space. What happens if, as it seems likely, we discover that gravity is quantized? [Chris Overstreet, Peter Asenbaum, Joseph Curti, Minjeong Kim, “Observation of a gravitational Aharonov-Bohm effect”, Science (2022), editorial note: “The Aharonov-Bohm effect is a quantum mechanical effect in which a magnetic field affects the phase of an electron wave as it propagates along a wire. …”; Tobar, G., Manikandan, S.K., Beitel, T. et al. “Detecting single gravitons with quantum sensing.”, Nat Commun 15, 7229 (2024). “… Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity.”]
Recently I saw a lecture by Claudia de Rham at the Perimeter Institute and she said if you include the “vacuum energy” and Einstein’s GR, the universe should be expanding an order of 28x what is observed. She goes on to say that if gravity does not have infinite reach but instead if gravity has a limited, finite, range then we can explain the expansion.
So, my question is to try and combine this article ad her lecture, can a unified theory be formulated with ONE force having different ranges, much like radiation spectrum, making up all the forces we have measured thus far. And, yes, gravity would be the “flavor” of the unified force with the longest range (reach). This theory could suggest that there are infinite forces, a continuum that could explain the entire range of physics from below Plank’s scale to Einstein’s universe with GR.
On the second question, your answer does of course bring to mind the concept of the Alcubierre drive; I’ve heard mixed messages about whether the idea has been disproven or not, but do you have any more particular insights on it? Is there a fundamental flaw in the reasoning, or perhaps just a logistical problem in the apparent lack of “negative matter?”
For that matter, of it DID work, what would that even look like? What would an outside observer see when the bubble travels? What would happen if the bubble ran into something? What evidence might we look for astronomically that it would be possible, or even something like it already existing?
The real problem with faster-than-light travel, no matter how you run the details, is the risk of a breakdown in causality, whereby, by traveling faster than light and doing something very far away just a few seconds from now, you do something which (from some other observers’ points of view) was actually done in the past. That’s a basic issue in special relativity: what looks like faster-than-light-forward-in-time travel from one perspective looks like faster-than-light-backward-in-time travel from another perspective. And if that’s possible, then so is traveling into your own past, by going out and back. [slow but instructive animated gif: https://upload.wikimedia.org/wikipedia/commons/2/25/Ftl-time-travel-equivalence.gif ] So I would view the Alcubierre drive proposal, if correct, as evidence that negative matter is inconsistent with a causally sensible universe.
There are similar issues with closed time-like curves in general relativity.
For these reasons I don’t even think there are sensible answers to the scientific questions you asked in the last paragraph. It would be somewhat like looking for evidence of perpetual motion machines; they would long ago have destabilized the universe and any notion of cause and effect.
There have been a fee papers looking into this, and popular youtube channel PBS Spacetime did an episode on how we might detect warp bubble destruction via gravitational waves. Indeed since it (theoreticaly) works by warping space, a lot of its effects should mirror gravitational waves. Distortion and red\blueshifting of light as the bubble passed, disruption of matter caught in the bubble’s path and so on. It helps that in a lot of formulations you cannot accelerate either within th ebubble or the bubble itself, so you’re stuck in one inertial frame for the duration.
The only way you’d see evidence would be if things went wrong; the easiest way to build this is a lot of negative mass. The whole setup is quite extreme and has a lot of energy in it. Enough to simply splatter smaller things in its path, but if it hit a planet… oof. (Plus there’s the question of how you build and dismantle the bubble, which is stable as-is but prone to violently dissipating is poked too hard.) A bubble bursting would be a cosmic event.
Dr.Strassler:
This statement may be off the topic of “different building blocks” but I have read, mostly in biology journals, that carbon is the back bone of life, because of the many ways it can bond to itself, and many other elements. However, it is plausible that silicon could also be a back bone, somewhere in the universe, although the life forms built from a silicon back bone would have the odd property that their waste product would be sand like.
In interstellar, on the planets where time is running much slower, than earth time, wouldn’t the gravity gradient for people on that planet be very uncomfortable?
On your second point: no. Interstellar’s physics is correct on that issue. The tidal force from the gravity gradient, which could be so unconfortable, is proportional to the distance across the living creature, which is tiny relative to the scale of the object being orbited. Meanwhile the gravitational redshift, and ensuing time dilation, is a property of the spacetime only. So a small creature on a planet orbiting a black hole would be time dilated relative to us but would not feel much tidal force. Gravitational redshifts can be very large when you orbit outside a large rapidly rotating black hole, but the tidal forces become extreme only deep inside the horizon as you approach the singularity. Some discussion on precisely this point is found in https://relativitydigest.com/2014/11/07/on-the-science-of-interstellar/
But the point I make in this post is much simpler; the time dilation due to very rapid round-trip travel can be very large, without any issue of tidal forces, gravity, etc. Some of Interstellar’s time dilation has only to do with motional time dilation, not with gravitational time dilation.
Ah, ok. I thought it was the gravity of the planet itself that was causing the time dilation. So, it’s the gravity of the black hole, the planet is orbiting, causing the time dilation at the LOCATION of the planets orbit, is that correct understanding? Wouldn’t the planet itself cause some time dilation? I’m assuming it would be small compared to the black hole.
Yes, that’s correct.
Sure, the planet will cause some time dilation. So does Earth. https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment But this is tiny… and independent of the effect of the Sun or of the black hole, which gives an overall time dilation of the whole system relative to someone very distant from any gravitating objects.
I don’t think silicon is ‘plausible’; when your main source of mass is an unreactive crystaline solid rather than a mobile gas, that’s a big issue. There’s a reason carbon-based life doesn’t make too much use of silicon. (As opposed to, say calcium.) Plus, silicon HATES single bonds, its compound polymerize easily and are incredibly oxygen hungry. We’ve detected alcohol in space, but silicon always seems to be tightly bound to oxygen. I’d back nitrogen over silicon any day of the week.